SPATIAL AND TEMPORAL VARUTIONS IN SUSPENDED SEDIMENT AND
SEDIMENT DEPOSITION, CUMBERLAND BASIN, BAY OF FUNDY
A Thesis
Presented to
The Faculty of Graduate Studies
of
The University of Guelph
by JENNIFER FRANCES PARRY
In partial fülfilment of requirements
For the degree of
Master of Science
August, 200 1
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CONTROLS AFFECTING THE SPATIAL AND TEMPOM VARIATIONS IN SEDIMENT DEPOSITION, CUMBERLAND BASIN, BAY OF FUNDY
Jennifer Frances Parry Advisor: University of Guelph, 200 1 Professor J. Ollerhead Co-Advisor: Professor R. Davidson-Arnott
The purpose of this study was to examine the relationship of spatial and temporal variation in sediment deposition rates between several saltmarshes over a tidal cycle, within the Cumberland Basin to the following geomorphic controls: (1) wind waves and tidal currents, (2) suspended sediment concentrations, (3) vegetation, and (4) marsh morphology. Five diverse marshes were chosen along the length of the Basin. Sarnples of suspended sediment concentration (SSC) and sediment deposition were collected simultaneously over individual tidal cycles, while continuous measurements of wind direction and velocity were collected over the duration of the study. More detailed measurernents were made during several tidal inundations by an electronic instrument array. Results indicate that waves generated at the mouth of the Basin are of key importance to sediment availability in the water colurnn throughout the Basin. Spatial variation in sediment deposition between marshes is primdy a function of two intemal controls: marsh rnorphology and relative roughness index (ratio between vegetation height and mean water depth). However, no one geomorphic control cm be determined as dominant, as sediment deposition in the Cumberland Basin is the result of complex interrelationships between various controls. 1 would like to thank my advisors Dr. Jeff OIlerhead and Dr. Robin Davidson-
Arnott first and foremost for giving me the opportunity to become involved in the ongoing research project in the Bay of Fundy - it has been an invaluable expenence.
Moreover. 1 would like to thank them for their patience and guidance both on the field and in the process of analysis and writing. I would also like to thank Jaime Dawson for her experienced help and advise throughout the project and Danika van Proosdij for her time and support (you have been in this position too!). A great big thank you to my invaluable field assistant, Michelle Zehr who helped make things run srnoothly and for the additional help provided by Dr. Patrick Hesp, Erin Whittley and Kelvin Macquamie.
Also, thank you to Mario Finoro, our chief techincian who provided his techincal expertise and patience throughout the process of this research.
My thanks would not be complete without a big thank you to my farnily and friends (especially Mom, Dad and Pete) who were there to encourage me through the thick and the thin of it aI1.
Finally, I would like to acknowledge the financial support provided by a Latomell
Travel Grant, University of Guelph Graduate Scholarships and Ontario Graduate
Scholarship of Science and Technology, as well as NSERC grants to JWO and RDA. TABLE OF CONTENTS
ABSTRACT
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
CHAPTER 1 RESEARCH CONTEXT AND OBJECTrVES
1 .O Coastal saltmarshes
1.1 Significance of study
1.2 Directions of saltmarsh research
1.3 Controls on saitrnarsh sedimentation 1.3.1 Sediment supply 1.3.2 Wind stress and wave height 1.3.3 Tides 1.3.4 Vegetation 1.3.5 Morphology
1.4 Patterns and rates of inorganic deposition
1 -5 Study purpose and objectives 1S. 1 FieId work
CHAPTER II STUDY AREA AND SITE SELECTION 2.1 Cumberland Basin, Bay of Fundy
2.2 Site selection 2.3 Site characterization 2.3-1 Pecks Cove marsh 2.3.2 West Allen Creek marsh 2-33 Westcock marsh 2.3 -4 Tantrarnar marsh 2.3 -5 Fort Beauséjour marsh
CHAPTER III FUSEAIRCH DESIGN AND Ml3THODOLOGY
3.1 Research design
3.2 Instrumentation and procedures 3.2.1 Sediment deposition 3.2.2 Suspended sediment concentrations Rising stage bottles Optical Backscatterance Sensors 32.3 Wind and wave conditions Wave stdf Wave prediction 3.2.4 Tides Tide tables Pressure transducer 3 2.5 Vegetation 3.2.6 FieId mapping 3.2.7 Data analysis
CHAPTER IV RESULTS
4.0 Introduction
4.1 Temporal and spatial variation in suspended sediment concentrations (S SC) 4.1.1 Temporal variations in SSC 4.1.2 Spatial variations in SSC across each marsh 4.1.3 Spatial variations in SSC between marshes 4.1 -4 Variations in SSC as a fünction of local and regional wave height Local predicted wave height Regional predicted wave height 4.1.5 Variations in SSC as a function of tidal amplitude
4.2 Relative significance of each geomorphic control on sediment deposition 4.2.1 Wind waves 4.22 Tides 4.2.3 Suspended sediment concentrations 4.2.4 Vegetation
4.3 Spatial variation of sediment deposition 4.3.1 Variations in sediment deposition across each marsh 4.3.2 Variations in sediment deposition between marshes
CHAPTER V DISCUSSION AND CONCLUSIONS
5.0 Introduction
5.1 Temporal and spatial variation of suspended sediment concentrations (SSC) 5.1.1 Temporal variations in SSC 5.1.2 Spatial variations in SSC across each marsh 5.1.3 Spatial variations in SSC between marshes 5.1.4 Variation in SSC as a function of intense rainfall 5.1.5 Variation in SSC as a function of local and regional wave height 5.1.6 Variation in SSC as a function of tidal amplitude
5.2 Relative significance of each geomorphic control on sediment deposition 5.2.1 Susvended sediinent concentration 5.2.2 Wind waves 5.2.3 Tidal amplitude 5.2.4 Vegetation
5.3 Spatial variation of sediment deposition 5.3.1 Pecks Cove marsh 5.3.2 West Allen Creek marsh 5.3.3 Westcock marsh 5.3 -4 Tantrarnar mars h
5.4 Conclusions
REFEFWNCES
APPENDICIES LIST OF FIGURES
Ficure 1.1: Conceptual mode1 of the intrinsicfextrinsic factors and process variables affecting sediment deposition over the temporal scde of a tidal cycle. annually and decadally (modified from van Proosdij, 2001).
Figure 2.1: Map of the Cumberland Basin. Bav of Fundv (modified frorn Gordon et al.. 1985).
Figure 2.2: Pecks Cove marsh; a) marsh margin and b) location of the instnunent array.
Fiirure 2.3: West Allen Creek marsh; a) rnarsh margin and b) location of the instrument array.
Figure 2.4: Westcock marsh: a) aerial view of the rnarsh and b) the cliffed marsh margin.
Figure 2.5: Tantrarnar marsh; a) Tantrarnar river with the dark meen Iow marsh vegetation bordering the channel and b) location of the instnunent array.
Figure 2.6: Fort Beausejour marsh: a) the cliffed marsh margin and b) location of the instrument array.
Figure 3.1 (a andb): Plan view diagrarns of the transect orientation with respect to the marsh margin and relative location of the Stations, rising stage bottles, and sediment deposition traps.
Figure 3.2: instrument array on the H-fiarne. Shown from the right: EMCM, OBS, and pressure transducer. b) Custom-built wave staff.
Figure 3.3: Site set-up for Pecks Cove marsh.
Fiirure 3.4: Site set-up for West Allen Creek marsh.
Ficure 3.5: Site set-up for Westcock marsh. Figure 3.6: Site set-up for Tantramar marsh.
Figure 3.7: Site set-up for Fort Beauséjour marsh.
Figure 3.8 (a to e): Cross profile of transect on each marsh.
Figure 3.9: A sediment deposition trap is on the left. and a rising stage bottle is fixed to the stake on the right.
Fimre 3.10: Relationship between six risinp; stage bottle sarnples taken simultaneously at Pecks Cove marsh. Variation between sarnples is < 16% of the rnean.
Figure 3.11 : OBS calibration. Results of regression based on cornparison between in situ bottle sarnples and the OBS.
Figure 3.12: Exarnple of vegetation quadrat at West Allen Creek, Station 1 on June 29, 2000.
Fi~ure4.1: Exarnple of instrument summarv data for June 5.2000.
Fieure 4.2: Correlation of suspended sediment concentrations (SSC) across each marsh.
Fieure 4.3: Correlation of SSC at Stations III between marshes
Fieure 4.4: Anotnalous SSC results for Julian dav 186.
Ficure 4.5: ReIationship between predicted local wave height and SSC.
Fieure 4.6: ConeIation between wind speed and SSC.
Figure 4.7: Relationship between predicted regional wave height and SSC.
Figure 4.8: Relationship between wind direction and SSC.
Figure 4.9: Correlation between tidal amplitude and SSC. Fimre 4.10: Relationship between sediment deposition and tidal amplitude.
Fieure 4.1 1: Statisticallv significant relationships between sedirnent deposition and SSC.
Figure 4.12: Relationship between sediment deposition and relative roughness index.
Figure 4.13: Correlation of sediment deposition across each marsh. 95
Figure 4.14: Correlation of sedirnent deposition between marshes. 98
Figure 5.1: Conceptual mode1 of the aeomorphic controls influencing 1O0 sediment deposition in the Cumberland Basin, Bay of Fundy.
... Vlll LIST OF T-ABLES
Table 4.1: Correlation results of SSC sarnples behveen Stations across each marsh.
Table 4.2: Correlation results of SSC between sites.
Table 4.3: Results of the correlation between predicted locaI wave height and SSC.
Table 4.4: Results of the correlation between wind s~eedand SSC.
Table 4.5: Results of the correlation between predicted regional wave height and SSC.
Table 4.6: Geomorphic controls affecting concentrations of suspended sediment.
Table 4.7: Results of correlation between tidal amplitude and sediment deposition.
Table 4.8: Results of correlation between SSC and sediment deposition.
Table 4.9: Correlation results of sediment de~ositionbetween Stations across each marsh.
Table 4.10: Correlation of sediment deposition between Stations III between marshes. CHAPTER 1
RESEARCH CONTEXT, PURPOSE AND OBJECTIVES
1.0 Coastal saltmarshes
Coastal saltmarshes are transitional environments that occupy the intertidal
muddy substrate between land and sea (Allen and Pye, 1992). They occur dong the coast
of most continents, but are most prevalent high in the intertidal zone in temperate and
high latitudes. Saltrnarshes develop under Iow energy conditions. which allow for the
deposition of suspended sediment and establishment of vegetation. These conditions rnay
be found in tidal embayrnents. estuaries, lagoons, and deltas or behind barrier islands. As
saltmarshes are situated between aquatic and terrestrial ecosystems, their biota reflect the
influence of both the land and sea. Compositionally, they may be divided according to
the position of the mean water Iine, into Iower and upper marsh (Orme, 1990). The lower
marsh is usually dominated by one halophytic genus which is adapted to salinity and
repeated submergence, whereas the less harsh conditions of the upper marsh allow for
greater flonsiic diversity (Adam, 1990). Morphologically, saltmarshes are often
characterized by a low average slope. depositional features such as levees, and erosional
Features such as tidal creeks (Orme, 1990) and marsh cliffs (Allen and Pye, 1992).
1.1 Significance of the study
Saltmarshes are a valuable natural resource. Recognized as ecologically
important habitats, they act as a nursery and food source to a variety of coastal fisheries, wildlife and waterfowl (Brindley et al., 1998; Clarke and Sellers, 1998; Moreira, 1999). The environmental value of saltmarshes fùrther extends to include the buffering capabilities of marsbes to storm surge, which acts to protect coastal developments (Frey and Basan. 1978); and their temporary storage function, which acts to remove toxic metal ions from estuarine circulation. The latter fûnction may also double as an indicator of environmental quality in the bordering water body (Mortimer and Rae, 2000; Lewis et al.. 2001 ; Wall et al.. 200 1). Worldwide, saltmarshes have been under stress, primarily from human activity, and as such various marsh restoration and marsh management projects have been undertaken (e-g., Hughes, 1999; Johnson, 2000).
These valued areas depend on sedimentation for their creation and maintenance
(Schoellharner, 1996). Sediment deposition on marsh surfaces is a function of a variety of interna1 and extemal physical and biological controls and processes. Factors which have been suggested as influential to sedimentation include marsh morphology, proximity to the sediment source, tidal currents, wind and wave stress, suspended sediment concentration. ice, movement of relative sea level, biological activity, and vegetation (Gleason et al., 1979; Knuston et al., 1982; Stevenson et al., 1988; Cahoon and Reed, 1995; Reed, 1995; Allen and Pye, 1992; Leonard and Luther, 1995;
Woolnough et al., 1995; Ollerhead et al., 1999; van Proosdij er al., 2000). In more generaI terms, sediment deposition may be considered a function of both sediment availability and opportunity for deposition (Reed, 1989). The temporal scale of study determines the contribution of these factors to saltmarsh sedirnentation. Temporal scales may range from vcry short (individual tidal cycles to years), short (decades), medium
(centuries) and long (millennia) (Allen, 3000). Figure 1.1 illustrates generalized rel2tionships between process variables and the key intnnsic/extrinsic factors over a range of time scales.
PROCESS VARIABLES TiMESCALE INTRINSIC 1 EXRINSIC FACTORS
wind - source material and location
wave -- morphology and shoreline confÏÏuration
seasonal effects *.--- (e.g. ice)
tidal characteristics (range, reg im e)
sea level history
ohrrrebr vegeMion d'-t characteristics and i Longderm i glacial history dynamics i Ewlvtion ! 1
climateI change
Figure 1.1: Conceptual mode1 of the intrinsic/extrinsic fhctors and process variables affecting sediment deposition over the temporal sale of a tidal cycle, annuall y and decadally (modi fied fiom van Proosdij, 200 1). 1.2 Directions of saltmarsh research
To date, a significant arnount of research relating to saltmarsh sedimentation has consisted of long-term studies conducted in an effort to understand the rates of vertical accretion in the face of climate change and subsequent sea level change (cg., Reed, 1990;
Reed, 1995; Shaw and Ceman, 1999). One result of these studies has been the recognition of marshes as a sedi-ment sink (Pethick, 1992). According to Stevenson et al. (1988) this concept may be accurate over the long-term, but over the short-term saltmarsh sedimentation is considered to be highly variable over space and tirne. Where long-tem measurements make general trends in sediment accretion or erosion more readily apparent such studies do not offer suffrcient information regarding the geomorphic controls affecting the processes by which sedimentation and erosion occurs. However, short-term measurements of this variabiIity may provide information on the controls affecting deposition of sediment and therefore provide a better understanding of the long- term trends recognized in the saltrnarsh system. This view is supported in Christiansen et al. 's (2000. 3 15) study. which stated, "long term assessrnent of the fate of marshes that maintain their elevation by accumulating mineral sediment depends on accurate understanding of the physical processes that control minera1 sediment deposition on the mars11 sudace".
Most studies which have focused on short-term sediment dynamics in coastal marsh systems (e.g., Reed, 1989; French and Spencer, 1993; French et al., 1995;
Hutchinson, 1995; Leonard et al., 1995; Brown, 1998; van Proosdij, 20001, have examined temporal changes in sediment deposition on one marsh or the spatial change in sedimentation across one rnarsh. There appears however, to be little data on the sirnultaneous variability of sedimentation over several marshes within the same system
(with a few exceptions, e-g., Ward et al., 1998). Research dealing with this spatial aspect
rnay be useful in recognizing the different (or similar) response of marshes to various
geomorphic controls.
1.3 Controls on saltmarsh sedimentation
Sediment deposition is based on the availability of sediment and the opportunity
for deposition (Reed, 1989). Suspended sediment (and its source) is of primary
importance regarding sediment availability. Concentrations of suspended sediment may
Vary in response to currents associated with wind wave activity and tides (Wang et al.,
1993). Tides provide the medium and horizontal flow by which suspended sedirnent is
transported and dispersed over marsh surfaces. If currents are strong enough tides may
also entrain sediment. Waves rnay provide energy to suspend sediment (Schoellhamer,
1996: van Proosdij et al., 2000), and increase water flow velocities in which sediment is
transported and dispersed over the marsh surface.
Several geornorphic controls contribute to provide conditions suitable for
sediment deposition. Where wave activity and tidal currents rnay be a source of suspended sediment, they rnay also inhibit or promote sediment deposition based on their current velocities. These current velocities rnay be darnpened by vegetation, encouraging sediment deposition (Knuston et al., 1982; Pethick et al., 1990; Shi et al., 1995, 1996).
