Sediments are the primary determinants of biological activities in the Upper Bay of Fundy, notably benthic habitat and ecosystem processes; being able to forecast their behaviour is a high priority as indicated in the SEA report (Whitford, 2008). These areas are of high importance for primary productivity and functioning of the estuarine ecosystem. However, our current understanding of sedimentary processes operating in the upper intertidal zone is extremely limited and significantly limits our ability to accurately model far field effects of energy extraction. We know that a decrease in velocity will affect the transport and deposition of sediments and alter their properties (e.g. grain size, cohesiveness, organic matter content) however the relative magnitude of these changes in upper intertidal zones such as tidal creeks and salt marshes is unknown. In addition, we do not know if these changes would fall within the bounds of natural variability or how processes would be either amplified or dampened under a changing climate.
The purpose of this research project was to assess how the dynamics of sedimentation change in response to changes in energy between neap and spring tidal cycles. The differences in tidal prism and energy between neap and spring tidal cycles were used as a proxy for energy extraction due to in-stream tidal power devices. The overall goal therefore was to gain a better understanding of the factors controlling sediment transport and deposition within intertidal ecosystem and how these processes may or may not change with changing tidal energy or amplitude.
The research was conducted over a 3 year period at two intertidal sites within the Cornwallis Estuary. Starrs Point and Kingsport were chosen specifically for contrasting levels of wave exposure which would likely result in differences in sediment transport processes and ‘sensitivity’ to changing tidal conditions. Similar types of instruments available at the Intertidal Coastal Sediment Transport (In_CoaST) Research Unit were used for all three experiments however were deployed in different configurations and at different sampling rates to focus on a range of research priorities. Instruments used included: one Nortek shallow water bottom mounted Acoustic Doppler Current Profiler, 3 Nortek Acoustic Doppler Velocimeters, 2 Optical Backscatter Sensors (OBS 3+ Campbell Scientific), surface mounted sediment traps, 1 Teledyne ISCO automated water sampler deployed on a tower and 1 RBR temperature, turbidity and salinity probe.
From August 2009 to September 2011, a total of 73 tides were sampled over a range of spring to neap tidal cycles. Complete data sets (all instruments and traps function with the exception of the ISCO sampler) were collected for 40 of these tides. A total of 624 sediment deposition and 431 suspended sediment samples were collected with almost a third of these being processed for disaggregated grain size (DIGS) analysis. In addition, data were collected during a full range of natural variability in non-ice meteorological conditions. Data collected during these experiments represent the most comprehensive empirical data set ever collected within intertidal ecosystems in the Bay of Fundy.
Significant differences in sediment transport processes and controls on sediment deposition were found between the sheltered Starrs Point site (both within the creek and on the marsh surface) and the exposed Kingsport location. The differences in sediment deposition may be linked to differences in both the availability of sediment and the opportunity for this sediment to be deposited. Overall current velocities were very low 5-10 cm∙s-1 within the Starrs point tidal creek and less than 5 cm∙s-1 within the vegetated canopy at both sites. Measured shear stresses are insufficient to re-suspend sediment within the exception of the creek thalweg. Water depth and tidal stage play an important role in controlling the hydrodynamics within these systems, particularly within the tidal creek system. Tides which are restricted to the channel tend to be flood dominant while those that exceed the bankfull level are ebb dominant. Suspended sediment concentrations were highly variable ranging from < 50 mg∙l-1 to 5,800 mg∙l-1 particularly during flood and final ebb portions of the tide and during storms - mean during tides ~ 50-150 mg∙l-1. This suggests an ephemeral formation of a fluid mud layer which would dampen turbulence near the bed. At Starrs Point, there was approximately seven times the mass of sediment deposited within the tidal creek when compared to the marsh surface. In addition, approximately 1.5 times more sediment was deposited during overmarsh tides compared with channel restricted tides. The ADCP and OBS records indicate notable settling of sediment just after high tide which was not observed at the Kingsport site. Significantly less sediment (approximately 10 times) was deposited within the exposed Kingsport site and the highest values were recorded, not surprisingly, during lower amplitude tides. The grain size spectra between tides were very similar, and the highly flocculated nature of the material leads to more rapid settling with higher suspended sediment concentrations and more resultant deposition with a greater volume of water (e.g. depth). While sediment availability is enhanced during heavy rainfall events it does not necessarily lead to higher deposition immediately after the storm. Overall, these findings provide important baseline information regarding natural variability in biophysical processes and the ability of intertidal ecosystems to respond to changes in the environment (e.g. resilience) that can be applied to computer models currently being developed.
The amount of sediment deposited, particularly on the marsh surface, appears to be most sensitive to changes in water depth rather than changes in tidal energy. Therefore even a 5% reduction in tidal amplitude would reduce the number of over-marsh tides by a similar figure, and cause an increase in the occurrence of channel-restricted tides and result in significant changes in inundation time and flooding frequency on the marsh surface. The frequency of marshfull tides can potentially increase as well, in which case amplified erosion of marsh edges may create an additional sediment source. Decreased inundation frequency of high marsh surfaces may impose a sediment deficit in marsh systems, as less material is distributed to the marsh surface from tidal creeks. High marsh areas will likely be the most significantly impacted with a loss of sediment input. This can show impacts in marsh sedimentation and resulting elevation, channel equilibrium, vegetation community structure, and ecological productivity.
Potential decreased ebb-flow magnitude is likely to be associated with decreased tidal amplitude, due to less water being put into storage on the marsh surface, as well as less frequent inundation events. Lower magnitude ebb flows may show less capacity for sedimentary work, reducing sediment mobility during ebb phases. The result may be creek infilling and a reduction in bank steepness, which would likely have continued impacts on creek hydrodynamics and sediment transport. A continuation and exacerbation of this cycle would constitute a non-linear response of fine-grained materials in tidal creeks, and would impact the movement of water in and out of the estuary through changes in deposition and erosion patterns and the resulting basin geometry. Either a circumstance of decreased sediment supply to the marsh surface, or an increase in in-channel sedimentation, will impact the form and function of salt marshes
While this research has led to an increased understanding of the factors controlling sediment transport and deposition, it has also raised our awareness of the spatial and temporal complexity of these processes. Future field data collection efforts should try to get a better picture of the significant fluctuations in suspended sediment concentrations in the exposed marsh and mudflat system, given the challenges during the Kingsport deployment. It is recommended that the research be expanded to include a greater range of seasonal conditions and simultaneous measurements at multiple locations. It will be critical that computer models be tested for a range of environmental conditions and at a sufficiently fine spatial scale to resolve differences in sediment transport processes between un-vegetated creek or mudflat surfaces and the marsh surface. In addition, given the sensitivity of intertidal sediment transport processes and deposition to water depth, it is recommended that a Basin wide GIS assessment be conducted to identify the areas that will be most sensitive (e.g. upper marsh) to changes in tidal amplitude.