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Home , Longshore drift, Rip current

UNIVER S ITY OF QUEENS LA ND DEPARTMENT OF CIVIL ENGINEERING "'�"' ,;t,. .c B U L L E T I M M o. 7 - MOVE N I FRY, TA .;1 1 .B83 l N0,7 2

WAVE GENERATED CURRENTS

SOME OB SERVATIONS MADE INFIXED BED HYD RAULIC MODEL S

M. R. GOURLAY, B.Sc., B.E.(Syd.), M.E.

Lecturer in Civil Engineering

I TA I · 688 No.'7

�

CURRENT BULLETINS

Engineering Economics - A commentary on Hawken's "Economy of Purchase": J. H. Lavery.

2 Analysis by Computer - Prestressed concrete sections in flexure : J. L. Meek.

3 Analysis by Computer - Nonbraced multistorey building frames: J. L. Meek and E. W. Karamisheff.

4 Brittle Fracture of Steel - Performance of NDl B and SAA Al structural steels: C O'Connor.

5 Buckling in Steel Structures-1. The use of a characteristic imperfect shape and its application to the buckling of an isolated column : C. O'Connor.

6 Buckling in Steel Structures-2. The use of a characteristic imperfect shape in the design of determinate plane trusses against buckling in their plane: C. O'Connor.

Copies of these bulletins are available on application to-

Secretary, Department of Civil Engineering, University of Queensland, Brisbane, Q. WAVE GENERATED CURRENTS

SOME OBSERVATIONS MADE IN FIXED BED HYDRAULIC MODELS

by

M.R. GOURLAY, B.Sc., B.E.(Syd), M.E. Lecturer in Civil Engineering

This bulletin is a revision of a paper to be published in the Proceedings of the Second Australasian Conference on Hydraulics and Fluid Mechanics at the University of Auckland, New Zealand, 6th to 11th December, 1965. CONTENTS Page

ABSTRACT 1

INTRODUCTION 3

TYPES OF WAVE GENERATED 3

THE MODEL INVESTIGATIONS 4

MODEL OBSERVATIONS OF CURRENTS 5

PREVIOUS INVESTIGATIONS 16

DISCUSSION OF RESU LTS 17

CONCLUSION 19

ACKNOWLEDGEMENT 19

REFERENCES 20 ABSTRACT

Following a brief summary of the different types of currents associated with waves, some observations of wave generated currents made during three fixed bed wave model investigations are described. The data presented show the influence of wave direction, wave period, and changes in geometry on the form and magnitude of the currents. The trends from these model results are compared with the model and field observations and theoretical analyses of other investigators and their general agreement shown. In par ticular variations of wave height along a and the location of breaker zones are found to be important factors in the production of wave currents. 3

INTRODUCTION

It is well known that the form of a sedimentary coastline is determined by the wave climate to which it is exposed. is transported along a as littoral drift, the direction of transport depending upon the direction of the waves within the breaker zone. Most coastlines are held at various points by fixed rocky which cause modifications to the form of the . On a coastline undisturbed by man equilibrium conditions generally exist, with the amount of sand transported into a given zone being balanced by the amount being removed from it. However, this equilibrium may be disturbed when an engineering structure is built on such a coast.

Sand is moved along a coast by currents generated by waves. An engineering structure such as a breakwater will produce local modifications to these wave generated currents which will result in accretion or at certain points. It is the purpose of this bulletin to describe some observations of wave generated currents made in three separate fixed bed hydraulic models and the effect of proposed harbour works and other factors on the magnitude and form of the currents. As the measure­ ments were made by rather crude methods, and as the currents were not the primary phenomenon being measured in each case, the results are of qualitative significance only. However, as the author is unaware of any comparable published information of model observations of wave current patterns, it will be of interest to compare them with what is already known from field observations and more idealised model studies.

TYPES OF WAVE GENERATED CURRENT There are several forms of current associated with waves which have been described in the literature (l) . Briefly the following types of current may occur on a coast.

1. BEACH DRIFTING

Strictly speaking this is not a current, but a mechanism for transport. When waves break at an angle to the shoreline such that the uprush has a longshore component, then sand will undergo a sawtooth motion, being moved up the beach with a longshore component and falling back along the line of greatest slope under the action of gravity during the backwash. This action appears to predominate under the fztiOH of the relatively flat waves associated with the so called summer profile an 3) .

