Chesapeake Bay Shore Erosion

     Hurricane Isabel resulted in irregular erosion of the Chesapeake Bay shoreline in Maryland. In the aftermath of the storm, the Governor’s Chesapeake Bay Cabinet, concerned about environmental degradation due to the influx of sediment into the Bay, requested an estimate of sediment input from shore erosion. The Maryland Geological Survey (MGS) polled local officials and county planners throughout the State and, based on limited quantitative information, endeavored to supply that estimate.
     From available photographs, MGS deduced that: (1) erosion was uneven in occurrence and amount, (2) the storm surge afforded two opportunities for erosion, once as water inundated low-lying coast lands and again as floodwaters ebbed, (3) erosion control structures commonly remained intact, but failed to prevent bank erosion, (4) the storm disrupted nearshore sedimentary structures, and (5) not all changes were erosional.
     The most extensive assessment of shore erosion was conducted by Baltimore County, using aerial surveys. Erosion occurred along an estimated 18,300 ft of county shoreline, about 1.5% of the county’s total shoreline length. No other county prepared as comprehensive an assessment of storm-related erosion.
     To approximate the amount of sediment delivered to the Bay as a result of Isabel, MGS used the percentage of affected shoreline in Baltimore County, an assumed mean retreat of 5 ft, and an average bank height of 5 ft to estimate the area and volume of sediment lost. These figures probably overestimate losses, but they provide some indication of the impact of Isabel relative to sediment delivery during other extreme events. Extrapolating Baltimore County shoreline losses to the Western Shore of the Maryland Chesapeake Bay, MGS estimated that 20 acres of land were lost, contributing about 81,000 metric tonnes of fine-grained sediment to the Bay.