The effect of modifications to a groyne area in the Nieuwe Waterweg

Abstract

In the Nieuwe Waterweg an ongoing pilot takes place in one of the groyne areas. This pilot is a part of the “De Groene Poort” project. The idea for this project is to create intertidal areas by modifying and nourishing groyne areas. These artificially formed intertidal areas will create a new habitat for wildlife and attract different vegetation. For this project, several modifications have been made to the groyne area: a dam has been placed in front of the groyne area, the groynes on either side have been partially heightened and a nourishment has taken place. To better understand the effects of these modifications to the groyne area, research has been done on the effects of the tide and waves caused by shipping on the flow and sediment behaviour inside the groyne area.

Two different measurement campaigns have been carried out for this research. In the first measurement campaign six ADVs were placed in the groyne area between March 24 and April 12 ,2022. These ADVs measured the velocities and pressures with a frequency of 8 Hz. Also, sediment samples were taken during this campaign, which have been analyzed using laser diffraction. In the second campaign, ADCP measurements were carried out with a floating ADCP attached to a jet ski. With this ADCP the vertical velocity profiles have been obtained for several cross-sections at different stages of the tide.

It was found that the flow in the groyne area has changed due to the dam and the nourishment. Typical flow in an emerged groyne field with the eddies is unrecognizable during lower water levels in the modified groyne area. The exchange of water between the main channel and the groyne area is through the gap in the dam, which removes the mixing layer that separates the groyne and the main channel during submerged conditions. The flow in this groyne area is lateral and divided into two main flows along either side of the nourished island. Rocks and a higher entrance in the downstream groyne compared to the entrance in the other groyne, gives the groyne area different flow velocities between ebb and flood. The neap and spring tidal cycle enhances Ebb and flood differences in the groyne area even more. These velocities differences also impact the sediment transport and its absolute direction in the groyne area. The dam also helps reduce the wave influence on the groyne area, although these waves still significantly influence the velocities. Waves also propagate in a lateral motion from groyne to groyne through the groyne area. The water can move material from the nourishment, but the spreading of this material is limited to a number of locations in the groyne area. For the largest part, the nourished material is still at the location where it is deposited.