2. LONGSHORE CURRENT

When waves break at an angle to the shore, the resultant transformation of the oscillatory wave into a translatory wave results in a movement of water parallel to the shore within the breaker zone. This current will result in the transport of the sand put into in the br ker zone, particularly when steep waves break (� t obliquely on an offshore bar . In thl5 case quite a significant current may form within the trough landward of the bar < ) . 4

3.

By this term is meant a seaward return current along the bottom. Its reality has been subject to debate, but it is possible that it may be identified with the seaward bottom return current which is developed during heavy with an onshore . The latter sets up a landward surface current due to direct shear stress, which tends to pile water up along the shore. A seaward current then develops along the bottom to compensate for this and recent model investigations(5) show that this is a significant factor in beach erosion. 4. RIP CURRENTS

The water transported shorewards by the breakers must be balanced by some form of return current and this appears to occur primarily in the form of rip currents rather than undertow. A rip is a narrow seaward current generall� located at a point of low wave height and spaced at intervals along a beach(6, 7, and 9). They generally coincide with breaks in the offshore bar and are fed by inshore feeder currents from either direction parallel to the beach. Thus they may cause local reversals of the longshore current. 5. WAVE CURRENTS OR LATERAL EXPANSION CURRENTS

The refraction of waves by offshore bottom topography or their diffraction around or headlands may result in local variations in breaker height along a coast. Under these conditions the local mean water level inshore of the breakers will tend to increase at the higher wave height zones due to the transport of increased volumes of water landward. This will result in a lateral expansion current parallel to the shore towards the zones of lower wave height accompanied by a at the latter point to return the water seawards. Indeed it is probable that it is this type of mechanism which causes rip currents(8) in which case the latter may be considered as part of the lateral expansion current system or vice versa. The local reversal of littoral current in the lee of an obstacle is also primarily a lateral expansion current(lO). 6. MASS TRANSPORT CURRENTS

It has been shown theoretically(ll) and experimentally(12) that for waves of finite height there is a net landward movement of water particles along the bottom and at the water surface with a compensating seaward movement at mid-depth. These currents are known as mass transport currents and tend to move sand along the bed from offshore towards the breaker zone under swell or offshore wind conditions. However, if the sand is thrown up into suspension by large waves the tendency is to remove it seawards(5 and 13).

THE MODEL INVESTIGATIONS

The observations of wave generated currents presented in this bulletin were made during three fixed bed model investigations carried out at the University of Queensland. All three models concerned the design of small boat harbours on coast­ lines subjected to heavy wave and swell action. Further, the proposed structures were to be located within the breaker zone in each case. The currents observed included longshore currents, rip currents and lateral expansion currents. 5

Two of the models, Moffat Beach(14) and Mooloolaba (in progress at the time of writing, April 1965) were built for the purpose of designing a harbour for a shore based pilot service for the port of Brisbane as well as providing facilities for the expansion of the fishing industry and general recreational activities. Both models were built to an undistorted scale of 1 in 120. The third model was an exploratory one for determining the best site for a fishing harbour behind the short coral on the southern shore of Norfolk (15). The scales of the latter model were 1 in 200 horizontally and 1 in 100 vertically and the waves were reproduced to the vertical scale for both height and deepwater wave length. Currents were assumed to follow the normal Froude law relations, i. e. V = L2 for the undistorted models r and V = D21 for the distorted scale model. * r r r In all three situations the harbour sites were exposed and subject to continuous swell action, while storm waves were largely a function of the weather patterns as land masses were too far away to limit the fetch. Consequently, in each case tests were made with several offshore wave heights for a given wave direction and period, the directions being selected having regard to refraction effects and meteorological observations and the periods from limited visual observations (subsequently confirmed for Moffat Beach and Mooloolaba by recordings). Wave heights were measured by capacity probes, the output of which was observed visually on an oscilloscope (Moffat Beach) or recorded on a pen recorder (Norfolk Island and Mooloolaba). In situations where the recorded wave varied with time, as is usually the case in breaker zones and areas in the lee of them, the wave height was taken as the maximum recorded in a record of arbitrary length, generally about twenty waves. Currents were measured by timing the motion of small floats with a stopwatch between the lines of a grid painted on the bottom of the model. The form of the current patterns and the breakwater zones were observed visually with the aid of the previously mentioned grid.