The effectiveness of vegetation as a baffle is detennined by plant height, density and morphology (Gleason el al., 1979; Leonard and Luther, 1995; Boorman et al., 1998).
Marsh morphology is also an important factor that rnay influence the opportunity for sediment deposition. Marsh elevation (Frey and Basan, 1978; French and Spencer,
1993). tidal creeks (French and Stoddard. 1992; Schostak et al., 2000), nature of the marsh margin. slope and the presence or absence of man-made dykes al1 influence the hydrodynarnics of tides on marsh surfaces, and therefore sediment transport and depositional dynamics.
The aim of this literature review is to provide some background information on the geomorphic controls that both influence sediment availability and the opportunity for deposition. The relative importance of the geomorphic controis affecting sedimentation on saltmarsh surfaces is largely determined by the temporal scale over which it is studied.
At the scale of a tidal cycle, the literature has highlighted the roles of suspended sediment concentration, tides, wind stress and wave height, vegetation, and morphology as important factors to consider in coastal saltrnarsh sediment dynamics.
1.3.1 Sediment sirppiy
Prior to discussing the factors controlling sediment deposition, consideration of the sediment itself is necessary. There are two types of sediment input to coastal saltmarsh systems, organic and inorganic. Organic sediments are generally produced and deposited in situ. The composition of this sediment is primarily plant detritus derived from above and below ground primary production and fecal and carbonate matter fiom fauna. This fonn of sediment is often more abundant in warrner climates, or where inorganic sediment supply is limited (Orme, 1990). Inorganic sediments however, are of greater significance to this study. Inorganic sediments brought into coastal saltmarsh systems are primarity suspended fine silts and clays derived from a variety of sources. These sources may include offshore marine deposits, riverine sediment (Brueske and Barrett, 1994), estuarine and coastal cliff erosion (Amos, 1987) and aeolian sediment (Orme, 1990). Fine sediments derived fiom these sources are easily transported in suspension and as such may travel great distances, mixing with many additional sources of sediment before reaching marsh surfaces. Cornparatively, barrier over-wash, ice rafting and ice rnelt-out
(Olierhead et al.. 1999; Pejnip and Anderson, 2000) are often the source of coarser sediments. The contribution of al1 these sources to the marsh system may Vary as a function of weather conditions, climate, tides, and the seasonal balance between marine and fluvial processes.
1.3.2 Wind stress and wave heiglrt
Wind stress and the resultant wave heights function to both bring sediment into suspension as well as influence the opportunity for deposition. Currents associated with wave orbital velocities rnay be sufficient to bring sedirnent into suspension, increasing available concentrations in the water column. However, it has been suggested that only at Iower wave heights cm sediment actually be deposited (van Proosdij et d.,2000). The extent to which wind stress and wave height affects sedimentation depends on fetch, wind strength and directional variability, and timing with respect to the tidal stages (Allen and
Pye, 1992).
According to Pethick (1992) coastal saltrnarshes often develop in protected areas that are fetch limited, reducing the maximum period and size of the waves that impact marshes. In a study of saltmarshes in the Cumberland Basin: Dawson et al. (1999) found fetch length in itself to be of limited importance in marsh development as the sites with the largest fetches had the most extensive marshes. In this case, the rnarshes developed on a wide tidal ramp where energy was sufficiently dissipated to allow for deposition.
Wave height, is considered to have the rnost direct impact on marsh sediment dynarnics (French et al., 2000; van Proosdij et al., 2000). Waves create turbulence in shallow waters as a result of the interaction of wave oscillations with the bed. Increased turbulence is often associated with increased shear velocity at the bed, which acts to bnng sediment into suspension. A study by van Proosdij et al. (2000) found increased wave energy results in an increase of the mean arnount of sediment in suspension thereby increasing the amount of sediment available for deposition. However. van Proosdij et al.
(2000) also found sediment deposition only occurred where wave height was small (< 15 cm). In contrast, where wave height exceeded 15 cm, sediment was brought into suspension but not deposited because oscillating current velocities remained too high.
Frey and Basan (1978) suggest that the force of everyday tides or wind generated waves which have been dampened by marsh vegetation are probably insufficient to lift and re-suspend sediment deposited on marsh surfaces during slack tide. If this is the case, only during storm events (with high winds and therefore increased wave heights) will energy be suffkient to erode and re-distribute surficial sediment. Storrns are ofien associated with a net import of material, as sediment is eroded from adjacent mud and sandflats and deposited high in the marsh. Several studies have observed the role of storms and their contribution to inflated levels of suspended sediment concentration and sedirnent deposition (e.g., Reed, 1989; Goodbred and Hine, 1995; Schoellhamer, 1996; Roman et al., 1997; Day et al., 1998; Goodbred et al., 1998). However, Stevenson et al.
(1988) also observed that hurricanes, and even thunderstoms, rnay lead to an export of material from marsh systems. PowerfuI wind and waves associated with storms rnay quarry large blocks of sedirnent and cause undercutting and collapse of saltrnarsh cliffs
(Allen and Pye, 1992). Hi& energy events rnay also damage surface vegetation, or uproot seedlings, which then exposes marsh substrate to scouring by currents (Keddy,
1982).
Wind and waves contribute most to the entrainment and transport of sediment when they correspond with the tidal stage, by causing tidal asymmetry or enhancing a current asymrnetric tidal regime. Leonard et al. (1995) and Adam (1990) suggested the persistence of strong onshore winds rnay lead to elevated water levels, whereas prolonged periods of marsh emergence rnay occur with continued offshore winds. The elevated water levels due to onshore winds may act to enhance the flood stage of the tidal cycle and retard the ebb, leading to a net landward movement of sediment. In contrast, offshore winds rnay depress the flood and enhance the ebb, leading to a net export of sediment. It is important to note that this rnovement of sediment occurs under the assumption that fiow velocity is suffrciently increased by the wind-generated waves to cause sediment entrainment.
1.3.3 Tides
Sirnilar to waves, tides have a dual role in coastal saltrnarsh sedimentation, both influencing the concentration of sediment available in suspension and the opportunity for deposition. Tides provide the medium and horizontal flow by which suspended sediment is transported and dispersed over marsh surfaces. Higher tidal currents rnay bring
sediment into suspension, while lower currents rnay allow sediment to fall out of
suspension. Tidal current velocities Vary as a function of tidal stage (flood, slack, and
ebb). on a cyclical basis (spring and neap tides) or spatially.
The coasts on which saltrnarshes exist rnay be divided into micro- (< 2.0 m),
meso- (2- 4 m), and macro- (> 4 m) tides based on tidal range. Tides rnay also be diurnal
or semi-diurnal. meaning that high tide occurs once every 24 hours 50 minutes or once
every 12 hours 25 minutes respectively. Each tide consists of three stages: flood, slack
and ebb. As tides progress vertically through these stages there are associated changes in
the horizontal velocity and consequent changes in suspended sediment concentration.
During the flood stage of a tidal-cycle the incoming water rnay reach the marsh
surface by two means: via tidal creeks or by overtopping of the marsh margin. Where
tidal creeks are present, flow enters creek channels and remains confined there until such
time as it enters tidal creek heads or over-spills the natural levees. WhiIe flow is
confined within the channels, relatively high current velocities have been observed (Frey and Basan, 1978; Stevenson et al., 1988; Orme. 1990; Pye, 1995) - with some exceptions
(cg.. Schostak et al., 2000). Because these velocities are ofien sufficiently high above the critical shear velocity for suspended sediment entrainment and transport, currents rnay re-suspend sediment that was deposited durhg the previous ebb tide. This entrainment results in a gradua1 increase of suspended sediment concentration over the first half of the flood tide (Reed et al., 1985; Pethick, 1992).
Once tidal stage exceeds channel capacity, sheet flow will occur across the marsh surface. In this later stage of the flood, many studies have observed a decrease in flow velocity and total suspended sediment coupled with increased rates of sediment
deposition over a marsh surface with increased distance from tidal creeks (French and
Spencer. 1993; Leonard et al., 1995). According to Frey and Basan (1978), this decreased velocity is the result of vegetation and a lack in channelization.
In cornparison, where no iidal creek networks have been developed floodwaters reach the marsh surface via the marsh margin. In this instance, flow velocity may decrease with increased distance from the rnarsh margin as currents are baffled by vegetation (Knuston et al.. 1982), producing different spatial patterns in suspended sediment concentration and sediment deposition across the marsh surface than the previous exarnple.
At high tide (or slack water) tidal energy is substantially reduced. and flow velocities ofien decrease to a minimum. During this time, sediment rnay have the opportunity to settle out of suspension (Luternauer et al., 1995), although the short duration of tidal sIack does not permit deposition of al1 the suspended sediment (Onne,
1990). While many studies contribute to the generalization that the highest rate of sedimentation occurs during sIack tide, van Proodsdij et al. (2000) offered alternative evidence. In their studies of the high rnacro-tidal environment in the Bay of Fundy, during moderate waves (0.1 -O. 15 m) the highest rates of sedimentation occurred at the begiming and end of the tidal cycle rather than during the slack period of high tide. van
Proosdij et al. (2000) suggested that at the beginning and the end of a tidal cycle, water depth over the marsh is lower allowing the relative roughness of the marsh vegetation to be at its maximirm, thereby promoting deposition. FIow reversal occurs on the ebb tide. Afier inundation, many areas of salt marsh drain slowly, pnm&ly because local topography is rarely conducive to rapid drainage
(Adam. 1990). In other words. gentle slopes and shallow channels of the high fiats inhibit s~vificurrents, resulting in the initial seaward flow of the ebb tide to be in the form of low-velocity sheet flow across the marsh. Once the height of the water colurnn has been lowered and if tidal creeks are present. velocity may increase as flow becomes progressively confined to the channels (Frey and Basan. 1978; Pye, 1995). According to
Leonard et al. (1995) sediment present in the water colurnn during ebb tide is limited to material incapable of settling out of suspension during the slack period. The greater the frequency of tidal inundation. the greater the potential for suspended sediment to enter the marsh system. Moreover, the longer the water column resides over the marsh surface, the greater the potential for the sediment to settle out of suspension.
It is important to note that the above discussion of tidal forcing is not representative of al1 situations. Tides and morphology in combination with rnany other factors form a complex system that is not always predictable at one site, nor are generalizations always transferable frorn one site to another. The flood and ebb stages of a tidal cycle are not always equal in strength and duration, and often one dominates the other. resulting in tidal asymmetry. This asymmetry is recognized as either a flood or ebb dominated tidal regime. Flood dominated tides are characterized by higher velocities in the flood than the ebb, resulting in landward sediment transport. Conversely, ebb dominated tides are characterized by a higher velocity in the ebb than the flood, resulting in seaward sediment transport. According to Stevenson et al. (1988), it is not obvious why the channels of some coastlines would have greater tendencies toward flood domination. Pethick (1980) suggested that velocity surges and asyrnmetry in tidal channels are caused by a combination of independent tidal stage inputs and channel morphology. Allen and Pye (1992) however, suggested tida! regime and water depth rnay determine flood or ebb domination. In their example of the British coasts, they indicated that where a meso- or macro- tidal regime exists, in combination with an intertidal zone of comparative shallowness, a flood regime would dominate. Several researchers have attempted to address the velocity and discharge variations associated with flood or ebb dominated tidal regimes (e-g.,Pestrong, 1965; Bayliss-Smith et al.,
1979).
Tidal forcing is not constant throughout tirne; rather it varies considerably - either on a cyclical basis or by a change in climatic conditions. Cyclical variations in tidal amplitude occur over the period of a fortnight and the two extremes are known as spring and neap tides. Spring and neap tides represent a temporal increase in amplitude as a result of increased force associated with the gravitational pull of the moon. As a consequence of the higher amplitude and potential higher velocities (Pethick, 1992) associated with spring tides, suspended sediment concentrations are often observed to be greater than those experienced during lower velocity neap tides (Schoellhamer, 1996).
According to Schoellharner (1996), as spring tide is approached there is an accumulation of suspended sediment in the water column, whereas when neap tide is approached there is a net deposition of suspended solids. 1.3.4 Vegetation
Salt marsh vegetation contributes to sedimentation in three main ways: 1) promoting inorganic deposition, 2) stabilizing accreted material, and 3) contnbuting organic matter to the soi1 (Adam, 1990). For short-terrn rneasurements of sedirnent deposition (i-e., over the temporal scale of a tidal cycle) in mid to high latitudes, the roIe of vegetation as a geomorphic control on sediment deposition is of greater significance than its stabilizing roIe or organic contribution. Variations in plant morphology (Leonard and Luther, 199.3, vegetation density (Gleason et al., 1979), and height (Boorrnan et al.,
1998) are important variabIes infiuencing the ability of plants to buffer current velocities
(Knuston et al., 1982; Lutemauer et al., 1995) and modiQ flow hydrodynamics in the plant canopy, encouraging (or in some cases inhibiting) deposition (Pethick et al., 1990;
Shi et al., 1995). In other words, the presence of vegetation may provide opportunistic conditions for sediment deposition.
In a study by Leonard and Luther (1995) on the response of hydrologic flow patterns to plant morphology, stem diarneter and leaf spacing were found to be important factors in determining whether vegetation promotes deposition or erosion. More specifkally, they found that small basal stem diarneters and closely spaced leaves reduced turbulence promoting deposition. In contrast, large basal stem diarneters and sparse leaves caused flow near the substrate to become turbulent and therefore more likely to cause sedirnent erosion than deposition.
With respect to plant density, a lab experiment by Gieason et al. (1979) found sand accumulation on the marsh surface afier 60 waves was a positive, non-linear function of stem density. In other words, higher stem densities more effectively dissipate wave energy, increasing the sediment deposition rate. This positive relationship between plant density and deposition was observed to increase at a declining rate. Brueske and
Barrett (1994) corroborated this relationship, observing the highest rates of sedimentation at their site occurred where vegetation was most dense - the cattail stands.
Variations in plant height rnay also influence wave energy and current velocities.
Boorrnan et al. (1998) observed that at low velocities the retardance of water flow by vegetation was proportional to the height of the vegetation, but with increased velocity the retardance of ta11 plants were generally less than that of short plants. The explanation given is that at higher velocities ta11 plants may bend with the flow, reducing their effectiveness as a roughness element. The rnost effective vegetation for flow retardance is short (mean height = 50 mm) and stiff so as not to bend with increased flow velocities
(Boorman et al., 1998).
The ability of vegetation to 'baffle' wave energy and tidal current velocities decreases the potential of sediment re-suspension and at Iow enough velocities permits deposition (Stevenson et al., 1988; Adam, 1990; Luternauer et al., 1995). According to
Boorman et al. (1998) depth of the water colurnn relative to plant height is key to darnpening wave and current velocities. Where depth of the water colurnn above the marsh surface is large in cornparison to vegetation height, the ability of vegetation to act as a macro-roughness eIement and sIow current velocities is reduced. As a result, wave activity may perpetuate across the surface of the marsli, and retain the sediment in suspension (van Proosdij et al., 2000). Conversely, where water levels are suff~ciently low, plants are able to baffle wave energy and current velocities, thereby providing conditions more suitable for deposition. According to the US.Arrny Corps of Engineers (as noted in Knuston et al., 1982). over 50% of wavr energy is dissipated in the first 2.5 m of marsh. Therefore, the greater distance of marsh vegetation through which water flows. the greater the percentage of wave height and energy loss. Boorman et al. (1998) suggested vegetation reduced flow veIocities in proportion to the increase in sudace roughness that it created. As a result of decreased wave energy and flow velocity, deposition may result.
Many authors have noted that difficulties that exist in rneasuring how fluid flow behaves through saltrnarsh vegetation due to shallow water depths, instrument Iimitations in measuring slow flow, and the overall complexity of the system. In an attempt to avoid these difficukies, a number of studies conducted on specific flow hydrodynamics in and above the vegetative canopy have been carried out in a flume (cg.Pethick et al., 1990;
Shi et al.. 1995, 1996). Resuits of these studies have been supported by field investigations (e-g., Leonard and Luther, 1995).
