The Recent Sedimentology of Scolt Head Island, Norfolk

THE RECENT SEDIMENTOLOGY OF SCOLT HEAD ISLAND, NORFOLK

Peter Stanton Roy

VOLUME II

A thesis submitted for the degree of Doctor of Philosophy at the University of London

Geology Department, Imperial College of Science & Technology

February 1967 LIST OF CONTENTS - VOLUME II Section Page 5. THE MORPHOLOGY AND THE HISTORICAL DEVEL- OPMENT OF THE NORTH NORFOLK COAST 253 5.1 Coastal Morphology 253 5.2 The Historical Development of the North Norfolk Coastline 255

6. SCOLT HEAD ISLAND 261 6.1 Physiographic Setting 261 6.2 The Evolution of Scolt Head Island 262

7. THE BEACH ENVIRONMENT 272 7.1 Introduction 272

7.2 The Morphology of the Scolt Head 277 Island Beach Environment

7.3 The Scolt Head Island Back-beach 282 Sub-area 7.3.1 General Discussion 282 7.3.2 Back-beach Ridges and Runnels 283 7.3.3 Transverse Ridges 284 7.3.4 Sediment Movement Associated with 285 a Ridge and Runnel System

7.3.5 Possible Mode of Origin of a 288 Transverse Ridge 7.3.6 Beach Cusps 290 7.3.7 The Origin of the Gravel 291 7.4 The Scolt Head Island Fore-beach 296 sub-area 7.5 Sediment Movement in the Beach and 300 Near-shore Zones Page 7.5.1 General Discussion 300 7.5.2 On-shore Sediment Movement 303 7.5.3 Along-shore Sediment Movement 309 7.6 Holkham Beach Sub-area 316 7.7 Brancaster Golf Course Beach Sub-area 320 7.8 The Grainsize Characteristics of the Sediments of the Beach Environments of Holkham, Scolt Head Island and 322 Brancaster Golf Course 7.8.1 Introduction 7.8.2 Sediments Collected along the Holkham Beach Profiles 324 7.8.3 Sediments Collected along the Scolt Head Island Beach Profiles 326 7.8.4 Sediments Collected along the Brancaster Golf Course Beach 331 Profiles

7.8.5 Samples Collected along Scolt Head 332 Island Beach at Low Water Level 7.8.6 General Characteristics of the Beach Sediments 336 7.8.7 Conclusion 338 7.9 The Brancaster Harbour Bar Sub-area 362 7.9.1 Morphology and Dynamic Conditions 362 7.9.2 Sand Tzansport 372 7.10 The Grainsize Characteristics of the Sediments Comprising the Brancaster 374 Harbour Bar Subarea 7.10.1 Conclusion 403 7.11 Burnham Harbour Bar Sub-area. Morphology and Sediment Movement 410 7.12 The Grainsize Characteristics of the Sediments Comprising the Burnham 416 Harbour Bar Page

7.12.1 General Discussion and Conclusions 430

8. THE DUNE ENVIRONMENT OF SCOLT HEAD ISLAND 443

8.1 Morphology 11'13 8.2 Grainsize Characteristics of the Sediments of the Dune Environment ii)I8 8.3 Origin of the Dune Sands 463 8.4 Wind Blown Sand Measurements 467

9. THE TIDAL INLET ENVIRONMENT 479 9.1 Introduction 479 9.2 Genesis of a Tidal Inlet Environment 481 9.3 The Salt Marsh Sub-environment 489 9.4 The Inlet Channel Sub-environment 490 9.5 Mow Creek, Norton Creek and Brancaster Harbour Channel 492 9.6 Marsh and Marsh Creek Sedimentation 501 9.7 Detailed Investigation of Specific Depositional Features within the 513 Inlet Channel Sub-environment 9.7.1 Cockle Bight Bar - Physiography and Dynamics 516 9.7.1.1 The Western Subsidiary Bar 527 9.7.1.2 The Eastern Subsidiary Bar 527 9.7.2 Sediment Movement over Cockle Bight Bar (excluding the subsidiary bars) 534 9.7.2.1 Brancaster Channel 535 9.7.2.2 Cockle Drain 537 9.7.3 Norton Creek Bar 545 9.7.3.1 Theoretical Considerations as to the Genesis of the Sediments 551 Comprising the Norton Creek Bar Page

9.7.4 Overy Cockle Strand 568

9.8 Mega Ripples 572

9.9 Direct Measurements of Current Action and of Sand, both in Suspension and on 581 the Channel Bed

Station 1 588 Station 2 591 Station 3 595 Station 4 598 Station 5 602 Conclusions 625

9.10 General Conclusions 636

10 STATISTICAL ANALYSIS 639

10.1 General Discussion 639

10.2 Average Grainsize Characteristics 642 of the Sediments occurring in the Various Sub-areas 10.3 Average Modal Distributions Character- ising the Sediments of the Various 646 Sub-areas 10.4 C.M. Plots 652 10.5 Investigation of the Non-environmental Relationship between the Grainsize 655 Parameter Values 10.6 Theoretical Considerations on the Mechanism of Sediment Fractionation 656

11.1 Conclusion 691 11.2 Limitations 693 11.3 Possible Applications of this Study 695

REFERENCES 699 Other Works Consulted 708 253 THE MORPHOLOGY AND THE HISTORICAL DEVELOPMENT OF 5 OF THE NORTH NORFOLK COAST

5.1 Coastal Morphology 5 . 1

The N. Norfolk coast extends from S.E. of Cromer W.N.W. to Blakeney Point, westwards to Hunstanton and then southwards into the Wash. The coastal physiography changes in character from E. to W.: cliffs with a steep gravel beach are found to the E. of Blakeney Point, wide, predominantly fine grained, sand beaches backed by gravel ridges between Blakeney Point and Hunstanton, and gently sloping sand, and mud flats to the S. of Hunstanton. This change in coastal physiography, and in sedimentary environments, reflects a progressive decrease in effective wave energy towards the W. (see section 3.4). A strip of sandy beach, which varies in width from 800 feet at Scolt Head Island to over 2 miles off Stiffkey, occurs between Blakeney Point and Hunstanton. The low water level, and to some extent the high water level, show a sinusoidal configuration with seaward convexities, or lobes, occuring at Holme, Scolt Head Island, Wells and Blakeney (see sheet 2). Holkham Bay and Brancaster Bay occupy the shallow concavities formed between the lobes of Wells, Scolt Head Island and Holme. Examination of the latest Ordnance Survey map shows that the eastern sides of these lobes, as denoted by the I.W.O.S.T.L.I are approximately straight, while the western sides are more irregular. The eastern side of Blakeney 254 lobe is orientated at 113°, that of Wells lobe at 105°, and that of Scolt Head Island at 102°. The characteristic straightness and west-north-westward trending orientation of these features may be explained (as first suggested by V.W. Lewis (1931), and demonstrated experimentally by Bruun (1953)), by the tendency of gravel and sand ridges, when exposed to wave action, to orientate themselves perpendicular to the direction of strongest wave approach. It has been established in section 3.4 that dominant waves approach the N. Norfolk coast from the N. and N.E. Because of the orientation of the Y. Norfolk coastline, the effective energy of these waves, expended on the Y. Norfolk coast, will decrease towards the W. It seems probable that the higher the wave energy, the more rapidly and effectively will the reorientation of a gravel ridge be achieved. It is proposed, therefore, that the difference in orientation between the lobes of Blakeney Point and Scolt Head Island reflects the relatively greater success of the former towards achieving a N.W.-S.E. trending orientation. Furthermore, wave refraction will cause a slightly more northerly direction of wave approach at Scolt Head Island than at Blakeney Point (Steers 1960). Thus, in the case of Scolt, an equilibrium orientation normal to the local direction of dominant wave appreach will be attained at a lower angle than in the case of Blakeney Point. These seaward convexities or lobes are commonly associated with gravel ridges which are superimposed upon the sandy beach. Wind blown sand has accumulated to form dunes on the top of these gravel ridges. However, a broad 255 beach development, such as occurs between the Blakeney Point and Wells lobes, and between the Wells and Scolt Head Island lobes, may afford sufficient protection from storm waves to allow dunes to form without the presence of a gravel ridge. Dune ridges, formed under either one or the other of the above mentioned conditions, occur at the top of the beach, between Wells and Brancaster, and also at isolated patches on Blakeney Point and at Holme Beach. Unconsolidated salt marshes, often reclaimed, lie behind the sandy beaches. These marshes abut against, and overlap undulating Pleistocene boulder clay deposits. These boulder clays extend northwards beneath the North Sea. and overly the glacially eroded chalk. During the rise in sea level, gravelly sand barriers formed along the coastline. Fine grained sediments have accumulated behind these barriers to form marshes.

5.2 The Historical, Development of the Y. Norfolk Coastline 5.

The historical development of the N. Norfolk coastline has been studied by using the Ordnance Survey maps for the years 1827, 1871, 1907, 1950 and 1959. These have been reproduced on an E-W scale of 1/75000with an exaggerated Y.TS scale of 1/30160 (or 1 inch = 0.477 miles), (see sheet/. Reservations are held by the author as to the accuracy of the L.W.M.O.T. level indicated on the maps produced prior to the utilisation of aerial photography. However, as long intervals of time separate consecutive surveys, any major trends observed are believed to be 256 qualitatively significant, although of only limited quantitative value. Sheet 7A shows the 25 ft. contour line, the H.W.M. 0.T. level for the years 1871 and 1959 and the L.W.M.O.T. levels for the surveys of 1827, 1871 and 1959. The L.W.M. 0.T. levels of consecutive surveys have been compared graphically in sheet 7B on the same scales as in sheet 7A. The deposition and erosion between the consecutive surveys is indicated by the displacement of a curved line above or below the horizontal ordinate, respectively. The significance of the amount of erosion or deposition shown in this way is dependent upon the beach slope. The horizontal displacement of the shore line on a steep beach is indicative of a greater sediment movement than is a similar displacement on a gaily sloping beach face. An approximate quantitative estimate of the relative volumetric changes has been presented in sheet 70 by summing the periodic changes in consecutive surveys shown in sheet 7B, and by dividing this total distance by the average width of the beach. The 25 ft. contour in sheet 7A represents the almost straight E-W orientation of the chalk mainland or "basement" on which Pleistocene, and later, Recent sediments have accumulated. The 25ft. contour is shown as a discontinuous line in which the breaches represent the positions of glacially(?) formed valleys occupied at present by the Glaven, Stiffkey and Burn Rivers. It is tentatively proposed, based upon the conjunction of these main river valleys 257 and the seaward convexities, or lobes, of Blakeney Point, Wells and Scolt Head Island, (sheet 7A), that the latter features were originally of fluvioglacial, deltaic origin. The post glacial streams which occupied these valleys produced outwa.sh fans to the N. in the wake of the retreating glaciers. West (1963) and Hardy (1964) have proposed a similar (although a more directly glacial) origin for the Blakeney Ridge feature. At the beginning of the Atlantic period approximately 6000 years ago, the sea transgressed over these deltas carrying in front of it reworked glacial debris. The refraction of waves around these features tended to concentrate the gravel to form ridges. Processes of winnowing and sorting removed the mud and silt fraction to the lower energy environments (found behind the protective gravel ridges) to form salt marshes. A tendency probably also existed for the fine grained sand to accumulate in the shallow indentations of Holkham and Brancaster Bays (note the broad sand flats to the W. of Blakeney Point). The gravel accumulations, which in the case of Scolt Head Island formed a barrier island (Shephard 1952), may be regarded as "fossil" accumulations (Hardy 1964). However, as sea level is still rising slowly, this process is active and its effectiveness may be judged by the relatively large amounts of erosion of Blakeney Point and Scolt Head Island in the past 140 years. The progressive landward migration of the main dune ridge on Scolt Head Island and elsewhere between 1906 and 1951 has been noted by Steers (1960). 258 It appears obvious from an examination of the changes in the L.W.O.S.T.L. shown in sheets 7B and 70, that the general tendency is one of coastal recession. The most significant phenomenon displayed in sheet 7B is the consistent trend from 1827 to 1959 of accretion in Holkham Bay, and the equally consistent displacement of this area, of accretion towards the W. Deposition of sediment in Holkham Bay may be explained in terms of the cicrulation pattern, mentioned in section 3.6, which results from the deflection of the eastward flowing flood current by the Bridgirdle (see sheet 2). Fine grained sand transported by this current will tend to accumulate in Holkham Bay and, undeF the combined influence of the clockwise circulation and wave action will migrate shorewards (see Fig. 3.1.). The pronounced area of erosion which occurs immediately to the E. of this area of deposition represents the continuous erosion of the western edge of Wells lobe since 1827. The westward migration of both the area of deposition and that of erosion (although less marked in the latter case), bears witness to the local dominance of the ebb current which flows to the W. in the near-shore zone at this locality. This residual westward direction of sediment movement is supplemented by the dominant westward littoral drift (see section 3.3). Bruun (1962) has proposed a theory relating coastal erosion to a rise in sea-level, based upon the concept of the equilibrium profile (Bruun, 1954, 1955). Upon a rise in sea level, material will be eroded from the coast and re-distributed on the sea floor which will tend to neutra- FIGURE 5.1,

Fig. 5.1 Illustrating tbe-influence of a rise in sea level (from S.Z.: to S.L')-:interms of erosional Icrosshatched area) and depoSitional (black)- effeCts Upon a coastal and near-shore zone (after Bruun; 1962) 260 lise the increase in water depth and effect a return to equilibrium conditions (see Fig. 5.1). Wave tank experiments carried out by Schwartz (1965) support Bruun's theory. Present vertical movements, apparently due to isostatic readjustments, together with eustatic changes due to the melting of the ice caps, are, according to Valentin (1953), producing at present a rise in sea level, relative to the coastline of England, between 1.7 and 2.3 mm/year. Thus it is to be expected that coastal erosion will result. The achievement of equilibrium conditions, which is the end product of Bruun's theory, has been pro- hibited in the area under discussion by the dominantly erosive action of the strong tidal currents in the near- shore zone. That this state of disequilibrium has persisted is demonstrated by the general erosion and retreat of the N. Norfolk coast since 1827 (Steers, 1960 and sheet 70). A further manifestation of this rise in'sea level may appear as a possible increase in the tidal regime with a subsequent increase in current velocity. This in turn may possibly be the direct cause of the general erosion effecting the Burnham Bank Complex (see section 4.5). This offshore erosion and deepening will tend to increase the disequilibrium between the balancing forces, of rising sea level, coastal erosion and near-shore deposition, implied in Bruun's theory, thus increasing the rate of coastal recession. 261 6 SCOLT HEAD ISLAND 6

6.1 Physiographic Setting 6.1

Scolt Head Island, which has been selected for the detailed sedimentological studies described below, is situated on the N. Norfolk coast between the villages of Burnham Overy Staithe and Brancaster (sheets 2 and 8). At high tide it forms an E-N elongated barrier island. As shown in sheet 8, a beach, which varies in width at low tide from between 800 to 1200 ft., extends in a gentle arc, convex to seawards, for 3.6 miles in an E-W direction along the front, or seaward side of the island. Dunes have developed on a gravel ridge which occurs on the landward edge of this beach. Numerous lateral gravel and sand dune ridges are seen to diverge to the S. and S.W. from this main ridge. At either end of the island, the extension and expansion of the beach produces broad flats of sand (Brancaster Harbour Bar) in the W., and of mixed sand and gravel (Burnham Harbour Bar) in the E. East of Burnham Harbour the beach becomes increasingly wide reaching a maximum width at Holkham Gap. Immediately S.W. and W. of the island, Brancaster Golf Course beach extends to the W. The tidal inlets of Braneaster Harbour and Burnham Harbour are connected behind the island by Norton creek, and so isolate Scolt Head Island from the mainland. Salt marshes have developed behind the main dune ridge on Scolt Head Island, and on the mainland side of Norton Creek. 262 These marshes are dissected by numerous tidal channels and marsh creeks. The nomenclature of the physiographic features on Scolt Head Island and the adjacent areas is shown on sheet 8.

6.2 The Evolution of Scolt Head Island 6.2

The evolution of Scolt Head Island has been thor- oughly investigated by Steers (1960) who compared the position of the H.W.O.S.T.L. of various maps of the island published between 1825 and 1953. Two main trends are apparent: (1) the island has progressively grown towards the W., (that part of the island to the W. of Long Hills 4see sheet 84., has apparently formed after 1825; (2) there has been a general shoreward retreat of the island both at the low and high water levels (see Steers, 1960 and sheet/. The westward growth of the island is believed, to be directly related to the changes in the Burnham Bank Complex. As proposed in section 3, this triangular bank complex was originally much larger and included Race Bank further to the N. At present the Burnham Bank Complex causes the dominant wave trains approaching the N. Norfolk coast from the N. to be refracted and concentrated near the western end of Scolt Head Island (see Fig. 3.4 D and E). Due to this refraction, coarse grained material will be concentrated in the area of high energy as a result of converging directions of beach drifting. However, owing to littoral water movements away from the area of high energy 263 waves, fine grained sediment will be transported towards the lower energy areas to either side of this high energy area. Under the original bank configuration, it is believed that wave refraction caused the concentration of wave energy to occur some distance to the E. of its present position. As the bank complex gradually decayed this high wave energy area migrated westwards, and with it, the area of coarse grained sediment accumulation. When the original high energy area existed at the eastern end of Scolt Head Island, the area to the W. of this point would most probably have been influenced by a residual eastward direction of beach drifting. This would explain the existence of the remnants of a gravel ridge system found within Brancaster marsh behind Scolt Head Island. These remnants appear to belong to the Brancaster Golf Course ridge system (see section 9.4). The orientation of the lateral ridges in the Brancaster Golf Course ridge system, together with trends observed within historical time, indicated that these gravel ridges have been produced by an eastward migration of coarse sand and gravel. As Steers suggests, the relative rate of westward growth of Scolt Head Island is indicated by the lengths of the lateral ridges. Conditions conducive to the formation of a long lateral (such as Long Hills, see sheet 8), indicate a. cessation in the westward growth of the island for a. comparatively prolonged period. The short, poorly 264 developed laterals, which occur to the W. of Long Hills, probably reflect a period of relatively rapid westward growth. This westward growth has taken place by the progressive accretion of gravel to the western end of the main shingle ridge. The effect of beach drifting of gravel, under the influence of winds from the N.E., has been demonstrated by field experiments using marked pebbles (Steers 1960). Periodic N.V. storms have at various times reeurved the western ends of the main gravel ridges (partly pro- tecting them from further erosion), to form laterals. The low energy environment produced behind these ridges induces mud flats and salt marshes to develop. (Note the occurence of fine grained sediments behind the newly formed gravel ridges shown in Fig. 7.31). Detailed surveys of the Ternery end of Scolt, carried out between 1928 and 1958 (Steers 1960), demonstrate the complexity of the changes which occur over a short term period. During the last three years the main gravel ridge has migrated westwards, and has been recurved southwards, producing two small lateral gravel ridges on which embryo dunes have begun to form. Providing that the new gravel laterals can withstand the destructive effect of N.W. storms and gales, new salt marshes will eventually begin to form in their lee, further adding to their stabilization (see below), and another step in the phenomenon of westward extension of Scolt Head Island, as described by Steers (1960), will have been achieved. -n GRRVEL GRRVEL AIME 0 RIDGE SIRGE 2. .0., STAGE I. .., 0 ,..

• 0 0 0 0 r,„ o ° o 0. 0 0 0 ''''' 0 0 0 ..0' . 0 0 N i 0 0 a MHWOTL aNgi 0 0 0 0 0 0 0 0 a ...- oe op ,.. , , _ _ '41. 0 0 o w 0 0 • ..,. MARSH .0#4. oN 0 0 0 *•• ••••• .00. 0/111111r1IIIII.... • 0 IIIIIIIIIIII ...., °... BEAU.' 0 ' 0 .,...... ",...1°. 0...... :...0,..._°•-•••••:-......

0 '0 : °-:.---: f 7.#' .. • : • : .• •

MLWOTL.

•.•

Fig. 6.1. Illustrating the shoreward migration of a gravel ridge undet retrograding conditions, and the resulting exposure of salt marsh on the beach face.

0 266 The development of a salt marsh behind a dune ridge is essential for the stabilization and permanence of physiographic features such as Blakeney Point and Scolt Head Island. Since the high energy conditions which form a gravel ridge also cause it to retreat shorewards, the presence of an adjoining marsh allows this latter process to take place without a decrease in the elevation of the ridge above sea level, which is such a. critical factor in its survival (see Fig. 6.1). This is demonstrated by the movement of shingle at the W. end of the island. Although large quantities of gravel have periodically extended as ridges to the W. of the Ternery (Steers 1960), strong winds from the Y. and N.W. have spread the shingle over a broad area to the S.W. extending southwards to Far Point.

In the last 4. years, however, a ridge has maintained itself long enough to allow a marsh to begin to form in its lee. It is in this manner that the island has grown to the W. The main dune ridge of Scolt Head Island, between Burnham Harbour and Hut Hills, is orientated slightly Y. of W. To the W. of Hut Hills, however, the coastline is orientated slightly to the S. of W. (sheet 2). This change in orientation is believed by Steers (1960) to have resulted from the tendency of the island to reorient- ate itself in accord with the direction of dominant wave approach. This tendency will force the distal end of the island into deeper water (Brancaster Road) where it will be subjected to increased wave (and current) action. FIGURE 6.2.

1886

1965

Fig. 6.2 Showing the configuration of: Sco1 Head Island (at the low and high tide levels) at various stages of its growth. The directions of dominant wave action on the coast., a:(0 current action in Brancaster Harbour, boaeter with the resulting direction of growth, arE indeated by arrows. 268y This will subsequently drive it shorewards again. /9 It seems possible that the changing position of Brancaster Harbour (which has migrated to the W.), may also be responsible, to some degree, for this change in orientation (Fig. 6.2). During the time Brancaster Harbour mouth was situated in the vicinity of Long Hills and Wire Hills (Fig. 6.2A), the orientation of the channel was approximately N-S. The jetting action caused by the ebb tide flowing out of the harbour mouth would tend to transport sediment northwards. Once outside the harbour mouth this direction of transport would have been opposed by wave action, thus causing sediment to accumulate near the harbour mouth. Subsequently, the long shore forces of tidal current and littoral drift would have tended to disperse the sediment in much the same manner as it does today. The progressive westward migration of the Ternery subsequently deflected Brancaster Harbour channel to the W. (see Fig. 6.2 B and C). The jetting action of the ebb current at the harbour mouth thus afforded less and less opposition to the on-shore sediment transporting action of the waves. This is reflected by the southward deflection of the coastline to the W. of Hut Hills. At the present time the jetting action acts at an acute angle to that of the direction of wave approach, with the result that sediment is moved onshore and to the W. 270 by the waves and currents of the North Sea. Since no large rivers exist in the vicinity by which significant quantities of new material may be added to the system, it is felt that the general uniformity of the sediments in a spatial unit (i.e. an environment) in this area, reflects a. relatively constant energy level. The primary considerations in the following study have been directed towards gaining an understanding of the relationship between the existing dynamic controls (wind, waves and currents), and the resulting sedimentary deposits. Direct measurements of the physical energy conditions (or the level of physical activity), existing in the study area have been made only superficially. The relative effects of wave and current action on the mechanics of sedimentation (transport, erosion and deposition), have been implied by the application of the results of previous experimental and field observations of similar relationships. The sedimentary deposits occuring in the study area have been delineated primarily upon their broad morphological characteristics into Beach, Dune and Tidal Inlet environments. Further division into sub-environments have been made using more detailed sedimentological and morphological criteria. FIGURE 7-1 E RIDG

ERCH

-B DUNES K R 6FIC H O

HWOSTL ERC TEA -B E

a FOR -J _I

w AIN a .w wz 0 x 0 z M z = 0 in n a o a et •TOE OF BEACH u. re . . L WOST L t...o h

FORE - bERCH IIRCK -13EFIC I-1 NEFIR- (or tow - tide. SHORE tcrrace) ZONE FORE -. SHORE BRCK-SHORE

Fig. 7.1 Beach terminology used -this discuSsion 272 7 THE BEACH ENVIRONMENT 7

7.1 Introduction 7.1

The Beach environment discussed in this section includes not only the Beach on Scolt Head Island, but also the Holkham and Brancaster Golf Course Beaches (see sheet 8). The Beach environment has been divided into the fore-beach and back-beach sub-environments. This sub- division does not conform to the recognised terminology of back-shore and fore-shore used in the U.S.A., (Shephard 1963; Ingle 1966). Since, however, a sharp change in the gradient, together with different grainsize properties, naturally separates the Beach environment along the front of Scolt Head Island into two zones, it is considered advisable to sub-divide the beach according to these natural zones rather than to conform with those of the recognised beach terminology (see Fig. 7.1). Y. Davies (Ph.D. Thesis, 1962) used a. similar terminology in describing the beach morphology at Gibraltar Point, Lincoln. shire. Many beaches along the Lincolnshire and Norfolk coasts conform to this beach type, which is typified by the Scolt Head Island Beach. In the type area the back- beach sub-environment, found adjacent to the main dune ridge on Scolt Head Island, is composed of coarse grained sand and gravel, and includes the berm as well as the steeply sloping beach face. As shown in section 3.5, these deposits have formed under high energy conditions. 273 The fore-beach sub-environment is composed of fine grained sand and occurs to the seawards of the toe of the beach on Scolt Head Island. Thus sub-environment coincides with the low tide terrace (see Fig. 7.1). These deposits have formed under comparatively low energy conditions (see section 3.5). Owing to abnormal energy conditions at various localities in the study area, the normal relationship between the back-beach and fore-beach sub-environments is modified or absent. Because of these abnormalities the study area has been divided into geographical sub-areas. These sub-areas have been described separately, and an attempt has been made to assess the local variations in dynamic conditions which have produced them. On the basis of the morphological, sedimentological and dynamic conditions characterising each sub-area, it has been assigned to either one or the other of the beach sub-environments. The following sub-areas have been distinguished:- (1)The Scolt Head Island back-beach sub-area. This sub-area coincides with the back-beach sub-environment of the type area. (2)The Scolt Head Island fore-beach sub-area. This sub- area coincides with the fore-beach sub-environment of the type area. (3)The Holkham Beach sub-area. On the basis of physiographic (but not sedimentological) properties this sub- area has been divided into both back-beach and fore-beach sub-environments. 274 (4)The Brancaster Golf Course Beach sub-area incorporates, in a modified form, both the back-beach and fore-beach sub- environments. (5)The Brancaster Harbour Bar sub-area has been a si gned A to the fore-beach sub-environment. (6)The Burnham Harbour Bar sub-area has been almost entirely asigned to the back-beach sub-environment. The location and extent of these sub-areas are shown in sheet 8. In the following discussion the morphology of the Scolt Head Island back-beach sub-area is described in section 7.3, and the morphology of the Scolt Head Island fore-beach sub-area is described in section 7.4. The sediment movement occuring in both of these sub-areas is described in section 7.5. This is followed by a description of the morphology and sediment movement occuring in the Holkham Beach sub-area (section 7.6), and in the Brancaster Golf Course Beach sub-area. (section 7.7). The grainsize characteristics of the sediments occuring in these various localities are then described together in section 7.8. The morphology and sediment movement occuring in the Brancaster Harbour Bar sub-area are discussed in section 7.9, and the grainsize characteristics of the sediments in this sub-area are described in section 7.10. The morphology and sediment movement existing in the Burnham Harbour Bar sub-area are described in section 7.11, which is followed by a description of the sediments in this sub- area (section 7.12). FIGURE 7-2

Ht./OSTE-

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PROFILE 1. TERNERY POINT. L 140STL.

14v4OST

PROFILE 2 HUT HILLS

LwOST I.

SCALE FEET 10

40 8o izo too zoo FEET FIGURE 7-2

WdOSTL

Ns. PROFILE 3. HOUSE HILLS 1.340STL.

t4WOSTL

PROFILE 4. ....\ ..*...,

BREAKTHROUGH .... \„. ./' "...... :4„ WenTi. '......

Fig. 7.2 Profiles 1 to 4, surveyed during neap tides (long dashes) and spring tides (short dashes) in March 1964, and resurveyed in November 1965 (unbroken line). The crosshatching in profile 4 indicates the approximate position at which ancient marsh deposits are exposed on the beach. 277 7.2 The Morphology of the Scolt Head Island Beach 7.2 Environment

The Beach environment on Scolt Head Island includes the Bra.ncaster Harbour Bar, and Burnham Harbour Bar sub areas, as well as the Scolt Head Island fore-beach and back-beach sub-areas. However, only the back-beach and fore-beach sub-areas, which occur as sub-parallel zones extending longitudinally along the front of Scolt Head Island, are included in the type Beach environment. For this reason the morphology of these sub-areas is considered to be typical of the fore-beach and back-beach sub-environments. A study of the beach profiles, surveyed at a number of points along the Scolt Head Island Beach during 1964. and 1965, presents a picture of the general beach morphology, as well as of the variations in morphology from place to place along the beach (see Fig. 7.2). The location of these profiles is shown in sheet 8. The average slopes of the back-beach and fore- beach sub-environments on each profile are as follows: (1) the Ternery profile, 1:22and 1:80; (2) Hut Hills profile, 1 :17 and 1 :87; (3) House Hills profile, 1:17 and 1:61 and (4.) the Breakthrough profile, 1:15 and 1 :60. These back-beach slopes are slightly steeper than those recorded as being characteristic of the Lincolnshire coast by Barnes and King (1955). A relationship between beach slope and the grainsize of the sand comprising the beach has been shown to 278 exist by Bascom (1951). King (1959) visualises the mechanics of this relationship in the form of a balance between the beach gradient, rate of percolation of water through a given material, and the difference between the energy of the swash and back-wash. As shown in the general beach profile (Fig. 7.1), as well as in the four profiles in Fig. 7.2, a astinctive runnel, or trough, exists at the base of the beach face (at the toe of the beach). This runnel typically occurs at the boundary between the fore-beach and back-beach sub-environments. Both sub-environments are characterised by elongated ridges and runnels, the distribution of which is limited to the fore-shore zone as defined by King (1959), Shepherd (1963) and Ingle (1966), (i.e. to the zone between L.N.Y.T.L. and H.W.M.T.L.). The cross-section of a ridge is characterised by a more steeply dipping landward slope than seaward slope. The ridges and runnels in the back- beach environment are composed of mixed sand and gravel, and generally show a greater relief than the fore-beach ridges and runnels, which are composed of finer grained sediment. In the fore-beach sub-environment, these features have been termed "balls" and "lows" by Guilcher (1958) and "balls" and "swales" by Van Straaten (1953). Two, and occadonally three ridges form on the fore- beach (Fig. 7.2, profile 1, spring tide survey), while the back-beach usually displays only one ridge. An exception to this latter case is seen in Fig. 7.2, profile 2, (neap 279 tide survey). The stetp beach face gradient of this profile, and the occurence of two pronounced ridges are, according to Bascom (1951), indications of progradation. King (1959) has suggested that ridge and runnel development is restricted to beaches which are subject to a considerable tidal range, but are protected from long swell waves. Their position upon the beach face is also thought by King to be related to the various levels at which the tide remains comparatively stationary for prolonged periods, i.e. H.W.S.T., H.W.N.T., L.W.Y.T. and L.W.S.T. This latter relationship is apparently qualified by local factors to such an extent, that it is seen in nature only rarely, or incompletely. Under favourable wave conditions, a tendency exists for the back-beach (gravel) ridge to develop above H.W.O. S.T.L. to form a. berm. This subsequently produces a. comparatively flat back-shore area. (see Fig. 7.2, profiles 1, 2 and to a less extent 3). A slight tendency is shown in profiles 1, 3 and 4, for the beach face to form a steeper gradient under neap tide conditions than during the spring tides. The sparcity of the beach profile surveys, carried out during 1964 and 1965, does not allow an accurate assessment of the seasonal changes effecting the Beach environment. However, general field observations of these changes appear to be in broad agreement with the seasonal trends displayed by the beaches along the Lincolnshire coast, (barnes and King 1955). Field observations suggest FIGURE 7.3 .

MANBY

,J) <

GORELSTON - ZOO hours

'150

-100

cc dw Ez to z 2 z z 41

Fig. 7.3 Seasonal wind frequency diagram for winds stronger than 11 knots from directions between NW and NE. Based upon wind data from the meteorological stations of Manby and Gorleston. 281 that ridge and runnel features of both sub-environments are best developed during summer. In winter the beach commonly suffers retrogression, causing a diminishment in the width, together with an increase in the height, of the berm (see Fig. 7.2, profiles 2 and 3). The results of previous work on the seasonal changes occuring in the beach profiles by A.T. Grove (in Steers, 1960), suggest that the broad changes occuring on Scolt Head Island reflect the response of the beach to changing weather conditions. The comparatively quiet wind conditions which exist during summer (Fig. 7.3), produce accretion to the beach. The strong wave action (implied by the wind distribution shown in the above figure) during winter, spring and autumn, probably cause a flattening of the beach profile and a subsequent movement of material onto the fore-shore and into the off-shore zone (King 1959). An"off-shoren bar upon which waves break at low water has also been observed to form during periods of strong wave action but is conspicuously absent in summer. Old marsh muds are exposed by retrogradation of the beach face in front of the Breakthrough (see sheet 8 and profile 4 in Fig. 7.2). It is believed that the retrogradation at this point is produced by a local abnormality in the wave pattern, rather than by seasonal changes in the weather. 282

7.3 The Scolt Head Island Back-Beach Sub-Area 7.3

7.3.1 General Discussion 7.3.1

The back-beach sub-area on Scolt Head Island is shown, in sheet 8, to lie immediately seawards of the main dune ridge. It extends northeastwards from the end of Norton Hills, where it joins the Burnham Harbour Bar sub-area, to the western end of the island where it curves southwards around the Ternery Point. As previously mentioned, the sediments of this sub- area are characteristically coarse grained, ranging from medium grained sand through pea-shingle to coarse gravel. The composition and origin of this gravel is discussed by Coles Phillips (Steers 1960), who states that "pebbles other than flint constitute less than a. fraction of a percent of the beach". These flints have been derived locally by glacial erosion of the Chalk. Sections through the shingle ridge at Blakeney Point by Hill and Hanley (1914) show mixed gravel and sand overlying and intercalated with an underlying (largely gravel free) sand. Hardy (1964), describing the same general area, refers to the shingle as forming an "exo-skeleton" on a seaward sloping sand foundation. This composite picture is believed to be fairly typical of the sand and gravel accumulations which constitute the back-beach sub- area of Scolt Head Island. 283

7.3.2 Back-Beach Ridges and Runnels 7.3.2

The back-beach sub-area is characterised by the presence of one, and occasionally two parallel ridge and runnel systems, especially in the western half of the island (approximately W. of House Hills). These ridges commonly diverge towards the N.N.N., from the orientation of the coastline. The diagonal trend of the ridges is partly a result of the increase in width of the runnel as it drains westward. This usually causes the ridge, which is confined to the back-beach sub-area, to "pinch out" where the runnel meets the fore-beach. However, this diagonal orientation may also reflect the response of a ridge to form at right angles to the direction of dominant wave action. In the case of Scolt Head Island this displays a dominant component from the N.E. sector (section 3.2), and subsequently produces a residual sediment movement in a westward direction. The existence of a close relationship between ridge orientation (especially of those composed of the coarse material), and the direction of wave approach has been demonstrated in wave tank experiments by Kemp (1962), and has been inferred from the numerous field observations of Lewis (1931), Steers (1946) and others. Per Bruun (1954.) has proposed a theory, based upon experimental work in a natural (although small scale) environment, which explain the orientation of ridges in terms of the littoral drift produced by wave action. This phenomenon has also been reported to exist at Gibraltar point, Lincolnshire by King (1959) and W. Davic-5(1962). 284 The internal structure of these back beach ridge features has not been investigated.. However, the structural composition of similar features has been observed at Gibraltar Point by Davit (1962) and probably applies to the ridges on Scolt Head Island.. Daviesfound that the bedding roughly paralleled the external outline of the ridge; dipping gently (5° to 10°) on the seaward face, horizontal under the crest, and on the inner (lee) slope dipping more steeply (10° to 30°) towards the dunes. However, by virtue of the landward migration of these features, it would seem that the lee slope would be more commonly preserved. This is supported by impregnations taken of the fore-beach ridges which display shoreward dipping laminations.

7.3.3 Transverse Ridges 7.3.3

A prominent diagonal or transverse ridge occurs periodically in front of Hut Hills on Scolt Head Island. It commences as a berm on the back-shore, but with the development of a. runnel on its landward side, it changes to a ridge as it traverses the back-beach. Its prolongation onto the fore-beach is accompanied by a decrease in height and an increase in width, until it appears only as an indistinct undulation at the low water level. This ridge is orientated at an acute angle (285°) to the orientation of the coastline (277°). The on-shore migration of this feature is therefore manifested as a westward retreat of the head of the runnel which lies immediately behind the 285 ridge. A less distinctive feature has been observed further to the E. in front of House Hills where locally the beach gradient extends uniformly and unbroken across the beach environment. It is note-worthy that these rare features transgress uninterruptedly across the physical boundary between the back-beach and fore-beach sub-areas, and are unique in this respect. Similar features have been reported by Sheaard (1963).

7.3.4 Sediment Movement Associated with a Ridge and 7.3.4 Runnel System

The formation of a beach ridge (especially in the back-beach sub-area) is believed to be the sole result of wave action. The waves, breaking on the seaward face of the ridge (i.e. on the beach face), will move sediment upwards and downwards under the influence of the swash and backwash. The resultant movement of sediment either on-shore (accretion), or off-shore (erosion)is determined by the balance between swash energy and backwash energy. This balance is in turn a function of wave steepness, beach gradient and sediment composition. (The grainsize of the sediment largely dictates the rate at which water percolates downwards towards the water table, Duncan 1964.) According to the observations of others, as well as those of the author, accretion to the beach occurs by the successive on-shore movement of ridges ( King and Williams, 1949, and Bascom, 1960). This phenomenon has been reported (King 1959; Davis 1962) to occur mainly 286 during summer. Measurements of the rate of accretion of similar beach ridges to the back-shore zone of St. Vincent Island, have been carried out by Tanner (1961), who estimated the addition of a new beach ridge once in every 70 yrs. in the case of this emergent coastline. The mechanism of ridge migration appears to reflect the lateral as well as shoreward migration of sediment under the action of the swash and backwash of waves approaching at an oblique angle to the coast. Individual particles moving mainly as a bed load follow (on a small scale) a. zig- zag transport path transversely up the beach face (Steers 1960 and others). A large scale zig-zag interchange of sediment also exists between the top and bottom of an en-echelon ridge and runnel system. Sediment "drifts" transversely up the ridge face under wave action and is subsequently washed into the runnel where it is swept seawards by current action, until it is incorporated a-new in the next ridge and runnel. The sediment movement, which produces the on-shore migration of a ridge, is instigated when the waves break on the crest of the ridge and the swash flows into the runnel. Since the runnel will channel the swash seawards, the landward component of the waves (the swas4 will be unopposed and material will be transported across the ridge and deposited in the runnel, thus forming a steep, landward migrating lee-slope on the inside edge of this ridge. It appears that, although wave induced transport of material is by bed-load, the mechanism of ridge migration 287 is similar to that of a ripple or sand wave produced solely by current action. Movement of sediment on the crest of a ridge is invariably associated with processes of sorting and winnowing. Due to the mechanisms enumerated above, a tendency exists for the finer material at any point to be preferentially mobilised and deposited in the runnel. Field observations indicate a horizontal grading to exist across the upper part of the ridge crest with coarsest gravel concentrated on the seaward edge, decreasing in size to pea- shingle on the inner edge. This trend possibly reflects a decline in swash energy across the crest of these typically flat topped features. A trend also exists of increasing grainsize along the crest of a ridge. This is especially well shown by the transverse features, and is apparently produced by the increase in wave energy from low to high tide (see section 3.5). The entrapment of the swash in the runnel behind a ridge produces a relative increase in the seaward velocity of flow within the channel as the tide falls, and subsequently a decrease in the velocity of the flood current moving up the runnel during the rising tide. During high tide, when high energy waves are over-flowing the ridge top, these effects will be emphasised in the head of the runnel where it is narrowest. The presence of the ebb formed mega ripples in the bottom of the runnel are a direct result of these currents. Field measurements indicate an 2811 amplitude of up to 4 ft. and a wave length of 30 ft. at the head of the runnel, decreasing seawards to 20 ft. at its mouth. Their decreasing wave length and more symmetrical cross-section seawards, reflects the declining of current velocities in this direction (Van Veen, 1931). The central part of the crests of the mega ripples are commonly deflected down stream due to the high rate of sand transport in the centre of the channel. This, in extreme cases, may cause a ripple to bend sufficiently down stream to coalesce with the preceeding ripple thus isolating a "pot-hole" against the side of the channel. Photograph of mega-ripples occuring in a runnel on the Norfolk coast near Muudsley display a similar phenomenon (Cornish 1901).

7.3.5 Possible Mode of Origin of a Transverse Ridge 7.3.5

The development of a. transverse ridge extending uninterrupted across the back-beach and fore-beach appears, from field observation, to be the only phenomenon capable of modifying the distinctive physical discontinuity between the two beach sub-areas on the Scolt Head Island beach. The development of such a feature may possible result when a sufficient amount of coarse material has been isolated on the rear ridge of the fore-beach. A stage will be reached when the amount of coarse grained material avalanching into the runnel exceeds the amount of material carried seawards by the current in the runnel. The ridge will then commence to migrate landwards, and will eventually

FIGURE 7.4.

FORE- • • • • • • r . • BEACH " o • 0 >1 '0 • • • o • v• • 0' 0 . 0 RIDGE t a. •o• op 0 0. • • o . • • ..0. • . o •O• • • • ,„ If • FORE. $ERCH 'RUNNEL o o e3 0 0 ° 0 0 0o es O ° o c' BACK- r, 0 o 0 0 0 0 0 BEACH (' C:21 0 0 - 00 0 0 00 RUNNCL 0 0 ° g 0. 0 0 0 cg 0 0000 0 0

• 'FORE- .o . BEACH RIDGE .RutilvEL Oo 00 ea !;; °• 0. • 0 0 9 Cr 0_ -00000 060 c, 0 0 o 0 r--) 0 0 C.3 0 ei 0 0 0 0 L-(11 oc) ° 0 0L-) 4:70 •0 0 0 BACK BEACH 0 0 0 000:0 4Q RUNNEL- o .6. 0 d o o (3o 0 tg 0

Fig. 7.4 Showing a loCal concentration :of gtavel on the main fore-beach ridge which coalesces with the back-beach gravelly sand deposit. The foteH: beach runnel i$ ,thet0forediverted seawards and a transverse featute is formed. 290 coalesce with the back-beach (see Fig. 7.4). This causes the runnel at the base of the back-beach to be blocked and rerouted to the seawards of the newly mobilised fore- beach material. This phenomenon causes the fore-beach ridge crest, on either side of the break, to be re- orientated. The down drift (in this case the western) side moves landwards, while the up-drift side is forced seawards by the new drainage channel (Fig. 7.4). It is suggested that this process is in operation in front of Hut Hills where (possible due to the presence of Brancaster Roads deep off-shore) slightly higher wave energy is effective in concentrating coarse grained material on the fore-beach. The greater berm development on the back- shore in front of Hut Hills also suggests a local mechanism of shingle accretion in this area.

7.3.6 Beach Cusps 7.3.6

Beach cusps form a comparatively rare, but characteristic feature of the back-beach sub-area. Cusps commonly form on the seaward face of the back-beach ridge at about half-tide level, and appear from field observations to result from a process of sorting by wave action. These features are composed of a superficial layer of sand, alternating with gravel. They are formed during one tide cycle, but retained, under favourable conditions, during succeeding ones. The gravel forms a succession of small, seaward pointed horns (the cusps) along the beach face and the sand, or finer gravel, is isolated as a thin 291 superficial layer in the "bays". Beach cusps, formed on both sand and shingle beaches, have been fully discussed by Johnson (1910), Krmbein (1944), Guilcher (1958), Allen (1964), and Otvos (1964). Small regular pebble ridges or mounds were observed on the seaward face of the Ternery in much the same position as occupied by cusps (which on this occasion were absent). These features were orientated at right angle to the coastline at a spacing of 1-12- to 2ft., and are similar to those recorded by Otvos (1964). Each ridge showed a horizontal grading from coarse material in the W. to fine in the E., and also an imbricate structure from top to bottom which indicates that they were formed by flow up the beach by the swash. Folk and Ward (1957) noted imbricate structure in a superficial layer of pebbles on (but not within) the pebbly sand deposits of the Bazors River. These authors propose that imbricate structure is produced by the removal of sand from around the pebbles, and is therefore indicative of an erosional process. In view of this proposal, the imbricate structure observed in the small pebble mounds on the beach face may have formed as a result of the erosional action of the swash.

7.3.7 The Origin of the Gravel 7.3.7

The origin of the extensive, although isolated deposits of gravel which occur at Scolt Head Island and Blakeney Point, as well as at other places along the English coast, presents an enigma. Hardy (1964) believes 292 that these gravel accumulations are "fossil" deposits formed at an earlier time when the post-glacial sea level was rising and transgressing across the recently formed boulder clay deposits. However, it appears conclusive from numerous field observations that this gravel moves en masse along the Scolt Head Island beach in response to oblique wave action. In accepting Hardy's "fossil" origin, the dynamic stability, which apparently accounts for their long-term persistence, begs investigation. The westward growth, in historical time, of both Scolt Head Island and Blakeney Point raises the question of gravel loss and its possible replenishment. Hardy believes that the gravel oscillates eastwards and westwards between Sheringham and Blakeney Point, in response to the changing direction of wave approach: any residual movement being insignificant. However, in view of the dominant oblique wave action from the N. and N.B. that effects Scolt Head Island (section 3.3), together with the island's westward growth in historical times, movement to the W. and a subsequent loss of gravel from its eastern end seems to be an inescapable conclusion. As shown in sheet 7.C, strong retrogression has affected both Blakeney point and Scolt Head Island during the last 150 years. Although these changes have been measured at L,W.O.S.T. level, they also reflect a retreat of the back-shore area. This is corroborated by the active dune erosion which has caused the retreat of the dunes, especially in the vicinity of Norton Hills, the 293 Breakthrough and as far west as Hut Hills (section 8). As stated by Steers (1960), awry large proportion of the gravel on Scolt Head Island is incorporated in the lateral ridges enclosed within the marshes. The retrogression of the beach has exposed these laterals, and the gravel made available in this way has replenished, at least in part, the mobile gravel of the beach face. It is interesting to note that the part of the island suffering most spect- acularly from erosion, namely the Breakthrough, lies in an area without lateral ridges (sheet 8). This absence of laterals may, however, only be an apparent effect since 4 ft. trenches dug through the marsh muds, which outcrop on the beach face in this area, in all cases exposed gravel extending beyond the lower limit of the pit. The mechanism of sediment sorting and on-shore transport, proposed below, suggests that a slow but fairly constant addition of coarse sand and gravel from off-shore may contribute towards replenishing these back-beach shingle deposits. Gravel movement in off-shore waters has been shown to exist by Kidson, Carr and Smith (1958), Kidson and Carr (1959), and Kidson, Steers and Fleming (1962). A slight shoreward, and along-shore residual movement has been generally observed, although the rate of movement is far slower than that ofi sand. The movement of gravel within the intertidal zone has been measured, using marked pebbles, by Steers (1960) on Scolt Head Island, by Kidson and Carr (1961) and by Hardy (1964) near Blakeney Point, who have found that the direction of shingle move- 294. ment is dictated by the local wave conditions. Residual movement will presumably be determined by the dominant direction of wave approach. Holkham Beach affords indirect evidence of an off-shore origin for the gravel which occurs scattered over its surface. Here, in view of the uniformly fine grained nature of the beach sands, and the uniform or convex beach gradient (section .7.6), wave energy only increases very slightly, if at all, as the tide rises. Under these circumstances, reworking of the buried back- beach gravel deposits, which have been observed to occur under a superficial covering of sand on the berm surface (see section 7.3 below), seems to be an unlikely. mechanism to produce the scattered pebble deposits which occur at lower levels on the beach face. Furthermore, as Steers (1960) has previously noted, a long shore origin, in this case, appears to be equally improbable. Since the dominant direction of beach drifting is towards the W., pebbles, derived from Blakeney Point must necessarily traverse the broad, low energy, sand flats, as well as the channels of Blakeney and Wells Harbour4 before reaching Holkham Beach. It is felt that these extremely gently sloping sand flats present a. formidable barrier to the lateral transport of coarse material. Therefore, to explain the presence of shingle on Holkham Beach and elsewhere, an offshore source seems the only alternative. 7.5

- AmemillIMIIIIINameimmemarripash-

' - • . 40USE I.' • HILLS

Fir. 7.5 Beach opposite House Hills on Scolt Head Island sLGwing a diagonally orientated fore-beach runnel which op :Ls towards the west. Scale: 12 inches approximately equals mile. 296 7.4. The Scolt Head Island Fore-Beach .Sub.-Area 7.4

The fore-beach sub-area stretches along the front of Scolt Head Island (see sheet 8). On the basis of the presence of sediments of similar grainsize, this sub-area is considered to be continuous with the Brancaster Harbour Bar sub-area. The fore-beach sub-area is characterised by a fairly uniform areal distribution of fine grained, well sorted sand, and by an average slope ranging between 1:60 and 1:90. For the sake of simplicity this sub-area has been arbitrarily limited along its seawards edge by the L.W.O.S.T. level. However, its characteristic slope and grainsize extend uniformly some distance into the near-shore zone (Fig. 3.8). On its landward edge, the contact between the fore-beach and back-beach sub-areas has been arbitrarily delineated by the runnel which occurs at the base of the back-beach slope. This feature cor.. responds approximately to the point at which the beach gradient steepens (Fig. 7.1). However, the change in grainsize between these two sub-areas usually occurs over a narrow transition zone which encompasses the runnel as well as the crest of the associated fore-beach ridge. In actual fact this grainsize discontinuity could be more accurately placed along a line lying slightly seawards of this ridge crest (section 7.8.3). Ridge and runnel systems form en-echelon sub- parallel undulations on the fore-beach of Scolt Head Island, and are commonly continuous longitudinally for up to a mile or more. The intervening runnels widen to 0)

Fig. 7.6 Beach on Scolt Head Island displaying discontinuous fore-beach ridges, breached by small drainage channels which have been deflected to the west as a result of westward littoral drift. Photographed by Huntings Surveys Ltd., June 16.0'. Scale: 6 inches approximately equals mile. 298 the W. as they approach the low tide level (see Fig. 7.5). This subsequently causes the adjoining ridge to decrease in height and width towards the W., which in turn produces a. slight diagonal trend in its orientation. Occasionally, however, these features may form parallel to the shore line, at which time the ridges may be transacted by numerous small drainage channels (Fig. 7.6). This phenomenon is induced during periods when waves approach normal to the shore-line, causing onshore-offshore water movements. Consequently, the wave induced long-shore currents and beach drifting processes are minimised. This, in turn, produces a negligible lateral movement of sediment. Lateral water and sediment movements appear to be essential in the maintenance of diagonally trending ridges and runnels. The absence of lateral water movements, on the other hand, appears to be a prerequisite for the creation of these features in a parallel orientation. The gentle relief ofthe fore-beach ridges and runnels probably reflect the general relationship established by Bascom (1951), between fine grained sand and a gentle beach gradient. However, the increased significance of tidal and long-shore currents, as opposed to the beach drifting (which is dominant in the back-beach environment), may contribute towards maintaining the subdued relief of these fore-beach features. The shoreward dipping laminae found in impregnations of a fore-beach ridge are believed to have been deposited at the lee face of a. landward migrating ridge. This 299 suggests a similar mode of shoreward migration to that proposed in section 7.3.4. in relation to the comparable features in the back-beach sub-environment. The tendency for these features to migrate shorewards has been mentioned by Van Straaten (1961) and others, and is thought to occur under the action of small quiet weather waves. Of the fore-beach features, the rear ridge is invariably the most prominently developed (Fig. 7.1) and usually displays a somewhat coarser than normal grainsize distribution on its crest. The associated runnel which lies to the landwards of this ridge at the toe of the beach face is also usually well developed. Current scour has, in places, exposed very coarse gravel in the bottom of this runnel (Steers 1960). Large irregular flints, which retain in part their original shapes are commonly included in these tightly packed and immobile (in terms of current action) gravel deposits. Similar deposits have also been observed in the bottom of a back-beach runnel near the Ternery. The origin of this gravel is open to conjecture, however, it has the appearance of an old, and now buried, beach deposit. The presence of ridges and runnels constitutes a marked difference between the fore-beach sub-environment of Scolt Head Island and the fore-beach of Gibraltar Point, Lincolnshire, as described by Davies(1962). This lack of conformity is further emphasized by the presence, in the latter area, of ripple marks and of burrowing organisms such as Arenicola marina and Lanice conchilega (Davies1962); 3001 and also by the occasional deposition of thin mud laminae over the fore-beach sand flats. Although ripple marks do occur on the Brancaster Harbour Bar, their absence on the ,Scolt Head Island fore-beach probably reflects the influence of somewhat higher energy conditions along the N. Norfolk coast. An off-shore bar has been observed to form, during winter, in the near-shore zone along the front of Scolt Head Island (Fig. 7.6). According to the experimental work of Saville (1950), and the observations of others, this phenomenon results from the seaward movement of material eroded from the beach under the destructive wave conditions which are usually prevalent during winter.

