Design Methodology for Meeting Return to Predevelopment Hydrology Requirements
For highways outside of cities, infiltration within a right of way (ROW) may be one of the few means of meeting the return to predevelopment hydrology requirement under the Energy Independence and Security Act of 2007 (EISA). Right now, hydraulic design procedures for highways focus on the design of channels to convey stormwater runoff away from roads, but are not intended to assist with the design of ROWs for infiltration. Tasks for developing such a design process may be expected to include: (1) determining a process to incorporate a discharge-based means to quantify the runoff from the roadway surface (as differentiated from the static depth-based means currently in use for best management practices); (2) dividing the surface of an ROW into a rectangular grid to enable geospatial calculations; (3) developing a calculational strategy accounting for the simultaneous processes of stormwater running onto a pixel, infiltrating from that pixel and the remainder flowing on to the next downslope pixel; (4) developing a means to track the subsurface flow from the infiltration; (5) determining how to "collect" the infiltrated water that may flow into a drainage channel; and (6) converting infiltration and flow results into a required width for an ROW. There are additional policy issues that have technical implications (e.g., whether precipitation falling on the ROW must also be infiltrated in order to show compliance) that must also be addressed.
EISA requires that federal agencies comply with a return to pre-development hydrology requirement for all new projects and substantial rehabilitation. There is a precedent for laws written for federal agencies being applied to state DOTs because of the receipt of federal funding (e.g., National Environmental Policy Act). State DOTs report conversations with EPA that this requirement will be applied to them in the future. The implementation of this requirement has already found its way into the National Pollutant Discharge Elimination System stormwater permit, albeit with a slightly lesser storm depth than dictated in EPA's Guidance for implementation. As currently implemented, actual infiltration is not the compliance criterion. For example, stormwater quality improvement through filtration with the water the being redirected to a storm sewer can count towards compliance. It is thus imperative that an infiltration-based design process be established for those roadway locations where filters and/or storms drains do not exist and would be difficult to implement.
Kathleen M. Trauth
University of Missouri, Dept of Civil & Env Eng
May 9, 2017
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