Synthesis of Hydrologic and Hydraulic Impacts: Technical Report

A substantial portion of the cost of highway projects [approximately 40%, according to one in-house Texas Department of Transportaiton (TxDOT) estimate] is for drainage infrastructure, which is intended to minimize any adverse hydrologic and hydraulic (H&H) impacts of the project. Yet, adverse H&H impacts are not well-defined, despite the cost invested in drainage structures for mitigation of such impacts. This report provides guidance towards defining adverse H&H impacts and recommendations for their mitigation. A minimalistic approach towards defining and mitigating adverse H&H impacts would fulfill minimal legal requirements while taking into account the sovereign immunity enjoyed by state agencies such as TxDOT. A literature review and surveys of state DOTs as well as TxDOT districts suggest that this minimalistic approach be discouraged in favor of an emerging “No Adverse Impact” approach. For better defining and mitigating adverse impacts TxDOT is to refine coordination with local floodplain administrators, optimize site visits by District Hydraulic Engineers and other designers because of centralization, continue to improve H&H modeling and documentation, disseminate best practices regarding drainage system design, and have ongoing training relating to particular H&H issues. Implementation of the recommendations may require a change in the status of the District Hydraulic Engineers.