7.5 Sediment Movement in the Beach and Near-Shore Zones 7.5

7.5.1. General Discussion 7.5.1

Brunn (1954. ) has summarised the conclusions of the experimental work of Bagnold (1946), Johnson (1 948), Saville (1950), Brunn (1951) and Beach Erosion Board (1954) on the effects of wave action upon a beach profile:- "(a.) Waves with high steepness ratio produce a bar profile. Waves with low steepness ratio produce a beach ridge profile. (b)Waves with high steepness ratio erode the beach; waves with low steepness ratio build up the beach. (c)Waves with high steepness ratio can be identified as winter waves. They form a storm, or winter profile. Waves with a low steepness ratio can be identified as summer waves, they build up a. swell or summer profile. The 3n1

intermediate profile occurs at a steepness ratio of about HO /L0 = 0.026. (d)There are two types of littoral drift, bed load transport on the beach itself (beach-drift) due to uprush and back wash, and suspended load transport in the breaker zone due to the breaking waves and the generated longshore currents. (e)In equilibrium storm profiles, transport is mainly of material in suspension. In equilibrium swell profiles, transport is mainly by beach drift. The transition between these different types of sediment transport is sudden and occurs at a steepness ratio of about 0.03. (f)The sediment transport along summer profiles is much greater than than along winter profiles for waves with the same energy. The peak transport occurs at a steepness ratio of from 0.02 to 0.025 and is almost entirely beach drift. (g)The transport rate decreases very rapidly for steepness ratios less than 0.02. (h)The transport rate increases with increasing wave energy, other conditions retainiug the same. (i)The transport occurs almost entirely in the area shoreward from the 'breaker zone f ." Brunn (1954). Experiments carried out with irradiated quartz sand by Inman and Chamberlain (1959), and Inose, Sato and Shira.ishi (1959) and others, indicate that, under the influence of waves, sand on the sea floor will be moved in a direction parallel to that of the wave approach, with 3m2 a residual movement on-shore. However, the experiments of Saville (1950) and others suggest that the direction of residual sand transport is modified by wave steepness, maximum on-shore movement being produced by low steepness swell waves. More recently Ingle (1966) has published the results of a comprehensive series of experiments using fluorescent sand to trace sediment movement in the breaker, surf and swash zones. These experiments were designed to explore the processes instigated by wave action, but exclusive of the effects of tidal currents, involved in the transportation of sand. Dominant sediment movement was found to be concentrated in the breaker and swash zones, and the direction of movement was found to be related to longshore current action in the former zone, and beach drifting in the latter zone, produced by oblique wave approach. Under weak wave action, sediment moved mainly within the breaker zone, while strong wave action causes dominant transport to occur in the swash and surf zones. In the former case, sediment in the surf and swash zones shows a tendency to migrate seawards into the breaker zone - but not beyond. Miller and Zeigler (1958, 1964) have studied the processes of sorting by correlating theoretical, laboratory and field data in an attempt to predict size and sorting patterns in the off-shore breaker and swash zones. Ingle (1966) has used different coloured sand fractions to trace the movement of various size grades shorewards of, and within, the breaker zone. Although the results of these 303 tracer experiments are far from conclusive, they suggest that fine grains move on-shore while coarser grains move towards the breaker zone. Sediment movement in theoffshore zone has been implied in the work carried out by Shepherd and Inman (1951, 1953), and has been measured by others, including Ingle (1966), using fluorescent and radioactive sand and pebble tracers. In general, a shoreward direction of residual movement has been noted with an oblique component depending upon the direction of wave approach. In the following discussion, the on-shore sediment movement and the along-shore sediment movement have been considered separately.

7.5.2 On-shore Sediment Movement 7.5.2

It is significant that the modal grainsize of the medium to coarse grained sand of both the Scolt Head Island back-beach sub-area, and the adjacent dunes, are similar (section 8.3). Obviously the latter has been derived from the former. If the fine grained sand, which is characteristic of the fore-beach, existed in any quantity in the back-beach, it would be preferentially mobilised by the wind to form dunes. Since this is not the case it appears that practically no fine grained sand is deposited in this latter sub-environment, (a conclusion substantiated by grainsize analysis in section 7.8.3). Assuming that the sand of the back-beach, as well as of the fore-beach, is derived from off-shore, nn effective sorting mechanism must exist between the two sub-environments by which coarse 304 sediment is concentrated in the former and fine grained sand is retained in the latter. This proposal implies the ability of wave action to transport across the fore-beach sand flats, material which is coarser than the sediment apparently in equilibrium there. Such a concept is superficially improbable in the light of sand tracer experiments which were carried out by Ingle (1966) to explore the processes of sorting within the zone of breaking waves. These experiments indicate a residual movenent of coarse sand seawards towards the breaker zone, while fine grained sand remained in suspension in the breaker zone or migrated shorewards. Ingle tentatively proposes that these trends may be conceived of in terms of the differential movement of different grain sizes in which "each grain apparently sought a position of hydrodynamic equilibrium under the existing surf and slope conditions", Ingle (1966). Such a concept was first proposed by Cornaglia (1898) but, apparently due to the multitude of subsidiary factors existing in the zone of shoaling waves, the theoretical grainsize distribution that he proposed was rarely, if ever found in nature.. Cornaglia's theory subsequently fell out of favour. The experiments of Ingle, however, do not, as yet, explore the movement of sediment produced under such complex dynamic conditions as those existing at Scolt Head Island, where a tidal regime causes a breaker zone to transgress across a foreshore which displays two distinct gradients. Waves will experience the maximum effect of bottom 30.5 friction (and consequently maximum wave refraction), during low tide when the off-shore water depth is at a minimum. Waves of a given size will, therefore, exert least effective energy on the beach face at low tide (sections 3.2 and 3.4). Thus beach drifting, which produces on-shore, off- shore sand transport (as well as long shore current action, which causes lateral sediment movement), will be least effective under these tidal conditions. Subsequently, it would appear that the quantity and mean grainsize of sediment capable of being mobilised by the wave action will also be at a minimum at low tide. With the commencement of the rising tide, however, the increasing water depth off-shore produces a gradual increase in the energy. Since, as Ingle (1966) has shown, sediment transporting capacity increases with wave energy, it is apparent that both wave capacity and competency will increase as the tide rises. To explore this phenomenon, the ideal conditions of uniform long...tam wave actions have been assumed. Under a tidal regime, the most seaward limit of the breaker zone represents the seawards extent of the intertidal (including the inner part of the near-shore) zone over which sediment is mobilised by the direct action of breaking waves. This seaward limit is achieved during low tide at which time new sediment may be introduced into the intertidal zone. Although wave energy increases and the plunge point moves landwards as the tide rises (see section 3.5), it would seem most improbable (on the basis 306 of experimental and theoretical considerations) that the sediment transporting ability of the waves during low tide at a point in the low tide breaker zone, will be exceeded at successive stages of the tide when this point will lie to the seawards of the breaking waves. It appears logical to assume that the maximum sand transporting energy available to the low tide breaker zone controls the maximum grainsize of the sediment introduced from off-shore. Thus despite the increasing wave energy resulting from the rising sea-level, the sediment subsequently transported shorewards across the fore-beach is only that sediment originally introduced into this area by the action of the low tide waves. Naturally, as wave conditions never remain uniform for long, it would appear, theoretically, that the coarsest material present in the fore-beach reflects the energy conditions, at low tide, of the strongest (constructive) waves which have affected the area. The characteristic uniformity of the modal grain size of the fore-beach sub-area superficially supports this model. The comparatively limited occurrence of pebbles scattered over the fore-beach (most commonly on the crest of its rear-most ridge), may result from the action of strong, although rare storms. The pebble tracing experiments of Kidson, Smith and Steers (1956), Kidson, Carr and Smith (1958), Kidson and Carr (1959) and Kidson, Steers and Flemming (1962) indicate that very restricted pebble movement does in fact occur in off-shore areas under the influence of even "normal" wave conditions. It is, there- 307 fore, possible that some pebbles are eventually moved into the zone affected by breaking waves and are then gradually moved across the fore-beach. However, an alternative, or supplementary mode of origin of the pebbles in this sub- area is feasible: the erosion of the back-beach by storm waves may comb some coarse material seawards, depositing it in the fore-beach sub-area (Hardy, 1964). Once sediment is introduced into the fore-beach zone it will, under constructive wave conditions, tend to migrate shorewards in front of the transgressing breaker zone. This sediment will eventually be transported, as bed load, onto and over the main fore-beach ridge and into the runnel which separates the fore-beach and back- beach sub-areas. Fine grained sand will be maintained partly in suspension by the wave turbulence on the ridge crest, and partly transported seawards by the lateral current in the runnel. The breaker zone, retreating as the tide falls, will remobilise the finer sediment, and redistribute it over the fore-beach sand flats as the wave energy decreases. These lateral currents will be especially active during the falling tide. By this mechanism, the sorting and differentiation of fine from coarse grained sediment in the beach environment is achieved. It seems unlikely that much sediment will move across the runnel under these conditions. However, during strong wave action, when sufficient coarse material is moving over the ridge crest to be deposited in the runnel, 308 this ridge may migrate shorewards and coalesce with the back-beach. This mechanism, acting locally on parts of the ridge, has been tentatively proposed in section 7.3.5 to account for the formation of a transverse ridge. Whether this method of addition of coarse grained material, to the back-beach is the only one operative seems doubtful, and conceivably some sediment does manage to traverse the runnel in conjunction with the transgressing breaker zone. However, such a process seems somewhat improbable, since wave energy, both on the crest of the fore-beach ridge and on the bask. beach slope. will be greater than that acting on the bed of the intervening runnel. The significance of this ridge and runnel system appears to lie in its intimate association with the mechanisms of sorting or differentiation, by which fine grained sediment is localised on the fore-beach and coarse material on the back- beach. It is significant that where this runnel is absent, as is the case in front of House Hills (see section 7.3.3), the grainsize distribution of the beach sands show a progressive increase in mean and modal size from low to high water level (section 7.8.3). From the foregoing discussion, it may be appreci- ated that the grainsize distribution of the beach material, the gradient of the fore- and back-beach sub-environments, the wave energy and the tidal regime, are all closely interrelated. The primary factors in this relationship appear to be: (1) the tidal regime which causes a variation in wave energy; and (2) the presence of a strongly 3n0 bimodal sediment distribution. It is noteworthy that, where the local sediment grainsize distribution is unimodal, as is the case on Holkham Beach, a uniform or convex beach slope results (section 7.6). The existence of the two sub- environments, which characterise the beach environment, may be understood, therefore, in terms of the interaction of the dynamic conditions with the sedimentological characteristics of the particular beach deposit. The bimodal property of the beach deposits may reflect either a parent sediment with a bimodal grainsize distribution, or the existence of two parent sediments with distinct grainsize distributions. Upon the basis of the grainsize characteristics, the boulder clay in this vicinity would seem to satisfy the former alternative (see section 10.5).

7.5.3 Along-Shore Sediment Movement 7.5.3

It has been previously suggested by Steers (1923), and proved theoretically by the author, that wave action on the N. Norfolk coast is most effective in producing a long-shore component for waves approaching from the N.E. This is the direction of greatest fetch which is least affected by the expanse of shallow water (the Burnham Bank Complex) lying to the Y. and N.W. of Scolt Head Island. Furthermore, it is believed that the deeper water off the Cromer, Sheringham coast, compared with that further to the W., will, in general, cause the intensity of wave energy to decrease towards the W. (However, the analysis of the wind information discussed above 310 indicates that the offshore banks act rather as a lens which locally concentrates the energy of waves, approaching from the N., onto the central part of the N. Norfolk coast in the vicinity of Scolt Head Island.) A residual westward direction of sediment movement along the N. Norfolk coast is implied by both of these factors, and is supported by the measurements, made by Steers (1960) of the movement of gravel on the beach of Scolt Head Island. No direct measurements of the residual direction of sand transport have been carried out on this coastline. However, it seems likely that sediment transport by wave action will be significantly modified by current action, especially in the case of the fine grained sediments of the fore-beach (Steers 1923). According to measurements made by Steers (1960), (see Fig. 3. 23.A), the strongest tidal current flows towards the E. in front of Scolt Head Island. This would appear to produce a residual sediment transport in the opposite direction to that induced by wave action. This strongest current flows to the E. during the upper tidal range, at which time the fore-beach lies to the seawards of the breaking waves. During the lower tidal range, however, waves (although much smaller than during high tide), breaking on the fore-beach, will put fine grained sand into suspension, and will produce a residual westward flowing long-shore current. These two factors will supplement the sediment transporting ability of the less strong tidal current which flows to the W. at this time. In 311 addition, it has been shown in section 3.8 that the shallow water depth over the fore-beach during the lower tidal range significantly increases the sand transporting ability of this westward flowing tidal current. It is believed that the combined effects of wave action, tidal range and current action will produce a residual movement of sediment to the W. in the fore-beach sub-area. Since, however, the turbulence produced by breaking waves is restricted to the intertidal zone, and to a small part of the near-shore zone, it is believed that the strongest tidal current will be the dominant factor in the transportation of sediment in the off-shore and near-shore zones. The overall picture will, therefore, appear as a residual sediment movement to the E. in the off-shore and near-shore zones, and to the W. in the lower part of the intertidal (fore-beach) zone (see Fig. 3.19). The orientation of fore-beach ridge and runnel systems (runnels open to the W. - see Fig. 7.5 and 7.6) and the occurrence of the eastward facing mega ripples that occur in the trough between the off-shore bar and the beach at the eastern end of Scolt Head Island (section 7.8.5), together with the observed ebb-formed ripples noted by Kidson, Steers and Flemming (1962), in the near-shore zone, support this conclusion. The opposite directions of residual sediment movement in these adjacent zones constitute a coastal circulation system (see Fig. 3.19). As sediment is transported eastwards in the near-shore zone by the flood tidal current, it is constantly affected 312 by wave action, which bestows a slight landward component on its direction of migration. Thus sediment within the near-shore zone is moving eastwards and shorewards, and only commences to move westwards once it is accreted to the fore-beach sub-environment. This mechanism is pro» posed in section 4.5.3 to explain the stability throughout historical times of the intertidal sand flats to the W, of Scolt Head Island. The presence of the fine grained sand deposits (fore-beach) that extend along the N. Norfolk coast may also be attributed to the balance imposed by this phenomenon (see section 11.2).

In an attempt to resolve the question of the direction of residual sediment transport along the Y. Norfolk coast, the weathered product of the Carstone suggested itself as a natural tracer. The Carstone outcrops on the coast at Hunstanton and, as suggested by the off-shore sampling of Donovan (personal communication), probably also outcrops in Lynn Deep at the position shown in Fig. 7.7, which is based upon the calculations of R. Gallois (personal communication). Samples of the Carstone were collected from the beach face at various positions both S. and E. of Hunstanton, and traces of the characteristic limonite rich oolites were found in both directions. On Scolt Head Island they were observed to be concentrated at the base of the eroding dunes at the Breakthrough. The possibility of a significant source of Carstone oolites located N. and E. of 313

FIG. 7.7 Position of bottom samples collected by D.T. Donovan, in Lynn Deep, and the submarine outcrop pattern of the Kimmeridge Clay, Sandringham Sands, Snettisham Clay, Carstone and Chalk, as predicted by R. Gallois from structural contours.

o 0 25SE FIGURE

7-7. 4P A (41

SCALE 1• 75000 CHALK ) f' I 'PEBBLES / 1 CHrAuLKDI t "../." RED CHALK/ CARSTONE i mucksrowE / S1ETTISHAM I i • f /I 1 11 1% SANDRINGHAM ,/ lit \ ././ SANDS pEBBLESNUir // ETC N \ \t‘ I ‘ 4 1 HELL crRRvE1-• • ) KIMMERIDGE/ , 1 CLAY ,/ Rey MUDSToNE. ,' 1 / 1 1 ,- / t pEbBLES ,/ / % MUD. / ia PEBBLES HUD. / 5300N 1 CLAY ‘ / • SILT. /.; Ni

/ • I ‘ ‘ f II GREY SHALE. ,,67 r - • I „ .. c- / /1 . „tc+o i \ Qc r \ n / ,x., \_„. —I

14UNsTAtiTON

9 Fig. 7.8 Holkham Beach from Holkham Gap westward showing the irregular ridges and runnels on the lower beach face, and the approximate position of the berm which extends shorwards of the dotted line. Silts and muds have accumulated in Holkham Gap, at the rear of the berm, in the partial protection of the embryo dune. Scale: 6 inches approximately equals 1 mile. Photographed by Huntings Surveys Ltd. June 1960. 316 Scolt Head Island has been discarded. The existing distribution of oolites is felt to contribute support to the concept of the coastal sediment circulation pattern mentioned above, in which fine grained sand is carried E. by the dominant near-shore current, at the same time migrating shorewards under the influence of wave action, finally to be incorporated in the westward littoral tuft on the lower beach face.

7.6 Holkham Beach Sub-area 7.6 Holkham Beach extends from Burnham Harbour eastwards as far as Wells Harbour and reaches its maximum width at Holkham Gap. The beach face slopes very gently and fairly uniformly. A wide berm occurs to the landwards of the beach face and is especially well developed at Holkham Gap (see sheet 8, and Fig. 7.8). The beach sand is surprisingly uniform in grainsize, and is similar in this respect to the fore-beach sub-area of Scolt Head Island. However, morphologically Holkham Beach may be divided into two zones. The gently sloping beach face along its seaward side has slope, and grainsize characteristics, and has formed under dynamic conditions similar to those found in the fore-beach sub- environment. The rear part of the beach is flat topped and is morphologically identical to the berm of the back- beach sub-environment. The Holkham beach has, therefore, been subdivided into fore-beach and back-beach sub- environments (sheet 8) along the seaward edge of the berm. This division corresponds to the classical break between 317 the fore-shore and back-shore zones used in describing the beaches on the W. coast of N. America and elsewhere (Sheppard 1963). Ridges and runnels appear as discontinuous and, in the lower part of the beach, somewhat irregular features (fig. 7.8)4 Field observations suggest that these irregular ridges and runnelS occur on an almost flat zone which appears to be strongly influenced by current action. The ridges are often superficially capped with scattered, small pebbles. On the back-shore berm surface, fine grained sand appears to thinly overlie, and to be intermixed with a coarse gravel deposit. This gravel occurs at a level only reached by spring tides. In an area with such a gentle gradient, it seems unlikely that wave energy ever attains a sufficient intensity to affect it. The recent deposition of silts and muds on parts of this berm near Holkham Gap, further support the existence of a very low energy environment. Holkham Dunes (or Meals as they are locally named) border the landward side of the beach. These show a prominent line of youthful dunes along their seaward edge. This attests to the long term trend of accretion which has been shown to occur at this locality in sheet 7 (see section 5.2). One exception to this general picture of accretion occurs in the vicinity of Gun Hill (see sheet 8), where the migration of the Burnham Harbour channel has caused local erosion of the dunes. Another exception occurs in Holkham Gap where the deposition of 318 a thin layer of silt and mud on the berm surface (Fig. 7.8) has protected the underlying sand from wind erosion, thus causing the adjacent dunes to be under-nourished. The Holkham Beach sub-area differs from the Scolt Head Island Beach in possessing a convex, rather than concave beach slope and a uniform distribution of fine grained sand over its upper as well as its lower parts. It is proposed that this configuration has resulted from continuous addition of fine grained sand to the beach by a progressive shoreward migration of ridges. This process has, with time, completely buried a once existent back- beach gravel accumulation under an all-enveloping fore- beach deposit. Beaches which display an extensive flat topped berm are shown by Bascom (1960) to have been formed during conditions of constructive wave action, by the successive onshore migration and accretion of ridges. Bascom shows that the accretion of ridges may produce an increase in the gradient of the beach face. This, in turn, may conceivably lead to an increase in wave energy, which may cause erosion of the beach face and eventually, to the attainment of a balance between beach slope and wave energy for a given grainsize of the beach sediment (i.e. an equilibrium profile). According to Bruun (1954) ... "It seems as if an increase in steepness is sooner or later accompanied by an increase in the retrogradation of the shoreline." Since, however, this beach has been accreting within historical times, it may be assumed that 319 the gradient of the Holkham Beach face has, at all times, been in equilibrium with the dynamic conditions. The presence of extensive deposits of coarse gravel, now thinly covered by fine grained sand, in the back-shore zone suggest that it accumulated under appreciably higher energy conditions than now exist. An original environment, similar to that now existing on Scolt Head Island, is envisaged. However, under local conditions which allowed fine grained sediment to accumulate faster than it was removed by lateral sediment transport, the fore- beach sub-environment gradually built up until it covered the back-beach sub-environment. This eventually resulted in a uniform or convex beach gradient and an overall fine grained sand distribution. The introduction of excess sediment to this area may be related to the current circulation, and subsequent local reversal in the residual (eastward) off-shore sediment movement which has been suggested in section 3.6 (Fig. 3.19) to exist in Holkham Bay to the E. of the Bridgirdle. In view of the very gentle beach and near-shore gradient, it is possible that the Holkham Bay area is subject to slightly lower wave energy than is Scolt Head Island and Blakeney Point (Hardy 1964). This possibly acts as a low energy focal point towards which fine grained sediment gravitates away from the centres of high energy. The historical variations in the position of the L.W.O.S.T.L. shown in sheet 7, show a significant trend in the Holkham Beach sub-area. Here a narrow zone of 320 accretion, and an adjacent zone of erosion, have progressively moved westwards along the low tide level. This trend is unique along the Y. Norfolk coast, and may possibly reflect, or be related to the particular conditions producing the accretion to Holkham Beach.

7.7 Brancaster Golf Course Beach Sub-area 7.7 Brancaster Golf Course is made up of a complex gravel and sand dune ridge system, in which laterals show a small deflection towards the E.S.E. It is bordered to seawards by a beach, similar to that on Scolt Head Island, and on its landward side by the Brancaster Marshes. The stretch of beach to be discussed extends, unbroken, to the W. of this point (see sheet 8). The major part of the stretch of beach under discussion lies behind (i.e. to the S.) of Brancaster Harbour Bar, the Ternery and Cockle Bight (from which it is separated by Brancaster Harbour Channel), and is, therefore, partially protected from wave action - especially towards its eastern end (see sheet 8). The beach shows a coarse grained, pebbly back- beach zone, and a fine grained fore-beach zone. In addition, fine grained sand flats have formed at the base of the beach, adjacent to Brancaster Harbour channel, and grade into the fore-beach zone (see Fig. 7.30). It is believed, however, that these sand flats are more closely related, genetically, to the tidal currents in Brancaster Harbour, than to wave action. Historically the eastern end of Brancaster Golf Course Beach has been accreting (Fig. 7.27). The direction 321 of growth and the orientation of the gravel laterals, which underlie the dune complex of Brancaster Golf Course, is related to a local reversal of the direction of wave induced beach drifting. Owing to the protection afforded by Scolt Head Island from the N.E. winds, the significant waves approach this stretch of coastline from the N. and N.W. These waves, as shown in Fig. 3.6, are especially effective during high tide and produce a movement of the coarse grained sediment in the back-beach environment towards the E. As a consequence of this local reversal in sediment transport, the beach at the head of Brancaster Bay (further to the W.) suffers "undernourishment". This is indicated by the exposure of ancient peat and clay deposits (see sheet 8). The relatively slight southward deflection of the gravel laterals forming Brancaster Golf Course (sheet 8) indicate that storm wave action from the N. and N.E. is considerably less effective than that affecting Scolt Head Island. As mentioned above, this is due to the protection, afforded by Scolt Head Island, from N.E. waves, which only reach Brancaster Golf Course Beach as a result of refraction around the western end of Scolt Head Island. Marsh enclosed fragments of gravel ridges and dunes (the Nod, Little Ramsey and Great Ramsey) outcrop in Brancaster Marsh and Overy Marsh further to the E. of Brancaster Golf Course (see sheet 8). The orientation of these fragments of gravel ridges indicates that dominant wave action from the Y.W. was responsible for their form- 322 ation. These gravel ridges apparently formed under similar conditions to those which produced the Brancaster Golf Course ridge complex, at a time when Scolt Head Island extended no further W. than the position now occupied by House Hills. The fine grained sand of the fore-beach sand flats of the Brancaster Golf Course Beach may have accumulated through the action of longshore currents, etc., produced by the local reversal in the direction of dominant wave approach mentioned above. In addition, the dominant W. to E. inlet current in Brancaster Harbour (which floods strongly during the last two hours of the rising tide) may also cause a westward sediment movement within the harbour itself. However, it is felt that the majority of this sand was originally derived from further to the E. and has subsequently been introduced onto the Golf Course Beach by the mechanism of by-passing mentioned below (section 7.9.2). Once deposited in this sub-area further westward transport seems unlikely and remobilisation of the sediment will probably imply another cycle of recirculation over Brancaster Harbour Bar.

7.8 The Grainsize Characteristics of the Sediments 7.8 of the Beach Environments of Holkham, Scolt Head Island and Brancaster Golf Course.

7.8.1 Introduction 7.8,1 The gra.insize characteristics of the sands of the Beach environment have been investigated using the hydro- 323 dynamic settling tube method of grainsize analysis. This method of analysis is restricted to those size grades between 2 mm. and 0.062 mm. diameter. The gravel fraction, where present, has been ignored, while the grades finer than 4 phi ( .063 mm.) are naturally excluded from these sediments. Samples have been collected at various times between 1964 and 1966. Their approximate location is shown in sheet 8. Samples have been collected so as to explore various topographic features rather than in accord with a controlled grid system. The sampling positions have, therefore, been described accurately in reference to specific topographic features, rather than in regard to a rigid geographical framework. Beach samples have been collected on sampling profiles A and B on Holkham Beach, C, D, E, Fl, F2 and G on the Scolt Head Island beach, and on sampling profiles H and I on Brancaster Golf Course Beach (see sheet 8). In addition, samples have been collected at L.W.0.S.T.L. on an off-shore bar (which is a semi-permanent feature of the eastern end of the island), and along the seaward edge of the fore-beach between the Breakthrough and Smugglers Gap. Other samples were collected at scattered positions over the Scolt Head Island Beach at various times. These include a. group of samples (samples 608 to 616, see Table 7.4). collected at evenly spaced intervals over a grid of 100 feet by 65 feet, situated on the fore-beach sand flats (shown in sheet 8). Sample 617 represents a composite of these samples. These samples were obtained in order to record the natural degree of variation in the values of the grdnsize para- X24 meters over a small area of the beach environment which appears in the field to be essentially uniform.

7.8.2 Sediments Collected Along the Holkham Beach 7.8.2 Profiles These generally fine grained sands range from well to poorly sorted and are invariably negatively skewed (see Table 7.1). The frequency curves, shown in Figs. 7.16, 7.17A and 7.17B, indicate that the variations in the mean grainsize and sorting parameter values (shown in Table 7.1) are more apparent than real, and result from the addition, in varying proportions, of a coarse sand and gravel fraction to a uniform, predominantly fine grained and well sorted sand component with a mode of 2.1 phi. Those samples containing a significant amount of coarse material were obtained from near the crests of ridges on the beach face (compare Table 7.1 with Fig. 7.15). The samples collected at the seaward and landward ends of both profiles (profiles A and B) display the best sorting. The well sorted sediments found at the seaward ends of these profiles occur on very gently sloping sand flats (see profiles in Fig. 7.15), which are believed to be formed dominantly by current action. The well sorted sediment occurring at the landward ends of these profiles is, to a large degree, the result of wind action which winnows and separates the finer grades from the coarser material, and subsequently concentrates them in the dunes and on the tops of the berm. The gently convex beach slope reflects the generally

• • A 0 • LB • a .5 •• 0. •

16 • • • • • • 14 • • . .o 12 3 .-1 * • 10 "w • • S -a • • -wLa" HOLKHAM BEACH PROFILES A AND B • 4 • • 2s. • •

0 •

1 ZO B • 18 • • SCOLT HEAD ISLAND - BEACH • 16 • • -PROPILES C,D,E RND G. • 14 • • 12 • • 0 10 3 • • • •W • • 8 - o . • • ID • , • • • • 6 .1- • us W Ls. • • 0 -Z • 4 • • I— X ••• - tp op • • • L73 • 6 2 • • 0 •• • • a 1.0 1.1 12.. 1.3 1-4. 15 1.6 1.7 1.8 Pc) 2.0 21 22. MEAN GRRINSILE IN PHI UNITS

Fig. 7.-9 Relationship between pear .graine and beigbt Habovel low water leyel .for ..sa:ep.les collected on 'profiles A and B on Holkhap. .Beach and c) . profiles C,,- D, E and G ot Scolt HeadIsland beach. 32 uniform character of the sediments forming the beach (Bascom 1951). This low beach gradient will subsequently subdue wave action at all stages of the tide, and is, therefore, partly responsible for the relatively minor amount of coarse material transported across the beach. The presence, however, of a small proportion of coarse grained material in a number of these samples (see especially profile B., Fig. 7.17A and B) implies the existence of a mechanism, apparently related to wave action, by which coarse grained sediment may, under very high energy conditions, migrate shorewards across the beach.

7.8.3 Sediments Collected Along the Scolt Head Island 7.8.3 Beach Profiles. Profiles 0, D, E, F and G are distinctly different from profiles A and B,in being composed of sediment which increases in mean grainsize from the seaward to the landward part of the profile. The relationship between mean grainsize and the approximate height of samples above L.W.O.S.T. level is shown in Fig. 7.9A and Fig. 7.9B. A clearly defined relationship exists showing increasing mean grainsize with increasing height of the samples above L.W.O.S.T.L. for samples collected on profiles 0, D, E and G on Scolt Head Island (see Fig. 7.9B). This trend is, however, subdued and almost absent in the samples collected on Holkham Beach (Fig. 7.9A). The difference in character between the sediments comprising the beaches of Holkham and Scolt Head Island are further emphasised by comparing their frequency curves, shown in Pigs. 7.16, FIGURE 7.10

X 0 4.0.1 X 0 ▪ X X 5k, 0 0 0 O X X o 0 X X X x 0 0 X O X -- 03 0 x 0

X X x. 0

x 0

Jik o 2.0 X s X 0 O

o X 0

BFICK- BERCN SUB-ENVIRONMENT

FORE-13ERCH St/13-ENVI RONM ENT.

Fig. 7.10 Showing the grainsize parameter values of samples collected on profiles C (X) and G(o) 32L 7.17, 7.18 and 7.22 respectively. Figure 7.10 shows the relative uniformity in mean grainsize and standard deviation values of the fine grained sediments of the lower part of the beach profile on Scolt Head Island. At a point just seawards of the crest of the main fore-beach ridge, however, the sediments begin to coarsen and to become more poorly sorted. Generally these properties reach a maximum in the vicinity of the crest of the back-beach ridge. The skewness values, shown in Fig. 7.10, become more positive as they are traced landwards from L.W.O.S.T.L. These changes in grainsize reflect the changes in wave energy which have been shown tp occur as a result of the combined effect of a concave beach profile and a large tidal range (see Fig. 3.9). A mound of apparently well sorted "pea shingle" (i.e. pebbles with diameters in the order of 5 to 10 mm.) was observed to form at the commencement of a spring tidal cycle at the base of the back-beach ridge on the landward side of the runnel which arbitrarily divides the fore-beach and back-beach sub-environments.(see Fig. 7.11). This material appeared to have been derived from the fore-beach sub-environment, and to be in the process of migrating shoreward into the back-beach sub-environment. Sample 677 was collected from this mound of pea shingle, and sample 678 was gathered from the crest of the rear fore-beach ridge (which is separated from the location of sample 677 by a deep runnel) at a level which is vertically equivalent to the position of sample 677. The frequency FIGURE 741.

677

FORE-BEACH 678

-0.1 0 0.2. 0.4 0.6 0.8 I 0 1.Z 1.4 16 18 2 0 22 24 26 2.8 GRRINSIZE IN PI-II UNITS

0. PER SHINGLE 677 678 RUNNEL NORTH BACK-BEACH (sEriw R S)

Fig. 7.11 Showing the position on the beach, , and the grain size distribution (as frequency curves), of samples 677 and 678: 53() distribution curves of these samples (Fig. 7.11) indicate that the fine grained mode which predominates in samples 678 (and is characteristic of the fore-beach sediments) also occurs as a secondary mode in the bimodal grainsize distribution of sample 677, which lies in the back-beach sub-environment. This implies that, under certain conditions, fine grained sediment may (temporarily?) cross the runnel from the fore-beach into the back-beach sub- environment. Diagonal profiles D and F have been located on the diagonal ridges that occur in front of Hut Hill and House Hills on Scolt Head Island, respectively, and which have been previously described in section 7.3.3. These features locally traverse the fore- and back-beach sub-environments and disrupt the usually sharp physical discontinuity between these two zones. The frequency curves in Figs. 7.19 and 7.21 show that the modal grainsize of the sediments occurring on these features progressively increases from the lower to the upper end of the profiles (see also Table 7.1). This constant increase in grainsize distin- guishes these isolated features from the majority of the beach sections on Scolt Head Island (compare the frequency curves of samples collected over profiles D and F with those of profiles C and G in Fig. 7.18 and 7.22), and is undoubtedly related to their comparatively ulform gradient. An origin of these features has been tentatively proposed in section 7.3.5. 33• 7.8.4 Sediments Collected Along the Brancaster Golf 7.8.4 Course Beach Profiles

The mean and modal grainsizes of sediments collected on profile H, on Brancaster Golf Course Beach, show slightly less variation from the seaward to the landward ends of the profile (see Table 7.1, and Fig. 7.23)9 than do sediments gathered on profiles across the Scolt Head Island Beach. Since wave action at this locality is comparatively subdued, owing to the protection afforded by Scolt Head Island, the influence of current action is reflected in the beach sediments to a relatively high degree. Strong tidal currents in BrancadEr Harbour tend to deposit fine grained sediment towards the upper end of theirofiles This tendency modifies the effect of wave action which normally concentrates the coarse material in the back-beach sub-environment (compare the modes of samples 302, 303 and 304 of profile H with the back-beach samples of profile G, Fig. 7.22. Sample 301 in profile H was collected inihe bed of Brancaster Harbour Channel and reflects the effect of strong current action alone. Samples 310 to 317, collected in the vicinity of profile I (Fig. 7.24) further indicates the increased influence of tidal current action over wave action. These samples are situated on the sand flats which physiographically correspond to a fore-beach sub-environment; however, their mean and modal grainsizes are considerably coarser than are the equivalent fore-beach samples in profiles C and G on Scolt Head Island. 332 7.8.5 Samples Collected Along Scolt Head Island Beach 7.8.5 at Low Water The samples shown in Table 7.2 were gathered in December 1964, along the crest of an off-shore bar which formed approximately 20 yards off-shore at L.W.O.S.T. at the eastern end of the Island (see sheet 8). This feature shows a slightly asymmetrical cross-section similar to that of a typical fore-beach ridge in which the landward side slopes slightly more steeply than the seaward side. A channel separates this off-shore bar from the beach. The bottom of this channel is covered by broad, E. facing mega ripples formed by the flood current. The grainsize parameters of these sands are shown in Table 7.2, and their frequency curves are shown in Fig. 7.25. Except for sample 463, from the channel bottom, these sediments are moderately fine grained, very poorly sorted and strongly negatively skewed. The majority of them are characterised by a pronounced coarse grained tail which, in samples 461, 462, 464 and 465, reaches sufficient development to create a secondary coarse mode of approximately 1.7 phi. The fine mode is rather variable, ranging from between 2.1 and 2.4 phi. It has been proposed by many workers (e.g. King, 1959), that an off-shore bar forms as a result of beach retrogradation. This occurs most commonly during winter when destructive wave action causes the beach profile to become less steep. The sediment of an off-shore bar is generally thought to have been derived from the beach face. The wide range of grainsizes of the sediments 333 of this feature indicates that this material has, in fact, been derived from the back-beach as well as the fore-beach sub-environments. A supplementary origin may be tentatively proposed. The poorly sorted sediments of the off-shore bar may represent material derived from off-shore, moved on-shore, and subsequently concentrated in a bar feature under specific wave conditions. This proposal finds some small support in samples 661, 666 and 667 (Table 7.7 and Fig. 7.38, section 7.10), which were collected at depths of 12 feet or more beneath L.W.O.S.T. level off the W. end of Scolt Head Island (sheet 8), and are believed to be fairly typical of the off-shore sediments. With the exception of sample 666, these sediments, however, which are also very poorly sorted, display only slightly negatively skewed grainsize distributions as opposed to the strongly negatively skewed distributions of the off-shore bar sediments. Samples 516 to 527 were collected on the 23rd December, 1964, along the L.W.O.S.T.L. from the Breakthrough westwards as far as Hut Hills (see sheet 8). Previous to this time the N. Norfolk coast had been subjected to numerous winter storms and has suffered considerable retrogradation. It is proposed that the abnormal coarseness and positive skewness of the sediments collected at this time (see Table 7.3 and Fig. 7.26), indicate the partial removal of the fine sand fraction from the lower beach face under these erosive conditions. This positive skew- FIGURE 7.12A.

-07 -0.6 -05 -04 -03 -02 -0.1 0 +01 +02 +03 5k

Fig. 7.12 A. Relationship between the'Mean grainsize (M2) and skewness (Sk ) of all the beach samples, 1 excluding' those collected at the low water level on the Scolt Head Island beach. FIGURE 7.12 B.

2:2 2.1

2.0

1q 1.8

1.7 1.6 Niz (1,F10 1-5 1.4

1.2

11 1.0

0.9 0.8

02 03 04 05 0.6 07 as aq 1.! 1.2 t3 (1'4")

Fig. 7.12B Relationship between the mean grainsize (M ) and standard deviation (a) of all the beach samples,, excluding those collected at the low-w4ter level on the Scolt Head Island beach. 336 ness is apparent in samples 856 to 864 (profile D, see Fig. 7.19), which were also collected at a time when the beach was suffering erosion. These samples were gathered transversely across the beach face from the low to the high tide level and suggest that erosion, which preferentially removed the finer fractions, operated at all levels of the tidal range. The frequency curves of samples 522, 523, 524 and 526 (Fig. 7.26) show a bimodal distribution of grainsizes which appears to be produced by the addition of a coarse mode to the normal fine grained mode normally typical of the lower fore-beach sub-environment. It is possible that this coarse grained mode has been derived from the back-beach sub-environment and has been combed seawards, as a result of beach erosion. This has subsequently been mixed with the normallbre-beach sediments.

7.8.6 General Characteristics of the Beach Sediments 7.8.6 The relationship between the mean grainsize, the standard deviation and the skewness of the sediments from the various beach profiles are shown in Figs. 7.12A and 7.12 B. The former plot indicates that an improvement in sorting occurs with decreasing grainsize. The latter shows a slight, although erratic trend of more negative skewness with decreasing grainsize. Similar plots for samples collected along the low water level and on the off-shore bar, Fig. 7.13, also show a relationship between improved sorting and more negative skewness with decreasing mean grainsize. These general relationships are also shown by the sediments occurring in the Tidal Inlet environ- FIGURE 713.

1 • 1 -F0.3 • .. • •. • 0, • • • • • • • 1, • • • • • • • • •• • • •

•

1.4 1.5 1.6 17 1-8 R zo Z.3 M01.1)

07 0.6 0.5 GI (P") 04 03

Fig, 7.13 Relationship of bOth atandard, deviation .(or) and skewness (Sk ) to :the 1 mean grainsiz e of:Samples collected. onot . the off-shore ar, and along the low water level on Scolt Head Island beach. 338 ment (see section 9). The relationship between improved sorting and decreasing mean grainsize has been previously reported by Inman (1949). Phleger (1965) has observed this phenomenon to occur in sediments collected along a traverse across a barrier island.

7.8.7 Conclusion 7.8.7 By relating the theoretical distribution of wave energy, predicted in section 3.5, across the average concave beach profile, which is typically found on the Scolt Head Island Beach, to the general distribution of grainsizes occurring across the beach, it is apparent that the former is closely related to, and it is believed, controls the latter. The progressive increase in wave energy from L.W.O.S.T.L. to H.W.O.S.T.L., produces a corresponding increase in grainsize of the sediments deposited over this interval. This conclusion is modified in the case of Holkham Beach, where a gently sloping and convex beach profile is believed to subdue wave energy during the upperkalf of the tidal range. In this way, the accumulation of coarse grained material in the back-shore zone is prevented. It is proposed in section 7.6 that the beach profile of Holkham Beach has been produced by the long- term accretion of fine grained sediment to this part of the coast. A current (and sediment) circulation located in Well's Cut, to the Y. of Holkham Beach, is believed to be responsible for this phenomenon. The sediments collected across the eastern end of Brancaster Golf Course Beach also show a modified dis- 339 tribution as compared to those gathered on the Scolt Head Island Beach. This is attributed to strong current action in Brancaster Harbour which naturally tends to concentrate fine grained sand at a high level on the channel side and coarse grained sediment in the channel bed. This tendency is the reverse to that produced by wave action. The very characteristic runnel which forms at the toe of the beach face, between the fore- and back-beach sub- areas on Scolt Head Island (as shown by the various surveyed beach profiles), plays a significant role in creating local dynamic conditions (i.e. longitudinal water movements) which are responsible for separating the fine grained sand of the fore-beach sub-area from the coarser grained sediment of the back-beach (see section 7.5.2). The direction of littoral dirft, predicted theoretically from the average wave energy affecting this stretch of the N. Norfolk coast (section 3.3), indicates that a westward direction of sediment movement may be expected to occur inboth the fore- and back-beach sub-areas of Scolt Head Island. These theoretical predictions are verified by the field evidence of the orientation of ridges and runnels in both sub-area. This is, in addition, strikingly illustrated in the back-beach sub-area by the rapid westward migration of vast amounts of gravel, as well as of large pieces of debris, under storm conditions. The direction of residual sediment movement in the fore- beach zone is complicated by the predicted eastward move- 340 ment of fine sand by the strongest (eastward flowing) flood current, which acts during the lower half of the tidal cycle. This has been illustrated in section 3.8 to occur despite the modifying influence on sediment transport produced by the abnormal phase relationship between the times of peak velocity and the tidal range. The physiographic evidence mentioned above (i.e. the orientation of fore-beach ridges and runnels) suggests that the tendency for the fine grained sand of the fore-beach to be moved in a residual eastward direction by the tidal currents is more than counterbalanced by the residual westward direction of sediment movement produced in this zone by wave action. This balance is, however, apparently reversed in the near-shore zone where current action outweighs wave action and produces an eastward residual sediment movement. This is indicated by the occurrence of eastward facing mega ripples in the trough separating an off-shore bar from the beach at the eastern end of Scolt Head Island. It may be concluded as a general principle that in a tidal environment where the off-shore and intertidal gradients combine to produce a concave profile, wave energy will increase as the tide rises. This will be reflected in the distribution of coarse and fine grained sediment, the former of which will naturally move toward the crests of the high features. The complicated sediment movement pattern caused by the combination of a large tidal range and strong tidal currents marks a significant difference to the more simple sediment movements described by other workers as occurring on the Pacific coast beaches of N. America. 341 Table 7.1 Grainsize parameters of samples collected on sampling profiles A and B on Holkham Beach; C, D, E, F1, F2 and G on Scolt Head Island Beach; and H and S on Brancaster Golf Course Beach. Table 7.1 Phi Mean Phi Standard Sample Grainsize Deviation Skewness (Mz) '(5-1) (SKI) Profile A 571 1.97 0.28 -0.15 572 - - - 573 1.98 0.30 -0.23 575 1.96 0.26 -0.17 576 1.87 0.32 -0.19 577 1.91 0.41 -0.34 578 1.89 0.59 -0.51 579 1.65 0.87 -0.61 58o 1.99 0.25 -0.17 581 2.02 0.24 -0.16 582 2.11 0.23 -0.10 583 2.03 0.24 -0.20 Profile B 730 2.00 0.24 -0.09 729 2.04. 0.22 -0.12 728 1.86 0.32 -0.15 727 1.97 0.40 -0.22 726 0.87 1.13 -0.09 725 1.26 1.00 -0.66 724 1.92 0.34 -0.21 723 1.98 0.29 -0.18 722 1.88 0.50 -0.40 721 1.92 0.52 -0.43 720 1.87 0.30 -0.22 719 1.86 0.34 -0.19 718 1.86 0.36 -0.22 717 1.82 0.35 -0.23 716 1.84 0.33 -0.17 715 1.72 0.51 -0.33 71 43 1.76 0.42 -0.19 1.10 1.03 -0.1 2 1.87 0.41. -0.29 11 2.01 0..)1 -0.27 10 1.9 0.35 -0.30 0.2349 78g 1.98 0. -0.25 707 1,g1 0.41 -0.30 706 1. 0.1 -0.4! 705 2.i 0.26 -0.1 704 2.17 0.24 -0,0 703 2.15 0.25 -0.1 342 Profile C 695 1.64 0.32 0 694 1.48 0.31 +0.15 693 0.96 0.77 -0.21 692 1.29 0.37 +0.13 691 0.70 0.86 -0.04 690 1.43 0.42 -0.10 689 1.63 0.47 +0.03 688 1.73 0.53 -0.07 687 2.02 0.39 -0.14 686 2.13 0.36 -0.15 685 2.05 0.4.1 -0.19 684 1.80 0.84 -0.4.9 683 2.18 0.36 -0.23 682 2.14 0.41 -0.32 681 2.10 0.48 -0.35 680 1.58 0.96 -0.42 Profile D (Diagonal Ridge) 864 1.46 0.37 0 863 1.34 0.40 0 862 1.52 0.39 +0.03 861 1.71 0.34. +0.15 860 1.69 0.41 +0.03 859 1.84 0.38 0 858 1.75 0.37 +0.06 857 1.82 0.34 +0.05 856 1.90 0.35 +0.01 Profile E 2 1.48 0.4.0 -0.04 3 0.74 0.76 -0.11 4 0.84 0.71 -0.10 5 0.73 0.78 +0.09 6 0.85 0.75 +0.03 8 1.44 0.64. -0.29 9 1.16 0.78 -0.18 10 1.82 0.59 -0.33 11 1.88 0.77 -0.49 Profile F1 (Diagonal Ridge) 209 0.86 0.76 -0.08 210 1.50 0.51 -0.24 211 1.68 0.42 -0.17 212 1.87 0.39 -0.15 213 2.05 0.38 -0.20 343 Profile F2 (Runnel) 258 1.06 0.70 -0.15 259 1.15 0.70 -0.15 260 1.03 0.77 -0.11 261 1.43 0.47 -0.24 262 1.69 0.36 -0.09 263 1.83 0.30 -0.09 264 2.03 0.31 -0.20 265 2.29 0.29 -0.35 Profile G 29 1.23 0.36 +0.14 28 1.45 0.35 +0.07 27 1.29 0.39 +0.01 26 1.03 0.70 -0.14 25 1.14 0.54 -0.17 24 1.38 0.56 -0.15 23 0.39 0.49 +0.63 22 1.41 0.72 -0.20 21 1.86 0.34 0 20 1.81 0.37 -0.10 19 1.86 0.36 -0.11 18 2.10 0.31 -0.14 17 2.12 0.39 -0.36 16 2.14 0.31 -0.15 15 2.15 0.32 -0.16 14 2.24 0.30 -0.23 13 2.06 0.41 -0.26 12 2.10 0.35 -0.11 Profile H 302 303 1.82 0.37 0 304 305 1.96 0.36 -0.06 306 1.87 0.39 -0.19 307 1.80 0.37 -0.06 308 1.95 0.34 -0.10 309 1.96 0.35 -0.09 301 1.62 0.30 +0.04 Profile I 315 1.60 0.42 -2.12 316 1.89 0.32 -0.01 317 1.91 0.38 -0.04 310 1.99 0.29 -o.o9 311 1.73 0.38 +2.21 312 1.91 0.32 -0.09 313 2.04 0.33 -0.07 314 1.87 0.36 -0.11

FIGURE 7.14.

695 4WOSTL

694- 693 692

691 690 689 688 657 686 PROFILE C. 685 684 681 BREAKTHROUGH LuJOSTI- 683 682 - 63° 14W051-1- 3 4 6

PROFILE E. 10

HUT HILLS 1-1405T1..

111.40511-.

Iq 18 PROFILE G. 17 15. 16 14 TERNERY POINT 1312 twOSTL.

Fig. 7.14 Position 'of samples collected on sampling profiles C, E and G. The scale used is the same as that in Fig. 7..2. IUN ES 575 573 572 571 411405T1- 577 L 578 379 NORTH 580

583 532. 581 DIFIGRAMATIC REPRESENTATION OF PROFILE A. SCRLE I INCH 2.0 FEET VERTICAL RND 400 FEET HORIZONTAL

1.41,105TL

723

NORTH

Dinci RpriATIc REPRODUCTION OF PROFILE B. Lu../0511- SCRLE :1 INCH =10 fE.ET VERTICAL AND ZOO FEET HORIZONTAL FIGURE 7.16 PROFILE A .

571

572 573

575

576

577

578 579

580

581

582.

583 .2. 0 az 0.4 0.6 0•a I0 1.2 1.4 1.6 1.8 2.0 2:2 24 2:6 GRRIN51ZE 1N PHI UNITS

Fig, 7.16. Frequency curves of samples collected on profile A. FIGURE 7.174 PROFILE B .

716

715

714

715

712.

711

710

709 708

707

706

705 704

703 -az 02 04 0.6 0.8 1.0 11 14 1.6 1.8 2.0 2-2 2.4 2.6 GRPINSIZE N. PHI UNIT

Fig. 7.17 A and B. Frequency curves of samples collected profile B. FIGURE 7.17B PROFILE B .

730

72q

728

727

726

725 724

723 722

721 720 719

718

717

-02. 0 02 04 06 08 10 1.2. 1.4 F6 I:a 2.0 2.4 26 GRAINSIZE IN PI41' UNITS FIGURE 7.18. PROFILE C.

695

694

693

692,

691

690

689 688

687

686 685

684

683 652

681

680 0 Qal. 04 0.6 GS HD 1-2 14 1.6 1.8 20 2.2 2.4 26 Z8 GRRINSIZE PI41 UNITS Fig. Frequency curves of samples colleced o pcofile C. FIGURE 7.19

PROFILE D.

864

863

862

861

860

959

858

857

856

02 0.4 0.6 0.8 10 12 1.4 1.6 1.8 20 22 24 Z6 GRRINSIZE IN PHI UNITS

Pig. 7.19 FrecrJe:tcL curves of samples collected on profile D. (Transverse feature situated to the N. of House Hills) FIGURE 7.20.

PROFILE E.

4 5

-02 0 02. 04 as 08 1.0 1.2. 14 1.6 IS 2 0 2.2. 24- 2.6 GRMNSIZE W PHI UNITS

Fig. 7.20 Frequency curves of samples collected on profile FIGURE 7.21 .

PROFILE RIDGE CREST. 20q

210

211

212

213

PROFILE Fa. RUNNEL_ 258 25q

260

161

262

263

2.64-

a65

-01 0 02 04 0.6. 0.8 1.0 1.2 M. 1.6 1.8 2 0 2:2. 24 i6 GRRrNSIZES IN PHI UNITS FIGURE 7.22. PROFILE G.

26 25

2.4 23

17 16 15

14 13

12. -02 0 oa '04 06 0.8 10 ia 14 1.6 1.8 ZO 2 -4- 2.6 phi Satit. baikti4 02 3CONi. FIGURE 7.23.

03 FORS - BIR‘H f, SAPID FIAT 301- 20p1i• R. U 305 ARBO

306 307 308 31 H 09 3. 301 4

PROFILE H.

