The use of pile groynes to reduce sediment exchange between river and harbour

Abstract

The use of pile groynes to reduce sediment exchange between river and harbour Introduction To reduce siltation in harbours located in rivers or waterways in order to decrease the high costs of dredging and maintenance of its basins, a need of searching measures to decrease the sediment exchange between river and harbour arises. Research One of the measures to reduce the sediment exchange is the use of pile groynes structures located in the river stream, upstream the entrance of the harbour basin. Due to the blocking of the groynes, the velocities of the river will increase. Because of this, the navigability along the river stream can be altered. The objective of this research is to determine the effect of the groyne at the interface harbour-river and the mixing layer which develops along the entrance. Experiments To analyze the effect of the groyne on the mixing layer dynamics, a schematized physical model of a harbour basin was built in a laboratory flume. Different configurations of the pile groyne were performed to devise the effect of different geometries of the groyne. In order to relate the exchange process to velocity distributions, Particle Tracking Velocimetry measurements were performed to obtain the velocity field at the water surface. The analysis of data was approached on the exchange which takes place via the mixing layer and the primary gyre, that are formed at the river-harbour interface and the harbour basin, respectively. On the basis of the experimental results can be concluded that the best configurations to reduce the entrainment of water and the velocity gradient along the entrance are the groynes located close to the upstream corner and with spacing between piles lower than its width. Numerical modelling Several configurations of the groyne were simulated using state of the art of a 2D numerical model, FinLab, in order to perform a comparison between experiments and simulations, providing an indication as to what extent the exchange processes are properly incorporated into this model. A good estimation can be obtained for the case without groyne in relation with the basic structures and turbulence properties of the mixing layer at the interface river-harbour. However, it was not possible to simulate the effect of the groyne by increasing the roughness coefficient at the groyne location. For this reason, some recommendations for further studies about that topic are given.