Physical Model Research on Energy Dissipation Downstream of the New Weirs of Denderleeuw in Belgium

The weir lock complex of Denderleeuw is situated on the river Dender in Belgium upstream of the city of Aalst. The renovation of the weir lock complex encompasses the building of two new weirs approximately 55 m upstream of the existing weirs. The new weirs consist of steel-rubber gates with a width of 7 m. Downstream, the weirs of Teralfene will be abolished. This will result into a lowering of the downstream reference water level with 0.56 m and into an increase of the head of the new weirs to 2.76 m.

To investigate the energy dissipation downstream of the steel-rubber gates a physical model of the new weirs on scale 1:12 was built in a current flume at Flanders Hydraulics Research. Physical model tests were carried out for three different hydraulic conditions consisting of a combination of discharge, upstream and downstream water level.

Lowering of the downstream bottom level as well as the presence of a stilling basin were tested as counter-measures to fixate the hydraulic jump immediately downstream of the new weirs. The tests included three different downstream bottom levels, two different bottom levels and end slopes of the stilling basin and the presence of energy dissipation blocks in the stilling basin.

During the tests, the discharge, the upstream and downstream water level and the velocity above the bottom in six locations downstream of the new weirs were measured. At the same time video cameras near the model recorded the current pattern downstream of the weirs. By means of image processing techniques the location of the water line was determined from the recorded current pattern. All these measurements were used to compare the different tested configurations.

The results of the physical model tests let conclude that both the lowering of the bottom level downstream of the new weirs and the presence of a stilling basin are good counter-measures to fixate the hydraulic jump immediately downstream of the falling jet. The near bed velocity downstream of the stilling basin is influenced by the bottom level and the slope of the end sill of the stilling basin. The presence of energy dissipation blocks in the stilling basin shortens also the length of the hydraulic jump.