Coastal erosion is expected to increase in many European countries owing to a rise in sea level. This will lead to an increased demand for sand for the nourishment of beaches and dunes. Mining of sand from the middle and lower shoreface (depths of 10 to 30 m) in large-scale mining pits will be required to meet this increased demand in future. It is expected that large-scale mining through the dredging of artificial sand pits or the removal of existing sand banks will have a significant impact on the nearfield and far-field morphology (increased coastal erosion). To this end, the Delft3D model was upgraded using state-of-the-art transport formulations and was verified using a number of field and laboratory experiments. These tests were used to establish the efficacy of morphodynamic models in predicting the behaviour of large-scale mining pits so as to contribute to improved coastal zone management. A typical validation result is shown below (simulation performed with default parameter settings).
The validation results presented in this study have shown that the model is able to produce reliable predictions of the morphological development of pits and trenches in deeper water if the undisturbed transports are scaled to measured or realistic values. The Brier skill scores indicated that the performance of the model was ' excellent ' when TR2004 transport formulations were used. Furthermore, it was shown that the TR2004 predictors for bed roughness and the suspended sediment diameter resulted in accurate and robust predictions of the sediment transport (always accurate within about 50%). Although experience with the updated Delft3D-Online model is still limited, we concluded that at its present status, the model may be applied with confidence to model the morphological behaviour of pits/trenches and to assess the near field and far field impacts. However, sensitivity studies have shown that the final predictions may be significantly affected by small inaccuracies in the applied hydrodynamic forcing conditions and a number of model settings.
Sand extraction is often performed at sandbanks (tidal banks, shoreface connected ridges, head banks) due to their composition and their location. To assess the impact of such mining activities it is imperative to monitor abandoned sand extraction locations to investigate recovery of the sandbanks. However, the associated morphological time scales at which these recoveries typically take place are on the order of decades, which imposes a severe limitation for the implementation of regulations. It is therefore vital to improve and validate existing models to obtain insight into the reliability of the predictions made with numerical models. To this end, the Delft3D model was applied to an artificial sand ridge about 6 km off of the Dutch coast which has been monitored for over 20 years. The model was found to be capable of providing a good estimate of the observed morphological development. |
