Lessons Learned
The feasibility phase taught two important lessons with respect to the design of sand engines in tidal channels.
Although the sand engine succeeded in reducing the flow through the Onrust channel, the transport rate of sediment from the Onrust shoal towards land due to waves is limited in the numerical model because of the large water depth over the shoal, even at low tide. When examining similar cases, the inclusion of waves in a numerical model may only be useful if long shore transport rates are thought to be important, or the depth of the sand engine or shoal is in the order of the incident waves.
The placement of sand engines in tidal channels should be done with considerable care, since the position can greatly affect the tidal sediment transport asymmetry. In the feasibility study of this sand engine, the effect of the nourishment on the flood and ebb flows is discussed and the subsequent effect on the net direction of transport. Since the gross sediment transport rates in tidal channels is large, more or less asymmetry in the tidal sediment transport may have a larger effect on sedimentation and erosion patterns than initially intended.