Low impact development in urban areas for integrated watershed management across scales

Urbanization and land use changes have increased impervious surfaces and resulted in more frequent flooding and higher contaminant loads in hydrologic systems. According to the National Resources Defense Council, stormwater runoff rivals or exceeds discharges from factories and sewage plants as a source of pollution throughout the United States. The Environmental Protection Agency identifies urban stormwater as the second largest source of water quality damage to open water bodies. In recent years, the use of low impact development (LID) practices, that aim at maintaining or closely replicating predevelopment hydrology, have become widespread. LID practices promote stormwater infiltration, filtration, evapotranspiration, and on-site storage through a variety of small-scale technologies that, when integrated throughout a watershed, can holistically address shortcomings associated with and/or complement conventional practices. Notable examples are rain gardens, green roofs, and permeable pavements. This thesis monitored meteorological and runoff data for one type of LID technology, a green roof, located at the Business Instructional Facility at the University of Illinois at Urbana-Champaign. The data was used to calibrate the moisture retention parameters of the green roof substrate using Hydrus-1D. The model is capable of simulating runoff and evapotranspiration for continuous periods of time allowing it to take into account the antecedent moisture content of green roofs at the beginning of each storm. The general features of the runoff hydrographs were captured by the model. Hydrus 1D was used to generate runoff hydrographs for green roofs, and these were incorporated into the Calumet Dropshaft 51 (CDS-51) model that was developed for the Village of Dolton, IL using the Illinois Urban Hydrologic Model (IUHM). IUHM was modified to incorporate the response of green roofs and study their impact on the catchment runoff behavior. Hypothetical uniform and triangular storms were used to study the influence of rainfall intensity and the temporal distribution, respectively. Wet and dry year precipitation for CDS-51 were also tested, and the results show that the volumetric retention of stormwater by green roofs is not constant and depends on the antecedent moisture content, even for storms of similar depths. Benefits realized from replacing conventional roofs with green roofs in CDS-51 were then quantified using benefit transfer. The analysis was based on the hydrological analysis at the watershed level scaling-up from the local level, a nonlinear process. The private benefits considered are the benefits from reduced energy use and the external benefits considered are the benefits from reduced stormwater treatment volume, reduced combined sewer overflow treatment and storage volume, and consumer willingness to pay for increased infiltration and decreased street flooding. Results indicated that external benefits are substantial and quantifying the value of other nonmarket benefits (not considered in this study) could make the case for using green roofs and other green infrastructure for stormwater management more favorable