Compound weirs

Abstract

Two control structures in the form of compound weirs were built near Pannerden and in the Hondsbroeksche Pleij. These compound weirs consist of multiple adjacent gates with individually configurable weir heights. To make optimal use of the flexibility of the structures, research must be done into how different compound weir configurations affect the stage-discharge relationship. Both perfect and imperfect flow are researched.
This thesis has the following research question: “How is the flow over a compound weir affected by the configuration of the individual weirs?”
The research on imperfect weirs was caried out by constructing two analytical models based on a combination of momentum- and energy conservation, and a combination of Carnot losses and energy conservation. This was done for two cases, one case in which the equations were solved explicitly by using the average weir height, and the second case in which the equations were solved by solving a system of equations (one for each gate). The research on perfect weirs was caried out by making use of the Rehbock formulation, again for both the individual weir heights and the average weir height. To validate the models and to research interaction between adjacent gates, experiments were carried out in a 3-meter-wide flume with a scale model of the control structure near Pannerden.
In the experiments it was found that in the case of perfect weir flow, the discharge coefficients hardly change for different configurations with the same average weir height. The recorded maximum change was 1.9%. The best performing model for perfect flow was the model based on the average weir height. In the case of imperfect flow, it was found that the discharge coefficient can vary up to 13%, so the configuration influences the discharge coefficient significantly. No unambiguous results were found on how certain configurations affect the discharge coefficient. The models for imperfect flow performed well for an average weir height of 10 cm, but not for an average weir height of 7.5 cm. The least performing model was the model based on individual gates with the momentum equation.
With PIV (Particle Image Velocimetry) it was found that lateral flow starts at a greater distance from the weir if the lateral travelling distance is bigger. Furthermore, the streamlines hit the weir at an increased angle if the transition between high and low weir heights was sudden instead of gradual.
Further research should focus on adding more forms of energy head loss to the analytical model. Focus should be on one specific average weir height and more data should be collected on fewer configurations to get more insight into the behaviour caused by specific properties of a configuration.