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Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada
Continental ShellResearc71.Vol.7. Nos. 11/12.pp. 1295-13(10,1987.
I)278-4343/87 $3J~) + 0.00 © 1987PergamonJournalsLtd.
Printcd in Grcat Britain.
Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada CARL L.
AMOS*
(Received 5 September 1986; in revised form 12 February 1987; accepted 1 April 1987) Abstract--This paper describes measurements of suspended sediment fluxes at a total of 32 stations situated on four reference sections in the turbid estuary of Chignecto Bay, Bay of Fundy, Canada. The purpose of the study was to determine the sediment budget (sources, transport paths and sinks) and the seasonal variations in particulate fluxes. The major sources of sediment are the eroding cliffs surrounding the bay (1,0 x 106 m 3 y-l) and the seabed (6 × 106 m 3 y ~). There are no present-day sinks within the estuary; sediment is principally moved in suspension to the wider part of the Bay of Fundy. Residuals in sediment mass transport are strongly affected by storms. These disrupt the logarithmic longitudinal sediment concentration profile which is normally present, and cause sediment to be transported out of the estuary. Well-defined turbid ribbons occur which meander unpredictably through the sampling sites; estimates of sediment mass transport are thus dubious.
INTRODUCTION
CHIGNECTO BAY is a turbid estuary at the head of the Bay of Fundy (Fig. 1). The sedimentology of Chignecto Bay was virtually unknown until a multi-disciplinary study of the bay began in 1978 (GORDON and DADSWELL, 1984). Details of the methods used in the sedimentological part of this study are given by AMOS and ASPREY (1981). The purpose of this work was to examine the budget of sediments (sources, sinks and transport paths) and to document the distribution patterns of suspended material in Chignecto Bay. Chignecto Bay is a "mUddy" estuary. This is due to the nature of the sediment sources, eroded cliffs which surround the bay are composed of Paleozoic mudstones and sandstones (PLINT, 1986). The seabed, which is also suffering erosion, is composed of laminated silts and clays which add to the supply of sediment in suspension (AMOS and ZAITLIN, 1985). Sediment transport pathways are complex. Net transport of both bed and suspended material is headward along the bay margins and seaward through the centre (AMos and ASPREY, 1981). Nepheloid (fluid mud) layers, which typify and control the Bristol Channel sediment distribution, have not been found despite acoustic and optical surveys of the near-bed water column. Sand occurs in subtidal bay margins and moves headward
* Geological Survey of Canada, P.O. Box 1006, Dartmouth NS, Canada B2Y 4A2. 1295
1296
C.L. AMos
j~
/')
I
L,rxLE
• OOTE=C.,GNEC~O
SHEPODY BAY / S I
~#
INNER CHIGNECT0
IlOOC
,5}, e
~ /" "CUIdBERLANI)~'I
,,~
/-i
/ SEAWATER ANALYSIS
ee
MAY - SEP 1978
//J"
O SINGLE CUIq'RENTMETER M ~ I ~ I I WATER ~NITO~ING STATION(iB M,~UR) I WAVERIDERBUOY
• TIOE GAUGE
7
{ ~------'': I7"-- .~y.OO''~
""
J..... da*
Fig. 1. Chignecto Bay and sub-basins: Shepody Bay and Cumberland Basin. Monitoring stations were occupied along the four sections illustrated in the figure. A conceptual sub-division (inner and outer) of the system into zones distinctive in sedimentary character is also shown.
