Road retention projects can provide an efficient, cost-effective solution for your flooding problems. They provide an alternative to the trend of replacing culverts with larger, more expensive culverts and bridges with larger bridges which transfer the flooding problems with damages downstream. This scenario was vividly shown in the large area of Minnesota that was damaged by the 1993 floods, along with the flooding that occurred on the Mississippi and Missouri Rivers.
The member counties of Area II Minnesota River Basin Projects have been replacing culvert and bridges with smaller sulverts on township roads, county roads and state highways since 1989 as part of an overall floodplain management plan. The project sponsor purchases flowage easements from the upstream property owner(s) to allow the water to pond behind the road crossing for periods of 24-72 hours. This time allows the structure to "meter" out the flow, thus reducing flood peaks, velocity, and water surface elevations downstream.
Road retentions are most effective when the locations are carefully selected and designed to maximize the interaction with other culverts and bridges both upstream and downstream; creating a watershed-based design. Structures may be retrofitted to existing culverts in good condition, as part of new culvert installations, and as an integral part of sight distance improvements that usually require the roadway elevation to be raised.
These structures can have many configurations, but principally two different designs. Retrofitted designs of multiple culvert crossings are accomplished by plugging one or more lines of culverts or modifying the inlets of existing culverts by providing a weir box for larger flows and maintaining a small culvert for low flows. New culvert installation can be on two different designs: 1) a single culvert with standpipe weir, or 2) a smaller than existing culvert allow the larger storms to be impoinded by the roadway and small storms to pass through unimpeded. (Refer to the below diagrams.) These designs allow the engineer to custom fit the specific type of flood control that is best suited and most beneficial to the watershed.
Costs of installing retention structures range from slightly more than the cost of replacing an existing culvert to cosiderably less than replacing an existing bridge. For most road projects, the newly installed culvert will be smaller and less expensive, although the road height may require raising and increasing the cost for additional fill material. Flowage easement costs are normally about $200/acre for noncrop areas and $400/acre for crop ground with encouragement to sight projects where cropland can be avoided. Area II, in conjunction with the participating county engineers, have been providing professional engineering services for hydrologic and hydraulc design, which reduces the cost of hiring a consultant engineer. Most roads and highways in need of upgrading for sight distance corrections and safety enhancement can be fitted for retentions as additional roadway height is usually warranted.
Amendments to Town Bridge Law M.S. 160.80 in 1992 allows the use of state funds to pay for additional cost above and beyond the cost of a typical bridge or culvert replacements when smaller appurtenances are installed to provide flood damage reduction. Counties which have constructed these types of project have found the costs to be less than typical replacements due to the savings in pipe cost alone.
Road retention structures do require the design of a professional engineer with a strong background in hydrology and hydraulics. For more information regarding Road Retention Techology or the design process, please contact Area II Minnesota River Basin Projects or the Board of Water and Soil Resources (BWSR).