Wave attenuation by oyster reefs in a shallow coastal bay

Oyster reefs influence local waves and sediment transport in estuarine environments. Because of their potential for attenuating wave energy, building oyster reefs close to eroding intertidal marshes has been considered as a means of slowing or reversing shoreline erosion. Our study was designed to address the effectiveness of fringing oyster reefs to attenuate wave energy in shallow coastal bays. Our primary focus was on 2 well established reefs constructed of shell material and a newly emplaced reef constructed of cement “oyster castles” that were all located near marshes. We measured wave conditions on both sides of the study reefs and at a control site for periods of 3-4 weeks. Differences in wave heights across the reefs were used to quantify the effects of reefs on waves under varying tidal and wind conditions. The reefs reduced wave heights by an average of up to 17% but the reductions were largest at intermediate water depths. At low water depths, waves were small regardless of wind speed and were completely interrupted by the emergent reef. At water depths greater than about 0.3 m above the reef, the reefs in our study area had significantly less impact on wave heights. The pattern of wave height change as a function of water depth and wind speed was very consistent for the three study reefs, despite differences in the material comprising the reefs, their orientation relative to the shoreline, and wind conditions. The waves most likely to drive marsh retreat are those that reach the marsh when the elevation of the water surface is close to the elevation of the marsh edge, therefore marshes with an edge elevation near mean sea level are most likely to benefit from reductions in wave energy associated with oyster reefs like those in our study area. Marshes with a high edge scarp may not experience significant reduction in the wave energy driving marsh retreat from the presence of a fringing oyster reef.