302,

303

304,

305

306

307

308

30q

310

301

I I 0.2 0 02 04 0.6 0.8 PO 1.2 1.4 1.6 1.8 20 2'2 2.4 2.6 PM UNITS

Fig. 7.23 Diagramatic crossection and frequency curves of samples collected along profile H. FIGURE 7.24.

PROFILE

317

316

315

314

312 311

310

02 0.6 0.8 1.0 12 1.4 1.6 '1.8 20 22 2.4 2.6 2•S GRAINSIZE 1N PHI UNITS

Fig. 7.24 Frequency curves of samples collected on profile I. 35 Table 7.2 Grainsize parameters of samples on the off- shore bar that is located in front of Norton Hills at the eastern end of Scolt Table 7.2 Phi Mean Phi Standard Sample Grainsize Deviation Skewness (Mz) ) (SKI) 452 1.88 0.46 -0.11 453 1.85 0.45 -0.06 454 1.88 0.60 -0.37 455 1.75 0.61 -0.26 456 1.73 0.64 -0.23 461 1.62 0.72 +0.19 4.62 1.76 0.60 -0.23 464 1.73 0.54 -0.06 465 1.80 0.52 -0.14 457 2.12 0.48 -0.13 458 1.75 0.72 -0.37 459 1.90 0.47 -0.22 460 1.90 0.52 -0.25 463 1.89 0.32 +0.02 FIGURE 7-25. OFF-SHORE BAR.

WEST 465 464

462

461 456

455

454-

453

452. EAST

NORTH 457 453 456

4561

460

463 SOUTH 0 02 04 06 08 I0 12 /4 l•6 /13 20 22 24 2.6 2.8 GRAIN5IZE IN PHI UNITS

Fig. 7.25 Frequency curves of samples collected on the off-shore bar that Occurs at the eastern end of Scott Head Island. 358 Table 7.3 Grainsize parameters of samples collected along the L.I‘T.O.S.T.L.from in front of the Breakthrough westwards as far as Smugglers Gap Table 7.3 Phi Mean Phi Standard Sample Grainsize. Deviation Skewness (Mz) (11 ) (SKI ) 516 1.72 0.37 +0.16 517 1.71 0.35 +0.14 518 1.84 0.33 +0.12 519 1.46 0.38 +0.24 520 1.70 0.34 +0.13 521 1.76 0.39 +0.06 522 1.77 0.39 +0.09 523 1.68 0.41 +0.16 524 1.59 0.45 +0.19 525 1.75 0.47 -0.07 526 1.65 0.43 +0.13 527 1.90 0.46 -0.20 FIGURE 7-26.

LOW WATER LEVEL y

WEST 52,7

526 525

524-

52 522

521

5Z0

519

518

517

516 Ensr I 0.8 10 1.2 14 1.6 1.8 ZO 22 24 2.6 GRMNSIZE W PHI UNITS

Fig. 7.26 Frequency curves of samples collected along the lOw water level of the Scolt Head Island beach between the Breakthrough and, Hut Hills. 360 Table 7.4 Grainsize parameters of samples collected at scattered positions mainly within the fore-beach sub- area of the Scolt Head Island Beach Table 7.4 Phi Mean Phi Standard Sample Grainsize Deviation Skewness (Mz) (G-1) (SK1) 608 2.16 0.33 -0.21 609 2.10 o.37 -0.22 610 2.13 0.33 -0.26 611 2.20 0.31 -0.24 .. 612 2.06 0.37 -0.19 613 2.09 0.34 -0.19 614 2.10 0.34 -0.20 615 2.10 0.33 -0.21 616 2.08 0.37 -0.24 617 2.09 0.35 -0.25 247 1.88 0.37 -0.13 249 1.92 0.33 -0.06 250 1.98 0.45 -0.28 252 1.89 0.36 -0.17 253 1.55 0.47 -0.20 254 1.53 0.43 -0.13 646 1.80 0.37 -0.03 251 1.94 0.45 -0.19 248 1.47 0.30 -0.10 647 1.40 0.37 +0.07 677 0.98 1.13 -0.26 678 1.95 0.46 -0.15 FIGURE 7.27.

‘1'.\\ \\ VC

:::-- • . .

HWMOT grad LWMOT Ifici I • 11907 . 14153 LWOSTL 1464

Fig. 7.27 Showing the various positions of the L.W.O.T.L. and H.W.O.T.L. between 1891 and 1964 at the western end of Scolt Head Island. Scale: 3 inches equals 1 mile. 362

7.9 The Brancaster Harbour Bar Sub-area 7.9

7.9.1 Morphology and Dynamic Conditions 7.9.1 At the western extremity of Scolt Head Island, the sand flats of the Brancaster Harbour Bar are continuous with the frontal fore-beach sub-area of Scolt Head .Island (see sheet 8). At the western end of the Ternery the main gravel ridge bends southwards,and displays a similar relationship to the Bar as exists between the fore-beach and back-beach sub-areas described in section 7.2. Unlike the fore-beach along the front of Scolt Head Island, the Bar lacks a gradient, being an essentially flat topped feature. Numerous discontinuous and irregularly orientated ridges occur over the Bar, especially in the western half. Since the rate at which these ridges migrate across the Bar is determined by local variations in wave and current conditions, their irregu- larity is understandable. A comparison of various Ordnance Survey maps and a compass traverse, made in 1964, indicates a long-term direction of growth towards the W. and S.W. (Fig. 7.27). The same processes which control sediment movement on the fore-beach (section 7.5) also act on the Brancaster Harbour Bar. However, current action appears to be more important than that of waves in this sub-area. A phase relationship exists between the tidal current flowing E.-W. in front of Scolt Head Island and the tidal currents of the inlet of Brancaster Harbour. FIGURE 7.33.

Orr ig in al tra.N5VErS ridge STAGE A

Ebb current is preferentially concentrated- in the centre Ebb of +he- rage cau5iNg 0- c:1°1.4n-51-ream bowtrig

5TAGE B

Flood current is deflected either side o> the. nose of the. r40)1 .

Flood

5TTIGE C. I i i Ebb Flood FLOOD CHANNEL

Ebb

EBB CHANNEL

Flood Ebb

Fig.7,;33 Showing the .develOpment of an ebb and flood channel system as a result of the deformation' of a transverae,sand.ridge by ebb and flood current action. •...... 10.1ippleminimpuripp...- '-'11RN. II, • 7.32

ko.111•••••••••••••• 4-

• 365 The current movements in the inlet are termed "inlet currents" to distinguish them from "tidal currents" which comprise part of the North Sea circulation. This phase relationship between inlet and tidal currents has been determined from the tidal measurements made by Steers (1960) in the off-shore area, and those made by the author in Brancaster Harbour channel (see Figs. 7.28 and 7.29). An E. to W. tidal stream flows in front of Scolt Head Island during the low half of the tidal cycle, which extends approximately from 3 hours after high tide (+3) to 2 hours before the next high tide (-2), (tidal times are related to high tide Immingham). For most of this interval the inlet currents are restricted to the main channel of Brancaster Harbour, ebbing to the W. as the tide falls. A W.-E. tidal current flows in front of Scolt Head Island during the higher half of the tidal cycle, which extends from approximately 2 hours before high tide (-2) to 3 hours after high tide (+3). The water level during this interval is above the Harbour Bar, and from 2 hours before to 1 hour after high water the inlet currents flood diagonally across the Bar from N.W. to S.E. For 2 hours following high tide (+1 to +3 hours) the water level is still above the surface of the Bar, when the water mass enclosed in Brancaster Inlet commences to ebb. At this time the tidal currents are still flooding to the E. which subsequently causes the currents flowing westward out of Brancaster Harbour to FIGURE 7.29.

LOW HRLE OF THE TIDAL RANGE

EIM1 TIDE (+3 to -2 hours

211211ai,e

HIGH HALF OF THE TIDAL RANGE

TERVIERY Pt L.

HWOSTL

Fig. 7.29 Showing the duration and direction of water . movements in the vicinity of the Brancaster Harbour Bar;_ during the lower atd the upper parts of the tidal range. FIGURE 728.

---3 FRTHogi

STEERS TIDAL STRTION 0

• . . L osTi.

..—%......

.. ..- . • .• .• .• • .

FLOOD ColtOgr4 FLowifig To Twit E. STEERS TIDAL MEASUREMENTS EBB CURRENT +-z: F1,.01,-11t46 •-) to THE W.

FLOOD CuRRENT Fi.owiNc, -ro "NE E. fivi_ET rpm. STATION N°.3.

+I Eas CtiRRENT 3i FLOWING, To Tim W.

LW. HW LW HW

Fig. 7.- 28 Comparison between the tidal current Velocity to the N. o. SColt Head Island, and those in Brancaster Harbour channel. 5RANcrISTER GOLF cooKSE -ESERCH.

qoL.F. CLUB NOUSE 41011k4, .4111-W „meat, odsiVap. Anatillicacto Fig. 7.30 Brancaster Harbour Bar, Brancaster Harbour Channel, the harbour mouth bar and Brancaster Golf Course Beach. Photograph taken about half tide by Huntings Surveys Ltd., June 1960. Scale: 6 inches approximately equals mile. ag• • . 0 . - • IRRrG,utRR F TERNERs 'WS POINT -:7‘4°4 AND LOWS

cwie grnweci.

StclimeNfati OAI

'- WRECK ) TL'i

Zara' 171F-

-• 7.31 Brencaster llerbour Bar (eastern end, adjacent to the Ternery), showing the configuration of the highs and lows Fnd the progressive increase in size towards !lorth of the train of mega-ripples in the bed of the ebb-channel. Fine grained hove begun to -ccumulate behind the recently formed gravel ridges at the Ternery. inches pproximately equals I mile. 37f be deflected to the N. and E. around the Ternary (see Fig. 7.29). The interaction of these two currents (i.e. the inlet and the tidal currents) appears to cause a concentration of the inlet current near the Ternary at the eastern end of the Brancaster Harbour Bar. A broad funnel-shaped ebb channel (shown in Fig. 7.30) is formed by this flow. This ebb channel is orientated approximately Y.-S. and is characterised by the development of ebb formed mega ripples. These increase in wave length and amplitude towards the N. (Fig. 7.31)and, according to Van Veen (1931) afford a criterion for indicating the presence and direction of residual sand transport (which, in this case is towards the N.). The scouring which exposes gravel at the southern or Brancaster Channel end of the feature is also characteristic of an ebb channel configuration. Further evidence of local inequality between opposing flood and ebb current velocities is afforded by the typical "S" shaped ridges which have formed on the N. end of the Bar (Fig. 7.32). These features are apparently generated by the preferential action of a current upon one part of a transverse "ridge" or sand wave, thus causing unequal sand transport over the feature. This produces a bowing in a down current direction (stage A, Fig. 7.33). The opposing current is then deflected around the nose and along one or both sides of the "ridge", causing in turn dominant sediment transport in an opposite direction to occur at these points (stage B, Fig. 7.33). The opposing ebb and flood currents are in this manner confined to local ebb and flood channels (stage C,Fig.7.33). 371 Ebb-current ripple marks, which are almost completely absent from the fore-beach, commonly form on the Brancaster Harbour Bar. These are orientated with their lee sloepes facing between 030° to 055° near the Ternery, while further to the W. the orientation swings towards the N., and at the western end of the bar ripples are facing about 350°. This divergence in ripple orientation parallels the variation in the direction of water movement over the Bar during the beginning of the ebb current out of the inlet, which occurs during the last 2 hours of the eastward flowing tidal current in front of the Island. The westward growth of the Brancaster Harbour Bar has, at times, been interrupted and even reversed. This process represents a cyclic phenomenon, similar to that displayed by the BurnRam Harbour Bar discussed in section 7.11 below. That this is so has been indicated by the surveys of the western end of Scolt Head Island carried out by R.F. Peel in 1933, 1934 and 1935 (Steers 1960). In this interval the channel mouth is shown to out flow about TIT mile to the E. of the present position of the "Wreck". This undoubtedly represents the result of breaching of the western Bar due to deepening of the channel mentioned above. That such a phenomenon will occur is, theoretically, predicted by Bruun and Gerritsen (1961), who discuss the relative decrease in inlet channel stability that results from the natural process of channel extension under the influence of littoral drift conditions. 372 Under a given tidal regime a maximum value exists for the relationship between increasing channel length, decreasing cross-sectional area and increasing bottom shear stress which, when exceeded, will produce shoaling in the channel and cause diversion of the flow into a shorter and broader exit. The result of this comparatively rare phenomenon is to spasmodically add a large quantity of fine grained sand to the undernourished beaches of Brancaster Bay further to the west.

7.9.2 Sand Transport 7.9.2 The implications of these above mentioned water movements in terms of sand transport suggest an anticlockwise circulation: (1) residual littoral drifting of material westwards along the front of the Bar; (2) current induced movement south-eastwards across the surface of the Bar during most of the rising tide; (3) current induced seaward transport, northwards across the Bar for about 2 hours at the beginning of falling tide. This circulation pattern will produce a "terminal" for sediment transported from the E., thus accounting for the westward growth of the Bar. The entrapment of this westward moving sediment causes the undernourishment of the beach in Brancaster Bay further to the W. This general picture of sediment movement under the influence of the tidal and inlet currents is modified by wave action which will generally tend to move sediment shorewards across the Bar and, depending upon the direction of wave approach, bestow upon it a component 373 towards the S.E. or S.W. A residual shoreward and westward migration of sand is thus produced and is supported by the long term changes in the shape of the Bar (shown in Fig. 7.27). It is proposed that sediment, transported S.W. across the bar, will move down the steep slope bordering the S. side of the Bar, under much the same mechanism as that which produces the migration of a sand wave. Although the inlet currents will redistribute this sand, the long term trend shown in Fig. 7.27 indicates that the rate of addition of sediment to this "lee" slope outbalances that removed by current action. It seems probable that the westward transport of sand is not completely terminated on the Brancaster Harbour Bar by the circulation outlined above, and that some natural sand "by-passing" does occur. Bruun and Gerritsen (1961) state two mechanisms for by-passing sand across tidal inlets: (i) "by-passing via off-shore bar" occurs when the tidal flow at the inlet is small and wave action is prominent, while (ii) "by-passing by tidal flow" is operative in a strong tidal environment* The latter mechanism would seem to apply to the Brancaster Harbour Bar where inlet current velocities of up to 4 knots have been observed. However, the anticlockwise sediment circulation system which exists over the Bar would minim- ise the transference of sediment across the Brancaster Harbour channel by this method. During low tide, when the inlet current velocities are small, the process of littoral 374 beach drifting will be especially effective under strong wave action from the N. and N.W. Refraction diagrams (Figs. 3.4 D and E) for waves approaching from the N. show a concentration of energy to occur at the western end of Scolt Head Island. Since waves approaching from the N. are predominant (section 3.2), and as a small arcuate bar occurs across the Brancaster Harbour mouth (Fig. 7.30) it seems most probable that by-passing, if it takes place at all, does so under the influence of wave action on this off-shore bar. The fairly extensive fine grained sand flats which occur at the base of the Golf Course beach (on the S. side of Brancaster channel) may have accumulated by sand moving across the channel by this process.

7.10 The Grainsize Characteristics of the Sediments 7.10 Comprising the Brancaster Harbour Bar Sub-area Samples were collected on the Brancaster Harbour Bar on a number of separate occasions in 1964 (see Tables 7.10, 7.11 and 7.12, and Figs. 7.36, 7.37 and 7.41). The samples shown in Table 7.5 and Fig. 7.34 were collected during neap and spring tides. With the exception of samples 233 and 233A, the sediments collected during neap tides are slightly coarser grained, and very slightly less well sorted than the samples collected during the spring tides. The possibility that small inaccuracies in the positioning of the spring tide samples may be responsible for the variations between respective neap and spring tide samples is

FIGURE 7.34

243A S 243

241 A

241 N

240A

240

237A

237

237A

235 /

233A

233 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 24 2.6 2.8 PHI UNITS . - Fig. 7.34 Frequency curves of samples. collected' on the Brancaster Harbour Bar during' neap tides (TAI), and at approximately the same positions during spring tides(S). 376 Table 7.5 Parameter values of samples collected on Brancaster Harbour Bar during neap tide (N) and at similar positions during the following spring tides (S) Table 7.5

Phi Mean Phi Standard Sample Grainsize Deviation Skewness Tide 233 1.84 0.26 -0.18 233A 1.72 0.27 -0.16 235 2.06 0.32 -0.23 235A 2.09 0.30 -0.33 237 1.88 0.41 -0.33 237A 2.13 0.29 -0.28 24.0 2.05 0.31 -0.19 240A 2.11 0.29 -0.14 241 1.94 0.27 -0.16 241A 1.99 0.26 -0.15 243 2.12 0.34 -0.24 243A 2.18 0.30 -0.31 rendered unlikely by the consistency of this trend. Further evidence for this relationship is presented in Table 7.6 and Fig. 7.35, which shows the parameter values and frequency curves of sediments repeatedly sampled over a small area (120 feet by 75 feet) on a sand bar in Mow Creek (see sheet 8) during neap and spring tides. Except in the case of the sample pair 562 and 700, sediments collected during the neap tides also display a slightly coarser mean grainsize, and a very slightly less well sorted grainsize distribution than do sediments collected during the spring tides. This relationship may, therefore, be considered to be a valid function of the tidal cycle. This is the reverse of that found by Kestner (1961), who sampled coarser grained sediments during spring tides than during neap tides in the Lune FIGURE 7.35 .

7-' / \ / \ / \ / \ v f—N .---- 702 / 1 564 / \ / \ z \ .-- 701 ..______•-',\ \ \ 563 / \ / \

_..,- / \ 700 / \ / \ 562 / \ N / \ "" .....---.... , / \ \N` 699 _-.- / \ S / \ 561 / \ N V „ \

...... 698 . / \ • \ 560 N

697 .....,...... S 559 N II I 1 t i 12 14 1.6 1.8 2.0 22 2.4 2.6 2.8 3.0 PHI UNITS

Fig. 7.35 Frequency curves of samples collected in Mow Creek during neap tides (N), and ate the same positions during spring tides (S) (see sheet [) 378 Table 7.6 Parameter values of samples collected on a grid over a small area in-Movi Creek at neap tide (N) and at spring tide (S). The location of the sampling area is shown in sheet 8. Table 7.6 Phi Mean Phi Standard Sample Grainsize Deviation Skewness Tide 559 2.08 0.29 -0.11 697 2.21 0.27 -0.10 560 2.13 0.31 -0.19 698 2.20 0.27 -0.16 561 2.10 0.27 -0.12 699 2.21 0.27 -0.12 562 2.22 0.27 -0.09 700 2.20 0.26 -0.10 563 2.18 0.24 -0.09 701 2.26 0.26 -0.10 564 2.07 0.31 -0.14 702 2.18 0.28 -0.10 1111.10l

Estuary. The explanation of this phenomenon is somewhat obscure. The parameter values and frequency curves of samples collected in Brancaster Channel and off the western end of the Bar are shown in Table 7.7 and Fig. 7.38. Their locations are shown in sheet 8. The sediments collected in Brancaster Harbour channel (301, 652 and 653) are very coarse grained, while sediments from the mouth of the channel and to the N. of the Bar are typically fine to very fine grained. This distinction is apparently caused by the higher current velocities within the channel. Many sampls are poorly sorted (e.g. 652, 659, 661, 666 and 667); this, however, is unrelated to water depth as both well sorted and poorly sorted samples occur at all depths. During Fig.7.36 _Brancaster Harbour Bar showing sampling positions (Drawn from a compas's traverse survey carried out on the 25.3.64)

MAG. N. 246 244 266 243 242 240

238 WRECK • 245 TERNERY .0'07 023i 114414 / e I A

4, / / / If •233 •236 •235 •234 •232

BRANCASTER CHANNEL

SCALE : 6 INCHES = 1 MILE Fig. 7.37. Brancaster Harbour Bar showing sampling positions and the location of the detailed sampling area. (Drawn from compass traverse _surveys carried out on the 2.12.-64 and on the 7.12.64)

'505

387 • "3:4 WRECK 388

:361 .353 66 i 354 356. BRA NC ASTER CHANNEL FIGURE 7.38.

667 -15' -12' 666 -12' 661

660

659

-11' 658

657

656

.655

653

652 301 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 28 3.0 3.2 PHI UNITS Fig. 7.38 Frequency curves of samples collected below. L.W.O.S:T.L. in Brancaster Harbour Channel, and in the: near-shore waters off the western end of Brancaster Harbour Bar .

382 Table 7.7 Brancaster Harbour Ohannel and off-shore samples Table 7.7 ,pea•Nalir aev.pablificniw= Phi Mean Phi Standard Approximate Sample Grainsize Deviation(_:...1 Skewness Depth of (Piz) ) (SK1) Sample beneath L.W.L.O.S.T. (in feet) 301 1.62 0.30 +0.04 4 652 1.39 0.54. -0.22 4 653 1.49 0.33 +0.07 5 655 2.05 0.32 -0.08 5 656 2.07 0.35 -0.11 5 657 2.01 0.38 -0.21 4 658 2.36 0.32 0 11 659 2.04 0.41 -0.19 8 660 2.4.6 0.33 -0.02 12 661 2.14 0.52 -0,04 12 666 2.12 0.72 -0.30 12 667 2.06 0.54 +0.02 15 654 pebbles 662 pebbles 663 pebbles 665 dark grey clay

this sampling programme patches of gravel were commonly encountered (e.g. in the vicinity of sample 654). The pebbles were usually well rounded and occasionally coated with dark grey clay, probably indicating the presence of exposed boulder clay deposits. Some pebbles were encrusted with marine growth suggesting considerable stability under local wave action. Similar occurrences of encrusted pebbles have been observed by Kidson, Steers and Fleming (1962) to occur in moderately deep water (30 to 40 feet at high water spring) to the Y. of the Island. The sediments collected around the periphery of the Bar show a statistical trend of increasing mean grain-

FIGURE 7.39 .

Mz (PHI) 2.3 - O 0 0 2.1 0

1.9 0 0 1.7 0 1. 5

61 0 (PHI) 0 0 0 0 0.3 0 O o O 0 0.2

-0.1

- 0.2 0 0

0 0 -0.3 0 0 SKI 0 N) N N.) N) 1\.) (...) (A) (A) CA) (A) CA) •IN ▪ (3) ▪ (.11 0 1\3 (▪ A) • O) CT) SOUTH NORTH EAST PoWET EAST Fig-7-3g- Graphic presentation of the parameter values of samples collected around the periphery of Brancaster Harbour Bar in a clockwise direction, starting with sample 232 in the S.E. corner and ending with sample 246 fn the N.E. corner. 384 size in an anticlockwise direction, starting at the Y.E. corner of the Bar (from right to left in Figs. 7.39 and 7.40). 1 less distinct trend of increasingly more positive skewness values is also apparent in this direction. Sediments are finest grained and most negatively skewed on the Y.E. edge of the bar at a position northwards of the Ternery. Their grainsize distribution becomes coarser and slightly more positively skewed towards the 7!T. along the seaward side of the Bar. This is best displayed by samples 509 to 500 in Fig. 7.40. However, at the western end of the Bar, sediments 369, 371, 372 and 373 locally become abnormally fine grained (Fig. 7.40). From the W. end of Brancastcr Harbour towards the E., along its inner landward side, the sediments display a weakly defined trend of increasing mean grainsize and more positive skewness. The standard deviation of these sediments however, shows no pattern. This progressive increase in mean grainsize in an anticlockwise direction coincides with the direction of sediment circulation proposed in section 7.9.2. However, the direction of sediment movement in such a circulation is normally accompanied by a trend of decreasing grainsize in the direction of residual sediment transport. The contradictory trend described above may be explained by a combination of two factors:- (a) The grainsize distribution of these sediments probably largely reflects variations in the local energy level. Tidal current energy on the southern flank of the Bar is FIGURE 7.40.

2.3 0

2.1 0 o 0 o 0 0 0 1.9 0 0 0 0 0 Mz 0 117 (PHI) 1.5 0 0 1.3 0

0.4 0 a 0 61 0 0 o 0 o 0 0 0 0 0.3 0 0 (PHI) 0 0.2

+0.1 - 0

51-ct 0 o 0 o o 0 0 -0.1 0 0 0 -0.2 0 0

-0.3 0 • Lb.) CO Low (A) Lots3U1L0 t..k) LAW1U1 (.710101U1C71 CD 01 01 Ul 0.) (T) -•••1 *N1 "NI 0 0 0 0 0 0 0 ro (A) A .1a. 07 r\.) co 0 n) A Cr) c0(0 -SOUTHEAST WEST * NORTHEAST

g. 7.40. Graphic presentation of the parameter •values of samples collected around the periphery of Brancaster Harbour Bar in a clockwise direction starting with sample 352 in the S.L. corner and endirg willb sample 509 in the r.H corner. 386 apparently greater than wave energy on its northern side. This is especially true in the ebb channel that occurs in the S.E. side of the Bar, and through which a considerable proportion of the inlet ebb current is channeled in a northward direction during the early part of the falling tide (see section 7.9.1). It is in the broad mouth of this ebb channel that the coarsest grained sediments occur (sample 232, Fig. 7.39, and sample 352, Fig. 7.40). (b) As previously mentioned, this ebb channel represents a zone of scouring and non-deposition (section 7.9.2). Fine grained sediment is transported, in this zone, northwards and north-westwards across the surface of the Bar. However, as the tide falls, wave action along the front or seaward edge of the Bar will become increasingly important, and will oppose the northward migration of current borne sediment, thus causing its deposition. It is proposed that the fine grainsize of samples 246, 266 and 244 (Fig. 7.39), and 509, 508, 506 and 504 (Fig. 7.40) may be attributed to this phenomenon. The wide range of parameter values of the samples shown in Tables 7.10, 7.11 and 7.12 appear superficially to be more closely related to the minor variation in relief on the surface of the bar than to their relative geographic position. In order to investigate this possibility, a small area at the S.W. end of the Bar was closely sampled on the 7.12.1964. This area incorporated a. number of low relief mounds and depressions which have, for convenience, been

FIGURE 7.41 . 466 • • •\ / • \ 11497 •

• \ .0496 • 75YDS. \ • •

• . . . w Z:7-__.- • I • . •495 1 . -_,_ DETAILED 498 .7.2---<- . •' . SAMPLING •494 U:----- LOCATION ON I Ct BRANCASTER \. D — HARBOUR BAR I • O= co-: (LOW AREAS ARE 489 • \.... 493 CC-- STIPPLED) •-7 <- •1 488 -I= •-- 492 490 48 •479 W. • i-- •481 0= / •/ •478 • . . < •. 491 ' U::: 74 -- •480 ' • Z:-<....---7_ • • • . .•482 4.86 • 473 CC= • 475 487 476 472

471. •• 477

470 .

• .••••'" / • • .•••••• 469 \.• / /. • . • 468 / •

/.. / • . /• : • 0 467 I. N 388 termed "highs" and "lows". Superficially, these features appear to be the product of ebb and flood current action; some, however, are aligned E.-W. normal to the direction of wave approach, and may be produced by wave action. The location of this detailed sampling area is shown in Fig. 7.37 and the positions of samples 466 to 498 collected within it are shown in Fig. 7.41. Their mean grainsize, standard deviation and skewness values are shown in Table 7.12. Frequency curves have been drawn for all samples, but are only reproduced to illustrate specific relationships. The various modal sizes displayed in these frequency curves have, however, been tabulated, and are displayed elsewhere (see Table 8.10). Of all samples collected over the surface of Brancaster Harbour Bar, the majority are fine grained (the mean grainsize varies between 1.95 and 2.15 phi and the dominant modal grainsize varies between 2.1 and 2.2 phi) well sorted (average standard deviation value of about 0.30 phi), and from moderately to strongly negatively skewed. In addition to these fine grained sediments, medium to coarse grained, less well sorted and more positively skewed sediments occur randomly. In order to investigate the possible existence of any relationships between the grainsize characteristics and the surface topography of the Bar, the coarse grained sediments which, as mentioned above, occur fairly rarely, have been grouped into three categories according to their 389 Table 7.8A Coarse grained sediments collected from the crest of high features (mainly situated on the northern side of the Bar). Table 7.8A Phi }1ean Phi Standard Sample Grainsize Deviation Skewness 1-1odal (Nz) (/5 ) 1 (S1(1 ) Sizo 37A 1 .98 0.31 +0.01 2.10 375 1 .79 0.38 -0.05 1 .97 376 1 .73 0.40 -0.05 1 .8A. 377 1.91 0.34 -0.13 382 1 .71 0.44 -0.14 1 .92 386 1 .83 0.26 +0.05 1.94 387 1 .78 0.27 -0.03 2.01 ------"'----~~_. Table 7.8B Coarse greined sediments that occur in depres sions (loca 1:Ed mainly on the southern side of the Bar) Table 7.8B Sample (l'~z ) Ivlodal Size ------~--_._- 233A 1 .72 0.27 -0.1 6 2.08 352 1 .46 o .3A. ~0.0L1 1 .70 353 1 .78 0.27 ~0.04 2.02 361 1 .85 0.31 +0.01 2.10 470 1 .73 0.34 +0.01 1 .92 471 1 • 71 0.29 +0.12 1 .88 A.77 1 .75 0.33 +0.08 1 .95 A.80 1 .67 0.31 +0.02 1 .92 481 1 .61 0.34 +0.14 1 .70 4.82 1 .78 0.28 +0.05 2.00

Table 7.8C Coarse gnined sediments collected at low water level Table 7.8C

Sample (Mz) (()1 ) Modal Size 35L1. 1 .32 0.31 -0.02 1 .50 1 .78 0.30 +0. 11 1 .~O ~~~ 1 .44 0.31 -0.04 1 • co 366 1 .74 0.41 +0.05 2.00 500 1 .87 0.30 +0.05 1 .90 ------~

FIGURE 7.4g.

2_2

2.0 Mz NO (PHI) 1.8 -o

1.6 0--

0.4 r 6i 0.3 0, -a (PHI) -0- 0.2

+0.1 - o

SKI 0 0

- 0.1 '0- - -0'

359 - 361

357 358 NORTH LWOSTL BRANCASTER 356 BRANCASTER SOUTH CHANNEL HARBOUR 301 100 YDS.

Fig.7.42.• Graphic presentation of the parameter values of samples collected at various positions across the southern edge of Brancaster Harbour Bar. 391 topographic location. (A) Samples collect on the crest of the high features, which occur randomly over the surface of the Bar, are shown in Table 7.8A. (B) The parameter values of those coarse grained samples collected in the bottom of lows or depressions on the surface of the Bar are shown in Table 7.8B. (C) The parameter values of coarse grained samples collected at approximate low water level are shown in Table 7.80. A marked dissimilarity between the grainsize charactersitic of the sediments within each category is obvious, and is further emphasised if the various fine grained sediments, which also occur in these topographic localities, are included. The occurrence of coarse grained sediments on the surface of the Bar is obviously determined by a number of different factors other than, or in addition to the local topography. The variation in the distribution of grainsizes of sediments collected at closely spaced intervals, across a number of isolated topographic features, was next investigated. Two classes of features have been arbitrarily distinguished: Class A. The southwestern side of the Bar is characterised by an essentially continuous ridge, the southern side of which slopes steeply into Brancaster Harbour channel. Samples 356 to 361 have been collected up this slope and sample 301 (from the bed of Brancaster channel) has also been included, although it does not occur exactly on this traverse line (see Fig. 7.0). In addition, samples have been collected at various FIGURE 7.43.

0 2.1 0 0 0 1.9 Mz 0o .0.." 1.7 ( PHI) 0 1.5 0 - 0 0 1.3 0 0.5

61 0.4 0 (PHI) o. 9 0.3 o 8 0 0 0.2

- 0.1

SKI 0 0.1

- 0.2

- 03 363 365 367 NORTH SOUTH LWOSTL 36B BRANCASTER 362 CHANNEL 364 BRA NC ASTER 366 HARBOUR BAR 301 652 653

Fig.'; .43 Graphic presentation of the parameter. values of samples collected along a traverse across the southern edge of Brancaster Harbour. Bar. 393 positions on the bed of Brancaster Channel along the L.W.L., and on the adjacent crest of this ridge (see Fig. 7.4.3)• It is apparent from these figures that the sediments occurring on the channel bed are generally coarser grained and more positively skewed than those occurring at L.N.L. and on the ridge crest. Standard deviation values show an essentially random variation in both figures. Class B. Closely spaced samples have been collected transversely across a number of approximately E.-W. orientated highs and lows, situated over the W. end of Brancaster Harbour Bar. Samples 377 to 382 comprise a traverse located on the N. side of the Bar. Their parameter values are graphically portrayed in Fig. 7.44 and their frequency curves are shown in Fig. 7.45. Samples

4.67 to 474, 483, 488, 489, and samples 477, 476, 486, 4.91 and 492 comprise two traverses on the S. side of the Bar within the detailed sampling area. Their parameter values are graphically illustrated in relation to topography in Figs. 7.46 and 7.4.7, and their frequency curves are shown in Fig. 7.4.8. In addition, the possibility that current action may cause some variation in the grainsize properties laterally along the crest of a high and in the trough of a low has been investigated (see Table 7.9 and Fig. 7.4.9). An absence of any consistent trend in these values is, however, apparent. From a close examination of figures 7.44., 7.4.6 and 7.47, it is obvious that fine grained sediments occur

FIGURE 7.44•

2.2

2.0 MZ. PHI 1.8

1.6

61 0.5

(PHI) 0.4 - o---- - °------o------0 0,3

0 -0:1 SKI 0 -- -°- -- --o-_ -o -0.2

382 381 380 379 378 377 SOUTH NORTH

1 100 YDS

Fig.7.44 Graphical presentation of the parameter values of samples collected along a traverse located on, the northern side of Brancaster Harbour Bar. FIGURE 7.45

382

381

380

379

378

377 0.8 1.0 1.2 14 1.6 1.8 2.0 22 2.4 2.6 PHI UNITS 382 381 37 77 380 379 SOUTH NORTH 100 Y DS

Fig. 7.45 Frequency curves of samples collected along a traverse located on the northern side of Brancaster Harbour Bar.

FIGURE 7-46.

Mz ( PHI) - 2.2 0 o- - 2.0 - 1.8 1.6

6I(PHI) - 0.4 - - 0------0' 0-o------0 0.2

- SKI -+01 • 0 _e

0—

`Os • 0 /

EBB CHANNEL FLOOD CHANNEL 467 468 469 ,, 473 474 488 489 470 471 j..,.74.---.-7- .''7",:. •• .-7-.-..,_,....,/483 ...:-::---1. - . -...... : ...... t.7r-777...... -..: : ..: ' : :.*:.:....'...... :::...... ,:.:-.: ...... :.:.: .; .'... .::.

100 YDS.

Fig. 7.46 Graphical presentation of the parameter values of samples collected along a traverse located in the detailed sampling area. FIGURE 7.47.

2.0

MZ 1.8 (PHI) 1.6

0.4 — 61 ----- c)- (PHI) 0.3 - 0.2

+0.1 SKI 0

0.1

—0.2 FLOOD. CHANNEL EBB CHANNEL 476 ' 491 492 47 7 -- : : ....- 4?6.._*„....!..:, :...... ,,,_ 4 SOUTH r7..." .:: -...:.:::-:-:...... • ....-...1..:,.1:.....:.1:::-..---.1NORTH

100 YDS

Fig. 7.47 Graphic presentation of the parameter values of samples collected along a traverse in the detailed sampling area. FIGURE 7. 48.

492

491

48E

476

477

489

488

483

4 74

473

472

471

470

469 468

467

0.8 1.0 1.2 1.4 1.6 1.8 2.0 22 2.4 2.6 2.8 PHI UNITS

Fig. 7.48 Frequency curves of samples collected along two traverses in the detailed sampling area. 400 Table 7.9 Parameter values of samples 374 to 377; 476, 475, 474, 478, 479; and 491, 490, 488, 493, collected from W. to E. along the crests of three highs or ridges. Samples 487, 486, 485, 483 and 484 were collected along the trough of a low Table 7.9 Phi Mean Phi Standard Sample Grainsize Deviation Skewness (Mz) ('-1) (SK1 ) 374 1.98 0.31 +0.01 375 1.79 0.38 -0.05 376 1.73 0.40 -0.05 377 1.91 0.34 -0.13 476 2.04 0.29 -0.17 475 1.92 0.29 -0.03 474 2.06 0.29 -0.11 478 2.08 0.25 -0.12 479 2.14 0.24 -0.12 491 2.00 0.34 -0.20 490 2.06 0.32 -0.24 488 2.07 0.30 -0.13 493 2.06 0.31 -0.17 487 2.01 0.39 -0.21 486 2.02 0.36 -0.18 485 1.99 0.42 -0.22 483 2.08 0.37 -0.29 484 2.10 0.32 -0.27 401 both in the troughs of the lows, and on the crests of the highs. The latter case has also been seen to apply to fine grained sediments occurring on the crest of the ridge adjacent to Brancaster Harbour on the southern side of the Bar (see Figs. 7.42 and 7.43). Coarse grained sediments similarly occur both in the trough of the depressions and also on the crest of the highs. The presence of coarse grained sediment on the crest of a high feature has been noted in the case of the beach ridges that occur in the beach sub-areas on Scolt Head Island (see section 7.8.3). This phenomenon is related to wave action, and it is proposed that the same process is responsible for the concentration of coarse grained sediment (samples 382, 375 and 376) on the highs that occur on the Y. (seaward) side of Brancaster Harbour Bar. The occurrence of fine grained sediments (e.g. samples 380, 483 and 486) in the bottom of lows is also tentatively attributed to wave action, Such lows may, under these circumstances, be equated to the fore-beach runnels. The occurrence of coarse grained, usually positively skewed sediments (samples 470, 471, 477, 480, 481 and 482) in the bottom of a depression may be attributed to scouring by current action. The broad depression (see Fig. 7.41) in which these sediments occur leads northwards off Brancaster channel and probably functions in a similar fashion to the large ebb channel that occurs over the S.E. part of the Bar (see section 7.9.1 and Fig. 7.30). The GURE 7.49

484 483 485

486 487

479

478 474

475

476

377

376

375

374 I 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.G 28 PHI UNITS

Fig. 7.49 Frequency curves of samples collected along the crests of two highs (samales 374-377, and 476-479), and along the trough of a low (samples 487-494). FIGURE 7.50.

2.3 •

2.2 • • • • • 2.1 • • : • • S. • • • • • • 2.0 s • • • • • • • • 11 9 • • •

• • • • •

• 2 1.4

1.3 , I . . . I 0.22 0.26 0.30 0.34 0.38 0.42 0.46 STANDARD DEVIATION ( PHI)

Fi. 7. 7.50 Rc lation_Fhip bctTrn mcan 9r ins 17,:, and 5-7 -tandard. deviation Of al 1 !,:anipic:: co 1 on Brancater Harbour Ba 403 coarse grained, positively skewed sediments that occur at various positions along the low water level on the southern side of the Bar, adjacent to Brancaster channel, as well as those sediments that occur on the channel bed, are apparently also produced by tidal current scouring. Since currents will normally flow at peak velocity along the centre, or deepest part of such depressions, and will decrease slightly in strength over the intervening highs, the deposition of fine grained sediments on the crest of the highs may also be attributed to current action.

7.10.1 Conclusion 7.10.1 The sedimentary deposit comprising the Brancaster Harbour Bar has apparently accumulated as a result of a circulation produced by combined wave and current action which traps sediment migrating westwards along the front of Scolt Head Island. The variations in the grainsize properties of the sediments collected on the Bar are also believed to be related to the combined action of waves and tidal currents; the latter of these factors, however, appears to be dominant. The relationship of both standard deviation and skewness to mean grainsize of all samples has been plotted in Figs. 7.50 and 7.51. A rather random distribution typifies both plots; however, a poorly defined trend of more positive. skewness with increasing grainsize is shown in Fig. 7.51, and is slightly more clearly defined in samples 466 to 498, collected in the detailed sampling area FIGURE 7.51 .

+0.2

+0.1

• + • +

• • •

•

• • -b, 3 •

•

a 0.4 1.2 1.4 1.6 1.B 2. 0 2.2 2 .4 MEAN GRAIN SIZE ( PHI)

Fig. 7.51 Relationship between mean grainsize and skewness of all samples collected on Brancaster Harbour Bar. (Samples collected from the detailed sampling area are denoted by a cross). 405/6 at the S.W. end of the Bar. The general trend of more negative skewness with decreasing grainsize is similar, although less well defined to that of the sediments occurring in the Beach environment on Scolt Head Island and in the Tidal Inlet environment.

Table 7.10 Parameter values of samples collected at neap tide (25.3.1964) around the periphery of Brancaster Harbour Bar Table 7.10 Phi Mean Phi Standard Sample Grainsize Deviation Skewness Kurtosis (Mz) (SKI) (Kg) al191:0:11*.111,.. 232 1.63 0.34 -0.20 1.00 233 1.84 0.26 -0.18 0.88 234 1.64 0.26 -0.09 0.99 235 2.06 0.32 -0.23 0.93 236 2.12 0.25 -0.22 0.91 237 1.88 0.41 -0.33 0.91 238 2.15 0.29 -0.30 0.93 239 2.05 0.31 -0.30 0.97 240 2.05 0.31 -0.19 0.90 241 1.94 0.27 -0.16 0.90 242 2.17 0.32 -0.37 1.01 243 2.12 0.34 -0.24 0.94 244 2.22 0.29 -0.30 0.94 245 1.98 0.27 -0.11 0.86 266 2.22 0.28 -0.35 1.02 246 2.22 0.31 -0.32 407 Table 7.11 Parameter values of samples collected over the surface of Brancaster Harbour Bar in December 1964 Table 7.11 Phi Mean Phi Standard Sample Grainsize Deviation Skewness Kurtosis (Hz) (6-1 ) (SK1 ) (Kg) 352 1.46 0.34 -0.04 1.01 353 1.78 0.27 -0.04 1.12 354 1.32 0.31 -0.02 0.88 355 1.50 0.31 +0.06 0.85 356 1.78 0.30 +0.11 0.97 357 2.10 0.24 -0.08 0.94 358 2.10 0.25 -0.07 0.89 359 2.12 0.25 -0.13 0.95 360 1.98 0.27 -0.11 0.96 361 1.85 0.31 +0.01 0.90 362 1.4.4. 0.31 -0.04 0.97 363 1.98 0.33 -0.09 0.90 364 1.79 0.33 -0.02 0.89 365 2.04 0.26 -0.01 0.90 366 1.74 0.41 +0.05 0.88 367 2.06 0.30 -0.11 0.88 368 2.15 0.32 -0.18 0.97 369 2.08 0.27 -0.02 0.89 370 2.07 0.30 -0.13 0.88 371 2.28 0.24 -0.11 0.96 372 2.06 0.29 -0.06 1.13 373 2.09 0.30 -0.11 1.03 374 1.98 0.31 +0.01 0.88 375 1.79 0.38 -0.05 0.92 376 1.73 0.40 -0.05 1.04 377 1.91 0.34 -0.13 0.93 378 1.98 0.35 -0.16 1.03 379 2.03 0.36 -0.16 1.18 38o 1.98 0.37 -0.13 0.93 381 1.80 0.44 -0.12 0.95 382 1.71 0.44 -0.14 1.18 383 1.99 0.35 -0.12 0.94 384 2.03 0.31 -0.11 0.99 385 1.97 0.27 -0.04 0.93 386 1.83 0.26 +0.05 1.93 387 1.78 0.27 -0.03 1.01 499 1.94 0.38 -0.15 _ 500 1.87 0.30 +0.05 - 501 1.90 0.35 -0.09 - 1.95 0.32 -0.11 - 503 1.98 0.28 -0.08 _ 504 2.05 0.30 -0.14 - 2.0Q 0.28 -0.10 - 2.03 0.32 -0.19 - 550 1.86 0.27 -0.08 - 508 2.01 0.32 -0.18 - 509 2.12 0.28 -0.32 - 408 Table 7.12 Samples collected on the 7.12.64 over a small scale area at the S.W. end of Brancaster Harbour Bar Table 7.12 Phi Mean Phi Standard Sample Grainsizo Deviation Skewness (Mz) (61) (SK1 ) 466 1.88 0.33 0 467 1.90 0.30 -0.02 468 1.93 0.31 -0.11 469 1.92 0.32 -0.03 470 1.73 004 +0.01 471 1.71 0.29 +0.12 4.72 1.94 0.31 -0.11 473 2.00 0.28 -0.13 474 2.06 0.29 -0.17 475 1.92 0.29 -0.03 476 2.04 0.29 -0.11 477 1.75 0.33 +0.08 478 2.08 0.25 -0.12 479 2.14 0.24. -0.12 480 1.67 0.31 +0.02 4.81 1.61 0.34 +0.14 482 1.78 0.28 +0.05 483 2.08 0.37 -0.29 484 2.10 0.32 -027 485 1.99 0.42 -022 486 2.02 0.36 -0.18 4.87 2.01 0.39 -0.21 488 2.07 0.30 -0.13 489 2.07 0.31 -0.17 4.90 2.06 0.32 -0.24 491 2.00 0.34 -0.20 492 2.01 0.34 -0.16 493 2.06 0.31 -0.17 494. 2.05 0.31 -0.14 495 2.04 0..34 -0.16 496 2.05 0.31 -0.14 497 2.05 0.30 -0.13 498 2.03 0.31 -0.16 FfGURE 7.52.

)

• ••••,..

•

LWMOT Clad. 14WMOT 1891

1907 I/ ,it II 1953

Fig. 7.52. Showing changes in the high and low water lines between 1891 and 1964 at the eastern end of Scolt Head Island. Scale: 3 inches equals 1 mile. C 7.11 Burnham Harbour Bar Sub-area 7.11

Morphology and Sediment Movement

At the E. end of Scolt Head Island, an easterly prolongation of the back-beach has produced Burnham Harbour Bar. This differs markedly from its western counterpart: in its opposite sense of orientation, in its coarser grained composition, and in its generally higher elevation. Only a narrow strip of fine grained sand (corresponding to the fore-beach sub-area) exists along the seaward edge of the Bar (note the distribution of beach environments shown in Figs. 7.55A and B). Periodically, this Bar migrates to the E., displacing, in this direction, the mouth of Burnham Harbour. According to local reports, this migration is a cyclic process with an approximate period of 20 to 30 years (a more frequent cycle than that of the Brancaster Harbour Bar). After an interval of eastward growth, when the harbour mouth may be displaced as much as 1 to 12 miles, and the channel of Burnham Harbour has become elongated, the bar is breached close to the end of Norton Hills, and the process recommences. (Refer to the discussion on channel stability in section 7.9.1). Various stages in this cyclic process are shown by comparing the Ordnance Survey maps of 1887 and 1907, and the map produced by R.J. Small and P. Haggett in 1953 (see Fig. 7.52). The local absence,in the zone along the seaward edge of the Burnham Harbour Har, of the fine grained sand which usually characterises the fore-beach sub-environment, 7J m

Fig. 7.53. Off-shore topography and locally dominant current directions in front of Scolt Head Island. The paths of sand moving under the combined influence of waves and dominant currents are indicated by dotted lines. The fore-beach and back-Leach sub-environments are indicated by light and heavy stippling, respectively. 412 may be at least partly explained by the effect of the off- shore physiography of the Bridgirdle and Brancaster Road upon the local current configuration, and subsequently upon the near-shore movement of sediment. As previously described in section 3.7, and illustrated in Fig. 3.19, two zones of sediment movement exist in the intertidal zone and in the near-shore waters off the N, Norfolk coast. Sediment is transported to the E. by the dominant flood current in this nearshore zone (and at the same time migrates shorewards as a result of wave action), and is returned to the W. in the intertidal zone by the processes of littoral drift. The deposits of fine grained sediment, which occur in the fore-beach sub-environment, reflect the dynamic balance between these two conflicting processes. However, it is proposed that this system is locally interrupted by the physiographic features of the Bridgirdle and Brancaster Road to the N. of Scolt Head Island. In Brancaster Road, the near-shore flood current dominates over the near-shore ebb current which flows in the opposite direction (i.e. to the W.). Sediment is therefore transported from the W. into this locality. In the vicinity of the Bridgirdle, however, the dominant flood current is deflected to the E.N.E., and subsequently transports sediment away from the coast into the northern lobe of Wells Cut (see Fig. 7.53). In this case the W. flowing ebb current will be locally dominant in moving sediment in the near-shore zone opposite the eastern end of Scolt Head Island. In this area, there- 413 fore, a residual sediment transport towards the W. will be locally produced. This will supplement the effects of westward littoral drift along the beach face, and consequently cause the observed deficiency of fine grained sand along the seaward edge of the Burnham Harbour Bar. A number of other factors possibly contribute to this phenomenon of local undernourishment occurring at the E. end of Scolt Head Island. (a) Owing to the erosionally formed bottom topography of the S.E. side of the Burnham Bank Complex, and to the scarcity of fine grained sand in this scoured environment, the W. flowing ebb current is possibly undersaturated in terms of its potential sand transporting capacity. This current will, therefore, function essentially as an eroding agent. (b)As proposed in section 7.6, Holkham Beach, to the E. of Burnham Harbour Bar, is an area of progradation. The preferential accumulation of sediment at this point will help cause the undernourishment of the area immediately down drift, which corresponds to Burnham Harbour Bar. (c)Since the zone of residual sediment movement in the nearshore waters of the N. Norfolk coast is eastwards with an on-shore component due to wave action, the distance from the shore line of this main zone of sediment movement will affect the amount of sand actually reaching the shore. As this zone is deflected northwards (i.e. away from the shoreline) by the Bridgirdle, the distance sediment is obliged to move under wave action, before it is Summar-I HARbouR BAR

RUNNEL il Pig. 7.54- Beach at the eastern end of Scolt Head Island displaying local reversals in the rientation of both the fore-beach, and back-beach ridges and runnels. Photographs taken ky K.S.J., May 1960. Scale: 12 inches approximately equals 1 mile. 415 deposited in the intertidal zone, is subsequently greatly increased. As shown in Fig. 7.53, this sediment, as it migrates shorewards over the Bridgirdle, comes under the influence of the locally dominant W. flowing near-shore current. Some of this sediment will, therefore, be transported towards the W. into Brancaster Road where it will once again come under the influence of the dominant E. flowing current; and so will be recirculated again. Being composed predominantly of coarse grained sand and shingle, Burnham Harbour Bar is influenced by wave action mainly during high tide (section 3.4). An eastward migration of the coarse material is indicated by the orientation of local back-beach ridges and runnels, the latter of which drain towards the E. (Fig. 7.54.). Since the overall direction of beach drifting along the Y. Norfolk coast is to the west, this local anomaly implies that a divergence in the direction of sediment transport exists. This causes a depletion of the coarse sand and shingle comprising the back-beach in front of the Breakthrough. The primary cause of this anomaly has not been ascertained, but may possibly be related to the Bridgirdle. Erosion of the back-beach in turn renders the dunes in the area of the Breakthrough vulnerable to storm wave attack during high tides and explains their prolonged erosion and retreat. Since the sand supplying the main dunes is derived from the adjacent back-beach, this phenomenon also accounts for the small development of dunes in 416 this area. The orientation of the small westward curving fore- beach spit which deflects the low water outlet of Burnham Harbour (Fig. 7.55 B), indicates that the process of westward migration of fine grained sand in the fore-beach sub- environment is uninterrupted, although diminished in volume. Since the Bar is composed of coarse grained material, and as a result of its moderately high elevation, it appears that, unlike its western counterpart (Brancaster Harbour Bar), the phase relationship between inlet and tidal currents plays an insignificant role in sediment movement over the Bar. Wave action tends to transport sand and shingle across the top of the bar, as migrating ridges which, when they reach the inner edge, avalanche into Burnham Harbour channel. This accounts for its landward movement and the consequent erosion which is at present taking place atlhe base of Gun Hill. Since, however, there does not appear to be any appreciable accumulation of gravel in the inner part of Burnham Harbour, it is probable that strong ebb current action in the channel transports, at least some gravel seawards again, where it is incorporated once more in the Bar.

7.12 The Grainsize Characteristics of the Sediments 7.12 Comprising the Burnham Harbour Bar The Burnham Harbour Bar is composed of coarse grained sand and gravel. Gravel occurs on the crest of the ridge on the southern side of the Bar, and also along its southern edge adjacent to Burnham Harbour channel. A thin 117 123 A122 103 BURNHAM 121 + 107 PROF{ LE HARBOUR 120 BAR 2.

