Chantal Donnelly

State of knowledge on measurement and modeling of coastal overwash

Abstract A critical review is presented on the state of knowledge and calculation capability for coastal overwash. Overwash and overwash deposits (washover) accompanying hurricanes and severe storms can devastate coastal communities and habitat, but in many areas these processes are essential for maintaining the integrity of barrier islands while creating new habitat. This review covers general studies of overwash processes, studies from a geological perspective, physical modeling, field studies including measurements of washovers and related hydraulics, and the state of numerical modeling capability to predict overwash. Although significant literature exists describing individual overwash events and locations experiencing frequent overwash, complete hydrodynamic and morphologic documentation of an overwash event is lacking. A limited number of algorithms or models exist to quantify overwash occurrence, deposited sand volume, and upper beach profile evolution. Existing models of overwash occurrence and one-dimensional beach profile evolution have been shown to perform successfully against available data, and areas of improvement are identified. Models must be made capable of simulating the various washover morphologies that have been produced by different hydrodynamics, overwash spreading based on dune topography, friction and percolation, and interaction between swash bores. Comprehensive laboratory and field data sets to achieve these aims are still lacking.

Numerical Modelling of Beach Profile Change caused by Overwash

Beach profile change due to overwash has a major impact on both developed and undeveloped coasts following storms. The ability to predict such profile change has been further developed within a numerical beach profile change model, taking into account lateral spreading and infiltration on the back slope of the beach, and overwash where the beach crest becomes inundated. These new capabilities were tested for overwash of a low flat barrier and where dunes are present, reproducing well both the volume and shape of the overwash deposits.

Characterisation and modelling of washover fans

Pre- and post-storm topography and aerial photography, collected in regions where new washover fans were formed, were studied to determine the extent of morphologic, vegetative and anthropogenic control on washover shape and extent. When overwash is funnelled through a gap in a dune ridge and then spreads laterally on the back barrier, decelerating and depositing sediment, it forms washover fans. Fans were shown to primarily occur at pre-existing gaps in the foredune. During overwash, these gaps, or overwash throats, widened and deepened. The shape and extent of the fan was shown to depend on not only the pre-storm topography, but also the existence of beach tracks, roads and other anthropogenic influences and vegetation. The cross-shore overwash profile change model by Larson et al. and Donnelly et al. was modified to include pre-storm throat widths and a lateral spreading angle estimated from the pre-storm topography as inputs and tested using cross-shore profiles through the fan centres. These new inputs make the model more generalised, such that the calibrated model is applicable to a wider range of cross-shore profiles.

One-line modelling of complex beach conditions: An application to coastal erosion at Hai Hau beach in the Red River Delta, Vietnam

The long-term shoreline evolution at Hai Hau Beach in the Red River Delta, Vietnam was simulated using the one-line model GENESIS (Hanson, 1989). Dikes were modelled as seawalls and the sediment continuity equation in GENESIS was modified to take into account the offshore transport of fine-grained sediment. The model performed well, reproducing the magnitude and trend of the long-term shoreline change. Additionally, a net shoreline transport to the southwest was calculated which is in agreement with observations and the estimates of local engineers.

Modeling Shoreline Evolution at Hai Hau Beach, Vietnam

Abstract The coastline of Hai Hau District, located on the northeast coast of Vietnam with about 30 km of shoreline, is chronically eroding. Previous studies have tried to highlight the main causes of the erosion along this coastline, and several hypotheses exist. To examine the hypothesis that gradients in the longshore sediment transport rate and cross-shore fine sediment lost offshore are the main causes generating the serius erosion at Hai Hau Beach, a newly developed numerical model of shoreline change based on the one-line theory was applied and compared with data. Sea dike segments, reinforced by stones and mortar, were modeled using a seawall boundary condition, and the sediment continuity equation was modified to take into account the offshore transport of fine-grained sediment. The simulated shorelines agreed well with the measured shorelines, both for the calibration and validation periods. The calculated sediment budget shows that the net sediment transport is in the southward direction and that a large amount of fine-grained sediment is lost into deep water. These two sinks of sediment are believed to be the main causes of the serious erosion at Hai Hau Beach.

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.

ANALYTICAL MODELING OF DUNE RESPONSE DUE TO WAVE IMPACT AND OVERWASH

Abstract in Undetermined An analytical model of dune erosion was developed based on impact theory, where the weight of eroded material is assumed proportional to the impact force from waves hitting the dune. The model was validated with data from several laboratory and field studies on dune erosion covering a wide range of hydrodynamic, sediment, and geometric conditions. Overwash was then included in the model and the evolution of a rectangular‐shaped dune subject to both wave impact and overwash was investigated. Finally, an analytical solution for the case of a schematized barrier islands exposed to overwash was developed. Analytical models are useful in the initial stage of a project when approximate estimates are required. (Less)

Analytical Modeling of Dune Response Due to Wave Impact and Overwash

Abstract in Undetermined An analytical model of dune erosion was developed based on impact theory, where the weight of eroded material is assumed proportional to the impact force from waves hitting the dune. The model was validated with data from several laboratory and field studies on dune erosion covering a wide range of hydrodynamic, sediment, and geometric conditions. Overwash was then included in the model and the evolution of a rectangular‐shaped dune subject to both wave impact and overwash was investigated. Finally, an analytical solution for the case of a schematized barrier islands exposed to overwash was developed. Analytical models are useful in the initial stage of a project when approximate estimates are required. (Less)

Analytical Modeling of Dune Response due to Wave Impact and Overwash

Abstract in Undetermined An analytical model of dune erosion was developed based on impact theory, where the weight of eroded material is assumed proportional to the impact force from waves hitting the dune. The model was validated with data from several laboratory and field studies on dune erosion covering a wide range of hydrodynamic, sediment, and geometric conditions. Overwash was then included in the model and the evolution of a rectangular‐shaped dune subject to both wave impact and overwash was investigated. Finally, an analytical solution for the case of a schematized barrier islands exposed to overwash was developed. Analytical models are useful in the initial stage of a project when approximate estimates are required. (Less)