The Elkhorn Slough Tidal Wetland Project is a collaborative effort to develop and implement strategies to conserve and restore estuarine habitats in the Elkhorn Slough watershed. It involves over a hundred coastal resource managers, representatives from key regulatory and jurisdictional entities, leaders of conservation organizations, scientific experts and community members. The main goals of the Tidal Wetland Project are to: (1) conserve existing high quality estuarine habitats, (2) restore and enhance degraded estuarine habitats, and (3) restore the physical processes that support and sustain estuarine habitats. Particular emphasis in the restoration planning process has been placed on the first goal, which aims to stop the ongoing marsh loss and estuarine habitat erosion in Elkhorn Slough. The Tidal Wetland Project builds upon a number of past planning reports and efforts including the 1989 Elkhorn Slough Wetland Management Plan. The conservation and restoration of Elkhorn Slough�s estuarine habitats are considered a priority to California due to the loss of approximately 80 percent of coastal marshes in the state alone.
In November 2006 the Philip Williams and Associates, Ltd. (PWA) team was retained to investigate potential restoration alternatives to address these problems of tidal erosion and marsh loss. The alternatives considered in this study include existing conditions and three additional restoration alternatives.
DELFT3D was selected as the primary hydrodynamic modeling tool for the Elkhorn Slough TWP restoration evaluation. DELFT3D models unsteady tidal flows, flow through hydraulic structures, and drying and flooding of intertidal areas. The model employs a curvilinear flexible-mesh grid system, which provides the ability to fit the computation grid to the variable bathymetry of Elkhorn Slough.
The largest project task was the development of a hydrodynamic computer model of the Slough to provide a tool to predict the response of the system to possible restoration alternatives.
The goals of the modeling effort are to: (i) characterize the hydrodynamic response of the tidallyaffected portions of the Elkhorn Slough system to various restoration actions; and (ii) inform the projection of future slough evolution and morphology.
The methodology developed and implemented for this study combines predictions of bed shear stress from the DELFT3D hydrodynamic model with an assessment of past rates of geomorphic change using bathymetric data collected by SFML. For each alternative, the developed methodology is applied to predict the change in bed elevation in the main Slough channel between Year 0 and Year 10 and between Year 10 and Year 50. DELFT3D models unsteady tidal flows, flow through hydraulic structures, and drying and flooding of intertidal areas. The model employs a curvilinear flexible-mesh grid system, which provides the ability to fit the computation grid to the variable bathymetry of Elkhorn Slough.
The modeling effort provides a calibrated and validated hydrodynamic model that will serve as a tool for a variety of future studies. Of primary initial importance is the evaluation of restoration alternatives and quantification of various hydraulic characteristics of the proposed management actions. Conceptual models developed by the Science Panel will help refine both the computer-based hydrodynamic modeling efforts and the geomorphic assessment. Specifically, the model will quantify changes to some of the primary physically-based cause-and effect pathways, which include:
*Channel Scour. Modifications to the tidal inlet that decrease bed shear stress along the main channel and result in slower rates of channel deepening and widening.
*Loss of Marsh Edge (Bank Erosion). Modifications to the tidal inlet that decrease current velocities and bed shear stress and result in slower rates of marsh loss (edges).
*Loss of Marsh Interior (Marsh Dieback). Modifications to the tidal inlet and/or placement of sediment on the marsh plain that decrease tidal inundation and result in slower rates of marsh loss (interior).
In addition to evaluating how these conceptual model linkages respond to proposed restoration actions, the hydrodynamic modeling will also inform future design refinements of the restoration alternatives. These may include the optimal geometry of a barrier under Highway 1, the appropriate size of a natural tidal inlet mouth, or channel dimensions. Results from the hydrodynamic model will also provide input to the long-term stability assessment of new ocean inlets by quantifying the effective tidal prism under restored conditions.
Overall, the DELFT 3D model provided an accurate representation of the Slough hydrodynamics. The differences between observed and predicted parameters presented in the calibration report are not considered significant, especially when compared with changes observed in the Elkhorn Slough system since the completion of Moss Landing Harbor in 1947 and when compared with changes under the proposed restoration alternatives. Additionally, sensitivity analyses demonstrate that model predictions are relatively insensitive to typical freshwater discharges. Extreme storm events and the associated runoff were not evaluated as part of model calibration and validation. Ongoing, daily tidal exchange, rather than extreme events, is assumed to control long-term morphologic change. DELFT3D is a widely used modeling tool for estuarine simulations and has been validated in
numerous studies. However, all numerical models rely on approximations which introduce sources of uncertainty in the model results. Uncertainties may be present both spatially and temporally, and may result from a variety of factors, including physical characteristics of the model domain, specification of boundary conditions, or limitations in the model's numerical formulation. For the specific application of a hydrodynamic model of Elkhorn Slough, it is important to assess the modeling uncertainties and assumptions made in applying the model to understand the extent to which these uncertainties could affect model predictions.
The primary goal for the next year of the Tidal Wetland Project is to reach agreement on the best course of action to restore and enhance estuarine habitats in Elkhorn Slough. These decisions will identify which conservation strategies to further consider or to eliminate. Advancements made during the current phase of the Tidal Wetland Project- technical analysis of ecosystem dynamics and restoration alternatives, have largely been guided by the Strategic Planning Team, a group of key stakeholders, with expert input from the Science Panel, a large group of university, government and consulting scientists. The next steps are interpreting the results of technical analysis in light of the restoration alternatives and making a group decision about the optimal course of action to pursue. The TWP team is dedicated to ensuring that this multi-dimensional process is executed with rigor from both a technical and social perspective.
The results suggest that historically a potential for closure existed, and that the inlet persisted in a regime where episodic depositional events could have resulted in significant morphologic changes over seasonal and annual time scales. This migration could be managed by the construction of one or more jetties to limit shoaling within the inlet throat and inhibit lateral migration.
Philip Williams & Associates, Ltd., H.T. Harvey & Associates, 2nd Nature, E. Thornton, and S. Monismith, 2008. Elkhorn Slough Tidal Wetland Project, Hydrodynamic Modeling and Morphologic Projections of Large-Scale Restoration Actions: Final Report. June 6, 2008. http://elkhornslough.org/tidalwetland/ESTWP/ESTWP_PLAN_050207_lres.pdf
ElkhornSlough\Project Report\100_percent_final\ElkhornTWP-Report Final v4.doc
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