1.3.5 Morplzology
Marsh morphology also contributes to circumstances where sediment may have the opportunity to settle out of suspension. Marsh elevation, dope, tidal creeks, and the nature of the margin are a11 intepal components of marsh morphology, which together influence sediment dynamics within the marsh. Numerous researchers have recognized the influence of marsh elevation on sedimentation, where marsh elevation relative to the height of the tidaI prism infiuences the depth and duration of tidal inundation and potential exposure to wave activity (Stoddart et al., 1989; Leoncrrd, 1997; Christimsen et al., 2000). Cahoon and Reed (1995) and French and Spencer (1993) observed that frequency and duration of tidal inundation was an important influence on the potential for
suspended sediment to be deposited on the marsh surface. However, according to van
Proosdij et al. (2000). areas with the greatest inundation time do not always relate to the
highest rates of sediment accumulation, suggesting depth of the water colurnn over the
marsh surface may support wave propagation. thereby increasing shear stress at the bed
and inhibiting sediment deposition.
As a sediment source, and conduit for watedsediment transport ont0 the marsh,
the role of tidal creeks to saltrnarsh sedimentation has been relatively well documented
(e-g.. French and Spencer, 1993: Reed et al.. 1995; Christiansen et al., 2000; Schostak et
cd.. 2000). However witl5 a few exceptions (e.g., Allen, 2000), comparatively Me
information is available on the contribution of marsh slope and the nature of the marsh
margin to sediment dynarnics.
Marsh morphology bas often been recognized as indicative of the physiographic
stage in marsh development. whether in a linear or cyclic pattern of evolution. Allen
(2000) provides a comprehensive review of conceptual models developed for 'youthfùl' and 'established' saltmarshes. In his review, development îrom a youthfùl to established marsh involves a "progressive elaboration and increase in density of tidal creek networks during marsh build-up". Moreover, as marshes develop, sedirnent accumulation increases the elevation of marsh platforms relative to the height of the tidal frame, thereby decreasing the annual nurnber of floods. Frequency of flooding leads ta variations in vegetative assemblage, where fiequently flooded marshes are low in the tidaI fiame and are dominated by low marsh vegetation (a young marsh) and infrequently flooded marshes have an increasing expanse of high rnarsh vegetation (a mature rnarsh). This observation has been a focus for mmy 'age' classifications of marshes (e.g., Frey and
Basan. 1978).
1.4 Patterns and rates of inorganic deposition
According to Adam (1990) and Pethick (1992) most coastal saltmarshes
accumulate only a few millimeters of sediment per year. As previously discussed, coastai
saltmarshes are influenced by many geomorphic controls. which in turn affects the
availability of sediment and rates of sediment deposition. These controls are not constant
triroughout time and space and as such thcre are spatial and temporal variations in
sedirnent deposition.
Several long(er)-terrn trends (fiom weeks to years) in salîrnarsh sedimentation have been recognized. The first trend, widely recognized by researchers, is an exponential decrease in sediment deposition rates with increased marsh elevation
(Pethick. 1992; French et al.? 1995; Woulnough ef al., 1995; Allen, 1997). According to
Adam (1990), the rate of sedimentation in upper saltmarshes decline not only because of fewer flooding tides which reach this extent of the upper tidal zone, but ais0 because significant arnounts of sediment are trapped in the Iower marsh before reaching a higher elevation. Contrary to this trend, Brown (1998) and van Proosdij (2001) observed upper marshes to have the greatest net deposition, although they are subject to fewer tidal inundations, while net deposition decreased toward the marsh front despite the increased nurnber of tidal inundations.
A second frequently observed trend is a decrease in deposition rate with increased distance from tidal creeks (French and Spencer, 1993; Brueske and Barrett, 1994; Luternauer et al., 1995; Ward et al., 1998). Additionally, sediment deposition rates may decrease from the location at which a river enters an estuary to the mouth of an estuary
(Stevenson et al.. 1986). For example, Ward et al. (1998) observed a general down- estuary decrease of deposition rates in the channel margin marshes along the axis of the
Naticoke river (an estuarine tributary systern). This trend only holds tme where there is a sediment rich, up-estuary river. What these trends al1 have in comrnon is a decrease in sediment deposition with an increased distance from a source of sediment - whether it is water in the estuary, tidal creeks or sediment rich rivers.
On a short-term scale of observation (individual tidal cycles), several authors have suggested that trends in deposition are less apparent as deposition is highly variable within the tides and at similar velocities during different tidal cycles (Reed et al., 1985;
Brown. 1998). As opposed to the resultant sedimentation patterns associated with long- term trends. short-term trends are more process-related relationships whereby controls such as waves, tides, suspended sediment and vegetation, may be observed to either promote or inhibit deposition. Over the past decade some short-term trends have been recognized allowing for several biophysical controls to be postulated as influences on saltmarsh sedimentation and, based on these findings, the development of models on saltmarsh morphodynamics (Allen, 1990; French, 1993; French et al., 1995; Woolnough et al., 1995; Allen, 1997). Suggested controls include wind, waves and tidal currents, suspended sedirnent concentrations, and vegetation. In addition, iiï~shrnorphology is considered an important control influencing surface topography, slope, and height and duration of tidal inundation. Recognition of these controls provides a better understanding of the processes by which sediment is transported and deposited on marsh surfaces.
1.5 Study purpose and objectives
AIthough many studies have been conducted to measure long and short-tem sediment accretion/deposition over individual marsh surfaces, few have focused on short- term sediment deposition (ie.. at the scale of a tidal cycle) over several marshes of the sarne system. A study of the spatial distribution of sediment deposition at the scaie of a tidal cycle ni11 provide valuable information regarding the geomorphic controls affecting rates of sediment deposition.
The purpose of this sinidy was to examine the relationship of spatial and temporal variation in sediment deposition rates between several saltmarshes over a tidal cycle within the Cumberland Basin to the following geomorphic controls: (1) wind wave and tidal currents, (2)suspended sediment concentration, (2) vegetation height and density, and (4) marsh morphology. In this case, marsh morphology includes the nature of the marsh margin, the pattern of tidal creeks, the dope of the marsh, and the elevation of the marsh in the tidal regime.
From this purpose statement four hypotheses were developed:
1) Higher waves and higher tidal current velocities result in increased suspended
sediment concentrations, thereby increasing the sediment available for deposition.
2) As wave height and tidal current velocity increase, the amount of sediment deposited
on a saltmarsh surface decreases. 3) As vegetation height and density increase, the relative proportion of sediment
deposited on a saltmarsh surface increases.
4) Sediment deposition rates Vary due to differences in marsh location and morphology.
In order to address the four research hypotheses, the following objectives were
developed:
Determine the location of five marshes, representing both morphological diversity
and spatial variation along the Cumberland Basin.
Determine the spatial variability of suspended sediment concentrations across each
marsh and between marshes as a function of wind wave conditions and tidal
amplitude.
Examine the relative significance of each geomorphic control on sediment deposition
rates in the saltmarshes of the Cumberland Basin.
Determine the spatial variability of sediment deposition across each marsh and
between marshes.
1.5.1 The Field Work
This research was carried out fiom late May to mid July 2000, in five selected saltmarshes located along the Cumberland Basin, Bay of Fundy. The study sites represent a diversity of saltmarshes in the Cumberland Basin which are subject to a variety of geomorphic controls affecting sediment dynarnics. This research will be a valuable contribution to a larger body of research on salt marsh sediment dynamics in the Bay of
Fundy being conducted by the Geography departments at the University of GueIph and Mount Allison University. More specifically, this study will complement a recent pubIication by van Proosdij et al. (2001). van Proosdij et al. 's study focused on controls of sediment deposition on Allen Creek rnarsh over single tidal cycles in the Cumberland
Basin. This study expands on her work by providing insight on the geomorphic controls affectinp sedimentation patterns over several rnarshes of the same system, in the
Cumberland Basin at the scale of a tidal cycle. CHAPTER II
STUDY AREA AND SITE SELECTION
2.1 Cumberland Basin, Bay of Fundy
This study was conducted in the Cumberland Basin, Bay of Fundy (Figure 2.1).
The Bay of Fundy is a large, macro-tidal embayment situated on the east Coast of Canada.
Located between the provinces of Nova Scotia and New Bninswick, it comprises the northeast extension of the Gulf of Maine. The Bay is characterized by a high concentration of suspended sedirnent (approximately 0.3 gliter), which is subject to seasonal variations (Ollerhead et al., 1999). This sediment is derived from cliff line erosion of Pâleozoic mudstone and sandstone and an eroding seabed of laminated silts and clays (Amos, 1987). Comparatively, contributions of sediment by river sources are minor (Gordon and Cranford, 1994). The suspended sediment is composed of 95% coarse silt, 2.5% clay and 1.5% sand, with a mean grain six of 4.5 phi (0.036 mm) (van
Proosdij et al., 1999). High sediment supply and large tidal range has allowed for the development of extensive saltmarshes, mudflats and sandflats. Over the past few centuries however, the majority of saltmarshes present in the Bay of Fundy have been dyked and drained for development and cultivation, reducing the area of 'natural' tidally tlooded saltniarshes from 395 km2 to 65 km' (Gordon and Cranford, 1994). Presently, the intertidaI zone of the Bay of Fundy is composed of 17% saltmarshes, 40% mudflats and 43% sandflats (Amos and Tee, 1989).
The head of the Bay of Fundy divides into Minas Basin and Chignecto Bay and
Chignecto Bay further divides into Shepody Bay and the Cumberland Basin. The
Cumberland Basin is a 1 18 km2 turbid estuary, with a semi-diurnal tide of 10-13 m. Saltmarshes occupy the upper intertidal zone whereas extensive mud and sand flats occupy the middie portion of the intertidal zone and coarse lag deposits are found near
Iow water (Gordon and Cradord, 1994). In a description by Gordon and Cranford
( 1994). high marshes in the Cumberland Basin are located at elevations greater than 1 m above the mean high water (MHW). They further described these zones as having a very
low average slope (virtually horizontal) and to be dissected by drainage channels.
According to Gordon and Cranford high marshes flood only during very high spnng tidrs. with maximum tidal inundation being about 2 hours per tide and maximum flooding depth about 1 m. In cornparison they described Iow marshes to be located less than 1 m above MHW. These marshes are characterized by gentle slopes and frequent flooding. Maximum tidal inundation is approximately 4 hours, with maximum flooding depth in the region of 4 m. Due to variations in flooding frequency. different vegetative communities have become established on high and low marsh. Growing season for the veçetation is generally May to October. with maximum biomass established by late
August (Gordon and Cranford, 1994).
Saltrnarshes in the Cumberland Basin are subjected to varying degrees of exposwre. Fetch varies from < 5 km to > 30 km, with the dominant wind direction from the southwest during the summer and an average wind speed of 15 km/hr. Seasonal temperature differences for nearby Moncton, New Brunswick range on a monthly average fiom a daily maximum of 25OC to a daily minimum of -13.6 OC (Environment
Canada, 2000) which aIso contributes to the character of the marshes in the Basin, as vegetative die back occws around November with the onset of winter and is reestablished around May. Figure 2.1: Map of Cumberland Basin, Bay of Fundy showing the distribution of saltrnarshes and tidal rnudBats. The study sites are 1) Pecks Cove; 2) West AIlen Creek; 3) Westcock; 4) Tantramar; and 5) Fort Beausejour rnarshes. The numbers represent the approximate location of each study site (modified from Gordon et al., 1985). 2.2 Site selection
The saltmarshes chosen for the study were selected on the basis of three criteria:
1) they had to provide a spatial representation of the Basin as well as represent
the diversity of marshes in the Cumberland Basin,
2) total travel time between marshes had to be short enough to permit sampling
of al1 the marshes during one tidal cycle,
3) they had to be accessible.
In trying to provide adequate spatial coverage of the Cumberland Basin, travel tirne between marshes was an important consideration, because each nui required each site to be visited both pnor to and following tidal inundation. Time traveled had to remain feasible given cost and time constraints. As a result of these constraints marshes were chosen on the west side of the Basin (closest to our base in Sackville). A fwther determinant in site selection was accessibility to the marshes. Most marshes were not accessible by main roads. and therefore considerations of property ownership and the condition of tracks (e-g., logging tracks, dyke roads, etc.) fiu-ther narrowed the potential number of sites.
A total of five marshes were chosen, including Pecks Cove marsh. West Allen
Creek marsh, Westcock rnarsh, Tantrarnar marsh and Fort Beauséjour marsh (Figure 2.1).
It would have been ideal for the chosen marshes to be in a 'natural' state (i.e., not dyked).
However, since the majority of marshes in the Bay of Fundy have been dyked and roads providing access to the marshes were generally those used for dyke maintenance or farmers, such antliropogenic influences could not be easily avoided. Pecks Cove and
West Allen Creek marshes are the only 'natural' sites. Natural in this definition means there were no obvious anthropogenic influences on marsh development - specifically dykes.
2.3 Site characterization
2.3.1 Pecks Cove marsil
Of the five chosen sites, Pecks Cove marsh is located closest to the mouth of the
Cumberland Basin. In comparison to the other chosen marshes, Pecks Cove marsh is relatively wide, with a width of approximateIy 100 - 120 m (distance from the landward edge to the seaward extent of the marsh vegetation). This marsh is characterized by a gentle siope, which is maintained across the marsh surface and extends out into the mudflats (Figure 2.2). Located low enough in the tidal frame to be inundated with every tide, the mmh suface at Pecks Cove is at Ieast partially flooded for an average of 3-4 hours during each tidal event with a maximum water depth of approximately 3-4 meters.
As a result of this consistent flooding, the marsh supports an almost exclusive growth of the salt toIerant, low marsh species, the short form of Spcrrtina aiternrflora. The landward vegetative boundary ends abniptly dong a sandy beach deposit, whiIe the seaward marsh margin is irregular in shape due to new mudflat colonization by Spartina alternzflorn. No tidal creeks dissect the marsh surface within the study area; however, Pecks Cove tidal creek is located several hundred metres to the southwest of the study area. At this site, creek development is generally limited to the mudflats, seaward of the marsh margin.
