Morphology of Pits, Channels and Trenches Part III: Investigation of the longshore and cross-shore impact of various pit designs

For several years the large-scale mining of sand from the Dutch Sector of the North sea is in discussion related to the need of sand for shoreface, beach and dune nourishment and large-scale engineering works at sea (Maasvlakte extension, airport at sea). The mining methods considered, basically fall into two categories: wide, shallow or small, deep mining pits. Presently, most sand mining pits with a limited depth, not deeper than about 2 m, are excavated beyond the 20 m depth contour. Deep mining pits have not yet been made extensively. One of the large uncertainties is the morphological response of sand mining pits in different water depths. Although sand mining is only allowed beyond the 20 m depth contour, an onshore migration could results in an increasing morphological response. As a consquence this could adversely affect the stability of the upper parts of the coastal zone. The stability of sand mining pits at deeper water depths is the main focus in this study. From the comparison between a morphodynamic area and 2DV model (Delft3D and Sutrench) it was concluded that the main differences found in the model simulations are caused by the fact that flow contraction was not accounted for in the Sutrench simulation. On the relative small time scale of 1 year, over which this comparison was made, this primarily results in a different morphological development of the pit slopes. However, as the backfilling rates are over-estimated in the Sutrench simulations it is expected that this will result in an overestimation of the migration rates of the pit and an under-prediction of the morphological time scale of pits where flow contraction plays an important role. It emphasises the inherent limitations of the 2DV concept of Sutrench. The Sutrench model can be used to obtain a first order estimate of the occurring bottom changes. However, reliable predictions can only be made if flow contraction is taken into account. Both the longshore and cross-shore morphodynamic simulations have shown that the depth at which pits are constructed has a large influence on the pit stability. Moreover, if the uncertainty of the boundary conditions and model parameters is taken into account, the accuracy ranges show a significant increase for shallower pits. This implies that model predictions for pits in shallower water are associated with an increased uncertainty.