Modeling and control of algae detachment in regulated canal networks

Algae development in open-channel networks induce major disturbances because of clogging issues on hydraulic devices (pipes, weirs, filters,...). An original strategy to manage these algae developments consists in flushing the fixed algae. The flush is carried out by increasing the hydraulic shear conditions using the hydraulic structures of the canal network. In response 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. The present work aims at providing a design method for these flushes, based on a simplified detachment model. The efficiency of a flush will depend on its amplitude and duration. The design objective consists in maximizing the algae detachment using as little water as possible, and without overcoming a maximal turbidity level. We have developed a physical model to assess the impact of a flush on fixed and drift algae dynamics. We propose in this article a simplification of this model using linearization around a reference steady state regime. The simplified linear model can be used for automatic control design. The model parameters are identified on a real network: the branch of Marseilles North, part of the Canal de Provence, located in Southern France. The calibration is based on continuous monitoring of a quality parameter: the water turbidity. The calibrated model is then used to design the upstream discharge flush as an open-loop controller. Finally, flushes designed using the open-loop are tested on the simulation model, showing the interest of the method.