The marsh morphology, fiequency of flooding and vegetative characteristics suggests that Pecks Cove marsh is comparatively 'young' in development and will likely change over the next few years, as the vegetation limit extends seaward. Figure 2.2: Peck Cove rnarsh; a) marsh margin and b) the location of the instrument amy 2.3.2 West Allen Creek marsh
West Allen Creek marsh is a part of the larger Allen Creek marsh system (Figure
2.3). In comparison to Pecks Cove marsh, marsh development at West Allen Creek is not as extensive (although it becomes considerably more extensive to the east of the study site), with a width in the region of the study area of approximately 50 m. Similar to
Pecks Cove marsh- West Allen Creek marsh is characterized by a gentle dope extending across the marsh surface and out into the mudflats and is also Iocated low enough in the tidal frame to be mostly inundated with every tide. For every tidal event, the majority of the marsh surface is covered by water for approximately 2.5-3 hours. Due to regular tidal flooding, the vegetation is exclusively short Spartina alterniflova. The landward extent of marsh vegetation is bordered by small rocky headlands interspersed with small sand and grave1 beaches. In some locations bedrock outcrops dissect the marsh surface. The seaward marsh margin exhibits both erosion of marsh substrate (as evidenced by exposed roots) and new marsh development on the adjacent mudflats. West Allen Creek has several sinall to moderate sized creeks dissecting the marsh surface and the adjoining mudflats. Based on the above observations, West Allen Creek may also be considered relatively 'young' in development and appears to be growing in extent, as evidenced by newly colonized areas of vegetation on the mudflats. Figure 2.3: West AUen Creek marsh; a) marsh margin and b) the location of the instrument array 2.3.3 Westcock ntarsh
The character of Westcock marsh is significantly different than Pecks Cove marsh
and West Allen Creek marsh (Figure 2.4). This rnarsh is approximately 45 -50 m wide
with a virtually horizontal rnarsh surface. An earthen dyke bounds the back of the marsh
and a two-tier marsh cliff approximately 3 rn in height delimits the seaward extent of the
marsh. Westcock marsh sits significantly higher in the tidal heand as such is only
inundated during very high spnng tides. Due to infrequent flooding of the marsh surface a much higher diversity of vegetation has become established. which is representative of high marsh. The dominant vegetation is Sparrina patens. with lesser arnounts of
Triglochin maritimum, Plantago rnaritima and Pirccinellia maritirna. Short Spartina niternljloï-a also exists in a broad, shailow creek that runs along the back of the rnarsh, parallel to the dyke, along the rnarsh rnargin and along certain paths on the marsh surface where early flood waters are directed. Westcock marsh may be characterized as a being at a 'mature' stage in tems of development, as evidenced by the high marsh vegetation and presence of an eroding marsh cliff. Figure 2.4: Westcock rnarsh; a) aerial view of the marsh and b) the cliffed rnarsh margin. 2.3.4 Tnntrarnar marslr
Tantramar marsh is located along the Tantramar River. which is situated near the head of the Cumberland Basin (Figure 2.5). Although not located on the Basin per se,
Tantramar marsh provides a site that is influenced by the processes in the Basin, yet is sheltered from direct waves. Tantrarnar rnarsh is approximately 50 m in width at the location of the study site, with an essentially horizontal high marsh surface. Like
Westcock marsh, earthen dykes border Tantramar marsh; however, on the seaward extent of the high marsh there is a significantly smalier marsh cliff (approxirnately 0.4 m high) at the base of which is a narrow strip of low marsh (approximately 2-3 m wide). Beyond this the dope becomes significantly steeper as the mud banks form the channel of the
Tantramar River. Tantramar marsh is located high in the tidal frarne, and therefore is inundated only during high spririg tides. The low marsh. however, is flooded over a
Larger range of spring tides than the high marsh. As a result the hig1.i marsh exhibits diverse vegetation, where Spnrtina patens dominates and smaller concentrations of ilrriplex pntula, Jtrnczïs gerar-di and Puccinellin maritirna exist. In addition, Spartina alfernzflora is found in the broad shallow creek following the base of the dyke. In cornparison the low marsh is entirely ta11 Spnrtina alternzj7ora. A large secondary tidal creek leading into the Tantramar River is Iocated to the east of the transect, and a smaller tidal creek intersects the marsh surface to the west. Figure 2.5: Tantrarnar marsh; a) Tantramar river with the dark green Iow marsh vegetation bordering the channel and b) the location of the instrument array. 2-3.5 Fort Bea rise/our marsh
Fort Beausejour is located at the head of the Cumberland Basin and is therefore the most exposed to wind and waves of the five chosen sites (Figure 2.6). It is approximately 400 m in cvidth, delineated by an eartben dyke along the back and sides of the marsh and a two-tier marsh cliff at the seaward extent that measures approximately
3 m in height. The high marsh surface is nearly horizontal. cvhile the adjoining mudflats have a steeper dope (0.06). Of the five marshes chosen for this study, Fort Beauséjour is located hiçhest in the tidal frame and is inundated only during the very highest sphg tides. As a result of the infrequent flooding of the rnarsh surface, the vegetation is typical of a high marsh. Spartinn patens dominates, with lesser arnounts of Triglochin rncrritimzun, Distichlis spicatu, Plantago maritimum, Plantczgo maritirna and Lirnoniurn ncrshii. As observed at al1 the marshes with dykes used in this study, a broad and generally shallow creek follows the base of the dykes around the rnarsh, supporting the growth of Spartina alternflora. A small tidal creek, following the base of the dyke is in close proximity to the study area. However. based on Schostak et al.3 (2000) study of tidal creeks it was assumed to have no major impact on the study site. Similar to
Westcock marsh, the presence of predominantly high marsh vegetation, the marsh cliff, and the broken chunks of turf scattered on the rnudflats below as a result of erosion, indicate that Fort Beauséjour marsh is at a 'mature' stage of development. Figure 2.6: Fort Beauséjour marsh; a) the cliffed rnarsh margui and b) the location of the instrument array. CWAPTER III
RESEARCH DESIGN AND METHODOLOGY
3.1 Research design
Data were collected over 33 individual tidal cycles over an 8-week period, fi-om
May 3 lst to July End.2000. A 45-50 m transect extending from the marsh margin,
landward was established on each of the five selected marshes. Samples of suspended
sediment concentration and sediment deposition were coilected simultaneously over the temporal scale of individual tidal cycles. Additionally, continuous measurements of wind direction and velocity were collected at one location over the entire duration of the study.
More detailed measurements were made during several individual tidal inundations by an electronic instrument array, alternateiy located at one of the five marshes throughout the study.
Within each of the five chosen sites, a 45 to 50 m transect was established extending fi-om the seaward marsh margin, toward land. On each marsh the transect was positioned at a maximum distance fiom tidal creeks. This decision was made in an effort to reduce the effects of sedimentation patterns associated with tidal creek flooding - a pattern which has been recognized by numerous authors (e-g., French and Spencer, 1993;
Brueske and Barreît, 1994).
Along the transect four stations were established. Station III was located at the marsh margin, Station II was 20 m landward of the marsh margin and Station 1 was 40 m inland from the margin. Station IV was located on the mudflat, 5 or 1O m seaward of the marsh margin - depending on the marsh morphology. For Pecks Cove and West Allen Creek marshes, Station IV was located 10 m seaward of Station III. At Westcock marsh and Fort Beauséjour marshes Station TV was Iocated 5 m seaward of Station III on the marsh 'step' and at Tantramar marsh no Station IV was established due to the steep muddy bank that extended beyond the marsh rnargin. Suspended sediment samples were collected at Stations 1 and III, while sediment deposition was measured at Stations 1, II,
III and occasiondly at Station IV. Due to the infrequency of marsh surface flooding at
Westcock and Fort Beauséjour marshes. additional samples of suspended sediment were coIIected at Station IV, on the marsh margin 'step'. Figure 3.1 provides schematic diagrams illustrating transect orientation on the marsh surface and the relative sampling locations of suspended sediment and sediment deposition.
More detailed data were collected during tidal inundation by the instrument array.
Alternately located at only one of the five marshes, the instrument array consisted of an
H-frarne supporting a Marsh McBirney 5 12 bi-directional electromagnetic current meter
(EMCM)' a D&A Instruments mode1 OBS-jT"Optical Backscatterance Sensor (OBS), a sensitTMpressure transducer (type M6420-015PA-03) and a custom built resistance type wave staff (Figure 3 -2).
Figures 3.3 to 3.7 provide a plan view of the location of the traps (which are CO- tocated with the Stations) and instruments on each marsh surface. From a different perspective, Figure 3.8 (a to e) provides a cross-profile of the transect on each marsh, aiso indicating Station locations.
The H-frame was located as close to 5 m landward of the marsh margin as possible, in an effort to collect data that was representative of conditions at the marsh margin during the flood tide. However, in the case of Pecks Cove and West Allen Creek marshes, the cables were not long enough to reach the ideal location. At Pecks Cove marsh the instrument array was set up almost parallel to Station 1 and at West Allen
Creek marsh the instruments were paraIIel to Station II. The cables that ran from the instruments on the H-frarne and the wave staff were secured to a number of wooden stakes. This measure was taken in an effort to minimize rnovement of the cables by wind wave and tidal currents, thus reducing the potential for interference in voltage output.
The cables were comected to an A/D board and the program E~S~AG*~was used to log the data. The OBS, EMCM, pressure transducer and wave staff were run off a 12-volt battery and a grounded generator powered the desktop computer. Data were recorded at a rate of 4 Hertz for approximately a 5-minute period. This method of data collection was done every 15 minutes at Pecks Cove and Allen Creek. However, due to the much shorter tidal inundation at the remaining sites, data were collected in back-to-back instrumentation runs. EMCM, OBS and pressure transducer data were collected during altemate instrumentation runs with the wave staff. as experience suggested that the voltage oritput from a wave staff may interfere with the other instruments. For the purpose of this study a 'mn' refers to each 5-minute block of data collected by the electronic instruments. Due to the logistics of power supply and availability of equipment, measurements using the electronic instrument array could only be conducted one marsh at a time. Station Station 1 20 m B T T
MARSH MUDFLATS
Figure 3.1 a): A plan view diagram of the transect orientation with respect to the marsh margin and relative location of the Stations, rising stage bottles (B), and sediment deposition traps (T). This diagram is representative of Pecks Cove and West Allen Creek marshes.
Station Station
T
MARSH
Figure 3.1 b): A plan view diagram of the transect orientation with respect to the marsh margin and relative location of the Stations, nsing stage bdes (B), and sediment deposition traps (T). This diagram is representative of Westcock, Tantramar and Fort Beausejour marshes. Figure 3.2: a) instrument array on the H-frame. Shown fiom the right: EMCM, OBS, and pressure transducer. b) Custom-built resistance type wave staff.
Pecks Cove marsh
1 Low marsh +4 Mudflats-b 3 -
Horizontal distance (m) al
West Allen Creek marsh ------
3 . Lotv marsh Mudf3at.s
Horizontal distance (rn) b) -- - - Westcock marsh
Horizontal distance (rn)
Figure 3.8 a) to c): Cross-profile of transect on each marsh. Tantrarnar mars h
Horizontal distance (m)
4 - .- . Fort Beauséjour rnarsh
Horizontal distance (m) e) -- .- -
Figurc 3.8 d) to e): Cross-profile of transect on each marsh. 3.2 Instrumentation and procedures
The folIowing section describes the equipment and procedures used to collect data on sediment deposition, suspended sediment concentration, wind and wave conditions, tides. and vegetation. Field mapping techniques used in this study are addressed and lastly. the statistical tests used in the analysis of these data are provided.
3.2. I Sediment deposition
L2ccordingto Reed (1989), the coIIection of deposited sediment is preferred over measurement of vertical accretion for short-term sedimentation studies. This is because trapping of deposited sedirnents avoids the problem of compaction during sarnpling, allows for more fiequent sampling and consequently provides a better base for the evaluation of event controlled deposition. ln this study, sediment deposition was nieaswed as the amount of sediment that settled out of the water column over an individual tidal cycle. With respect to sediment deposition (and suspended sediment concentration), the relative arnount of organic material is minimal (van Proosdij, 2001), and therefore the terrn 'sediment' will be assurned to contain both inorganic and fine organic material.
Sediment fallout was collected on a surface-mounted University of Guelph sediment trap (van Proosdij, 2001) (Figure 3.9). These traps were designed after Reed's
(1989) petri dishes, but were modified to improve handling, ease of deployment and ruggedness. Each trap consisted of three 9-cm diameter filter papers of a known mass placed on a thick woven mesh (scour pads) for drainage. The filter papers and mesh were secured between a 15 x 30 cm aluminum plate (on top), which was cut by three 8 cm diarneter holes, and 4 alurninum strips (on the bottom). The entire ensemble w-as held together by four threaded legs each approximately 10 cm in length. Qualitative VWR
brand filter papers were used in the sediment traps (Cat.No. 28310-048). The filter papers rneasured 9 cm in diameter with a pore size of 5 Fm. Each paper was Iabeled and
its mass recorded pnor to deployrnent in a trap.
The sediment deposition traps were set approximately 0.5 to 1 m away from the stake marking the Station in an effort to minimize disturbance of the marsh surface in close proximity to the traps. Each sediment deposition trap was aligned parallel to the marsh margin. Once exposed by low tide, the traps were collected and the filter papers transferred to individually labeled petri dishes. The petri dishes were taken to the laboratory and the lids removed to allow the samples to air dry for 24 hours. Dry filter papers were then re-weighed and their mass recorded in grarns. Deposition amount was then calculated as mg/crn2/tidal cycle. Similar to the methods of Hutchinson (1995), samples of deposited sediment were not washed of salts because the filters could not be rinsed without min since the sedirnent was not consolidated. Al1 traps were processed in the same way. It is important to note that the sediment deposition is only a relative nieasure, as digerences in the surface characteristics of the filter papers and the marsh have some effect on the nature of the measurements (Reed, 1989).
Between tidal cycles, the sediment traps were disassembled and washed thoroughly to avoid contamination of filter papers for the next deployment. If perchance it rained afier the traps were exposed on the low tide, the sediment samples were discarded because loss of sediment on the filter papers as a function of raindrop splash was evident. Figure 3.9: A sediment deposition trap îs on the te& and the rising stage bottle is fixed to the stake on the right .
3.2.2 Suspended sediment concentrafrafron
Suspended sediment concentration was measured by two methods: Rising stage bottles and Optical Backscatterance Sensors (OBS). Rising stage bottles provided a water sample representing the leading portion of the flood tide, and the OBS provided data on the fluctuations of suspended sediment concentration throughout a aven tidal cycle.
Risinp; stage bottles
Rising stage bottles were the primary method of suspended sediment data collection. The bottles were constmcted of copper tubing, a rubber bung and a 500 ml
Nalgene bottle (Figure 3.9). At Stations 1, III (and IV) a rising stage bottle was fixed to a wooden stake by a hose clamp with the base of the bottle set IO cm above the bed. The intake and exhaust pipes were oriented onshore so as to limit the potential of floating vegetation obstructing the pipes on the rising tide. The bottles collected a water sample on the rising tide, filling within the first minute to minute and a half after submersion of the intake pipe (which occurred at a water depth of 28 cm). Once the tide was low enough to fully expose the bottles, the samples of suspended sediment were collected and transported to the Iaboratory. The sediment samples were then filtered through 9.0 cm diameta- VWR brand qualitative filter paper (Cat-No. 283321-077) with a pore size of 1
Pm. Prior to filtration, the filter papers were IabelIed and the mass recorded. A Welch
Duo-Seal Vacuum Purnp (mode1 #1400) facilitated the filtering process. Afier filtration, the filter papers were left for 24 hours to air dry. The sarnples were then re-weighed and the mass recorded. Suspended sediment concentration was calculated in mg/l/tidal cycle.
Variation in sampling was tested using 6 rising stage bottles aligned parallel to the marsh margin at Pecks Cove marsh. Suspended sediment concentrations were found to vary no more than 16 % of the mean (Figure 3.10). Pecks Cow marsh
Sample bottle num ber
Figure 3.10: Relationship behveen six rising stage bottle samples taken simultaneously at Pecks Cove marsh. Variation between sarn~lesis < 16% of the mean.
Optical Backscatterance Sensors (OBS)
A D&A Instruments OBS-3 was used in this experiment. OBS's use the reflectance of light frorn sediment in suspension to measure the 'turbidity' of the water.
According to Maa et al. (1992), OBS's are very sensitive to grain size, sediment composition and air bubbles; however, they do provide detailed, high frequency and point specific records of change in suspended sediment concentration. As a part of the electronic instrumentation array, one OBS was secured to an H-fiame with the sensor oriented offshore.
Similar to the methods employed by Schoellharner (1996) and Fishman and
Friedman (1 989), field calibrations of the OBS were conducted. Bottle sarnples were taken by hand close to the OBS at the beginning of each 5-minute instrument record. The sarnples were processed in the same manner as the rising stage bottle sarnples and were then used to calibrate the OBS data. The calibration was based on 21 sarnples taken over 8 tides at Pecks Cove and West Allen Creek marshes, enconipassing a range of wave and tidal conditions. The results of a regression between the initial 20 second mean of OBS voltage data and suspended sediment concentrations rneasured by the sarnpling bottles gave an r' value of 0.9 1, which is significant at the 99 % confidence level (Figure 3.1 1).
O -- -. O 0.5 1 1.5 2 2.5 Voltage output (Hz)
Figure 3.11: OBS calibration. Results of regression, based on cornparison between in situ bottle samples (mg/L/tidal cycle) and the Optical Backscatterance Sensor (Hz).
.3.2.3 Wimi urrd wave conditions
Wave conditions were both measured and predicted over a range of conditions.
Direct measurements of wave height were made using a wave staff. Predictions of wave height were calculated based on wind speed, wind direction and fetch. Wave staff
One custom-built, resistance type wave staE was deployed alternately at each of
the five sites as a part of the electronic instrument array. The wave staff was positioned
approximately 3 cm above the bed and secured by hose clamps to a metal pole firdy
sunk into the marsh. Located approximately 10 rn fiom the H-frarne parallel to the marsh
margin. the wave staff was considered to be a suffkient distance from the H-fiame to
avoid interference. It was positioned in an effort to experience wave conditions
comparable to those experienced by the instruments on the H-frame. The wave staff was
calibrated in a sono-tube filled with seawater.
Unfortunately the data collected by the wave staff could not be used because of
noise in the data which produced a spurious signal.
