Evolution of Equilibrium Slopes at Calvert Cliffs, Maryland A Method of Estimating the Timescale of Slope Stabilization

摘要

The present study documents the evolution of coastal bluffs from steep, actively eroding, to low-angle stable slopes at Calvert Cliffs, Maryland, and provides historic age control for the rates of these changes. We measured slope angles at intervals along the bluff line northward from Cove Point and Flag Ponds Nature Park, and from the updrift harbor structure at the Flag Harbor Marina. In the Cove Point and Flag Ponds areas, coastal bluffs were protected from wave action for several centuries by prograding depositional landforms. Measured slope angles varied between 25°-37° with a mean of approximately 31°. There was no progressive decrease in slope with age on the centennial time-scale. This consistency of slope angles suggests that steep, eroding bluffs quickly change to stable slopes (at the angle of repose) once they are protected from toe erosion. In the Flag Harbor area, a least squares regression of slope angle vs. distance showed a decrease in angle from steep 60°-70° slopes north of the harbor to stable 30°-40° at the harbor. A relationship between time and distance along the shoreline allowed us to estimate a slope stabilization time for this area of 35-40 yrs.