MODEL OBSERVATIONS OF CURRENTS

1. NORFOLK ISLAND MODEL In the Norfolk Island investigation waves were reproduced from three different directions - east, south east and south west - with two different periods - 10 and 15 seconds. Wave heights varied between 6 and 19 feet for 10 second waves and 5 and 12 feet for 15 second waves. As seen in Figure 1 waves from the east are subject to considerable refraction since their deep water direction is very oblique with the shore. South easterly waves are affected significantly by Nepean Island offshore which results in two separate wave trains reaching the shore after modification by both diffraction and refraction. The reef is open to the south west and waves from this quarter roll straight in and break directly on the reef.

* L represents horizontal scale, D the vertical scale and V the velocity scale. r r r 6 + -N- I

FEET S� L.... FIG l SOUTHERN SHORE OF NORFOLK ISLAND

The currents set up by waves are consistent with the above conditions (Figure 2). Firstly, the obliquity of the easterly waves sets up a westward flowing littoral current on the seaward side of the reef. At the same time breakers are higher at the western end of the reef since the eastern end is she ltered by Point Hunter. This results in an eastward flowing current behind the reef which returns seaward through one of the entrances near the eastern end. Similarly, breakers on the western end of the reef set up a clockwise circulation in the lee of the pier, while a similar clockwise circulation also occurs in Emily which is sheltered by Point Hunter. With minor variations this pattern is maintained for all wave heights tested for both 10 and .15 second waves (Figures 2a and 2b).

A similar current pattern occurs with 10 second waves from the south east (Figure 2c). However, a change occurs with 15 second waves as, owing to the greater refraction and diffraction effects around the southern side of Nepean Island, the highest breakers on the reef are now at the eastern end (Figure 3) and the current behind the reef is westward except for a small clockwise circulation at the western end (Figure 2d).

10,-----::-..---.------.----"-T"-----.----,.----, t LEGEND 0 5ft. - 15Sec.Wavts l a1---�--+----+----+��-+----+---1 .a. 12ft. - 10Sec. Waves c,. 12ft. - 15Sec Waves ■ 15 ft. - 10 Sec. Waves Current Oirect1or'I: •-10 Sec. Q- 15 Sec.

Distance along Line from end of Pier to Point Hunter - Feet FIG. 3 WAVE HEIGHTS AND CURRENT DIRECTION BEHIND REEF -SE.WAVES 7

(a) iOSEC. EASTERLY WAVES

(b) 15SEC. EASTERLY WAVES

(c) 10SEC. SOUTH -EASTERLY WAVES

(d) 15 SEC. SOUTH-EASTERLY WAVES

4�.�,"{! '-�/;-,: ,,,- ...-.:- '·'�,

�..-- Curren t Direct,on • Wave 0_ire ct,on (f)

NORFOLK ISLAND 8

More pronounced changes occur with south westerly waves (Figures 2e and 2f), the principal one being the reduction of the westerly littoral current seaward of the reef and the development of more pronounced seaward currents through the reef entrances. The latter have the appearance of rip currents, and tend to move seaward through the breakers. Behind the reef eastward currents are set up with 15 second waves, while with 10 second waves this current tends to be in the opposite direction. The circulation in Emily Bay is now anticlockwise except for the lowest waves (5 to 6 feet) of both periods.

While it was not possible to relate the magnitude of the currents to the wave characteristics in all cases due to incomplete data, it was evident that the current velocity was a function of local breaker height. Figure 4 shows the variations in current velocity with ocean wave height for different current systems. Wherever sufficient observations were available, the maximum observed velocity and the mean velocity for a particular current are given. In certain cases the velocity at specific points is also shown. The number of observations determining the mean velocities is variable between two and five, so these velocities are not necessarily of equal . Also plotted in Figure 4 are the heights of the breaking waves which cause the currents. These values represent the average of several wave heights measured within the particular breaker zone which appeared to be the one primarily causing the current. It is evident from Figure 4 that both the currents and the breaker heights vary in the same manner with changes in offshore wave height.

