The Mississippi Department of Transportation (MDOT) selected the State Route 18 stream crossing of White Oak Tributary 1 for replacement based on the need to widen the road to support requirements for evacuation routes and the future Average Daily Traffic count. The site is north of Port Gibson, and north of the Bayou Pierre crossing along SR 18. The existing crossing was a double box culvert design that was installed in 1935 with a drainage area of roughly 2 square miles. At first glance this seems to be a simple alternative analysis to determine if widening the culverts or a bridge is required; however, the environmental investigation led to a unique partnership with the United States Fish and Wildlife because the crossing sits in the Bayou Pierre Watershed, the only place on earth you find the threatened Bayou Darter (Etheostoma rubrum). Erosion and high water for larger recurrence events created headcuts and steep riverbanks within the Bayou Pierre watershed which damaged Bayou Darter spawning habitat. These problems were compounded by decades of poor management decisions related to gravel mining and livestock raising along the river. This site, because of an existing ten-foot headcut at the outfall of the culvert, was selected for redesign and replacement including aquatic organism passage due to it being critical habitat for a threatened species and the individual needs of the species. The project included several alternatives, but each had to meet clear design standards, pass the design recurrence events for MDOT with respect to required freeboard, resolve the head cut, and allow passage of the Bayou Darter during low flow events with respect to its biological limitations. In this presentation we will walk through the partnership of the state and federal agencies, the design process and evaluation of the site, and show the final design with finished construction of the first step-pool fish passage in the Southeast.
