Getting Stormwater Off the Main Span

If you’ve driven over the main span of the new bridge, you likely noticed the camber (i.e. arched shape) of the main span, which is slightly higher in the middle than at the approach structures. What you may not have noticed was the virtually imperceptible slope of the bridge deck that angles from the median toward the outside edges to enable stormwater run-off during rain.

While dealing with stormwater run-off may not seem like a big deal, the 2,000-foot length of the main span means that – when a severe rain event occurs – water could build up fast, and it can’t remain standing on the road surface for safety concerns. Because of environmental regulations (stormwater from a road surface will have oil and grease contaminants) and safety protocols, stormwater cannot be discharged directly into the channel, 205 feet below. Run-off has to be captured and piped to a treatment facility.

The above photo shows part of the deck drainage system. The smaller “90-degree” elbow connects a grated drain box to the main drainage pipe, the bigger pipe. That main pipe collects stormwater from drain boxes located along the deck overhang and transports it to one of the two Austin sand filter vaults on both sides of the Bridge Project. Those vaults are built into the ground and are the size of a large house. Stormwater enters them, then slowly filters through the sand and other media to remove the contaminants.

One fun side note, to prevent rain water from landing on the cable stays then dropping hundreds of feet onto your vehicle with a loud thwack, little ridges curve around the white casing of each stay to enable the water to safely travel down the stays in a spiral path and drop safely.