Baffle Chute Design Protects Resident and County Infrastructure

Leavenworth County, Kansas wanted to add value to their community by upgrading one of their rural, gravel roads. Together, we smoothed out the road’s profile, making it less hilly and improved it to an asphalt surface. Along with designing these elements, the Affinis team was tasked with improving the stormwater conditions. The road crossed a creek in some areas, which flowed downstream to residents’ properties.

The county wanted to add a larger RCB to manage stormwater, in keeping with their standards. With the new design, water would no longer pass over the roadway. Instead, it was all directed to flow through the box. Increased flow combined with an 18-foot vertical drop from one side of the road to the other would result in substantial exit velocity. It had the potential to cause erosion in not only the right-of-way, but near residents’ properties.

When water comes out of a pipe or culvert too fast, it can cause scour. Depending on the velocity or duration, water can wash away gravel, rock, creek beds, etc. Whatever it is pointing at needs to be reviewed for stability. Our goal was to slow down the water, returning it to its semi-natural state, so it wouldn’t compromise the streambank. Typically, we use rip rap, or rock, to reinforce an outflow, but the project’s steep geometry would have been unfeasible with rip rap.

Given the site constraints, we selected the U.S. Bureau of Reclamation’s design for a baffle chute. Using guidance from HEC 14, Hydraulic Design of Energy Dissipators for Culverts and Channels, we were able to slow down the water in a short distance. In a low or typical rain event, tiny flows ping pong through the structure’s baffles, the concrete rectangular shapes shown here. In large flows, the water churns and sprays upward, after hitting the baffles. The churning and spray is the energy loss necessary to mitigate the acceleration due to gravity.

The design strategy used on 147th Street improves the roadway for residents, while still protecting the county’s natural assets. With the baffle chute, we met our goal to meet the existing outlet velocity and protect the roadway from larger storm events. Solutions, like this one, help move communities forward.