Ty V. Wamsley

A formula for longshore sediment transport in the Swash

A formula is proposed for the longshore sediment transport in the swash based on the study by Larson et al. on swash-zone transport and foreshore evolution. Hydrodynamics needed to predict the local transport rate were derived from ballistics theory, where both the cases with and without friction were investigated. The formula was validated with data from two experiments carried out in the Large-scale Sediment Transport Facility (LSTF) of the Coastal and Hydraulics Laboratory. In total five experimental cases were reproduced using a typical value on the transport rate coefficient from Larson et al. A simple exponential decay with distance offshore was employed to model the transition between the swash zone and the inner part of the surf zone. The LSTF data on transport rates measured in this region supported such a simplified model.

Surge Generation Mechanisms in the Lower Mississippi River and Discharge Dependency

AbstractThe Lower Mississippi River protrudes into the Gulf of Mexico, and manmade levees line only the west bank for 55 km of the Lower Plaquemines section. Historically, sustained easterly winds from hurricanes have directed surge across Breton Sound, into the Mississippi River and against its west bank levee, allowing for surge to build and then propagate efficiently upriver and thus increase water levels past New Orleans. This case study applies a new and extensively validated basin- to channel-scale, high-resolution, unstructured-mesh ADvanced CIRCulation model to simulate a suite of historical and hypothetical storms under low to high river discharges. The results show that during hurricanes, (1) total water levels in the lower river south of Pointe a La Hache are only weakly dependent on river flow, and easterly wind-driven storm surge is generated on top of existing ambient strongly flow-dependent river stages, so the surge that propagates upriver reduces with increasing river flow; (2) natural le...

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.

Guidance for Developing Coastal Vulnerability Metrics

ABSTRACT Wamsley, T.V.; Collier, Z.A.; Brodie, K.; Dunkin, L.M.; Raff, D., and Rosati, J.D., 2015. Guidance for developing coastal vulnerability metrics. Appropriate coastal zone management and storm-damage risk reduction requires the assessment of vulnerability in natural and human environments. Confusion arises, however, as vulnerability is conceptualized in many different ways and is closely related to other concepts such as risk and resilience. This paper defines nomenclature, presents a conceptual definition of vulnerability, and lays out a proposed conceptual approach for identifying and defining meaningful metrics to ensure a complete assessment of coastal vulnerability. While the focus of this paper is developing metrics for assessing vulnerability to coastal storms, the approach is valid for a wide range of systems and hazards at multiple scales and can explicitly consider the impacts of climate change. The approach is demonstrated through application to a simply coupled human-environment system on the coast and explicitly considers natural and nature-based features.

Morphologic Classification of Coastal Overwash

A set of pre- and post-storm beach profile data was assembled and the profiles classified into seven different cross-shore morphology change types resulting from overwash. These were crest accumulation, dune/berm translation, dune lowering, dune destruction, barrier accretion, short-term barrier rollover, and barrier destruction. Pre- and post-storm barrier profile sets from recent laboratory experiments also fell into the new classification system. Forcing mechanisms for the different categories are suggested. Understanding of the mechanisms leading to different types of cross-shore morphologic change is useful in developing cross-shore profile numerical modelling capabilities.

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.

Performance of Experimental Low Volume Beach Fill and Clay Core Dune Shore Protection Project

Abstract An experimental shoreline protection project in Jefferson County, TX was constructed and monitored as part of the National Shoreline Erosion Control Development and Demonstration Program. The project determined the effectiveness of a nontraditional, low volume beach fill at reducing erosion of underlying clay layers and evaluated the performance of a clay core dune compared with that of a sand filled dune. Four cells were constructed on the beach face having 0.18 or 0.25 mm sand and fill volumes of 6 or 12 cy/ft. A fifth control cell had no fill. A 2500 ft long dune was constructed, with half being composed entirely of sand and half being composed of a clay core and sand cap. The project was constructed during the summer of 2004 and was impacted by Hurricane Ivan in September 2004 and later by the same storm as Tropical Storm Ivan in October 2004. An evaluation of profiles and other data taken before, between, and after these storms shows that the clay core dune survived the storms much more intact than the dune constructed entirely of sand. The clay core dune suffered minor scarping at its seaward toe, while up to the seaward half of the sand dune was removed in some sections. The results of the low volume beach fill were also promising. The fill performed well by protecting the underlying clay layer from erosion, but interpretation of the results is complicated by the presence of geotextile tube groins that were placed to contain the fill.

COASTAL BARRIER BREACHING: COMPARISON OF PHYSICAL AND NUMERICAL MODELS

Two movable-bed physical model experiments of breach growth in a sandy barrier were conducted in a 3.3-m wide, 64-m long flume with a wave generator capable of generating 0.5 m waves. Breaching was initiated both without waves (Case BR1) and with waves (Case BR2) in separate tests by maintaining a head across the barrier. Breach channel deepening was controlled by the head difference across the barrier. Breach widening was found to occur primarily through episodic avalanching. The avalanched material was transported by the rapid flow both along the breach channel and transverse to the flow. In the presence of waves, breach widening was accelerated and dominated by swash processes. The numerical model of Kraus and Hayashi (2005) was assessed through comparison to measurements. The analysis furthered understanding of breach processes and identified several enhancements to improve predictive capability of the numerical model. (Less)

A HIGH RESOLUTION UNSTRUCTURED MODEL TO STUDY STORM SURGE IN THE PACIFIC ISLAND OF GUAM

A high resolution storm surge modeling framework for Guam has been developed. The objective is to provide reliable information on the future storm hazard. Located nearly 3,000 miles west of Hawaii, Guam boasts the westernmost point of U.S. territory. Guam hosts the Apra Naval Base and Andersen Air Force Base - now major components of the US Pacific Command (PACOM). Because of its location and physical characteristics, the Pacific island of Guam is subject to strong meteorological, hydrologic and geologic extremes. Meteorological hazards such as tropical storms and typhoons (or hurricanes) pose the most frequent and visible threat and cause major economic losses in Guam. Development of a high-resolution modeling framework for Guam, therefore, supports a variety of applications in the context of emergency management and development decision-making related to hazard and risk information. Reliable information on maximum probable wave height and storm surge of a given storm with sufficiently high resolution to show variation along the Guam coast and inland is essential for the cost-effective design and operation of military and public works projects.

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.