Watershed Approach to Mitigating Hydrologic Impacts of Highway Projects
The traditional technique for addressing hydrologic impacts of highway projects is to construct detention basins. While the basins are effective at reducing peak flows and flood issues, they are less well suited to maintaining or reestablishing relatively "natural" hydrology for streams, and may in fact lead to local degradation (Sovern and MacDonald 2001). They also require considerable right-of-way close to highways, add to the maintenance burden, and can become attractive nuisances. Full infiltration on-site is frequently not feasible due to physical constraints. Other approaches, such as using compost amended vegetated filter strips for volume reduction, are promising, but still require right-of-way that may not be available. This project would investigate the capability of landscape modifications elsewhere within a watershed to off-set the hydrologic effects of the increase in impervious surface and changes in drainage associated with highways. Examples include floodplain reconnection, use of strategically placed and designed created wetlands, and reforestation. The goal of the research would be to identify effective actions, develop techniques for quantifying the benefits of the actions and assessing their ability to compensate for project impacts, and develop guidance for evaluating and selecting the appropriate approach for the affected watersheds. This research would be a companion to NCHRP 840 "A Watershed Approach to Mitigating Stormwater Impacts". Anticipated tasks for the project include: • Literature search and survey to identify and evaluate existing research relevant to the project, and any existing projects that may provide useful examples or information on use and effectiveness of this approach. • Gap analysis to identify what additional information may be needed to complete the project • Development of techniques to quantify the benefits of potential actions • Development of guidance on the evaluation and selection of actions Sovern, D.T. and A. MacDonald, 2001. Can in-stream integrity be obtained through on-site controls? In Linking Stormwater BMP Designs and Performance to Receiving Water Impact Mitigation. B. Urbonas, ed. Engineering Foundation Conference 2001, August 19-24, 2001 | Snowmass Village, Colorado, United States p. 47-59.
The adverse effects of the flashier hydrology that result from increased pavement have been getting increased attention. The consequent stream channel changes harm aquatic habitat, and can reduce water quality by increasing sediment loads, caused by accelerated erosion during runoff events, and reducing low flow discharges. While EPA did not promulgate federal rules to regulate stormwater volume directly as a pollutant, ESA compliance conditions, some NPDES MS4 permits and some state regulations do have hydromodification requirements. There is also interest in reducing pollution, particularly sediment, by preventing excess erosion. Implementation of a watershed approach to addressing hydrologic impacts would have benefits both environmentally and for transportation agencies. The approach is well suited to advance measures, providing a temporal gain of benefits. Stream corridor enhancement hydrologic processes are more "natural" than detention basins, and would be more likely to result in less artificial runoff patterns. The watershed approach is also well suited to collaboration with cities, counties and watershed councils, with opportunities for cost sharing. Costs can also be reduced by incorporating hydrologic mitigation with wetland or fish habitat mitigation.
William B. Fletcher
Oregon Dept. of Transportation
503 731 3024
May 8, 2017
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