Designing flood resilient road-stream crossings: a case study from the July 2016 northwest Wisconsin flood

Abstract

The July 2016 flood in northwest Wisconsin caused the failure of over 40 road-stream crossings and damaged 300 miles of roads on the Chequamegon-Nicolet National Forest. Peak flood flow estimates were obtained at 26 road-stream crossing sites including 17 sites designed using stream simulation (SS). Peak flood magnitudes ranged from 9 to 2,629 cubic feet/second/square mile (cfsm). Floods at 21 sites were over 100 cfsm which were 1.4 to 14.4 times greater than the estimated 1 percent annual exceedance probability flood. Only 1 of 17 SS sites failed, although 13 overtopped. Infrastructure survival was attributed to the large capacity associated with SS, ability to pass debris, and the ability to pass large flows over the road, usually to one or both sides of the structure. Substantial differences in flood flow rates among the 26 sites were correlated to differences in watershed characteristics, including slope, storage, soils, and drainage density, which are related with landtype association (LTAs). Watersheds with more rapid runoff are anticipated to be more vulnerable to increasing flood flows from more-intense rainfall in the future. Using this concept, LTAs in Wisconsin were placed into four vulnerability classes as an example of how to anticipate future flood flow increases during road-stream crossing design. These classes were combined with site characteristics to illustrate how culvert and bridge design can be adapted to accommodate a changing climate. Based on our findings, we propose SS design as a primary design criterion, and suggest adding supplemental design features as watershed and site vulnerability to flooding increase. This approach has the potential to help road managers meet infrastructure and environmental objectives in the short term and long term.

Citation

Higgins, Dale; Donner, Deahn M. 2025. Designing flood resilient road-stream crossings: a case study from the July 2016 northwest Wisconsin flood. Gen. Tech. Rep. NRS-231. Madison, WI: U.S. Department of Agriculture, Forest Service, Northern Research Station. 69 p. https://doi.org/10.2737/NRS-GTR-231.