Testing produces design guidance: Fid-Bau Portal

Testing produces design guidance

Robeson, Michael D.

Outlet erosion protection of culverts can be a serious problem due to the increased erosive potential of outlet flows. Determination of the scour potential should be common practice with the design of culverts. A new transition geomat product with the application of culvert and storm-water outlet scour protection was tested at Colorado State University. The objective of the testing was to determine the hydraulic and sediment stability threshold conditions. Exit velocity can be considered the main factor in determining the need for erosion protection at a culvert outlet. Culvert performance can be affected by the downstream water surface elevation or tailwater. For this article, low tailwater conditions are examined. Low tailwater conditions result when the flow exits at approximately one-third of the culvert. Two types of scour can occur in the vicinity of culvert outlets including local scour and general channel degradation. Local scour can be considered a direct result of the high-velocity flows at the culvert outlet while general channel degradation results from changes to the river regime by natural processes or human activities. This article focuses on the local scour that could occur at culvert outlets. The test program is summarized and engineering design guidance is offered as a result. Design guidelines for riprap scour protection downstream of a culvert outlet are explicated and a design example is given.