Simulation of Water Flows in an Agricultural Drainage System with Hydraulic Structures

Dynamics of surface water is rationally understood as transport phenomena of mass and momentum, and the cross-sectionally averaged shallow water equations are commonly applied to one-dimensional open channel flows. This study develops an efficient numerical model based on the shallow water equations to simulate water flows in an existing agricultural drainage canal collecting runoff from paddy fields. Hydraulic structures installed in the canal include a wooden overflow weir, hydraulic drops, and a by-pass system immediately downstream of a hydraulic drop creating different micro aquatic environments. The water in the canal originates from a perpetual spring and from the paddy fields which are ephemeral water bodies. The resulting agricultural drainage system is modeled as a multiply connected (looped) locally one-dimensional open channel network consisting of seven reaches and two volume-less junctions with lateral inflows from the paddy fields. The numerical scheme employs the standard Galerkin method for the continuity equation and a simple cell-centered finite volume method for the momentum equation so as to consistently handle the junctions as implicit internal boundary conditions. The water balance in the paddy fields is also simulated to generate the runoff, which is regarded as the lateral inflows into the canal. Steady and unsteady flows in the agricultural drainage system are successfully computed with assumed model parameter values. A transcritical flow and a still water area in the by-pass system are reproduced in the steady flow under the condition of constant water depths in the paddy fields, agreeing well with field observations. The unsteady flow computed for a heavy rainfall event indicates a significant flood mitigation effect of the paddy fields.