The Effect of Coastal Storms on Tidal Flooding and Turbidity in a Mid-Atlantic Salt Marsh

Hydrologic and oceanic influences on water level and turbidity fluctuations in the St. Jones River, a subestuary of Delaware Bay, were examined using 11-yr timeseries (1996 - 2006) of tide heights, offshore winds, river discharge, and the National Climate Data Center's storm event database. The purpose was to isolate factors that deliver large amounts of sediment to the fringing marshes of the river. Time-series analysis was used to remove the effect of semi-diurnal tides, and coherence analysis was performed to determine the relative influences of river discharge and winds on subtidal (tide-filtered) water levels. Statistically significant water level and turbidity events, defined as peaks above the 95% confidence interval for the water level and turbidity datasets, were compared to the NCDC database. Subtidal water levels in the tidal river are strongly influenced (40-60% of total variance) by remote wind forcing, primarily winds blowing along-shelf from the northeast, which leads to coastal setup. Easterly winds within the Bay have a secondary influence (10-40%) on subtidal water levels in the St. Jones River. Freshwater river discharge contributes less than 20% of the total variance in subtidal water levels; the tidal portion of the St. Jones River is affected more by offshore wind forcing than river discharge. Of the 38 water level and turbidity events that occurred during the study period, 25 were attributable to nor'easters, 9 to strong winds blowing from the northeast, and 3 to tropical storms. There were 46 nor'easters in the NCDC database, with 39 producing significant high water levels and 26 producing significantly high turbidity. Because nor'easters produced most of the anomalously high water levels accompanied by high turbidity, the results of this study imply that nor'easter storms are critical to delivering sediment to salt marshes in Delaware's tidal wetland coast.