in the form of sand waves, megaripples and sand ribbons. Gravel is found dispersed throughout the upper part of the bay a n d is distributed by ice-rafting during winter months (GORDON and DESPLANOUE, 1983):' The system is transgressive and sea-level rise in the study region is approximately 0.15 m y-1 (GRANT, 1970). The resulting change in bay geometry has led to an increase in tidal range over the last 6000 years (ScoTr and GREENBERG, 1983) with an associated dramatic change in sedimentation patterns. Present-day surface tidal currents are in excess of 1.7 m s-I during spring tides and maximize during mid-stages of the flood and ebb. Notwithstanding these strong tidal flows, the bay is wave influenced. Deep water waves, which are generated principally in winter storms, propagate to the head of Chignecto Bay. Largest observed waves reach 3 m significant wave height and 10 s period. SEDIMENT B U D G E T
The major sources of sediment in Chignecto Bay are from wave erosion of the cliffs (in the outer part of the bay) and current scouring of the seabed. The mean cliff recession rate is high: 0.37 m y-1. So too is the sediment input from this source: 1.03 × 106 m 3 y-i (HILDEBRAND et al., 1980). Input is greatest in the spring and autumn seasons when waves are largest. Added to this is the erosion of the seabed off Cape Enrage, which releases 6.0 x 106 m3 of fine material annually into the water column. Fluvial input of sediments is low. The catchment area around the bay is 5000 km 2 and freshwater
Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada
1297
discharge from any river is no greater than 28 m 3 s-1. The associated sediment concentrations are <50 mg 1-1, so that fluvial input is only 0.3 × 106 m 3 y-1. Input is greatest during spring freshets when both discharge and sediment concentration are highest. Chignecto Bay is a net exporter of sediment. A volume of'5.5 × 10¢' m3 y-~ is transported in suspension to the Bay of Fundy during fair-weather conditions. A further volume of 1.8 × 106 m 3 y-1 is discharged during storms. There are no major sinks of fine-grained sediment in Chignecto Bay. Intertidal mudflats are ephemeral; most salt marshes have been reclaimed, and remaining surfaces are lagged by coarse material (by the process of erosion) or poorly developed sand flats. Areas where flow is artifically reduced, such as near causeways and reclamation dykes, are infilled with sediment within 1-2 years (BRAY et al., 1982). Remaining sediment is kept in suspension until it can escape to quiescent, deeper parts of the Bay of Fundy, where fine-grained sediments are presently accumulating (FADER et al., 1977). METHODS
Suspended sediment flux was determined at a total of 32 stations across four reference profiles of Chignecto Bay (Fig. 1). Stations were occupied for periods from 9 to 40 h between June 1978 and June 1980. Three reference stations were re-occupied to evaluate seasonal effects due to waves on sediment flux. Samples were taken hourly from 5 depths using 5 i Niskin (3 depths) and 1.2 1 Knudsen (2 depths) sampling bottles. They were then split, filtered and analysed gravimetrically for sediment concentration. Temperature, salinity, current speed and direction, and attenuance were also measured hourly against depth. Wave data were abstracted from two Datawell waverider buoys in the bay. Currents were monitored for long-term variations with Aanderaa RCM-4 meters deployed on 12 moorings for periods in excess of 30 days. AMOS and ASPREY (1981) presented methods of calculating sediment flux from these data; they also tabulated the observations and results and discussed sources of errors. Areal patterns of suspended sediment concentration were determined from calibrated transformations of the Coastal Zone Colour Scanner aboard NIMBUS-7 (AMos and TOPLlSS, 1985) and from the Landsat Multispectral Scanner (MUNDAY et al., 1979). The scanners, which image the entire bay almost instantaneously, have spatial resolutions of approximately 1000 and 60 m, respectively. The images were thus useful in discriminating differing scales of patterns in surface sediment concentrations. R E S U L T S AND D I S C U S S I O N
Subsamples were analysed and compared to determine the reproducibility of the method of analysis. The mean variation over the range of concentrations was _+16%. Concentrations derived from sub-samples were compared with those derived from analysis of duplicated whole samples. Below 40 mg 1-1 no error was apparent; between 40 and 300 mg 1-1 split samples indicated a lower concentration by a mean of 20%; above 300 mg I ~split samples were 50% lower. This indicated that, despite bottle agitation and rapid decantation, settling within the 5 1Niskin bottles took place. Flux calculations were based only on samples free from settling errors. Suspended particulate matter in upper Chignecto Bay is composed largely of silt (7090%) with lesser amounts of clay (10-20%) and sand. Microscopic examination showed
1298
C.L. AMOS
that less than 5% of particles were flocculated and the predominating grain diameter was between 10 and 20 tam. Particles were predominantly siliceous and particulate organic carbon was low (KElzER et al., 1984). Thus, only the flux of total suspended particulate matter is reported.
