Sea-level rise, progradation, and shoreline erosion: A history of Horse Island Marsh, Rehoboth Bay, Delaware

Stratigraphy, radiocarbon age estimates, and a local relative sea-level curve for Rehoboth Bay establish a three-stage evolutionary sequence for Horse Island marsh, Rehoboth Bay, Delaware. An initial transgressive sequence, accompanied by shoreline erosion, formed the marsh about 2000 ybp. Subsequently, a period of marsh shoreline progradation occurred which expanded the areal extent of the marsh. Over the past 200 years, erosion is once again the dominant shoreline process causing rapid shoreline retreat.;The shoreline of Horse Island marsh is currently eroding even though the marsh surface sedimentation rate is equal to or greater than the rate of local relative sea-level rise. This implies that other factors, such as wave climate and nearshore lagoonal sedimentation rates, play a significant role in shoreline processes. Depending upon the interactions of these variables, a marsh shoreline can either (1) drown, (2) retreat by erosion, or (3) prograde.;The marsh shoreline in western Rehoboth Bay, Delaware, is rapidly eroding due to wave attack. A 30--90 cm vertical scarp characterizes the shoreline and exposes the present-day rootmat and the underlying mud unit. Over a three-year period, average erosion rates ranged from 14 cm/yr to 43 cm/yr. Long-term erosion rates from nine different marsh shorelines were then plotted against the estimated wave power for each site to produce a regression equation that can be used to predict erosion rates for other marsh shorelines.;Three styles of shoreline erosion were observed. (1) Cleft and neck formation---V-shaped notches are cut into an initially "straight" shoreline. Between adjacent clefts, marsh necks, up to three meters in length, occur creating an undulatory shoreline geometry. (2) Neck cut-off---marsh necks can be cut off from the marsh creating a small marsh "stack." (3) Undercutting with rootmat toppling-wave action erodes the lower mud unit faster than the overlying rootmat creating an overhang that eventually topples into the bay.