Wave prediction
Wave height (H,,) was predicted for the 6-hour period prior to high tide using
averaged wind speed, wind direction and fetch in combination with AC ES^^ (Automatic
Coastal Engineering System). software of the U.S. Army Corps of Engineers. Hm, is
defined as the zero moment of wave height. Calculation of Hm, is derived from frequency analysis, and is very similar to significant wave height (H,), which is derived fiom time domain analysis (Karnphuis, 2000). Wave height was calculated in metres for each tidal cycle at the margin of each marsh. C.T/ind speed and direction
Wind data collected at Amherst, Nova Scotia by the Atlantic Climate Centre,
Environment Canada (2000) were the primary source of wind speed and direction data for this study. Supplementary wind data were collected by a Davis Instruments anemometer and wind vane mounted on the rooftop of a two-story house situated along the shore of the Cumberland Basin between West AIlen Creek marsh and Westcock marsh. Data were recorded by a data logger and downloaded ont0 a laptop compter once a week. The data were then interpreted using the Davis Instruments' program
Weather Wizard II'". Wind speed and direction were averaged over a 6-hour penod prior to high tide.
Fetch
Fetch was determined from a 150000 map of Amherst, New Brunswick/Nova
Scotia (2 1 W 16, Edition 3) and a 1 :ZO 000 rnap of Amherst (2 1 H, Edition 3) both published by Energy, Mines and Resources Canada. A line was drawn paralle1 to the shore and azimuth readings of fetch with respect to grid north were measured radidly at
10 degree intervals from a designated point on the marsh surface to the adjacent shoreline. Fetch was measured from marsh margin to marsh rnargin, or where there was no marsh, to the coastline. Appendix A provides the list of calculated fetch Iengths for each marsh. 3.2.4 Tides
Tide tables
The primary source of tide data came from the year 2000 predictive tide tables for
Pecks Cove by the Canadian Hydrographie Service. These data included the predicted time and height of hiph and low tide.
Pressure transducer
A sensitTMtype M6420-015PA-03 pressure transducer with a range of O to 15 PSI
ABS was used to provide a measurement of water depth, and duration of rnarsh inundation as the tide rose and fell. These measurements w-ere made over individual tidal cycles and are representative of conditions at the location of the instrument array on the rnarsh surface. The pressure transducer was secured 8 cm above the bed by a hose clamp to a vertical leg of the H-frame. Vegetation beneath the pressure transducer was clipped to avoid contact with the sensor. The pressure transducer was calibrated in a 3 rn high static tube.
3.2.5 Vegetation
Vegetation height and density were measured at approximately two week intervals throughout the duration of the study period. Each vegetative survey was carried out on the sarne day at each Station on each marsh. The location of the quadrat was marked such that surveys were carried out at the same location on the rnarsh sudace each time. Vegetation density was counted at ground level in five randomly chosen 20 cm x
20 cm squares within a 1 m2 quadrat (Figure 3.12). The total number of stems was multiplied by five to obtain an estimate of stem density for 1 m2. At the same time, average vegetation height was detemined based on height measurements taken in five squares using a ruler. In addition, approximately half way through the study period, samples of the species of vegetation on the marsh surface were collected in and around the quadrat at each station on each marsh, and were identified upon return to the lab.
The role of vegetation in buffering current velocities is well documented in the literature. Its effectiveness as a buffer is a fùnction of both vegetation height and density and depth of the water column. Therefore, for the purpose of analyzing the effect of vegetation on sediment deposition rates, vegetation height and density and depth of the water column were incorporated into the calculation of the Relative Roughness index
(W:
where H is the mean vegetation height and h is mean water depth. Although density is not directly incorporated into the ratio, increases in vegetation density are closely related to increases in vegetation height. In other words, vegetation becomes both taller and denser through the growing season. Mean water height was calculated from the pressure transducer data, and is based on average water height during marsh inundation. RR was calculated for the location of the instrument array on each marsh. On the days of instrument runs, vegetation height was estimated based on growth trends of vegetation at the Station located closest to the instrument array. Figure 3.12: Example of vegetation quadrat at West Allen Creek, Station 1 on June 29, 2000.
3.2.6 Field mapping
The five sites were rnapped using GPS, standard survey techniques and a
photographic journal. A Trimble Pathfinder proxlZTMGPS system was used to generate
a map for each site. These maps indicated the location of the seaward and landward
marsh margins, the location each Station along the transect (which is also indicative of
the location of the sediment traps and rising stage bottles), position of the H-frame and
wave staff, as well as any other significant features (e-g., creeks). This system has a
positional accuracy of 0.3-1 .O rn and an elevational accuracy of 0.5-2.0 m.
A Leica TC600 total station was used to determine the elevation of the above mentioned features for each transect at each site and to create a profile for each transect.
Lastly, photographic characterizations of the marshes (taken from the air and on the growid) provide useful visual prompts to facilitate general comparisons between the marshes. 3.2.7 Data analysis
Non-pararnetric statistical tests were used to analyze the contribution of wind waves and tides to sediment availability, and the geomorphic controls acting sediment deposition. Spearman's rank-difference correlation was used to determine the degree of association between variables. &rd Wilcoxon matched-pairs, signed ranks test was used to determine if there were statisticdly significant differences between significantIy correlated data sets.
Spearman's rank-difference correlation and Wilcoxon matched-pairs, signed ranks test were used to compare results of suspended sediment concentrations and sediment deposition for the Stations across each marsh, per tidal cycle. While Spearman's rank alone was used to recognize relationships between suspended sedirnent concentrations and sediment deposition at Station III (or IV where applicable) between marshes for each tidal cycle. Furthemore, Spearman's was used to determine relationships between each geomorphic control and both suspended sediment concentrations and sediment deposition at Station III (or IV) for each marsh per tidal cycle. Statistical tests were conducted using
STATISTICA~~(version 5) and, unless otherwise defined, al1 statistics were calculated using a 90% cod~denceIevel. CHAPTER IV
RESULTS
4.0 Introduction
Spatial and temporal variations in each geornorphic control and interactions between these controls contribute to a complex system of sediment dynarnics in coastal saltmarshes. Due to this compIexity and some of the limited data sets of this study, a statistical isolation of the actual contribution of each geomorphic control to sedirnent deposition is not fëasible. Therefore, results in this section reflect the presence or absence of relationships between variables, the strength of that relationship and their significance at the 90% confidence leveI.
Results here are divided into three sections based on the objectives. As variations in sediment deposition are a hnction of both the availability of sediment and opportunity for deposition (Reed, 1989), the first two sections atternpt to address these aspects in relation to sediment deposition on the Cumberland Basin marshes. More specifically, section one analyzes the availability of sediment (suspended sedirnent concentrations) with respect to wind waves and tidal currents and section two evaluates the contribution of several geomorphic controls in providing the opporhinity for sediment deposition.
Finally. the Iast section provides insight on the presence or absence of patterns in sediment deposition both across and between marshes. 4.1 Temporal and spatial variation in suspended sediment concentrations (SSC)
Al1 other things being constant, the higher the concentrations of suspended sediment, the more sediment that is available for deposition. Currents associated with wind waves and tides are considered an important influence on concentrations of sediment in the water column, as the currents are responsible for both entraining and transporting sediment to the marsh surface. Results in this section are divided into four components. including a bnef examination of the temporal change in SSC at each marsh, an investigation of variations in suspended sediment concentrations across each marsh and spatially throughout the Basin, and an evaluation of the contribution of wind waves and tidal amplitude to variations in suspended sediment.
4.1.1 Temporal variations in SSC
In order to provide a better understanding of suspended sediment dynarnics at the study sites, OBS data supplied information on the temporal variation in SSC (at one location on the marsh) over the duration of individual tidal cycles. Results of the OBS data indicate different trends in SSC over the scale of a tidal cycIe between sites. At
Pecks Cove and West AlIen Creek marshss, generally higher concentrations of SSC occurred on both the leading edge of the flood tide and the later portion of the ebb tide.
In cornparison, SSC at Tantramar and Westcock marshes remained relatively constant or decreased slightly over the tide. SSC at Fort Beauséjour marsh did not change appreciably throughout the period of inundation (Figure 4.1) Figure 4.1: Example instrument summary for Pecks Cove marsh, June 5,2001.
EMCM (x-axis)
Time relative to high tide (minutes)
-Mean ----t---Standaid deviation
Time relative to high :ide (minutes)
Time relative to high tide (minutes)
Pressure Transducer
Time relative to high tide (minutes) 4.1.2 Spatial variations in SSC across each marslt
In an effort to establish a representative value of suspended sediment concentration (SSC) for each marsh per tidal inundation? it was important to deterrnine if
SSC at stations 1 and III (or IV where applicable) were similar. Test results indicated that
SSC values across Pecks Cove (PC) and West Allen Creek (AC) marshes were positively correlated (Table 4.1) but significantly different (Figure 4.2). At Pecks Cove and West
Allen Creek marshes, values of SSC at Station 1 were consistently higher than at Station
III. suggesting that sediment was brought into suspension as tidal waters flowed over the saltmarsh. With respect to the remaining marshes, sarnples of SSC across Westcock
(WC) demonstrated a high correlation (r = 0.8). however the relationship was not statistically significant (which may be a function of the srnaII sample size). Meanwhile,
Tantramar (TR) marsh showed no significant correlation, while a Iack of data prevented analysis of results for Fort Beauséjour marsh (FB). Based on either lack of correlation or a significant difference between rising stage bottle sarnples, SSC samples should not be averaged across each marsh.
Pair of Variables Tidal cycles Spearman's r p-value PC1 and PC3 27 0.83 0.0000 AC1 and AC3 27 0.84 0.0000 WC? and WC3 5 0.80 O. 1041 TRI and TR3 5 0.10 0.8729 FBI and FB 4 3 n/a n/a
TabIe 4.1: Correlation results of SSC sarnples betrveen Stations across each marsh. SSC across Pecks Cove marsh (PC) and across West Allen Creek marsh (AC) are significantly correlated, while there is no significant correlation across each of Westcock (WC) and Tantramar (TR) marshes. Insufficient data were available to test Fort Beauséjour (FB). Significant correlations were determined at the 90% confidence level, and are in bold print. Pecks Cove marsh West Allen Creek marsh
4 0- 1:l Iine 1:1 line #
O do00 2000 3000 O 1000 2000 3000 SSC at PC1 (m@) SSC at AC7 (mg/L) a) b)
Figure 4.2: Correlation of suspended sediment concentrations (SSC) across eacli marsh. Graphs a) and b) illustrate the significant correlations between SSC at Stations 1 (PCl, ACI) and III (PC3, AC3) across Pecks Cove and West Allen Creek respectively. In both situations SSC is significantly higher at Station 1 than Station III, suggesting sediment is brought into suspension as the tide flows across the marsh, Statistical difference of these correlated samples was deterrnined at the 90% confidence level.
4.1.3 Spntinl varintions in SSC between marshes
For the purpose of comparing SSC between marshes, rising stage bottles, which were first to be inundated and/or located at the furthest seaward extent, are used for further analysis. This includes Station III at Pecks Cove, West Allen Creek and
Tantramar marshes, and Station IV at Westcock and Fort Beauséjour marshes. By choosing SSC sampling locations closest to the open water of the Basin, samples should be more reflective of floodwater conditions in the Basin and the influence of site specific variables such as vegetation may be reduced.
Using the same statistical tests as in the previous section, variations in SSC between sites were analyzed. A strong to moderate significant positive correlation was found between SSC for al1 sites with the exception of Tantramar marsh (Table 4.2), where an increase (or decrease) in SSC occurs throughout the Basin. Significant correlations between the sites (excluding Tantramar) suggest that these sites were collectiveIy influenced to some degree by one or more of the same geomorphic controls.
Conversely. lack of correlation between Tantrarnar marsh and the other four sites suggests that geomorphic controls affecting SSC at Tantramar may be different or at least partially different than those affecting the other sites.
Of those sites with significantly correlated SSC, results indicate significant differences in SSC between al1 sites (as can be seen by the deviations from the 1: 1 line in
Figure 4.3), with the exception of Pecks Cove and Fort Beausejour, and West Allen
Creek and Westcock marshes. It is important to note, however. that just because these sites are not statistically different in terms of SSC, it does not mean that their comparable levels of SSC arise by the same geomorphic controls.
Based on the results, significant correlations between sites suggest that to some degree marshes throughout the Basin were collectively influenced by the same geomorphic control(s), although the dominance of that influence may be Iessened with distance (accounting for the significant differences). Controls affecting SSC at Tantramar may be site specific as indicated by its difference fi-om al1 the other sites. 1 Pair of Variables 1 Tidal cycles 1 Spearman's r 1 p-level 1 PC and AC PC and WC PC and TR PC and FB AC and WC AC and TR AC and FB WC and TR WC and FB TR and FB
Table 4.2: Correlation results of SSC between sites. Samples of SSC at al1 sites are significantly correlated at the 90% confidence ievel, with the exception of Tantramar mardi. Significant correlations are in bold print. ------Pecks Cove and West Allen Creek Pecks Cove and Westcock marshes mars hes
1:l line
O 500 1000 1500 2000 O 500 1000 1500 2000 SSC at PC (rng/L) SSC at PC (mg/L) b)
.. - - - -- Pecks Cove and Fort Beauséjour marshes West Allen Creek and Westcock marshes ' 1:l Iine - 1:1 line
SSC at PC (mg/L) SSC at AC (mg/L) c) d) - ---
Figure 4.3: Correlation of SSC at Station III behveen sites. Graphs a), b), c), d), and e) exliibit significantly correlated SSC between sites over the scale of individual tidal cycles. SSC between Pecks Cove (PC) and Fort Beauséjour (FB) marshes (graph c), and West Allen Creek (AC) and Westcock (WC) marshes (graph d) are not significantly different at the 90% confidence level, while SSC at PC and AC (graph a), PC and WC (graph b), AC and FB (graph e), and WC and FB (graph f) are significantly different. - West Allen Creek and Fort Beaiséjour marshes
/+ 1:l line / 0 #' #' 0 0 0
SSC at AC (mg/L)
Westwck and Fort Beauséjour marshes
/ 1:l line 0 0 / / 0 / 0
SSC at WC (mg1L) As a Mer note, in scatterplots developed to illustrate the nature of these relationships. a data point was found to significantly deviate from the trends. This anomalous data point, occumng on JuIy 5- 2000 (Julian day 186), was characterized by low tvave height and low to average SSC at Pecks Cove and West Allen Creek marshes.
Hocvever. very high SSC was measured at Westcock and Fort Beauséjour marshes. This day was unique during the study period as it experienced a localized thunderstorm bringing heavy intermittent rain. Based on field observation, the thunderstorm appeared to be situated over the upper portion of the Cumberland Basin. Figure 4.4 illustrates the nature of the relationship between SSC at Pecks Cove marsh and both Westcock and Fort
Beauséjour marshes. This anomalous data point suggests that rainfall may be an important geomorphic control on SSC; however, with only one observation, this control cannot be further explored. Based on the relationship between min and SSC in the
1iterature (e-g., Evans and Collins, 1975; Wolaver et al., 2998) and the lack of observed rain activity during the remainder of tidal cycles data were collected, data for day 186 were excluded from the data set. Pecks Cove and Westcock marshes Pecks Cove and Fort Beausejour marshes Day 186 l:? iine Day 186 & 0 ,= 1:l line
O 500 1000 1500 2000 O 500 1000 1500 2000 SSC at PC (mg/L) SSC at PC (mg/L)
-- -
Figure 4.4: Anornalous SSC results for Julian day 186. Note the significantly higher SSC at Westcock marsh (WC) and Fort Beauséjour marsh (FB) for day 186 than experienced at Peck's Cove marsh (PC).
4.1.4 Variations in SSC as a frriictiori of local and regional wave lrei't
Unfortunately, a comparison between predicted waves (H,,) and measured wave staff data could not be done, as there was a quality control problem with the wave staff data. The problematic data were recognized as measured wave heights were considerably different than observed wave heights. The possible source of error could not be detemined, however it rnay be a function of interference or instrumentation problems.
For the most part, predicted waves matched observations of wave activity in the field and as such the predicted waves are used for analysis in this study.
Local predicted wave hei-ht (H,,)
A geomorphic control that may contribute to local variation in SSC is local wave height. Local waves in this context refer to predicted wave height (Hm) for the marsh margin of each study site, and is a function of fetch length, wind direction and wind speed.
In a correlation between predicted local wave height and SSC for each marsh, only West Allen Creek marsh exhibited a significant correlation (r = 0.43), while the remainder of the sites showed no significant relationship between locally generated wave height and SSC (Table 4.3, Figure 4.5). However, when wind speed alone was correlated with SSC for each marsh, al1 relationships (with the exception of Tantramar marsh) were significant (Table 4.4, Figure 4.6). This suggests that waves generated elsewhere in the
Basin are a dominant control on SSC at the marsh rnargins.