Turbid fronts, ribbons, plumes and boils The across- and down-bay gradients in sediment concentration are complex and irregular. To describe these patterns a series of definitions is given. A turbid front is the interface separating water masses exhibiting differing sediment concentration gradients. They are usually found during the ebb stage of the tide where water masses from adjoining basins meet. The water masses of Cumberland Basin and Shepody Bay are separated by a turbid front visible in central Chignecto Bay during low water. Such fronts are well-defined and can be linear or sinuous. A turbid front was monitored as it passed through a sampling site (12) at the mouth of Chignecto Bay (Fig. 1). Attenuance at 4 m below the sea surface increased significantly within a 30 s period (between sampling) as the front passed the site. The boundary of the front was vertical (within the limits of detection from a stationary profile) and extended 20 m below the sea surface. It was underlain by less turbid water, although the water column was homogeneous with respect to temperature and salinity. A turbid ribbon is a region of constant sediment concentration which is more turbid than the surrounding waters, and which exhibits a length-to-width ratio greater than 10. The major axis of the ribbon is parallel to the dominant flow. Turbid ribbons in Chignecto Bay are up to 1 km wide, 15 km long and have well-defined lateral boundaries. These are predominantly found in the more turbid (>50 mg l-l), narrower parts of the bay (Cumberland Basin). They appear to be the result of low lateral shearing and eddy diffusion (in relation to longitudinal transport) coupled with locally high resuspension rates resulting from changes in channel geometry and bed character. They are diagnostic of bed erosion by tidal currents. A turbid plume is a region within a water mass which is characterized by elevated sediment concentrations and which exhibits significant lateral diffusion. A turbid plume was observed during the ebb stage of the tide off Cape Enrage (Fig. 1). The plume is formed by turbulent upwelling from localized basins created by seabed erosion. Nearbed turbid water is injected into less turbid surface waters whereupon it disperses both longitudinally and laterally. The point of upwelling is fixed in position through time. Small-scale (spatial lengths <10 m) fluctuations in turbidity have also been detected in Chignecto Bay using near-surface attenuance meters recording hourly in 30 s bursts at 1 Hz. Fluctuations in turbidity were variable and often large. The high fluctuations corresponded with observations of turbid boils (near-circular regions of relatively high turbulent flow characterized by a higher concentration of suspended sediments than the surrounding water). Boils are approximately 1-5 m in diameter and are advected at the speed of the local flow. They are seen throughout the bay, but only during conditions of high wave activity, when they are associated with deceleration of the flood and ebb stages of the tide. Suspended sediment concentration and transport Near-synoptic surface measurements of sediment concentration (KE~zER et al., 1984) show an exponential decrease down-estuary from a maximum of 6 g 1-1 at the head.
Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada
1299
Spatially averaged concentrations derived from Landsat data show similar gradients (AMos and ASPREV, 1981). The value of the exponent-defining sediment gradient remains remarkably constant down-estuary and through time within the inner and outer estuarine water masses. Changes of slope and offset in sediment concentration are related to estuary geometry, and typify turbid fronts. The gradient is disrupted during storms, indicating that it is in balance with fair-weather conditions of tidal flow and sediment sources and sinks. Sediment concentration at any station was greatest near the bed during low water and least near the surface during high water. Particle settling was significant at current speeds <0.5 m s-l; at these times sediment concentration decreased exponentially with height above the bed. Above speeds of 0.8 m s-I particulate matter moved upward through the water column; between 0.5 and 0.8 m s-1 concentration with depth remained steady. The measured net transport of sediment during either the flood or ebb periods varied by up to 4.1 x 107 g m-1. The residual mass transport over a tidal cycle was between 11 and 80% of the net (flood or ebb) transport. Residual transport values in Inner Chignecto Bay were seaward, with rates per tide varying between 1.3 and 3.3 x 107 g m-~; residual transport values per tide in the Outer Bay were up to 0.5 × 107 g m-1. The cycle-to-cycle coefficient of variance of residual sediment transport was _+41% (n = 3) and the seasonal was +50% (n = 6). High variances were the result of wave activity during storms. Due to the method of sampling (from low water to successive !ow water) storm resuspension resulted in a bias towards ebb dominance. This is a consequence of the asymmetric response in sediment resuspension. Resuspended bottom sediment was observed at the surface within 1-6 h after the critical significant wave height was reached, whereas subsequent sedimentation during fair-weather conditions took between 36 h (20 m of water) and 72 h (50 m of water). The critical significant wave height for bed erosion was 0.5 m in Inner Chignecto Bay and 0.7 m in the Outer Bay. Thus, half the stations occupied were influenced by the effects of storm wave resuspension. Many were occupied after the passage of a storm, in which case a difference in mean concentrations between flood and subsequent ebb was to be expected due to settling of the storm-derived sediment component. Data at the remainder of the storm-influenced stations were affected by waves reaching threshold conditions during the periods of observations. In either case, short-term estimates of residuals from these stations are inappropriate for long-term interpretations.
Acknowledgements--I wish to thank Drs D. C. Gordon Jr and G. Daborn for support and guidance in this study. My thanks also go to K. W. Asprey, P. Keiser, R. B. Murphy and the crew of CSS Dawson who contributed to field mobilization and operations. The manuscript was reviewed by Drs D. Greenberg and B. J. Topliss prior to submission; I thank them for their valuable input. This publication is Geological Survey of Canada contribution no. 30086.
REFERENCES AMOS C. L. and K. W. ASPREY (1981) An interpretation of oceanographic and sediment data from the upper Bay of Fundy. Bedford Institute of Oceanography Report Series BI-R-81-15, 143 pp. AMOS C. L. and B. J. TOPLISS (1985) Discrimination of suspended particulate matter in the Bay of Fundy using the NIMBUS-7 coastal zone cotour scanner. Canadian Journal of Remote Sensing, II, 85-92. AMOS C. L. and B. A. ZAITLIN(1985) The effect of changes in tidal range on a sublittoral macrotidal sequence, Bay of Fundy, Canada. Geo-Marine Letters, 4, 161-169.
1300
C . L . AMOS
BRAY D. I., D. P. DEMERCHANT and D. L. SULLIVAN (1982) Some hydrotechnical problems related to the construction of a causeway in the estuary of the Petitcodiac River, New Brunswick. Canadian Journal of Civil Engineering, 9, 296-307. FADER G. B., L. H. KING and B. MACLEAN (1977) Surficial geology of the eastern Gulf of Maine and Bay of Fundy. Geological Survey of Canada Paper 76-17, 23 pp. GORDON D. C. and M. J. DADSWELL (1984) Update on the marine environmental consequences of tidal power development in the upper reaches of the Bay of Fundy. Canadian Technical Report of Fisheries and Aquatic Sciences 1256, 686 pp. GORDON D. C. and C. DESPLANQUE (1983) Dynamics and environmental effects of ice in the Cumberland Basin of the Bay of Fundy. Canadian Journal of Fisheries and Aquatic Sciences, 40, 