1------,-----::-1-�=;o�.,,f-''------;l"--l9t·� 61----+---+--+-+-� :,:� �u 1--�-i-=-t------1=�0-f'----;1.� � ,'-----t=M---1 � C'.: I

12 16 2 u 0 � -----,------/c---..0 ·� j ,c-----+--,...8-�,2 0 4 8 :,: Wove Height - Feet 1-----!,"'=,-""-+-��-+-"'�'J..'----0-- Max. current velocity o Eastward current behind reef-E/end □ Circular current behind pier ><---+----;_ Av. current velocity □ Circular current behind pier "' Eastward current behind ree f-Slrughter Bay 0------

In certain cases the data given in Figure 4 was sufficient to indicate that the current velocit y is proportional to the wave height. Figure 5 shows the current velocity plotted as a function of breaker height and, while there is scatter of some of the results, the general trend in every case indicates that the velocity is directly proportional to the wave height. The constant of proportionality is of course variable due to the different relative effects of the other variables which influence the magnitude of the velocity. These other factors include breaker angle and the cross- 9

sectional area of the flow after the water leaves the breaker zone. In a couple of cases the velocity-breaker height line does not pass through the origin , thus indicating a finite current velocity at zero wave height. In these situations it is probable that the wave height measured was not that of the breakers causing the current, but of the breaking surge some distance landward of th e break-point.

9,----,------r-- �----,----,----,--,------, �u �6f-- -+-- --+--t-----b-"--7"'t...... ,.--t--7 �I - - --t-- 7 r�4 >- --¥""�--+--->---+------;..� - �4f---+-- --+- --/-,.,....-,.',11-+----+-

1: .,..- �2f-- -+-..,,C..,,..-4--t---+--- -+---t- 7 ±-- + 8 +- - Breaker Height -Feet 2 4 6 8 10 12 2 4 6 8 10 12 14 (a) SOUTH EAST - 10 SEC. (c) EAST - 15 SEC.

---....--, .,.- � Mox. current x- +-A-->f Av. current Current at a - """----t------l 0------0 point 21-- --+-----"L,,.l""--=--,:l---' Note: For locations of currents see Figure 4. Breaker Height - Fee t 10 2 4 6 8 10 12 ( b) SOUT H WEST - 10 SEC.

FIG. 5 RELATION BETWEEN CURRENT VELOCITY AND BREAKER HEIGHT NORFOLK ISLAND

With regard to the influence of wave period on the current velocity, there was insufficient data to give a definite relationship. However, there was some evidence that for breakers of equal height, the currents were smaller for the longer period 15 second waves.

2. MOFFAT BEACH MODE L

In this model a proposed breakwater was first tested with 10 second waves of various heights from the east. Tests were then commenced with north easterly waves and it was found that substantial modifications would be necessary to the breakwater, both to provide adequate protection and to eliminate an undesirable current which formed within the harbour entrance (Figure 6a) . High waves breaking at a slight an gleto the beach due west of the breakwater head caused a circular current in the lee of th e northern extremity of the breakwater . This was augmented by waves breaking in the shallow water to the north of the breakwater head.

Two attempts at modifying this current were made , the first involving a short western breakwater to deflect the current and the second an extension of the main breakwater . The first was partially successful , while the second merely accentuated both the area and magnitude of the current (Figures 6b and 6c). The final design (Figure 6d) , which was a compromise between protection, navigation and current pattern requirements deflected the main current from the entrance, although a smaller current still occurred within the western wave trap owing to the presence of a small breaker zone inside the outer western breakwater. Observations showed that the 10

� ('� {--,, ,\ (l _.,

(b)

Waves under 12ft. (d)

I I '

' \ ....,- o,,- ,< ,,_ .,,,:,q,- ,<.-s.

Waves over 12 ft. (e)

LEGEND �Breaker Zones. ..,_.,_Current Direction. + Wave Direction. FIG. 6. MOFFAT BEAC H WAVE CU R.RENT PATTERNS 11

removal of th e inner western breakwater resulted in this current occupying the whole harbour area (Figure 6f) . A westward current across the harbour entrance still occurred with waves 12 ft and higher (Figure 6c) and its path varied with the location and extent of the breaker zone off the head of the main breakwater.

Further investigation of this arrangement with easterly waves showed that the circulation to the north of the outer western breakwater was still weakly present together with a more diffu sed northerly current from the breaker zone to the north of the breakwater head (Figur e 6g) . The circulatory systems within the harbour had disappeared since the small breaker zone within the western wave trap had also disappeared. Of interest al so are the current patterns observed in the final tests from the east without any breakwaters, where a strong northerly current was observed along Dickey Beach (Figure 6h) in place of the southerly currents with the breakwaters.

In these tests the relationship between local wave height and current velocity is clearly shown (Figure 7) for the circular currents within the harbour area, both with and without the inner breakwater and for the southward current on the seaward side of the br eakwater. The two circular currents, both originating from the same breaker zone plot as a singletr end line (Figure 8) except for one point which is significantly lower. In this test it was noted that for some unexplained reason the path of the current was different to the others, passing through deeper water where the velocity was lower than would have been otherwise expected.