102

100 SCALE: 1 I NCH 650 FEET

(A) COMPASS SURVEY 16.3.64

454 -453 ---' • 4 411 452 434 401 .407 4ic .\40, 412 4324r, • 433 - 431 408 • 429 *435 400 409 423 BURNHAM • 0442 427 •441 iN HAROUR 444 399 426 • • • • • BAR • *438". • • • 398 PROFILE 425 450 3. •394 396. 413 424 393 •392 395- NORTO 390 391 414 HILLS 415 oo4 16 417 389 418 388 420 419 421 SCALE: 1 I NCH 65 0 FEET 422 • 423 (B) COM PASS SURVEY 5.12.64

Fig. 7.55 A and B. Showing the location of samples collected on the Burnham Harbour Bar. The broken line delineates the zone containing some fine grained sand on the seaward edge of the Bar. 418 zone of finer grained sediment commonly occurs along the seaward edge of the Bar, and forms a continuation of the Scolt Head Island fore-beach sub-area. Samples 99 to 127 and 388 to 434 were collected on two occasions, on the 16.3.64 and the 5.12.64. respectively, over the surface of the Bar, and in the case of samples 404. to 423 along the edges of the Burnham Harbour channel (see Tables 7.13 and 7.14, and Fig. 7.55A and B). In addition a small scale area was selected for detailed sampling samples 434 to 454. (see Table 7.15). The location of this detailed sampling area. is shown in Fig. 7.55B and the positions of the samples collected on it are shown in Fig. 7.61. The combined parameter values of sediments, collected along three approximately N.S. orientated profiles (profiles 1, 2 and 3), are illustrated graphically in Fig. 7.56. The positions of these traverses are indicated in Fig. 7.55 A. and B. A faint trend of increasing mean grainsize from N. to S., across the surface of the Bar, is indicated. Both standard deviation and skewness values display a wider range of variability on the southern side of the Bar adjacent to Burnham Harbour than on its seaward side. Frequency curves of the samples collected along profiles 1, 2 and 3 are shown in Figs. 7.68 and 7.69. Samples were collected at various positions along the Burnham Harbour Channel at L.W.O.S.T.L., and at adjacent positions on the crest of the Bar (see Fig. 7.57). Clear trends of decreasing mean grainsize, improved sorting and 2.2 FIGURE x 7.56 o 18 • • x • • • • z 14M 0 (PHI) X to 0 0.6

0.7

0.6

0 0.5 CTI X • o (PHI) x o • • 0.4 e • x 0 xo 0.3 •

+0.4 x - *0,3 • *0.2

• - +0.1 • • • • • • SKI 0 0 x x 0 -0.1 x -0.2 0 0 - 0.3 BURNHAM NORT H HARBOUR BAR

102 118 120 121 122 123 115 PROFILE 1.00 104 127 126 125 124 112 PROFILE 2.(°) 390 424 425 426 427 428 429 430PROFILE 3.(v)

Fig. 7.56 Showing the variation in the grai.nsize parameter values of samples collected along profiles 1, 2 and 3 across the Burnham Harbour Bar.

FIGURE 7.57. 1.7

1 6 0 Mz -0 (PHI) 1.5 0 1.4

0.7 0 0.6

0.5 0-t (PHI) 0,4 0-

0.3

0.2

+0.1 SKI 0

-0.1

-0,2

- 0.3

BURNHAM HAR BOUR CHANNEL NORTH SOUTH 391 BURN HAM 393 HARBOUR 395 BAR

- Fig. 7.57 showing the variation in the grainsize parameter values of samples collected at three localities on the top and bottom of the ridge along the southern edge of Burnham Harbour Bar. 421 more positive skewness occur from the top to the bottom of this slope. The trends shown by the grainsize parameter in this case are the reverse to that shown by samples collected at comparable positions along the southern edge of Brancaster Harbour Bar (compare with Figs. 7.38 and 7.39). The explanation for these conflicting phenomena may reside in the predominance of wave action involved in the transportation and deposition of the sediments forming the Burnham Harbour Bar, while current action is the dominant mechanism of sedimentation in the case of the Brancaster Harbour Bar. A series of samples, collected at intervals from the inner part of Burnham Harbour eastwards and northwards around the inner edge of the Bar to the mouth of the Harbour, have been grouped to form traverses 1 and 2; and samples collected along the southern and eastern edge of the Burnham Harbour channel comprise traverse 3 (see Fig. 7.58). Generally the grainsize parameter values of these samples show no consistant variation although sediments occurring near the mouth of the Bar appear in general to be somewhat more poorly sorted than those occurring in the central regions of the Harbour channel. The majority of these sediments are positively skewed, which is probably related to the strong current action existing in the channel of Burnham Harbour. Samples were collected at irregular intervals along the L.W.O.S.T.L., from the E. to the W. of the Harbour mouth (see Fig. 7.59). The parameter values of these sediments indicate a clearly defined trend of improved sorting, FIGURE 7.58 2.0 • • 18 x o O X* x x • •x A 1.6 8 x xo 0 A_ MZ 1.4 0 0 (PHI) 0 t2 0 0

1.0

• 0.7 • 0.6 • 0 0.5 X x • • 0 0.4 0 0 • 0 • X (PHI) 0 x ox 0 x • 0 x 0.3 x x • 0.2

• - +0.3 • +0.2 Zi• 0 0 o 0 x 0 o - +0.1 0 )( 0 0 x • x 0 x SKI • • o 0 x • -0.1 • 0 • • -0.2 • CHPNNEL MOUTH. 99 100 102 103 104 105 107 108 109 110 TRAV.1. 388 389 390 391 393 395 397 398 399 400 401402 4031R4V.2. 423 421 420 417 416 415 414 413 408 407 406 404 TRAV.3.

422 419 418 405

Fig. 7.58 showing the variation in the grainsize parameter values of samples collected along traverses l(*), 2(x) and 3(o) along Burnham Harbour channel.

FIGURE 7.59.

1.9 - 0

1.8

0 1.7 0 M Z 0 .10 1.6 0 (PHI) 0 1.5 8

1.4

a5 0 0 0 0 0 0 0,4 0 0 (PHI) 0 0 0 0 0.3

+0.2

+0.1 SKI 0

-0.1 HAR BOUR MOUT H 452 432 431 430433 405 434 404 406 411 412 WEST 403 402 EAST

Fig. 7.59 showing the variation in the grainsize parameter values of samples collected near the L.W.O.S. T. L. along the seaward edge of the Burnham I-tarbour Bar, (both to the E, and to the W. of the harbour mouth) 424 associated with increasingly more positive skewness values (except for samples 4.06 and 412), from W. to E. The mean grainsize generally increases from either direction reaching maximum coarseness values in the vicinity of samples 403 and 405 at the Harbour mouth. The meaningfulness of this abnormal relationship between improved sorting and more positive skewness, which has been previously noted in samples 391 to 396 collected on the southern edge of the Bar (see Fig. 7.57), is at present obscure. The parameter values of samples 110 to 117, which have also been collected along the seaward edge of the Bar to the west of the Harbour mouth, are shown in Fig. 7.60. The standard deviation value of these sediments also displays an improvement from W. to E., although their mean grainsize and skewness values are generally non-trending. The distribution of the values of the grainsize parameters of the samples collected in the detailed sampling area (see Figs. 7.61 to 7.64) appear to be almost completely randomly distributed in relation to the surface topography of the Bar, with the exception of the positive skewness values of samples collected along traverse 4, which is aligned in a zone extending south-eastwards across the Bar (Fig. 7.64). Positively skewed sediments also occur along profile 3, further to the W. of the detailed sampling area (see Fig. 7.56). This characteristic property appears to be completely unrelated: to the grainsize of the particular sample, to the topographic relief of the surface of the Bar, FIGURE 7.60.

0 0 0

0 0 1.6

0.6 0 0 0 05 0 0 0 0 0.4 (PHI) 0.3

0 0.2

+0.1

0 SKI

WEST 117 116 115 114 113 112 111 110 EAST HARBOUR MOUTH

Fig. 7.60 showing the variation in the grainsize parameter values of samples collected near the L.W.O.S.T.L.along the seaward edge of the Burnham Harbour, (froM the harbour mouth towards the W.) FIGURE 7. 61

LW CST L . • 434 • ' „ . •

435 •

400 • . • 436 • • • . •

437 i . 1 N. • I .1 • 441 . ; 444* 443 •. S. • 1%439\ * 440 •4 /44 8 . • • • . I 447 446 450 . • • I LE '• 451 • • PROFILE •0 .

394 (1) 396

• . • IMO L • 3 9 3. • BURNHAM CHANNEL

394 446 75 YDS 445 443 NORTH 437 43 5 436 434

Fig. 7.61. Diagramatic map of the detailed sampling area on the Burnham Harbour Bar showing the sampling positions and the general relief. (Low areas are stippled). FIGURE 7.62

MEAN GRRINS1ZE 1.71 (IN PHI UNITS)

1.61

1'36 • 144

1.52 •

155• *FM

• 1.42. (. 5g

Fig. 7.62 Distribution of the mean grainsize values of samples collected in the detailed sampling area. FIGURE 7.63

STRNDARD DEVIRT1ON 035 (IN PHI UNIT)

0.42 • 0.48 0-45

• 0.65 0.50

60.46 0.53 • 0.81 0.3q *0.49 0.48 •0•5Z 0.7c1 • 0.64 0.34 0.65 0.4-6 • 0.37 0.68

Fig. 7.63 Distribution of the standard deviation values of samples collected in the detailed sampling area. FIGURE 7-64

+0.04 SKEWNESS

• +0.02 0

• —0•12, +012

0-0.02

— 012 go — 0.37 +0.04 -0.04 • +0.05 •-0-17 —0.27 • +0.14 —0.07 +0.05

• —011

—01/ —0.26

+0.01 • +0.05

Fig. 7.64. Distribution of the skewness values of samples collected in the detailed sampling area. 430 or even (as has been previously noted in connection to sediments occurring along theedges of Burnham Harbour) to strong current action. The parameter values of samples collected on profiles 4 and 5 (see Fig. 7.61) are illustrated in Fig. 7.65 and their frequency curves are shown in Fig. 7.70. In general, the parameter values shown in Fig. 7.65 appear to be unrelated to the topography of the Bar.

7.12.1 General Discussion and Conclusions 7.12.1 It may be concluded that the sediments comprising the Burnham Harbour Bar are generally coarse grained, with an average mean grainsize in the order of 1.6 phi, and display a wide variability of sorting. The majority of these sediments are characterised by a positively skewed grainsize distribution. These grainsize properties, together with an obvious topographic continuity, suggest a moderately close affinity to the sediment accumulation forming the Scolt Head Island back-beach sub-area. However, some samples, collected near the seaward edge of the Bar in both March and December 1964, contain a distinct fine grained mode or fraction. The frequency curves of these samples are shown in Fig. 7.71. This fine grained fraction indicates the existence of low energy conditions similar to those of the fore-beach sub-area. On this basis, a narrow fore-beach sub-environment has been delineated along the seaward edge of the Bar (see Fig. 7.55 A and B). These fore-beach deposits are more extensively developed in March FIGURE 7-65. 1.7

1.6

0 1.5

0 0 1.4 M 0 0 0 1.3 (bhi) 0

0 0 0 it

0.8

0.7 0 0.6

0 0 0.5 Ci1 • 0 (phi) 0 0 0.4 0 0 0 0.3

+0.2

394 44-6 - 3q6 450 445 443 44 439 438 437 436 435 434. SOUTH 393 NORTH 39S

Fig. 7.65 Showing the variation in the grainsize parameter values of samples collected along profiles 4 (o) and 5(+) i the detailed sampling area. '12e.. Pg. -1.61 j. FIGURE 7.66.

• 0.80

0.76

0.72.

0.68

0.64 •

0.60

0.56

062 (phi) • 0.48 •

0.44 I •• • •• • • . • • 0.40 • • • • • • • • • • • • •• • 0.36 • • • • • • • • • • • 032 • • • •• 0.28 • • 024

0.20 • 1.0 1.1 12 13 1.4- 1.5 1.6 1.7 1.8 1.9 2.0 2.1 22 Mz IN UNItS.

Fig. 7.66 Relationship between the mean grainsize ) and the standard deviation 0) of ail samples collected on the Burnham Harbour Bar. 433 (Fig. 7.55A) than in December (Fig. 7.55B). To the E. of the Harbour mouth samples 406, 411 and 412 display a fine grained mode, which further to the E. occurs dominantly in the fine grained sediments comprising Holkham Beach. Since, over a fairly long term period, current action may be considered to be a uniform and consistent factor in sedimentation processes, the changes in the- volume of fine grained sediment that occurs on the seaward side of the Burnham Harbour Bar most probably reflect seasonal variations in wave conditions. The partial absence of fine grained sediment shown in Fig. 7.55B was observed to occur in December (mid winter). Winter storms, and the accompany- ing destructive wave action, were apparently instrumentive in moving sediment from the beach seawards into the near- shore zone to form the off-shore bar that was observed at this time. The relationships of both standard deviation and skewness to mean grainsize for all samples collected on the surface of the Bar have been plotted in Figs. 7.66 and 7.67 respectively. These plots are characterised by a completely random distribution which is contrary to the normal statistical trends of improved sorting and more negative skewness with decreasing grainsize, shown by sediments occurring in the Scolt Head Island ;beach and Inlet Channel sub- environments. The fact that the coarse grained sediments comprising the Burnham Bar subarea suffer constant remob- FIGURE 7.67.

+0:3

• 4-0.2

• • • • • • • • • +01

• • • • • • • • OD •• • • • • • • • 00 0 • • • • • 0 SKI • • • • • • • • . • • • • • • • • • • • • • • -0.1 • • •

•

• -0.2

• • • • -0.3

- 0.4 1.0 1.1 1.2 1.3 1.4 1.5 16 1.7 1.16 1.9 2.0 2.1 2.2 Mz (PHI)

Fig. 7.67.- Relationship between the mean grainsize (M ) and skewness (SK ) of all samples collected on z 1 the Burnham Harbour. 435 ilization, and are apparently recirculated, may in some way be responsible for the absence of a relationship between mean grainsize, standard deviation and skewness. The high degree of randomness in the mean grainsize, standard deviation and skewness plots of sediments occurring on the Brancaster Harbour Bar (see Fig. 7.50 and 7.51), which is also influenced by a sediment circulation (although the rate of reworking is slower than in the case of the Burnham Harbour Bar), may possibly also support this proposal. Implied in this tentative proposal is the sugges- tion that the relative stability of a sediment accumulation may be indicated by the relationship between mean grainsize, sorting and skewness. Sediments which show a well defined trend between decreasing mean grainsize, improved sorting and more negative skewness values are comparatively stable, in a dynamic sense, while a random distribution between these parameter values apparently indicates instab- ility of the sediment accumulation. 436 Table 7.13 Parameter values of samples collected on Burnham Harbour Bar on the 16.3.64. Table 7.13 Phi Mean Phi Standard Sample Grainsize Deviation Skewness Kurtosis (Mz) (,)1) (SK1) (Kg) 99 1.94 0.33 -0.03 0.90 100 1.92 0.44 -0.15 1.00 102 0.62 0.61 +0.36 0.85 103 1.64 0.22 -0.01 0.93 104 0.78 0.67 -0.26 0.66 105 0.54 0.56 +0.26 0.81 107 1.55 0.31 -0.19 0.97 108 1.60 0.42 -0.19 1.16 109 1.64 0.34 +0.05 0.98 110 1.73 0.40 -0.09 0.92 111 1.75 0.20 +0.01 0.89 112 1.81 0.42 -0.10 0.93 113 1.73 0.48 -0.07 0.88 114. 1.87 0.46 -0.14 0.86 115 1.92 0.45 -0.09 0.90 116 1.64 0.54 +0.02 1.01 117 1.67 0.54 -0.05 0.90 118 1.57 0.39 -0.07 0.99 120 1.09 0.67 -0.17 0.84 121 1.65 0.31 -0.06 1.04 122 2.04 0.32 -0.11 0.95 123 1.94 0.38 -0.12 0.95 124 1.84 0.34 +0.02 1.01 125 1.72 0.30 -0.04 0.94 126 1.64 0.39 -0.11 1.04 127 1.25 0.48 -0.27 1.12 437 Table 7.14 Parameter values of samples collected on Burnham Harbour Bar on the 5.12.64. Table 7.14 Phi Mean Phi-Standard Sample Grainsize Deviation Skewness (Mz) (01 ) (SK1) 388 1.76 0.40 +0.17 389 2.15 0.39 -0.04 390 1.58 0.26 +0.23 391 1,64 0.26 +0.08 392 1.53 0.38 393 1.64 0.31 -72,:g 394 1.59 0.41 -0.11 395 1.62 0.32 +0.01 396 1.47 0.68 -0.26 397 1.69 0.32 +0.04 398 1.56 0.29 +0.10 399 1.58 0.30 +0.06 400 1.44 0.45 0 401 1.52 0.45 -0.07 402 1.52 0.35 +0.07 403 1.44 0.34 +0.04 404 1.50 0.38 +0.09 405 1.45 0.44 +0.09 406 1.74 0.41 +0.02 407 1.56 0.42 -0.01 08 1.1 0.54 -0.13 09 1.6 010 -0.0 10 0. 2 -0.1 11 1:g 0. ; +8.1 112 1.79 0. 413 1.30 0.34 +0.11 414 1.66 0.30 +0.07 4.15 1.49 0.31 -0.02 416 1.66 0.34 417 1.15 0.29 418 1.68 0.28 7(1+0.05- 419 1.52 0.36 +0.09 420 1.16 0.29 +0.09 421 1.51 0.31 +0.06 422 1.44 0.38 0 423 1.28 0.39 +0.16 424.. 1.54 0.36 +0.06 425 1.51 0.36 +0.02 4.26 1.61 0.61 427 1.58 0.52 2:241 L28 1.72 0.39 +0.06 L29 1.62 0.11 +0.10 L30 .n 0. 5 +0.01 431 1. 0. 4 -0.06 432 1.89 0.39 -0.04 433 1.58 0.41 434. 1.71 0.35 +0.04 438 Table 7.15 Parameter values of samples collected on 5.12.64 from the detailed sampling area on Burnham Harbour Bar Table 7.15 Phi Mean Phi Standard Sample Grainsize Deviation Skewness (Mz) (C51 ) (SK1) 434 1.71 0.35 +0.04 435 1.61 0.42 0 4.36 1.36 0.48 +0.02 4.37 1.41 0.50 +0.12 4.38 1.52 0.48 +0.05 439 1.54 0.39 +0.04 44.0 1.31 0.52 -0.17 441 1.46 0.4.6 -0.02 442 1.39 0.65 -0.21 443 1.19 0.81 0.37 4.4.4 1.52 0.53 -0.12 445 1.28 0.79 -0.27 4.4.6 1.51 0.65 -0.29 447 1.59 0.64 -0.07 448 1.55 0.49 -0.04. 449 1.50 0.34 +0.12 4.50 1.39 0.4,6 +0.05 451 1.42 0.37 -0.11 452 1.88 0.46 -0.11 453 1.85 0.45 -0.06 454. 1.88 0.60 -0.37 FIGURE 7· 68.

115

123 '-r-----~ 122.

~ ~121

"----....;;~ 120 ----+------+------=::=::::..1 1/8

102.

"--_--..." 112

\------1 12.4

~-~112S

126

\27

PROFfLE Z.

Fi.g. 7.68 prequency curves o~ samples collected along profJ.les 1 and 2. FIGURE 7. 69.

430

429

428

427

426

425

424

PROFILE 3. 390 Oa 0.4 0.6 0-8 1.0 1.2. 1.4 1.6 1.8 Z O 2.2 24 2.6 GRIIINSIZ.E IN PHI UNITS

Flg. 7.65, Frequency curves of samples collected along profile 3 across the Burnham Harbour Bar. FIGURE 7.70.

435

437

436

438 439 449

450

396 PROFILE 4. 395

44-3

445

446

PROFILE 5 3c14- 393

04 0.6 0,8 FO 1.2 14 1.6 /.8 2 0 2 2:4 2.6 GRPINSI-ZE IN P141 UNITS.

Fig. 7.70 Frequency curves of samples collected along profiles 4 and 5 across the Burnham Harbour BaT: (within the detailed sampling area). FIGURE 7.71 ,

110 ►II 112 113

114 115

116 117

122

123

412,

11 406 --` 432 452_ 453

0.2 0.4 0.6 0.8 1,0 1.2 14 1.6 1.8 2 0 2.2 24 2.6 GRRINSIZE IN PHI UNITS.

1,. jg. 7.71 Frequency curves of samples possessing a significant fine-grained fraction (mode), which were collected on the seaward side of the Burnham Harbour 3

8. THE DUNE ENVIRONMENT ON SCOLT HEAD ISLAND 8

8.1 Morphology 8.1

The dunes on Scolt Head Island fall into two dis- t tinct catagories:- (a)Those dunes which occur in a continuous frontal, longitudinal, E.W. zone or ridge extending from Burnham Harbour in the E. to the Ternery in the W., and form the physiographic "back-bone" of the island. These dunes separate the Beach environment of Scolt Head Island from the Marsh sub-environment and include, from E. to W., Norton Hills, Low Hills, Smugglers Gap, Hut Hills and Bight Hills (as shown in sheet 8). (b) Those dunes which occur within the marsh sub-environment on the inner, or landward side of the longitudinal ridge (see sheet 8), have either developed on the recurved lateral ridges as in the case of House Hills, Butchers Beach and Long Hills, or comprise the inner part of a local broadening of the longitudinal ridge as in the case of Norton Hills. The highest and most extensive dune development occurs in Norton Hills and Hut Hills, where dunes reach heights of approximately 40 to 50 feet respectively. It is possible that Norton Hills were once higher but have, over the last 150 years, at least, been subject to constant erosion on their seaward, and highest side, which has resulted in their progressive.landward retreat. Hut Hills have also been eroded (as in fact has the whole intervening 444 dune ridge), and it is only the western half of Bight Hills (to the W. of sample 132, sheet 8) that displays the residual effects of accretion to their seaward edge (Steers, 1960). The dunes forming Low Hills, between Norton Hills and House Hills, are low, narrow and generally under developed. A breach, the Breakthrough, was made in these low dunes during the 1953 storm surge, at which time gravel from the back-beach sub-environment was spread fan-wise over the marsh surface. The dunes, which subsequently reformed and closed the breach, suffer periodic erosion along their seaward edge and are, as a result, very narrow (only 10 to 15 feet wide in places). They thus afford little real protection against future storm surges. House Hills, which belong to the lateral dune category, comprise an irregular mass of dunes. They reach greatest development (with heights in the order of 25 to 30 feet) around the periphery, and enclose a shallow basin in the interior. The dunes forming Smugglers Gap are also poorly developed, similarly in this respect to Low Hills, although field observations suggest that these latter dunes are subject to greater erosion than are those of Smugglers Gap. Bight Hills, which rarely exceed 25 or 30 feet in height, are widest adjacent to Hut Hills and at the Ternery, but are rather narrow at the head of Cockel Bight. A diagrammatic section of the longitudinal dune ridge, as it appears viewed from the south, is shown in Fig. 8.1. Dunes which have formed on lateral ridges vary considerably in height and in Long Hills and Butchers Beach 445 reach maximum heights of approximately 30 to 35 feet. In isolated parts of Long Hills, wind erosion has completely removed the dunes leaving the underlying gravel exposed. Steers (1960) has described the dunes on Scolt Head Island and mentioned the effects of wind erosion in producing blow-outs. He also briefly discussed the role played by vegetation in dune development, a subject which is further discussed by V.J. Chapman in the chapter on "The Plant Ecology of Scolt Head Island" in Steers (1960). In accord with the proposed evolution of Scolt Head Island (Steers 1960, and section 6.2), the age of the dunes increases progressively towards the E. (with the exception, according to Steers (1960), of some minor dune development at the E. end of Norton Hills. Steers (1960) has suggested that the extensive development of Long Hills (see sheet 8) may be attributed to an hiatus in the westward growth of the island. During this hiatus, sand, blown off sand flats to the W., accumulated on Long Hills. This proposal may also explain the extensive development of Hut Hills which have been formed mainly by the accumulation of sand blown off the beach. Such an explanation would seem, however, somewhat unsatisfactory, and incomplete. Field observations indicate that the back-beach in front of Hut Hills is slightly higher than elsewhere. This factor may in turn indicate a locally high rate of accretion, possibly associated with the development of the transverse ridge described in section 7.3,3 which occurs at this locality. It is believed that the 144A

Fig. 8.1 Graphic presentation of the mean grainsize, standard deviation, and skewness parameter values of samples collected on the crest of the Longitudinal Dune ridge. II FO r-C BIGHT HUT HILLS THE NORTON HILLS TERNERY HILLS SMUGGLER71 LOW GAP HILLS WEST ---"\EAST wwwwwwwwwro F3 (A) Ul 01 (.11 Ul. Ul Ul a) 07 CD 6) 6) -1 -1 -A -4 - co OoNw.pwmoN).pmo M 0) 0) 0) 0) 9,), ••1 CD co cr) Ul A um) U1 1.9 o w m co 0 0 1.8 0 0 1.7 0 0 0 0 0 Mz 1.6 0 0 0 0 0 0 (PHI) 0 1.5 0

0 1.4 G 0 0 0 1.3 0

0.4 0 0 0 0 o -n 6. O o 0 ° 0 0 0 0 0 oo 0 o 0 0 0 0 0 0 0 0 0 0 (PHI) 0.3 0 0 • 0 • 00 dis w• • • • • • • • • • • • • • • -0.1 • SKI • • • •• • • - 0.2 448 size and extent of these dunes is in some way related to existing dynamic conditions, rather than to a past event in the evolution of the Island.

8.2 Grainsize Characteristics of the Sediments of the 8.2 Dune Environment.

The parameter values of samples collected along the crest of the Longitudinal Dune ridge are shown in Table 8.5 and are illustrated graphically in Fig. 8.1. By comparing these values with the general dune relief, which is also diagrammatically shown in this figure, it is apparent that a. relationship exists between increasing dune height and decreasing mean grainsize. This trend, however, is reversed at the W. end of the island in the vicinity of the Ternery. A very tenuous correlation also exists between increasing dune height and more negative skewness values. The distribution of the standard deviation, which varies between 0.3 and 0.4 phi however, displays no relation to dune topography. The parameter values of samples collected from House Hills, Long Hills and the inner part of Norton Hills are shown in Table 8.6. These samples represent the Lateral Dune Category. The parameter values of the samples shown in Table 8.1, which were collected on two traverses across Bight Hills and the embryo dune on Holkham Beach respectively, have been illustrated in relation to the dune topography in Fig. 8.2. Except for awry faint trend of increasingly more positive skewness towards the dune crest, these values FIGURE 8.2

1.9

MZ 1.8

(PHI)17

1.6 0 0

0 0 0 1.5

1.4 0

035 o 0 0 o 0 0 0 61 030

0.25 ( PHI ) 020

+0.2 - 0 0 +0.1 0 0 0 SK -0.1

- 0.2 SOUTH NORTH 19 G18(0) 584

Fig. 8.2 Showing the variation in the grainsize parameter values of samples 584-390 collected across Bight Hills on Scolt Head islar d, and of samples 618-626 collected across an embryo dune on Holkham Beach. 450 display a random distribution. The slightly more positive skewness values near the dune crest may reflect the preferential removal of fine material by wind erosion which will be more active on the higher, than on the lower parts of the dune face. The frequency curves of these samples are shown in Figs. 8.5 and 8.6 respectively.

Table 8.1 Parameter values of samples 584 to 590 and 618 to 626, collected transversely across an ambryo dune on Holkham Beach and across Bight Hills respectively Table 8.1 Mean Standard Sample Grainsize Deviation Skewness (Mz) ) (SK) 584 1.91 0.31 -0.21 585 2.02 0.24 -0.15 586 1.92 0.28 -0.13 587 2.00 0.25 -0.15 588 1.97 0.26 -0.12 589 2.05 0.24 -0.14 590 1.97 0.28 -0.16 618 1.50 0.30 +0.08 619 1.52 0.33 +0.11 620 1.37 0.31 +0.07 621 1.48 0.32 +0.14 622 1.50 0.35 +0.10 623 1.60 0.35 +0.15 624 1.60 0.34 +0.10 625 1.53 0.34. +0.14 626 1.53 0.36 *0.03

Samples 638 to 645 and samples 648 to 651 were collected at various levels in the near vertical faces of blow-outs in Hut Hills (longitudinal dune category) and Long Hills (lateral dune category) respectively. Samples 638 and 651 were collected near the base of these respective dunes. The parameter values and the frequency curves of these sediments are displayed in Table 8.2, and Fig. 8.7. FIGURE 8.3 .

(PHI)

0.45 0 0 0

0.40 - 0 0 0 0 0 0 0 0 00 00 0 0 0 0 0 A 0,35 0 0 00 0 0 0 0 0 0 0 0 0 0 0 00 (;) 00 0 0 0 0 0.30 0

1,3 1.4 1,5 1,6 1.7 1,8 1.9 MZ (PHI)

0 0 0 +0.15 O 0

o +0 .10 o 0 0 0 +0.05 O 0 0 0 0 0 0 0 0 B 8 00 0 0 0 0 -0,05 - 0 O 0 0 -0.10' 0 0 0 O 8 - 0.15 - 0 • 0

- 0.20' 0 SKI -

Fig. 9.3 Relationship of standard deviation (dr) and skewness (SR ) to the mean grainsize (Mz) of samples collected in theLongitudinal Dune category.

452 With the exception of the standard deviation values of samples 638 to 645 which statistically improve from the crest towards the base of the dune, these parameter values are generally non-trending.

Table 8.2 Parameter values of samples 638 to 645 and 648 to 651 collected at various heights in two near vertical blow-outs in Hut Hills and Long Hills respectively Table 8.2 Mean Standard Sample Grainsize Deviation Skewness (Mz) (61 ) (SKI) (BASE ) 638 1.81 0.31 -0.03 639 1.48 0.30 +0.03 640 1.60 0.34 +0.18 641 1.74 0.32 +0.17 642 1.89 0.36 -0.02 643 1.57 0.39 +0.08 644 1.74. 0.42 -0.15 645 1.74 0.37 +0.03 ( RASE) 651 1.86 0.37 -0.05 650 1.81 0.38 -0.06 649 1.51 0.43 +0.06 648 1.96 0.32 -0.01

The relationship between standard deviation, skewness and mean grainsize is shown in Fig. 8.3A and B in the case of samples collected on the longitudinal dune ridge, and in Fig. 8.4A and B in the case of those samples collected from the lateral dunes (together with sediments gathered across the small embryo dune on Holkham Beach). These plots display, in general, a random distribution with the exception of Fig. 8.4A, which indicates a slight trend of improved sorting with decreasing grainsize to exist in the samples of the lateral dunes ridges. It is apparent from the above mentioned tables, FIGURE 8•4.

61 (PHI) 0.50 0 0.45 0 8 o • 0 o A 0.40 • oo O o 0 o o 0.35 600 0 0 0 0.30 00

1.5 1.6 1.7 1.8 1.9 2.0 2.1 MZ(PHD

+0.05

0 0 - 0.05 0

B -0.10 0 0 0 0 0 0 O ... 0 o oO uo o 0 -0.15 00 0 0 o 0 -0.20 0 SK.

Fig. 8.4 Relationship of standard deviation (0") and skewness (Sk ) to the mean grainsize (Mz 1 ) of samples collected in the Lateral Dune category; (including samples from the embryo dune on Holkham Beach). 454 and especially from the frequency curves, shown in Figs. 8.8 and 8.9, that the sediments forming the longitudinal dune ridge are coarser grained (mean of the mean grainsize values equal 1.60 phi), than those occurring on the lateral ridges (mean of the mean grainsize equals 1.9 phi). The frequency curves shown in Fig. 8.6 indicate that the sediments comprising the dune on Holkham Beach possess a finer grained mode and are better sorted than the sediments forming either the longitudinal or the lateral dune ridges. In general, however, the standard deviation values of the sediments from these categories are rather similar (except for the Holkham Beach dune sediments) averaging 0.34 and 0.35 in the case of the longitudinal and lateral dunes respectively. The skewness values, however, vary considerably for the sediments from the longitudinal dunes, while the sediments from the lateral dunes (including the Holkham Beach dune sediments) are usually strongly negatively skewed. These characteristics of the typical skewness values may be explained in terms of the bimodality of the sediments occurring in these categories. In the longitudinal dunes those sediments which display bimodality,contain, in general, greater quantities of the coarser mode than do the bimodal samples from the lateral dune category. This factor tends to produce a more symmetrical grainsize distribution (i.e. a more positive skewness) in the longitudinal dune sediments. A large number of the sediments from both dune categories, excluding those from the Holkham Beach dune, are characterised by a bimodal grainsize distribution. 455 The modal sizes of these samples, estimated from the frequency curves shown in Figs. 8.5, 8.7, 8.8 and 8.9, are shown in Table 8.7, in the case of the longitudinal dune samples, and in Table 8.8, in the case of the lateral dune samples. The similar modal sizes that occur in a_ significant number of samples may be grouped into four classes in Table 8.7 (longitudinal dune samples) and into three classes in Table 8.8 (lateral dune samples). The mean size of the modes occurring in each class are also shown in these tables, together with the number of modes (expressed as a percentage of the total number of samples in each dune category) that occur in each class. It is obvious from these tables that, in both categories, two modal sizes predominate. In the longitudinal dune category (Table 8.7), the dominant modal sizes are 1.33 phi and 1.7 phi. The latter, fine grained mode occurs twice as commonly as the former, coarser grained mode. In the lateral dune category (Table 8.8) the dominant modal sizes correspond to 1.69 phi and 2.12 phi. Again the latter, finer grained mode occurs almost twice as commonly as the former, coarser grained mode. It is significant that the mode composed of grains in the order of 1.7 phi size, is common to both categories. It is perhaps also significant that the finer grained modes in each case are twice as abundant as the coarser grained modes. In addition to these dominant mean modal sizes, the longitudinal dune sediments display minor modes of 1.5 and 1.08 phi. This latter fine grained mode coincides fairly closely to the dominant mode occurring 456 in the lateral dune samples. In the lateral dune sediments, a minor mode of 1.4 phi occurs, but does not have a close counterpart in the longitudinal dune sediments. By comparing the modal sizes of individual samples with the mean modal sizes characterising the sediments of each dune category, it is apparent that many samples deviate from the norm. In general, the samples collected near the crest of the longitudinal dune ridge (Fig. 8.8A and B) display fairly consistant bimodal distributions which, in general, conform closely to the mean modal sizes shown in Table 8.7. Exceptions are seen to occur in Figs. 8.5 and 8.7, which show the frequency distribution of samples collected transversely across Bight Hills, and in the near vertical sections up Hut Hills. These sediments appear, in general, to be characteristically unimodal. Samples 52, 53 (Fig. 8.8A) and 645 (Fig. 8.7), which occur near the highest parts of Hut Hills (longitudinal dune ridge), display bimodal peaks of comparable size to those typifying sediments of the lateral dune ridges (i.e. with sizes in the 1.7 and 2.1 phi). This may be attributed to the com- parative remoteness of the peaks of the high dunes from the beach sands, from which they have apparently been derived. This factor will naturally limit the proportion of coarse (heavy) grains transported by the wind to these heights. Similar reasoning may, in part, be applicable in considering the origin of the finer grained sediments of the lateral dune ridges, although in such cases as Long

Hills and Butchers Beach, which are up to 2 mile from the FIGURE 8.5.

B1G147 HILLS

618

619

62.0

621

6ZZ

625 62.4

625

626 0:8 10 1.2 14 116 1.8 2.0 22 214 GRAIN SIZE IN PHI ONITS

Fig. 8..5 Frequency curves of samples collected across Bight Hills, (which form part of the longitudinal dune ridge). FIGURE 8.6.

HOLKHAM BEACH DUNE

NORTH

584

585

586

587

588

589

1 • 1 0 / " 590 / / • \ 1 44 /I 1 • ' e / 1 N SOOT I-1 e ' 1 4. ‘ 1 ..,... / , % /.. 4. / 1 / 1 .• LS :tin 573

_ 4.....• .. ?.44. 574 a 1 ' . • 1 1.0 1.2. 1.4 /.6 /.8 ZO 2A- 2:6 GRAIN SIZE IN PHI UNITS

Fig. 8.6 Frequency curves of samples collected across an embryo dune on tiOlkham Beach. (Sample 573 and 374 were collected from the adjacent beac surface) FIGURE 8.7.

0.8 1.0 1.6 . 1.8 20 2.2. 2.4 2.6 GRAN 5174 114 PHI. UNITS Fig. 8.7 Frequency curves of samples collected up the - vertical .face of blow-outs in Long Hills, (Lateral dune category), and Hut Hills, (Longitudinal category). I FIGURE 8 8 A .

128

12.9

130

131

132

133

134

1'55

136

l 38

13q

0.4 0.6 08 10 1.2, 1.4 1.6 1.8 20 2:2 24 2.6 GRPIN SIZE IN PI-11 UNITS FIGURE 8.8 B..

(LATERAL DUNE CATEGORY)

q5 LON GITUDINFIL DUNE RIDGE (cowl-Nun)) 173

172-

171

170

169

16S

167

166

165

163

66 I 04 06 0.8 I.0 I z 1.4- 1-6 1.8 2 0 2-2. 24 2,4

Fig 8.8 A and B. Frequency curves of Samples collected near the crest of the longitudinal dune ridge. Samples 95 to 98, however, were collected from the inner part of Norton Hills and belong to t..i e Lateral dune category.

FIGURE 8.9

HOUSE HILLS

ISO

179

178

177

176

175

174

LONG HILLS

147

14-6

145

144

143

14-2_

141

140 0-8 1.0 1-2. 1.4 16 1.8 20 2.2. 2.4- 2.6 a GRAIN SIZE IN PHI UNITS.

Fig. 8(.) Frequency curves of samples collected from House Hills and Long Hills which belong to the Lateral Dune category. 463 beach, a more local source than the beach sands of Scolt Head Island must be invoked (see below). Some doubt is cast on the general theory mentioned above by samples 638 (Fig. 8.7), and 649 (Fig. 8.7). Sample 638 occurs at the base of Hut Hills, and displays similar fine grained modes to those of samples 52 and 645 collected from the crest of these hills. Sample, 649 is located about 8 feet beneath the crest of Long Hills, and displays an abnormally coarse grained bimodal distribution. The modes occurring in this latter sample &re more typical of those characterising the longitudinal dunes (see Table 8.7). Sample 96 (Fig. 8.8B), collected from an interior dune in Norton Hills, displays a very poorly sorted distribution, and a flat topped mode which ranges in size from 1.5 phi to 2.35 phi, thus encompassing the modal sizes typifying sediments from both dune categories.

8.3 Origin of the Dune Sands 8.3

The significance of the bimodality and the high degree of consistency of the modal sizes, occurring in a large number of the dune sediments, may be understood in terms of the potential source material from which they are derived. On Scolt Head Island, two possible source areas will be considered: the beach along the northern side of the Island (including Burnham Harbour Bar), and the broad sand flats of Brancaster Harbour Bar which occur at the western end of the Island. It seems highly probable that sand flats, similar to the present feature, occupied 464 comparable positions at almost all stages of the island's evolution. The various modal sizes occurring in the samples collected along the beach traverses C, E and G (section 7.8.3) are shown in Table 8.9. A distinction has been made, in compiling this table, between the fore-beach and back- beach sub-environments. The modes occurring in the sediments collected on the Brancaster Harbour Bar are shown in Table 8.10. The modal values of these sediments have been estimated from frequency curves drawn of all samples collected over these localities. Unfortunately, a limitation may be introduced into this analysis by the randomness of the sampling positions which will produce a non-statistical representation of the sediments typifying both these localities and, in particular, the Beach Environment. As shown in these tables, the various modal sizes naturally fall into one of three size ranges or classes. The mean grainsize of the modes that occur in each class, together with the number of modes in each class (expressed as a percentage of the number of samples in each population), have been computed, and are shown as a total in Tables 8.9 and 8.10. The sediments of the fore-beach sub-environment (Table 8.9) are almost entirely unimodal with a mean modal size of 2.25 phi. Those of the back-beach sub-environment, however, are commonly bimodal and dsplay dominant mean modal sizes of 1.27 phi and 1.7 phi. Of these, the coarser grained mode occurs slightly more commonly than does the 465 finer grained mode (70% compared with 55%, see table 8.9). Sediments occurring over the Brancaster Harbour Bar consist of two dominant modes (1.75 phi and 2.17 phi), of which the finer grained mode predominates (88% compared with 22%). The previously mentioned similarity of the fore-beach sub- environment, and the Brancaster Harbour Bar sub-area, is emphasised by the similarity of their dominant modal sizes (i.e. 2.25 phi and 2.17 phi respectively). The finer grained of the two dominant modes occurring in the back-beach sub- environment (1.7 phi) is also present, although less abundantly, in the Brancaster Harbour Bar sub-area. The mean modal sizes, and their relative abundance, of the Dune and Beach environments are compared in Table 8.3 below.

Table 8.3

Sedimentary Category Mean Modal Sizes

longitudinal dunes 1.33 1.5 1.7 2.08 category (39%) (1 6%) (78%) (10%) lateral dune category 1.4 1.69 2.12 (21%) (58%) (100%) fore-beach sub- 2.25 environment (100%) back beach sub- 1.27 1.7 2.05 environment (70%) (55%) (1 0%) Brancaster Harbour Bar 1.75 2.17 sub-area (22%) (88%)

Upon the above evidence of modal similarity, it is proposed that the sediments of the longitudinal dune category are genetically related to the back-beach sub- environment. The lateral dune sediments on the other hand 46A are genetically related to sand flats, similar to those of the Brancaster Harbour Bar, which apparently existed off the W. end of Scolt Head Island at earlier stages of the islands evolution. In the case of Long Hills these sand flats have since been covered by the finer grained silts and muds which have accumulated in Cockle Bight. It is noteworthy that the relative proportions of the dominant coarse, to the dominant fine mode in the back-beach sediments (70%: 55%) are reversed in the longitudinal dune sediments (39%:78%). This suggests that the sand forming the fine mode in the beach is preferentially mobilised by wind action. In the lateral dunes, however, the coarse dominant modal size (1.69 phi) occurs in relatively greater proportions (58%) than does the comparable mode (1.75 phi) in the Brancaster Harbour Bar sediments (22c), A coarse grained minor mode of 1.4 phi also occasionally occurs in the lateral dune sands. The presence of this mode together with the large proportions of the 1.69 phi mode may be attributed to the presence, in the lateral dunes sands, of some sediment derived from the back-beach sub-environment. The close relationship between the dune sediments of the embryo dune on Holkham Beach, and the beach sands from which they have been derived, is shown in Fig. 8.6 by comparing the frequency curves of these dune samples with those of samples 573 and 575 which were collected from the nearby beach surface. The general significance, however, of the close relationship between the modal sizes of the source material and the modal sizes of the derived dune sediments, lies in 467 the implied, relatively non-discriminative nature of wind action in erosional and depositional processes. These results support the conclusions of Shephcard and Young (1961) who refuted the statements of Folk and Mason (1958) and Friedman (1961) regarding the distinction of beach and dune sediments on the basis of their grain size characteristics.

8.4 Wind Blown Sand Measurements 8.4

6. series of experiments were carried out to measure directly the quantity and grainsize characteristics of the sand carried by winds of different velocities blowing over the fore-beach, and back-beach sub-environments. The equipment used in these experiments is illustrated in Fig. 8.10 and consists of a horizontally mounted "tear-drop" shaped container, open at both ends (the inlet being of smaller diameter than the outlet) in which baffles have been fixed. The operation of this apparatus is based upon the assumption that the pressure of the air moving through the inlet opening will decrease in the expanded interior of the container. This pressure difference will be manifested by a decrease in wind velocity inside the container, which will consequently cause a decrease in the sand transporting capacity of the wind. Sand will, therefore, be deposited inside the container. The baffles have been incorporated to further increase deposition. . The outlet has been constructed larger than the inlet in order FIGUR 8-10.

WIND STREA

INLET OUTLET

COLLECTION SCALE :10C MS = 3CMS BOTTLE

WIND- BLOWN ANEMOMETER SAND TRAP

HEIGHTOF TRAP ABOVE BEACH SURFACE

Fig: 8.10 Showing a vertical crossection through the wind bloWn sand-trap, and illustrating the apparatus as positioned on the` beach surface. 469 to reduce possible build up of back-pressure inside the container. As shown in Fig. 8.10, the sand trapped in the bottom of the container is funneled into a. small collection bottle. Contemporaneously with the collection of the blown sand, measurements were made of the wind velocity using a vane type wind recorder (see Fig. 8.10). An effort was made to measure the wind velocity at the same height as that at which the sand was trapped. The objective of this experiment was to collect samples of sand at various heights above the beach surface over an interval of time during which the velocity of the wind remains essentially constant. It was hoped to obtain a series of such measurements at various points over the beach surface during different wind conditions. Unfortunate- ly, owing to the limited number of measurements, the small quantity of sediment collected, the variability of the wind velocity, and variations in the dampness of the beach sediment, the results of these experiments are extremely inconclusive. The quantity of sand trapped, and the velocity of the winds, are shown in Table 8.4. Grainsize analysis was carried out on the few samples collected in sufficient quantities, however, the analysis of such small quantities of sand is rather inacurate. The frequency curves of samples A, B and C, which were trapped on the back-beach in front of Bight Hills, are shown in Fig. 8.11. The modal sizes of these sands remain fairly consistent as the height at which the sample was collected above the beach FIGURE 8.11.

+3'

+1"

794 !•2 1.4 1.6 1.8 2 0 ZZ 24 26 2:8 GRAIN SIZE IN PHI UNITS

Fig. 8.11 Frequency curves of wind blown sand samples A, B and C collected at 1", 3" and 6" above the beach surface. Sample 794,was collected from the beach surface a short distance up-wind. 4-71 surface, increases. A sample, a few millimeters thick, was collected at this time from the beach surface some distance down-wind of the sand trap apparatus (sample 794). The modal size of this beach sand is, however, distinctly coarser grained than the mode of the wind blown material. Samples H and I were collected at scattered positions on the fore-beach, and samples J to M were collected at scattered positions on the back-beach. These samples, and to a less degree samples E, F and G, were collected at a time when the beach sand was still somewhat damp after a prolonged period of rain. This factor undoubtedly contributes towards producing the comparatively small quantities of sediment trapped at these times. Alternatively, the large amounts of wind-blown sand collected in samples A and B is most probably due to the gusty nature of the winds blowing at this time. The velocities shown in Table 8.4 only represent an average wind speed over an interval of time, field observations, however, suggest that the maximum velocity reached momentarily during gusts is a more significant factor in the mobilization of the beach sand. 472 Table 8.4 Showing the quantity of wind blown sand collected at various positions on the beach of Scolt Head Island at various wind velocities Table 8.4 Wind Wind Quantity Height in Locality in Blown velocity collected inches of which sample Sand (in feet (in one hour samples was collected Sample per second) in grams) above beach surface A 22.2 906.0 1 back-beach

B 23.3 116.3 3 • • • •

0 24.2 4.95 6 • • • *

D 25.2 0.52 9 • • • •

E 22.0 5.78 6 • • • •

F 18.9 5.30 12 • • • •

G 21.7 0.5 15 • • • • H 22.5 0.86 3 fore-beach I 23.5 2.47 3 .. .. J 18.0 1,34 1 back-beach K 6.0 0.04 5 .. L 20.0 0.06 5 .. Y 10.5 0.4.1 5 •• .•

It is believed that a more extended series of measurements, along these lines, may contribute significant information upon the grainsize relationships between the dune sands and the beach material from which they are derived. 473 Table 8.5 Parameter values of dune samples collected along the longitudinal dune ridge extending from the Ternery in the W. to Burnham Harbour in the E. Table 8.5 Mean Standard Sample Grainsize Deviation Skewness Kurtosis (Mz) (0-1) (Kg) 139 1.82 0.31 -0.11 0.96 138 1.66 0.34 -0.01 0.94 136 1.70 0.35 -0.02 0.95 135 1.64 0.33 -0.08 0.97 134 1.60 0.33 -0.05 1.00 133 1.73 0.30 -0.04 0.97 132 1.60 0.30 -0.04 0.96 131 1.53 0.30 -0.07 0.96 130 1.65 0.36 -0.12 0.91 129 1.57 0.34 -0.09 0.94 128 1.60 0.31 -0.11 0.86 30 1.88 0435 -0.15 0.90 50 1.82 0.33 -0.14 0.89 52 1.87 0.36 -0.07 0.94 53 1.68 0.38 -0.03 0.86 54 1.62 0.34 -0.03 0.92 56 1.40 0.37 -0.14 0.99 58 1.41 0.35 -0.03 1.03 60 1.41 0.31 -0.03 1.10 62 1.30 0.40 -0.01 0.95 64 1.49 0.37 -0.01 0.95 66 1.44 0.38 -0.05 0.92 69 1.52 0.31 -0.02 0.97 163 1.36 0.43 -0.06 0.94 165 1.58 0.31 +0.02 1.01 166 1.32 0.44 -0.22 1.01 167 1.70 0.30 -0.07 0.98 168 1.74 0.37 -0.14 0.89 169 1.59 0.33 -0.03 0.98 170 1.55 0.31 -0.13 1.01 171 1.74 0.29 -0.07 0.94 172 1,65 0.33 -0.02 0.97 173 1.51 0.33 -0.06 0.98 474 Table 8.6 Parameter values of dune samples collected from the dune systems of House Hills, Long Hills and the interior part of Norton Hills, which belong to the Lateral Dune Category Table 8.6 Mean Standard Sample Grainsize Deviation Skewness Kurtosis (Mz) (61) (SK1) (Kg) 174 1.86 0.37 -0.13 0.89 175 1.95 0.41 -0.14 - 176 2.00 0.32 -0.18 0.94 177 2.09 0.33 -0.15 0.91 178 2.06 0.33 -0.15 0.92 179 2.03 0.35 -0.16 0.93 180 1.91 0.39 -0.16 0.91 140 1.92 0.32 -0.13 0.86 141 1.97 0.33 -0.20 0.94 142 1.88 0.34 -0.15 0.95 143 2.00 0.32 -0.15 0.91 144 1.89 0.36 -0.16 0.90 145 1.81 0.38 -0.14 0.94 146 1.94 0.34. -0.12 0.93 147 1.99 0.28 -0.16 0.92 95 1.96 0.34 -0.04 0.89 96 1.66 o.45 -0.004 0.86 97 1.94 0.35 -0.15 0.86 98 1.94 0.37 -0.11 0.86 475 Table 8.7 Approximate modal sizes of sediments collected from the longitudinal dune ridge on Scolt Head Island Table 8.7 Sample Modal sizes in phi Sample Modal sizes in phi units units 139 1.3 1.8 168 1.4. 1.9 138 1.35 1.7 169 1.3 1.7 136 1.35 1 .7 170 1.7 135 1.35 1 .7 171 1.8 134 1.35 1.7 172 1.3 1.75 133 1 .8 173 1.3 1.7 132 1.35 1 .7 618 1.65 131 1.3 1 .7 609 1.7 130 1.3 1.8 620 1.6 129 1.35 1 .7 621 1.65 128 1.7 622 1.6 30 2.1 623 1.75 50 1.6 2.0 624 1.75 52 .75 2.1 625 1.7 53 1.3 1.75 626 1.75 54 1.35 1.7 638 1.7 2.1 56 1.45 639 1.4 58 1.5 640 1.5 60 1.5 641 1.7 62 1.0 1.5 642 1.75 64 1.3 1.7 643 1.5 66 1.0 1.6 644 1.8 67 1.3 1.65 645 1.7 2.1 69 1.3 1.7 163 1.5 Percent- 165 1.3 1.7 age no. 166 1.5 of samplffi 39% 16% 78% 10% 167 1.8 containing the modes Mean modal 1.33 1.5 1.7 2.08 size 476 Table 8.8 Approximate modal sizes of sediments collected from the lateral dune ridge, excluding samples collected on the Holkham Beach Dune Table 8.8

Sample Modal sizes in phi units

174 1.5 2.1 175 1.7 2.1 176 1.7 2.1 177 2.15 178 1.7 2.15 179 1.7 2.2 180 1.6 2.1 140 1.6 2.1 141 2.1 142 1.5 2.1 143 1.7 2.1 144 2.1 145 1.4 2.0 146 2.1 147 1.7 2.1 95 1.7 2.2 96 1.3 2.1 97 1.7 2.15 98 1.7 2.2 Percentage number of 58% 100% samples con- 21% taining mode

Mean 2.12 modal size 1.4 1.69 477 Table 8.9 Approximate modal sizes of sediments collected on the beach of Scolt Head Island along traverses 0, E and G (see section 7.8.3 Table 8.9

Fore-Beach Sub-Environment Back-Beach Sub-Environment Modal sizes in phi Modal sizes in phi Sample units Sample units 11 2,25 2 1.5 12 2.25 3 1.25 1.7 13 2.15 4 1.3 1.7 14 2.2 8 1.3 1.7 15 2.2 9 1.3 1.7 16 2.15 10 1.7 2.1 17 2.3 21 1.7 2.0 18 2.2 22 1.2 19 1.95 24 1.25 1.7 20 1.95 25 1.25 680 2.4 26 1.25 681 2.35 27 1.35 682 2.35 28 1.2 683 2.35 29 1.15 684 2.3 689 1.7 685 2.2 690 1.65 686 2.3 692 1.2 687 2.1 693 1.45 694 1.4 695 1.7 Percent- Percentage age No. number of of samples - - 100% samples 70% 55% 10% contain- containing ing mode mode Mean mode Mean modal 1.27 1.7 2.05 size 2.25 size 4.78 Table 8.10 Approximate modal grainsizes of sediments collected on the Brancaster Harbour Bar Table 8.10 Modal sizes in phi Modal sizes in phi Sample units Sample units 233 2.0 470 1.7 2.1 233A 1.9 471 1.7 2.1 235 2.3 472 2.1 235A 2.3 473 2.15 237 2.15 474 2.2 237A 2.3 475 1.95 240. 2.2 476 2.15 240A 2.3 477 1.75 241 1.75 2.1 478 2.2 241A 1.75 2.1 479 2.2 243 2.3 480 , 1.7 243A 2.3 481 1.5 352 1.5 482 1.8 353 1.8 483 2.25 354 1.3 484 2.2 355 1.3 1.7 485 2.2 356 1.7 486 2.2 357 2.2 487 2.2 358 2.2 488 2.2 359 2.2 489 2.2 360 2.15 490 2.2 361 1.7 2.1 491 2.25 362 1.4 492 2.2 363 2.15 493 2.2 364 1.8 494 2.2 365 2.1 495 2.2 366 1.7 496 2.2 367 2.2 497 2.2 368 2.2 498 2.2 369 2.1 499 2.15 370 2.2 500 1.8 371 2.35 501 2.T 372 2.2 502 2.05 2.2 03 1. 2.2 50 2.2 1. 2.1 J5 2.05 50 2.15 2.1 2.05 2.2 ) 2.2 2.2 509 2.2 2.2 1.7 2.15 Percentage 1.7 2.1 2.1 No. of - 24% 88% I ° .2 samples con- taming modes, g21 .. i 2.7.1 7 Mean modal L66 1. 2.1 size 1.75 2.17 2.0 ig 2.05 469 2.0 470 9 THE TIDAL INLET ENVIRONMENT 9

9.1 Introduction 9.1

This sedimentary environment is located behind the barrier ridges occurring on Scolt Head Island and Brancaster Golf Course. It is made up of the Salt Marsh and Tidal Channel sub-environments. The Marsh sub-environment on Scolt Head Island is made up of individual marsh units separated by lateral gravel ridges. From E. to W. these are Great Aster Marsh, Plantago Marsh, Plover Marsh, Hut Marsh, Missel Marsh and Cockle Bight. A more or less continuous marsh fringes the mainland, and corresponds to Overy Marsh in the E., and Brancaster Marsh in the W. These marshes are separated from those on Scolt Head Island by Norton Creek. Marsh deposits are composed mainly of fine grained sand, silt and mud with the proportion of sand increasing downwards. Their surfaces, which are near horizontal, occur (with the exception of Cockle Bight which is in an immature stage of development) near H.W.O.N.T. level, aid are vege- tated by halophytic plants. The Tidal Channel sub-environment includes both the large channels and the marsh creeks, between which a distinction has been arbitrarily defined. Marsh creeks form a complex drainage system in each marsh, while the tidal channels form large scale features situated between the individual marshes, and into which the marsh creeks flow. The tidal channels comprise Overy Creek and Burnham Harbour 48(1 in the E., connected by Norton Creek to Brancaster Harbour and Mow Creek in the W. Commonly, a fairly narrow transition zone of mud flats occurs between the Salt Marsh and Tidal Channel sub-environments. This gently sloping, and dominantly unvegetated zone represents the preliminary stages of peripheral marsh growth and displays sedimentological and physiographic characteristics which grade from one sub- environment into the other. Sedimentation in the Tidal Inlet environment on Scolt Head Island is in many ways similar to that occurring in both the Lagoonal and Estuarine environments. Inter- tidal marsh and mud flat deposits usually characterise all these environments. However, sediments deposited in a tidal channel may differ somewhat from those occurring below low water level in an estuary or a lagoon. The main distinction between sediments deposited in a lagoon and those deposited in a tidal inlet arise from the generally quieter, sometimes almost stagnant conditions often existing in the former. Owing to the relatively large amount of water impounded in a lagoon, the recircu- lating effect of tidal exchange through the comparatively narrow barrier inlet channel, or pass, is far less apparent than in the case of inlets such as those occurring on the N. Norfolk coast. Consequently, sediments will accumulate on the bottom of a lagoon under less turbid and commonly more variable conditions than normally existln the channels of a tidal inlet where the water mass is almost entirely recirculated and renewed during each tidal cycle. In the 481 former case the influence of weak current action will commonly be reflected in the sediments. Owing to the quieter conditions existing in a lagoon, sediments will suffer less reworking than in an inlet, and as a result, biogenic components (shells, etc.) will comprise a relatively greater proportion of sediments. In addition, these quieter conditions will promote greater bioturbation of the sediments forming in a lagoon. Differences between the sediments accumulating in an estuarine channel and those in an inlet channel may be largely attributed to the existence of a river in the upper reaches of an estuary. The influence of the tidal currents may in this case be modified by the introduction of river water and by the presence of a salt wedge. This forms as the denser salt water moves up the estuary with the rising tide beneath the less dense river water. This phenomenon results in a mixing of the sediments carried by the river, with those introduced by the flood tide.

