Design of an Orifice and Weir Outlet for Poorly Drained Forested Watersheds

Abstract

Orifice-weir structures at ditch outlets are being used to reduce peak drainage rates and to store water during the growing season in poorly drained managed pine plantations. Earlier studies have shown their effectiveness on reducing drainage outflows while conserving water during the growing season. This study reports on criteria and preliminary guidelines for designing such outlets. The guidelines were developed using both observed data and DRAINLOB (a forestry version of DRAINMOD) simulations for multiple combinations of outlet sizes and four different poorly drained organic and mineral soils. Main objective functions chosen were reducing frequency of peak drainage rates and wet days with water table within 30 cm from the surface, and maximizing the days water table were within desired range of 45 to 120 cm depth during the growing season. Results show that the size and the depths of the orifice and the weir at the outlet primarily depend upon the main objective function chosen followed by the soil type and drainage area of the watershed. Wet days increased with smaller orifice sizes and shallower weir depths, Analyses showed that for the same drainage density and area, a larger orifice may be required in the organic soil to achieve the same reduction in wet days as in the mineral soil.

Citation

Amatya, D.M.; Skaggs, R.W.; Hughes, J.H. 1999. Design of an Orifice and Weir Outlet for Poorly Drained Forested Watersheds. Written for presentation at the 1999 ASAECSAE-SCGR Annual International Meeting Sponsored by ASAE Toronto, Ontario, Canada Paper No. 99-5049