The topic which is considered in this thesis is: understanding the influence of beach morphology on the alongshore variance in wave run-up on an intermediate reflective beach, considering bars and cusps. The focus of this thesis is laid inside the swash zone, in which the water motion is present of waves that run up and run down on a beach. Energy from wave run-up could deliver erosion to the beach. It is relevant to know what the magnitude is of run-up during extreme events, in order to protect the beach. Several studies are done to wave run-up. There are relations which specify run-up, however, the alongshore variability is not studied in detail and less knowledge is available about this topic. At Anmok beach in South-Korea an intermediate reflective beach is present containing beach cusps and crescentic sandbars. A rhythmic bar and beach state contains the most complex morphology, furthermore the morphology changes a lot within intermediate beaches (Wright and Short [1]). The characteristics of this beach are used to perform an analysis to the influence of cusp and bar morphology on alongshore variation in wave run-up.

Run-up is composed of setup and swash. Setup is the super elevation of the mean water level, swash is ‘’a time-varying location of the intersection between the ocean and the beach’’ according to Stockdon, Holman [2]. Swash can be decomposed into two parts, incident band swash and infragravity band swash. Swash and setup depend on beach slope, deep-water wave height and the deep water period[2].

To analyse the alongshore variability multiple bathymetries have been generated on which 500 waves are modelled for 60 different wave condition. First of all a reference case is modelled with a uniform bathymetry. Secondly beach cusps are used as input with different length scales and at last a beach cusp with crescentic sandbar is used. The length scales of the cusps are 452, 300 and 100 metres. The bathymetries are idealized bathymetries with the characteristics of Anmok beach. The run-up and components are calculated and an analysis is done to the magnitude of run-up and the standard deviation along the beach.

The magnitude of run-up is lower for a cusp system compared with the reference situation and even lower for the cusp bar system. Furthermore there are no large differences in magnitude of run-up between different cusp lengths. A larger alongshore variance is observed when a cusp (bar) system is present. A cusp system of 452 metres contains larger run-up at the horn compared with the embayment, this holds for large and small wave heights. The difference is 18% and 8.4% respectively. However, when a cusp bar system is present less alongshore variability is visible and an opposite behaviour is visible for small wave heights. In this case the same pattern can be seen for large wave heights. A difference of 3.68% is seen when the horn is compared with the embayment. However, for small wave heights the run-up is 10.5% smaller at a horn compared with an embayment.

A cusp length of 100 metres shows different behaviour compared with a cusp length of 452 metres. Run-up is larger at an embayment compared to the horn. This holds for large wave heights. The alongshore variance is in this case larger compared to larger cusp lengths. A cusp of 452 metres and 300 metres leads to similar results, whereas a 100 metres cusp shows deviations. It could be edge waves which could have an influence on a cusp with a length of 100 metres.

A very common observation is the episodic erosion of beaches during storms and the slow recovery (accretion) afterwards (Yates et al. 2009). Morphodynamic models parameterize physical processes in order to relate the fluid motions (hydrodynamics) to the bed level changes (morphodynamics) over a wide range of spatial and temporal scales. Despite recovery of the beach profile being a slow process, accretion mechanisms in the swash zone are complex to represent by a numerical model due to the shallow, rapidly varying flows and high concentration gradients (Brocchini & Baldock 2008). The swash zone on most beaches is readily accessible, but this accessibility does not translate into a broad knowledge of the underlying physical processes (Chardón-Maldonado et al. 2015). This research focused on assessing the representation of physical processes in the swash zone of intermediate-reflective beaches during erosive and accretive conditions in the non-hydrostatic version of the XBeach model (XBeach). This is a depth-averaged phase resolving model, mainly used for calculations of storm impact (hours-days) on sandy and gravel beaches. Based on conclusions in literature, relevant physical processes that contribute to accretion were determined. Using a simple planar beach bathymetry, the sediment transport formulations in XBeach and the individual influence of groundwater effects, bed slope effects, sediment response time and wave breaking induced turbulence were assessed. The results showed that for both accretive and erosive wave conditions, XBeach predicts erosion in the swash zone. Groundwater infiltration and wave breaking induced turbulence are likely to enhance onshore transport significantly Reniers et al. (2013), Turner &Masselink (1998). To verify the findings of the planar beach modeling approach, in the second part of this thesis the morphodynamical predictions of XBeach were compared to the dataset collected during the Bardex II experiment. Bardex II was performed in 2012 in the Delta Flume in the Netherlands and the dataset contains observations of a series of experiments focusing on the effect of varying wave, sea level and beach groundwater conditions on a sandy beach (D50=0.42 mm)(Masselink et al. 2013). Stored data and published resultswere used for comparison with the morphodynamical prediction performance of XBeach. Two timescales have been analyzed, the total morphological response of the beach on a timescale of 100 minutes and the intra-swash sediment transport processes on a timescale of 10 seconds. Two experiments from the series have been reproduced. The first, experiment A4, has an almost stable, slightly erosivemorphological response throughout the swash zone. The second, experiment A8, shows accretion in the upper swash and a stable profile in the rest of the swash zone. XBeach erroneously (over)predicted erosion above the mean sea level (MSL) for both A4 and A8 conditions. This conclusion was related to the results of the intra-swash sediment transport assessment. The modeled velocity in both uprush and backwash were higher than the Bardex II measured velocity and XBeach extremely underpredicted uprush sediment concentrations suggesting that turbulence induced by the bore is not enough taken into account. Over-predicted backwash sediment concentrations for both accretive and erosive conditions suggested that groundwater infiltration was not strong enough. However, enhancing wave breaking induced turbulence and groundwater infiltration did not lead to an improvement of the predictions of sediment concentrations in the swash. The sediment transport formulations of XBeachwere developed using a long wave resolving (short wave averaging) model, andwidely validated and calibrated on field and experimental data (Soulsby 1997, van Rijn et al. 2007, Van Thiel De Vries 2009). Therefore, in the last part of this thesis two possible improvements of the two sediment formulations of XBeach (van Thiel-van Rijn and Soulsby-van Rijn) when applying them in a short wave resolving model are assessed using a 1D sediment transport model. The decomposition of the velocity signal in amean and a fluctuating part improved mainly the predictions using Soulsby-van Rijn where the separate calibration of turbulent kinetic energy resulted in better predictions for both transport formulations. This analysis was performed only at one point on the cross-shore domain (slightly above MSL). Although the results for this position were promising, comparison of modeled sediment transport with Bardex II observations at different positions throughout the upper and lower swash zone is needed to give a full validation of the proposed adaptations to the transport formulations.