The Emory-Georgia Department of Transportation (GDOT) Bottomless Culvert study reviewed the efficacy of bottomless culverts, their cost, and relative maintenance needs in contrast to that of embedded box culverts. In the past, GDOT has almost exclusively used box culverts when necessary for watershed management. This study showed that bottomless, or three-sided, culverts can be cost saving across the lifetime of the structure, as well as more suitable for aquatic wildlife connectivity. Information gathered from scientific journals, Federal Highway Administration publications, interviews with stakeholders, and other sources was compiled into a final report comprising a rigorous cost-benefit analysis. The report discusses methods for avoiding and minimizing adverse environmental impacts from GDOT’s projects, opportunities for streamlining the preconstruction approval process, and potential for cost savings from mitigation and maintenance expenses.
Installation costs and site conditions were the most important factors when deciding which stream crossing type to install according to multiple stakeholders; however, cost-benefit analysis revealed that while bottomless culverts do have higher installation costs, this may be offset by reduced mitigation costs and maintenance savings. Compensatory mitigation pursuant to Section 404 of the Clean Water Act represents a significant cost for projects and maintenance, and mitigation credits from banks in Georgia are becoming more expensive. As Georgia requires more credits per linear foot of impact, and the number of available credits decreases, 404 mitigation will represent a larger and larger proportion of overall project cost. Bottomless structures have the potential to negate mitigation costs, thus allowing projects to continue that may otherwise have been put on hold. Bottomless culvert installation was found to be more conducive in locations that had shallow bedrock present for the foundation and footings. A larger structure that spans the bankfull width of the stream is necessary to streamline consultation with Fish & Wildlife Service and maximize mitigation savings. With that inherent oversizing comes extra cost, but also more open area beneath the culvert, thus reducing debris and maintenance costs while having a similar or smaller length of stream impact compared to embedded box culverts. The presence of shallow bedrock and non-erosive soils may indicate bottomless culverts may be suitable, and a holistic review during the preliminary design process will decide if they are beneficial at that site. Bottomless culverts are recommended where their capital costs are less than typical box culvert options, in areas with high incidence of listed species, and in situations where a box culvert would not meet the environmental and economical requirements set by GDOT or other DOTs.