Characterization of Tidal Wetland Inundation in the Murderkill River Estuary

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The inundation of a salt marsh with tidal water is a simple concept, but quantifying this process in time and space is difficult due to the difficulty of adequately sampling a dynamic and spatially heterogeneous flow system on a vegetated surface having microtopography. This analysis is in support of a numerical hydrodynamic and water-quality model developed to investigate low dissolved oxygen in the tidal Murderkill River. A parameterization of inundation is developed for the 1,200 hectares of tidal marsh along the 12-kilometer reach of the tidal Murderkill River between Frederica and Bowers Beach. A parsimonious modeling approach is used to bridge the gap between the simple “bathtub model” of instantaneous inundation using only water elevations from Delaware Bay and the complexity of hydrodynamic modeling of overland flow in tidal wetlands. A more complex parameterization or process model is not warranted due to the lack of data to document inundated marsh areas along the extensive marsh platform of the 12-km river reach. Having a simple parameterization within a more complex hydrodynamic model provides flexibility in sensitivity testing of the extent of hydrologic and biogeochemical interactions between the marsh and the river. Project resources do not need to be committed to modeling a complex process that is unconstrained by observations. The parameterization can also be useful for understanding and evaluating anomalies in the conservation of water mass and phase offsets in tidal discharge that may result by not explicitly modeling the dynamic flow and storage of water in tidal wetlands.

In the parameterization, the marsh is divided into “marsh zones” (n=31) based on hydrologic character and position along the river. A cumulative probability distribution of wetland elevation is calculated from a digital elevation model for each marsh zone. These cumulative probability distributions serve as a simplification (parameterization) of the critical information contained in the raster data sets of marsh zones and elevation. Each marsh zone is related to an adjacent river reach and the area in the zone that is below the stage of its related reach is instantaneously inundated. Marsh zones are aggregated into two sets of marsh “groups” (n=22 and n=4). This methodology incorporates the spatial and temporal variation in water levels in the Murderkill River but the results are put into a structure more conducive to analysis and visualization.