1331-1342. GRANT D. R. (1970) Recent coastal submergence of the maritime provinces, Canada. Canadian Journal of Earth Sciences, 7,676-689. HILDEBRAND L. P., E. B. MACDORMAND, D. R. NELSON, C. G. POWELL, N. A. RODGERS and J. A. WALKER (1980) Activities of the Job Corps Program; Fundy tidal power development. Available from Bedford Institute of Oceanography library. National Research Council of Canada Project No. 16-01-002N, 176 pp. KEIZER P. D., J. C. GORDON and E. R. HAYES (1984) A brief overview of recent chemical research in the Bay of Fundy. In: Update on the marine environmental consequences of tidal power development in the upper reaches of the Bay ofFundy, D. C. GORDON and M. J. DADSWELL, editors, Canadian Technical Report of Fisheries and Aquatic Sciences 1256, pp. 45-63. MUNDAY J. C., T. T. ALFOLDI and C. L. AMOS (1979) Bay of Fundy verification o1 a system for multidate Landsat measurement of suspended sediment. In: Satellite hydrology, M. DEUTSCH, D. R. WIESNET and A. RANGO, editors, Publ. American Water Resources Association, pp. 622-640. PLINT A. G. (1986) Slump blocks, intraformational conglomerates and associated erosional structures in Pennsylvanian fluvial strata of eastern Canada. Sedimentology, 33,387-399. ScoJ-r D. B. and D. A. GREENBERG (1983) Relative sea-level rise and tidal development in the Fundy tidal system. Canadian Journal of Earth Sciences, 20, 1554-1564.
I)278-4343/87 $3J~) + 0.00 © 1987PergamonJournalsLtd.
Printcd in Grcat Britain.
Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada CARL L.
AMOS*
(Received 5 September 1986; in revised form 12 February 1987; accepted 1 April 1987) Abstract--This paper describes measurements of suspended sediment fluxes at a total of 32 stations situated on four reference sections in the turbid estuary of Chignecto Bay, Bay of Fundy, Canada. The purpose of the study was to determine the sediment budget (sources, transport paths and sinks) and the seasonal variations in particulate fluxes. The major sources of sediment are the eroding cliffs surrounding the bay (1,0 x 106 m 3 y-l) and the seabed (6 × 106 m 3 y ~). There are no present-day sinks within the estuary; sediment is principally moved in suspension to the wider part of the Bay of Fundy. Residuals in sediment mass transport are strongly affected by storms. These disrupt the logarithmic longitudinal sediment concentration profile which is normally present, and cause sediment to be transported out of the estuary. Well-defined turbid ribbons occur which meander unpredictably through the sampling sites; estimates of sediment mass transport are thus dubious.
INTRODUCTION
CHIGNECTO BAY is a turbid estuary at the head of the Bay of Fundy (Fig. 1). The sedimentology of Chignecto Bay was virtually unknown until a multi-disciplinary study of the bay began in 1978 (GORDON and DADSWELL, 1984). Details of the methods used in the sedimentological part of this study are given by AMOS and ASPREY (1981). The purpose of this work was to examine the budget of sediments (sources, sinks and transport paths) and to document the distribution patterns of suspended material in Chignecto Bay. Chignecto Bay is a "mUddy" estuary. This is due to the nature of the sediment sources, eroded cliffs which surround the bay are composed of Paleozoic mudstones and sandstones (PLINT, 1986). The seabed, which is also suffering erosion, is composed of laminated silts and clays which add to the supply of sediment in suspension (AMOS and ZAITLIN, 1985). Sediment transport pathways are complex. Net transport of both bed and suspended material is headward along the bay margins and seaward through the centre (AMos and ASPREY, 1981). Nepheloid (fluid mud) layers, which typify and control the Bristol Channel sediment distribution, have not been found despite acoustic and optical surveys of the near-bed water column. Sand occurs in subtidal bay margins and moves headward
* Geological Survey of Canada, P.O. Box 1006, Dartmouth NS, Canada B2Y 4A2. 1295
1296
C.L. AMos
j~
/')
I
L,rxLE
• OOTE=C.,GNEC~O
SHEPODY BAY / S I
~#
INNER CHIGNECT0
IlOOC
,5}, e
~ /" "CUIdBERLANI)~'I
,,~
/-i
/ SEAWATER ANALYSIS
ee
MAY - SEP 1978
//J"
O SINGLE CUIq'RENTMETER M ~ I ~ I I WATER ~NITO~ING STATION(iB M,~UR) I WAVERIDERBUOY
• TIOE GAUGE
7
{ ~------'': I7"-- .~y.OO''~
""
J..... da*
Fig. 1. Chignecto Bay and sub-basins: Shepody Bay and Cumberland Basin. Monitoring stations were occupied along the four sections illustrated in the figure. A conceptual sub-division (inner and outer) of the system into zones distinctive in sedimentary character is also shown.
