Technical Note: Calibrating Agridrain Water Level Control Structures Using Generalized Weir and Orifice Equations

Abstract

Control structures have been used for drainage management in subsurface (tile) drainage. In these structures, the outlet level can be adjusted manually or electrically. To determine flow rates through these structures, we developed weir equations for manual, and an orifice equation for automated water level structures. These discharge equations were based on measured flow rates and water depths in AgriDrain structures ranging in size from 152 to 610 mm (6 to 24 in.). For each structure, a two-parameter equation for weirs with end contractions was used for low flow rates, and a one-parameter equation at higher flow rates, with the transition occurring at the point of inflection of the two-parameter equation. The fitted equations for 152-mm structures, for 203- to 610-mm structures, where Q is the flow rate (L/s), L (cm) is the width of the gate, and H (cm) is the flow depth above the gate. The discharge equation for the orifice-type opening in the structure tested was with head measured relative to the bottom of the structure, where Q is in L/s, and the units of H0 and H are cm. H is equal to H1 for unsubmerged conditions, and to the difference between H1 and the depth of submergence otherwise. This equation is consistent with the standard orifice equation if the head is measured relative to the center of the gate. These proposed equations can be used for the determination of tile flow rates where AgriDrain drainage control structures are installed, eliminating the need for additional flow measuring devices.