Long-Term Simulation of Coastal Evolution

University dissertation from Water Resources Engineering, Lund University

Abstract: There is a strong relationship between regional sediment transport and the local processes at tidal inlets and around coastal structures. Many coastal projects require quantitative understanding of these processes and the interactions between them. Regional sediment transport and shoreline evolution models that fully include important coastal processes at local scale are lacking at present. The overall aim of this thesis is to develop a new numerical model of regional sediment transport and shoreline evolution for complex conditions with a simulation domain that may extend over hundreds of kilometers and cover several inlets and river mouths including development of flood shoal and ebb shoal complexes, shoreline response in the vicinity of inlets, barrier elongation and different shore protection measures.

The new numerical modeling tool is composed of a one-line model of shoreline change, an inlet reservoir model, and a spit growth model. The shoreline change model was based on the one-line theory (Pelnard-Considere, 1956), employing algorithms for the numerical solution developed by Hanson (1987) and including regional shoreline features introduced by Larson et al. (2002a). The inlet reservoir model is based on a reservoir analogy approach developed by Kraus (2000, 2002) and then refined by Larson et al. (2006) through the introduction of the flood shoal and associated coupling coefficients describing the transfer of sediment between the morphological units. A mathematical model of spit growth and barrier elognation supplied by sediment coming from the longshore sediment transport (LST) was developed based on the spit growth model suggested by Kraus (1999).

In order to realistically reproduce the coastal evolution at local scales, an attempt was made in this study through the introduction of several new methods and modifications to improve the above mentioned models. Fine grain size sediment lost into the deep water was included in the shoreline change model through a loss parameter combined with gradients in the LST rate (Donnelly et al. 2004). Wave sheltering effects from the bars was included in the empirical formula for LST through an attenuation parameter for breaking wave height in lee of the bars. Direction of ebb jet at inlets was taken into account in the empirical formulas for the distances from the inlet to the downdrift and updrift attachment bars. Onshore movement of sediment from attachment bars was also included in the inlet reservoir model. For the spit growth and barrier elongation model, a relationship between maximum depth of the channel and the depth of active LST was used to estimate LST rate bypassing the spit.

The model was validated against measurements at Hai Hau beach in Vietnam, at Long Island coast in the United States, and at the Badreveln spit in Sweden. At the Hai Hau beach, a 20-year time series of offshore waves hindcasted from the recorded wind data was used to reproduce the nearshore wave climate; measured shorelines in 1910, 1965, and 2000 were employed to calibrate and validate the model. At the Long Island coast, hindcast wave data of 20-year time series from three WIS stations along the coast was used as input for the model; measured shorelines in 1933 and 1983, measured volume growth of the flood shoals and ebb shoal complexes at several occasions between 1931 and 1998, net longshore transport estimated from measurements, and measured barrier elongation, were used to compare with the modelled simulations. At the Badreveln spit, measured spit locations at a number of occasions from 1860 to 1994 were used to compared with the analytical solution of the model.

The model simulations were generally in good agreement with the measurements. The modified formulas and new methods introduced by this study were performing well at these study sites. The modelled results show that fine sediment lost into the deep water and gradients of the LST are main causes of the severe erosion at Hai Hau beach; onshore sediment transfer and wave sheltering effects from the attachment bars are main contributions to produce the salient-type feature in the downdrift areas of the inlets; dredging work to maintain the navigational channel at the Fire Island Inlet located at the south end of the Long Island coast is the main reason for the barrier stable period connected with the period of dredging from 1954 to 1994. The model applications show the capability of the model to simulate regional sediment transport and shoreline evolution for complex conditions including several inlets and river mouths, different coastal protection measures, barrier elongation, and shoreline response in the vicinity of inlets.