Chesapeake Bay Shore Erosion

contact: Stephen Van Ryswick (stephen.vanryswick@maryland.gov)

Hurricane Isabel and Shore Erosion in Chesapeake Bay, Maryland
by Lamere Hennessee and Jeffrey P. Halka

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Abstract of this Article

     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.

Introduction

On September 18, 2003, a tropical cyclone, Isabel, made landfall as a Category 2 hurricane along the North Carolina coast. The storm then weakened and accelerated. Within 24 hours Isabel had dissipated, but not before ravaging coastal communities all along the Western Shore of Maryland’s Chesapeake Bay. Shore erosion was an unmistakable and widely reported effect of Isabel’s passage over the State. Damage to shoreline structures alone was assessed at $84 million ^[1]. Government agencies and citizens groups were concerned about the possible deleterious effects of an influx of suspended sediments and nutrients on the Bay ecosystem, particularly given the near-record extent of the summer’s anoxic “dead zone.” The Governor’s Chesapeake Bay Cabinet requested an estimate of sediment input contributed by shore erosion. The Maryland Geological Survey (MGS) endeavored to supply that estimate.

Severe erosion threatens road. Bay Ridge, Chesapeake Bay, Anne Arundel Co. [2]

1. Maryland Department of Planning, 2004,. Baltimore, MD. 29 p. Lessons learned from Tropical Storm Isabel
2. J. Stein (photographer), 2003, Anne Arundel Co. Soil Conservation District.

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