Numerical Model of Beach Topography Evolution due to Waves and Currents: Special Emphasis on Coastal Structures

Detta är en avhandling från Lund University (Media-Tryck)

Sammanfattning: The beach topography change in the nearshore zone may be induced by natural phenomena such as wind, wave, storm, tsunami, and sea level rise. However, it can also be caused by man-made structures and activities, for example, groins, detached breakwaters, seawalls, dredging, and beach nourishment. Therefore, understanding the beach topography evolution in this zone is necessary and important for coastal engineering projects, e.g., constructing harbors, maintaining navigation channels, and protecting the beach against erosion. During the latest decade, advanced numerical models have been used as useful tools for simulating the beach morphological evolution. A number of such numerical models have been developed and applied through the years in many practical applications. However, the hydrodynamical and morphological processes are extremely complex in the nearshore zone and still beyond our current knowledge to describe in detail. Thus, these numerical models often include a limited set of processes characterized by certain time and space scales. Furthermore, high-quality and synchronized data sets from laboratories and the field are also limited, making model validation difficult. The overall objective of this study was to develop a robust and reliable numerical model of beach topography evolution due to waves and currents with the emphasis on the impact of coastal structures. Such a model should describe the effects of both longshore and cross-shore sediment transport over time scales from individual storms to seasonal variations. In order to facilitate this, a number of sub-models were developed and improved, including (i) a random wave transformation model, (ii) a surface roller model, (iii) a nearshore wave-induced current model, (iv) a sediment transport model, and (v) a morphological evolution model. These sub-models were coupled together and validated against detailed, high-quality data from the Large-scale Sediment Transport Facility (LSTF) of the Coastal and Hydraulics Laboratory in Vicksburg, Mississippi, United States. The obtained results predicted by the numerical model were satisfactory and in good agreement with measurements. The simulations showed that the calculated wave conditions and longshore current were well reproduced for all investigated test cases with and without structures. The calculated cross-shore current somewhat underestimated the measurements, however, it was in good agreement with observations in the lee of structures. Although the calculated wave setup overestimated observations, the absolute differences between calculations and measurements were relatively small. The predictions of beach morphological evolution under waves and currents in the vicinity of a detached breakwater and a T-head groin agreed rather well with measurements. Both salient and tombolo formation behind these structures were well reproduced by the numerical model. In the future, the model will be further validated against available data from the laboratory and the field. However, already in its current state it is expected that the model can be applied in coastal engineering projects for predicting the beach evolution in the vicinity of coastal structures with some confidence.