Conversion from Discharge to Gate Opening for the Control of Irrigation Canals

The paper reviews several methods to convert discharge into gate opening. A control algorithm for one or several reaches of an irrigation canal sometimes uses a discharge as the control action variable even though the device to be manipulated is a gate or a weir. In this case a slave controller has to convert the discharge into a gate opening or a sill elevation in case of a weir. This is usually done by inverting the static relation between discharge and gate opening. An improved method can be based on the characteristics theory to estimate the water levels deviations. However, both methods underestimate the gate opening deviations required to deliver a desired discharge deviation, because water levels vary continuously along time when the gate is operated. The paper proposes a method to take account of this dynamic behavior of the pool-gate interaction by using a simple linear model for the pools dynamics, the IDZ model. The proposed method enables to better estimate the gate opening necessary to get a desired average discharge. The method is evaluated in simulation and on a gate of the Gignac Canal, located in the South of France. A dimensionless analysis of the problem is finally performed to evaluate the methods’ applicability. Introduction In the design of controllers for irrigation canals, it is well-known that using the discharge as the control variable enables to partially decouple the pools (Malaterre, 1995; Schuurmans, 1997; Malaterre et al., 1998; Malaterre and Baume, 1999). With such a choice as control variable, one needs to use a master-slave structure, where the master controller computes a discharge that is delivered by a local slave controller operating the gate. The functioning of this local slave controller is rarely mentioned in scientific publications, because it appears at first sight as a simple problem. Indeed, if the water levels upstream and downstream of the gate are measured, it seems sufficient to invert the gate discharge equation in order to compute the required gate opening to let a desired discharge flow through the gate. However, this is only Researcher, Unité Mixte de Recherche, Gestion de l’Eau, Acteurs, Usages, Cemagref, B.P. 5095, 34196 Montpellier Cedex 5, France. e-mail: [email protected] Researcher, Unité Mixte de Recherche, Gestion de l’Eau, Acteurs, Usages, Cemagref, B.P. 5095, 34196 Montpellier Cedex 5, France. e-mail: [email protected] Hydraulic Engineer, Unité Mixte de Recherche, Gestion de l’Eau, Acteurs, Usages, Cemagref, B.P. 5095, 34196 Montpellier Cedex 5, France. e-mail: [email protected] Assistant Engineer, Unité Mixte de Recherche, Gestion de l’Eau, Acteurs, Usages, Cemagref, B.P. 5095, 34196 Montpellier Cedex 5, France. e-mail: [email protected] 1 Journal of Irrigation and Drainage Engineering, 2008, 134(3):305-314 author-produced version of the final draft post-refeering the original publication is available at http://scitation.aip.org doi: 10.1061/(ASCE)0733-9437(2008)134:3(305) ha l-0 04 53 78 5, v er si on 1 5 Fe b 20 10 Author manuscript, published in "Journal of Irrigation and Drainage Engineering 134, 3 (2008) p. 305 p. 314" DOI : 10.1061/(ASCE)0733-9437(2008)134:3(305)