The Effect of Entrance Resistance on the Hydraulic Characteristics of Flow in Subsurface Drainage

In order to determine and analyze the head losses caused by the combined effect of entrance resistance and approach flow convergence to drains, some known models were studied. The field data of an existing sub-surface drainage system were applied for evaluating the models and selecting the best one. The hodograph solution was employed to compute the water head, directly above the drain, while the Oosterbaan model was used to validate the hodograph results. Total head loss between two drains, was determined using Ernst equation. The results showed that increasing total head loss caused reduction of hydraulic head in approach flow region in a non-linear manner. Application of Youngs and Hoffman equations as well as EnDrain program based on energy and Darcy equations could simulate drain discharge to some extent. Hoffman equation gave some promising results among the others in term of R (0.928), RMSE (0.0007) and SI (1.09).