Hybrid Drainage Design for Highway Underpass

Drowning fatalities in traffic accidents often occur at roadway sag curves or depressed sections. An increasing concern is how to quickly and safely remove the stormwater from low points of urban area roadways. In accordance with the green concept of Low-Impact-Development, in which a drainage system is designed to pass the full spectrum of runoff flows, this paper presents a new algorithm to improve the highway underpass drainage design for both safety and hydraulic efficiency. It is recommended that the underpass drainage facility be composed of two systems: an underground sewer line be sized to collect the minor storm event through gravity and a pump system be prepared to lift the excess water during a major storm event. Although the pump system in practice may be a permanent station or a mobile unit, the pump system prepared for a highway underpass must be documented in the emergency operational instructions. To design such a drainage system for multiple storm events, all individual drainage elements are to be sized individually, and then the entire drainage system must be evaluated under the tailwater effects. In this paper, a design example is presented to illustrate how to apply two hydrologic methods, including the rational method used to size the sewer pipes and sump pumps and the EPA Storm Water Management Model 5 was adopted to verify the performance of the proposed system under minor and major storm events, respectively.