MMS ENVIRONMENTAL STUDIES PROGRAM: ONGOING STUDIES

Investigation of Dredging Guidelines to Maintain and Protect the Integrity of Offshore Ridge and Shoal Regimes/Detailed Morphologic Evaluation of Offshore Shoals

Hayes and Nairn (2004) completed a review of the current understanding of the maintenance of linear ridge and shoal features on the continental shelf of the mid-Atlantic Bight and the northeastern Gulf of Mexico. Linear ridges are typically oriented parallel to the predominant wave approach direction, suggesting a common process for both origin and maintenance. Most researchers have concluded that ridges are derived from shorefaces of barrier islands as they retreat across the shelf in response to rising sea level and are subsequently maintained by storm-driven, downwelling currents.

The widely cited ridge formation theory of Huthnance (1982) requires a sufficient sand source, currents to move the sand, and an irregularity on the sea floor around which the ridges are initiated. Moslow and McBride (1999) postulated that one of the initial irregularities is a segment of an ebb-tidal delta abandoned by inlet migration. Nonetheless, these theories of origin provide little information on how these features maintain their form once they are detached from the shore, yet remain in a zone of active wave attack (in depths less than 20 m). Snedden et al. (1999) indicate that shoals in water depths less than approximately 20 m are migrating shoreward under the influence of Stokes Drift in fair-weather conditions. However, this model does not explain the maintenance of the form of linear shoal and ridge features.

To assess the impacts of dredging on these features, it is essential that a better understanding of the processes that maintain these features be developed. A new conceptual model presented in Hayes and Nairn (2004) demonstrates how waves shoaling and refracting up either side of a ridge off the coast of Maryland and Delaware result in convergence of sand transport over the crest of the ridge. The possibility that these ridges might deflate or disappear as a consequence of dredging, resulting in dramatic changes in wave conditions along the shore, is a major concern.

A substantial field program will be implemented to document the in situ morphologic evolution of Isle of Wight Shoal, offshore Maryland. Hydrographic surveys and surficial sediment sampling will be used to document morphologic and textural changes, whereas three bottom-mounted ADCPs will be used to measure waves and currents. Aerial photography will also be used at opportunistic times to capture wave refraction and shoaling patterns over and around the shoal.

A sophisticated numerical modeling program will be undertaken using nested, phase-resolving wave, hydrodynamic, sediment transport, and morphologic modules. Model runs will consider the long-term (~30 years) evolution of these features and demonstrate the influence of extraction on their morphologic integrity. Modeling, using a wave input schematization, will be tested against oceanographic observational data, quantitative data from aerial photography, and hydrographic data.

The final task will be to develop the guidelines criteria and guidelines based on the field observations and modeling results.

1. At what rate are ridge and shoal features created, potentially replacing those that are diminished through dredging?

2. What are the relative roles of waves and tides in maintaining the morphologic integrity of existing ridge and shoal features?

3. Is there a critical threshold for dredging that once crossed, ridge and shoal features may deflate, losing their morphologic integrity?

4. Is it possible to develop universal criteria for protecting the morphologic integrity of these features, possibly related to dimensionless characteristics such as crest depth to shoal height, length to width, volume removed to initial volume and storm wave height to crest height?

If universal criteria can be established, such criteria could provide the basis for MMS to develop guidelines to protect the morphologic integrity of these features from excessive dredging operations. If universal criteria cannot be established, it may be necessary to specify site-specific analysis following a recommended set of procedures to evaluate each case individually.