in the form of sand waves, megaripples and sand ribbons. Gravel is found dispersed throughout the upper part of the bay a n d is distributed by ice-rafting during winter months (GORDON and DESPLANOUE, 1983):' The system is transgressive and sea-level rise in the study region is approximately 0.15 m y-1 (GRANT, 1970). The resulting change in bay geometry has led to an increase in tidal range over the last 6000 years (ScoTr and GREENBERG, 1983) with an associated dramatic change in sedimentation patterns. Present-day surface tidal currents are in excess of 1.7 m s-I during spring tides and maximize during mid-stages of the flood and ebb. Notwithstanding these strong tidal flows, the bay is wave influenced. Deep water waves, which are generated principally in winter storms, propagate to the head of Chignecto Bay. Largest observed waves reach 3 m significant wave height and 10 s period. SEDIMENT B U D G E T
The major sources of sediment in Chignecto Bay are from wave erosion of the cliffs (in the outer part of the bay) and current scouring of the seabed. The mean cliff recession rate is high: 0.37 m y-1. So too is the sediment input from this source: 1.03 × 106 m 3 y-i (HILDEBRAND et al., 1980). Input is greatest in the spring and autumn seasons when waves are largest. Added to this is the erosion of the seabed off Cape Enrage, which releases 6.0 x 106 m3 of fine material annually into the water column. Fluvial input of sediments is low. The catchment area around the bay is 5000 km 2 and freshwater
Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada
1297
discharge from any river is no greater than 28 m 3 s-1. The associated sediment concentrations are <50 mg 1-1, so that fluvial input is only 0.3 × 106 m 3 y-1. Input is greatest during spring freshets when both discharge and sediment concentration are highest. Chignecto Bay is a net exporter of sediment. A volume of'5.5 × 10¢' m3 y-~ is transported in suspension to the Bay of Fundy during fair-weather conditions. A further volume of 1.8 × 106 m 3 y-1 is discharged during storms. There are no major sinks of fine-grained sediment in Chignecto Bay. Intertidal mudflats are ephemeral; most salt marshes have been reclaimed, and remaining surfaces are lagged by coarse material (by the process of erosion) or poorly developed sand flats. Areas where flow is artifically reduced, such as near causeways and reclamation dykes, are infilled with sediment within 1-2 years (BRAY et al., 1982). Remaining sediment is kept in suspension until it can escape to quiescent, deeper parts of the Bay of Fundy, where fine-grained sediments are presently accumulating (FADER et al., 1977). METHODS
Suspended sediment flux was determined at a total of 32 stations across four reference profiles of Chignecto Bay (Fig. 1). Stations were occupied for periods from 9 to 40 h between June 1978 and June 1980. Three reference stations were re-occupied to evaluate seasonal effects due to waves on sediment flux. Samples were taken hourly from 5 depths using 5 i Niskin (3 depths) and 1.2 1 Knudsen (2 depths) sampling bottles. They were then split, filtered and analysed gravimetrically for sediment concentration. Temperature, salinity, current speed and direction, and attenuance were also measured hourly against depth. Wave data were abstracted from two Datawell waverider buoys in the bay. Currents were monitored for long-term variations with Aanderaa RCM-4 meters deployed on 12 moorings for periods in excess of 30 days. AMOS and ASPREY (1981) presented methods of calculating sediment flux from these data; they also tabulated the observations and results and discussed sources of errors. Areal patterns of suspended sediment concentration were determined from calibrated transformations of the Coastal Zone Colour Scanner aboard NIMBUS-7 (AMos and TOPLlSS, 1985) and from the Landsat Multispectral Scanner (MUNDAY et al., 1979). The scanners, which image the entire bay almost instantaneously, have spatial resolutions of approximately 1000 and 60 m, respectively. The images were thus useful in discriminating differing scales of patterns in surface sediment concentrations. R E S U L T S AND D I S C U S S I O N