1----' #-!+ - -+ 4 4 - - --+- --+- -+-- ---,s :g. 1------11--.,,_d� - =- =---: "i+ _-"-,1,"' -..:::"'-- ,,...- ++- -.....---,a lt:-_- _ � 2>-----,<,<,'L----+--+-- --+----+- �4 � - .x 0 "'-- --+ "'- ii 4 8 12 16 20 240 Oo --'4 ------!8- 12�-----"-c16-�20 �2 r Ocean Wave Height - Feet (a) CIRCULAR CURRENT WITHIN HARBOUR ( b) CIRCULAR CURRENT WITHIN HAROOUR (c) SOUTHERN CURRENT ALONG WITH INNER WESTERN BREAKWATER WITHOUT INNER WESTERN BREAKWATER SEAWARD SIDE OF BREAKWATER

J..EQfl!D � Max. current velocity � - --'(-- + - - Av. current velocity • • ■ Breaker height FIG. 7 VARIATION OF CURRENTS AND BREAKER HEIGHT WITH O=C.... E..._A .....,N...__WA...... Y.... E ._ .....HE -.. 1.....GH ...... T...__ ( N, E. WAVES ) MOFFAT BEACH

u s�-�--�-�--�--��� LEGEND Circular current within harbour with a{ 0 <:: � i'.;'. Inner Western Breakwater. 0 I o Max. Current Velocity. � � 41----+---+--�-�--��+------l . x Average Curren_t Velocity. . !_ 4 -u Circular current within harbour without .e 0 .; Inner Western Breakwater. � > c, Max. Current Velocity. � - 21-----+------,,.--S.--"'-'-----+---+---+------l Y Average Current Velocity. 2 >----+-��-+>-.c-,,.- t--=- ----1 C Southward current on seaward side 1: of Breakwater. t::, t::, o Max. Current Velocity. u u + Average Current Velocity. 4 6 8 10 12 Breaker Height - Feet Breaker2 Hei4ght - Feet 8 (a)SOUTHWARD CURRENT ON SEAWA RD SIDE OF BREAKWATER ( b)CIRCULAR CURRENT INSIDE HARBOUR FIG. 8 RELATION BETW N CUR NT BREAKER HEIGHT %.E WAYE�� �Bfi�/YBE��g 12

3. MOOLOOLABA MODEL

As the general wave conditions at this site were similar to Moffat Beach, the same procedure of testing with 10 second waves of various wave heights was adopted. Tests were made from both the north east and east for existing conditions and several possible breakwater alignments. On account of the more open nature of the area modelled the current patterns showed a greater variability with wave height for a given wave direction and break­ water geometry (Figure 9). This is largely a result of changes in location and extent of the breaker zones with waves of various heights. However, for both east and north east waves the same trends in current directions at the mouth of the Mooloolah are evident. There is a westward current along the edge of the rock shelf on the north side of Point Cartwright with an anticlockwise in the lee of the causing an eastward current along the . The introduction of a breakwater deflects the first current northwards into deeper water and tends to reduce the intensity of the current along the spit (Figures 10a and 10b), the velocity of the latter becoming smaller the longer the breakwater. Several rip currents were observed along the beach between Point Cartwright and Alexandra Headland with easterly waves, their location corresponding to zones of lower breakers (Figures 9i and 9j).

-- - � - 0 i:: 0 .-·� ,:._ o 4 8 12 16 20 0 4 8 12 16 20 � (a) EASTWARD CURRENT ALONG SPIT - N.E. WAVES 1:;- ( b) EASTWARD CURRENT ALONG SPIT - E. WAVES �8�E 1f8§ I16 ·g > LEGEND Currents : o No Breakwater o Breakwater N?1 js 12 ll Breakwater �2 :, ,;; Breakwater N?J u x 8 reokwoter N°3A !"' + 8 reakwoter N?J B 4 I·.; 8 Breaker Height , L ■ Breakwater N?1 I f -"� • Breakwater N?2 ..- Breakwater 1,/.i ,/ N?J _/ ,,,_,, / 4 2 \\ I /( / ,," 1' / ,.,.___ I�I ....,. , . 0 f / o , e 12 16 28 °o�" -·· -�, --�e--1�2 -�16-�20 Ocean Wove Height - Feet Ocean Wove Height - Feet (c ) NORTHWARD CURRENT ALONG HEAD OF BREAKWATER - N.E.WAVES (d) NORTHWARD CURRENT ALONG HEAD OF BREAKWATER - E. WAVES FIG. 10 VA RIAT)ON OF CURRENTS ANO BREAKER HEIGHT WITH OCEAN WAVE HE IGHT - MOOLOOLABA