9.2 The Cienesis of a Tidal Inlet Environment 9.2

The origin of the Inlet environment on Scolt Head Island is envisaged as resulting from the development, in post glacial times, of the sand and gravel barriers which now form the "back-bone" of Brancaster Golf Course, Scolt Head Island, and Halkham Meals, etc. According to Steers (1960), off-shore bars, or barriers, formed and migrated landwards in front of the rapidly transgressing North Sea, which followed the melting of the Pleistocene ice sheets. The inlet marsh deposits have accumulated under the pro- 482 tection from wave action afforded by these barriers. Pre- ceeding this landward migration, wide spread peat deposits formed under swampy conditions over large parts of the North Sea floor. Based upon pollen measurements, Godwin (1960) dates the oldest of three layers of peat that occur in the vicinity of Scolt Head Island as belonging to a period which occurred approximately 8000 years ago, in late Boreal and early Atlantic times. It has been proposed that the sea level rose rapidly following the melting of the last ice sheets, but subsequently rose more slowly in fairly recent times. It is a matter for conjecture (although it seems improbable) whether marshes had time to form behind the gravel ridges during the early, rapid rise in sea level; however, it seems most probable that such was the case in the last two thousand years or so, when sea level was rising more gradually. Furthermore, it is conceivable that the early, rapid rise in sea level was associated with both erosional and local depositional effects which would tend to produce a variable and complicated stratigraphic succession. The presence of exposed marsh deposits low down on the beach face at the Breakthrough on Scolt Head Island, as well as elsewhere along the N. Norfolk coast, attests to the continuation, although at a subdued rate, of this landward migration of the barrier ridges in front of a now slowly rising sea level. According to a radio carbon date obtained by K.A. Joysey of Cambridge (personal communication), the ancient marsh deposit which is exposed in the fore-beach sub-environment opposite the Breakthrough on Scolt Head 483 Island is between 1400 and 1800 years old. The top of this old marsh deposit is approximately 10 feet below the present level of the high tide (see Fig. 6.4), lUalcating a 10 ft. rise in sea level in the past 1600 years. This approximate rate of 0.006 feet or 2 mm. per year is in close agreement with the rate of eustatic rise in sea level. The marsh sediments were apparently deposited behind a gravel ridge which existed at that time some distance to the seawards of the present coastline. This picture is somewhat complicated at Scolt Head Island, which has grown progressively veotwards.(section 6.2), as well as migrating shorewards. It seems probable that the various fragments of shingle and dune ridge scattered over Brancaster and Overy Marshes (the Nod, Little Ramsey and Great Ramsey (sheet 8) represent buried remnants of a. once largely continuous ancient gravel ridge system. This ridge probably connected with Brancaster Golf Course, and possibly originated at a time when Scolt Head Island extended no further W,than the Breakthrough (see section 7.7). The erosion and destruction of this ridge system apparently occured in conjunction with the dominant westward growth of Scolt Head Island, and the subsequent expansion of Brancaster Harbour and the associated channel complex. The fact that the remaining fragments of ridge are still exposed above the marsh surface suggests that the destruction of this ridge system occurred comparatively recently, and accretion to the marsh surface (in response to the rising sea level) has not yet had time to bury them. The erosion of this gravel ridge by the

FIGURE 91.

0 >•• a 0 -.1

H cc LA RS A hi> L43 .2 ui a W MA in0 tu FIND (11

TER rr sL

RS Z Z EK u. m MR1NL Er ac-

RNC d 0 Lac

CRE M 0 • w BR • n• • 0 Z Ye MOW a

•0 o •••a6;.v •• • _ L2.

I CM. 10 FEET VERT. SCR le I cm. .300 FEET HOR1Z. N

SAND mmiii MARSH DEPOSITS

PEBBLY SRND bAoa BOULDER CLEW

Fig.9.1 Diagramatic section across Brancaster. Marsh showing the observed gradient of the mud./sand interface, and the inferred configuration of the boulder clay basement. 8 485 existing tidal channels and marsh creeks, may explain the erratic occurrence of abnormally coarse grained sediments occurring in the upper reaches of Mow Creek and elsewhere (i.e. samples 281, 282, 557 and 558, sheet 8, table 9.2). It has been established, by Steers (1960), and also by the author, from sections measured in the channel sides and from augering in the marsh surface that sand, and occasionally gravelly sand appears to underly the marsh muds except wherelhey abut against the mainland. A N.-S. diagrammatic section has been reconstructed from field measurements made across Brancaster Marsh just to the W. of Dial House, Brancaster Staithe (Fig. 9.1). The deposition of this underlying sand and of the overlying marsh sequence may be partly explained in terms of local current energy conditions. The current energy at any point in the inlet environment is determined by two factors: (a) its elevation above, low water level; and (b) its relative position along the length of the inlet. Normally, current velocities in a channel reach a maximum around mid-tide and decrease to minimum values at high and low tide. A point at high tide level will naturally be subjected to lower energy conditions than one at, say, mid-tide level. It would seem logical to assume that the stage of marsh development at which sand accumulation is superceeded by the deposition of silt and clay is determined by a local energy threshold level, which in turn is approximately related to a certain tidal height. Once the upward growth of the marsh exceeds this tidal height, the accumulation of silt and clay, in significant quantities, commences. 48g The sloping sand, mud interface displayed in Fig. 9.1 may, therefore, partly represent this "threshold" tidal height during a period of rising sea level. The thickness of the marsh at any point along this section would, therefore, be roughly proportional to its age. The oldest and most mature marsh development is seen to occur adjacent to the mainland, and apparently decreases in age seawards. This trend (shown in Fig. 9.1), although modified to an unknown degree by the lateral migration of Scolt Head Island and of Brancaster Golf Course, is also believed to apply generally to the marshes on Scolt Head Island which are presumably younger than Brancaster and Overy Marshes to the S. These tentative suggestions may be most profitably investigated by carrying out a series of borings across these marshes and by analysing the pollen content of samples collected at various depths. The sloping sand mud interface displayed in this figure may also be in part attributable to the normal sand surface which develops in the early (sandy) stages of marsh development. Once a gravel ridge forms a protective barrier against wave action, sandy marsh sediments begin to accumulate. However, in addition to current borne material, wind action supplies sand to the marsh surface immediately adjacent to the dunes on the gravel ridge. This causes the part of the marsh adjacent to the dunes to build up more quickly than the part nearer to the channel, and in this manner may produce a slight landward gradient such as is displayed in Fig. 9.1. 487 The bottoms of tidal channels, and marsh creeks, usually lie within the underlying sand layer. However, near the mainland where the marsh muds are especially thick (i.e. to the W. of sample 556 in Mow Creek), the channels have transgressed the mud sand interface and lie wholly within the mud strata. Based upon an examination of Cockle Bight, which displays a youthful stage of marsh development, it seems that the marsh creeks first form during the preliminary stages of sand deposition. They subsequently appear to retain their position as further sand, and later, silt and clay build the marsh surface upwards around them (Steers 1946). The relative stability of the beds of tidal channels is, however, a matter for conjecture. Phleger (1965) states that coastal lagoons (comparable in some respects to tidal inlets) eventually become filled with sediment. This process is rapid in the early lagoona.l history, but decreases.as the lagoon develops. The rate of infilling is dependent upon the balance between the rate at which sediment is supplied by the flood tide and the rate at which It is re.-oInd ebb. "The principU deposition is occurring in the marshes which are slowly accreting, eventually to reach levels above high tide. As the marshes expand in area the volume of the tidal water interchange between the lagoon and the ocean is gradually reduced, and thus current velocities decrease" (Phleger 1965). As a result of accretion to the marsh surface and expansion of the marsh area, the tidal channels and marsh creeks become increasingly sharply defined by progressively more 488 steeply sloping banks. As stated above, the upward growth of the marsh is accompanied by a decrease in the tidal capacity of the inlet, which would normally produce a decrease in current velocity. However, it is proposed that the constriction of the tidal channels (i.e. a decrease in their cross-sectional area) will tend to counter- act this tendency. Since, at any point in the channel, peak current velocities occur in the deepest part, deposition will preferentially take place on the sides and erosion will occur preferentially on the channel bed. As soon as accretion to the marsh surface significantly decreases the tidal capacity of the inlet, and subsequently produces a decrease in channel current velocity, deposition will occur on the sides of the channels. This will tend to reduce their cross-sectional area so that the itbial velocities are retained. Bruun and Gerritsen (190) discuss channel stability in terms of current velocity, channel length, cross-sectional area, bottom roughness and bed shear stress. Shoaling will result from a prolongation of the channel if it is not accompanied, and balanced, by a decrease in cross-sectional area. Assuming ideally, channel length, bottom roughness and bed shear are maintained constant, it is tentatively proposed that under equal ebb and flood current velocities, the decrease in tidal capacity of the inlet (which results from marsh accumulation) is balanced by a decrease in the cross- sectional area of the inlet channels, such that the current velocity in the channels remains constant. The observed inequality between ebb and flood 489 current velocities (see section 9.9 in which current measurements indicate strongest velocities to occur during the flood) is posdbly responsible for the reported shoaling which has effected not only Brancaster and Burnham Harbours, but also Wells and Blakeney Harbours over the last 50 to 100 years. Shoaling is not, however, an ubiquitous trend, as is indicated by isolated instances of erosion in Cockle Drain on the S. side of Cockle Bight Bar.(see section 9.7.1) and in the Deep Hole at Beach Point, where boulder clay has recently been exposed (see section 9.7.1). In general, however, the channel bottom sediments appear to be maintained in a state approaching dynamic stability by the balancing factors mentioned above.

9.3 The Salt Marsh Sub-Environment 9.3

In view of the comprehensive investigations carried out previously by others, further studies of these salt marsh sediments have been limited to a cursory sampling programme of the surface and sub-surface marsh and creek sediments occurring over Cockle Bight. The results of this study are discussed in section 9.6, and are used as a com- parative basis to evaluate the degree of modification experienced by sediments deposited in the bed of the inlet channels. Steers (1960) has described the salt marshes occurring on Scolt Head Island, their possible mode of origin and their large scale internal structure. In addition, Slater (1931) has studied the general character of 490 sediments accumulating on Cock"4e. Bight at the W. end of Scolt Head Island, and the processes controlling their deposition. Joysey, et al. are at present investigating the biological and zoological processes active in the Scolt Head marshes, and the roles played by these factors in marsh development. Van Straaten (1951, 1954), and Van Straaten and Kuenen (1957, 1958) have described the processes involved in the deposition of the fine grained sediments accumulating in the mud flats of the Wadden Sea, and in shallow marine environments generally. Inglis and Kestner (1958A and B) have measured the rate of accumulation of fine grained sediment in the intertidal zone of the Wash. Evans (1965) has further described the character of the sediments, the processes of sedimentation, and both the internal and external structural features characterising a succession of salt marsh, sand flat and mud flat deposits in the Wash.

9.4 The Inlet Channel Sub-Environment 9.4

Introduction

The Inlet channel sub-environment may be envisaged as a river system made up of tributary marsh creeks converging to form a central channel which has its mouth at the sea. The channel and creek bottoms are usually composed of clean, moderately well sorted and commonly rippled sand. The distribution of sediments in this complex ideally show a general longitudinal trend of decreasing 491 grainsize with increasing distance from the channel mouth. This is a reflection of an equally ideal decrease in current velocity in the same direction. A lateral gradation is also generally apparent, in which coarse material in the channel bed grades transversely into the fine grained marsh sediments. This ideal picture is considerably modified by variations in the current energy (produced by bends, con- fluences, etc., in the channel configuration) which is manifested as local erosion and deposition on the channel bed. The grainsize characteristics of the channel sediments have been investigated with two objectives in view: (A) to distinguish the channel sub-environment from other coastal and intertidal sub-environments on the basis of the grainsize characteristics of their sediments; (B) to investigate the relationship between the grainsize characteristics of a sediment and the dynamic conditions responsible for its deposition. Objective A is discussed more fully in sections 10.2 and 10.3. Objective B has been investigated along a number of different avenues of approach. (1)In order to study the overall variations in grainsize of the sediments within the inlet channel sub-environment, samples have been collected in the bottoms of Mow Creek, Norton Creek and Brancaster Harbour. (2)In view of the complex nature of the channel deposits, a cursory study was made of the surface and subsurface sediments occurring in the immature marsh creeks of Cockle Bight. In addition, samples were collected from the marsh itself for purposes of comparison. These investigations 492 are based upon the assumed similarity of the dynamic conditions existing in both the large scale tidal channels, and the small scale marsh creeks. (3)A detailed sampling programme has been carried out over a number of specific depositional channel features with the aim of relating the observed variations in the distribution of grainsizes to local fluctuations in the energy level associated with the specific feature. (4)The wide spread occurrence of mega-ripples in the inlet channel sub-environment suggests their significance in regard to sediment movement, and to at least one possible process of sediment differentiation and sorting. The distribution of grainsizes across, and within a number of mega- ripples have, therefore, been investigated. (5)Since the sediments occurring within this sub-environment are dominated by the action of the tidal currents, a series of measurements have been made in an attempt to directly relate current action with the sediments occurring both on the channel bed, and in suspension just above the bed.

9.5 Mow Creek, Norton Creek and Brancaster Harbour 9.5 Channels In order to investigate the overall range of grainsizes characteristically existing in the Inlet Channel sub- environment, samples were collected at positions, shown on sheet 8, in Mow Creek, Norton Creek and the lower part of Brancaster Harbour channel. In addition, samples collected over Cockle Bight Bar (Fig. 9.18) were included as characteristic of the upper part of Brancaster Harbour. 493 Generally, the samples were collected in the bottom of the inlet channels, however, some samples in Mow Creek were gathered transversely across the channel (see sheet 8). The inlet channel environment at the lower end of Brancaster Harbour grades imperceptibly into the fore-beach sub- environment of the Brancaster Golf Course beach. The fore- beach sub-environment will presumably be influenced by wave action as well as current action, while the channel sediments will be dominated by current action alone. Although this distinction may theoretically be made in terms of dynamics, it is natural that no clear cut sedimentological differences may be discerned between sediments occurring in these adjacent localities. The mean grainsize, standard deviation, skewness and kurtosis parameter values are displayed in Tables 9.2, 9.3 and 9.4 and in the case of Cockle Bight Bar, in Table 9.11, section 9.7.1. It was expected that the grainsize parameter values of these samples would reflect, in general, the predicted increase in energy from the upper to the lower reaches of the channel (i.e. from the upper part of Mow Creek to the lower part of Brancaster Harbour). The samples collected along the bed of Norton Creek, which links Brancaster Harbour to Burnham Harbour, were expected to show some indication of a predicted "watershed" or minimum energy region somewhere near the mid-point of the Creek. This water shed is assumed to exist since the tidal currents flood at approximately the same time from either end of Norton Creek, and ebb from the vicinity of its mid-point. 494 It is obvious from Tables 9.2, 9.3 and 9.4 that these individual channel sediments display no consistent longitudinal trends as predicted. However, the averages of the grainsize parameter values, shown in Table 9.1, Indicate that the sediments in Brancaster Harbour (including Cockle Bight Bar) are coarser grained and more pol.rly sorted than those in Mow Creek or Norton Creek. The arerage parameter values of the Norton Creek sediments indicate a similar mean size and standard deviation to those of Mow Creek; however, a. strong negative skewness is characteristic of the former samples.

Table 9.1 Average and standard deviation of the grainsize parameter values of the various samples collected in Mow Creek, Norton Creek, and Brancaster Harbour. Table 9.1 Sub-area and Average Average Average the number Mean standard Skewness of samples grainsize deviation Sk collected Ms (phi) Cr; (phi)

Mow Creek 1.96 0.38 4.03 (25) (0.045) (0.003) (0.012)

Norton Creek 1.90 0.33 -0.15 (19) (0.027 (0.003) (0.049)

Brancaster H Harbour (including 1.68 0.40 -0.05 Cockle (0.064) (0.017) (0.027) Bight Bar) (46)

The wide range, and apparently haphazard distribution of the grainsize properties displayed by these channel sediments may be attributed to the mode of origin and

FIGURE 9.2.

PHI 23 2.1

Mz. F9

1.5 x

PHI

0.4- x x a. 03 x 02

11411 111I1111141

285 2 X88 28786 289 290 293 295 294

Fig. 9.2 Grainsize parameter values of samples collected at three localities across the bed of Mow Creek. 40 development of the channels themselves. Unlike a river system the channels cal not erode their beds to achieve their present position, but rather remain relatively stable while the marshes buildup on either side. As previously discussed in section 9.2, normal marsh development commences with the formation, under moderately high energy conditions, of a medium grained sand flat similar to that of the present Brancaster Harbour Bar. Subsequent deposition in the protection of a gravel ridge increases the height of this deposit, causing a decrease in the energy level and subsequently a decrease in the mean size of the sediment accumulating. Since the marsh creeks and the embryo channels are believed to form at a very early stage of this process, the sediments comprising their beds are generally related to a primary stage of sand deposition. Subsequent local reworking, deposition, and erosion within the channel, considerably modifies their original grainsize characteristics, thus accounting for the observed erratic variations in the grainsize parameter values shown in Tables 9.2, 9.3 and 9.4. In some isolated cases, the migration of a channel has resulted in the erosion of an ancient shingle ridge which has been partly buried by the more recently formed marshes. The Nod in Brancaster Marsh is believed to be a case in point. Such erosion would result in the local occurrence of an anomalously coarse grained deposit in the channel bed. Samples which were collected across Mow Creek (samples 287, 286, 285; 290, 289, 288;. and 295, 294, 293),

FIGURE 9.3. PHI • A. 0.7 • • 0.6 •

• 05 ax X • • a. • • X *

0.4 * X • X X >>?< X )SC • • go xx )0 • X.X. • • . • *),0 ) ( • • 0.3 X X • •>Z< 4)( x a X • • X. X X • •

1.1 1.2 13 1-4 F5 1.6 1.7 F8 1-9 2.0 2-I Z2 23 PHI

• B. • 0.3

+ 0.2 X X X X • X X 00 • 4 XX + 0.1 x X X • • • a • • 11( AK X Ski. 0 • • •II • • l< • • x'x x • X X 0. x • - 0I • XX X x 4?( •x Z• • x•SC x x • • x x - 0-2. • x Xx - 0.3 • • • x •

Fig. 9.3 Relationship of both standard deviation (61) and skewness (Sk) to the mean grainsize (Mz) of samples collected in Mow Creek and Norton Creek, (X), and on Cockle Bight Bar, (0). 498 display a trend of decreasing mean grainsize, improved sorting and mare negative skewness values from the channel bed towards the marsh edge (Fig. 9.2). This trend reflects the normal lateral decrease in current velocities from the centre of a channel towards its sides. The increasing negative skewness values are apparently Ixoduced by a relative increase in the proportion of fine to coarse material in the sample. The mean grainsize of all the individual inlet channel samples has been related to their standard deviation and skewness values in Figs. 9.3.. and B respectively. Both plots display a vague trend of improved sorting, and more negative skewness with decreasing mean grainsize. These trends are similar to those shown by the parameter values of the individual samples collected across Mow Creek. 499 Table 9.2 The grainsize parameter values of samples collected in Mow Creek above Cockle Bight Bar. Table 9.2

Mean Standard Sample Grainsize Deviation Skewness Kurtosis (Mz phi) (01 phi) (SK1 ) (Kg) 556 1.71 0.50 +0.17 - 557 1.56 0.51 +0.17 - 558 1.65 0.42 +0.11 - 281 1.33 0.30 +0.22 1.07 282 1.50 0.40 +0.10 0.83 283 1.82 0.32 +0.08 0.99 284 1.82 0.34 +0.13 0.94 285 2.23 0.25 -0.11 0.99 286 1.95 0.32 -0.02 0.86 287 1.99 0.40 -0.09 0.94 288 2.26 0.30 -0.23 1.06 289 2.08 0.31 -0.15 0.90 290 1.96 0.37 +0.15 1.02 291 2.10 0.27 -0.06 0.96 292 1.62 0.43 -0.07 1.20 559 2.08 0.29 -0.11 0.94 293 2.13 0.33 -0.19 0.91 294 1.94 0.36 -0.04 1.03 295 1.51 0.44 +0.11 - 297 1.91 0.38 +0.02 0.83 298 1.94 0.37 -0.06 0.94 299 2.01 0.38 -0.15 1.14 300 2.09 0.31 -0.11 0.98 350 1.72 0.34 -0.10 1.03 34.9 1.88 0.29 -0.08 0.89 348 2.04 0.27 -0.03 0.95 593 1.91 0.33 -0.05 - 594 1.87 0.35 +0.03 - 595 1.80 0.35 +0.03 - 596 1.58 0.31 +0.10 - 597 1.72 0.33 +0.09 - 598 1.86 0.31 N-0.03 - 34.6 2.01 0.39 -0.05 - 347 2.09 0.26 -0.03 - 500

Table 9.3 The grainsize parameter values of samples collected along the bed of Norton Creek Table 9.3 Mean Standard Sample Grainsize Deviation Skewness Kurtosis (Mz phi) (Q phi) (SK1 ) (Kg) 150 1.95 0.27 -0.15 1.06 152 1.92 0.30 -0.18 0.96 153 2.12 0.30 -0.29 0.95 154 1.88 0.37 -0.11 0.86 155 1.98 0.36 -0.23 0.94 156 1.66 0.47 -0.07 0.85 157 2.06 0.30 -0.21 0.89 158 1.76 0.39 -0.06 0.91 159 1.94 0.33 -0.21 0.90 160 2.32 0.23 -0.27 0.90 161 1.80 0.37 -0.18 0.91 162 1.84 0.33 -0.11 0.94 280 1.70 0.37 -0.15 0.93 279 1.80 0.32 -0.10 0.99 278 1.83 0.31 -0.05 0.93 277 1.75 0.30 -0.10 0.92 276 1.89 0.37 -0.14 0.87 275 2.15 0.33 -0.20 1.13 274 1.81 0.35 -0.07 0.87

Table 9.4 The grainsize parameter values of samples collected in the bed of B:ancaster Harbour channel to the seawards (i.e. to the W.) of Cockle Bight Bar Table 9.4 Mean Standard Sample Grainsize Deviation Skewness Kurtosis (Mz phi) ( 1 phi) (SK1) (Kg) 605 1.52 0.35 +0.03 - 318 1.12 0.47 +0.32 1.58 311 1.73 0.38 +0.01 1.06 824. 0.60 0.31 +0.26 - 310 1.99 0.29 -0.09 0.96 309 1.96 0.35 -0.10 1.01 652 1.39 0.54 -0.22 - 653 1.49 0.33 +0.07 - 501 9.6 Marsh and Marsh Creek Sedimentation 9.6

A continuous process of intertidal flat deposition is instigated after the construction of a gravelly beach ridge which provides local protection from strong wave action. Primarily, sand is deposited on the landward side of this gravel ridge, but as sediment accumulates the energy level progressively subsides, and, together with this change, the mean grainsize of the material deposited progressively decreases. The narrow zones occupied by the marsh creeks offer the only exception to this progressive decrease in the energy level as the marsh builds upwards. It is believed that this sequence of depositional events, which eventually produce a typical marsh deposit, represent the least complicated example of sedimentation in the intertidal and near-shore environments. It is presumed that the marsh sediments will display a moderately consistent variation in grainsize characteristics which may be related to successive stages in the depositional history of the marsh. Although the marsh creeks have also passed through similar stages of evolution, their relatively high current velocities would have caused the normal marsh sediments to be considerably modified and reworked. Based upon the assumpton that, in terms of energy conditions, a marsh creek is basically a. small scale facsimile of an inlet channel, the grain size distribution of sediments occurring in both the:. creeks and the inlet channels should display common FIGURE 9.4

111111111111111111111miumll I I iu

RPPPDX. LWL FORE-BEACH

0000 0 o 0 BACK-BEACH 0 0 O 0 0 0 03.'"441.0 / o O / 0 0 TERNERY POINT

0 O

to 0 °14-6 2.23, 224 I%. • ‘1 I ° 191-1‘ 1 0 14 MY MY 220, • BIGHTGHT N 222 183- 186 49111111111144w FRR PT. en" 1111 \ M7182

Scale : 61Nchcs 4Proximaiely cpuo.15 1 mile. mc, z Muddy gravel, MC,

Fig. 9.4 Sampling positions on Cockle Sight marsh. The map is based upon areal photographs flown by Hunt:ings Surveys Ltd. in June 1960. 503 characteristics. An advantage in studying the grainsize characteristics of marsh creek sediments is afforded by the presence of adjacent undisturbed marsh sediments, from which the degree of modification experienced by the creek sediment may be gauged. Samples shown in the tables below have been collected mainly from Cockle Bight (Fig. 9.4) which is a youthful marsh, having only recently attained that stage of sedimentation when silts and clays are deposited. These samples have been collected from the marsh surface and sub-surface and from the creek surface and sub- surface. Samples 214, 215 and 216; 228 and 229, represent sediments at different depths in two newly formed embary- -lents to the W. of Ternery Point. In addition, three samples (35, 41 and 255) were collected in the bed of a creek in Hut Marsh (see sheet 8). Since the majority of these sediments (except those sampled from the creek beds) contain significant quantities of silts and clays, the grainsize analysis of the sand fraction alone reveals only an incomplete picture of the grain size distribution. Superficially, the relationship between decreasing mean grainsize and more negative skewness (shown in Fig. 9.5B) displayed by the surface and sub-surface samples, may be attributed to the exclusion of the fine fraction from the grainsize analysis. However, the fact that some of these grainsize distributions are positively skewed, despite their containing an unanalysed silt and clay fraction, suggests that this limited analytical procedure modifies rather than completely FIGURE 9.5.

P141 1 045 A

046 0 O 0 0 0 044- 0 042 0 0 0 0 0 0 0.40ti O 0 Ul. 0.38 0 00 . 0 0 0.36 0 0 0.34 0

0.32 0 0

0.30 0 0

Mz. 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 23 2.4 Pm UNITS B

0 0 0 0 o 0 0 + o °0 00 0 + O +0 0° — 0.1 o 0 o + o —0.2 + o 00 0 —0.3

Fig. 9.5. Relationship of both standard deviation, (IT ), and skewness, (Sk ) to the mean grainsize, (Mz) l of surface and sub-surface samples collected from the marsh (0) and marsh creeks (+) of Cockle Bight. 505 obscures the real character of the grainsize distribution. Supporting this proposal is the similarity of the relationship between mean grainsize and skewness value for samples collected within the marsh and creeks (Fig. 9.5B) and samples collected in the inlet channel sub-environment (Fig. 9.3), the latter of which contain only an insignificant quantity of silt and mud. Samples 35, 41 and 255 (Table 9.9 were collected from the surface sediments of the marsh creek in Hut Marsh. Sample 35 is situated in the upper reaches of the creek, and sample 255 is located at its lower end adjacent to Norton Creek.

Table 9.5 Showing the grainsize parameter values of samples collected from the bed of a creek in Hut Marsh Table 9.5 Mean Standard Sand frac- Sample Grainsize Deviation Skewness tion as a (Mz phi) (Sk) % of the (61- phi) total sample wd_ght 35 1.90 0.36 -0.04 88% 41 1.68 0.39 +0.01 90% 255 1.45 0.35 +0.18 100%

The frequency curves of these grainsize distributions are shown in Fig. 9.6. The progressive increase in mean grainsize from the upper to the lower reaches of the creek may be produced by a combination of three factors. (*) The progressive removal by winnowing of the finer grades as is implied in the decrease in the FIGURE 9.6.

0.6 OS 1.0 12 1.4 I.6 1.8 20 2.2 24 2.6 28 GRAINSIZE - PHI UNITS

Fig. 9.6. rrequency curves of surface samples collected down a marsh creek in Hut Marsh. 5A7 proportion of the silt and clay fraction. This factor may also be responsible for the more positive skewness of the coarser grained sediments (i.e. sample 255). (Refer to the discussion of this point in section 10.6). (b) The increasing depth of the marsh creek will allow it access to increasingly coarse grained primary marsh sediments. (c) The higher current velocities at the lower end of the creek will naturally cause the deposition of coarser grained sediments. The sparcity of samples over this section of marsh creek introduces a severe limitation to the above conclusions. The undisturbed Cockle Bight marsh sediments (Table 9.6) show a progressive increase in grainsize with depth as predicted. Samples 214, 215 and 216, however, which were collected in a small ombayment to the, W. of the Ternery, display a reverse trend of decreasing grainsize wittx This is attributed to the addition of a coarse grained wind blown sand fraction from the nearby dunes. The addition of this coarse grained sand causes the grainsize distribution to become more positively skewed as it increases in mean size. 5r)3 Table 9.6 Grainsize parameter values of the undisturbed marsh surface and sub-surface samples collected from Cockle Bight Marsh and from the embryo marsh immediately to the W. of the Ternery. Table 9.6 Mean Standard Sand frac-, Sample Grain- Devi- Skewness Sample ti on as a % size ation (Sk1 ) Position of the total (Mz in (Ci in sample phi) phi) weight 181 1.99 0.45 +0.02 surface 71% 182 1.88 0.41 +0.04 2" sub- 50% surface 183 surface 184 2.39 0.4.1 -0.24 2" sub- 42% surface 185 2.18 0.46 -0.24 5" ... 186 2.03 0.43 -0.13 .. .. 94% 189 2.16 0.39 -0.07 surface 60% 190 1.87 0.45 +0.01 4" walp-im 86% surface 191 2.28 0.41 -0.12 surface 43% 192 2.26 0.37 -0.08 2" sub- 78% surface 193 2.03 0.47 -0.19 6" . 85% 194 2,12 0.42 -0.23 18" . . 95% 199 2.03 0.38 0 surface 39% 200 1.94 0.32 +0.08 3" sub- 82% surface 203 1.95 0.40 +0.05 surface 51% 204 1.78 0.45 -0.06 3" sub- 60% surface 214 1.71 0.4.1 +0.16 surface 64% 215 1.84 0.35 +0.05 2" sub- 86% surface 216 1.83 0.37 +0.03 4" .. .. 97% 228 2.08 0.30 -0.12 surface 229 1.93 0.37 -0.18 2" sub- Mt. surface

Surface and sub-surface samples 222, 220; 224: 223; 226 and 225, were collected at various positions in the marsh creeks in Cockle Bight (Table 9.7, Fig. 9.7). 509 Table 9.7 Grainsize parameter values of Marsh Creek surface and sub-surface samples from Cockle Bight Table 9.7 Mean Standard Sample Grainsize Deviation Skewness Sample (Mzln phi) (01 in phi) (Ski ) Position 222 2.07 0.36 -0.16 creek surface 220 1.96 0.47 -0.22 18" sub-surface

224 1.73 0.34 -0.09 creek surface 223 2.04 0.45 -0.31 12" sub-surface 226 1.86 0.34 -0.07 creek surface 225 2.17 0.33 -0.24 18" sub-surface

Except for samples 222 and 220, the mean grainsize is coarser at the creek surface than it is in the sub- surface samples. It is tentatively proposed that this trend reflects the influence of the high current velocities in the creek, which cause winnowing of the finer sand grades to produce a relatively coarse grained lag deposit on the creek bed. This tentative proposal is further supported by the more positive skewness, and (in general) the improved sorting of the creek surface samples as compared with the sub-surface samples (refer to the discussion of the fractionation mechanism proposed in section 10.6). It is significant that samples 220 and 222 were obtained from the bed of a small creek in the lower part of Cockle Bight. This small marsh creek will be subjected to much lower velocities than will the large creek in which the other samples were collected. The distinction between these sample pairs is FIGURE 9.7. ,

M.S.S. 196 CS. 22

C.S.S. 22 If

M.SS.'

200 C.S. 224

C.S.S. 225 ------

M.S.S.

C.S. 226

C.S.S. 225 ...... 0.8 FO 1.2 V4 vb 1.8 2:2 2:4 2.6 2:8 GRPINSIZEIN PM UNITS

Fig. 9.7 Frequency curves of samples collected from adjacent positions in the marsh sub-surface (M.S.S.), the creek surface (C.S.) and in the creek sub-surface (C.S.S.). 511 illustrated in the frequency curves shown in Fig. 9.7 in which adjacent marsh surface s=imples have also been included for comparison. The occurrence of a fine mode of 2.35 phi in each of the sub-surface creek samples is apparently responsible for their strong negative skewness; however, the reason for the consistency of this mode is obscure. Table 9.8 Comparison of the grainsize parameter values of samples 199 and 189, collected on the marsh surface with samples 224 and 226 from the surface of the adjacent marsh creek Table 9.8 Mean Standard Skewness Sample Sample Grainsize Deviation (Sk) Position (Mz in phi) (GI in phi) 199 2.03 0.38 0 marsh surface

224 1.73 0.34 -0.09 creek surface 189 2.16 0.39 -0.07 march surface 226 1.86 0.34 -0.07 creek surface

The creek sediments are, as expected from the higher current velocities, coarser grained and slightly better sorted than the sediments occurring on the marsh surface. The marsh sub-surface samples display a similar mean grainsize and a slightly more positive skewness value than do the adjacent creek surface samples. Although the relative level at which adjacent samples were collected is not exactly the same, the similarity between their grainsizes supports the general proposal that the beds of marsh creeks and tidal channels remain 512 Table 9.9 Comparison of the marsh sub-surface samples (186, 200 and 190) with the adjacent marsh creek surface samples (222, 224 and 226). Table 9.8 Mean Standard Sample Grainsize Deviation Skewness Sample (Mz in phi) (01 in phi) (Sk) Position 186 2.03 0.43 -0.13 marsh sub- surface 222 2.07 0.36 -0.16 creek surface 200 1.94 0.32 +0.08 marsh sub- surface 224 1.73 0.34 -0.09 creek surface 190 1.87 0.45 +0.01 Marsh sub- surface 226 1.86 0.34 -0.07 creek surface at a constant level while the marsh surface builds up around them. The mean grainsize has been related to both the skewness value and the standard deviation of all marsh samples in Fig. 9.5A and B respectively. A trend of more positive skewness with increasing mean grainsize is apparent in the former plot. As previously mentioned, this trend is similar to that shown in Fig. 9.3 by the individual samples characterising the inlet channel sub- environment. The relationship between mean grainsize and standard deviation exhibits a random distribution when both marsh and marsh creek samples are considered. However, thesamples collected in the creeks alone show a slight trend of improved sorting with decreasing grainsize. It may be of significance that for any given grainsize a marsh creek sediment will tend to be slightly 513 better sorted and more negatively skewed than an undisturbed marsh sediment with the same mean grainsize.

9.7 Detailed Investigation of Specific Depositional 9.7 Features within the Inlet Channel Sub-environment

As mentioned in section 9.2, the original channel sediments have, in places, been reworked, remobilised and redeposited as a result of local fluctuations in the current velocity produced by changes in the general configuration of the channel. Typically, one of three general types of modifications to the original channel topography may be produced by these local fluctuations in the current velocity. (a) Erosion of depressions in the channel bed at the confluence of two channels. (b) Deposition of sediment to form a spit-like (or point bar-like) accumulation on the inner side of a bend in the channel. Such deposits occur as a result of a lateral decrease in the current energy and are usually associated with erosion on the outer side of the bend. (c) Deposition and accumulation of segment to form a bar in the middle of a channel. These features are usually associated with a local expansion of the channel and its bifurcation into two subsiduary channels with the subsequent deposition of sediment between. Although the balance between the dynamic factors and the physical configuration of the channel is continu- ally altering, over a short time interval it would be expected that the grain size distribution of the sediments comprising the separate features in the channel 514

Fig. 9.8 Topographic and structural components of Cockle Bight Bar and the surrounding locality. This diagram is based upon a compass traverse survey carried out in 1965, as well as upon the areal photographs. The directions of residual sediment movement have been inferred from the orientation of meg-ripples (small arrows). Lag deposits of mixed sand and gravel are distinguished by the symbol (sc).

Scale: one inch approximately equals 500 feet.

.—... /.....s. ( 0 0 Ni 0 1 M FIGURE 9.8. a 0 "."--.• I ' e4c, 0 TERNERY P'4% Cu• 0 .° e1/4'c, 10‘ o ° P t . LONGS I 0 COCKLE BIG MARSH I 0 o 0 M.G. M.G. \ 0 1) HILL BRA NCASTER ° Gk. \,. ° 0 HARBOUR BAR . i 0 , / , M 1 . ,/,'o / ..15,Rp ,tc.R STE R c 1..034,414_ M I--:- / /- c, 0 / AL' , - - _

/ r WESTERN 0 o HIGHEST f a I 5u8St ItuFIRy PRRTas OF "14- ERSTE R.N \ . 1 0 1311R. 0 E SUE5511DuRRY c BAR. 1 c) 0 ...... 3)0rniNIAN-1- cup.n.ENT I if — ...---...... 0 c. , .. , :::. 1 RECT I ON E.156, F. FLOOD / a SC."- 0,. -.-,.-...-„.-..‘ -.., MEGA-RIPPLE ORIENTATION ;., / O COCKLE 40. 0 - • --'..:.s.- i... I S ., M.G. - 0 0 FAR Pt BIGHT ••••••.: re r • • • BAR • Stn. .4 eStn.5. ' ‘ -• •-• --, - --' -, - ,, .„...... , _ , , , Stn 5. .., ,,•' - • ,, , c-_:;;) __ _.---s„ ,5„ ' "%::•i2i.../::- -- 0 i t• - -2 , 0 .... - etv- , _ _ __ . MG. O a GPSIvE L- . - v.e- 9'. - - : 0 ,;.' a 0 • F 0 0 - 00 - - 0 0 0 o SAND FLATS ...„. muSsE L5 . G c.::. 0 . , .. o •>" 4v DUNES . .4-- . 14 A-- • . 0 BACK- BE RCN 0 „ o BEFICH Pt. RS 14 MUD i ---- . ' 0 a M MR . MG MUDDY qRAVE L- .---- ...---- / c..., /".`k iuN 'eN ..,>`" 0 SEEP ^.• , 0 k.: .,•""'t .e'-'*4 ^, ° .5 C. SCOURED BED FORE - BEACH .../ .... .?44, ,,... 1/4. /".4 ,00.. 0 516 would, in some way, reflect the local energy distribution. A close surface sampling programme was carried out at a number of channel localities in the anticipation that trends in the grainsize characteristics of the sediments would be revealed, which could, with a reasonable degree of probability, be related to the currents which produced the feature. Three main potentially mobile sand accumulations within the channel system were investigated:- (1)Cockle Bight Bar, which forms a large sand deposit in Brancaster Harbour, between Far Point in the W. and Beach Point in the E. (2)Norton Creek Bar, which forms an elongated, lens- shaped mass of sand situated in Norton Creek opposite Hut Marsh. (3)Overy Cockle Strand, which comprises a broad gently sloping sand body abutting against Overy Marsh at the E. end of Norton Creek where it joins Burnham Harbour.

9.7.1 CocNb Bight Bar - Physiography and Dynamics 9.7.1

Cockle Bight Bar forms a large sand and gravelly- sand deposit which has accumulated in Brancaster Harbour. This Bar is considered to be analogous to the characteristic shoal which forms at the inner (lagoonal) end of a barrier inlet where the channel broadens to form a lagoon (Bruun and Gerritsen 1960). The main Bar is bordered to the N.W., X. and Y.E. by Brancaster Harbour channel, which forms a large bend or loop between Far Point and Beach Point (Fig. 9.8). On its southern side, the Bar is separated from Brancaster Golf Course Beach 517

Fig. 9.9 Areal photographic mosaic of Cockle Bight Bar, based upon a survey flown by K.St. Joseph in May 1960. Arrows indicate the proposed sediment transport paths.

Scale: one inch approximately equals 500 feet. -

_ 519 by Cockle Drain which is shallower than Brancaster Channel, being exposed approximately half an hour before the time of low water, spring tides. This general bar and channel configuration has been called "D" shaped by Cornish (1901), who described similar features from the Dovey, Mawddach and Montrose Estuaries. Cornish further comments on the currents characterising these channels: "down the straight stroke of the D runs the scouring ebb current; the silting current of the flood swings around the bow of the D". Two small subsidiary bars (the eastern and western subsidiary bars) occur in Brancaster Channel on the Y. side of Cockle Bight Bar. These features are semi-detached from the main Bar, being more closely related to local current (and sediment) circulations with— in Brancaster channel (Figs. 9.8 and 9.9). At either end of Cockle Bight Bar the Harbour channel has been constricted by Far Point in the W. and Beach Point in the E. The high current velocities, which are subsequently produced at these constrictions, have caused local scouring and erosion of the channel bed. The "deep" to the west of the Bar has been produced as a result of the periodic addition of gravel to Far Point, which has subsequently migrated southwards forcing the channel to likewise bend and to be constricted against Brancaster Golf Course Beach. The coarse grained sediment accumulating at Far Point is derived from beach material which moves westward along the beach front of Scolt Head Island under wave action from the N.E., then southwards around the Ternery Point under the

FIGURE 9.10.

4w 071- iCt5

-1 I TERNERY V POINT V (1.40 , / ici53 8 .` • ...... RANCPSTrii • C6 , 1.1407L. ' _se' C.E5425. lilt, ,...... ,...... - — ,,,, N. — -... 1407 < -- \ \ - ) 907 \\\

/89j' JAPINC FIST E R. GOLF COURSE

Fig. 9.10 Map showing the configUration of Brancaster Harbour channel, Ternery point and Brancaster Golf Course beach at various times since L891. (The 1960 outline has been taken from areal photographs flown in 1960 by Huntings Surveys Ltd.) Scale: 3 inches equals 1 mile. 521 influence of N. or N.W. storms. The Deep Hole which occurs adjacent to Beach Point at the E. end of the Bar is attributed to two separate phenomena: (a) at the confluence of Mow Creek and Norton Creek. The converging ebb streams in these channels produce vertical water movements which, according to Van Straaten (1952), are responsible for bottom scouring; (b) as mentioned in section 7.7, Brancaster Golf Course shows a long term trend of eastward growth which will subsequnmtly result in the constriction of Brancaster channel against Beach Point, thus producing a similar phenomenon to that which occurs at Far Point. It would appear from a comparison of Ordnance Survey maps of the years 1886, 1907 and 1954 (Fig. 9.10), that the Bar is genetically related to the Ternery - Far Point gravel ridge. Since the formation of this ridge (around the early nineteen hundreds) the Bar has increased in volume at the expense of Cockle Bight Marsh, the edge of which has been eroded by the northward migration of Brancaster channel. As this channel became elongated and more tortuous, its stability decreased, with the result that a subsidiary channel (Cockle Drain) has formed by scouring a more direct route between Far Point and Beach Point. Cockle Drain may, in time, replace in importance the existing main channel of Brancaster Harbour. Topographically, the Bar is roughly half saucer- shaped; lowest along its southern side and surrounded on the W., N. and N.E. by a higher rim. The undulating 522 surface of the bar terminates in the west against the high relief of a mussel (mytilus edulis) bank. This mussel bank forms the western end of Cockle Bight Bar and forms the highest relief of the feature. Such mussel *olonies accumulate large quantities of mud, in the form of faecal pellets, which is rerr'llrd stable under high current velocities by the addition of an organic secre- tion produced during the feeding process (Slater 1931). Although these mussel patches develop in areas where extensive sand transport presents the possibility of burial, their high surface-roughness causes an increase in the bed shear which locally increases erosion and sediment transport over their surface. Towards the N. and N.E., however, the surface of the Bar is gently inclined and culminates in the crest of a S.E. striking ridge, the steep scarp slope of which faces N.E. (Figs. 9.8 and 9.9). This ridge reaches its highest relief at its N.W. end where it forms the most northerly part of the main Bar. The general configuration of Cockle Bight Bar is illustrated photographically in Fig. 9.9, and diagrammatically in Fig. 9.8. The former is based upon a photographic survey flown by K.St.Joseph in 1960, while the latter, which differs slightly from the former, is constructed from a compass traverse survey carried out in 1965. Ebb formed mega-ripples dominate the surface of the Bar and are especially well developed immediately to the S. of the sand ridge formed on its N. and N.E. sides. Large, lobate, flood-formed mega-ripples occur in the narrow zone formed by the N.E. facing scarp slope 525 of this ridge. These ripples are especially well developed at the S.E. end of this scarp slope, where it widens slightly. Samples have been collected over the surface of the Bar at positions shown in Fig. 9.18. The grainsize parameter values of these sediments are shown in Table 9.11. In addition, more detailed sampling has been carried out on the eastern subsidiary bar (Fig. 9.14). The grainsize parameter values of these sediments are shown in Table 9.10, and their grainsize distributions are illustrated as frequency curves in Fig. 9.15. Composite samples 669 to 672, collected over the western subsidiary bar, represent the average grdnsize distribution of each quarter of the bar. Samples 320 to 326, on the other hand, are spot samples collected around the outer edge of this feature. Samples 824, 826, 842 and 850 represent the average sediment collected at two locations (stations 4 and 5) by grab sampling during the series of tidal measurements discussed in section 9.9. The grainsize parameters shown in Table 9.11 exhibit a wide range of values which superficially show no clear trend. Generally, however, the sediments on the N.W. side of the Bar are coarse grained, those on the N.B. side are fine grained, while in the central and on southern side they generally appear to be medium grained. Gravelly sand, however, occurs over the south eastern part of the Bar. This gravel is apparently introduced into the harbour by eastward beach drifting 524 along Brancaster Golf Course Beach, the back-beach sub- environment of which increases in width towards the E. where it lies immediately adjacent to Cockle Drain. It would appear that erosion of these coarse grained beach sediments, mainly by the ebb current, is responsible for their redistribution over the south eastern part of Cockle Bight Bar. A more extensive accumulation of gravelly sand forms the western subsidiary bar on the N. side of Cockle Bight Bar. The sediment forming this feature has been derived from Far Point as a result of N. and Y.W. storm activity which transports sediment southwards along the Ternery - Far Point gravel ridge and eventually washes it into Brancaster channel at this point. The configuration of this subsidiary bar, however, is genetically related to an ebb and flood channel system within Brancaster channel. Two distinct ebb and flood channel systems may be distinguished in Brancaster channel on the northern side of the Bar (Fig. 9.8), and are associated with the eastern and western subsidiary bars mentioned above (Figs. 9.13 and 9.11). Although the ebb channels, which, in both cases, lie to the N. of each subsidiary bar, are deeper than the corresponding flood channels, it is felt that this is the /Atult of prolonged ebb drainage (see station 3 , Fig. 9.40, section 9.9), and does not in fact reflect the direction of the residual sediment movement along the N. and N.E. side of Cockle Bight Bar.

