Streamlining river groynes

Abstract

Groynes are hydraulic structures commonly applied in the Dutch rivers. Depending on the water depth, groynes can be emerged or submerged. When a groyne is emerged, it has two main functions: maintaining the water depth in the main channel and preventing the river from eroding its bank. When the discharge in the river increases, the groynes become submerged. A submerged groyne acts as an obstacle to the flow, resulting in additional resistance and an increase in the upstream water level.

To increase the flow capacity of a river during extreme conditions, a couple of measures can be considered: increasing the height of the dikes and embankments, reducing the overall resistance of the river cross-section and increasing the area of a river cross-section.

This thesis focuses on reducing the overall resistance of the river cross-section, and more specifically, on investigating the effect that streamlining the downstream slope of a groyne has on the groyne-induced resistance. Hypothetically, streamlining the groyne by decreasing its downstream slope would decrease the intensity of flow processes like downstream flow separation, which could increase the discharge capacity of the groyne, and thus of the river cross-section, as a result of the decrease in groyne-induced resistance. An increase in discharge capacity would decrease the flood risk during extreme conditions.

To investigate the effect of streamlining a groyne by decreasing its downstream slope, a physical model experiment was set up in the 5 x 40 m^2 flume at the Hydraulic Engineering Laboratory of Delft University of Technology. The physical model represents part of a river cross-section. From the total 5 m width, the main channel and groyne field both took up 2 m and the floodplain took up 1 m. In streamwise direction, the flume contained six groynes and five full groyne fields. The groyne fields had transverse bed slope of 1:25. Electromagnetic flow meters (EMF meters) were used to measure the flow velocity at many points in both streamwise and transverse direction. The water level was measured by laser altimeters.

Decreasing the downstream slope of a groyne to 1:8 compared to a reference situation with a downstream slope of 1:3 resulted in a 4 % increase of the discharge capacity over the crest of the groyne. This increase of discharge capacity was not uniform over the transverse axis. Near the groyne tip, this increase of discharge capacity was more than 6 %, whereas at the end of the groyne, the discharge capacity decreased slightly compared to the reference situation.

A more detailed analysis of the data gave insight in why decreasing the downstream slope of a groyne increases the discharge capacity of a groyne. Streamlining the groynes resulted in a significant decrease of the relative turbulence intensity along the downstream slope of the groyne. This was consistently around 50 % lower for the streamlined groynes compared to the reference situation, indicating less intense turbulent structures. Moreover, auto-correlation functions of the flow velocity signals showed a shorter correlation length in conjunction with lower amplitudes and smaller periods of the fluctuations for the streamlined groynes. This indicates a signal with less correlation and higher frequencies. These results explain why the discharge capacity of a streamlined groyne is larger than of a reference groyne.