Subsamples were analysed and compared to determine the reproducibility of the method of analysis. The mean variation over the range of concentrations was _+16%. Concentrations derived from sub-samples were compared with those derived from analysis of duplicated whole samples. Below 40 mg 1-1 no error was apparent; between 40 and 300 mg 1-1 split samples indicated a lower concentration by a mean of 20%; above 300 mg I ~split samples were 50% lower. This indicated that, despite bottle agitation and rapid decantation, settling within the 5 1Niskin bottles took place. Flux calculations were based only on samples free from settling errors. Suspended particulate matter in upper Chignecto Bay is composed largely of silt (7090%) with lesser amounts of clay (10-20%) and sand. Microscopic examination showed
1298
C.L. AMOS
that less than 5% of particles were flocculated and the predominating grain diameter was between 10 and 20 tam. Particles were predominantly siliceous and particulate organic carbon was low (KElzER et al., 1984). Thus, only the flux of total suspended particulate matter is reported.
Turbid fronts, ribbons, plumes and boils The across- and down-bay gradients in sediment concentration are complex and irregular. To describe these patterns a series of definitions is given. A turbid front is the interface separating water masses exhibiting differing sediment concentration gradients. They are usually found during the ebb stage of the tide where water masses from adjoining basins meet. The water masses of Cumberland Basin and Shepody Bay are separated by a turbid front visible in central Chignecto Bay during low water. Such fronts are well-defined and can be linear or sinuous. A turbid front was monitored as it passed through a sampling site (12) at the mouth of Chignecto Bay (Fig. 1). Attenuance at 4 m below the sea surface increased significantly within a 30 s period (between sampling) as the front passed the site. The boundary of the front was vertical (within the limits of detection from a stationary profile) and extended 20 m below the sea surface. It was underlain by less turbid water, although the water column was homogeneous with respect to temperature and salinity. A turbid ribbon is a region of constant sediment concentration which is more turbid than the surrounding waters, and which exhibits a length-to-width ratio greater than 10. The major axis of the ribbon is parallel to the dominant flow. Turbid ribbons in Chignecto Bay are up to 1 km wide, 15 km long and have well-defined lateral boundaries. These are predominantly found in the more turbid (>50 mg l-l), narrower parts of the bay (Cumberland Basin). They appear to be the result of low lateral shearing and eddy diffusion (in relation to longitudinal transport) coupled with locally high resuspension rates resulting from changes in channel geometry and bed character. They are diagnostic of bed erosion by tidal currents. A turbid plume is a region within a water mass which is characterized by elevated sediment concentrations and which exhibits significant lateral diffusion. A turbid plume was observed during the ebb stage of the tide off Cape Enrage (Fig. 1). The plume is formed by turbulent upwelling from localized basins created by seabed erosion. Nearbed turbid water is injected into less turbid surface waters whereupon it disperses both longitudinally and laterally. The point of upwelling is fixed in position through time. Small-scale (spatial lengths <10 m) fluctuations in turbidity have also been detected in Chignecto Bay using near-surface attenuance meters recording hourly in 30 s bursts at 1 Hz. Fluctuations in turbidity were variable and often large. The high fluctuations corresponded with observations of turbid boils (near-circular regions of relatively high turbulent flow characterized by a higher concentration of suspended sediments than the surrounding water). Boils are approximately 1-5 m in diameter and are advected at the speed of the local flow. They are seen throughout the bay, but only during conditions of high wave activity, when they are associated with deceleration of the flood and ebb stages of the tide. Suspended sediment concentration and transport Near-synoptic surface measurements of sediment concentration (KE~zER et al., 1984) show an exponential decrease down-estuary from a maximum of 6 g 1-1 at the head.
Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada
1299
Spatially averaged concentrations derived from Landsat data show similar gradients (AMos and ASPREV, 1981). The value of the exponent-defining sediment gradient remains remarkably constant down-estuary and through time within the inner and outer estuarine water masses. Changes of slope and offset in sediment concentration are related to estuary geometry, and typify turbid fronts. The gradient is disrupted during storms, indicating that it is in balance with fair-weather conditions of tidal flow and sediment sources and sinks. Sediment concentration at any station was greatest near the bed during low water and least near the surface during high water. Particle settling was significant at current speeds <0.5 m s-l; at these times sediment concentration decreased exponentially with height above the bed. Above speeds of 0.8 m s-I particulate matter moved upward through the water column; between 0.5 and 0.8 m s-1 concentration with depth remained steady. The measured net transport of sediment during either the flood or ebb periods varied by up to 4.1 x 107 g m-1. The residual mass transport over a tidal cycle was between 11 and 80% of the net (flood or ebb) transport. Residual transport values in Inner Chignecto Bay were seaward, with rates per tide varying between 1.3 and 3.3 x 107 g m-~; residual transport values per tide in the Outer Bay were up to 0.5 × 107 g m-1. The cycle-to-cycle coefficient of variance of residual sediment transport was _+41% (n = 3) and the seasonal was +50% (n = 6). High variances were the result of wave activity during storms. Due to the method of sampling (from low water to successive !ow water) storm resuspension resulted in a bias towards ebb dominance. This is a consequence of the asymmetric response in sediment resuspension. Resuspended bottom sediment was observed at the surface within 1-6 h after the critical significant wave height was reached, whereas subsequent sedimentation during fair-weather conditions took between 36 h (20 m of water) and 72 h (50 m of water). The critical significant wave height for bed erosion was 0.5 m in Inner Chignecto Bay and 0.7 m in the Outer Bay. Thus, half the stations occupied were influenced by the effects of storm wave resuspension. Many were occupied after the passage of a storm, in which case a difference in mean concentrations between flood and subsequent ebb was to be expected due to settling of the storm-derived sediment component. Data at the remainder of the storm-influenced stations were affected by waves reaching threshold conditions during the periods of observations. In either case, short-term estimates of residuals from these stations are inappropriate for long-term interpretations.
Acknowledgements--I wish to thank Drs D. C. Gordon Jr and G. Daborn for support and guidance in this study. My thanks also go to K. W. Asprey, P. Keiser, R. B. Murphy and the crew of CSS Dawson who contributed to field mobilization and operations. The manuscript was reviewed by Drs D. Greenberg and B. J. Topliss prior to submission; I thank them for their valuable input. This publication is Geological Survey of Canada contribution no. 30086.
REFERENCES AMOS C. L. and K. W. ASPREY (1981) An interpretation of oceanographic and sediment data from the upper Bay of Fundy. Bedford Institute of Oceanography Report Series BI-R-81-15, 143 pp. AMOS C. L. and B. J. TOPLISS (1985) Discrimination of suspended particulate matter in the Bay of Fundy using the NIMBUS-7 coastal zone cotour scanner. Canadian Journal of Remote Sensing, II, 85-92. AMOS C. L. and B. A. ZAITLIN(1985) The effect of changes in tidal range on a sublittoral macrotidal sequence, Bay of Fundy, Canada. Geo-Marine Letters, 4, 161-169.
1300
C . L . AMOS
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