Operational Sediment Budget for the Outer Coast of Louisiana: Raccoon Point to Sandy Point


Effective design and management of beach restoration requires reliable sediment transport estimates for predicting project response under storm and normal conditions. From a project planning perspective, net sediment transport rates are critical for predicting project longevity and developing timing for future restoration needs. The purpose of this Scope of Services is to develop an operational sediment budget for the barrier shorelines of south Louisiana between Raccoon Point and Sandy Point (Figure 1) to determine net littoral sediment transport rates for use with shoreline restoration planning and design, and estimates of project longevity. All survey data required for the project are available through BICM, so the project will build on the invaluable data collection efforts completed since 2005. Required data sets include historical shoreline surveys, topography data developed via Lidar, bathymetric surveys, navigation channel dredging quantities, and barrier island restoration sand placement quantities. The shoreline/topographic and bathymetric data sets will be combined to create surfaces upon which temporal comparisons will be made. Once survey elevations are adjusted to compensate for relative sea level rise between the 1930s and modern surveys, change surfaces will be calculated to identify net sediment transport patterns and quantities for developing an operational sediment budget. Applied Coastal personnel have completed similar projects in support of the USACE MsCIP program and to evaluate the impact of channel dredging on adjacent shoreline response at Mobile Pass, AL.

The use of detached breakwaters is an integral part of coastal protection in Louisiana, having been constructed at a number of locations along the outer sandy coast of south Louisiana in an attempt to mitigate beach erosion. Breakwaters reduce energy associated with incoming waves before they impact the beach, thereby reducing sand transport and shoreline change. Unfortunately, an unintended consequence of breakwater placement is accelerated beach erosion at the downdrift end of the breakwater system where protection against incoming wave energy ends. Decreased sand transport rates landward of detached breakwaters increases beach erosion susceptibility downdrift of breakwaters, thereby accelerating shoreline recession. A primary question associated with breakwater placement is whether these structures impact the local sediment budget. To address this question, we are evaluating beach response to breakwater placement along Raccoon Island for the period 1986/97 to the most recent bathymetric surveys as a proxy for evaluating structure placement and potential beach impacts for other sandy shoreline locations in coastal Louisiana.