Site Tidal cycles Spearman's r p-value PC 27 -0.1 6 0.4353 AC 27 0.43 0.0248 WC 18 0.35 0.151 1 FB 18 0.36 0.1474
Table 4.3: Results of the correlation bebveen predicted local wave height (H,,) and SSC. Tantramar was not iiicluded in this analysis as it is sheltered from wave activity. Significance was determined at the 90% confidence Ievel, and are in bold print.
-- Site Tidal cycles spearman's r p-value PC 0.49 0.021 1 AC 0.80 0.0000 WC 0.46 0.0577 TR 0.31 0.3550 FB 0.47 0.0516
Table 3.4: Results of the correlation between wind speed and SSC. All sites but Tantramar (TR) eshibited a statistically significant positive correlation. Significance was determined at the 90% confidence level, and are in bold print. - -. Pecks Cove marsh West Allen Creek marsh
Predicted local Hmo (rn) Predicted local &, (m) a)
-. - - -- Westcock marsh Fort Beauséjour marsh
Predicted local Mio(m) Predicted local Hm (m) c)
Figure 4.5: These graphs illustrate the reIationsliip between predicted local wave height (H,,) and SSC. None of these correlations are statistically significant at the 90% confidence level. - -. - Pecks Cove rnarsh Mst Allen Creek marsh
Wind Speed (Whr) Wnd Speed (krn/hr) a) b)
Wtcock marsh Fort Beauséjour marsh
Wnd speed (kmlhr) c) Wind speed (kmhr)
Figure 4.6: Correlation between wind speed and SSC. Graphs a), b), c) and d) al1 exhibit a siçnificant correlation between wind speed (averaged over 6 hours prior to bigh tide) and SSC at the 90% confidence IeveI. Wind speed and SSC at Tantramar marsh was not significantly correlated. Reeional predicted wave height (H,,)
Predicted regional wave height was calculated for an arbitrary point centered in the mouth of the Cumberland Basin. When predicted regional wave height was correlated with SSC. a significant strong to moderate relationship was found at al1 sites but Fort
Beauséjour (Table 4.5 and Figure 4.7). The results of these tests suggest the importance of regional waves to the level of SSC observed at the marsh margins; in other words, waves at the mouth of the basin account for a significant arnount of the variation in SSC throughout the Basin.
An exarnination of wind direction and SSC lends credence to the idea that regional waves are an important influence on SSC at the marsh margins. In Figure 4.8,
SSC is plotted in relation to wind direction. When winds are fiom the west at Pecks Cove marsh it is considered an offshore wind of very small fetch (see Appendix A for fetch lengths). yet concentrations of SSC are at their highest. However, when viewed from the mouth of the Basin, westerly winds correspond to the longest fetch, and therefore larger waves and greater concentrations of suspended sediment result. This sediment is then advected to the marsh margins via the tides. The same trend cm be observed at a11 sites but Tantramar marsh. Site Tidal cycles Spearman's r p-value PC 27 0.62 0.0005 AC 27 0.80 0.0000 WC 18 0.53 0.0250 TR 11 0.64 0.0325 FB 18 0.38 0.1 104
Table 4.5: Results of the correlation between predicted regional wave height (W,,) and SSC. Significance was determined at the 90% confidence level, and are in bold print. ------. Pecks Cove rnarsh West Allen Creek rnarsh
i- 1500 F 1200 V O a 900 -m 600 CI) 300 O
Predicted regional Hm, (m) Predicted regional Hm, (m)
Westcock marsh Tantramar marsh
m 600 - . CI) 300 . . .O*
Predicted regional Hm, (rn) Prediced regional Hm, (m) c >
Figure 4.6: These graphs illustrate the relationship between predicted regional wave height (H,,) and SSC. All of these correlations are statisticaIIy significant at the 90% confidence level. Predicted regional wave height (H,,) and SSC were not significantly correlated for Fort Beauséjour marsh. North
West
North
West 8 East
W Allen Creek marsh O SSC(mg/L)
Figure 4.8: Relationship between wind direction and SSC. North
Westcock marsh South SSC (mg/L)
North
West East
Tantramar marsh South I
Figure 4.8: Relationship between wind direction and SSC. North
West
Fort Beauséjour marsh South SSC (mgIL)
Figure 4.8: Relationship behveen wind direction and SSC. 4.1.5 Variations iiz SSC as a fùnction of tidal amplitude
Whereas regional waves appear to be the primary mechanism influencing SSC,
tidal amplitude may also play an important. aibeit lesser role, in influencing levels of
suspended sediment throughout the Basin. Variations of tidal amplitude are reflected in
tidal current velocity. A higher current velocity is experienced during spring tides as the tidal prism rxpands, and conversely, lower current velocities occur during neap tides when the tidal pnsm is smaller. Based on the results of Schoellhamer (1996), it \vas espected that increased SSC would correspond with higher current velocities, which occur during spring tides.
To isolate the effects of tides from wave activity, the correlation between SSC and tidal amplitude was based on tidal cycles where local wave height was < 15 cm.
Correlation results of tidal amplitude and SSC suggest that West Allen Creek, Westcock and Fort Beauséjour marshes are moderately influenced by variations in tidal amplitude
(Figure 4.9). SSC at Pecks Cove and Tantrarnar marshes exhibit low to no relationship with tidal amplitude. However. the scatter of data p~intsat these sites may be attributed to the influence of regional waves and other geomorphic controls. masking a possible relationship between SSC and tidal range. Table 4.6 presents the results of the analysis between each of wind speed, predicted local and regional wave height, tidal amplitude, and SSC. .- - -- West Allen Creek marsh Westcock marsh
1000 1100 1200 1300 1400 1500 Tidal amplitude (cm) Tidal amplitude (cm) b)
Fort Beauséjour marsh
Tidal amplitude (cm) c>
Figure 4.9: Correlation behveen tidal amplitude and SSC. West Allen Creek marsh (graph a), Westcock marsh (graph b) and Fort Beauséjour marsh data are statistically significant at the 90% confidence Ievel. Pecks Cove and Tantramar marsh data are not significantly correlated. Marsh Wind S~eed wave height (Hmo) wave height (Hm*) -r value tidal cycles r value tidal cycles r value tidal cycles r value 0.49 22 -0.16 27 0.62 27 O. 32 0.80 22 0.43 27 0.80 27 0.52 0.46 18 0.35 18 0.53 18 0.50 0.31 11 nla nla 0.64 11 0.13 -0.47 18 0.36 18 0.39 18 0.74
Table 4.8: Geoinorpliic coiitrols affectiiic coriceiitrations of siis~eiidedsediiiieiit. Correlatioiis are siniiificant at the 90% confidence level atid are in bold priiit. 4.2 Relative significance of each geomorphic control on sediment deposition
The high temporal and spatial variability of sediment deposition was recognized in the results of Objective 1. As noted by many authors, sediment deposition is a function of both the availability of sediment, and its opportunity for deposition.
Objective 2 concentrated on the role of wind waves and tides, and their influence on the availability of sediment in the water colurnn. Objective 3, which is the focus of this section, attempts to examine how several geomorphic controls, including wind waves, tides. SSC, and vegetation affect the opportunity for sediment deposition. As this study encompasses several marshes within the same system, it is anticipated that a large part of the variation in sediment deposition throughout the Basin will be controlled by sirnilar extemal forcing factors. However, since each of the marshes is situated at a different location relative to mean high water Ievel, interna1 factors such depth and duration of tidal inundation, vegetative characteristics and morphology will create site specific differences.
4.2.1 FVitt d waves
Based on the literature, it was anticipated that at lower wave heights, sediment deposition would be greater at the sites nearest the marsh margin (Stations III) than fùrther inland. This was considered as wind wave velocities would be low, inundation time would be longest and height of the water column is greatest (and therefore more sediment is available for deposition). When waves are higher it was anticipated that sediment deposition would decrease at the marsh margin as curent velocities would hold sedirnent in suspension: however, as the wave energy is dissipated across the marsh
sediment deposition would increase at Station 1.
However. results of Spearman's correlation between predicted local wave height
(H,,) and sediment deposition for stations 1 and III on Pecks Cove, West Allen Creek and Westcock marshes over individud tidal cycles indicated no significant relationship at the 90% confidence level. Fort Beauséjour and Tantramar marshes were not included in the analysis as insufficient data were available to test the relationship for Fort Beauséjour marsh and Tantramar marsh was not subject to local wind waves.
4.2.2 Tides
As previously discussed, tidal amplitude rnay be considered indicative of tidal current velocities where higher current velocities are associated with spring tides and lotver current velocities occur during neap tides. In order to partially isolate the effects of tidal currents on sediment deposition over the scale of an individual tidal cycle, a comparison of these variables were made based on data collected from days with relatively calm conditions (O to 15 cm waves). By choosing calm days, velocities associated with wind waves would be removed and the resultant velocities would be a truer representation of tidal currents.
Results of the correlation indicate essentially no significant relationship between tidal amplitude and sedirnent deposition, with the exceptions of Station III at Pecks Cove marsh (r = -0.47, p = 0.08) and Tantramar marsh (r = 0.76, p = 0.01) (Table 4.9). At
Pecks Cove marsh (Station III) it appears that as tidal amplitude increases, sediment deposition decreases (Figure 4.1 1). Conversely, at Tantramar marsh as tidal amplitude increases so too does sediment deposition. More significant relationships may have been found if tides with waves < 5 cm were chosen, Merreducing the effect of wind wave associated velocities; however, such a restriction tvould leave insufficient data for testing.
Site Tidal cycles Spearman's r p-value PC1 15 -0.30 0.2834 PC3 15 -0.47 0.0786 AC1 14 0.22 0.4542 AC3 14 -0-14 0.6261 WC1 7 0.43 0.3374 WC3 nla nia nla TRI nla nla nla TR3 11 0.76 0.0062 FBI n/a nla nla FB3 nia nla nla
Table 4.9: Results of the correlation between tidal amplitude and sediment deposition. Insufficient data was available to test Station III at Westcock marsh (WC3), Station 1 at Tantrarnar marsh (TRI) and both Stations 1 and III at Fort Beauséjour marsh (FBl and FB3). Significance was deterrnined at the 90% confidence Ievel, and significant results are in boId print. - -- - Station III at Pecks Cove marsh Station III at Tantramar marsh
1050 1150 1250 1350 1450 1050 1150 1250 1350 1450, Tidal amplitude (cm) TidaI amplitude (cm)
Figure 4.10: Reiationship between sedirnent deposition and tidaI amplitude. Graphs a) and b) are the only statisticaliy significant reIationships (at the 90% C.L.) of al1 possible combinations, with r-values of -0.47 (p-vaIue 0.08) and 0.76 (p-value 0.01) respectively. Note the different y-axis.
4.2.3 Sirspended seniment concentration (SSC)
Wind waves contribute indirectly to sediment deposition by affecting the arnount of suspended sediment in the water column, which in turn afTects the arnount of sediment available for deposition. Cornparison of sediment deposition and SSC in this study was limited to tidal cycles characterized by predicted waves < 15 cm. This was based on findings by van Proosdij et al. 's (2001) study in the Cumberland Basin, where they found
15 cm waves acted as a threshold either promoting or inhibiting sediment deposition.
More specifically, as wave height increased from O - 15 cm, SSC and sediment deposition increased. Where waves exceeded 15 cm in height, SSC continued to increase but remained in suspension. Results of this analysis indicate a moderate positive correlation between sedirnent deposition and SSC at Station 1 for West Allen Creek, Westcock and Tantrarnar marshes
(Table 4.8, Figure 4.1 7). In other words, an increase in SSC was concurrent with and increase in sediment deposition. Insufficient data were available to include analysis of
Station III at Westcock and both Stations at Fort Beauséjour marshes.
Site Tidal cycles Spearman's r p-value PC1 15 0.04 0.8994 1 PC3 15 0.2597
AC 1 I 14 0.0470 AC3 14 0.9703 WC1 6 0.0188 WC3 n/a n/a TRI 5 0.0374 TR3 11 0.81 1O FB 1 nla nla FB3 l n/a nla
Table 4.8: Results of the correlation between suspended sediment concentrations and sediment deposition. Insufficient data was availabIe to test Station III at Westcock marsh (WC3) and both Stations I and III at Fort Beauséjour marsh (FB I and FB3). Significance was determined at the 90% confidence Ievei, and significant results are in boki print. ------Station I at West Allen Creek marsh Station I at Westcock marsh
0 .-- O 500 1000 1500 2000 2500 SSC (mg/L) b ) SSC (mg/L)
Station I at Tantramar marsh
Figure 4.1 1: Stat isticaIly sign i ficant relationshi ps between sediment deposition and SSC. Note the higher concentrations of sediment deposition on the x and y-axis of graph c). 4.2.4 Vegetuation
The influence of vegetation on sediment deposition was determined using the
Relative Roughness (RR) index, which is the ratio of vegetation height to mean water depth (Appendix B provides data for height of the water column). It is important to note that this RR index does not directly account for vegetation density. However9a measured increase in density is imately tied to the increase in vegetation height at these sites. Ln other words, as the vegetation grew in height it also grew in density.
van Proosdij (200 1) suggested that the relationship between sediment deposition and RR is of greater significance during calm conditions. Unfortunately, the limited data set from this study does not allow for such a cornparison. However, using data associated with dl wave heights, a correlation of sediment deposition and RR provided a significant moderatelstrong relationship (r = 0.79, p = 0.004) (Figure 4.12). This correlation was based on a combination of data collected fiom al1 sites but Tantrarnar marsh. Although inclusion of the data fiom Tantramar marsh increases the strength of the correlation (r =
0.84. p = 0.0006) when graphed the high sediment deposition values at Tantramar marsh significantly change the slope of the relationship. Thus, results of this analysis suggest chat an increase in the RR index relates well to an increase in sediment deposition, irrespective of predicted wave height. O -- - 0.00 0.05 0.10 0.15 0.20 RR index
1 + Pecks Cove marah West Allen Creek marsh
3 Fort Beauséjour marsh
Figure 4.12: Relationship between sediment deposition and relative roughness index (RR) (including data from al1 sites). This relationship is statisticalIy significant at the 90% confidence level, with an r-value of 0.79 and p-value of 0.004. The sample size is limited by the data available from the pressure transducer. 4.3 Spatial variation of sediment deposition
Over the time scale of a tidal cycle. several authors have recognized sediment deposition as highly variable both temporally and spatiaily in response to the simultaneous influence of several geomorphic controls. However, amid this variability some general trends may be recognized and therefore it is the aim of this section to highlight patterns of sediment deposition both across and between marshes in the
Cumberland Basin.
4.3. I Vrrriafiortsin sediment deposition across eacfi rnarsh
Non-parametric correlaticns were used in order to determine whether sediment deposition (SD) samples taken across each marsh surface (at Stations 1, II and III) were similx enough that they may be averaged to provide one sediment deposition value for each marsh per tidaI inundation. Results of the test (Table 4.9) indicate that most sediment deposition samples were not significantly correlated across the marsh surface - where an increase in one sampIe was not necessarily followed by an increase in another sample. However, Stations II and III at Pecks Cove marsh and al1 combinations of
Stations across West Allen Creek marsh exhibited a significant low to moderate positive correlation. In other words, at these locations there was a concurrent increase or decrease in sediment deposition between samples collected at each place on the marsh surface per tidal cycle. Stations 1 and II at Tantramar, and Stations 1 and III at Fort Beauséjour demonstrated a high correlation, however the relationships were not statistically significant, which may be attributed to the small sample size. Tidal cycles Spearrnan's r p-value PC 1 and 2 PC 1 and 3 PC 2 and 3 AC 1 and 2 AC 1 and 3 AC 2 and 3 WC 1 and 2 WC 1 and 3 WC 2 and 3 TR 1 and 2 TR 1 and 3 TR 2 and 3 FB 1 and 2 FB 1 and 3 FB 2 and 3
Table 4.9: Correlation results of sediment deposition behveen Stations across each rnarsh. Stations II and III at Pecks Cove marsh (PC), and al1 Stations across West Allen Creek marsh (AC) are sign ificantly correlated. Stations across Westock (WC), Tantramar (TR) and Fort Beauséjour (FB) marshes are not significantly correlated. Significant results (at the 90% confidence level) are in bold print.
