Out of sight most of the time, we expect culverts to carry flow beneath the roadways, whether in drought or in flood. As long as they do their job, most of us never give culverts a second thought.
IIHR Research Engineer Marian Muste thinks about culverts a lot. Culverts in Iowa are frequently blocked by sedimentation and vegetation, which obstruct the flow and can cause the culvert to overtop during storms, producing local flooding and property damage. Clean-up of the culverts is costly in terms of time and effort, and needs to be repeated often at many Iowa sites.
Muste and his team have developed a culvert design that prevents blockage by sedimentation and vegetation. Muste says his goal was to create a self-cleaning system that would flush out the sediment deposits using the power of the stream flow itself. The solution does not require maintenance and can be implemented at the time of construction or retrofitted after the culvert is built.
Muste’s work began in 2009 with a study designed to understand the mechanics of the sedimentation process at culverts for various flow events. The problem, he said, was that although midwestern multi-box culverts are frequently blocked by sediment deposits, the mechanics of the sedimentation process (when and how it occurs) had until recently barely been investigated. Moreover, Muste wasn’t able to find an existing solution in the literature or elsewhere, so he set out to create a new solution that would meet the Iowa Department of Transportation’s (DOT) requirements.
Muste and his team used a triple set of tests to find a working self-cleaning culvert design. First, he built a 1:20 scale three-box culvert model to test the new designs. Next, he used numerical simulations to refine the geometry of the design and test it under a range of flow conditions. Finally, Muste and his team built a 1:5 scale model to assess the performance of their selected design.
He says they developed a self-cleaning design that proved its reliability and efficiency through an extensive series of tests. Rounded segments or fillets are set on the stream bottom immediately upstream of the culvert. The fillets slope toward the banks in an attempt to bring the culvert site back to its original shape. By doing so, the fillets direct the sediment through the central channel and restrict the extent of low-velocity areas on the stream side that are prone to sedimentation, while maintaining the effectiveness of the flow conveyance over a range of flows.
The Iowa DOT has implemented Muste’s design as a culvert demonstration project on Highway 1 in Iowa City. Before deploying the new design, the team cleaned and monitored the culvert for two years in order to have baseline data for the demonstration. They used an Iowa Flood Center stream-stage sensor to provide real-time data on water levels, and a web camera to continuously record the culvert’s operation. Layers of sediment began to accumulate upstream from the two side channels almost immediately after it was cleaned.
Two years ago, the culvert was cleaned again, and the team installed the self-cleaning retrofit. The Iowa DOT and the city of Iowa City chose a new lining material for this purpose, concrete cast using flexible fabric forms. The revetment material consists of a “mat” filled with concrete, which results in a web of narrow flat moldings and raised ridges on the stream floor just upstream of the culvert. Muste’s team continues to monitor the culvert with special attention to sedimentation.
Since the self-cleaning design was implemented, Muste’s team has observed impressive improvement. “It is working very well,” Muste says. After the deployment of the new design, the sedimentation has almost disappeared. “We are very pleased to see that the culvert is now nearly ‘squeaky clean,’” Muste says.
The success of the new design has attracted the attention of the road hydraulics community and prompted an invitation for Muste to present a keynote lecture at the recent National Hydraulic Engineering Conference held in August 2014 in Iowa City.