Bridge Deck Drainage Guidelines

Bridge Deck Drainage

• Bridge drainage design directs surface rainwater and snow melt toward inlets or openings -- scuppers -- in curbs or barriers. The number of inlets and scuppers needed is set according to calculations for the amount of runoff predicted for the bridge. The surface is usually sloped longitudinally and to the sides, to ensure water flows from the traveling portion. The runoff generally discharges to either an open or closed system, depending on site conditions, location of the bridge, and environmental concerns.

Open Drainage System

• Storm water free falls through the inlets in the open system. This is sufficient when the location is suitable, and is often used when the bridge spans water. When the water body is large, the amount of storm water generated from the bridge is not usually considered harmful to the waters. Free-falling discharge isn't allowed onto travel lanes or sidewalks under the bridge, or a railway right-of-way, and erosion measures are needed for certain conditions.

Closed Drainage System

• Where there are concerns about aquatic habitat or water quality, or where bridges are located over other roads, highways or railway tracks, a closed drainage system is used. Storm water enters pipes installed beneath the deck, sloped to maintain a prescribed velocity and carry water to an acceptable discharge point, such as the municipal system. The material for the pipes is usually specified as galvanized, or metal that resists rust, such as stainless steel.

Bridge Deck Design and Maintenance

• Design for bridge deck drains needs to be coordinated with the structural engineering, so that the drainage inlets and scuppers are located without conflict with concrete reinforcement or structural members, where water can corrode the reinforcement, or cause condensation or leaking. Pipes for a closed drainage system may be contained within a facade for more pleasing aesthetics. Grates are used to keep drainage openings clear of obstructions and debris, and access for maintenance tasks has to be considered.