ASSESSING THE EFFECT OF ACCELERATED SEA LEVEL RISE ON A TIDAL MARSH, WACHAPREAGUE, VA

Tidal marshes are vulnerable to erosion and inundation due to sea-level rise because of their low elevation. Changes in the marsh area will alter the tidal prism which, in turn, will affect inlet configuration and the stability of the adjacent barrier islands.

The area of our research is the marsh near the Wachapreague Inlet region on the Eastern Shore of Virginia. The main focus of this research is to investigate how sea-level rise presents a threat to the marshes, causing them to fall below sea level and/or become inhabitable to marsh vegetation. This two-fold study focuses on field observations and historical analysis using aerial photos.

Eustatic sea level is rising about 3.1 mm/yr (IPCC, 2007). However, at Wachapreague, sea level is increasing faster due to subsidence of the land. Analysis of average monthly water levels from the Wachapreague tide station from 1978-2000 shows 6 mm/yr of sea-level rise. A similar study 16 km south of our study site reported marsh accretion rates of 1.8 mm/yr (Kastler & Wiberg, 1966, Estuarine Coastal and Shelf Science, 42, 683-700). This regional data infers a subsidence rate of 4.2 mm/yr in our study area.

Using ArcGIS, 5.74km² of the marsh was delineated from the 2007 Virginia Base Mapping Program orthophotos. Earlier aerial photos from Google Earth™ were available from 1994 and qualitatively, document changes in the marsh are made over that time span. The aerial photo interpretation of the marsh is complicated by variations in tide stage, short-term v. long-term variations, and resolution of the photos.

A detailed field study was conducted in September 2009 along a transect extending from the water’s edge towards the interior of the marsh. Preliminary results show that vegetation changes with elevation, making plant species a helpful indicator of changes in marsh elevation. Spartina alterniflora is present at higher elevations, while Spartina patens are present at lower elevations. Core samples show that soil horizons also change with elevation in the marsh region. At lower elevations, organic matter dominates; as elevation increased, the core samples contain more sand.

The results from this study may be useful to compare with other areas affected by accelerated sea-level rise over the next century and may help define regional variability of sea level change and marsh response.