Abstract
Sediments are the primary determinants of biological activity in the upper Bay of Fundy. Gaining an understanding of their behavior under varying environmental and energy conditions will provide a critical baseline to model anticipated effects of tidal energy extraction on sensitive intertidal ecosystems such as tidal flats and salt marshes. For example, changes in tidal energy or tidal range can induce changes in hydrodynamic forces, the structure and location of biotic communities and rates of sedimentation and erosion. These changes are most likely to be felt within intertidal communities at the upper reaches of the vertical influence of the tides. Since the processes of sedimentation and erosion are spatially and temporally variable, field data are required over a range of suspended sediment concentrations, current velocities, water depths, topographies, biotic communities (e.g. vegetation and benthos) and wind-wave energy. Any potential energy extraction or change in the tidal range will exert an influence on sediment dynamics within the system. However the magnitude of the change in intertidal areas is currently unknown and may or may not occur within a range of natural variability therefore it is vital to model these processes over a range of environmental conditions.
The purpose of this research project is to assess the implications of tidal energy extraction on sedimentary processes within shallow intertidal ecosystems. We specifically address OERA ’s research priority area regarding the relationships between tidal energy extraction and inshore areas (e.g. tidal creeks, marshes, intertidal flats). While previous modelling and field validation efforts have been able to resolve hydrodynamic and sedimentary processes at the basin scale (Smith and Mulligan, 2010), we focus on processes within the relatively shallow upper intertidal zone to gather much needed insight into hydrodynamics over smaller scale and ecologically sensitive areas. Our previous field work has already confirmed significant spatial and temporal variability in sedimentary processes between tidal creek and exposed salt marsh/mudflat ecosystems (van Proosdij et al. 2010). The additional data in this project have helped to refine preliminary empirical relationships including seasonal variability in processes, particularly suspended sediment concentration and sediment composition (e.g. grain size and floc fraction).
This study significantly advances our understanding of the seasonal variability in intertidal ecomorphodynamics: the interaction and adjustment of topography, vegetation, fluid and hydrodynamic processes, morphologies and sequence of change dynamics involving the movement of sediment. In addition, it provides the first numerical model in the Bay of Fundy that effectively integrates near and far field hydrodynamic processes and serves as an important step towards three-dimensional modelling the full impacts of tidal energy extraction in these important ecosystems.
In the field study, the seasonal control on deposition was strongest in the channel, seen at the creek and marsh bank stations. At these two stations, deposition and suspended sediment concentration were higher and this occurred in the winter, because of rapid deposition from high sediment supply. On the high marsh, the amount of sediment in floc form decreased and the seasonal control was less prominent. The period of October to February was the most active period in terms of high suspended sediment concentrations and sediment re-suspension. Episodic events with strong winds and heavy rainfall were effective at changing the grain size distribution of deposited sediment, this re-suspension also changing the characteristics of the sediment in suspension and therefore the incoming sediment on following tidal cycles. In addition, these episodic events appear to play an important role in maintaining equilibrium and a balanced annual sediment budget within the salt marsh tidal creek channel. However, this study also demonstrates that using only one scale of data (e.g. tidal cycle versus seasonal GIS digital elevation models) may lead to an in accurate estimation of the sediment budget, and more accurate sediment budgets should be developed by integrating over broad spatial and temporal scales.
The three-dimensional high-resolution hydrodynamic model (Delft3D) was used to simulate tidal currents and water levels, calibrated using acoustic observations over multiple tidal cycles in the intertidal zone at Kingsport. Using a system of three interconnected model domains at increasing resolution toward the intertidal zone, high resolution of the salt marsh and drainage channels was attained. A vegetation model that incorporated stem height, diameter and density was implemented and the resulting vegetation was determined to be crucial in reproducing currents in the marsh. A simulation that included turbines in Minas Passage was developed, representing the 2 .5 GW of tidal power extraction. The test produce d a 3.5% ( 0.2 m ) tidal amplitude decrease within the Kingsport marsh, suggesting that turbines may have impacts on intertidal water level elevations and inundation times. Future work will address sediment dynamics, tidal currents and surface waves in June 2013, corresponding to the dataset collected during the CCGS Hudson research cruise in Minas Basin.
The numerical model was also used to simulate the combined effects of surface waves, tidal currents and sediments with results indicating that storm events have major impacts on sediment transport, with winds that generate fetch-limited surface waves. The waves are important for re-suspension over the shallow tidal flats in the basin, by inducing wave orbital velocities at the seabed that in addition to tidal currents, create strong shear stresses on the bed. The results indicate that locally generated wind waves, can vary significantly over seasonal timescales, contributing up to 1-5 Nm-2 to the bottom shear stresses on tidal flats. The added shear stress due to waves leads to increased erosion of the tidal flats around the rim of the basin and increases the suspended sediment concentrations by 100-200 gm-3 in intertidal areas and by 10-20 gm-3 in deeper areas of the basin, representing a doubling in concentration in these areas. Predicting sediment transport processes in macrotidal environments is therefore de pendant on accurate simulation of the combined tidal flow and surface wave field properties.
The results of this project add tremendous value to industry and government partners involved in the Fundy FORCE initiative, but also build on two successful OERA projects led by Danika van Proosdij in collaboration with Peter Smith (Bedford Institute of Oceanography) and Ryan Mulligan (Queen’s University). In addition, it contributes directly to building local capacity in environmental effects monitoring and model ling by enriching our baseline data sets, adopting leading edge technologies and, providing “real world” training grounds for young scientists in Canada. This project was the foundation for training two bright young scholars at the Masters level: Emma Poirier at Saint Mary’s University and Logan Ashall at Queen’s University. An additional three undergraduate research assistants were trained in the field within the In_CoaST research unit. All team members have presented at regional, national and international conferences, disseminating our research widely and numerous papers will be submitted to referred journals in the near future. We have demonstrated that our team has taken full advantage of OERA ’s funding opportunity. By building on previous project investments and acquiring new data at key sites, we have developed a high resolution hydrodynamic model allowing us to integrate the field and model results that will help simulate anticipated TISEC installations while minimizing environmental impacts. Continuing work develop a combined wave/current/ sediment transport model that can be used to investigate the impacts of turbines on ecologically sensitive intertidal areas. Finally, the project results will benefit Nova Scotians by serving as a baseline that can be used to understand whether or not future changes associated with commercial scale tidal power generating structures in the Minas Passage are outside the range of natural variability of stressors that intertidal ecosystems have adequate resilience to respond.