At Pecks Cove and West Alien Creek rnarshes, sedirnent deposition increased
with increased distance landward of the marsh rnargiii. Statistically, there was no
significant difference in the absoiute values of deposited sedirnent over a tidai cycle
between Stations II and III, and 1 and II at Pecks Cove and West Allen Creek marshes
respectively. However, a statistically significant difference in sediment deposition was
found between Stations 1 and III, and II and III at West Allen Creek marsh. In other words, sediment deposition is higher at Station 1 than the similar arnounts deposited at
Station II and III at Pecks Cove marsh, and sediment deposition is higher for Stations 1 and II than Station III at West Allen Creek marsh. Figure 4.13 illustrates the nature of the relationships for the Stations correlated across Pecks Cove and West Allen Creek marshes, where the position of the data relative to the 1: 1 line ilIustrates the difference between the samples. Unfortunately, data sets for Westcock, Tantrarnar and Fort
Beauséjour marshes are limited. as their rnarsh surfaces are inundated only during high spring tide.
. -- -
1 O Pecks Cove rnarsh 10 - West Allen Creek marsh / / 1:1 line 1 :l line
SD at Station 1 (mg/cm2) SD at Station II (rng/cm2) b) a)-- . - --
10 West Allen Creek marsh 10 - West Allen Creek marsh
/ 1 :lline 1:l line
(mg/cm2) SD at Station II (mg/cm2) c) SD at Station I d) -- - Figure 4.13: Correlation of sediment deposition across each marsh. Graphs a), b), c), and d) eshibit significantIy correlated sediment deposition (SD) between Stations II and III across Pecks Cove marsh, and al1 combinations of Stations across West Allen Creek marsh. Absolute values of sediment deposition over the scale of a tidal cycle at Stations II and III at Pecks Cove marsh (graph a) and Stations 1 and II at West Allen Creek marsh (graph b) are not statistically different at the 90 % confidence level, while at West Allen Creek marsh, sediment deposition at Stations 1 and III (graph c) and II and III (graph d) are significantly different. Based on these results, sediment deposition appears to exhibit a significant relationship across Pecks Cove and West Allen Creek marshes but not across the other study sites. The lack of correlation in sediment deposition across Westcock, Tantrarnar and Fort Beauséjour marshes may be a function of the smalI sample size (due to infrequent flooding of the marsh surface) and/or differences in marsh morphology. With respect to morphology, Pecks Cove and West Allen Creek marshes are chxacteristic of a
'young' rnarsh, whereas the other sites are more typical of an 'older' marsh, with cliffs, tidal creeks (and dykes). The rnorphology affects the flow of water across the marsh surface and therefore may account for the differences in sediment deposition. As deposition sarnples are mostly not correlated across the marsh surface they should not be averaged ta produce one value of sediment deposition per marsh per tidal cycle. To be consistent between marshes, sediment deposition results were not averaged at any site.
4.3.2 Variations in sediment deposition between marslzes
For the purpose of comparing sediment deposition between marshes, sediment trap data at Station III for each marsh are used. Sediment traps located at Station III for each marsh were chosen because they were situated closest to the open Basin on the marsh/mudflat interface, therefore reduci~gpotential variability in sediment deposition associated with vegetative differences between marshes. Eliminating the potential effect of veçetation may allow for the comparison in sediment deposition to be more reflective of spatial location within the Basin.
Using the same statistical tests as in the previous section, variations in sedirnent deposition between sites were analyzed. According to the results of the test, only three of al1 the possible marsh combinations of sediment deposition were significantly correlated.
Those that demonstrated a significant positive correlation of rnoderate strength were
Pecks Cove and West Allen Creek marshes, Pecks Cove and Tantramar marshes, and
West Allen Creek and Tantramar marshes (Table 4.10). The remainder of combinations showed no significant correlation. The results of this test indicate that an increase in sediment deposition at one site is not necessariiy accompanied by an increase in sediment deposition at another site.
1 Pair of Variables Tidal cycles 1 Spearman's r p-value PC and AC 0.41 0.0837 PC and WC 0.70 0.1 881 PC and TR 0.53 0.0956 PC and FB 0.28 0.4581 AC and WC nla nla AC and TR 0.84 0.0022 AC and FB 0.32 0.4346 WC and TR 0.00 1.O000 WC and FB nla n/a TR and FB 0.38 0.3178
Table 4.10: Correlation of sediment deposition between Stations iII at each marsh. Pecks Cove (PC) and West Allen Creek (AC) inarshes, PC and Tantramar (TR) marshes, and AC and TU marshes demonstrated significant positive correlations. Significant correlations are determined at the 90% confidence level, and are in bold print.
The pattern of sediment deposition over a tidal cycle at Pecks Cove and West
Allen Creek marshes are similar to Tantrarnar marsh, however, the absolute values are quite different, where sediment deposition is lower at Pecks Cove and West Allen Creek marshes than Tantramar marsh. Sediment deposition at Station III on Tantramar rnarsh was rnzich higher than that collected at any other site within the study area (up to 10 tirnes greater). Wilcoxon matched pairs, signed rank test was conducted on the significantly correlated sarnples of sediment deposition collected over individual tidal cycles at Station
III between marshes. Based on this test, no statistically significant difference in the absolute values of sediment deposition at Station III between Pecks Cove and West Allen
Creek marshes was found. On the other hand, results show a statistically significant difference in sediment deposition between Pecks Cove and Tantrarnar marshes, and West
Allen Creek and Tantrarnar marshes. Figure 4.14 illustrates the nature of the relationship between the significantly correlated sites, where deviation of these data fiom the 1: 1 line are indicative of the difference in the absolute values of sediment deposition between sites.
Sirnilar to the previous results, the small data sets for many of the sites and/or rnarsh morphology rnay have been influential in determining relationships in sedirnent deposition at Station III between the marshes over individual tidal cycles. With respect to morphology, these results may demonstrate a relationship between Station III at Pecks
Cove, West Allen Creek and Tantramar marshes. Unlike the other sites, Station III at these locations are in the low marsh, which is both first to be inundated and last to be exposed during a tidal cycle. In contrast Station III at Westcock and Fort Beauséjour marshes is the last Station to be inundated and first to be exposed on their respective marsh surfaces due to the patterns of flooding. The variable deposition noted here is also representative of spatial and temporal variations in sediment availability (i. e., suspended sediment concentrations) as well as variations in the geomorphic controls (i.e., waves, tides, vegetation) affecting the ability of sedirnent to be deposited. ------Pecks Cove and West Allen Creek Pecks Cove and Tantramar marshes marshes
200 - 1O / / 1:i line 1:l line
West Allen Creek and Tantramar marshes
1:l line
Figure 4.14: Correlation of sedirnent deposition between marshes. Graphs a), b) and c) exhibit a significant correlation in sediment deposition between sites. Of the marshes exhibiting a correlation in sedirnent deposition, a test of differences found sediment deposition at Pecks Cove (PC) and West Allen Creek (AC) (graph a) are not s ignificantly different at the 90% confidence leve 1, while sediment deposition at PC and Tantramar marsh (TR) (graph b), and AC and TR (graph c) are significantly different. CHAPTER V
DISCUSSION AND CONCLUSIONS
5.0 Introduction
In this section, results of the study are discussed relative to findings in the literature. Cornparison of absolute measures must be viewed cautiously as small variations in rnethods may cause significant differences in the results, and as such the primary focus is on cornparison of reIative trends.
The objectives provide the fiamework for discussion of the results. More specifically, the discussion first highlights the role of wind waves and tidal currents in sediment suspension. This is followed by a discussion of the contribution of waves, tides. SSC. and vegetation to sediment deposition. Finally, attention is focused on how a combination of sedirnent availability and opporîunity for deposition has brought about spatial variation in sediment deposition along the Cumberland Basin. A conceptual mode1 helps illustrate these relationships (Figure 5.1). This mode1 brings together the main geornorphic controls influencing sediment deposition on the saltmarshes of the
Cumberland Basin. - - 1 ~eih ( 1 CYind direction 1 1 Wind spkd 1
Local wmes 1' Regional waves Ti& range /stage I I
Turbulence / bed shear velocity
Sediment Suspension 1/ surface
Advection of sediment I Suspended sediment avaaable at the rnarsh margin
Sediment ~eposition
Opportunity for sediment deposition ______)
Height and density
1 Vegetation 1
ZNTERNAL FACTORS
Figure 5.1: Conceptual mode1 of the geomorphic controls infiuencing sediment deposition in the Cumberland Basin, Bay of Fundy. 5.1 Temporal and spatial variation in suspended sediment concentrations (SSC)
Variability in SSC has been recognized to occur within individual tidal cycles and at similar current velocities during different tidal cycles (Reed et al., 1983; Wang et al.,
1993: van Proosdij, 2001). However, some general trends in the spatial and temporal variation of SSC may be recognized. This section provides a brief discussion on the temporal variation of suspended sediment concentration over the scale of an individual tidal cycle (as measured by the OBS), followed by a more in-depth explanation of the rising stage bottle results as they relate to spatial variations in SSC across and between mars hes.
5.1.1 Temporal variation in SSC
During tidal inundations, OBS data indicated that generally higher concentrations of suspended sediment occurred on the early flood and later ebb tide. Since rising stage bottles fi11 on the leading edge of the flood tide, the temporal variations in SSC noted by the OBS indicates that rising stage bottle sarnples of suspended sediment are not representative of SSC values throughout the entire tidal cycle, rather the higher values at the leading edge of the flood tide.
Similarly, in a study of Allen Creek, van Proosdij (2001) found higher values of
SSC during the flood tide. Furthemore, several authors have noted higher values of SSC during the later ebb tide (van Proosdij, 2001; Christiansen et al., 2000). On both the flood and ebb, the higher SSC values are presumably the result of recently deposited sediment becoming re-suspended in shallow waters, by either/or wave activity and tidal currents. 5.1.2 Spatial variations in SSC across each marsh
With respect to spatial variations in SSC over individual tidal cycles at Stations across each marsh, results indicated a statistically significant correlation in suspended sediment across each of Pecks Cove and West AlIen Creek. This relationship suggests that patterns of variation in SSC (over the scale of a tidal cycle) are similar. In contrast,
SSC sarnples were not significantly correlated between Stations across each of Westcock and Tantramar rnarshes.
The Iack of correlation in SSC samples at Stations across Westcock and
Tantramar rnarshes may be due to the morphology of the marshes, including the presence of dykes, which run along the back of these marshes, the cliffed marsh margins, and the substantial tidal creeks which intersect the rnarsh surface. These rnorphological characteristics modie the direction of incorning floodwaters. Where at Pecks Cove and
West Allen Creek rnarshes the flodwaters enter the marsh via the marsh margin, floodwaters at Westcock and Tantrarnar marshes are first directed into the primary tidal creeks, then spi11 into the broad shallow creeks that follow at the base of the dykes. As a result of this latter pattern of flow, the rising stage bottle Iocated at Station 1 for both marshes is filIed first. Once water levels become high enough, floodwaters may then enter the marsh surface via the marsh margin - at which point the rising stage bottle at
Station III is filled at Tantramar marsh. Station IV at Westcock marsh, however, fills prior to the marsh flooding, as it is located at Ieast one metre beIow the marsh surface, on the rnarsh cliff 'step'. In other words, for rising stage bottles at Station 1 to fill, the floodwaters are subjected to various influences, such as transportation through the tidal creeks and extensive transport through the vegetated canopy at the base of the dyke. Stations III and IV at Tantrarnar and Westcock marshes respectively, are filled fiorn waters coming directly fiom the open Basin. These variations in the source of suspended sedirnent samples may contribute to the lack of correlation found across Westcock and
Tantramar marshes.
Moreuver, the correlated samples of SSC across Pecks Cove and West Allen
Creek marshes indicate an increase in SSC as the waters flood the marsh, where samples at Station III are lower than those collected at Station 1. Presumably. unconsolidated sediment deposited on the marsh surface and vegetation during the previous tidal cycle was being re-suspended. Mean current velocities over the marsh surface, including both tidal and wind wave associated currents, were very Iow with the maximum mean flow over tidal cycles generally not exceeding 0.3 ds. van Proosdij (2001) noted similar results at Allen Creek marsh. tt would seem unlikely that these currents velocities would be above the threshold for sediment entrainment, however, when the shallow incorning floodwaters cross the marsh (at a depth below what was measured by the current meter) visually observed velocities appeared to be quite high.
This apparent increase in SSC between Stations agrees with the findings of Pye
(1995) in a study in the Wash, Eastern England, who found SSC on the flood generally increases in a landward direction, and suggested the increase was due to re-suspension of previously deposited material. In contrast, at Blyth estuary, French et al. (2000) found a decrease in the availability of SSC with increased distance from the marsh edge due to progressive particle settling as the water rnass moves across the marsh surface. 5.1.3 Spatial variation in SSC between marsltes
In tcrrns of correlation of SSC variation over tidal cycles between marshes, a
significant relationship was found between al1 sites but Tantramar marsh (Table 4.4).
Where SSC varied considerably between tidal cycles for the marshes dong the Basin,
variation in SSC at Tantrarnar marsh was relatively small. As this marsh is the only one
of the chosen sites not located directly on the Basin, Tantrarnar marsh rnay not be subject
to geornorphic controI(s) that are associated with the open water of the Basin and affect
the other four sites. The most notable difference is exposure to wind wave activity, as the
location of Tantramar marsh allows it to be largely sheltered fiom waves in the Basin. It
is possible therefore, that local wind waves rnay account for the differences in suspended
sediment concentrations behveen Tantramar and the other four sites.
In an examination of SSC concentrations along the Basin, there did not appear to
be a clear increase or decrease in SSC spatially with distance from the mouth of the
Basin. If al1 SSC sarnples were averaged for each site over the duration of the study period. Pecks Cove marsh was the highest, followed by West Allen Creek and then
Westcock marshes. SSC at Tantrarnar rnarsh was marginally higher than Westcock marsh and values at Fort Beauséjour were slightly higher again. However. in an examination along the central mis of the Basin, Amos and Tee (1989) found a consistent seaward decrease in SSC. The apparent lack of a trend in SSC along the marsh margins of the Basin that was observed in this study may be a fünction of sediments being brought into suspension as floodwaters cross the tidal flats. Therefore, based on distance and current speed, at which floodwaters traveI across the tidal flats and local wave activity, differences in SSC become apparent when the waters reach the marsh margin. The role of waves in re-suspension of material over mudflat surfaces has been well-documented
(Reed et al., 1985; Amos and Tee, 1989; Allen and DufTy, 1998), which lends credence to this possibility. However. for the marshes adjacent to the Cumberland Basin, this is speculation.
5.1.4 Variation in SSC as a fùnction of intense rainfall
Recall that in the results, an anomalous data point (Julian day 186) was found to be associated with thunderstorm activity and heavy intermittent rain. Although an investigation into this influence on SSC was beyond the scope of the paper. the result is not unprecedented. For example, in a study by Wolaver et al. (2998), inorganic suspended sediment concentration was exceptionally high following a major rain storm that occurred during the previous low tide. The author suggested that rain impaction probably caused sediment erosion from the marsh surface and creeks (also see Schostak,
1999) and that subsequent surface ninoff from the saltmarsh was the main contributing factor to increased inorganic SSC. Similarly, Evans and Collins (1975) noted that heavy rainfall might produce a slurried surface, which wilI lead to the surface sediment being more easily eroded. There remains, however, the need for a more thorough investigation into the contribution of intense rainfall to SSC.
5.1.5 Variation itz SSC as a fiinction of locnl and regionnl wave heighf
According to Dawson et al. (1999), marsh exposure to wind wave action within the Cumberland Basin is difficult to rneasure directly because it is highly variable over the tidal cycle and it is influenced by differing effects of sheltering and refiaction as the tide rises. For similar reasons prediction of wind waves is equally difficult. Wave prediction in this context does not account for differences as the tide nses and falls, rather it provides a general value of wave height as the flood tide reaches the rnarsh margin.
