Testing Bottomless Culverts

Bottomless, or three-sided, culverts offer promise as economic and environmentally friendly alternatives to traditional closed culverts. The Maryland State Highway Administration and the Federal Highway Administration have partnered to study the effects of scour on bottomless culverts. Laboratory models were developed to compare results from a simple rectangular shape to the results from shapes that are available commercially. The study was conducted in two phases. The first focused on measuring maximum scour depths at the culvert entrance and developing an analysis procedure to approximate prescour hydraulic parameters. The second phase expanded the investigation to include scour measurements at the entrance and outlets for submerged flow conditions. Additional tests were conducted to evaluate the use of various measures to reduce scour. Results showed that using the flow distribution at the culvert entrance to compute the primary scour depth component and adjusting with an empirical factor based on laboratory data appeared to be valid for bottomless culverts. Culvert shapes tested in these experiments did not significantly influence the scour, but the entrance conditions did. The use of 45 deg inlet wing walls considerably decreased the scour at the upstream corners.