A OI fl Bef 672 ZL6

671

670

111 -- 669 • -0'8 -0'6 -0'4 -0'2. 0 O'Z 0.4 0'6 0'8 1'0 1'2 /A- /.6 1'8 2.0 2:2 2:4 2:6 GI RAIN-512.E IN P141 V N ITS

Fig. 9.12. Frequency curves of the composite samples collected on the western subsidiary bar. 527

9.7.1.1 The Western Subsidiary Bar 9.7.1.1

On the evidence of wide spread flood-formed mega- ripples, the flood current, concentrated in the channel on the southern side of the western subsidiary bar (Fig. 9.11), appears to be dominant. Samples collected from the surface of this feature show a decrease in mean grainsize and an improvement in sorting from S.W. to N.B. The frequency curves of samples 669 to 671 (Fig. 9.12) indicate that this decrease in grainsize has been produced by a relative increase in the abundance of a fine mode of approximately 2.05 phi size. As shown by comparing figures 9.11 and 9.9, these sediments occur in the zone dominated by the flood current. Sample 672, however, collected at the N.E. end of the feature in a small zone influenced by the ebb current, is characterised by a distinctly different distribution of grainsizes than are samples 669, 670 and 671. Sample 672 is probably largely if not entirelyj composed of sediment derived from the E. rather than from the N. The general decrease in grainsize towards the N.E. supports the evidence of flood-formed ripples, and suggests that the coarse material, deposited in Brancaster channel from Far Point, moves dominantly northwards into the Harbour.

9.7.1.2 The Eastern Subsidiary Bar 9.7.1.2

The eastern ebb-flood channel system encloses the eastern subsidiary bar (Fig. 9.13), which may possibly be related to sedimentation by the flood current in the lee (i.e. to the E.) of a small mussel patch which has, Fie. 0 .1 7 ''Tnrth side of floc'-1e Bit -RPT. 10cotior of ti' er,stern subsidi9ry -her. The directiors and. the orientation nf t lerire and smell Prrow.7 /7.2eotivel7 ecuels I mile.

A 529 comparatively recently, grown into Brancaster channel from Cockle Bight Marsh. Samples 627 to 637 (Fig. 9.14) have been collected over this bar; their grainsize parameter values are shown in Table 9.10, and the frequency curves of their grainsize distributions are presented in Fig. 9.15. A regular variation of decreasing mean grainsize is apparent in Table 9.10. This trend is well shown in the progressive decrease in the peak modal size of the unimodal frequency distributions in Fig. 9.15. By equating the trend of decreasing mean and modal size to decreasing current energy conditions, an anticlockwise sediment circulation (shown in Fig. 9.14) may be proposed. This trend is associated with a process of fractionation by which fine grades are preferentially transported towards the protected (northern) corner of the feature. As the grainsize decreases, a tendency exists for the sorting to improve (see Fig. 9.16). The bimodality of sample 630, which was collected from the centre of the Bar, may possibly reflect the recirculation and remixing of the fine grades (shown in sample 627), with the newly introduced, coarser grained, and unfrac- tionated sediment. As in the case of the western subsidiary bar discussed above, a separate, small ebb- dominated zone may be distinguished - on the basis of ripple orientation - from a larger zone dominated by the flood current (see Fig. 9.14). The latter occurs over the western portion of the bar and is separated by a small ridge from the former. Unlike the western sub- FIGURE 9.14.

•627 ‘,;A- • A t\ • 628 630• • Nk. 637 62q t 632 P 633.` • 634 ki`t • al 635 ec" I •

4:-J a==1===1 636

--11" MEGR-RIPPLE ORIENTRTION

Pig. 9.14. Sampling positions on the eastern subsidiary bar. The ridge crest separates a zone influenced by the flood current in the N.W., from a zone influenced by the ebb current in the S.E. The proposed direction of sediment circulation is indicated by a broken arrow. FIGURE 9.15

627

631

632

633

635

636

634

629

628

630

657 1.. 1.4 1.6 1.8 20 2.2 24 2.6 2.8 30 GRAINS1ZE IN P141 UNITS Fig. 9.15 Frequency curves of samples collected on the eastern subsidiary bar.

FIGURE 9.16.

PHI 0 0.42 040 0.38

0.36 0 0 0.34 0 0 0.32 0

0.30 0 0 028 0 026 0 0.24 0

Mz Fai F9 2.0 2:1 2.2 PHI UNITS

—0.04 0 0 0 —0.06 0 0 0 —0.08 Sk,. 0.10 0 0 —0.12 0 0 —0.14

Fig. 9.16 Relationship of both standard deviation (6 ) and skewness, (Sic ), to the .1 1 mean grainsize (M2) of samples collected on the eastern subsidiary bar. 534 sidiary bar, however, no discontinuity exists in the character of the sediments on either side of this ridge. The sediments deposited on this feature comprise a small "closed" fractionation system into which sediment is apparently introduced from the W. by the flood current. It is perhaps significant that the coarsest grained sample occurring on this feature (sample 637), displays a modal size almost identical to that existing in samples 669, 670 and 671 located further to the W. on the western subsidiary bar. The ebb current, however, winnows and concentrates the finer grades of the existing sediment rather than introduces new material from the S.

9.7.2 Sediment Movement over Cockle Bight Bar, 9.7.2 (excluding the subsidiary bars)

The wide spread development of ebb-formed mega ripples over the surface of Cockle Bight Bar, implies a dominant seaward migration of sediment in a north westward direction across the top of this feature. The narrow zone of flood-formed mega-ripples and the adjacent zone of scoured pebbly-sand (Fig. 9.8) on the Y. side of the Bar, supports the proposed dominance of this flood current, and the subsequent eastward residual movement of sediment, in Brancaster channel around the N. side of this feature. Deposition at the boundary separating these two zones of semi-opposing directions of sediment transport has resulted in the formation of an approximately S.E. trending ridge. The N.E. facing scarp slope of this feature delineates the northern boundary of the 535 Bar. Additional indications of the current configuration and the transport paths of sediment, are supplied as a result of measurements made at stations 3, 4 and 5 (discussed in section 9.9) at the western end, eastern end, and on the southern side of the Bar respectively. It is tentatively proposed that, owing to its independent method of growth, the large mussel bank at the western end of the Bar is the key structural unit in producing the configuration of Cockle Bight Bar. Field observations indicate that the top of this mussel bank rises approximately 4 feet above the W. end of Cockle Drain, which is itself almost 3 feet above L.W.O.S.T. level, as measured at Station 3 opposite Far Point. The mussel bank subsequently acts as a partial barrier, or dam, to the water movements into, and out of the inlet during the low half of the tidal range. In the vicinity of this mussel bank the tidal stream bifurcates, flowing in Cockle Drain on the southern side of the Bar, and in Brancaster Harbour Channel around the N. side of the Bar. Since, however, the latter is deeper than the former, the tidal stream is restricted to Brancaster Channel during the lowest levels of the tidal range.

9.7.2.1 Brancaster Channel 9,7.2.1

As the tide ebbs, water is impounded over Cockle Bight Bar behind the mussel bank. With the falling water level an anomalous hydraulic gradient develops between the water flowing unrestrictedly out of the lower reaches of Brancaster Harbour, and the water mass 536 impounded behind the Cockle Bight Bar mussel bank (see further discussion in section 9.9). The resulting high current velocities coincide with the lowest stages of the tidal range when Cockle Drain has almost ceased to function, and the water dammed back over the Bar subsequently drains northwestwards and northwards into Brancaster Channel (see Fig. 9.45). These high current velocities, and high rates of sediment transport, are revealed in the measurements taken at Station 4 on the S.E. end of the scarp ridge shown in Fig. 9.41, and occur over an interval of less than 2 hours, as the last 5 feet of water drains off the surface of the Bar. This abnormal water gradient, and the subsequent high current velocity will be less apparent within Brancaster Channel than on the surface of the Bar. By comparing the concentration of suspended sediment at Station 3 and 4, it is apparent that peak sediment transport across the surface of the Bar during the ebb lags about one hour behind the time of peak sediment transport at Station 3An Brancaster Channel. It seems probable, therefore, that sedimentlransported across the top of the Bar will, in part, be deposited under somewhat lower energy conditions in Brancaster Channel and on the slope of the N.E. facing scarp. As shown by measurements at Station 3, in Brancaster Channel, at the W. end of the Bar (Fig. 9.40), the strongest current velocities occur during the flood. The dominance of this current is further supported by 537 the orientation of mega ripples in Brancaster Channel to the N. and N.E. of Cockle Bight Bar, and also by progressive variations in the mean grainsize of sediments occurring on the western and eastern subsidiary bars. Peak velocities of the flood current are achieved during the early stages of the flooding tide, prior to the submergence of the mussel bank, when the main water movements in the inlet are restricted to Brancaster Channel (see Station 3, section 9.9), and at a slightly later stage to Cockle Drain (see Station 5, section 9.9). Dominant sediment movement is, therefore, restricted to Brancaster Channel on the N. of Cockle Bight Bar, and sediment previously deposited in this zone by the ebb current is subsequently mobilised and recirculated in a clockwise direction by the flood current (see Fig. 9.9). As the rising tide submerges the S.E. striking ridge on the Y.E. side of Cockle Bight Bar, the abnormal hydraulic gradient operates in reverse and water floods southwards across the surface of the Bar. High current velocities and a high rate of sediment Transport are achieved for approximately one hour, during which time the tide rises about 2 feet above the ridge crest.

Q,7.2,2 Cockle Drain 9.7.2.2

On the southern side of Cockle Bight Bar, Cockle Drain acts as a combined ebb and flood channel. Two patches of ebb-formed mega ripples (Figs. 9.8 and 9.9), at either end of this channel, together with an intervening zone of scour, suggest that the ebb current is con- FIGURE 9.17

Fig. 9.17 Showing the embayment in the eastern end of Cockle Bight Bar, in which the scouring of the ebb current has exposed Boulder Clay Deposits (B.C.). Sediment deposited by the flood current has formed a small deltaic deposit at the eastern end of Cockle Drain (D.D.). Also shown are ebb-formed meg-ripples (M.R.), the back- beach sub-environment of the Brancaster Golf Course Beach (B.G.C.B.), and the location of Far Point (F.P.). (Photograph by K.S.J. May 1960).

Scale: 12 inches approximately equals 1 mile. 539 centrated along the southern side of the Drain, immediately adjacent to Brancaster Golf Course Beach. The "deltaic" deposit of coarse sand and shingle at the eastern end of the Drain (Fig. 9.17), together with the existence of ebb mega-ripples which have been strongly modified by the flood current (Fig. 9.42), reflect the importance of the flood current in a zone slightly to the N. of that occupied by the ebb in Cockle Drain (see Fig. 9.8). For some distance before the point where the ebb current enters Cockle Drain it flows over a scoured surface (Fig. 9.17). Cockle Drain itself has either a scoured bed or, as occurs at its eastern end, deposits of gravelly sand, originally derived from the back-beach zone of Brancaster Golf Course Beach. Although the velocity and the potential current capacity is high, the actual concentration of sediment transported by the ebb current is probably quite low, owing to a deficiency in the available source sediment. The understaurated capacity of this current is probably largely responsible for its erosion of Cockle Drain and the ambayment at the eastern end of the Bar. Although the sand flats at the W. end of Cockel Drain potentially afford ample supply of sediment capable of being transported eastwards into this channel,by the flood current, the presence of ebb orientated mega ripples over their surface suggest a reverse direction of residual sediment transport (see Pig. 9.8). Direct measurements 546 of sand in suspension at Station 3 (Fig. 9.40) show a surprisingly low concentration of sediment in suspension even at peak flood velocity. These indications imply a comparatively low rate of sediment transport by the flood current in Cockle Drain. Direct measurements of sediments transport at Station 5 in Cockle Drain (Fig. 9.43), indicates a local d, minance of the flood current. How- ever, this station is situated on the N. side of the Drain (see Fig. 9.8), where strong flood current has beeh predicted on the basis of physiographic evidence. Although direct supporting evidence is somewhat obscure or lacking, it seems probable that the ebb and flood current action are approximately balanced over the southern part of Cockle Bight Bar. Water movements in Cockle Drain appear to be dominantly engaged in erosion and scouring as opposed to those in Brancaster Channel, on the norther side of the Bar, which produce depositional features such as the eastern and western subsidiary bars. The currents flowing in Cockle Drain are undersaturated, while those in Brancaster Channel are over-saturated as regards sediment load. In conclusion, it seems apparent that a. clockwise direction of sediment migration exists over Cockle Bight Bar during the lower half of the tidal range. Residual movement in a N.W. direction across the northern half of the Bar is counterbalanced by S.W. residual movement around the N.E. flank of the Bar. The S.E. striking ridge along the northern side of Cockle Bight Bar represents a. temporary terminal for this mobile sediment. X3253a6 327 324 • 328 344.• 322 321, • .604 321 343 • .338 337 331 •31q • 8/6 824 •763,764 *318 850• 333 O 336 • 332 (.311 • 842 • *335 315 340• • 312 .316 •313

Fig. 9.18 Sampling positions over Cockle Bight Bar (excluding samples collected on the eastern subsidiary bar) 542 Table 9.11 Grainsize parameter values of samples collected on Cockle Bight Bar Table 9.11

Mean Standard Sample Grainsize Deviation incewness Kurtosis (Mzln phi) ( 1 in phi) (al) (Kg) 824 0.60 0.31 +0.26 - 318 1.12 0.47 +0.32 1.58 319 1.39 0.43 +0.11 0.98 320 1.15 0.63 -0.31 1.41 321 1.23 0.72 -0.32 1.46 322 1.59 0.38 +0.04 0.96 324 1.64 0.60 -0.30 1.26 325 1.60 0.66 -0.34 2.00 326 1.31 0.47 +0.09 1.33 669 0.79 1.04 -0.18 - 670 0.81 1.07 +0.09 - 671 1.16 0.99 -0.08 - 672 1.30 0.47 -0.10 - 327 1.82 0.30 -0.03 0.99 328 1.54 0.50 -0.14 1.11 604 1.78 0.36 -0.09 - 329 1.94 0.31 -0.13 1.00 330 1.79 0.34 +0.03 1.08 826 1.92 0.30 -0.03 - 331 2.03 0.29 -0.03 0.93 763 1.84 0.28 +0.01 - 764 1.73 0.31 -0.05 - 332 1.98 0.34 +0.02 0.89 333 1.67 0.36 +0.14 0.99 335 1.77 0.34 0 0.95 336 1.65 0.36 -0.03 .1.04 842 1.50 0.40 +0.01 - 850 1.72 0.26 +0.02 - 337 1.62 0.31 0 1.01 338 1.73 0.36 +0.06 1.16 343 1.63 0.32 -0.16 1.08 344 1.73 0.40 -0.19 1.21 34.0 1.97 0.26 0 0.95 315 1.60 0.42 -0.10 0.92 316 1.89 0.32 -0.01 1.10 311 1.73 0.38 +0.01 1.06 312 1.91 0.32 -0.09 1.23 313 2.04. 0.33 -0.07 0.93 FIGURE 9.19.

Fig. 9.19 Configuration and topography of the Norton Creek Bar showing dominant zones -of ebb .1 and flood. current activity. 'a.

2r FPrt of Norton CrePL- pnrr11,r-- nr C" "L- Tr!ra f I/11P 1.'1^ e•ws, ,ft,olor Arrnwe,. (r1,017)7T.Pn} 1,-, 7 . r•rr.‘ 545

9.7.3 Norton Creek Bar 9.7.3

Norton Creek Bar is an elongated mass of clean, fine to medium grained sand situated in the middle of Norton Creek. It is bounded on either side by small low-tidal drainage channels. This bar is generally highest at its N.E. end, in its centre, and along the central part of its southern side (Fig. 9.19 and 9.20). Its configuration has altered slightly in the interval of time between the photographic survey (Fig. 9.20) and the subsequent sampling survey carried out in 1965 (Fig. 9.26). Flood formed mega ripples are present over most of the feature (except in a broad embayment towards its eastern end), and possibly reflect a general dominance of the flood current flowing to the E., over the reverse ebb current. The presence of large, especially well developed, flood mega ripples on the N. and W. sides of this feature (Fig. 9.20), together with the local configuration of Norton Creek (Fig. 9.21), further suggest that strong flood current velocities dominate these parts of the Bar, while the southern side of the Bar is subject to relatively lower flood current velocities. During the ebb, however, the general configuration of Norton Creek to the E. of the Bar causes the ebb current to be concentrated over the southern side of the feature. The influence of the ebb current is seen at the E. end of the Bar in the broad embayment which is an apparent erosional feature. Samples 731 to 745 and 750 to 762 have been FIGURE 9.21.

Brancaster Marsh

Fig. 9.21. Configuration of Norton Creek in the vicinity of Norton Creek Bar, showing the zones of strongest ebb and flood current activity. The marsh edges are denoted by a thin continuous line and the channel bed is denoted by a dotted line. The map has been drawn from areal photographs taken in June 1960 by Huntings Surveys Ltd. ,Approximate scale: 6 inches equals 1 mile. 547 collected, on a grid system, over the surface of the Bar (Fig. 9.26). On the basis of grainsize characteristics, three sedimentary sub-areas have been delineated, sub- areas A, B and C (Fig. 9.26). The grainsize parameters kof these samples are tabulated in Table 9.14, and their grainsize distributions are illustrated in the form of frequency curves in Figs. 9.27, 9.28 and 9.29. Owing to the bimodality in a large number of these samples, the grainsize parameters are less useful than the frequency curves in distinguishing these sediments. The grainsize parameter values of samples 731, 733, 734, 735, 736, 737, 738, 743 and 744, shown in Table 9.14, indicate sediments which are typically coarse grained (average mean grainsize of 1.71 phi); poorly sorted (average standard deviation of 0.41 phi), and only slightly negatively skewed (average skewness value of -0.06). The frequency curves of these samples (shown in Fig. 9.27), are characteristically bimodal. The fine and coarse modes are rather irregular in size ranging between 1.9 phi and 2.1 phi in the case of the former, and between 1.5 phi and 1.7 phi in the latter case. These sediments occur over the N.W. part of the Bar and typify sub-area A. (Sample 475 has been excluded from the following discussion as it was collected in the drainage channel on the N. side of the Bar, and is, therefore, only partially characteristic of the Bar sediments). Samples 732, 739, 740, 741, 742, 743, 757, 758, FIGURE 9.22

PHI 0 A 0'46

0.44 0

0.42. 0 0 0 0 040 0.32 • 036 • 0.34 . ♦ 00 0 0.32 + + 0.30 o•zs • 0.26 • M. I4 1.5 1.6 1.7 1.8 1.9 20 21 2.2 23 PHI UNITS B +0.05 5k1. 0

—0.05 . • —0.10 O 0 —0.15 . . —0.20 .

Fig. 9.22 Relationship of both standard deviation, (al), and skewness, (Ski), to the mean grainsize, (Mz), of samples collected in sub-areas A(o), 13(0) and C(+) of the Norton Creek Bar. 549 759, 760 and 761 are fine grained (average mean grainsize of 2.09 phi), moderately well sorted (average standard deviation of 0.32 phi) and strongly negatively skewed (average skewness value of -0.15). Frequency curves of the grainsize distribution are, except in the case of sample 743, unimodal with a peak value ranging between 2.1 and 2.3 phi (Fig. 9.20. These samples occur over the central and S.W. side of the Bar and comprise sedimentary sub-area B. Samples 750 to 756 and 762, which are also typically unimodal with an average modal value of 1.85 phi, have mean grainsizes slightly finer than those of sub- area A (average mean size of 1.8 phi), and their sorting coefficients are approximately intermediate between those of sub-areas A and B (average standard deviation of 0.34 phi). The average skewness value of -0.036 indicates a somewhat more symmetrical grainsize distribution than that of sediments in sub-area A. These sediments occur over the E. end of the Bar and characterise sub- area 0 (see Fig. 9.29). The relationship between mean grainsize and skewness is shown in Fig. 9.22B, and indicates a trend of more negative skewness with decreasing grainsize. Except in the case of samples collected in sub-area A, this trend is not shown by the samples gathered from the separate sub-areas (which are indeated by different symbols in this figure). The relationship between standard deviation and mcia grainsize is also illustrated FIGURE 9.23.A

Fig. 9.23A. Illustrating the concept of mixing. whereby two components (thin line), are, combined. to produce a bimodal composite sediment (thick line). N.B. In this particular case, the peak of the fine grained mode in the composite sediment is slightly coarser than the modal size of the fine grained component. 551 in Fig. 9.22A, and shows a well defined trend of improved sorting with decreasing mean grainsize.

9.7.3.1 Theoretical Considerations as to the 9.7.3.1 Genesis of the Sediments of the Norton Creek Bar.

Theory 1. Mixing It may be demonstrated theoretically that, under specific conditions, a composite bimodal grainsize distribution may be produced by mixing two separate unimodal components. However, the separate grainsize distributions must be sufficiently different (preferably to such a degree that the modal grain size of at least one of the components (e.g. the fine component in Fig. 9.23A) is unrepresented in the grainsize distribution of the other component), for a bimodal composite distribution to result. If, on the other hand, the modal size of each original component is significantly represented in the grainsize distribution of the other, mixing will usually result in the creation of a unimodal composite grainsize distribu- ti,,7,n (Fig. 9.23B). As a consequence of mixing two components, the composite sediment produced in the case of model la, b and c (Fig. 9.23A), shows a slightly coarser grained fine mode than that occurring in the original fine grained component. The degree of bimodality will be most apparent when the components are mixed in equal quantities (model 1a, Fig. 9.23A), and decreases in significance as one component decreases in proportion to the other. The skewness value will become progressively FIGURE 9.23 B .

Fig. 9.23B Illustrating the concept of mixing whereby two separate components ,(thin lines), are combined in. various proportions to produce a unimodal composite sediment, (thick line). 553 more negative as the coarse component decreases, and as the fine component increases in abundance. The concept of mixing illustrated in Fig. 9.23A affords a potentially acceptable explanation for the bimodality of the sediments in sub-area A. Fine grained sediment, similar to that occurring in sub-area B, has apparently been mixed with a sediment with an indistinct coarse grained mode (possibly similar to that of samples 736 and 745 (see Fig. 9.27)). The fine mode occurring in these "composite" bimodal sediments is consequently somewhat coarser grained than the average mode occurring in the original fine grained component (compare the modal sizes displayed in samples of sub-area. A and B, Figs. 9.27 and 9.28). The concept of mixing in Fig. 9.23B (model 2a, b and c), is also a possible mode of genesis for the unimodal sediments occurring in sub-area C (discussed in more detail below). In order to investigate this theory further, a. number of samples have been selected from sub-areas A and B, so as to form a gradational series in which the rn.portions of the fine mode increase at the expense of the coarse mode. Samples 736, 735, 731, 733, 734 and 759 have been selected; their frequency curves are shown in Fig. 9.24, and their grainsize parameter values are shown in Table 9.12. The coarse mode occurs dominantly in sample 736 and the fine mode occurs dominantly in sample 759. FIGURE 9.24.

759

743

734

733

731

735

736 0-8 (0 Na 14 1.6 1.8 20 Z2 2.4 2.6 a ciRMNSIZE IN PHI urns

Fig. 9.24 Frequency curves of samples selected (from all of those samples collected on the Norton Creek Bar), to show a progressive variation in their modal distribution; decreasing abundance of the coarse mode associated with an increasing abundance of the fine mode. 555 Table 9.12 Changes in the grainsize parameter values of selected samples with increasing relative proportion of the fine to the coarse grades Table 9.12 Mean Sample Grainsize Sorting Skewness Trend (MzIn phi) ( 1 in phi) (Sk) 736 1.58 0.47 -0.01 735 1.67 0.41 -0.04 731 1.79 0.36 -0.05 733 1.82 0.39 -0.05 734 1.87 0.39 -0.12 743 1.93 0.37 -0.18 742 1.98 0.39 -0.09 759 2.11 0.33 -0.13

The fine mode displayed in the bimodal grainsize distributions of samples 735, 731, 733, 738, 734 and 744 of sub-area A, and 743 of sub-area B, is similar, although slightly coarser than that occuring in sub-area B (Fig. 9.28). This trend is compatable to that resulting from the concept of mixing proposed above (see Fig. 9.23A, model la, b and c). The observed relationship of decreasing mean grainsize associated with more negatively skewed grainsize distributions, is also compatible with the relationship derived as a result of this mechanism of mixing. Although the samples mentioned above, and shown in Fig. 9.24, have been specifically selected to show a relative increase in the proportions of the fine mode in relation to the coarse mode, the trends shown in Table 9.12 by the grainsize parameter values of these sediments, are similar to the trends of improved sorting FIGURE 9.25.

iparen,t-- seat tient

FLOOD InternedjaXC CURRENT plus int grained_ fractions B

coRR5 E MODE FINE MODE

GRCLINSIZE 'DECREASING

Fig. 9.25 Illustrating a concept of fractionation and remixing whereby an original parent sediment (A) is ,subjected to strong flood current action which removes the intermediate and fine grained fractions 0:9, leaving a coarse grained residual (C). The weaker ebb current returns the fine fraction (D), which is subsequently re-mixed with the coarse grained residual to form a bimodal sedimentary deposit (E).• 557 and more negative skewness associated with decreasing mean grainsize shown in Fig. 9.22 by all the samples collected over the Norton Creek Bar. These selected samples may, in this respect, be considered as representative of the range of sediments occurring over the surface of the Bar.

Theory 2. Fractionation Alternatively, variations in grainsize may be produced by a mechanism of sorting or fractionation (Doeglas 1946). In the case of a bimodal grainsize distribution, however, it is difficult to visualise a mechanism by which an intermediate grade may be preferentially removed leaving both coarser and finer grades to produce the bimodality observed in the sediments of sub-area A. One such special case has been proposed by R. Cloet (personal communication), (see also Inman 1949), and may conceivably apply genetically to the samples in sub-area A. Cloet suggests that under unequal tidal current velocities, the stronger flow may winnow and transport an intermediate, as well as a fine grade, while the weaker current velocity flowing in thecpposite direction may only be capable of returning the fine grade. (This concept is illustrated in Fig. 9.25). A residual removal of the intermediate grained sediment, but an overall dynamic stability of the fine grade, consequently results. Hypothetically, this fine grained grade may be equated with the fine mode occurring in sub-areas A and B, while the intermediate grade removed 558 preferentially, would be approximately equivalent in size to 1.8 phi. The coarse grained grade is equivalent to the coarse grained sand in sample 745 (Fig. 9.27) found in the drainage channel at the side of the Bar. In this model, the iegree of bimodality in a sample is increased as the intermediate grade is progressively removed. Although this theory does not satisfactorily account for the general increase in the coarseness of the fine mode in sub-area A, as compared with that in sub-area B, a mechanism of residual accumulation of a fine fraction with each tidal cycle may be relevant to the concept of mixing, proposed above, in relation to the sediments comprising sub-area A. This tentative proposal is discussed below. It is believed that the fine grained, well sorted and negatively skewed samples occurring in sub-area B reflect the product of a normal process of progressive fractionation whereby fine grained sediments are moved to a low energy environment (Doeglas 1946). The medium grained sediments of sub-area C may have also originated as a. result of fractionation, i.e. by the removal of the finer grades. However, the process of mixing illustrated in Fig. 9.23B may be equally applicable in this case, since the modal and mean grainsizes of these sediments lies between those of sub-areas A and B. The product of both mechanisms will, however, create increasing positive skewness values as the grainsize distribution coarsens (see section 10.6). If a fractionation genesis 559 (as opposed to a mixing genesis) is accepted for the sediments of sub-area B, it seems reasonable to assume that, where a sediment circulation is thought to occur (as in the case of the Norton Creek Bar), the coarse product of such mechanism will also exist locally. Fractionation is, therefore, tentatively proposed as the dominent mode of origin of the sediments of sub-area C, although the bimodality displayed in sample 753 implies a limited degree of mixing to have also taken place. Processes of fractionation are considered to be mainly responsible for the grainsize distributions of sub-areas B and C, while in sub-area A the effect of fractionation if present is obscured by mixing, in varying proportions, of a fine with a coarse grained mode. Assuming that the mean grainsize of a sedimentary deposit reflects its relative energy level, it is apparent from Table 9.13 that the sediments of sub- area A were deposited under slightly higher current velocities than sub-area C, while sub-area B indicates the existence of considerably lower en-rgy conditions than in either sub-area A or C. In view of the above discussion, a tentative genetic relationship between the grainsize characteristics of the sediment of Norton Creek Bar, and the local dynamics conditions may be proposed. The sediments of sub-area A are believed to be introduced by the flood current and to accumulate along 56n Table 9.13 Comparison of the average grainsize parameter values of samples collected in sub-areas A, B and C. Table 9.13 Average Average Sub-area Mean Standard Average Size (in phi) Deviation (in phi) Skewness

A 1.71 0.41 -0.06

B 1.80 0.34 -0.04

0 2.09 0.32 -0.15 the N. flank of the feature forming a train of eastward migrating mega ripples, thus indicating mass sediment movement. The constant addition of poorly sorted sediment to this sub-area largely prohibits the exhibition of the effects of fractionation and sorting in these samples. However, it seems probable that a proportion of fine material is winnowed out and redeposited under the lower energy conditions which probably exist over the centre and southern side of the bar (roughly corresponding to sub-area B) during the interval of flood current activity. As this coarse grained, poorly sorted sediment migrates eastwards over, and around the Y. side of the Bar, it becomes slightly better sorted and possibly slightly finer grained (sample 762, Fig. 9.29, Table 9.14). Part of this sediment is deposited at the eastern end of the Bar where it is influenced by the weakei ebb 561 current. The absence of mega ripples in this area suggests that the ebb current mainly separates a fine grained, well sorted fraction from the coarser grades, rather than causes mass sediment transport. This fine material is subsequently transported westwards over the surface of the Bar and is deposited in sub-areas B and A. In the former sub-area it is combined with the fine material previously deposited by the flood current. In sub-area A, however, its addition to the residual coarse grained poorly sorted sediment (originally introduced by the flood current), produces the bimodality which characterises the sediments occurring on the Y.W. side of the Bar. In conclusion, the Norton Creek Bar may be considered to have formed as a result of a. clockwise circulation of sediment. This is associated with a progressive decrease in grainsize produced by processes of fractionation which, in turn reflect variations in current energy. Comparison between photographic surveys carried out in 1955 by the Air Force and in 1961 by K.St.Joseph indicate that the Bar has slightly increased in size by accretion at its eastern end. This suggests that the dominant mass transport of sediment is dictated by the flood current, and is towards the E. 562 Table 9.14 Grainsize parameter values of samples collected on the Norton Creek Bar. (Categories A, B and C have been distinguished on the basis of grainsize characteristics) Table 9.14 Mean Standard Sample Grainsize Deviation Skewness (Mz in phi) (G) in phi) (Ski) A 745 1.38 0.41 +0.01 737 1.75 0.41 -0.13 736 1.58 0.47 -0.01 733 1.82 0.39 -0.05 738 1.70 0.44 -0.01 735 1.67 0.41 -0.04. 731 1.79 0.36 -0.05 734. 1.87 0.39 -0.12 744 1.82 0.42 -0.10 B 743 1.93 0.37 -0.18 742 1.98 0.34 -0.09 741 2.04 0.33 -0.16 732 2.26 0.26 -0.18 739 2.14 0.36 -0.21 740 2.05 0.33 -0.17 757 2.07 0.31 -0.15 758 2.23 0.28 -0.06 759 2.11 0.33 -0.13 760 2.12 0.30 -0.16 761 1.96 0.31 -0.16 C 753 1.77 0.39 -0.03 754 1.76 0.37 -0.07 751 1.90 0.33 -0.03 752 1.77 0.37 -0.08 750 1.73 0.38 0.04 755 1.86 0.32 0 756 1.78 0.28 0 762 1.80 0.32 +0.04 FIGURE 9-26.

• 750

• • 75Z 751 SUB-F1RER C. 753 • • 751- 755 761 762 * • • 758 760 759 • 7570 . 756 SUB-FIRER B. 74:5 • 745. 742 741 • 740

738 •

• 732. 734

731

Fig. 9.26. Sampling positions on the Norton Creek Bar, and the sedimentological sub-areas. FIGURE 9.27.

SUB-AREA A.

744

734

738

733

731

735

736

737

745 0.8 10 .12 14 1.6 IS 20 22 24 2& 28 GRRINSIZE IN PHI UNITS Fig. 9.27 Frequency curves of samples collected in sub-area A. on the Norton Creek Bar. FIGURE 9.28.

SUB -AREA B

"761

760 759

758

757

74-0

739

732

741

742

743 0.8 10 2 14 1.6 1.8 20 2.2 GRRINSIZE IN P141 UNITS

Fig. 9.26 Frequency curves of samples collected in sub-area B. on the Norton Creek Bar. FIGURE 9.29.

SUB-AREA C.

762

756

750

755

752

751

754

753

I I I 08 t0 1.2 NI. 1[6 [8 2.0 22 24 26 28 grumiSIZE IN PHI UNIT Fig. 9.29. Frequency curves of samples collected in sub-area C on the Norton Creek Bar.

FIGURE 9.30.

PNI PIN 2:2 ew4t. GREAT R5T'ER MARSH 0.0.61,1/4. 2.0 eu444 ""41s friz 1.8 773 O 16 779-6 . 775 • BANHAm . HARBOUR 776* . '7,8 / PHI • ....• 777* .- • • 0.5 773. do. .

• • • 7q0.• 0 4- • • • • •

a a 78Z 1 • • • • 780*. 03 . • - • . • • • - • • • • *781 02 . • . OVERY MARSH

• •

Fig. 9.30(a) Sampling positions on Overy Cockle Strand. Scale, 12 inches equals 1 mile. Arrows show the orientation of mega-ripples.

r O ) N

-NO N 0

Fig. 9.3000 Graphical presentation of the grainsize parameter values of samples collected along two traverses across Overy Cockle Strand. 568

9.7.4 Overy Cockle Strand 9.7.4

Overy Cockle Strand is situated at the E. end of Norton Creek, where it joins Burnham Harbour. This feature forms a large deposit of fine to mvdium grained, clean sand which abutts against Overy Marsh on its S. side, and slopes gently northwards into Norton Creek. Samples 773 to 791 have been collected along three N.S. traverses across the Strand (Fig. 9.30). Their grainsize parameter values are shown in Table 9.15. During the flood tide, water concentrated in Burnham Harbour channel diverges south eastwards into Overy Creek, and westwards into Norton Creek. The reverse water movements occur during the ebbing tide. Mega ripples formed in response to the flood current action, show a radial distribution around the inner end of the Burnham Harbour channel. Those ripples occurring on Overy Cockel Strand are best developed adjacent to Burnham Harbour where they show a general westward orientation (Fig. 9.30). Maximum current velocities are normally concentrated in the deepest part of a channel and decrease towards the sides. Current velocities, and, therefore, the energy level, will show a progressive decrease across the Strand from Norton Creek Channel southwards towards Overy Marsh. Overy Cockle Strand offers an opportunity to study variations in the grainsize distribution of sediments deposited as a result of a fairly uniformly lateral decrease in current energy which is 569 unrelated to a current, or a sediment circulation pattern. In the two main sampling traverses, the mean grain diameter shows a. progressive increase in size towards Norton Creek channel. The grainsize distributions show a trend in becoming more positively skewed as the grainsize increases; standard deviation values, however, show a more random variation in the sediments collected along the N.-S. traverse lines, although this parameter shows a general improvement from E. to W. across the Strand. Samples 773 and 787 were collected in the low tide drainage channel of Norton Creek, and are somewhat finer grained and more negatively skewed than expected from the general trend. These anomalies may possibly be attributed to remixing of additional fine grained sediment by the ebb current. The general relationships between decreasing grainsize, improved sorting, and more negative skewness (Fig. 9.31A) appears to be characteristic of many of the sediment mnulations of Scolt Head Island. The absence of bimodality in these sediments, possibly reflects the absence of mixing processes in the comparatively uncomplicated dynamic environment existing over the Strand. FIGURE 9.31. 0 A o•q-6 0 0.44

042 0 0 0 0.40 0

00 0

00 0.3+ 0 0 0 0 0.32 0 O 0 0.50 0 0 0.28

0 0.26

1.4 1.5 1.6 17 2.3 Mi .

+0.15 O 0 0 B t0•10

0 0 +005 0 0 O 0 O 0 o Sk, 0 0 0 O 0 -0-05

00 0 0 -0.10 0 co 0 0 - o•

-0.20

Fig. 9.31 Relationship of both standard deviation, (0-1), and skewness, (Sk1), to the mean grainsize (Mz), of samples collected on Overy Cockle Strand. FIGURE 9. 32 .

Fig. 9.32 Map showing the ebb and flood channel system at the eastern end of Brancaster Golf Course. The position of station 2 (at which current measurements were made) is also shown. Approximate scale: 1 inch equals 500 feet. Table 9.15 Grainsize parameter, values of samples collected on Overy Cockel Strand Table 9.15 Mean Standard Sample Grainsize Deviation Skewness (Mz in phi) (61 in phi) (Sk)

773 2.04 0.36 -0.10 774 1.68 0.39 +0.01 775 1.74 0.41 -0.06 776 1.82 0.33 +0.04 777 1.88 0.28 +0.05 778 2.03 0.26 +0.04 779 2.11 0.29 -0.08 780 2.20 0.30 -0.13 781 2.19 0.37 -0.24 782 2.18 0.29 -0.11 783 1.98 0.35 -0.06 784 1.67 0.37 -0.03 785 1.65 0.33 +0.13 786 1.52 0.41 +0.13 787 1.88 0.39 -0.05 768 1.39 0.46 +0.13 789 1.81 0.44 -0.01 790 1.82 0.40 +0.03 791 1.99 0.35 .-0.10

9.8 Mega. Ripples 9.8 A large spit-like deposit of medium grained sand is situated at the E. end of Brancaster Golf Course on the inside of the bend where Brancaster Harbour channel curves southwards into Mow Creek. Here a well developed ebb and flood channel system operates (as indicated by the orientation of mega ripples), to produce a sediment circulation (Fig. 9.32). Sediment transported by the flood current moves around the lower part of this sand mass, and is returned by the ebb current, via an ebb channel, at a. higher level. Samples 593 to 598 (Table 9.16), Fig. 9.33) have been collected FIGURE 9.33 .

, , , , , , , ,

598

597

3746

sqs

574

593 1.0 1.2 14 1.6 1.8 2 0 2.2 24 2.6 2-8 GRAIN SIZE IN P41 uNi-rs.

44 DOMINANT FLOOD CURRENT 596 597 5q5 598 594 sq

Fig. 9.33 Frequency curves of samples collected across a typical mega-ripple. 574 over a typical flood formed mega ripple with an amplitude and wave length of approximately 2' and 20' respectively.

Table 9.16 Mean grainsize, standard deviation and skewness values of samples 593 to 598 Table 9.16 Mean Standard Sample Grainsize Deviation Skewness (Mz in phi) (3'.1 in phi) (Sk)

593 1.91 0.33 -0.05 594 1.87 0.35 +0.03 595 1.80 0.35 +0.03 596 1.58 0.31 +0.10 597 1.72 0.33 +0.09 598 1.86 0.31 +0.03

The flood current flows from sample 593 towards sample 598. These two samples occur in successive troughs and sample 596 is situated on the ripple crest. Mean grainsize progressively increases up the st oss slope (samples 593, 594, 595), but decreases from the crest to the toe down the lee slope (sample 596, 597, 598). Sorting varies randomly while the grainsize distributions are typically positively skewed. Frequency curves (shown in Fig. 9.33), show, except in the case of sample 596, a bimodal distribution. The coarse mode is dominant except in the case of sample 593, but shows an inconsistant peak size. The secondary mode, however, is of a more consistant size (2.15 phi), and in sample 593 (in the ripple trough) attains predominance. Variations in the bimodal grainsize distribution of these samples appear to reflect two separate processes of fractionation. The consistency of the fine 575 grained modal size suggests that fractionation produces changes in the relative abundance of this mode; while the inconsistency in the modal size of the coarse fraction suggests that the process of fractionation is manifested by significant changes in the actual grainsize of this mode (as well as probable changes in its relative abundance). The grain size distribution becomes both coarser and more positively skewed, as the modal size of the coarse fraction increases in size, and as the relative proportion of the fine grained mode decreases in abundance. Samples 1 to 10 have been collected across a second broad ebb formed mega ripple, with a wave length of about 30 feet and an amplitude of 1.75 feet. The grainsize parameter values of these samples (Table 9.17) show no constant variation; they do, however, corroborate the characteristic positive skewness of the sediments of these ripples.

Table 9.17 Grainsize parameter values of samples 1 to 10. Table 9.17 Mean Standard Sample Grainsize Deviation Skewness (in phi) (in phi)

1 1.76 0.31 +0.22 2 1.64 0.29 +0.09 3 1.62 0.29 +0.07 4 1.63 0.22 +0.09 5 1.72 0.25 +0.13 6 1.66 0.23 +0.09 7 1.71 0.24 +0.11 8 1.67 0.25 +0.06 9- 1.64 0.25 +0.10 10 1.69 0.27 +0.19 FIGURE 9.34.

Fig.9.34 A Position of samples collected across a broad, low mega-ripple.

DIRECTION OF RIPPLE RDVRNCE

Fig. 9.34 B Internal structure of part of a mega ripple showing the two lee slope laminal from which samples A to G have been taken _(Drawn from an impregnation). 577 The cross-sectional shape of this ripple is shown diagrammatically in Fig. 9.34. The stoss slope is slightly uneven, with a secondary peak developed between samples 3 and 4. This suggests that the ripple, although dominated by the ebb current, is somewhat modified by the flood current. The subsequent reworking of the original grainsize distribution accounts for the erratic variations in the values of the grainsize parameters. Further investigation of the distribution of grainsizes down the lee slope of a mega ripple was carried out by analysing various parts of two lee slope laminae (laminae 1 and 2), preserved in an impregnation of a meg-ripple (Fig. 9.34). The parameter values of these samples, shown in Table 9.18, show, in both cases, a regular trend of decreasing mean grainsize from ripple crest down the lee slope (i.e. from samples A to D and E to G).

Table 9.18 Grainsize parameter values of samples A to G collected at various positions on two laminae deposited on the lee slope of a mega ripple. Table 9.18 Mean Standard Samples Grainsize Deviation Skewness (in phi) (in phi) A 1.90 0.33 +0.17 crest B 1.93 0.36 +0.09 Lamina 1.99 0.31 +0.08 (1) D 1.98 0.35 -0.01 E 1.91 0.36 +0.04 crest Lamina F 1.95 0.33 +0.04 (2) G 1.98 0.31 +0.08 578 Again positive skewness values typify the sediments incorporated in this mega ripple, while the standard deviation equally characteristically displays a random variation. According to Allen (1965), grains are transported up the stoss slope of a sand wave as a bed load and are momentarily put into turbulent suspension as they are swept over its crest. After leaving the crest the distance a grain travels before it falls is determined mainly by its size: coarse grains accumulate near the crest, while fine grains fall further down the lee slope, or in the trough. This mechanism of dissociation satisfactorily explains the observed decrease in grainsize down the lee slope of the mega ripplesstudied in the field. However, Allen further observed that, in tank experiments, the rate of deposition of coarse sediment on the upper part of the lee slope exceeds that of fine grained sediment occurring on its lower half. This eventually produces a steep and unstable lee slope which causes sediment to avalanche from the crest into the trough. Depending upon the current velocity, this avalanching may be intermittent or continuous. Allen found that intermittent avalanching tended to reverse the original distribution of grainsize down the lee slope: coarse grains tended to move at a relatively higher rate than do the finer grains, and are thereby concentrated in the trough. Continuous avalanching, however, which was found to occur when current velocities exceeded 60 cm. per second, produced a random distribution of grainsizes. FIGURE 9.35.

A 036 xx X x 0.34- A x 0.3Z

0.30

0,28 xx 0.26

0.24

0.22

Mz 1.6 1.7 1.8 1.9 ZO P111 UNITS +0.24

+0.16

+0.12

+0.08

x x X >re

5k, o X

X -0.08

Fig.9.35 Relationship of both standard deviation, (r), and skewness (Sk1) , to the mean grainsize,(Mz), of samplescollected over,and within the mega-ripples. 580 In view of the observed trend of decreasing mean grainsize down the lee slope of the mega ripples (Tables 9.16 and 9.18), it is tentatively proposed that avalanching does not oecur in these cases. Apparently, the sand is distributed at an even rate over the lee face, thus preserving a stable slope contemporaneous with mega ripple migration. Allen noted that the fine grained material transported over the crest of a sand wave is either incorporated in the toe of the lee slope (i.e. in the trough), or is transported down stream out of the immediate sphere of influence of the sand wave. Fine sand lost in this way will presumably cause the sediment of the sand wave to progressively improve in sorting and to increase in mean grainsize. Allen observed this phenomenon to occur at progressively higher current velocities. In the case of the samples tabulated above, a. trend of improved sorting and increasing mean grainsize is faintly discernable (Fig. 9.35A). This trend is in general agreement with the apparent partial loss of the fine sand grades mentioned above. The faint trend shown in Fig. 9.35A is somewhat anomalous when compared with the general relationship of improved sorting associated with decreasing mean grainsize, displayed by both Beach and Tidal Inlet sediment populations. Furthermore, it is probable that the cumulative effect of this winnowing action produces the characteristic positive skewness of these sediments. The plot of mean grainsize against 5R1 skewness value does not, however, reveal any consistant relationship (Fig. 9.35B). The general proposal that positive skewness is produced by the partial removal of the finer grades is supported by the changes in skewness value associated with the fractionation process operating on the beach face. Under the dynamic conditions described in section 7.5, fine grained sand is concentrated on the fore-beach sub- environment while coarse grained material is localised in the back-beach sub-environlv,mt. The fine grained beach sediments are characteristically negatively skewed while the coarse material is more positively skewed (see Fig. 7.12A, section 7.8.3). Samples 35, 41 and 255, collected down a marsh creek (section 9.6), also display an increasingly more positive skewness value, associated apparently with the process of winnowing, together with a general increase in the energy level. A fractionation mechanism to explain this observed phenomenon is proposed in section 10.6.

9.9 Direct Measurements of Current Action and of Sand, 9., both in Suspension and on the Channel Bed.

The following sub-section describes a series of experiments which were designed to explore the changes in the character of the sand fraction of the sedimwnts occurring both on the channel bed, and in "suspension" just above the bed; and to relate these changes to variations in the local tidal current velocity of a FIGURE 9.36.

A TERNERY ~PO1 NT

COCKLE t`/ "1 61GHT

LON& 14ILLS

FRR `POINT .11 O 1

BEACH POINT

4'041.0N c„Ik•

D1RL I-4005E

Fig. 9.36 Location of stations 1 to 5 occupied during current measurements. Mega ripples on Cockle Bight Bar are indicated. The channel sand deposits are represented by stippling. 583 spring tidal cycle. Five stations (station 1, 2, 3, 4 and 5) wrze occupied at various points in the inlet (Fig. 9.36) for an interval of approximately 12 hours during the spring tidal cycle. During this series of experiments, current velocity and water depths were recorded, and at half hourly intervals bed samples and suspended samples were collected. The bed samples were obtained by using a small Van Veen type grab. Suspended samples were collected from about 3 inches above the channel bed using a Li-inch (internal diameter) hose pipe connected at the surface to a manual bilge pump (see Fig. 9.37). The lower end of the hose was attached to a heavy iron tripod which anchored the inlet at a constant distance above the bottom. At the surface the water containing the suspended sediment was pumped through a 4 phi mesh sieve which retained the sand fraction. Laboratory and field experiments show that a pumping rate equal to, or exceeding two gallons per minute (which is equivalent to an internal velocity in the hose pipe of approximately 30 cm. per second) provided a sufficiently high velocity in the hose pipe to raise coarse sand. The sand fraction collected by this method was apparently moving over the bottom, partly by saltation, and partly in suspension. Samples of 20 gallons were usually pumped; however, this volume was decreased when high current velocities increased the concentration of sand in suspension. This technique of suspended load sampling is based upon a method used by Watts (1953). FIGURE 9-37.

FINE MESH SIEVE RETAINING THE SAND FRACTION

WATER DISCARDED WATER PLUS SURFACE THE SILT FRACTION

3" INTERNRL iLRMETER -Z. PLASTIC HOSE

THUMB SCREW TO RD7UST NOZZEL HEIGHT

HEAVY METAL TRIPOD

ANCHOR

Fig. 9.37 Diagramatio sketch of the apparatus used to sample sand in suspension just above the bottom 585 A number of inherent limitations are apparent in the methods of sampling, and of grainsize analysis of both the sediment in suspension and on the channel bed. (1)Since the settling tube method of grainsize analysis, used in this study, required a minimum sample size of 5 grammes it was necessary to discard the silt and clay component of the sediment in order to collect a sufficient quantity of sand from suspension. Even so, in a. large number of instances, the small quantities of sand transported by the lower current velocities rendered this method of grainsize analysis impracticable. As discussed in section 2.6.5, the accuracy of the settling tube method of analysis depends, to a large degree, upon the consistency of the sample size (ideally samples of approximately 15 grammes were used). Comparison, as in this case, between the results of analyses of samples of widely different amounts will produce maximum error. An additional disadvantage (previously mentioned in section 9.6) lies in the analysis of only the sand fraction of a sediment comprising both fine and coarse material. Only a limited picture of the grainsize distribution of the sediment is thus produced. (2)Owing to the manual methods of pumping, the water containing the suspended material, the collection of a sample took as much as 10 minutes, during wlich time it is conceivable that the channel bed may have suffered local erosion or deposition. Consequently the distance above the bottom at which sampling took place would alter. 586 It was considered that such bottom changes would most likely occur in areas of mega ripples (i.e. stations 2, 4 and 5), however, Cornish (1901), who measured the rate of advance of such features, records velocities of less than 4 feet per day. Such rates of migration would produce negligible changes in the bottom topography over an interval of only 10 minutes. The possibility that such effects may be cumulative during a tidal cycle was overcome by repositioning the tripod after each sample. (3)A subsidiary disadvantage of this method of collecting suspended material was found to result from the summer increase in the plankton population in the water, which eventually blocked the fine mesh sieve and, in fact, caused the premature termination of this series of experiments.

(4)It is reasonable to assume that, between the channel bed and the moving water mass, an interchange of sediment takes place. The residual direction of sediment interchange, whether erosion (i.e. movement of material from the bed into suspension), or deposition (i.e. movement of sediment from suspension to the bed), depends upon the bottom current velocity, bottom current acceleration (Kestner 1961), concentration of sediment in suspension, and the grainsize distribution of the bottom sediment. The surface film of sediment on the channel bed will %. exhibit the effects of this interchange. Since a grab sample scoops out sediment down to a depth of about 2 inches, the characteristics of this surface film will 587 be largely obscured by the bulk of the underlying sand. As an afterthought, it would seem that this disadvantage may be overcome by the use of greased cards in a similar manner to that employed by Ingle (1966) and others, in their fluorescent sand tracing experiments. In this series of experiments, an Ono recording type current meter was used to measure the tidal current velocities as near to the channel bottom as possible. Unfortunately, the tidal records from stations 1 and 5 (Tables 9.19 and 9.23) had to be measured on two separate days and subsequently combined. Slight variations in the position at which bottom samples were collected on these different days may contribute, to an unknown degree, towards producing the differences between the grainsize characteristics of the samples collected during the flood and ebb tide at these stations. In addition, at station 1, the range of the tides on the respective days differed by 12 feet (see Table 9.19). It is prcb- able that slightly higher current velocities will accompany the increased tidal range. In the graphical presentation of the results, sampling times, tidal ranges, current velocities (in knots), and suspended sample concentrations (in grammes per gallon) are related to the time of local high water as predicted in the Admiralty Tidal Handbook. Local high water at Scolt Head Island was determined by adding 4.0 minutes to the time of high water at Immingham. The grainsize parameter values of those suspended 588 samples collected in sufficient quantity to analyse are shown in Tables 9.19, 9.20, 9.22 and 9.23. The parameter values of the bottom samples collected at each station are shown in Table 9.26. In addition, the grainsize distributions of the suspended samples are represented as frequency curves in Figs. 9.54, 9.55 and 9.56). Each individual station is discussed separately in conjunction with the relevant figures and tables. The combined results and their implications are discussed later.

