Using measured erodibility values to assess scour below culverts
Culverts are used in our highway systems for two purposes: drainage and as bridge structures. There are thousands of culverts in every province in Canada. The large-sized culverts used as bridge structures can be up to $6,000,000 to install. As such, installation of a culvert can be a significant investment for our highway infrastructure. Failure of such a structure not only results in an economic loss, but is also a danger to the public.
One of the main causes of failure of culvert is undermining at the outlet due to scour of the soil in the outlet area. The culverts often provide a smaller cross-sectional area of flow for the flow to travel through, which accelerates the flow beyond that which was naturally occurring in the stream. This faster flow in the culvert causes the soil just downstream of the culvert to erode as the flow moves back into the downstream channel from the culvert. This erosion causes a scour hole to form below the culvert. To protect against this scour, typically large rocks (riprap) are placed to protect the riverbed and highway embankment. To do this properly, the potential size and extent of the scour hole formed by the flow must be estimated.
Current practice is to estimate the soil’s erosion resistance by assigning the soil at the site to broad categories of the material such as “stiff clay” or “loose sand” and finding the erosion resistance from a range of that parameter for those soil categories that are given in a table. However, the values given in these tables can be greatly different than measured values of erosion resistance. The objective of the research project described herein is to incorporate new techniques for determining soil erosion resistance into the design for scour protection at culverts. The goal is to improve the reliability of scour protection at culverts and to reduce capital construction costs with better determination of needs for riprap placement
