Continuous Distributed Modeling for Evaluation of Stormwater Quality Impacts from Urban Development
Understanding the impact of land use changes on nutrient and sediment loading from stormwater runoff to a water supply reservoir is the motivation for the study referenced in this chapter. Changes in the loading rate and the relative proportion of nutrients, e.g. nitrogen and phosphorus, can have important effects on eutrophication and algae production in the receiving water of the lake. Evaluation of stormwater runoff quantity and quality is performed for the 30.8 km2 (11.9 mi2) Rock Creek watershed located within the corporate limits of the City of Norman, OK. This watershed is part of the larger drainage area of Lake Thunderbird reservoir, which is operated by the Central Oklahoma Master Conservancy District and supplies drinking water to Norman and two other surrounding communities. The reservoir was constructed by the US Bureau of Reclamation in 1961-1965. The 2001 bathymetric survey determined Lake Thunderbird to have a maximum depth of 58 ft (17.7 m), mean depth of 15.4 ft (4.7 m), surface area of 5,439 acres (2,211 ha) and volume of 105,838 acre-feet (130,180,000 m3). Excessive algae production leads to taste and odor complaints about the finished water product.
Continuous simulation, using the physics-based distributed hydrologic model Vflo™, is used to identify runoff and loading rates for three development scenarios. Vflo™ is a commercial model that has been available since 2001. Distributed maps of topography, soils, and land use/cover represent the watershed surface characteristics at a model resolution of 60 m. Sample concentrations for selected water quality parameters have been tested at four locations within the watershed by the Oklahoma Water Resources Board. Sample concentrations are compared to the EMC values reported in the National Stormwater Quality Database, NSQD, reported in Maestre and Pitt (2005) and Pitt et al. (2003). While many constituents are within one standard deviation about the median, others are elevated at several locations within the watershed. Further comparisons are made for additional events for purposes of verification and evaluation of nutrients associated with the range of land use conditions and development found within the watershed.
Three scenarios are modeled for purposes of estimating potential impacts: (i) baseline conditions, (ii) two-thirds conversion to low-density housing; and (iii) full conversion. The baseline development scenario evaluated is consistent with the Norman 2025 planned land use. Future development scenarios consider build-out from country residential to low density housing. To represent each scenario, changes in the distributed model are made within specific areas of the watershed. Representation of the development scenarios within the model includes: (i) increased imperviousness; (ii) reduced hydraulic roughness due to developed surfaces; and (iii) changes in event mean concentration (EMC) for selected nutrients and other constituents. The three development scenarios are simulated under long-term continuous (1994-2005) conditions, design storms, and for specific events using precipitation input derived from rain gauge and radar.
This chapter presents the assessment of water quality impacts due to projected urban development using a fully distributed hydrologic model. The organization of this chapter begins with the methodology, input data, and study area, followed by results, discussion and summary sections.
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