The Offshore Sand Resources Study
|contact: Bob Conkwright (firstname.lastname@example.org)|
Our second objective in the Offshore Sand Resources was to determine which shoals were most likely to contain sand suitable for beach nourishment. Of particular interest to this study is a model introduced by McBride and Moslow (1991), which describes the formation of linear sand ridges. This model details the evolution of ridges formed when inlet shoals become submerged during sea level rise, and are subsequently reshaped by ocean waves and currents. The process can be summarized in six steps:
- ) A barrier island is breached, possibly by a storm event. Sand is transported by waves and currents along the shoreline in a predominant direction, the longshore transport vector. The island also migrates toward the mainland, due to sea level rise, in the direction of the transgression vector. On the Maryland coast, longshore transport is generally from the northeast to the southwest. Sea-level rise also pushes Maryland's barrier islands along a westward transgression vector.
- ) A shoal forms on the seaward side of the inlet. Sand is deposited on this shoal during ebb-tidal flow (ebb-tidal shoal). The shoal may also trap sand being moved by longshore transport.
- ) The inlet and its associated shoal migrate along a vector that is the result of both the longshore transport vector and the transgression vector. The long axis of the shoal parallels this new vector, which generally forms an angle less than 35 degrees from the existing shoreline. The shoal continues to grow in volume and length as long as sand is available.
- ) Eventually the inlet wanes and closes. Cut off from its ebb-tidal sand source, the shoal ceases to accrete. It may still trap sand carried by longshore transport.
- ) The barrier island continues to migrate toward the mainland, due to sea level rise. The water depth increases above, and the shoreline retreats from the shoal. Neashore waves and currents continue to reshape the shoal.
- ) The shoal becomes entirely detached from the front of the barrier island (the shoreface), and is submerged below the depth that average waves can affect it (the wave base). Only inner shelf currents and possibly large storm-generated waves reshape the shoal. The reshaping process elongates the shoal along its original axis, producing a characteristic linear sand ridge, oriented at an acute angle to the shoreline.
Linear ridges can evolve from several processes. The McBride/Moslow model offers a mechanism to explain why many linear ridges share common physical features. Ebb-tidal deltas that form at the mouths of inlets trap sand moving past and through the inlets. Therefore, sand ridges that have developed from ebb-tidal deltas have an excellent sand resource potential.
We applied the McBride/Moslow model to geologic data from the inner continental shelf to determine which sand ridges, or shoals, might have been ebb-tidally derived. These shoals have the best potential as sand resources. This method was employed primarily to narrow the number of shoals targeted for further investigation. Field operations to collect data on the shoals are expensive. Therefore, it was necessary to maximize the probability of locating sand resources before sampling projects were initiated. We also limited our search to within 24 km of the shoreline, and to water depths of less than 15 meters. These limitations were based on the economics and mechanics of dredging the shoals and emplacement of sand on the beaches. Eight shoals out of nineteen were targeted for sampling by applying the McBride/Moslow model to our geophysical data.