With easterly waves the northward current past the head of the breakwater attains a maximum during relatively low ocean waves (5 to 8 ft) and then falls off during higher waves (Figure 10d). This is because of the progressive extension of the breaker zone seawards and to the north of Point Cartwright, which results in a more diffuse current further offshore from the breakwater. The eastward current along the Spit is similarly reduced at high wave heights from the north east (Figure 10a). 13

Ocean Wove Height 8ft. Ocean Wove He ight 16ft.

Ocean Wove Height 8ft. Ocean Wave Breakwater N9 2. Breakwater N9 2.

H.W.M.

Ocean Wove Heght ­ ��

Ocean Wove Height 5 ft. Ocean Wove Height 12ft. Bre okwoter N9 2. Breakwater N9 2.

HY0-4.

Ocean Wove Height10ft, Ocean Wove Height 14 ft. ...__ Breakwater NQ 38. .,_ ....._'-...__� ---�� Breakwater N9 38.

FIG. 9. - --Current Direction. • Wove Direction. MOOLOOLABA WAV E CURRENT PATTERNS. 14

Unfortunately, from the point of view of studying the wave generated cu rrents, the regions where measurements of wave height were required did not always correspond with the most significant breaker zones so it has not been possible to relate current velocities to local breaker heights except for the northerly current past the breakwater head for north easterly waves (Figure 10 c). This current is caused by the breakers on the north side of Point Cartwright and its velocity was found to be proportional to the height of the breakers on the edge of the rock shelf at the base of the breakwater (Figure 11). The current also increases as the breakwater alignment becomes more easterly since the current is then deflected more and the cross-sectional area of flow tends to be reduced.

10�------�--�--�-----, LEGEND --0---- Breokwoter N9 1 NOTE : Breokwoter Number -e,- Breokwoter N9 2 i e --v/-- Breokwoter N9 J l���f� � iB���i���er more Westerly alignment 8>----�- --.------,----...... ,:...---=-- -+----+,,-L----+-----j

� � t 61---+----+----+------+-'"-----/---+�=---+�-=-----+---"'------l � ·;:;0 � 41---+----l------l,.L--,,L----+-___:,..."'------+-----+----+-----<

Breaker Height - Feet 6 8 10 12 14 16 FIG.11 RELATION BETWEEN CURRENT VELOCITY AND BREAKER HEIGHT FOR VA RIOUS BREAKWATERS (N.E.WAV ES) MOOLOOLABA

Observations in the model indicated that the westward current along Point Cartwright was basically a littoral current since the waves broke at an angle to the shore. On the other hand the eastward current along the Spit in the lee of the head­ land was basically a lateral expansion current since the breakers were parallel to the shore and the wave heights decreased along the direction of the cu rrent. These conclusions are confirmed by refraction diagrams for both North Easterly and Easterly waves (Figure 12) where the orthogonals show a distinct angle with the contours along Point Cartwright while they are normal to the shore along Mooloolaba beach and the Spit. Moreover, the spacing of the orthogonals tends to increase in the lee of the headl and thus indicating a decrease in wave height in agreement with observation, while the closer spacing of the orthogonals along Point Cartwright is consistent with the higher current velocities observed in this area. 15

.. ----.:.____,,,... CJ� & � ( o) NORTH EAST WAV ES

_N_

Wove Period : 10sec.

( b) EAST WAVES FIG. 12 WAVE REFRACTION DIAGRAM - MOOLOOLABA 16

PREVIOUS INVESTIGATIONS

The principal published observations of wave currents, in particular longshore currents and rip currents are those made on the Californian coast by Shepard(lti) and Inman(7, 8 and l 7), Inman and Quinn<18), and Putnam, Munk and Trayl or<19). other observations of longshore currents have al so been made by various Japanese investigators(20 , 21 and 22). These observations have shown that longshore currents vary with the angle the breakers make with the shore and the breaker height. The dependence on the latter factor is shown to be the reason for the unsteady and erratic behaviour of the currents since they are produced by wave trains which are themselves of randomly varying wave height.

