Analysis of the influence of the cross-shore profile shape of the surfzone on the longshore sediment transport rates

The longshore sediment transport (it will be referred also as LST) is a core issue in the field of coastal engineering. It is among the most important nearshore processes that control the coastal morphology, and determines in large part whether shores erode, accrete, or remain stable. Consequently, it is important to be able to calculate the LST rates. For this effect, there are various sediment transport formulae. One classification of LST formulations is the following: a) bulk transport formulas (assuming a simplified representation of the physical processes) and b) process-based formulas (including a large number of physical processes). The bulk LST formulas due to their simplicity are routinely used in coastal engineering studies. However, there is still great uncertainty in their estimations. One factor that may contribute to this uncertainty is the fact that the inputs to the bulk formulas are mostly wave characteristics at the edge of the surfzone, whereas morphologic features are usually not, or to a limited extent, included. The aim of this study was to analyze the influence of various morphologic features in the surfzone on the LST rates. It was investigated whether there are any significant correlations between LST rates and cross-shore surf zone features such as beach slope, the presence and number of bars and dimensionless parameters (i.e. the Iribarren parameter ? and the dimensionless fall velocity ? ) which may represent morphologic features. The process-based model UNIBEST-LT and the process-based formula Van Rijn 2004 were used. It can be assumed that reliable estimates for the actual sediment transport were provided because most of the physical processes known to influence LST are included in the models and because these models were calibrated with laboratory and field data. The influence of the tide was not included in this study. All calculations were performed using the same sediment characteristics, i.e. sand with D50 in the order of 200? m which is a typical value of the study area, the Holland coast. For the simulations, two types of cross-shore profiles were used: a) flat profiles and b) JARKUS profiles (measured profiles from the Dutch coast). The simulations with flat profiles were done using a single-wave condition climate. The range of the values of slopes were chosen according to the values of bars seaward slopes (measured from JARKUS profiles), because it is more likely that waves break over there. Generally, it was concluded that both the slope and the Iribarren parameter ? influenced the calculated LST rates in a similar way: the steeper the slope (or the higher the Iribarren parameter ? ), the higher the LST rates that were calculated. No clear trend between LST and ? was observed. However, the ? values were calculated with single wave conditions, whereas the ? is usually calculated for representative wave conditions that dictate how the cross-shore profile is. Also, some conclusions about the bulk formulas CERC and Kamphuis were made. Finally, it was studied how the slope and generally the cross-shore profile features are taken into account in the Van Rijn 2004 formula. The simulations with JARKUS profiles were done using a realistic wave climate (measured off the Dutch coast). The measured cross-shore profiles covered the area of the so-called Holland coast. No clear trend was observed between the computed LST rates and the number of bars of each cross-shore profile. Subsequently, the following slope parameters were studied: The average, the root mean square, the variance, the average downwards and the root mean square downwards slope. Significant scatter was observed between the computed LST rates and the various slope parameters. However, in all cases it was observed that the higher the value of the slope parameter the higher the computed LST. The strongest correlation was observed in the root mean square downwards slope results, while the highest scatter was observed in the average slope results. Furthermore, the variation of slope parameters (independently from the type of slope) played a more important role on the resulted LST for wave conditions of high ? (H^2.5×sin? 2a)? values than for wave conditions of low ? (H^2.5×sin? 2a)? values. Also, some conclusions about the bulk formulas CERC and Kamphuis were made. Finally, some recommendations for further research were provided (e.g. the investigation of bed and suspended load separately, the investigation of the influence of the number of bars, including their magnitudes and the investigation of the influence of the width of the surfzone or dissipation zone on LST rates). Additionally, based on the findings, it was concluded that in case the influence of cross-shore profile slope (or a related parameter) is to be implemented in a bulk LST formula, this influence (e.g. a parameter) should be related to the wave conditions: the higher the wave power (e.g. the quantity (H^2.5×sin? 2a)? ), the higher the influence of the cross-shore profile slope parameter. The implementation of such a parameter should be investigated further.