Results of this study indicated essentially no significant relationship between SSC and predicted locally generated wind waves. However, similar to the results of French et al. (2000) al1 sites demonstrated a significant positive correlation between wind speed and SSC. Correlation between wind speed and SSC suggests that waves are an important factor influencing SSC. Therefore, in a correlation of SSC and predicted regional wave height, al1 sites (with the exception of Fort Beauséjour) exhibited a significant positive correlation. These latter results suggest that waves generated at the mouth of the Basin account for a significant arnount of the variation in SSC throughout the Basin. Wave action at the mouth of the Cumberland Basin presurnably acts to mobilise sediment fiom the bed. mudflats and coastal cliffs, and the resultant suspended sediment is advected to marsh surfaces via tidal currents (as noted in the conceptual model, Figure 5.1). Reed
(1989) has also published similar findings. In a study of coastal saltmarshes near
Tenebome Bay, Louisiana, Reed (1989) found sediment brought into suspension in the bay was the primary source of sediment for marshes located on smaller bayous some distance from the open bay, as a result of tides advecting suspended sediment from the bay to the marshes.
This study recognizes the importance of regional wave activity to suspended sediment concentration, however wind direction and local wave activity should not be discounted. Based on knowledge of this system, it is to be expected that when winds are offshore to Fort Beauséjour (in a south-east direction), Pecks Cove marsh will have a higher suspended sedirnent concentration than the other sites. The concentration of suspended sediment would be a function of higher waves associated with a down estuary wind in combination with sediment suspended by regional waves. Meanwhile, Fort
Beauséjour would experience comparatively low concentrations of suspended sedirnent.
However, when winds are from the north-west suspended sediment concentrations between rnarshes in the upper and lower regions of the Basin are more comparable.
While Pecks Cove and West Allen Creek marshes are influenced by high concentrations of SSC from the mouth of the Basin, Westcock and Fort Beauséjour marshes are influenced by higher local wave heights which act to enhance suspended sediment concentrations in these areas.
5.1.6 Variations in SSC as a frirzction of Mal amplitride
Like waves, tides provide energy by which sediment may be brought into suspension, thereby increasing the availability of sediment in the water colurnn. Results indicated that West Allen Creek, Westcock and Fort Beauséjour marshes demonstrated a moderate, positive correlation between SSC and tidal amplitude. Correlations were based on tide data where wave conditions were < 25 cm, such that tidal amplitude would be more indicative of tidal current velocity.
Similar to these findings, several authors have found concentrations of suspended sediment to increase or decrease, based on the approach of spring tides or neap tides respectively (Reed, 1988; Powell, 1989; Uncles and Stephens, 1989; Schoellhamer, 1996;
Allen and DufSl, 1998; French et al., 2000). In exceptions to this trend, Hutchinson
(1995) found no consistent spring-neap pattern, and Christiansen et al. (2000) found that increased tidal amplitude effectively increased SSC along tidal creek banks, but did not affect concentrations in the marsh interior.
The lack of a consistent correlation between SSC and tidal amplitude for al1 sites along the Cumberland Basin may be in part due to the inabiiity in this study to undertake analysis for completely calm conditions (waves < 5 cm), which if possible may have made a relationship more recognizable. Wave activity, even if below 15 cm is likely to contribute to current velocity. Moreover, due to time constraints this study was unable to observe multiple springheap variations in tidal amplitude.
5.2 Relative significance of each geomorphic control on sediment deposition
Suspended sediment may be deposited as the result of a combination of various geornorphic controls, such as suspended sediment concentrations, low wave heights, low tidal amplitude (associated with low tidal current velocities), and high, dense vegetation
(which buffers currents). The contributions of these various controis to sediment deposition are discussed here.
5.2.1 Srïspended sediment concentration (SSC)
Correlation of SSC and sedirnent deposition for tidal cycles where wave heights were < 15 cm, indicated a moderate positive relationship for Station 1 at West Allen
Creek, Westcock and Tantrarnar marshes. Therefore, at these Iocations, when SSC increased, so too did sediment deposition. At this point it is not clear why there was a lack of correlation between SSC and sediment deposition for the remainder of the sites and stations. Examining this relationship from a slightly different angle, van Proosdij (2001) found a much stronger reiationship, where correlation between the mean decrease in SSC and sediment deposition over an individual tidal cycIe was significant at the 95% confidence level. From this relationship, she suggested that the sediment trap data was representative of the amount of SSC which was lost fiorn the water colurnn.
5.2.2 Wind waves
Wind waves not only provide energy to bring sedirnent into suspension, but depending on the arnount of energy may also inhibit or promote sediment deposition. As referred to in the 'Results' section, work on the Cumberland Basin by van Proosdij (2001) determined that wind waves 15 cm in height act as a threshold, beIow which sediment may be deposited and above which sediment may remain in suspension. In cornparison, results of this study found no significant relationship between predicted IocaI wave height and sediment deposition, either above or below the 15 cm threshold.
The lack of correlation between low wave height and sediment deposition may be due to the general nature of the wave data collected in this study. According to van
Proosdij (2001). the influence of wave activity will Vary across the marsh surface, since the impact of waves are linked both to the height of the wave and depth of the water column through which the wave is propagating. Therefore, a correlation of this nature may have benefited from more specific measurements of wave activity taken at several locations across each marsh. 5.2.3 Tinal amplitude
Similar to currents associated with wind waves, tidal currents, which are reflected in tidal amplitude, may act to inhibit or promote deposition. Results of this study found no significant correlation between tidal amplitude and sediment deposition with the exceptions of Station III at Pecks Cove and Tantramar rnarshes. However, those sites which e.xhïbited a correlation were not of a similar nature. At Pecks Cove marsh, an increase in tidal arnplitude is followed by a decrease in sediment deposition, whereas at
Tantrarnar rnarsh as tidal amplitude increases so too does sediment deposition. The latter relationship is in agreement to the observations of French and Spencer (1993) on tidal height and sedimentation.
The likely explanation as to why sediment deposition decreases at Pecks Cove marsh and increases at Tantrarnar marsh is wave related. At Pecks Cove, higher tides allow for waves to propagate hrther ont0 the marsh (van Proosdij, 2001), inhibiting deposition. At Tantramar marsh, higher tides means more sediment is available for deposition, and because of its sheltered location, local wind waves are not keeping sediment in suspension and therefore deposition may occur. The apparent lack of correlation between tidal amplitude and the remaining sites may be obscured by 1) sprinç-neap deposition and re-suspension cycles; 2) sediment mobilization by local wind waves: 3) high background sediment concentrations offshore (which effectively represent a lagged input fiom more distant rneteorological disturbances) (French and Spencer,
1993). 5.2.4 Vegetation
In this study, a significant positive correlation resulted between vegetation (in the
form of a RR index) and sediment deposition. Although van Proosdij (200 1) noted that
this relationship is more significant during calm wave conditions, the relationship that
was established in this study is strong, irrespective of wave height. In agreement to van
Proosdij (2001) the presence of vegetation acts to dampen wave and tidal currents,
allow-ing deposition to occur even on wavy days. As water depth decreases (or vegetation
height increases), the relative roughness of vegetation increases. The role of vegetation in
buffering current velocities and promoting sediment deposition is well documented in the
Iiterature (Knuston et al., 1982; Pethick et al., 1990; Shi et al., 1995, 1996; Christiansen et al., 2000).
Due to the spatial variability in vegetation across each marsh and between marshes, no apparent trend in sediment deposition as a function of RR can be found along the length of the Basin. This is primarily due the fact that rnorphology, which affects inundation time and depth and which in turn, deterrnines the vegetative characteristics of the marsh surface is interna1 to the marsh system and therefore quite site specific.
5.3 Spatial variation of sediment deposition
Sediment deposition requires the availability of sediment in the water colurnn and opportunity for deposition. Sediment brought into suspension by regional waves and advected by tides to the marsh surface is the primary control affecting sediment availability at marshes along the length of the Basin. The discussion of objective 3 illustrates several of tne geomorphic controls that influence deposition. However, it is important to note that these controls do not work independently of each other, rather it is a combination of several controls that dictate the resultant sediment deposition on marsh surfaces. The multitude of variables affecting sediment deposition. however, both internal and external to the marsh surface has provided an inconsistent picture of sediment deposition around the Basin. There is no apparent trend in sediment deposition dong the
Iength of the Basin as differences between marshes (e-g., morphology and vegetation)
Iead to differences in the internal forcing factors, and therefore some site-specific controls on deposition. The complexity of the system has been noted by several authors
(Reed, 1989; French and Spencer, 1993; French et al., 1995; Brown, 1998; van Proosdij,
200 1). whereby sedimentation cm Vary considerably between tidal cycles (short periods of time) at the same site, and between sites. In this study the variability was apparent both across individual marshes, as well as between marshes of the sarne system.
However, discussion of objective 1 will bring together both sediment availability and opportunity for deposition in an attempt to explain sedimentation patterns observed on the marshes throughout the Cumberland Basin. Each marsh will be discussed, with the exception of Fort Beauséjour, as the Iimited data set makes discussion of results difficult.
5.3. I Pecks Cove marsh
At Pecks Cove marsh, the negative relationship between sediment deposition and tidal amplitude exists due to the fact that as tidal amplitude increases, so too does depth of the water colurnn and tidal ciments. Unlike Tantramar marsh, Pecks Cove is not cornpletely sheltered fiom wave action. Therefore, as water levels increase either within an individual tide fiom flood to high tide or as amplitude increase from neap to spring, the depth of the water column over the marsh surface increases allowing waves to propagate fûrther inshore. Therefore, current velocities associated with wind waves may compound the effect of the tidal currents present. keeping sediment in suspension.
Moreovsr, the low RR index of this marsh provides little resistance to the wind wave and tidaI currents. However, by the time floodwaters reach Station 1 some of the wave and tidal current energy rnay have dissipated through interaction with the vegetative canopy.
In addition to the decreased current energy, the height of the water column above the surface is locver here and vegetation is taller, increasing the RR index, thus ailowing for a greater amount of sediment deposition to occur.
5.3.2 West Allen Creek marslr
In terms of sediment availability, West Allen Creek shows a positive relationship between SSC and tidal amplitude, and both local and regional wave heights. These relationships indicate that both tides and waves contribute to sediment availability, which may potentially be deposited over the rnarsh surface.
With respect to opportunity for deposition, results indicated a moderate relationship between SSC availabie in the water column and corresponding deposition over the scale of a tidal cycle. However, there is no significant relationship between sediment deposition and either tidd amplitude or predicted wave heights. This indicates that although both wave and tidal currents are responsible for supplying sediment to the overlying water column, a reduction in current velocity does not necessarily cause an increase in sediment deposition. The arnount of sediment deposited at West Allen Creek marsh is very similar to
Pecks Cove marsh. Across the marsh sediment deposition is highest at Station 1, yet correlates with the lower, comparable values of Stations II and III. The higher concentrations of sediment deposited in the upper reaches of the marsh are probably for the sarne reasons mentioned for Pecks Cove marsh. The effectiveness of vegetation to baffle wind wave and tidal currents increases as water floods across the marsh surface, with the height of the water colurnn relative to the vegetation decreasing landward and thus increasing the RE2 index.
5.3.3 Westcock rnarslr
AvailabiIity of suspended sediment is a function of both tidal amplitude and regional waves. Although there was a positive correlation between SSC and sediment deposition for Station 1, similar to West Allen Creek, there was no relationship between sediment deposition and either tidal amplitude or predicted local wave height. This perhaps suggests that, like West Allen Creek marsh, suspended sediment is being deposited, irrespective of wave and tidal currents at the site.
Results found no correlation of sediment deposition across the marsh surface.
However, recall that the nsing stage bottle sarnples were not correlated across this marsh surface either. As discussed earlier, Westcock marsh does not flood in the sarne manner as Pecks Cove and West Allen Creek. At Westcock marsh the incoming flood tide is routed through the tidal creeks and spills into a wide vegetated depression that runs dong the base of the dyke. It is by this route that the sediment trap at Station 1 is inundated during the majority of the flood and ebb tide. When water levels are sufficiently high, water will breach the marsh margin and inundate Station III. Although not tested, it is probable that these two locations were affected by quite different wind wave and tidal currents. which may account for the difference in sediment deposition.
5.3.4 Tmrtramar marsIr
As with each site along the length of the Basin, sediment deposition at Tantrarnar marsh is the result of a complex relationship of several geomorphic controls. As tidal range increases, Tantramar marsh in inundated by a greater volume of sediment rich water with the potential for deposition. As this site is sheltered fiom waves, the primary currents are tidal. The RR index, calculated for high spring tide at this location (as it is the only time that this marsh was inundated) is significantly higher than for any other location, indicating that vegetation on the marsh is highly effective in acting as a roughness element, slowing current velocities. These currents are very slow within the canopy, therefore providing conditions conducive to sediment deposition. In other words. at Tantramar marsh the higher the tidal range, the more suspended sedirnent brought into the marsh, whereby significant arnounts are deposited as a function of little to no waves and high RR which acts to buffer the tidal currents.
5.4 Conclusions
Sediment deposition throughout the Curnberlmd Basin is spatially variable.
External factors, specifically regional waves and tidal amplitude are the key factors influencing sediment availability spatially along the Basin. While the majority of the marshes are influenced by these sources of sediment, a complex relationship of intemal factors influence sediment deposition making the actual rates of deposition site specific.
The role the intemal factors, specifically marsh morphology, in influencing flow patterns and depth of the water colurnn across the marshes, as well as vegetation characteristics are of notable importance in providing the opportunity for sedirnent deposition.
Across individual marshes, morphology played a role in creating spatial differences in SSC and sediment deposition. Flow dynarnics associated with dyked and cliffed marshes (Westcock, Tantrarnar and Fort Beauséjour marshes) are considerably different than at 'natural marshes' (Pecks Cove and West Allen Creek marshes). In the former circumstance floodwaters are initially concentrated in the tidal creeks, and subsequently routed in the shallow depressions that follow the base of the dykes. In the latter circurnstance, floodwaters broach the marsh surface via the marsh margin. The significant variation in flow path of floodwaters as the rnarsh surface is inundated was found to contribute to interna1 variations in sedimentation.
Between marshes, rnorphological differences allowed for duration and depth of inundation to vary considerably. Pecks Cove and West Allen Creek sit Iow in the tidal frame and are flooded with every tide by a substantial depth of water, while the remainder of the sites are only flooded during high spnng tides by a comparatively shallow depth of water. Differences in flooding fiequency aliows for differences in vegetation, where Pecks Cove and West AILen Creek marshes have prirnady short
S'nrtinn alterniflora and the other marshes typically have high marsh vegetation which is both taller and denser (especially at Tantramar marsh). The hiyh marsh vegetation in combination with comparatively shallow flood tides produces a high RR index, contributing to favourable conditions for sediment deposition across the marsh surface. However, at Pecks Cove and West Allen Creek, deposition at Stations 1 rernained comparatively high as wave and tidaI current energy was dispersed across the marsh surface and the RR index becarne higher in the upper regions of the marsh.
In the Cumberland Basin, external factors are the key geomorphic control affecting sediment availability throughout the Basin. in contrast, variable sediment deposition rates at the scale of a tidal cycle, is more a function of interna1 geomorphic controls. No one geomorphic control can be determined as dominant, as sediment deposition in the Cumberland Basin is the result of complex interrelationships between various controls. REFERENCES
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Pecks Cove marsh Mouth of Basin Dearee 1 Kilometres Denree 1 Kilometres Dearee 1 Kilometres 30 0.00 35 0.50 40 1-25 45 7.50 50 1.63 55 8.85 60 17.73 65 9.60 70 8.75 75 10.00 80 8.25 85 5.55 90 6.38 95 4-50 1O0 4.05 105 3.95 110 3.80 115 3.60 120 3.45 125 3.05 130 3.38 135 2.70 140 5.28 145 2.65 150 6.38 155 3.00 160 6.48 165 5.75 170 8.50 175 6.20 180 1.70 185 6.40 190 1 .O0 195 7.05 200 11.50 205 10.00 215 1.10
Westcock marsh Fort Beauseiour marsh Deqree 1 Kilometres Dearee 1 Kilometres 74 1-60 125 0.00 84 1.65 135 0.00 94 2.25 145 3.90 104 3.10 155 5.75 114 4.45 165 7.05 124 6.15 175 3.65 134 7.25 185 3.50 144 5.15 195 3.55 154 4.45 205 4.20 164 4.45 215 6.35 174 4.80 225 18.00 184 5.50 235 10.85 194 6.80 245 7.1 5 204 8.05 255 5.00 214 16.00 265 4.40 224 2.90 275 3.75 234 2.00 285 2.60 244 1.10 295 2.30 254 0.00