Shepard<16) indicates that the median velocities of longshore currents are less than 1 knot for waves below 10 ft high with maximum currents up to 2 knots. Hom-ma, Horikawa and Sonu(20) quote longshore current velocities of the same order; i. e. , less than 1 knot for waves less than 1 metre high, 1 knot for waves 1. 5 to 2. 0 metres and 2 knots for breakers 2 to 2. 5 metres high. The same authors(21) al so record observations of currents up to 6 knots in a typhoon. Wave generated littoral currents of 1 to 2 ft/sec were observed with breakers 3 to 4 ft high in a fixed bed model of a semi enclosed bay at the mouth of Lake Macquarie, N. S. W. (23).

It has al so been observed, that longshore currents, while generally flowing in th e direction in which the waves approach the coast, show considerabl e local variation due to the formation of rip currents which may result in local reversals of (7). The occurrence of rip currents has been shown to be rl eated to the variations of breaker height due to refraction of the waves by offshore bottom contours(7). Water flows parallel to the coast away from zones of high waves and then seaward as rips at th e zones of lower wave height. Rip currents are more definite and widely spaced with long period swell wh ere refraction is considerable than with shorter period waves. On the other hand, the longshore current is more pronounced for the latter(7) since the breaker angle is greater due to smaller refraction effects.

Qualitative observations on the New South Wales coast near Sydney by McKenzie(9) confirm most of the American conclusions, while adding some further details. In particular he notes that at low the velocity of a rip is generally greater than at high tide since, although less water is moved shoreward, the return flow is more concentrated within the rip . The significance of coastal geometry on the form of rip currents is al so noted as is th e slope of the foreshore which infl uences the form of the breakers. More complicated current systems occur with flat than with steep ones where rips generally do not occur.

With regard to model investigations of wave currents, these generally relate to longshore currents on a strai ght beach . Most of those made with a moveable bed (e. g. , 2 and 3) were made with the purpose of determining the littoral transport rate and the currents were generally not measured. On the other hand, those made with a fixed bed(19 and 24) relate only to longshore currents al ong a straight plane beach with no bar. Lateral expansion currents and other wave currents in the vicinity of structures and headlands appear to have been measured systematically in only one 17

case (25) , although their influence on the development of littoral spits and has been shown in moveable bed tests(lO) Theoretical analyses for longshore currents have been made for plane beaches by Putnam, Munk and Traylor(l9) from energyand momentum considerations and by Mashima(26) and Eagleson<27) from a momentum point of view. Bruun(4) has put forward an analysis based on continuity for beaches with an offshore bar, the quantity of water transported over the bar by the breakers being balanced by the quantity returning seawards in the rips. The wave height distribution and lateral expansion current in the lee of a type structure have been analysedon the basis of diffraction theory by Shimano, Hom-ma and Horikawa(25) .

DISCUSSION OF RESULTS

The observations made in the three model tests described in this bulletin will now be summarised and compared with those of other investigators.

1. CURRENT PATTERN

While changes in wave direction, period and height are very significant, the local currents along a section of coastline are also very much a function of the coastline geometry as represented by coastal form and offshore bottom contours. For instance in the lee of a headland or breakwater the local drift current along the shore is generallydirected towards the headland regardless of wave direction. This reversal of drift in the lee of headlands has also been noted in California(l6) and in model tests(lO), Current patterns vary with the location of breaker zones and hence with variations of deepwater wave height. This factor is more significant when the off­ shore bottom is relatively gently sloped as off Mooloolaba than when it is steep as at Norfolk Island. 7 As observed by Shepard and Inman< ) rip currents are more prevalent when waves approach the shore wi th crests parallel to it as for example in the south westerly waves at Norfolk Island and easterly waves along Mooloolaba beach. Variations of wave height due both to refraction and diffraction have a significant effect on the current pattern, in particular being the dominant factor in semi enclosed areas such as harbours and bays. While the breaking of waves at an angleto the shore is the prime factor in generating a general littoral current as in the zone seaward of the reef at Norfolk Island or along the rock shelf on the north side of Point Cartwright at Mooloolaba, variations in wave height along the shore result in currents flowing from regions of high breakers into regions of lower breakers. For instance, the currents behind the reef at Norfolk Island are largely determined by variations in breaker height, while those on the seaward side depend on breaker angle. 18

Observations by the author in a ripple tank of lateral expansion currents behind a breakwater and an offshore island coofirm that the lateral expansion currents described by Sauvage de St. Marc and Vincent(lO) are basically due to the decrease in wave height in the lee of the structure. The closer the structure is to the shore, the greater the variation in wave height and the stronger the currents. This fact is clearly shown in the decrease in current velocity along the Spit at Mooloolaba with the construction of a breakwater . In certain situations with breakers at a localised point some distance from the shore in an otherwise protected area such as inside the Moffat Beach Harbour , the breakers act rather like a recirculating pump which sets up a circulation influencing the whole area.

