Turbidity management during flushing-flows: A model for open-loop control

Fixed algae developments induce strong constraints for the management of open-channel networks. They cause clogging issues on hydraulic devices and can sometimes lead to water quality alteration. An original strategy to limit the algal biomass is to carry out regular flushes. A flush is performed by increasing the hydraulic shear conditions using the hydraulic structures of the canal. Consequently to the shear stress increase, a part of the fixed algae is detached, then re-suspended into the water column, and finally transported into the canal network. This leads to a peak of turbidity that needs to be controlled. The present paper proposes a quasi-linear model of the turbidity response to a discharge increase, that can be used for automatic controller design. The model parameters are identified on a real network. The calibration is based on continuous monitoring of water turbidity. Flushes are simulated on the whole branch and on an intermediate reach in order to test the ability of the model to simulate the propagation of a turbidity peak. Then, the model is used to develop an open-loop controller of turbidity for flush design. The efficiency of a flush will depend on its amplitude and duration. The design objective consists in maximizing the algae detachment without exceeding a maximal turbidity level, and using as little water as possible. The designed flush is finally tested on a nonlinear model.