Date of Award

1-1-2010

Document Type

Campus Access Dissertation

Department

Environmental Health Sciences

First Advisor

Dwayne E Porter

Abstract

Coastal increases in population have generally resulted in conversion of forested lands to sprawling urban development. The impervious cover (e.g., roads, rooftops, driveways) that accompanies urban development leads to increases in the volume of stormwater and subsequent contributions of non-point source pollution to coastal water bodies (e.g., tidal creeks). State and local agencies have developed regulations to reduce the impact of stormwater and non-point source pollution on receiving waters through the use of stormwater Best Management Practices (BMPs). The curve number (CN) method is oftentimes used to estimate runoff from the 2- and 10-year design event which informs the size and design criteria of stormwater BMPs. Stormwater detention ponds are the most common BMP used along the South Carolina coast; however regional research suggests that limitations exist in their ability to address non-point source pollution. Low Impact Development (LID) practices (e.g., bioretention swales, pervious pavers, rainwater harvesting) have been suggested as a means for addressing stormwater quality yet their prevalence is limited along the coastal zone of the southeast. A study was conducted to identify the obstacles to the implementation of LID practices, define a method for evaluating their comparative performance at the catchment scale, and to evaluate their ability to retain stormwater and its associated pollutants regionally.

Interviews of stormwater professionals indicated that the educational needs of stakeholders (27 % of responses) and the regulatory process (22 % of responses) are the primary obstacles to regional implementation of LID practices. Participants suggested that the creation of guidelines for the design, permitting, and maintenance of LID practices would assist in overcoming these obstacles. The CN method provided a reasonably accurate estimate of runoff volume from the LID catchment over a small range of rainfall depths and intensities; 14.0 mm to 43.7 mm and 1.2 mm hr-1 to 7.6 mm hr-1, respectively. However, the CN method was not capable of estimating the volume of runoff produced from a high intensity rain event (13.7 mm hr-1) since the method does not include a temporal variant (e.g. rain over time). The LID practices at Oak Terrace Preserve reduced the concentrations of FC and TSS in the first portion of the runoff hydrograph (defined as the first 13 millimeters of rainfall) compared to subsequent portions of runoff. However, these lower concentrations did not translate into a difference in FC and TSS yields between the two catchments. Regulatory agencies were apprehensive of limited soil storage capacity and recommended the use of underdrains beneath the LID practices to prevent long-term ponding of stormwater. These underdrains quickly conveyed stormwater and associated pollutants offsite resulting in no difference in the volume of runoff or pollutant masses when compared to a curb-and-gutter system. In addition, seasonal contributions of pesticide runoff highlighted the over-riding impact of pollutant contributions from catchment residents.

Although BMPs have been shown to be effective at reducing stormwater peak flows and retaining particulate pollutant masses at the site scale, studies suggest that these impacts do not necessarily translate to mitigation at the watershed scale. This study highlighted the sensitivity of stormwater BMPs to design and suggests that we can no longer depend on an engineered solution to the ecological impacts of sprawling coastal development. Instead there is a need to be more thoughtful and strategic in the development of our coasts through greater land planning efforts to minimize impervious cover and maximize the conservation of open space and riparian buffers in coastal watersheds. Since coastal development cannot be prevented all together, stormwater management will remain an integral component to mitigating the impact of coastal development. Future Southeast coastal stormwater management programs should provide a comprehensive stormwater management strategy to promote: peak flow reduction through temporary stormwater retention, water quality enhancement by promoting pollutant filtration, sorption, settling, transformation, decay, and uptake, and the reduction of stormwater volume and pollutant loads at the source. LID practices provide an option for addressing water quality enhancements, but regionally relevant design guidelines and models should be developed and provided to engineers, developers, and regulatory agencies to assure their appropriate application. In addition, education and incentive programs should be incorporated into stormwater management programs to reduce pollutant loadings at the source.

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