2. CURRENT VELOCITY The velocity of the wave generated current is proportional to the height of the breakers causin it. This is in agreement with other observations and theoretical � analyses(16, l9 and 7), but the velocities observed in the models appear to be some­ what higher than those observed by investigators in the field quoted previously.

The maximum current velocity along a shoreline does not necessarily coincide with the position of the highest breakers. If the wave height varies along the shore due to refraction, then the significant factor will tend to be the difference in wave he�hts which causes a longshore water level gradient shoreward of the (20 an 25). The prediction of longshore current velocities and directions on the sole basis of consideration of the direction of the wave orthogonals at the breaker zone cannot therefore be considered reliable along an irregular coastline.

Little or no data was obtained roncerning the effects of variations in breaker angle and wave period, although it was evident that the currents increased as the wave became more oblique with the shore as reported by other investigators (19 and 27) and probably decreased with an increase in wave period in agreement with the theories of Putnam, Munk and Traylor(l9) and Bruun(4). In situations where the current flowed into relatively calm water it was also obvious that its velocity was inversely proportional to the cross-section of flow as in the somewhat analogous situation of an offshore bar and longshore trough with rip currents treated by Bruun(4). Indeed the flow behind the reef at Norfolk Island may be regarded as an extreme example of this situation. Similarly, currents along the end of the Mooloolaba breakwater increased with changes in breakwater alignment due to of the current. An interesting point is that the breaking of large waves some distance off shore generally results in a reduction in current velocity since the breakers near the shore have a lower wave height due to their partial dissipation further offshore. Moreover, the high breakers in the model tended to vary in height markedly with time even though the generated waves were relatively constant, and this resulted in a correspond­ ing unsteadiness in the model rip currents similar to that observed in nature(7). 19

3. SAND MOVEMENT The extension of a headland by a breakwater or groyn e results in a reduction of the velocity of the current in the lee of the structure, this reduction in velocity increasing with the length of the structure due to increased diffraction. The shore line will thus tend to build up if there is sufficient sand available(28) until a new equilibriwn form is established. On the other hand, if the sediment supply is cut off by the structure, then there will be no shoreline recession within the zone occupied by the eddy and the reversed drift current along the shore. This explains why it is possible to have a stable tidal at the updrift end of a beach subject to a significant littoral current, instead of in the usual position at the downdrift end of the beach. The Mooloolah River entrance is of this type and the sand bar at the end of the Spit has probably been built up by the lateral expansion current of the waves in conjunction with the tidal currents.

The Mooloolah River entrance is relatively stable since, while the wave currents vary in extent and magnitude with various wave conditions, their directions are fairly constant and it is probable that the littoral drift is relatively small. On the other hand, it is interesting to note that the region behind the centre of the Norfolk Island reef, Slaughter Bay, where the currents were observed to change direction with different wave ronditions, is also known to have an unstable sand bottom whose elevation may vary by several feet from time to time.

CONCLUSION Observations of wave generated currents in three fixedbed models showed that these currents can be quite extensive and may attain appreciable velocities. The observations are in agreement with those of other observers made in the field and in more idealised experiments, as well as with the general trends of theoretical analyses. In particular, variations of wave height along a shore and the location of breaker zones are found to be important factors in generating currents of the lateral expansion type in the vicinity of headlands and other coastal irregularities.

Further research on this type of current under simplified conditions, both experimental and theoretical, is desirable to define the relations between the magnitude and form of the current on one hand and the wave characteristics and coastal geometry on the other. Such information would be useful in helping to predict the possible movement of sediment in the vicinity of proposed harbour works from the undistorted fixed bed model tests required for a reliable assessment of the wave con­ ditions within the harbour, thus saving the cost and trouble of a moveable bed model study.

ACKNOWLEDGEMENT The experimental observations used in this paper were made by the author dur­ ing hydraulic model investigations carried out in the Hydraulic Laboratory of the Department of Civil Engineering, University of Queensland for the Department of Harbours and Marine, Queensland and the Commonwealth Department of Works. 20

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