CHARACTERIZATION OF TIDAL WETLAND INUNDATION IN THE MURDERKILL ESTUARY, DELAWARE, USA

A parameterization of inundation was developed for the 1,200 hectares of tidal marsh along the 12-kilometer reach of the tidal Murderkill River in Kent County, Delaware. A parsimonious modeling approach bridges the gap between the simple “bathtub model” of instantaneous inundation and the more complex modeling of overland flow in tidal wetlands. The parameterization was successfully implemented by others in a numerical model to create loading functions to represent import and export of chemical species to and from the wetlands. The parameterization was also used in the model to evaluate mass conservation and phase offsets in tidal discharge due to the dynamic storage of water in intertidal areas. In the parameterization, the marsh was divided into marsh tracts based on hydrologic character and position along the river. A cumulative probability distribution of elevation was calculated for each marsh tract from a LiDAR-based digital elevation model. Each marsh tract was related to an adjacent river reach; the area in the tract below the stage of its reach was instantaneously inundated. From the mouth of the Murderkill River at Bowers Beach to a location 12 kilometers upstream (Frederica) there was a decrease in marsh elevation with the mean elevation decreasing from 0.86 meters to 0.60 meters. This elevation trend is consistent with measured accretion rates at four sites in the study area that document an increase in accretion from near Bowers Beach (0.74 cm/yr) upstream to Frederica (0.33 cm/yr). Upstream marshes are flooded more frequently and for longer duration than downstream marshes and these patterns are also in agreement with site specific accretion data and other literature where more frequent and longer inundation results in higher accretion rates. Spatial trends in marsh-surface elevation are well-documented at the small length scale of less than two km with salt-marsh elevations typically decreasing with distance away from the channels and marsh edges. The bulk of these studies are for tidal channels having their headwaters within the salt marsh. No literature citations were found documenting the observed spatial trend at the scale of the estuary (12 km) with a flow-through channel rather than a blind-headed channel in the marsh.