Predicting Shoreline Retreat
As the shoreface part of the inner continental shelf retreats, its trailing edge forms a new surface which becomes an extension of the ramp. Waves are primarily responsible for shaping the concave shoreface at the shallow, most landward part of the shelf. Shoreface retreat occurs because of sand losses and/or sea-level rise. The ramp, located seaward of the shoreface, is usually a planar feature with a seaward inclination. Its slope, γ, at the time it is formed is dependent upon the long-term average retreat rate of the shoreface, s, and the long-term average rate of sea-level rise relative to the shoreface, a. The shoreface and ramp usually join asymptotically between 1 and 5 km from shore. Retreat of the shoreface may occur without a significant change in shape. Shoreline retreat on the order of kilometers is almost always the response of the shoreface to a long-term rise in sea level.
Recent shoreline retreat rates measured over a short time period (101 to 102 yrs) can be compared to long-term average past rates (103 to 104 yrs) to forecast shoreline behavior in the future, unless sedimentation or tectonism altered the ramp slope formed in the past. The recent shore retreat rate, sp, for a given recent sea-level rise rate, ap, is compared to the present average slope of the ramp, γp, where γp is assumed equal to a/s = γ. If γp » ap/sp, the present shore retreat rate is probably anomalously large compared to the long-term average rate for the reach of coast being considered. If γp « ap/sp, the present shore retreat rate is probably anomalously small compared to the long-term average rate for that reach. The future shore retreat rate, sf, can consequently be expected to increase rather than decrease in a trend more consistent with sf = af/γp. When the reason for present anomalous rates is established, a more definitive projection of the future rate can be made.
A test of the relationship of γp to ap/sp was made at five barrier island sites along the mid-Atlantic coast of the United States. At Smith and Assawoman Islands in Virginia, present shoreline retreat rates are similar to the long-term shoreline retreat rates of the past when referenced to the same relative sea-level rise rates. These islands are migrating landward (i.e., both ocean and lagoon shorelines are moving away from the ocean at about the same rate) as littoral sand is transported landward by overwash and tidal inlet processes. At Ocean City, Maryland; Sandbridge, Virginia, and a portion of the Outer Banks, North Carolina, the present shore retreat rate is anomalously low when compared to the present relative rate of sea-level rise. These islands are narrowing (i.e., ocean and lagoon shorelines are moving toward each other). When the islands reach a critical width (perhaps 350 m), island migration will begin and the ocean shore retreat rate will likely increase to five to eight times the present rate.
Figures & Tables
Sea-Level Fluctuation and Coastal Evolution - This Special Publication is the result of a symposium in honor of W. Armstrong Price held at the first SEPM Midyear Meeting at San Jose, California, on August 12, 1984. The factors controlling relative sea-level change along our shores are varied and, at best, imperfectly understood. Yet, the relative rate of change is what controls shoreline erosion, the arrangement of sedimentary facies of the coastal zone, and the character of deformities within the coastal stratigraphic record. Therefore, these papers address sea-level changes, shoreline responses, and the controls on the three-dimensional geometry of the consequent lithosomes; in short, the architecture of the coastal depositional systems.