Rolf Deigaard

LONG TERM MODELING OF SHORELINE RESPONSE TO COASTAL STRUCTURES

A new model concept for shoreline evolution is applied to the case of an offshore breakwater. The model is verified against bathymetrical surveys performed 4 years after the breakwater construction, and used to predict the consequences of an alternative design. The model concept is based on the combination of a one-line type of schematization of the nearshore morphology with a 2DH area modeling approach for waves, currents and sediment transport. This concept makes a model capable of taking into account: 1) variation in the offshore bathymetry and 2) the effect of coastal structures on waves and currents, and hence on sediment transport and shoreline response. The model results show very good quantitative and qualitative agreement with the observed morphology in the case of the original breakwater layout, and the results from the alternative breakwater layout demonstrate the model’s potential as a tool for shoreline assessment studies.

Shape and Size of Sandy Spits

A theoretical study is made of the shape, dimensions and growth rate of a spit. The spit is growing without changing its shape, being fed by the littoral drift along a long straight coast up drift of it. A one-line model for coastline development is used as basis for a simple analytical model. This is based on the assumption that the local longshore sediment transport is determined by the local orientation of the coastline relative to the incoming waves. The analytical model predicts an exponential shape of the spit. The analytical model gives a growth rate inversely proportional to the length scale and this model predicts an infinitely narrow spit to emerge. The sediment transport around the spit is investigated by two-dimensional models. The shape of the spit is still taken to be exponential and the growth rate is estimated on basis of the longshore sediment transport along the base of the spit. The maximum growth rate is found for a finite length scale of the spit and a spit of a finite width is therefore expected to emerge. INTRODUCTION This study concerns the morphological development at a coast with strong gradients or even interruption in the longshore sediment transport as can be encountered at smaller islands, at abrupt changes in the orientation of the coastline or where the coast is interrupted by an inlet. In such situations a spit or a sandy hook may be formed, which is 1) Ph.D.-student, Environment & Resources (Before 01.01.2001: Department of Hydrodynamics and Water Resources (ISVA)), Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark. dp@isva.dtu.dk 2) Associate Professor, Environment & Resources (Before 01.01.2001: Department of Hydrodynamics and Water Resources (ISVA)), Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark. 3) Professor, Environment & Resources (Before 01.01.2001: Department of Hydrodynamics and Water Resources (ISVA)), Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark,

Boussinesq-Type Modeling of Sediment Transport and Coastal Morphology

A two horizontal dimensional compound model is developed to simulate coastal sediment transport and bed morphology evolution due to wave action. The wave module is a higher-order Boussinesq-type model. The bed load in the surf zone is computed from an advanced semi-empirical formula while the suspended load can be calculated through the solution of the advection-diffusion equation for the sediment or alternatively from a simplified formula. The estimation of the sediment transport in the swash zone is based on the ballistic theory. The unified sediment transport module is valid under combined waves and currents including the wave asymmetry and phase-lag effects. The bathymetry is updated through the sediment conservation equation and the morphological accelerator factor technique accounts for extended simulation time. The model is validated against a number of short-term tests in one horizontal dimension. The response is generally good with most of the morphological features being reproduced in the cross-shore direction. A comparison between various sediment transport formulae and a sensitivity analysis are also performed illustrating the need for inclusion of the phase-lag effects.

Simulating 2DH coastal morphodynamics with a Boussinesq-type model

ABSTRACT A recently developed compound 1DH numerical model for simulating coastal sediment transport and bed morphology evolution is extended and validated in two horizontal dimensions. The wave module is a higher-order Boussinesq-type model. The bed load in the surf zone is computed from an advanced semi-empirical formula while the suspended load can be calculated through the solution of the advection-diffusion equation for the sediment or alternatively from a simplified formula. The estimation of the sediment transport in the swash zone is based on the ballistic theory. Extended simulation time is achieved through the application of the morphological accelerator factor technique. The validation tests showed a good response of the model with regard to longshore sediment transport under the combined action of waves and currents. A similarly good behaviour of the said model in the cross-shore direction was presented in a previous paper. In addition, the formation of a tombolo behind a detached breakwater is adequately predicted. A sensitivity analysis is also performed studying the equivalence between regular and irregular waves, as well as the effect of various parameters involved in sediment transport and morphological models.