Mary A. Cialone

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.

Engineering Study of Inlet Entrance Hydrodynamics: Grays Harbor, Washington, USA

An extensive field data collection effort was undertaken in Fall 1999 to examine wave propagation and currents through an inlet entrance. These data support a circulation and wave model for Grays Harbor, Washington, a jettied entrance with a large tidal prism. Both the field data and model results show wave attenuation in the inlet entrance, flood currents strongest on the north side of the inlet, and ebb currents more uniformly distributed. The influence of the tidal current and water level on wave transformation was also examined. Ebb current produces the greatest change at the inlet entrance, increasing wave heights by as much as 0.5-1.5 m. Flood current increases wave height at the seaward end of the entrance due to the ebb shoal redirecting flow offshore, but reduces wave height in the inlet throat. Water level has a minimal impact on wave height in the inlet entrance, but does control wave height in the back bay.

Quantifying impacts of forecast uncertainties on predicted storm surges

In this paper, we propose a framework for quantifying risks, including (1) the effects of forecast errors, (2) the ability to resolve critical grid features that are important to accurate site-specific forecasts, and (3) a framework that can move us toward performance-based/cost-based decisions, within an extremely fast execution time. A key element presently lacking in previous studies is the interrelationship between the effects of combined random errors and bias in numerical weather prediction (NWP) models and bias and random errors in surge models. This approach examines the number of degrees of freedom in present forecasts and develops an equation for the quantification of these types of errors within a unified system, given the number of degrees of freedom in the NWP forecasts. It is shown that the methodology can be used to provide information on the forecasts and along with the combined uncertainty due to all of the individual contributions. A potential important benefit from studies using this approach would be the ability to estimate financial and other trade-offs between higher-cost “rapid” evacuation methods and lower-cost “slower” evacuation methods. Analyses here show that uncertainty inherent in these decisions depends strongly on forecast time and geographic location. Methods based on sets of surge maxima do not capture this uncertainty and would be difficult to use for this purpose. In particular, it is shown that surge model bias can play a dominant role in distorting the forecast probabilities.

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.

Analysis of the Effect of Environmental Conditions in Conducting Amphibious Assaults Using a Ship Simulator/Vessel-Response Model Proof-of-Concept Study

Abstract : Significant technological advances have been made in estimating tidal current and water levels using numerical models as well as in estimating vessel maneuverability using ship/tow simulators. Benefits in applying these technologies to planning an amphibious assault include: 1) minimizing operational risk by testing the feasibility of the navigability of an area; 2) determining the capacity and timing of that operation; 3) defining the optimal axes-of-assault that best ensures navigational safety; and 4) developing a training platform for executing a particular plan. This report summarizes a proof-of-concept study for demonstrating the application of these technologies to allow commanders to determine the feasibility of surface amphibious operations and their use in a virtual amphibious assault near the Port of Anchorage, Alaska. This site was chosen because the environmental conditions at this location dictate that only a narrow window of time is available for conducting surface ship-to-shore operations. The vessel used in the virtual assault is the 1646-class Landing Craft Utility (LCU). The U.S. Navy (USN) provided two experienced craftmasters for piloting the LCU. Their assessment was that the simulator provided a realistic environment and the handling of the virtual LCU closely resembled that of a standard displacement landing craft.

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.

MISSISSIPPI COASTAL IMPROVEMENTS PROGRAM (MSCIP) STATISTICAL EVALUATION OF LINES OF DEFENSE

A multi-objective, comprehensive technical evaluation was conducted in close coordination and consultation with state and other federal agencies to determine the nature and level of hurricane protection and environmental restoration that lines of defense provide for coastal Mississippi. The definition of “lines of defense” in this context means ways of protecting coastal Mississippi from hurricane inundation. The intended approach was to investigate plans and designs that provide increased hurricane protection for the Mississippi Gulf Coast as well as avoid and minimize unintended consequences of taking such actions. This paper focuses on the development of state-ofthe-art atmospheric and coastal ocean hydrodynamic analysis methods that were used to determine the hurricane storm surge hazard along the Gulf Coast. These methods were developed as part of the Joint Coastal Surge Analysis Study in which the United States Army Corps of Engineers and the Federal Emergency Management Agency defined a unified technical approach to determine 100-yr storm water surface elevations across the region. Although other lines of defense were evaluated, the purpose of this paper is to assess the impact of flood protection structures on storm surge for storms of varying intensities and changes to the 100-yr flood elevations with flood walls in place. Nearly 200 synthetic storms were selected for simulation of the existing conditions along the Mississippi coast. Surge levels produced from these simulations populated the Joint Probability Method with Optimal Sampling (JPM-OS) surge response space and stage frequency curves were developed for 80 locations. By examining the actual surge responses from the simulated storms at a given location, the rank (which can be converted to return period) of the surge produced by each storm at that location was determined from the stage frequency curve. In this way, the range of return periods simulated for existing conditions could be covered for with-project (line of defense) conditions by applying the modeling system to a small subset of storms that span the full range of return periods, thus providing an efficient means to evaluate the lines of defense. The goal of this paper is to present an application of the JPM-OS by demonstrating how the Mississippi coast responds to different storms with and without lines of defense in place and how the lines of defense change stage frequency curves.