Influence of roughness changes on offtaking discharge in irrigation canals

The paper shows how a change in irrigation canal roughness can influence the flow passing through offtakes. HEC-RAS steady state model was calibrated with changing Manning's “n” and used for flow simulation in a modern irrigation system in Iran. Results showed that the Manning's “n” obtained from the model calibration procedure (0.025) was different significantly from its primary value (0.015) along the canal. The increase was due to poor canal maintenance. The changes in roughness of about − 20%, − 40%, 40%, and 100% of the primary value can significantly influence the upstream water level and the discharge passing through some offtakes. In order to better evaluation and comparison of offtakes, sensitivity indicator “offtake sensitivity to roughness” was introduced. An analytical approach helped us to define an equation for direct estimation of the sensitivity indicator under steady state uniform conditions for trapezoidal canals. Evaluation of water level fluctuations and the sensitivity indicator shows that only two of the offtakes are sensitive to roughness changes because of being located at the upstream of submerged cross-regulators. It can be concluded that the effects of roughness changes could be more important for offtakes located far from the downstream cross-regulators or near the submerged cross-regulators.