Station 1. Station 1 occupies a position in a stretch of Mow Creek (see Fig. 9.36), which is apparently unaffected by current circulations. The tidal regime may, therefore, by assumed to be fairly representative of normal, and uncomplicated inlet conditions. Ebb tide records were collected in the bottom of the channel. During the flood tide, however, the station position was moved slightly to the side of the channel bottom, which accounts for the finer grain size of the bottom sediments gathered at this time (see Table 9.16). The channel bed in the vicinity of station 1 is composed of partly rippled medium to fine grained, clean sand. As indicated in Fig. 9.38, this observation is considerably modified (if not negated) by an increase of 11 feet in the tidal range during the time the flood current measurements were recorded. This higher tidal range will naturally be associated with higher current velocities. This factor may possibly be the cause of the dominance of the

FIGURE 9.38, STATION 1.

6 FEET N I 4 EPTH

D 2 R TIDAL RANGE I 1 I t

WATE 0 om—.1-

ca zcc z • — 0.2. Z Z O ul Z cc La DC 5 01 Z Z CONCENTRATION

FLOOD CURRENT

TS FLOWING NO HOURS AFTER HIGH WRYER CURRENT S. VELOCITY + I +2 +3 +5 IN K 0 II 1 I EBB CURRENT ITY -6 -5 -4 -3 -2 HW FLowiNG, N • OC HOURS BEFORE HIGH WATER L E V 1

In .4- In.9 t- trri in In tn 1 t tt i SUSPENDED SAMPLES Iltli

0 ol K1 4 N cr o 3 0 0 0 00 0 o 0 00 o0 03 c:0 CP ea co 03 00 03 1 I 1 I I BOTTOM „SAMPLES II I I I

5t3 Table 9.19 Grainsize parameter values, concentrations and current velocites of suspended samples collected at Station 1. Tidal Range: ebb 16', flood 20'. Station 1. Table 9.19

Suspended Mz 1 SK1 Concen- Current Sample (phi) (phi) tration velocity in in knots grammes per gallon 1 2 3 Ebb samples contained a. negligible quantity 4 of sand in suspension 5 6 52 2.45 - - 0.29 1.5 53 2.36 - - 0.44 1.65 54 2.34 - - 0.37 1.6 55 2.33 - - 0.37 1.5 56 2.34 - - 0.23 1.3 57 - - - 0.08 1.1 591 flood current at this station. The negligible quantities of sand that were transported by the ebb current may partly reflect this difference between the peak ebb and flood current velocities, but is most probably due to the change in the sampling position which took place between the ebb and flood current measurements. The coarser grained sediments (samples 800 to 804), collected from the bottom of the channel during the ebb current, may indicate a lag deposit which is possibly only mobilised as a bed load (if at all) under the observed ebb current velocites (see Table 9.26). It is significant that these possible lag sediments are more positively skewed than are the finer grained bottom sediments. It is proposed that the fine grained suspended sediment, collected during the flood tide, is genetically related to the ct,mparatively finer grained bottom sand (samples 805 to 809) which was sampled slightly to the side of the channel bottom at this time (see Table 9.19). By comparing the curves showing current velocity, tidal range and the concentration of sand in suspension (Fig. 9.38), it is apparent that the majority of this suspended sediment is mobilised during the first 12 hours of the tide, at the end of which time the water level has risen approximately 8 feet above the channel bed.

Station 2. Station 2 is situated on the lower part of the sand bar which has formed on the inner side of the channel 592 where it bends southwards around the E. end of Brancaster Golf Course (see Fig. 9.36). The dominance of the flood current at this location is indicated indirectly by the orientation of mega ripples (i.e. steep lee slope) facing southwards into the inlet, as well as by the direct current, and suspended load measurements (see Fig. 9.39). At this station an increase in the peak velocity from 1.7 knots during the ebb to 3.1 knots during the flood has increased the concentration of sediments in suspension by a factor of 23 times, from 0.14 grammes per gallon (suspended sample 9) to 3.24 grammes per gallon (suspended sample 15). The former sample was collected when the water was approximtely 10 feet deep, while in the case of the latter sample it was only 4 feet deep. Sediment was only transported in significant quantities during the first 11 hours of the flood tide (see samples 13 to 19, Fig. 9.39). These suspended samples show an average mean grainsize of 2.1 phi, an average standard deviation of 0.27 phi, and are practically unskewed (see Table 9.20). Their average modal size corresponds to approximately 2.15 phi, and is rather consistant (see Fig. 9.54). The tide ebbed at this station for almost 42 hours, and flooded for approximately 3 hours. In the intervening 41 hours this locality was exposed above water level. The measured time of high tide occurred about 1,4 hours later than the predicted time of high water. Peak ebb current velocities occurred approximately 3 hours after high water, and peak flood velocities occur approximately

FIGURE 9.39.

12.

10 I^ $ LU z6

L4 3.5

IA 2 3.0 TIDRL RRNGE 3 z t.) 2.5 0 <.7) z 0 _t 2:0 _ V) Z t. I • 5 2 cc z cc . 0 A z ta cr 1/4.7 3 0.5 0z U. Z t.) CONCENTRATION

2, /IN

I-J1 0 Z FLOOD CURRENT HOURS AFTER HIGH WATER CURRENT ft-owits4G S. I V 1 +2. +3 +4 +5 +6 VELOCITY I I 0z t i i I i I 1 I I I I I -J ‘ —6 —5 —4 —3 —2., —I HW \ EBB CURRENT i I rLowit4G N. HOURS BEFORE HIGH WATER I— zUl ... \ et 1

r- m a- O - (\__t tel÷in Jc r- -- I t SUSPENDED SAMPLES 7 7i 7 I T 7 1 t

4 c0 1/40 0 — rl t41 -4- In q) eb. 00 Cr orm — c.t c•L - c•.( in &I oo iii a i ro 0 z ;5 a O6 .63 c76 co to oo co 1 I III I 1 BOTTOM SAMPLES 3 1 III I I I 594 Table 9.20 Grainsize parameter values, concentrations and current velocities of suspended samples collected at station 2. Tidal range: ebb 18'611 , flood 21'8". Station 2 Table 9.20 Suspended Hz SK Concen- Current Sample (phi) (phi) tration velocity in • in knots grammes per Fallon 7 - - - 0.02 1.2 8 - - - 0.08 1.5 9 - - - 0.14 1.7 10 - - - 0.11 1.2 11 - - - 0.004 1.0 12 - - - 0.002 0.8 soeilm.teenallesearam 13 2.05 0.29 -0.01 ' 2.60 1.7 14. 2.11 0.27 -0.05 2.02 2.2 15 2.09 0.27 -0.01 3.24 2.8 16 2.12 0.26 -0.01 1.43 2.15 17 2.10 0.26 -0.01 1.43 2.00 18 2.10 0.27 +0.01 1.35 1.85 19 2.15 - - 0.23 1.6 20 - - - 0.02 1.2 21 - - - 0.003 0.7 595 1 hour after the tide begins to rise. The water depths at these times are 10 feet and 5 feet respectively. As shown by the tidal range curves in Fig. 9.39, greater velocity acceleration occurs during the flood, than during the ebb current. The mean grainsize and standard deviation parameter values of the bottom samples (Table 9.26), although essentially similar, are, on the average, slightly more coarse grained and better sorted during the flood than during the ebb.

Station 3. This station is situated in the deep depression in the channel bed that occurs to the W. of Cockle Bight Bar, in the constriction of Brancaster Harbour channel (Fig. 9.36). As previously described, this depression has been produced by the southward migration of the gravel of the Ternery - Far Point gravel ridge into Brancaster Harbour. Because of the high current velocities at this locality (see Fig. 9.4.0), bottom scouring and erosion has exposed a lag deposit of gravel on the channel bed. No bottom sand samples were, therefore, obtained. Sample 824 was collected from a small zone of ebb formed mega ripples which occur on the southern flank of Brancaster Harbour Bar near this location (Table 9.26). Field observation of the channel bed further to the W. disclosed a stable gravel bed with occasional solitary mega ripples composed of coarse grained sand. Despite the extremely high velocities (the highest measured during this series of experiments) FIGURE 9.40.

0.10 STPTIO\ 3 16-

o. oq 1 4-

o.os 12- MUSSEL BANK ON 1— COCKLE BIGHT 10- ul 0.07 5 LL BRR EXPOSED cc z 0.06 0 F. ce

0'05 t 6- ut CC A 4- ct 0.048 • ••••.. •••••. Z z --- Q 0.03 2 2- c a TIDAL RANGE 0-0Z pus kft 0*01 c_t0 CONCENTRRTION

FLOOD CURRENT FLOWING E. _ CURRENT HOURS AFTER HIGH WATER VELOCITY H w +1 +2. +3 +4 +5 +6 I I I I , I I I -6 -5 -4 -3 -2. -1 H w HOURS HIGH WATER

EBB CURRENT - 1 FLOwING W.

1• O -tit ti- ?I\ • Z Ce •cf z -3 ti -

tn Lri cp h 00 r O — (Nt r41 (r) c‘t 4•1 (-•1 TO V) t4) t41 t NI, lilt tit It SUSPENDED SAMPLES tit 5907 Table 9.21 Grainsize parameter values, concentrations and current velocities of suspended samples collected at station 3. Tidal range: ebb 20', flood 21'. Station 3. Table 9.21

Suspended Mz 0-1 SY1 Concen- Current Sample (phi) (phi) tration velocity in in knots grammes per gallon 22 - - - 0.0025 1.0 23 - - - 0.0025 1.6 24 - - - 0.0040 2.2 25 - - - 0.0035 2.0 26 - - - 0.0015 2.7 27 - - - 0.0070 2.75 28 - - - 0.0060 3.2 29 - - - 0.0035 2.85 30 - - - 0.0015 2.4 31 - - - 0 1.9 32 _ - 0.0100 2.2 33 - - 0.110 2.9 34 - - 0.0640 3.4.5 35 - - 0.0360 3.0 36 - - 0.0370 2.8 59s recorded at this station, the concentration of suspended sediment was found to be surprisingly low (Table 9.21), reaching a maximum of 0.11 grammes per gallon, approximately * hour after the commencement of the flood tide (see Fig. 9.40). Peak ebb velocities (3.25 knots) occurred 4 hours after the predicted time of high water (according to Immingham) and peak flood velocities (of 3.45 knots) occurred 1* hours after the commencement of the flood tide. The maximum water depth measured at this station, however, occurred 1* hours after the predicted time of high tide. This measurement is in some doubt, and may c:nceivably be attributed to slight changes in the position of the boat. Current velocity measurements (shown in Fig. 9.4C)), indicate a prolonged period of ebb current flow which, for the 3* hours preceeding the commencement of the flood tide, reflect a period of drainage out of the inlet. As at station 1 and 2, maximum current velocity acceleration occurred during the rising tide.

Station 4. Station 4 is located on the N.B. edge of Cockle Bight Bar, adjacent to Brancaster channel (see Fig. 9.36). The almost equal flood and ebb current velocities (see Fig. 9.41) suggest that this station occupies a near-intermediate position between the zone on the N. flank of the Bar, which is dominated by the flood current, and the central zone of the Bar, which is dominated by ebb current action. The existence of these zones is implied by the FIGURE 9.41 .

STATION 4

8- 1.6

6. I4

4-- 1.2

et 2. 1.0 ec 3 0— oi

0.6

0.4

0.2 CONCENTRATION 0

FLOOD 0 CURRENT CURRENT FLOWING E. HOURS AFTER HIGH WATER z VELOCITY +2. +3 +4 +5 +6 0 I + I EBB CURRENT -6 -5 -4 -5 -z -1 H 0 FLOWING to HOURS BEFORE HIGH WATER

Z I Lu cc t.)

03 Cr 0 — te-N 1-("N r- cr 0 "d" tn t t 1 SUSPENDED SAMPLES tit t 1 I

v, oo cr 0 (NI M cn M *I' K1 tn 1.(1 tll M vt- 3 eP 00 oo ref d0 03 03 03 0 e0 00 MCA 00 00 I I 1 I II BOTTOM SRMPLES 11 II 11 60n Table 9.22 Grainsize parameter values, concentrations and current velocities of suspended samples collected at station 4. Tidal range: ebb 20'7", flood 21'7". Station 4, Table 9.22

Suspended Mz SKI Concen- Current Sample (phi) (phi) tration velocity in in knots grammes per gallon 38 _ - - 0.09 1.2 39 _ _ - 0.10 1.5 40 - - - 0.02 1.1 41 - - - 0.003 0.6 42 - - - 0.02 0.85 43 - _ - 0.02 1.05 44 2.19 0.23 -0.04 1.48 1.9 45 2.15 0.24 -0.06 1.10 1.7 46 2.05 0.31 -0.06 1.74 2.2 47 2.17 0.29 -0.12 1.87 2.0 4.8 2.24 0.22 -0.08 0.71 1.95 4.9 2.26 _ - 0.16 1.5 50 - - - 0.04.4 1.3 51 - - - 0.025 1.45 601 presence of flood and ebb formed mega ripples in the respective zones (see section 9.7.2.1). The ebb current reaches peak velocity of 1.5 knots, 1 hour after the observed time of high water, and about 2 hours after the predicted time of high tide. However, for reasons discussed below, a second and higher peak velocity (of 2.1

knots) occurs 4 hour before low water. A single peak flood current velocity of 2.3 knots is reached almost immediately the rising tide floods across the surface of the Bar at this le,cation (see Fig. 9.41). Maximum current acceleration occurs during the flood tide. Comparatively high concentrations of suspended sediment are mobilised during the peak velocities that occur shortly before, and immediately after low tide. Maximum suspended sediment was, however, sampled 20 minutes after the commencement of the flood tide (1.87 grammes per gallon), and was transported by a current of 2 knots (see Fig. 9.41). Suspended samples 44 and 45 (trapped during the ebb), and samples 46, 47 and 48 (trapped during the flood) were collected in sufficient quantities to analyse completely; sample 49 was, however, only partially analysed (Table 9.22). The mean grainsize of these sediments range between 2.05 and 2.26 phi, standard deviation values range from 0.22 to 0.31 phi, and in all cases these sediments are slightly negatively skewed. The modal peaks, displayed in the frequency curves of Fig. 9.55, vary from 2.10 phi to 2.32 phi. There appears to be no basic difference between the dis- 602 tribution of grainsizes carried by the ebb current, and those transported by the flood. The average grainsize distribution of the bottom samples 825 to 834, collected during the falling tide, appears to be identical to that of samples 835 to 840, collected as the tide rose (see Table 9.26). By comparing stations 2 and 4, it is apparent that, although the bottom sediment at station 2 is appreciably coarser than at station 4 (1.75 phi compared with 1.92 phi), higher concentrations of sediment are carried in suspension at the former station than at the latter (3.24 grammes per gallon at station 2 as compared with 1.87 grammes per gallon at station 4; compare Figs. 9.39 and 9.41). This phenomenon may be attributed to the higher velocities of the flood current at the former station (2.8 knots, as compared with 2.2 knots at station 4), which apparently outweights the reverse tendency, expected to occur in view of the greater ease with which the finer grained sediment at station 4 would be mobilised.

Station 5. Station 5 is situated in Cock? Drain on the S. side of Cockle Bight Bar (see Fig. 9.36). The actual station is positioned about 30 feet to the north of the scoured bottom of the Drain, on the edge of the low sand flats which occur over the centre of Cockle Bight Bar. As previously described in section 9.7.2.2, the bottom and southern side of the Drain appears to be dominated FIGURE 9.42

Fig. 9.42 Diagramatic illustration of a flood formed mega-ripple composed of pebbly sand, which has been modified by the superficial deposition of sand on its stoss side by ebb current action. 6n4 by the ebb current. Its northern side, however, which encompases this station position, displays evidence of slightly predominant flood current action. Flat-topped mega ripples, observed at this locality, appear to be composed of coarse grained pebbly sand which has been accumulated by the flood current. These features have, however, been modified by the ebb current, which has superimposed a. deposit of non-pebbly sand on the stoss (up current) slope of the flood formed ripple. A typical example of these features is diagrammatically reproduced in Fig. 9.42. Unfortunately, flood ucrrent velocity measurements were not obtained owing to a mechanical failure of the current meter. However, the dominance of the flood current is indicated by the predominant concentrations of sediment carried in suspension at this time (see Fig. 9.43 and Table 9.23). This may be a somewhat exaggerated difference, since the station position was altered between the time the ebb and the flood samples were collected. The station was positioned, during the rising tide, over an area of slightly finer grained bottom sediment (average mean grainsize of the flood bottom samples equals 1.72 phi) than during the ebbing tide (average mean grainsize of the ebb bottom samples equals 1.50 phi), (Table 9.26). Presumably, the finer grained bottom sediments occurring at the location occupied during the flood tide will be mobilised in greater quantities than the coarser grained sediments which occur at the position occupied by this station

FIGURE 9.4 3 ,

STPTION 3. T

6 IN FEE

TH 4 P E

R D 2. TIDAL RANGE

WITTE 0

07 eg

0.6

cc• 0.5 z 0. 04 Z v)• cc LL • 03 O tr z? • z 02 a

z 0.1 ZI CONCENTRRTION

C • 0

HOURS AFTER HIGH WRTER ÷5 +6 CURRENT VELOCITY i-vi +1 +2. +3 +4 0 0 I I 1 I , t II 1 I I I I EBB CuRRENT —6 —5 —4- —3 —2 —I H.W. FLOWIN G W. HOURS BEFORE HIGH WATER

ITY ELOC NT V RRE

CU 0 — +IA .$) r eQ ,r) Os) ko 1/40 kf) I ILI SUSPENDED SAMPLES

r- kr‘ In In tn tr) 013 copo 00 az ()a 00 BOTTOM SAMPLES 606 Table 9.23 Grainsize parameter values, concentrations and current velocities of suspended samples collected at Station 5. Tidal range: ebb 18'5", flood 17'. Station 5. Table 9.23 Suspended Mz -1 SK1 Concen- Current Sample (phi) (phi) tration velocity in in knots grammes per gallon 59 - - _ 0.11 1.9 60 2.09 - - 0.34 2.1 61 2.01 - _ 0.47 2.3 62 - - 0.12 2.0 63 - - - 0.02 1.2 64 2.00 0.25 +0.06 7.3 65 1.88 0.28 +0.03 7.12 66 1.91 0.34 +0.01 0.8 67 2.10 - - 0.73 68 2.00 - - 0.89 69 2.03 - - 0.48 70 - - - 0.07 607 during the ebb tide. As 't station 1, 2, 3 and 4, the maximum current acceleration also occurred during the rising tide, although the differences in acceleration between flood and ebb are less marked at this station. The mean grainsizes (Table 9.23) and modal grainsize (Fig. 9.25) of the suspended sediments (samples 59 to 69), display a significant degree of variability, ranging from 1.88 to 2.10 phi in the former case, and from 1.85 to 2.08 phi in the latter case.

General Discussions. By comparison between the values of the grainsize parameters in Tables 9.19, 9.20, 9.22 and 9.33, it is apparent that in all cases the sand fraction of the suspended samples is finer grained (by approximately 0.3 phi), and is us,:ally better sorted than the corres- poncing bottom samples (see Table 9.26). The former relationship is illustrated graphically in Fig. 9.44, in which the average mean grainsize of the suspended samples has been related to the average mean grainsize of the corresponding bottom samples, collected during the flood tide at stations 1, 2, 4 and 5. Also shown on this figure are the relative total quantities of sediment transported in suspension by the flood current. These values have been obtained from the curves showing the concentration of suspended sediment at various times of the tidal cycle (see Fig. 9.38, 9.39, 9.41 and 9.42) by adding the concentration values at 10 minuteintervals

FIGURE 9. 44

UI 0 STN. 1 (3.27 UNITS) e.3 Q

Laa z 2.2 0 STN. 4 (5-85 'UNITS) LL 0

-‘5). 0 STN.2 (Z0.8 UNITS)

z tit 2.0 0 STN. 5 (2.•3D UNITS)

UI t_o a 1.7 1-8 Fq 2.0 2.1 FIVERRGE NIFIN GRAIN SIZE OF BOTTOM sAmPLes IN PHI UNITS.

Pig. 9.44 Relationship between the average mean grainsize of the suspended samples, and the average mean grainsize of the bottom samples collected during the flooding tide at stations 1, 2, 4 and 5. The numbers, shown in brackets, give an indication of the relative amounts of sediment transported in suspension during the flood tide at each station. F;09

The values calculated in this manner do not, therefore, represent the real quantity of sediment mobilised, but are of use only as an indication of the relative mass of sediment moved at the various stations' positions by the flood current. It is apparent from this figure that a relationship exists between the grainsize of the sediment occurring on the channel bed and in suspension above the bed. It is proposed that this is a genetic relationship, i.e. that the suspended sediment is, in fact, derived from the bottom sediment occurring in the immediate vicinity. Alternatively, this genetic relationship may be envisaged in terms of the locally existing energy level. The current velocity at a point in the inlet determines the type of sediment transported or deposited at this point. In this way the bottom sediments reflect the locally occurring energy conditions which, in turn, control the type of sediment deposited, i.e. the character of the bottom sediments is determined by the sediment transpdrted by the current in this vicinity. It is significant that the quantity of sediment transported by the flood current at station 2 (see Fig. 9.44) is abnormally high (20.8 units), while its average mean grainsize is somewhat finer than would be expected from the trend shown in this figure by stations1, 4 and 5. It would seem logical to assume, from the plots of concentrations of suspended sediment and the mean grainsize of the suspended sediment against current velocity (Fig. 9.47 and 9.46), that if an abnormally fine grained 1310 sand fraction is available, it will be transported in correspondingly large quantities. However, the grainsize characteristics of the bottom sediments at station 2 (Table 9.26) in no way suggest the local presence of such a fine grained fraction. It is, therefore, tentatively proposed that the fine grained sediment mobilised at station 4 (average mean size of 2.18 phi) is transported south eastwards by the flood current and reaches the position of station 2, where it is combined with locally derived material. Alternatively, the abnormally fine grained suspended sediment mobilised at station 2 may be in some way related to the mega ripples which characterise this locality, The mechanics controlling such a relationship are, however, obscure. In view of the above proposed genetic relationship between sediment carried in suspension and the sediment occurring locally on the channel bed, it is probable that the abnormally low concentration of suspended material collected at station 3 (Table 9,21) is related to the absence of a sand fraction on the channel bed at this locality. This phenomenon is, none-the-less, somewhat m7sterious, since the flood current traverses the extensive potential source areas of the Brancaster Golf Course fore-beach, and also of the Brancaster Harbour Bar, further to the W. of station 3; while the ebb current passes over Cockle Bight Bar immediately to the W, of this station. As shown by direct measurements, as well as field observations, considerable quantities of sediment (1.5 grammes per gallon) are mobilised by the ebb 611 current at station 4 on Cockle Bight Bar. It is, therefore, strange that this sediment does not appear at station a distance of only 800 yards to the W. (especially since the ebb current appears to increase in velocity from 2.1 knots at station 4, to 2.7 knots at station 3, over this distance). Two tentative explanations for this pliniomenon are presented:- (a) NorrPaly, sand sized sediment is transported as bed load, in saltation and in suspension, and is mainly concentrated in a fairly narrow zone immediately above the bottom. However, under the very strong, and excessively turbulent current conditions which are believed to exist at station 3, it is possible that the suspended sediment is fairly evenly distributed throughout the water mass at this locality. If such is the case, suspended load samples collected near the bed at station 3 will display much lower concentrations than expected, and will be incomparable with the suspended load measurements made at stations 1, 2, 4 and 5, where less turbulent conditions exist. (b) As described below and in section 9.7.2.1, the sediment transported by the ebb current in a. northwest- ward direction across the top of the Cockle Bight Bar is probably at least partly deposited in Brancaster Channel on the Y. side of the Bar. This phenomenon has been attributed to the difference between the abnormally high current velocities which occur at the end of the ebb on the surface of the Bar, and the more normal 612 velocities that are presumed to occur, at this time, in the adjacent channel. It is possible, therefore, that the sediment mobilised at station 4 may in fact be deposited almost immediately in Brancaster Channel and will not be transported as far as station 3. Despite these considerations, the presence of an extensive train of ebb- formed mega ripples over the surface of Cockle Bight Bar, attests to a high degree of sediment mobility (see Fig. 9.9). It has, however, been suggested by Johnson of the N.I.O., and Cloet of the A.H.O. (in personal communications) that the overall rate of mass sediment transport in such a train of mega ripples is more directly related to the rate of migration of the ripples themselves than to the velocity of the current. Cornish (1901) has shown the rate of advance of mega ripples in the Dovey Estuary to be between 2 and 3 feet per day. Allen's (1965) work on experimental sand waves suggests that all except a small proportion of the sand (corresponding to the finest grades present) moves by a combination of saltation and bed load, and after travelling a short distance is re-incorporated in the preceeding sand waves. In view of these consideration it is probable that only a small quantity of sand is released from the lower (down current) end of the train of mega ripples on Cockle Bight Bar during each cycle of the ebb tide, and it is this sediment which was sampled in such small quantities during the ebb current at station 3. The current velocity measurements at station 4. EXPOSED Mt/5SLX BANK

I FOOT FIBOVE LOW WFITER LEVEL

Fig. 9.45 Showing the configuration of the ebb current over Cockle Bight Bar at three stages of the falling tide. 614 (Fig. 9.41), on the Y.E. flank of Cockle Bight Bar, show an abnormal distribution. The ebb current velocity reahces two peaks, one about 2 hours after predicted high tide, and the other only a few minutes before the water drains off the top of the Bar. The flood current achieves a single peak velocity a few minutes after the rising tide floods over the surface of the Bar. Maximum sediment transport coincides with these velocity peaks, and occur less than 1 hour before and after low water. These abnormal velocities are believed to be a local effect, attributed to the topographic configuration of Cockle Bight Bar. The general relief of this feature, as indicated by the water level at various stages of the tidal cycle, is shown diagrammatically in Fig. 9.45. It is apparent in this figure that once the large mussel bank at the western end of the bar is exposed by the falling tide (this occurs approximately 3 hours after predicted high tide), it tends to form a partial barrier or dam to the ebbing water movements in the inlet. The low central portion of the bar acts as a "basin" in which a large volume of water is enclosed. In order to escape seaward this water flows either to the N. into Brancaster channel, then westwards into the sea, or directly westwards around the southern edge of the mussel bank via Cockle Drain. As the tide falls further, the Cockle Drain outlet becomes rapidly shallower and (according to field observations) only functions as a narrow gutter 1 hour after the mussel bank is exposed. FIGURE 9.46. MERNGRR I NSI ZE IN PHI UNITS. the mean(o),andmodal(•)grainsize ofthesuspended diagrarnatically byastraightline. Fig. 9.46 samples. Thegeneraltrendofthe relationshipbetween mean grainsizeandcurrentvelocity isindicated 23 2.2 z•5 2.1 24 1 1.9 . 8 1.3 141.51.617 . Relationship CURRENT VELOCITYINKNOTS between currentvelocityand 1.8 1-clZO2 . 1 2 . 2 2.3 616 On the Y. side of the Bar, the falling tide rapidly exposes the N.N.-S.E. trending sand ridge which occurs along the N.E. flank of the Bar. This ridge acts as an extension of the barrier formed by the mussel bank, and forces the water trapped on the surface of the Bar to drain into Brancaster Channel further and further to the E. (see Fig. 9.45). Owing to these obstructions, the water in the mouth of Brancaster Harbour channel, to the W. of Cockle Bight Bar, drains into the sea at a faster rate than the water drains from around the high areas on surface of the Bar. A slight difference in the water levels, and hence an abnormal water gradient, is progressively instigated between the water in Brancaster channel to the N. and W. of the Bar, and the water dammed back over the surface of the Bar. This phenomenon, which is also produced in reverse during the early stages of the flood tide, is responsible for the high current velocities which have been measured at station 4. The grainsize parameter values of the suspended samples, and the velocity of the current flowing at the time they were collected, are reproduced in Table 9.24. The modal grainsize of the suspended samples (shown in Table 9.25), has been determined from the frequency curves shown in Figs. 9.54, 9.55 and 9.56. The competency (Hjulstrom 1939) of the inlet currents is demonstrated in Fig. 9.46, where the mean and modal grainsize of the sand in suspension has been plotted against the current velocity. Although some scattering occurs, FIGURE 9.47. 4.0 0 3.0 0

0 2.0 0 O 0, 0 0

0 1.0 z • 0.8 c.D 0.6

0 0 0.4 00 0 0.3 0 z 0

tri 0 0 0 0 0 • 0.1 z R 0.02) z ") 0.06 0 u. 0 0 6 0.04

0.03 0 wz • 0.0Z

1 2 3 4 CURRENT VELMITY • IN KNOTS

Fig. 9.47 Relationship between current velocity and the concentration of sand in suspension. The general trend of this relationship is indicated diagramatically by a straight line.

FIGURE 9.48.

10 8 0 0 6 ON LL A G 4 ER 0 3 0 AMS P GR 0

N 0 I 0

0 0 0 ON

SI 0 0 PEN

SUS 0.8 0

N 0 I

D 0.6

SAN 0 0 F 0.4 0 0 0 ON O 0.3 0 ATI 0 0 TR N E

0 CONC

2.0 2-1 z z 23 MERN GRRINSI2E IN P4 UNITS Fig. 9.48 Relationship between the concentration of sand in suspension and the mean grainsize of this s and. 619 especially of the modal values, a general trend exists showing, as expected, an increase in mean grainsize associated with increasing current velocity. Variations in the capacity (Hjulstrom 1939) of the currents are shown in Fig. 9.47, where the concentration of sand in suspension has been related to the current velocity. Again the expected trend of increasing concentration with increasing current velocity was verified. These two trends are less clearly illustrated in Fig. 9.48, in which the concentration of sand in suspension has been related to its mean grainsize. As indicated by comparing the graphs shown in Figs. 9.46 and 9.47, changes in the capacity of a current are far more significant than changes in its competency (distribution of grainsizes) at various current velocities. An increase of 1 knot in the current velocity (i.e. from 1.4 to 2.4 knots) produces in the order of a 1.35 fold increase in the mean grainsize of the suspended sediment (Fig. 946), and a 100 fold increase in the concentration (or quantity) of the sediment capable of being transported (Fig. 9.47). The results of both increasing grainsize, and increasing concentration of the suspended sediment, with increasing current velocity is in general agreement with the results of v. rious workers including Kuenen (1965) and Phleger (1965). The skewness values of the suspended samples display an irregular distribution when plotted against current velocity (Fig. 9.53). However, when this parameter

FIGURE 9.4 9 .

0.34- O

0.52

0.30 O O 0.28 0 (Y, 0 0 0.26 00 0 0.24 O 0.22 0

Mz 1.8 ZO 2.1 Z2 Z.3 PH1 UNITS

+0.06

+0.04 0 +0.0Z O 0 5k, o O 0,0 -0.0a

-0.04 0 0 -0.06 O 0

-0.10

Fig. 9.49 Relationship of both standard deviation (Ci) and skewness, (Ski) to the mean grainsize (Mr) of the suspended samples. 621 was related to mean grainsize in Fig. 9.49B, a slight trend of more positive skewness with increasing grainsize became apparent. The relationship between standard deviation and mean grainsize of the suspended sediments also displays trend of improved sorting with decreasing mean grainsize (Fig. 9.49A). These general trends are similar to those displayed by the grainsize parameter values of the inlet channel sediments by the marsh and marsh creek samples, and by the beach sands. At all stations at which current measurements were fully recorded (stations 1, 2, 3 and 4), a dominant flood current velocity occurred. The current acceleration (the rate of tidal rise), as indicated by the steepness of the tidal range curve, is also greater during the flood than during the ebb. The combination of these factors explains the typically higher concentration of sand in suspension during the flood,and are in accord with the observation of Kestner (1961), who investigated changes in sediment character during a tidal cycle in the Lune Estuary. Here the grainsize distribution of the sand carried in suspension by the ebb current was found to be distinctly finer grained and more poorly sorted than that carried by the flood current, owing to the inclusion of a considerable proportion of silt and mud. Kestner attributes these variations of grainsize and sorting to the immediate source from which the sediments have been derived. The sediment carried by the flood current reflects the erosion of the sandy channel bed, while the FIGURE 9.5 0 .

EBB SUSPENDED LORD

FLOOD SUSPENDED LOAD

1 i I 3.0 4.0 5.0 6.0 7.0 80 GRRIN SIZE IN PHI UNITS

Fig.9.50 Frequency curves of the suspended sediment carried by the ebb and the flood currents in the Lune Estuary, (based upon measurements made by Kestner, 1961) 623 ebb current transports sediment derived mainly from the marshes and mud flats. A closer examination of Kestner's results, however, indicates that the sand component of the composite sediment transported by the flood current is coarser grained and more poorly sorted than the sand fraction carried by the ebb current (see Fig. 9.50). Since the flood current velocity exceeded the ebb velocity, these results support the general observations of decreased grainsize, improved sorting and decreasing current velocity shown in Figs. 9.49A and 9.46 above. However, due to a deficiency of sufficient ebb tide data, these trends have not been related specifically to flood or ebb current conditions. A further point of agreement with Kestner's observations lies in the general decrease in the mean grainsize of the suspended sediments with increasing distance up the inlet. The average mean grainsize of the sand in suspension at stations 5, 4, 2 and 1 are respectively 2.00, 2.18, 2.10 and 2.36 phi. This trend simply reflects a parallel decrease in peak current velocity from the lower to upper reaches of the inlet. I.t is reasonable to assume that the sediment which is transported, mainly in suspension, in the zone near the bottom, includes a somewhat coarser fraction which moves by saltation. This relatively coarse grained sand will presumably only appear in significant quantities at high current velocities. It is perhaps significant that the frequency curves of suspended samples 13, 14, FIGURE 9.51 .

\ \ / \ / P-' r- i ' \ `\\ \ / ,--./ \ \ e56 __. / \ , \ \ 47 ---- ,'-- \,. \\ ,, \ \ 46 e' N STATION 4. ,-... 835 _./ \ / \ / \

I \\ /1 \ // \ r , \ \\ / \ N , N I I \ / / \ 8Z0 , \ . ..--, . / \ . , .. 17 ' \ / \ ,/ , \ / . /\ / 81q \. / . . . 15 - / / \

/ \ // \ \ 817 /1 \ \ / / \ \ / \ \ „, / ... 14 -- / \ \ \.-_- - - . . 13 -- _ -- STRTION 2. 8168 , t 1 1 i I 1 t 1.0 1.4 1.6 1.8 2.0 2.2. 2:4- Z6 2.8 PHI UNITS.

Fig. 9.51 Comparison between the grainsize distributions of suspended samples containing a subSidary coarse grained mode (broken lines) and bottom samples (solid lines) collected contemporaneiousty. 625 15, 17 and 18 from station 2, and samples 46 and 47 from station 4 (Fig. 9.51), show a small subsidiary, coarse grained mode of 1.9 phi. The corresponding current velocities are shown in Table 9.25, and in all cases exceed 1.7 knots. These suspended samples are compared with the channel bottom samples which were collected at approximately the same time. At station 2 suspended samples 13, 14 and 15 correspond to bottom samples 816B and 817, suspended samples 17 and 18 correspond to bottom samples 819 and 820. At station 4 suspended samples q.6 and 47 correspond to bottom samples 835 and 836. It is apparent that the dominant modal size of the bottom sample is similar to the grainsize of the subsidiary mode of the suspended samples. Both station 2 and station 4 are situated in the vicinity of mega ripples. It is tentatively suggested that this coarser subsidiary mode represents sand in saltation, measurable quantities of which are apparently mobilised at current velocities greater than 1.7 knots.

General Conclusions. The general results of this series of experiments are in agreement with Kuenen's (1965) and Phlege2s (1965) findings regarding current capacity and competency. Crude bottom sampling techniques, however, only permit a cursory comparison with the work of Kestner (1961) in the Lune Estuary. It is believed that the lack of relationship between current velocity and both mean grainsize and FIGURE 9.52,

2.2

2.0

1.8 TS O 1.6 KN N I k4-

4- + TY 1.2 ELOCI

V /.0

ENT 0.8 RR

CU 06

04 1.5 1.6 1:7 1-8 1.9 2.0 2•I MEAN GRAINSIZE IN PHI UNITS

Fig. 9.52 Relationship between current velocity and the mean grainsize of the bottom samples.

FIGURE 9- 5

+014 +0-12.

+010

+0.08

+0:06

+0.04-

-1-0.02. 0 Ski 4-+ O +0O 0 +4- + 4+ —0.02. + -H- + 0+

+ + o 0 —0.06

+ o —0.08 + —0.00 + ++ 0 —0•12.

—0.14

—0-16 —0•18 -

0 0.5 1 .0 1.5 20 Z.5 30 KNOTS CURRENT vv....ocrrY

Fig. 9.53 Relationship between current velocity and the skewness, (SKI), of both Lhe bottom samples, (+), and tFie suspended samples, (0). 628 skewness of the bottom samples, shown in Figs. 9.52 and 9.53 respectively, are a reflection of this crude bottom sampling technique. This limitation also made unreliable any comparison between the suspended sediments and those occurring on the channel bed. The effect of current action upon a sediment is normally restricted to a surface zone of only a few grain diameters thickness. An exception of this rule exists when ripples are for- d, in which case a zone, equal to the ripple height, is mobilised. In the past it has been customary to consider the effects of current action, etc., by examining a "surface sample". Generally such a sample consists of sediment down to, or even exceeding a depth of 1 inch. It is obvious, in view of the above consideration, that such a sample will commonly contain a very large proportion of material which has absolutely no chance of being affected by the locally existing current conditions. However, the existance of a prominent zone of mega ripples in the vicinity of some of the station positions (notably station 2) presents the possibility that the uniform and unchanging character of the bottom sediments throughout a tidal cycle is in fact a real phenomenon (at least within the range of accuracy of the analytical proceedure). This conclusion is, however, not favoured in view of the theoretical considerations of the mechanism of interchange of sediment between the channel bottom and the overlying water mass. 629 Table 9.24 Grainsize parameter values of the suspended samples and the velocity of the current at the time of sample collection Table 9.24 -ilia-a.1111111Milmalafilaft• Suspended Mean Standard Skewness Current sample Grainsize Deviation (Sic1) Velocity (Mz in phi) (CI in phi) (knots) 13 2.05 0.29 -0.01 1.7 14 2.11 0.27 -0.05 2.2 15 2.09 0.27 -0.01 2.8 16 2.12 0.26 -0.01 2.15 17 2.10 0.26 -0.01 2.00 18 2.10 0.27 +0.01 1.85 44 2.19 0.23 -0.04 1.9 45 2.15 0.24 -0.06 1.7 46 2.05 0.31 -0.06 2.2 47 2.17 0.29 -0.12 2.0 48 2.24 0.22 -0.08 1.95 64 2.00 0.25 +0.06 65 1.88 0.28 +0.03 - 66 1.91 0.34 +0.01 - 630 Table 9.25 Modal grainsize of suspended samples and the velocity of the current at the time of sample collection Table 9.25

Station Suspended Modal Subsidiary Current Sample Grainsize mode where Velocity (in phi) present in knots (in phi) 1 52 2.40 - 1.5 53 2.37 - 1.65 54 2.35 - 1.6 55 2.32 - 1.5 56 2.36 - 1.3 2 13 2.15 2.0 1.7 14 2.15 2.0 2.2 15 2.11 2.0 2.8 16 2.12 - 2.15 17 2.14 2.0 2.0 18 2.15 2.0 1.85 19 2.10 - 1.6 4 44 2.25 - 1.9 45 2.16 - 1.7 46 2.10 2.0 2.2 47 2.28 2.0 2.0 48 2.32 - 1.95 4.9 2.20 - 1.5 5 59 1.92 - 1.9 60 2.06 - 2.1 61 1.94 _ 2.3 62 1.92 - 2.0 64 1.89 _ - 65 1.85 - - 66 1.98 - - 67 2.08 - - 68 2.00 _ - 69 2.08 FIGURE 9. 54 .

53 52

1 ci

13 1.4 1.6 11 ZO 2.2 2.4 z.6 2-3 3 0 GRRIN SIZE IN PHI UNITS.

Fig. 9.54 Frequency curves of suspended samples collected at stations 1 and 2. FIGURE 9.55 .

42,

5TRTION 4. 4-4

1 .4 1.6 1.8 20 22. 24- 2-6 2.8 3.0 GRAIN Silt IN PHI UNITS

Pig. 9.55 Frequency curves of suspended samples collected at station 4. FIGURE 9.56

6c1

68 67

62,

60 SIR lON 5. 59 1,2. 14 1.6 1.8 20 2-2 24- 26 ZS 3 0 GRRIN SIZE IN PHI UNIT.5

Fig. 9.56 Frequency curves of suspended samples collected at station 5. 634. Table 9.26 Grainsize parameters of bottom samples collected during the ebb and flood tides at stations 1 to 5.

Station 1 Table 9.26 Ebb tide Flood bottom Mz 1 SKI tide Mz C5 SK1 samples (phi) (T'h (phi) bottom (phi) (phi) (phi) samples 800 1.74 0.45 0.0 805 2.2 0.33 -0.09 801 1.80 0.46 -0.06 806 2.05 0.33 -0.11 802 1.56 0.46 0.0 807 2.00 0.33 -0.09 803 1.71 0.43 0.0 808 1.99 0.34 -0.13 804 1.63 0.42 +0.08 809 2.01 0.35 -0.16 Average 1.69 0.44 - Average 2.01 0.34 - Station 2 810 1.87 0.33 -0.03 816B 1.81 0.35 -0.08 811 1.74 0.41 -0.09 817 1.79 0.35 -0.02 812 1.81 0.39 -0.09 818 1.64 0.33 +0.05 813 1.58 0.53 -0.19 819 1.77 0.31 +0.01 814 1.90 0.30 0.0 820 1.90 0.29 -0.04 815 1.86 0.36 +0.03 821 1.66 0.36 -0.04 816 1.65 0.42 -C.06 822 1.63 0.39 -0.01 823 1.68 0.47 -0.11 Average 1.77 0.39 - Average 1.73 0.35 - Station 3 Gravel bottom - sample 824 was collected from the side of the channel. 824 0.60 0.31 +0.26 Station 4. 825 1.91 0.28 -0,01 835 1.89 0.30 -0.05 826 1.92 0.30 -0.03 836 1.89 0.32 -0.01 827 1.96 0.28 -0.03 837 1.86 0.33 0.0 828 1.96 0.28 -0.03 838 1.89 0.30 -0.01 829 1.88 0.29 +0.03 839 2.01 0.26 -0.02 830 1.85 0.31 -0.05 84.0 1.98 0.28 -0.05 831 1.93 0.31 -0.02 832 1.95 0.30 -0.C4 833 1.90 0.30 +0.C2 834. 1.94 0.28 0.0 Average 1.92 0.29 - Average 1.92 0.30 635 Station 5 Table 9.26(cont.) Ebb tide Flood bottom Mz SK1 tide Mz :3K1 samples (phi) (phi) (phi) bottom (phi) (phi) (phi) samples 841 1.26 0.42 +0.04 847 1.74 0.27 -0.05 842 1.50 0.40 +0.01 848 1.70 0.30 -0.07 843 1.68 0.31 +0.13 849 1.79 0.24 +0.06 844 1.47 0.35 -0.02 850 1.72 0.26 +0.02 845 1.52 0.34 -0.04 851 1.59 0.37 -0.09 846 1.57 0.31 -0.02 852 1.73 0.28 -0.01 853 1.71 0.32 -0.11 854. 1.77 0.30 -0.02 855 1.66 0.31 -0.06 Average 1.50 0.35 - Average 1.72 0.29 636 9.10 General Conclusions 9.10

By virtue of the various procedures adopted in the study of the dynamics, and of the sedimentology of the Inlet Channel sub-environment, it is difficult to make a concise statement as to the relationship existing between these two factors. However, it would appear that the lack of homogeneity, or of a consistent overala trend in the distribution of sediments in the inlet channel sub-environment, is produced by variations in the primary sediments deposited during the eaxly stages of the development of the Tidal Inlet deposits, as well as by local variations in the current energy conditions. From the cursory study of the relationship between marsh and marsh creek sediments (section 9.6). it would appear that the particular conditions existing in the marsh creeks (and presumably also in the channels) produce a slightly improved sorting, and a more negatively skewed distribution in the sediments deposited on the creek beds as compared with those accumulating on the marshes. (These statements refer to the sand fractions of the respective sediments only.) The detailed study of Cockle Bight Bar, and of Norton Creek Bar, affords an illustration of the usefulness of grainsize analysis in conjunction with a study of the morphfology,and of the local dynamic conditions, in determining the genesis of specific sand accumulations. It is believed that this procedure would find broad application in the investigation (and prediction) of the changes occurring in off-shore banks and near-shore 637 bars, etc., which have not been observed at regular intervals over a long period of time. It is apparent from these detailed studies that the grainsize characteristics of a sediment are only meaningful in the context of the sediment population of which it forms part, and in this context does in fact reflect the dynamic conditions responsible for the deposition and accumulation of this deposit or population. The study of sediment movement under specific dynamic conditions illustrates the shortcomings of the normal sampling techniques. This study also gives the impression that sediment movement conforms to, and is controlled by measurable physical factors, and that the various sedimentary phenomena observed in nature may be explained and understood in terms of these dynamic factors. It is believed that with further study of the relationship between sediment properties, bed characteristics, current velocity (or wave energy) and the physical configuration of the environment, a much more profound understanding of the processes of deposition, erosion and movement of sediment may be obtained. In example of this type of thinking is displayed by the results of the investigation of the sediments of the mega ripples described in section 9.8. These sediments appear to be characterised by the property of positive skewness which is, more than likely, related to, and is induced by the dynamic conditions producing these features, rather than to an inherent property of the sediments themselves. 638 In addition, a relationship between improved sorting and increasing mean grainsize is shown by these sediments. The observations of other workers (e.g. Allen 1964) suggests that winnowing (i.e. the removal of a fine grained fraction), may be responsible for this phenomenon. It is reasonable to extend this explanation to account for positively skewed sediments found elsewhere, e.g. in the bottoms of the channels of Mow Creek, Brancaster Harbour and Burnham Harbour, and even to the positively skewed sediments in the Beach and Dune environments. The implications of this line of reasoning (which is contrary to previous considerations - see Folk and Mason (1958), Friedman (1961) and Daune (1964)), in terms of a general fractionation process, is discussed in section 10.6 below. It is believed that such sediment properties as skewness, when considered in relation to the specific physical environment, may supply useful information as to the conditions (whether erosional or depositional) affecting a. particular sediment accumulation at a given time. 639 10 STATISTICAL ANALYSIS 10

10.1 General Discussion 10.1 In order to use a statistical approach to distinguish between sedimentary environments, it is essential that the basic data be collected as objectively as possible. This would normally involve the collection of. data according to a grid system. However, as previously described in section 2.1, sampling carried out during this study has been based upon a subjective procedure. This subsequently made the construction of "trend surfaces" (which have been used by Miller (1956) to distinguish objectively between various sediment populations) an impracticable proposition. In view of general physiographic and sedimentological considerations, this subjective sampling procedure has been largely based upon various preliminary assumptions as to the location and limits of the sedimentary environments existing in the study area. The statistical analysis described below has been based upon the data collected from these predetermined environments, and will subsequently reflect any preconceived misconceptions inherent in the original assumptions. The study area has been sub-divided into the Beach, Tidal Inlet, and Dune environments. The Beach environment has been further sub-divided into the fore-beach and back- beach sub-environments. The former included the Holkham Beach sub-area, the Scolt Head Island fore-beach sub-area, 640 and the Brancaster Harbour Bar sub-area; while the latter is composed of the Burnham Harbour Bar sub-area, and the Scolt Head Island back-beach sub-area. The Tidal Inlet environment has been separated into the Inlet Channel and Tidal Marsh sub-environments, the latter of which has not been investigated in this study. The Dune environment has been divided into the Longitudinal Dune category and the Lateral Dune category. The statistical grainsize characteristics have been calculated from the samples collected in each of these eight sedimentary sub-areas and categories. The majority of the samples collected over the Scolt Head Island Beach naturally fall within either the fore-beach or the back-beach sub-environment. However, an intermediate transition zone exists in which the sediments show characteristics common to both sub- environments. The samples occurring in this zone have, therefore, been arbitrarily assigned to either one or the other of these categories (Samples located to the seawards of sample 688 (profile C), sample 859 (profile D), sample 10 (profile E),sample 210 (profile 1), and sample 21 (profile G) have been incorporated in the fore- beach category.) The samples collected along the low water level, on the off-shore bar, and at various positions over the lower part of the beach have also been included in this category. The remaining samples, which were collected over the upper beach face, make up the back-beach sediment 641 population. The statistical character of the sediments occurring in the Inlet Channel sub-environment have been investigated using all the samples listed in section 9, with the exception of those samples collected on Cockle Bight Bar and to the seawards of this point. These samples are believed to represent a combination of beach (wave dominated) and inlet channel (current dominated) material, and are, therefore, not representative of the Channel sub-environment as a whole. Those samples collected by grab during the series of current measurement, and suspended load sampling experiments (described in section 9.9) have also been included in the form of an average sample from each of the four station positions. Owing to the method of sampling, a danger exists that a certain sediment type within a sub-area will be either over emphasised or under emphasised depending upon the relative number of that particular type of sediment collected. A non-representative factor may have thus been introduced in the case of the Brancaster Harbour Bar, the Burnham Harbour Bar and the Inlet channel sub-areas, where detailed sampling within a small locality has been carried out. Rather than select specific samples from these localities, all samples have been included in the statistical analysis, the results of which must, in these instances, be considered in the light of such possible limitations. 642 10.2 Average Grainsize Characteristics of the Sediments 10.2 Occurring in the Various Sub-areas

The mean value and the standard deviation have been calculated for the mean grainsize, sorting (standard deviation) and skewness values of the various samples of each population. The term population is used here in the same sense as that used by Sahu (1964), and refers to the samples collected from a particular environment or sub- area. The standard deviation, which is the square of the variance, has been obtained using the formula:-

SD = V2 = (SX2) (SX)2 /N - 1 Where "S.D." equals the standard deviation, "V" equals the variance, "(SX2)" equals the sum of the squares of the components and "SX" equals the sum of the components, and "Y" equals the number of samples comprising the population. The results of these calculations are shown in Table 10.1, and are illustrated in Fig. 10.1. It must be remembered that the grainsize parameter values have originally been calculated from the sand sized material contained in each sample, and, therefore, in no way take into account the presence of either a mud or gravel fraction that may possibly also occur in certain samples. This qualifying statement applies only to a comparatively few samples which contain a significant proportion of mud, collected from Cockle Bight marsh (section 9.6); and also to those samples collected mainly from the FIGURE 10.1.

BU BB FB BR NB IC 1-0 LR • 1

0.9 1.0 t.1 19,, 1.3 1.4- 1.5 1.6 1,7 1.8 1.9 z:o R.1 2.2 MERN GARIN SIZE IN PHI UNITS

BU • BB FB BR HB IC LO LA

01 0.2. 0'3 04 0.5 0.6 0.7 0'8 SORTINGI CSTFINDBRD 1)VIFITION) IN PHI UNITS

BU BB FB BR • HB IC • LO LR I t -0.4 -03 -0'2 -0,1 0 -1-0,1 +0.2,.. 51,(WNE5S

Fig. 10.1 Illustrating the statistical mean value and the standard deviation of the mean grainsize, sorting and skewness parameter values of the sediment populations of the Burnham (BU) and Brancaster (BR) Harbour Bars, the Back-beach (BB) and Fore- beach (FB), the Holkham Beach (HB), the Inlet Channel (IC), the Lateral Dunes (LA) and Longitudinal Dunes (LO). 644 Table 10.1 Shows the statistical average and the standard deviation (in brackets) of the mean grainsize, sorting and skewness parameter values for the various sediment populations collected in the study area. Table 10.1 Sediment population and Average Average Average number of samples col- Mean Standard Skewness lected Grainsize Deviation (sorting) Fore-beach 1.905 0.434 -0.160 (73) (0.206) (0.129) (0.140) Back-beach 1.250 0.541 -0.003 (33) (0.345) (0.206) (0.175) Brancaster Harbour Bar 1.955 0.319 -0.128 (97) (0.204) (0.038) (0.111) Burnham Harbour Bar 1.560 0.423 -0.022 (94) (0.279) (0.117) (0.1 26)

Holkham Beach 11.867 0.407 -0.250 (39) (0.261 ) (0.226) (0.138) Inlet Channel 1.920 0.256 -0.080 (97) (0.205) (0.053) (0.114)

Longitudinal Dunes 1.596 0.34.1 -0.068 (50) (0.1 48) (0.036) (0.054) Lateral Dunes 1.918 0.347 -0.135 (23) (0.101) (0.035) (0.046) 645 back-beach sub-area and from the Burnham Harbour Bar sub- area, which contain varying quantities of material coarser than 2 mm. From the statistical range of values shown in this figure by the sorting (standard deviation) andEkew- ness grainsize parameter values, it is conspicuous that each population overlaps the others. The statistical range of the mean grainsize parameters of these various populationscalso overlap, except in the case of the Scolt Read Island back-beach sub-area. This environment may be distinguished, using ihis latter property, from all other populations except those of the Burnham Harbour Bar, and the Longitudinal Dune categories. It is, therefore, apparent that in only very special cases would the grainsize parameters of a sediment, collected from an unknown locality, be of any use in determining its origin. It is possible that better definition would be achieved if the absence, or presence of gravel was also recorded statistically. From field observations it is apparent that gravel does not occur in the Dune Environment, or in the Brancaster Harbour Bar sub-area; it is also practically non-existant in the upper reaches of the Inlet Channel sub-environment. It may, therefore, be stated that a sample containing gravel does not belong to any of these populations. However, the absence of gravel in a sample does not necessarily include it in one of these populations, as gravel free sediments also occur in the 646 fore-beach sub-environment. The internal structures of the sedimentary deposits occurring in these various sub- areas may also supply additional criteria, which, is used in conjunction with the statistical grainsize properties, may possibly allow a more discriminating distinction to be made between the various sediment populations. By comparing the statistical parameter values of the Holkham Beach population with the parameter values of individual samples collected along profiles A and B (section 7.5.1), it is apparent that the statistically poor sorting (0.407phi) and the high degree of variability (0.226) in this population may be attributed to the extremely poor sorting of about one quarter of the samples collected (i.e., samples 578, 579, 706, 713, 714, 716, 721, 722 and 726). The frequency curves of these samples show that, although the dominant sand grade is well sorted, a coarse "tail" occurs which gives the appearance of a poorly sorted sediment. This particular instance emphas- ises the misconceptions, and the wrong emphasis which may be implied as a result of describing a grainsize distribution using statistical parameters which give no indication of the modal characteristics of a sediment.

