Stormwater Symposium

Oral

397817 - Beyond the design storm: Annual hydrologic metrics for improved LID design and evaluation

Tuesday, June 5
8:30 AM - 10:00 AM
Location: Mirage Room
Co-Authors: William Hunt, Raleigh, NC – North Carolina State University

The goal of an LID project is to develop land such that its hydrology is minimally changed. LID projects are typically designed and evaluated using design-storm metrics, such as peak flow mitigation and/or required treatment volumes. Long-term hydrologic patterns, however, are difficult to predict, so they are rarely addressed. As a result, LID projects that ignore long-term patterns do not address all five components of the natural flow regime (magnitude, duration, frequency, timing, and rate of change). This research proposes a new LID approach that integrates two long-term metrics into the design and evaluation process: matching the annual pre-development water balance (% evapotranspiration, % infiltration, and % runoff) and matching the frequency of runoff events. A four-step procedure is proposed and will be tested as a case study in North and South Carolina, USA: (1) the proposed hydrologic metrics will be calculated at all the unregulated streams in the USGS stream gauge network; (2) each metric’s hypothetical pre-development value will be predicted using a random forests modeling approach that replaces each watershed’s disturbed areas with natural land cover; (3) a map will be developed to discretize the predicted pre-development metrics into hydrologic regions; and (4) the regional metrics will be translated into site-scale targets for LID design and evaluation. Initial results suggest that pre-development annual hydrologic metrics differ across the states, demonstrating the need for regionally-specific LID targets. The proposed approach is suited to achieve the goals of LID because it considers both short-term and long-term patterns in the pre-development hydrologic regime.

Charles C. Stillwell

Ph.D. Candidate
North Carolina State University

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