The purpose of this study is to quantify the cross-shore sediment transport, caused by washover flows on barrier islands in general and on the Wai San Ting sandbar in Taiwan in particular. Washover transport on a barrier island is the transport of sediment across the sandbar f r om its steeper seaward face towards the gentler sloping landward or lee-side, caused by overflowing water from one side to the other. The overwash current is generated by a water level difference across the bar, which is caused by wave set-up and/or a difference in tidal amplitude at both sides of the bar. To simulate the washover process and to quantify the sediment losses due to washover, hydraulic and morphological models are applied. For the quantification of the washover transport at specific cross sections of sandbars, a one-dimensional model is designed. This stationary wave and flow model is able to compute irregular waves, washover currents and washover transports, driven by tide and wave-induced water level differences across the sandbar. The model links the sediment transport directly to the overwash current, so that the consequences of various overwash events on the cross-shore barrier profile can be examined. The one-dimensional overwash model is verified by comparing the results with laboratory measurements and by three other mathematical models: MIKE21_HD, which computes the tide and wave-induced overwash currents; MIKE21_NSW, which computes the wave phenomena in near shore areas, and UnibestJLT, a one-dimensional model which is used to compare the wave parameters and wave set-up with the overwash model. To examine the overwash process in more detail a sensitivity study is performed. In the sensitivity study various barrier island geometries, wave and f l ow conditions are modelled and the effect of the different parameters on the washover process is examined. In the end of this thesis a case study of a regularly overwashed sandbar in Taiwan is made. The near shore wave model MIKE21_NSW and the hydrodynamic model MIKE21_HD are applied to simulate the two-dimensional wave and flow conditions over and around this sandbar. The results of this 2-D modelling are used for boundary conditions in the 1-D overwash model, which is applied to compute the washover sediment transport on a cross section of the sandbar. With the calculated sediment transport an extrapolation is made to find the yearly sand losses which contributes to the total movement of this barrier island.