Alison Sleath Grzegorzewski

Interaction of Barrier Islands and Storms: Implications for Flood Risk Reduction in Louisiana and Mississippi

Abstract Coastal barrier islands are natural lines of defense and an integral part of a comprehensive flood risk reduction and management plan. A high resolution numerical modeling system capable of representing complicated coastal landscapes and simulating all the primary relevant physical processes is applied to better understand the influence of barrier island restoration on hurricane surge propagation. Model results indicate that barrier island restoration may significantly alter surge pathways and flood volumes of surge reaching inland coastal areas as open water passes become the dominant flow mechanism during a storm event. However, the exclusion of the morphologic evolution of a barrier island during a storms passage is a significant limitation with the existing numerical models and is currently under development. The results in this paper demonstrate the need to include morphologic changes to fully evaluate the impact barrier islands have on water levels at the mainland coast.

THE INFLUENCE OF BARRIER ISLANDS ON HURRICANE-GENERATED STORM SURGE AND WAVES IN LOUISIANA AND MISSISSIPPI

A sensitivity analysis was performed to assess the impact of bathymetric and frictional resistance changes on ADCIRC-simulated peak surge elevations and STWAVEsimulated waves. Natural landscape features such as barrier islands have the potential to create frictional and bathymetric resistance and affect storm surge and wave energy even when submerged. The purpose of this study is to qualitatively assess the impact of barrier island restoration and degradation on storm surge and wave energy in Southeast Louisiana and Mississippi for storms of varying intensities. The study area includes the Chandeleur Islands as well as Cat Island, Ship Island, and Horn Island. Results from this modeling study support the use of barrier islands as a first line of defense against hurricane impacts, and may be used to optimize sustainable coastal protection strategies. However, verification of these simulations for hurricane surge and associated waves requires field data of the nearshore wave conditions under such extreme conditions. Verification of the numerical model results is not possible at this time due to lack of field data.

CONCEPTUAL REGIONAL SEDIMENT BUDGET FOR THE U.S. NORTH ATLANTIC COAST

A Conceptual Regional Sediment Budget (CRSB) was developed for coastal beaches, bays and estuaries extending from Virginia to Maine, USA, as part of the North Atlantic Coast Comprehensive Study. Hurricane Sandy made landfall on October 30th, 2012 near Brigantine, New Jersey and generated severe beach erosion and property damage throughout this region. A CRSB is the first step in understanding sediment transport patterns and magnitudes, and aligning dredging and placement operations to take advantage of natural processes and identify sediment deficiencies in a regional system. Optimizing regional sediment management practices in this region is critical to (a) improve beneficial use of dredged sediments; (b) reduce the risks of future storm damage and enhance the environment; and (c) reduce costs in maintaining coastal infrastructure. Data from 1990-2013 indicated an average of 18.7 million cu yd/year was dredged within the North Atlantic region. The CRSB is accessible via a web portal.

INFLUENCE OF SEA LEVEL RISE AND RIVER FLOW RATE ON STORM SURGE IN THE MISSISSIPPI RIVER, USA

A significant issue in the design of flood protection in southeastern Louisiana is the consideration of relative sea level rise (RSLR) due to eustatic sea level rise and local subsidence. The Mississippi River levee designs must also take into account the effect of the river flow rate at the time of landfall for a particular storm on surge levels in the river. This paper examines the combined effect of sea level rise and river flow rate on surge levels in the Mississippi River. The focus is on estimating the potential impact of RSLR and river flow rate on hurricane surge in the lower Mississippi River by examining the range of surge response to these two varying conditions. This is accomplished through numerical surge modeling of 17 hypothetical hurricanes for a base condition and with the inclusion of a projected sea level rise and various river flow rates.