10.3 Average Modal Distributions Characterising the 10.3 Sediments of the Various Sub-areas

Further investigation of the general character of the sediments occurring in the various sub-areas, sub- 647 environments and environments (i.e. in the fore-beach and back-beach sub-environments, in the Brancaster Harbour Bar and Burnham Harbour Bar sub-areas, in the Longitudinal and Lateral Dune categories and in the Tidal Inlet environment) have been carried out by comparing the average modal grainsizes characterising each sediment population. The modes of all samples have been approximately determined from frequency curves (a large number of which have not been included in this report) of their grainsize distributions. (As in the preceeding statistical analysis of the sediment grainsize parameter values, the sediments occurring on, and to the seawards of Cockle Bight Bar have been excluded from this analysis.) The modes of the samples collected from the above mentioned localities have been categorised into groups at 0.1 phi intervals (i.e. 1.0 to 1.1 phi, 1.1 to 1.2 phi, etc.). The number of modes occurring in each group or category has been expressed as a percentage of the number of samples making up that population. The distribution of the modes occurring in the various grainsize categories have then been illustrated in the form ofahistogram for each sediment population (see Fig. 10.2). As previously stated in section 8.3, where the origin of the dune sands have been investigated by a similar method, the non-statistical nature of the sampling procedure used in this study limits the degree of accuracy of this method. Never-the-less, the large number of samples collected in the majority of these localities (with the FIGURE 10.2. BURNHAM HRRBOUR BAR (97) 7r7rf:177. •=7:77.

LoNGITuDINFIL- DUNES (51)

BACK - BERN (zo)

177.7.77

TIDAL INLET (163)

BRRNCASTER HARBOUR. BFIR(12)

FORE-BEACH (ig)

LRTERRL DUNES (i9)

1.1 1.2 1 .3 H- (.5 1.6 1.7 1.8 1.9 2.0 21 2 2 2'3 24 PHI UNITS

•Fig. 10.2 Histograms shewinc7 the frequency distribution of the various modal sizes in the sediment populaions (number of samples is shown in brackets), representing :me Beach (stippled), •Dune (blank) and Inlet Cln,innel (hlack) environments. 649 exception of the fore-beach and back-beach sub-environments, where the spacing of samples was fairly uniform and is, therefore, believed to be reasonably statistically representative), to some extent, makes up for this limitation. Using the histograms shown in Fig. 10.2, it is valid to compare the relative abundance of the various modes in one locality with the relative abundance of these modes in another locality. However, it is invalid to make any statement as to the absolute abundance of a given modal grainsize occurring in the different sediment populations. This is because the volume of sediment contained in the various sedimentary deposits differs considerably and, as far as this work is concerned, is an indeterminable quantity. By the same token it is impossible to ascertain, with any degree of certainty, the absolute abundance of any given modal size in the sediments comprising the Recent deposits in the study area. It is apparent from Fig. 10.2 that modes between 2.1 phi and 2.4 phi occur relatively abundantly in the Tidal Inlet environment (this mainly applies to the sand of the channel sides and bottoms), in the Brancaster Harbour Bar sub-area, in the fore-beach sub-environment and in the lateral dune category. Modes between 1.7 and 1.8 phi occur relatively commonly in all except the fore- beach sub-environment, while coarse grained modes between 1.2 and 1.4 phi occur in the Burnham Harbour Bar sub-area, in the Back-beach sub-environment and in the Longitudinal FIGURE 10.3 .

858A

858 B

859

0.2 04 0.6 0.8 10 . 1.2 14 1.6 1.5 2.0 22 24 2.6, R.8 3.0 GRAIN SIZE IN P1-II UNITS.

Fig. 10.3 Frequency curves of samples of boulder clay collected in the side of the deep hole opposite Beach Point (858A and B), and on the mainland adjacent to Brancaster Marsh at a point a short distanceto the W. of Dial Cottage. 651 Dune category. Both the Burnham Harbour sub-area and the Tidal Inlet environment are characterised by a wide range of adjacent modal sizes in roughly equal abundance. In view of the above mentioned limitation, it is only possible to make a very tentative estimate of the overall abundance of the most commonly occurring modal grain sizes. It would seem that grainsizes of between 1.7 to 1.8 phi occur most frequently in the study area, and that grainsizes between 2.0 and 2.4 phi occur somewhat less abundantly. An even less prominent maxima appears to occur between a range of grainsizes of 1.2 and 1.4 phi. It is interesting to compare these tentative estimates of the absolute abundance of certain grainsize grades existing in the shady area with the frequency curves of three samples of boulder clay (of which the sand fraction only was analysed) collected from the vicinity (see Fig. 10.3). It is generally apparent that the dominant grainsizes occurring in the boulder clay incorporate the three estimated dominant grades occurring in the Recent sediments of this area. However, the relatively low abundance of sand grades between 1.4 and 1.7 in most of the sediment populations appears to be somewhat difficult to understand, if a boulder clay parent sediment is proposed as the original source for these va3ious populations. Despite this minor anomaly (which may be clarified by further analyses of samples of boulder clay), it is pro- 652 posed that the Recent sedimentary deposits fringing the N. Norfolk coast are (at least in the case of the sand fraction) composed of reworked and redistributed boulder clay. It seems also probable that a large proportion of the silt and clay, as well as the gravel components of these Recent sediments, are also genetically related to the locally occurring boulder clays.

10.4. C.M. Plots 10.4. In an endeavour to discriminate between various sedimentary environments Passega. (1964) has proposed the method of "C.M. Plots", which are assumed to represent the relationship, in terms of grainsize, between the peak energy, and the average energy conditions existing in an environment. This relationship is believed to be indicative of the characteristic energy conditions under which a particular sediment population will accumulate, and, therefore, to form a. criterion to discriminate between environments. By plotting the 1% percentile (C•) against the 50% percentile (N) of a cumulative grainsize distribution, Passega has endeavoured to relate the characteristic grainsizes which are mobilised by peak energy conditions to those which are mobilised at theeuerage energy level. Passega has had some success in differentiating between such widely different sediment populations as those comprising an outwa.sh fan, a glacial and a beach deposit. A modified version of this technique has been utilised in the present statistical analysis. Owing to the difficulty FIGURE 10.4_

2000 BEACH

1000

700

.500

300

LE 2000 I • • • BURNHAM , INLET ENT

C HARBOUR CHANNEL. BAR

PER WOO

5% 700

500

LL O 300 /-N 0

• 0 ty 2000 E z /000 ui ;h. 700 a 500

300

100 200 300 SOO 800 100 200 300 500 $00 MEAN GRRINSIZ.E IN MICRONS

Fig. 10.4 Plots of the mean grainsize (in microns) against the grainsize of the 5% percentile (in microns) of the samples making up the Beach, the Burnham Harbour Bar, the Brancaster Harbour Bar, the Dune and the Inlet Channel sediment populations. 654 in accurately determining the grainsize grades at the 1% percentile level in a. cumulative grainsize curve, the 5% percentile values have been used instead. These values have been related to the median grainsize of a. large number of samples gathered from the Brancaster Harbour Bar, the Burnham Harbour, the Scolt Head Island Beach, the Dune, and from the Inlet channel sub-environments. However, the various modified C.M. Plots shown in Fig. 10.4 indicate that, except in the case of the Burnham Harbour Bar, and to a less extent in the Scolt Head Island Beach, the distribution of points overlap. This method affords no better discrimination between sediment populations than does the statistical mean and standard deviation of their mean grainsizes shown in Table 10.1 and Fig. 10.1. Since the Kurtosis parameter of Folk and Ward (1957) was not calculated for all the samples collected, it was, unfortunately, impossible to apply the method of "discrim- inatory analysis" proposed by Sahu (1964) to distinguish between the beach and the a.eolian dune environments using the fomula:-

beach:aeolian = -3.5688Mz 3.7016 1 2 - 2.0766SK1 +3.1135Kg However, in view of the close genetic relationship which has been shown, in section 8.3, to occur between these two environments, the existence of a statistical distinction of this type seems somewhat unlikely. 655 10.5 Investigation of the Yon-Environmental Relation- 10.5 ship Between the Grainsize Parameter Values

An effort has been made to investigate the possibility that the three grainsize parameter values of the samples collected in this study may, in some way, be interrelated despite their being largely unrelated to a particular sedimentary environment. From the various sections describing the grainsize characteristics of the sediments occurring in different localities, it has been shown that a trend of decreasing mean grainsize is commonly related to a statistical improvement in sorting and to a more negatively skewed grainsize distribution. The values of these three grainsize parameters, of all the samples collected in the study area, have been related by plotting skewness against standard deviation for each of 10 mean grainsize categories of 0.1 phi interva73, between 1.4 phi and 2.2 phi. These plots are shown in Fig. 10.5 A to J. It is apparent from a comparison between these figures that the standard deviation (sorting) values, and the range of these values, tend to decrease (i.e. sorting improves and becomes less variable) as the mean grainsize decreases. The skewness values also display a general trend of more negative skewness with decreasing grainsize. These overall trends may be summarised as a general improvement in sorting, and a more negatively skewed grainsize distribution, with decreasing mean grainsize. Or, 656 stated in another way, in samples showing the same degree of sorting (i.e., in the case of well sorted distributions) the coarser sediment will, in all probability, be more positively skewed than the finer sediment; or similarly, in the case of strongly negatively skewed distributions, the coarser grained sample will be more poorly sorted than the finer grained sample. These relationships have been observed to exist in the sediments collected on the Scolt Head Island Beach, and in the Inlet Channel sub-environment, and have also been observed by Inman (1949) and others. In addition to these overall trends, a statistical tendency (which appears to be unrelated to grainsize) of more positive skewness,with improved sorting, is very faintly discernable in the majority of the individual mean grainsize categories. This trend is usually (althoush not exclusively) indicated by a low density scattering of points in the poorly sorted and strongly negatively skewed quarter of each plot (see Fig. 10.5). This phenomenon is somewhat anomalous in view of the reverse relationship, shown in the overall trend, between improved sorting and more negative skewness.

10.6 Theoretical Considerations on the Mechanism of 10.6 Sediment Fractionation

It is believed that the various sediments comprising the different sediment populations existing in the study area basically reflect the different levels under which they were deposited. Since these sediments appear Fig. 10.5 A to. J. Showing the relationship between standard deviation (6-11 ) and skewness (Ski ) for all samples collected in the study area, in the mean grainsize categories of: coarser than 1.4 phi, 1.4 to 1.5 phi, 1.5 to 1.6 phi, 1.6 to 1.7 phi, 1.7 to 1.8 phi, 1.S to 1.9 phi, 1.9 to 2.0 phi, 2.0 to 2.1 phi, 2.1 to 2.2 phi and finer than 2.2 phi. FIGURE 10 • 5 B

IV z: 1.41-01.5 IDhl

-.+0.2.

-4-0.1

---0.1

-0.2

-0.3

-04

0.2, 0.3 0.4 0.5 0.6 07 0.3 GI. FIGURE 1 0 . 5 C .

Mi 1.5 to. l• 6 1phi

5k,

•

- +0.1

• -

• ---0-3

• - 0.4

0-2. 03 04 05 0.6 07 0:8 (5; FIGURE 10 5 D.

N1 : I.6 to 17 Ipki.

Sk, +0.3

+0.1

---0.3

--0.4

02 013 0 4 0.5 0:8 (5; FIGURE 10.5 E. Mz : 1.7 to 1.8 h.hi.

Sk, -1-0.3

- +02

-+0.I

--W.

-0.5 • • -0.4

0,2. 0.3 64 0.5 0.6 0.7 0:8 CI FIGURE 10.5 F .

: I. F0 1.9 kiki.

02 0.3 04 015 6.6 0:7 0:8 61 FIGURE 10.5 G .

Mz.: K1 fo 2-0 bhi

Sk, -+o:5

-+p2

--0.z

02. 04 0.5 0.6 og ot FIGURE 10.5 H.

NZ : 2.0 to 2..1

Sk, +0.3

+0.2.

+0.1

• • 0

-0.1

-03 •

—04

02, 0.3 O.6 07 FIGURE 10. 5

Niz : 2-1 +o IDhi

5k, +0.3

+0.2.

+0•I

---0.1

\!• • --0•3

-0.4

0.2 0.3 0.6 0.8 cif FIGURE 10- 5 J .

Mz finer kar\ • 2 phi

k; - +0.5

4-0.2

- +0.1

--0•/

--0.3

0.2. 6.3 0:4 0S 06 0.7 0:6 (71 FIGURE 10.6.

FREQUENCY CUR.VE OF THE SEDIMENT REMOVED BY CuRRENCT .V . (E CURVE 14)

rREQuEN cy CURVE OF RESIDUAL. SEDIMENT 0.1 CURVE R

FREQUENCY CURVE OF PARENT - SEDIMENT (E CURVE P) CURRENT VEL0CI1Y •V REMOVES ALL. GRADES FINER THAN THIS GRRINS17..g.

GRAINSIZE DECRERSING

Fig. 10.6 Representing, in 'the form of frequency curves, the concept of fractionation proposed by Douglas (1,946). The terminology used in comparable to that defined in figure 10.8 below. 668 to have originated from a common parent sediment (i.e. the boulder clay), in their present form they represent successive stages in a fractionation series, each stage of which is charadnrised by a distinctive energy level and by a specific type of sedimentary deposit. This model corresponds to the mechanism of "progressive sorting" proposed by Russel (1939). Coarse sediments, associated with high energy conditions, occur at the upper end of this series, while finer grained sediments, associated with lower energy conditions, occur at its lower end. An endeavour will be made to explain the statistical relationship shown above between coarse grained, poorly sorted and positively skewed sediments, and well sorted, fine grained and negatively skewed sediments in terms of a. mechanism, or process of fractionation. In the past it has been generally accepted that if a. current is sufficiently strong to mobilise a given grainsize, all the grains of this size and smaller, contained in a given sedimentary deposit, will be removed by this current. Doeglas (1946) has illustrated this concept by plotting grainsize distributions in the form of cumulative curves which are truncated at their fine end by the removal (winnowing) of a fine grade as a. result of fractionation. This general idea has been illustrated in Fig. 10.6 by representing the grainsize distribution as a frequency curve. It will be noted in this figure that the fine grained sediment put into suspension will be strongly positively skewed, while 669 the coarser residual will be strongly negatively skewed. This theoretical relationship between grainsize and skewness is the reverse of that found extremely commonly in the sediments of the Beach, Dune and Inlet Channel sediment populations. Here the skewness values become statistically more negative as the sediment becomes finer grained. The positive skewness of the winnowed component produced in Doeglas's concept of fractionation is also in discord with the distribution of grainsizes characterising the suspended samples collected during the series of experiments described in section 9.9 above. These sediments are typically unskewed, or are slightly negatively skewed. In view of these considerations, an alternative concept of sand mobilization has been conceived in an endeavour to explain the observed distribution of grainsizes carried by a current, and also to explain the general trends displayed by the sediment populations described in this study. This concept is described in three stages (A, B and C), in which the very general aspects are succeeded by a description of the practical application of this concept.

(A) Primarily, a basic premise has been made: that the capacity of a given current velocity, when it is saturated with fine grained sand, is greater than when it is saturated with coarser grained sand. Stated another way: a given current flowing at a uniform velocity over two sediment deposits, one coarse grained and the other fine grained, 670 will mobilise a greater quantity of the finer sand than of the coarser material. This statement pre-supposes a similar abundance of the various grainsize grades, and that all the grainsizes under consideration are potentially mobile at this specific current velocity. There is, however, no implication as to the possible mechanism of sediment movement, whether as a bed load, in suspension or by saltation. A. limited investigation of the experimental work carried out in the Civil Engineering Department of Imperial College, and of that described in some recent publications, uncovered no experimental work along these lines to either verify or negate this premise. However, Dr. Johnson of the Y.I.O. (personal communication) supports this general statement. (The experimental investigation of this phenomenon presents an interesting line of research.)

(B) This primary proposal may be expanded to apply to the ideal case of a wide range of grainsizes (present in equal proportions), subjected to a uniform current velocity which is capable of mobilising only some of these grades. It is assumed that, of those grainsizes potentially capable of being mobilised, both the fine and coarse grains commence to be mobilised contemporaneously (in all probability this may not be the case under natural conditions, where the fine grains may be preferentially mobilised relative to the coarse grains). However, under the ideal conditions suggested, it is proposed that a

FIGURE 10.7.

PERCENTAGE PROMILITY OP MOBILIZATION 100

CURVE X 80-

60-

IDEAL PRRENT SEDIMENT

20- SEDIMENT REMOVED

GRRiWStZE -DECREASING

Fig. 10.7 Showing the theoretical probability of mobilization ' curve (curve X), fo .. current velocity V, which has been superimposed upon an ideal sediment (represented by a histogram) containing equal proportions of all grainsize grades. The predicted grainsize distribution of the sediment removed by the• current is indicated by the black part of the histogram. 673 trend remains consistant: i.e. the response increases as the grainsize decreases. The potential relative mobility of a specific grainsize at a given current velocity may, therefore, be described in terms of its "probability of mobilization". The probability of mobilization measures the quantity of a specific size grade of sediment that is removed (winnowed) from a sedimentary deposit by a current of given velocity, acting for, and in excess of a minimum unspecified time interval. The probability of mobilization is expressed as a percentage value of the total proportion, or quantity of this grainsize grade, originally present in the surface layers (of an unspecified thickness) of a sedimentary deposit). In the case of a single grainsize grade, this value increases as the velocity increases. In the case of a constant current velocity the probability that a grain will be mobilised increases as the grainsize decreases. The various probabilities of mobilization of a range of grainsize grades (which occur in equal abundance) subjected to a specific, uniform current velocity, may be represented by some type of an inclined (and probably curved) line. Such a line has been arbitrarily shown in Fig. 10.7 by curve X, in which the probability of mobilis- ation is expressed as a percentage on the abscissa, while the grainsize decreases from left to right along the ordinate. In this figure the ideal range of grainsize 674 grades is also represented as a histogram, in which all grades are present in equal proportions (i.e., the columns are of an arbitrary, but uniform height). From curve X, the amount of each grade mobilised may be determined, and is represented in Fig. 10.7 as a fraction, or a proportion, of the corresponding column in the histogram. Curve X in this figure may be considered to represent the current conditions existing at one stage in a fractionation series. Lower energy levels, or stages, in this series would be represented by probability of mobilization curves which are displaced to the right along the ordinate of this figure towards finer grainsize grades. This concept of probability of mobilization must be considered within the framework of various subsidiary factors:- (1) The grainsize distribution of the original sediment. If all the grains in a deposit are capable of being moved by a given current velocity, the whole deposit will be eventually removed. If, however, some grades are stable under these energy conditions, a stage will be reached when a lad deposit is formed of coarse, stable grains on the surface of the deposit which will protect the underlying and unstable grades from current action. As the surface film of material is removed and grains are mobilised from progressively deeper layers within the deposit, it is reasonable to assume that the rate at which a given grainsize is mobilised will decrease with time. A time factor 675 and a depth factor (both of which represent unknown quantities) are, therefore, introduced into this conceit. (2)The above considerations are based upon a closed system concept, in which sediment is progressively removed (i.e., winnowed) from a static deposit. Under such conditions an end point, or a static equilibrium condition will eventually be achieved in either one or another of the above mentioned ways. However, under natural dynamic conditions, at any stage or energy level in a fractionation series, sediment is being introduced from a higher energy level at the same time as somewhat finer grained sediment is being removed to a lower energy level. Under these conditions, a dynamic balance exists between the quantity and type of sediment being introduced, and the type and quantity of sediment removed. This balance determines whether this stage of the fractionation series exhibits residual erosion or deposition. It also determines the grainsize characteristics of the sediment deposited at this stage of the fractionation series, as well as the grainsize characteristics of the winnowed material which is removed to a lower energy level. (3)The degree of saturation of a current, in regard to its sand transporting capacity, may also be significant to this concept. It is probable that as a given current becomes increasingly saturated, the range and proportion of grainsizes that may be moved will progressively decrease. However, for the sake of simplicity, this factor has been FIGURE 10 . 8

PERCENTAGE PROBABILITY OF NIOStLizATIoN I0 PROBABILITY OF MOBILIZATION CURVE FOR VELOCIT Y 'V" (cuRv8 X )

RESIDUAL SEDIMENT wave R = CURVE P— CURVE FREQUENCY W. IISTRIBUTION OF 40 THe "nigoRETtem_ SOURCE SEDIMNT FREQUENCY (CURVE T)) 7)ISTRIBUTiON OF THE WINNOWED SEDIMENT (CURVE

GRRINSIZE 3)ECRAS

Fig.10.8 Hypothetical relationship between the distribution of grainsizes in a theoretical source sediment (curve P); the probability of mob8lization curve (curve X) at current velocity V, and the grainsize distribution of the sediment removed by the current (curve W). The sediment remaining after the winnowed fraction has been removed from the parent is shown by curve R. 677 disregarded in formulating the original theoretical concept which assumes that the upstream current reaches the deposit in an unsaturated state.

(C) This concept of probability of mobilization may be expanded further to include a more normal parent grainsize distribution in which the proportion of grainsizes varies between small proportions of the coarse and fine grades, and large proportions of the intermediate grades. The grainsize distribution of such an ideal parent sediment is shown in Fig. 10.8 by curve P. For convenience, this sediment has been represented by a symmetrical, or unskewed distribution. By imposing curve X (the probability of mobilization curve) on curve P, it is theoretically possible to determine the proportion of the various grainsizes contained in the parent which will constitute the material mobilised and transported by the current, V. The grainsize distribution derived in this manner is shown by curve W. in Fig. 10.8. Curve R represents the residual grainsize distribution which remains after "equilibrium" conditions have been achieved, and has been calculated by subtracting curve W liom curve P. Under natural dynamic conditions, the parent sediment (P curve) may be envisaged as the average grainsize distribution of the material introduced, over a long period of time, to a. particular stage in the fractionation series. Similarly, the winnowed material (w curve) represents the average sediment removed from this stage; while the R curve 678 may be considered to represent the average grainsize distribution of the sediment which accumulates over a long period of time (and under a constant energy level) at this stage. Before any statements may be made concerning the shape of the R and W curves and their significance in general terms, the effect of changes in the shape and relative position of the two primary curves (i.e. the P and X curves) must be explored. These properties are investigated firstly theoretically, and secondly by a trial and error method in which different shaped primary curves are combined and the changes in the resulting secondary curves are recorded. 1. In considering the hypothetical relationship between the curve of the probability of mobilization (curve X) and the grainsize distribution of an ideal natural sediment, a number of proposals may be made with a. reasonable degree of assurance. (a) In a normal fractionation series, the deposits forming at successive stages are progressively finer grained and reflect successively lower energy conditions. Under these conditions, it is reasonable to assume that the energy level on the "up-stream" side of a particular stage will be only slightly higher than that existing on its "down-stream" side. This statement implies that between successive fractionation stages there is no large jump, or sudden decrease between adjacent energy levels. This 670 means that the grainsizes introduced to a certain stage will possess only a small proportion of those grains which are incapable of being removed from this stage. Since curve P (i.e. the parent sediment) represents the sediment introduced to a fractionation stage, and curve X controls the grainsize distribution of the sediment removed, it follows that the coarsest grainsizes capable of being mobilised by curve X will be only slightly finer than the coarsest grains occurring in the P curve. The relative positions of curve X and curve P, shown in Fig. 10.8, may be considered to be fairly realistic. (b) It would seem that the shape of the probability of mobilization curve (curve X) is rather important. In the case of coarse grained, poorly sorted sediment, there will be a tendency for the coarse grains to shelter the fine grains from current action. If this is so, the probability of a fine grain being mobilised will be slightly less than if this same grain was included in a well sorted, fine grained sediment. In addition, a surface uneveness may be produced by a wide mixture of different sizelgrains which will tend to cause the coarse grains to be more exposed to current action than if these coarse grains were included in a well sorted coarse grained sediment. In addition, the affect of fluctuating energy conditions produced by increased turbulence at higher current velocities may be largely responsible for depositing finer grains and mobilising coarser grains than would otherwise 680 be expected; while at lower velocities the current will approach laminar flow conditions, and energy fluctuations either side of the average velocity will, therefore, be minimised. These factors will tend to produce a. rather more gently sloping probability of mobilization curve in the case of high energy conditions and a. poorly sorted, coarse grained sediment, than that produced under the conditions of low energy and a fine grained, well sorted sediment. This means that in the former case a broad range of different grainsizes will occur beneath the X curve (i.e. between grain sizes which can just be mobilised and those grainsizes of which 99.9% will be mobilised), while in the latter case a smaller range of grainsizes will occur beneath the X curve. In general terms, these proposals mean that high energy conditions will be less descriminating, in terms of differentiating between different grainsizes, than will low energy conditions. This general proposal is supported by the natural occurrence in the study area of a statistical trend between coarser grained, poorly sorted sediments (apparently deposited under high energy conditions) and finer grained, well sorted sediments (deposited under lower energy conditions).

2. Practical investigations of variations in the shape of the X and P curves are shown in Figs. 10.9 and 10.10. In Fig. 10.9 the X curve has been represented as FIGURE 10 .9 . nn PERCENTRCIE pERCENTRG,E DV PitorsABluTy OF PR0144:1131LITY OF MORILIZATION MOB11-17...RTtoN

R. -ve SK. R. + ve 5k. + ve - 40

2.0

GRRINSIzE TsCRERSIN

Type A Type 13

(Poorly sorted, coarse grained (Well sorted, fine grained parent sediment representing parent sediment representing high energy conditions) low energy conditions) 682 a straight line inclined at different angles (curves X1 , X2 and X3), while the grainsize distribution of the parent sediment (i.e. the P curve) has been expressed in two forms (types A and B) representing a coarse grained, poorly sorted sediment and a fine grained, well sorted sediment. Type A represents typical high energy conditions while type B represents lower energy conditions. They may be considered to be end members of a fractionation series. Upon superimposing, in these various combinations, the X curve upon the P curve, it is apparent that the resulting W and R curves change in character. In the cases of both the poorly and well sorted parents (types A and B), the amount of material removed (W curve) increases as the slope of the X curve increases. In the case of the poorly sorted parent sediment (type A), an increase in the slope of the X curve produces an increasingly more negative W curve and, subsequently, an increasingly more positive residual sediment (R curve). In the case of the well sorted sediment (type B), an increase in the slope of the X curve causes the W. curve to become progressively more positively skewed and the R curve to become increasingly more negatively skewed, i.e. the reverse trend to that produced in the case of type A sediments. The arrangement of P and X curves in Fig. 10.10 is similar to that in Fig. 10.9, except that the X curve is represented as a slightly curved (concave upwards) line.

FIGURE 10 /0 PERCENTAGE .10: PROBABILITY OF MOBILIZATION

R• UN 5k. - ve 5k.

GRAIN 517-E ZECRER5 I NG

Type A Type B (Poorly sorted, coarse grained (Well sorted fine parent sediment representing grained parent high energy conditions) sediment reprea- -enting low energy conditions 604 Again the residual curves (R curve) in the case of the poorly sorted sediment type (type A) are positively skewed, except in case X3, where it is unskewed. The winnowed fraction (H curve) is consistantly negatively skewed in the cases of both the poorly sorted and well sorted parent sediments, while the R curve in the latter case is consistantly negatively skewed. It must be noted here that the skewness trends observed in the model curves (Figs. 10.9 and 10.10) are not directly comparable to the general skewness trends shown by the sediments collected over Scolt Head Island. This is because the skewness values of the natural sediments have been determined using Folk and Ward's formula, which measures the skewness in relation to the median (50cA percentile) value. In the case of the model frequency curves, however, skewness values have been estimated in relation to the modal size, i.e. the volumes of sediment both finer, and coarser than the modal size have been compared. The "median" skewness values have also been determined for these model curves (shown in Figs. 10.9 and 10.10), and have been compared with the "modal" skewness values in Table 10.2. From this comparison, it appears that the "median" skewness values are a less descriminating measure of the natural shape of a frequency curve than are the "modal" frequency curves. The only advantage of the median and mean values is that they may be calculated simply and exactly from a 685 Table 10.2 Comparison between the skewness values of the frequency curves drawn in Figs. 10.9 and 10.10, determined in relation to the modal grainsize, and in relation to the median grainsize (using Folk and Ward's formula). Table 10.2 "Modal" "Median" "Modal" "Median" Skewness Skewness Skewness Skewness

Fig. 10.9 Type A Type B

R1 + ve unskewed unskewed unskewed - ve unskewed + ve + ve R2 + ve unskewed - VC unskewed

W2 - ve - ve + ve unskewed R3 + ve + ve - ve - ve W3 - ve unskewed + ve unskewed

Fig. 10.10 Type A Type B

Ri + ve + ve - ve unskewed

W1 ve ve - ve ve R2 + ve + ve ve unskewed

W2 - ve - ve - VC - VC R3 unskewed unskewed - Ye unskewed W3 ve unskewed - ve unskewed 6(736 cumulative curve. The estimation, by eye, of the modal value from a frequency curve (as carried out in this study) is a somewhat less exact procedure. This inexactitude arises because the shape of a frequency curve, as derived from a cumulative curve, depends to some degree upon the class intervals into which the distribution if divided. Different class intervals will cause the modal peak of the frequency curve to vary slightly. This, in turn, will cause the symmetry (i.e. skewness) of the frequency distribution also to vary slightly. The mean and median values of a grainsize distribution are mathematical measures, and bear an abstract relationship to the natural properties of the sediment. The modal grainsize, on the other hand, is a physical entity, and is, therefore, a. more meaningful measure of the physical character of a grainsize distribution than are the mean or median values. For this reason, the shape of the various model frequency curves shown in Figs. 10.9 and 10.10, have been described in relation to their modal size. From the above limited "practical" investigation, it is apparent that, by superimposing the probability of mobilization curve on an unskewed parent sediment, the W and R curves so produced may be either positively or negatively skewed. Generally, however, a positively skewed R curve and a negatively skewed W curve are produced in the case of a poorly sorted parent (which in the study area would correspond to a. coarse grained sediment). A€37 While in the case of a well sorted parent sediment (which would correspond to a. fine grained sediment in the study area), there appears to be a tendency for both the R and W curves to be negatively skewed. At first glance it would appear inevitable that by subtracting more material from the fine grained side of an unskewed grainsize distribution than from its coarse grained side a negatively skewed distribution will result. (The "centre of gravity" of a grainsize distribution is considered to be represented by its mean grainsize.) This is true if the modal grainsize is unaffected by this subtraction. Usually, however, (as is the case in the poorly sorted sediments illustrated above) such a subtraction will cause a shift in the modal size (relative to the mean grainsize) towards the coarse end of the distribution. Under certain conditions, this shift of the modal peak will counter balance the tendency towards producing negative skewness, in which case an unskewed or positively skewed distribution will result (i.e. the modal grainsize will correspond to, or be slightly coarser than the mean grainsize). It has been mentioned above that a coarse grained distribution is, in the natural conditions existing in the study area, characteristically poorly sorted and positively skewed, and will be associated with a gently sloping probability of mobilization curve (X curve). These conditions are satisfied by the residual curves in cases type Al in 688 Figs. 10.9 and 10.10. Similarly, a fine grained distribution is, in nature, usually well sorted and negatively skewed, and is associated with a more steeply sloping X curve. These conditions are approached by the residual curves (R curve) in the case of type B3 in Figs. 10.9 and 10.10. The most significant application of this concept of probability of mobilization lies in its extension to the case of progressive fractionation. Under these conditions, an originally unskewed parent sediment will be modified by the removal of a negatively skewed winnowed is fraction (w curve) which transported to a lower energy level where it is deposited. Fractionation of this sediment, in turn, will produce an even more negatively skewed winnowed fraction, and either a more, or less negatively skewed residual, and so on. In this way a fractionation series may be produced which will correspond to that found statistically in the study area where progressively lower energy stages in the fractionation series are characterised by progressively fine grained, better sorted and more negatively skewed distributions (i.e., R curves). In addition to the affects of progressive sorting which are manifested in the fractionation series described above, the effect of local erosion and deposition at any stage in the fractionation series may also be predicted using the general theoretical procedure mentioned. Pres- umably erosion will result when the normal energy level at 680 a stage in the fractionation series suddenly increases, while deposition occurs when it suddenly decreases. Such comparatively rapid changes in the energylevel commonly occur in the beach environment due to changing weather conditions, but will occur more slowly in the tidal Inlet environment where gradual changes in the configuration of a channel may be reflected in local erosion or deposition on the channel bed. This fractionation model presents a potentially fruitful line for further experimental research. By devel- oping a flume in which the width increases down current, it is possible to reproduce a progressive decrease in current energy down stream. If a layer of sand, with a known grainsize distribution, is placed in the narrow (up current) part of the tank and is subjected to erosional current activity, the resulting change in the grainsize distribution of its surface layers may be determined by a sampling procedure using greased cards,as mentioned in section 9.7.2.2 above. Similarly, an examination of the sediment deposited further down stream (where the tank broadens, and the current velocity is subsequently lower) will give an insight into the product of this erosion (the winnowed component). By repeating this procedure for various current velocities, and by using different parent sediments, a rather complete picture of the relationship between the effects, upon various grainsize distributions, of differential erosion and deposition may be 690 obtained. This experiment may be expanded to explore the effects of mixing by introducing a non-erodable (i.e. coarse grained) sediment of known grainsize at the down current end of the tank. Sediment winnowed from the up current deposit will be superficially mixed with the down current deposit and the effects of deposition on the former, and erosion of the latter may be explored by the above mentioned sampling procedure. By sampling both deposits at increasing depths it would be possible to explore the thickness of the zone of influence of both the erosional and the mixing processes. Furthermore, by varying the current velocity it may be possible to produce ripples, or even sand waves, and to investigate the resulting increase in the depth of this zone of influence. 691 11.1 CONCLUSIONS 11.1

An attempt has been made to determine the dynamic milieu which has produced the physiographic and sedimentological configuration of Scolt Head Island, and the changes in this milieu which have caused the physiographic development of part of the N. Norfolk coast. Using wind data from local meteorological stations, it has been shown that maximum wave energy is produced by waves approaching the coast from the N. However, owing to the orientation of the coastline, waves from the N.E. produce a dominant long-shore energy component towards the W. Because of the expanse of the off-shore banks to the N. of the N. Norfolk coastline, the variations in water depth, due to the large tidal range, allow higher energy waves to reach the coast during high tide than during low tide. The coarse grained fractions in the boulder clay, from which the sediments in this locality are derived, are, therefore, concentrated in the upper part of the beach while the fine grained fractions accumulate on the lower part of the beach. Since beach slope is largely controlled by the grainsize of the sediment forming it, this distribution of sediment produces a concave beach profile on the Scolt Head Island beach. Measurements of the topographic changes occurring in the off-shore bank complex, to the N. of the N. Norfolk coastline, show a regular erosional trend except at its S.W. corner where accretion has occurred within historical 692 time. These changes are caused by the tidal current pattern which has been determined from various tidal stations in the off-shore area. Changes in the bank configuration in turn have modified the current pattern. The accretion at the S.W. corner of the bank complex results from an anticlockwise current, and sediment circulation, while theaTsion of the rest of the bank is attributed to the absence of a. closed circulation system. Erosion of the Bank complex has caused changes in the wave refraction pattern, and the migration of an area of high energy along the coast towards the W. The migration of this area of high energy has caused the westward growth of Scolt Head Island. The beach, tidal inlet and dune environments in the vicinity of Scolt Head Island are distinguished by uniform physiographic, sedimentological and dynamic conditions. Wave action is the dominant factor in the deposition of the coarse grained sediments of the upper part of the beach. The combined influence of waves and tidal currents control the fine grained sedimentation on the lower part of the beach. In the almost complete absence of wave action, current action is the dominant process in the tidal Inlet environment. The accumulation of sediments in a number of isolated deposits in this environment has been related to local water and sediment movement patterns. Wind action is naturally the dominant factor in dune formation; however, it has been shown that the 693 immediate source sediment controls the composition of these dune sediments. An hydrodynamic procedure of grainsize analysis was developed to determine the grainsize distribution of the sand fraction of the sediments occurring in the study area. The sediment populations of the various sedimentary environments could not, however, be satisfactorily distinguished from one another on the basis of their grainsize characteristics alone. Finally, a theoretical fractionation mechanism which modifies existing concepts has been proposed to explain the general trends shown by the grainsize characteristics of all the sediments sampled in the study area.

11.2 Limitations 11.2 The accuracy of the results of this study are limited in a number of ways: (1) limitations in the primary data; (2) limitations in processing the data. 1(a) Since successive Admiralty charts and Ordnance Surveys of the off-shore and coastal areas, respectively, are separated by long intervals of time, only the major and long-term changes in topography are apparent. (b) The majority of the off-shore current measurements were made during the early part of the century. More recent topographic changes in the banks may have significantly altered the current pattern. (c) Current measurements made by the author in Brancaster Harbour were obtained over an interval of 694 only 11 hours, and are, therefore, far from representative of the average spring tidal conditions. (d) Since the wind records were obtained for only one year, the theoretically calculated wave action on Scolt Head Island (which is based on these records) is, therefore, not necessarily indicative of the average, long-term conditions. (e) The method of collecting samples by grab, when endeavouring to relate the character of the bottom sediments to the local energy conditions, was found to be unsatisfactory. This is due to the inclusion in these grab samples of sediment which is too deeply buried to be affected by the present energy conditions. A method of sampling using greased cards, which are pressed onto the surface of the deposit, is believed to be preferable. (f) The statistical manipulation of the results of the grainsize analysis is somewhat limited by the subjective sampling procedure used in this study. 2. The settling tube method of particle analysis is believed by many workers to be a less sensitive procedure than sieving. This opinion is not held by the author since some objective standards must necessarily be established before the respective merits of each method may be determined. Since the method of analysis used in this study only determines the grainsize distribution of the sand- sized material, the interpretation of sediments containing, in addition, a clay or gravel fraction is limited. 695 Inaccuracies in analysis of sand sized sediments may be caused by turbulence at the base of the settling tube. These inaccuracies will, however, only affect the analytical results when comparisons are made between analyses of sediments with markedly different grainsize distributions, or between samples of very different size. It is believed that such limitations in the processing of the data are largely counterbalanced by the relatively large number of samples collected and analysed in this study. It is felt by the author that one of the major limitations in the study of recent sedimentary deposits arises through the comparatively small number of samples collected. Since the manipulation of quantitative data has been infinitely simplified by the use of computer techniques, a premium should be placed upon the development of a rapid method of sediment analysis geared to a computer type output.

11.3 Possible Applications of this Study 11.3 The transport of sand, silt and clay by wave, current and wind action produces many practical problems. A great deal of money and energy is expended each year, all over the world, in preventing coastal erosion, maintaining harbour entrances, dredging out docks and stabil- izing dune sands. However, in other areas, these dynamic processes act in a beneficial way by building up intertidal flats and making valuable land available for reclamation. By gaining an understanding of the connection 696 between the movement of sediment and the controlling dynamic factors which produce this movement, it is believed that eventually man will be able to control this phenomenon for his own purposes. An understanding of the processes which are important on the N. Norfolk coast is extremely urgent in view of the fact that the total sealing off of the Wash has been contemplated. The possible changes that could be induced by joining Sunk Sand to the mainland may be considered as a case in point. At the moment, this off-shore lank is accreting and is (at least temporarily) trapping sediment as a result of a sediment circulation centred upon it. However, it would be possible to disrupt this circulation by constructing a barrier from Sunk Sand to the mainland. It is believed that this would result (as has occurred in historical time to Middle Bank to the N.) fin the erosion of Sunk Sand and the transport of sediment into the Wash. This would cause an increased rate of accretion to the intertidal sand flats in the Wash, which would in turn cause an increase in the rate of marsh build out and of land reclamation. A subsidiary effect of this interference with the present situation would be to reverse the dominant eastward current flow in the nearshore zone along the N. Norfolk coastline. The residual direction of sediment movement in this zone would then be towards the W., similar to that produced by wave action in the inter- 697 tidal zone. In other words, the coastal circulation system that now exists in front of Scolt Head Island would be disrupted. This would cause the rate of erosion of the N. Norfolk coastline to increase except in the vicinity of Holme, at thelp2stern end of this stretch of coastline, where accumulation would temporarily occur. Deposition at this point would, however, only occur for as long as Sunk Sand formed a barrier to westward sediment movement. Once this barrier is removed, sediment, transported westward along the N. Norfolk coastline, would be carried into the Wash. These proposals are based upon the assumption that the present current pattern will be maintained except where it has been directly interrupted by the barrier between Sunk Sand and the coastline. However, the possibility exists that this change will significantly alter the water movements elsewhere in the off-shore area. This possibility especially applies to the dominant westward flowing ebb current that influences the central and northern part of the bank complex. Conceivably, this dominant current may be weakened or even reversed. If this happened the erosion of the bank complex would decrease and sediment would only move westward along the coast under wave action in the intertidal zone. A further application of the type of study described in this thesis may follow once the grainsize characteristics of the sediments, as well as their areal distribution, are understood in terms of the dynamic 698 conditions which produced the deposit. Instead of rely- ing upon historical data, the direction of migration, as well as the areas of erosion and deposition of a sediment accumulation may, in this way, be ascertained directly. By using this information as a prerequisite in controlling sediment movement it would be possible to apply it to areas in which the evolutionary history is unknown. 600 REFERENCES

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Laboratory study of sea-level rise as a cause of shore erosion. J. Geolt 73,528-534. SHEPARD, F.P., 1952. Revised nomenclature for depositional coastal features. Bull. Am. Ass. Petrol. Geol. 36, 1902-1912. , 1963. Submarine Geology, 2 ed. Harper and Row, New York, N.Y., 557 pp. 706 SHEPARD, F.P. and INMAN, D.L., 1951. Nearshore circulation. In J.W. Johnson (Editor), Proc. 1st Conf. Coastal Engr. Council Wave Res., Univ. California, Berkeley, California, pp.50-59. 1953. Areal and seasonal variations in beach and nearshore sediments at La Jolla, California. U.S. Army Beach Erosion Board, Tech. Mem.) 39, 82 pp. SHEPARD, P.T. and MOORE, ., 1955. Central Texas coast sedimentation characteristics of sedimentary environments, recent history and diagenesis. Bull.lm. Ass. Petrol. Geol. 39, 1463-1593, SHEPARD, F.P. and YOUNG, R., 1961. Distinguishing between beach and dune sands. J. Sedim. Petro1.31,196-215. SKERTCHLEY, S.B.J., 1877. The geology of the Fenland. Mem. Geol. Surv. U.K. SLATER, L., 1931. Sedimentation in the salt marsh on Scolt Head Island. Trans. Norfolk and Norwich Nat. Soc. 13, 133-140.

STEERS, J.A., 1927. The East Anglian Coast. Geogr. J. 69, 30-42. , 1946. The coastline of England and Wales. Cambridge University Press (London). , 1960. Scolt Head Island. Heffer, London.270pp STRAATEN, L.M.J.U. Van, 1951. Texture and genesis of Dutch Wadden Sea. sediments. Proc. of 3rd Inter. Conf. of Sedimentology, Gronin,en-Wageniirge51. 225-244. , 1952. Current rips and dip currents in the Dutch Wadden Sea. Proc. K. ned. Akad. Net. Sect.B. 55, 228-238. , 1953. Megaripples of the Dutch Wadden Sea in the Arcachan (France). Geol. en Milnb. 15, 1-11. 1954. Composition and structure of Recent marine sediments in the Netherlands. Leid. geol. Meded. 19, 1-110. 1961. Directional effects of winds, waves and currents along the Dutch North Sea coast. Geol. en Mijnb. 40, 333-391. 707 STRkATEN, L.M.J.U. Van and KUENEN, Ph.H., 1957. Accumu- lation of fine grained sediments in the Dutch Wadden Sea. Geol. en Mijnb. 19, 329-254. , 1958. Tidal action as a cause of clay accumulation. J. sedim. Petrol. 28, 406-413. STRIDE, A.H., 1963. Current swept sea floors near the southern half of Great Britain. Quart. J. geol. Soc. London, 119, 175-199. and CARTWRIGHT, D.E., 1958. Sand transport at the southern end of the North Sea. Dock Hark. Auth. 38, 323-324. SWEETING, Y., 1943. Wave trough experiments on beach profiles. Geog. J. 101, 163-172. TANNER, W.F., 1961. Offshore shoals in area of energy deficit. J. Sedim. Petrol. 31, 87-95. TAYLOR, J.C.N., 1954. Petrography of the Shotover ironsands and associated beds between Oxford and Aylesbury. D.I.C. Thesis, Imperial College London University. VALENTINE, H., 1953. Present vertical movements of the British Isles. Geogr. J. 119, 299-315. VEEN, J. Van., 1931. Sand waves in the North Sea. ydro- graphic Review 12, 21-42. , 1936. Onderzockingen in de Hoofden in ver- band met de gesteldheid der Nederlandsche Must. Nieuwe Verhand. Bataafsch. Gen. Proefonderu. Wijsbeg. Rotterdam, 2e reeks, Deel 11, 252 pp. VOLLBRECHT, K., 1966. The relationship between wind records, energy of long-shore drift, and energy balance of the coast of a restricted body of water, as applied to the Baltic. Marine Geol. 4 119-147. WATTS, G., 1953. Development and field tests of a sampler for suspended sediment in wave action. U.S.. Amy., Beach Erosion Board! 34. WEST, R.G., 1963. Problems of the British Quaternary. Proc. Geol. Ass. 74, 147-186. WHITTACHER, W. and JUKES-BROWN, A.J., 1899. The geology of the borders of the Wash. Mem. Geol. Surv.II.K. WILLIAMS, W.W., 1960. Coastal Changes. Rutledge & Kpgan Paul, London, 220 pp. 703 WOODWARD, S. 1884. The country around Fakenham, Wells, and Holt. Mem. Geol. Surv. U.K. ZEIGLER, J.M. and. GILL, B., 1959. Tables and graphs for the settling velocity of quartz in water, above the range of Stokes' law. Woods Hole Ocean Inst. Rep. 59- (unpublished manuscript.

OTHER WORKS CONSULTED

DARBYSHIRE, J., 1956. An investigation into the generation of waves when the fetch of the wind is less than 100 miles. quart. J. Roy. Meteorol. Soc. 82,461-468. DOEGLAS, D.J., 1955. The origin and distruction of beach ridges. Leidse Geol. Meded. 20, 34-47.

FOLK, R.L., 1966. A. review of grainsize parameters. Sedimentology 6, 73-93. HANTZSCHEL, W., 1939. Tidal flat deposits in Recent marine sediments, Tulsa. Oklahoma. Rec. Mar. Sedim Spec. Pub. S.E.P.M., No.4., 1955. 195-206. INMAN, D.L., 1952. Measures for describing the size distribution of sediments. J. Sedim. Petrol. 22, 125-1455. JOHNSON, J.W., 1948. Yodel studies made at the University of California river and harbour laboratory. Trans. Am. Geophys. Union 29, 107-1 14. YESTNER, F.J.T., 1962. The old coastline of the Wash. Geogr. J., 128, 457-478. , 1966. The supply and circulation of silt in the Wash. Inter. Assoc. for Hydraulic Res. Conte. KING, C.A.M., 1951. Depth of disturbance of sand on sea. beaches by waves. J. Sedim. Petrol. 21, 131-140. KRUMBEIN, W.C., 1938. Size frequency distribution of sediments and the normal phi curve. !T. Sedim. Petrol. 2, 84-90. , 1959. Trend surface analysis of contour type maps with irregular control point spacing. J. Geophys. Res. 64, 823-834. YANOHAR, M., 1955. Mechanics of bottom sediment movement due to wave action. U.S. Ara...Beache.c Erosion :Board, Tech. Mem. 75, 121 pp. 709 MASON, C.L. and FOLK, R.L., 1958. Differentiation of beach, dune and aeolian flat environments, Mustangs Island, Texas. J. Sedim. Petrol. 28, 211-226. MENARD, H.N., 1950. Sediment movement in relation to current velocity. J. Sedim. Petrol. 20, 148-160. MoT'ANUS, P.A., 1963. A criticism of certain usage of the Phi-notation. J. Sedim. Petrol. 33, 670-674. MILLER, R.L., 1958. A study of the relation between dynamics and sediment pattern in the zone of shoaling wave, breaker and foreshore. Eclo,gap. Gaol. Hea_v. 51, 542-551. FUNK, N.H. and TAYLOR, M.A., 1947. Refraction of ocean waves, a process linking underwater topography to beach erosion. J. Geol., 55, 1-22. OTVOS, E.G., Jr., 1965. Sedimentation-erosion cycles of single tidal periods on Long Island Sound beaches. J. Sedim. Petrol. 35, 604-609. ROGERS, J.J.W., 1965. Reproducibility and significance of measurements of sedimentary size distributions. J. Sedim. Petrol. 35, 722-732. SCHLEE, J., 1966. Modified Woods Hole rapid sediment analyser. J. Sedim. Petrol., 36, 403-413. SHEPARD, F.P., 1950. Beach cycles in southern California. U.S. Army Beach Erosion Board Tech. Mem. 20,26pp. , 1950. Longshore bars and longshore troughs. U.S. Army Beach Erosion Board Tech. Item. 20. 'and INMAN, D.L., 1950. Nearshore water circulation related to bottom topography and wave refraction. Trans. Am. Geophys. Union, 31, 196-212. SPENCER, D.W., 1963. The interpretation of gra.insize distribution curves of elastic sediment. J. Sedim. Petrol. 33, 180-188. STEERS, J.A., 1933. 1933 Report, Scolt Head Island. Trans. Norfolk Norwich Nat. Soc. 13, 324-330. , 1954. Recent changes on the marshland coast of north Norfolk. Trans. Norfolk Norwich Nat. Soc. 17, 206-209. 710 STRAATEN, L.M.J.U. Van, 1950. Giant ripples in tidal channels. Ti:tdschi. ned. aardrilLsk. Genoot. 67(3), 336-341. and KUENEN, Ph.H., 1957. Accumu- lation of fine grained sediments in the Dutch Madden Sea. Geol. en Mi;inb. 19, 329-354. , 1958. Tidal action as a cause of clay accumulation. J. Sedim. Petrol. 28, 406-413. TANNER, W.F., 1964. Modification of sediment size distribution. J. Sedim. Petrol. 34, 156-162. WATTS, G., 1954. Laboratory study of effects of tidal action on wave-formed beach profiles. U.S. Ala Beach Erosion Board Tech. Mem. 52. ZEIGLER, J.M., WHITNEY, G.G., and HATES, C.R., 1960. Woods Hole rapid sediment analyser. J. Sedim. Petrol. 30, 490-495.