Bioretention cells (also known as rain gardens) are a Low Impact Development (LID) method for sustainable stormwater management. An increasingly popular form of urban stormwater infrastructure, bioretention cells use an engineered, vegetated-soil-system to both reduce quantity and enhance quality of stormwater. The ability of bioretention systems to remove common pollutants from urban stormwater runoff, and reduce runoff volume through evapotranspiration, in a temperature climate during plant senescence were assessed in this full scale field-based study. Stormwater run-off simulations were conducted for 5-, 10-, and 25-year return period storm events at a field site in Victoria, British Columbia, Canada. Tests were run on both, a vegetated cell planted with a mix of Betula nigra, Betula nana, and Salix lutea, and a control cell with turfgrass. Influent and effluent field parameters were recorded for pH and dissolved oxygen (DO), in addition to lab analyses conducted to quantify COD, TN, TON, TP, ortho-phosphate, and TSS removal from the stormwater. Water quality and hydrologic performance were results were compared between the vegetated and control cell using a Wilcoxon Signed Rank Test. In addition, hydrologic results were correlated with daily Evapotranspiration (ET) and meteorological station data using Spearman’s Rho Correlation. The vegetated cells were more effective (p value < 0.05) at retention of water volume, DO, COD, and orthophosphate, when compared to the control. Strong correlations (p value < 0.05) were found between the retention of water volume, and each of ET, maximum temperature, average temperature, minimum temperature, and average wind, for only the vegetated cells. / Graduate

Virginia's new stormwater regulations involve the use of the Runoff Reduction Method (RRM), which requires the product of the peak flow rate and runoff volume (Q*RV) from the one-year storm event in the post-development condition to be reduced to eighty percent of the pre-development Q*RV to protect against channel erosion. This study models different bioretention cell sizes in a developed watershed in Blacksburg, Virginia to determine the "performance" at both the sub-watershed and watershed levels. In addition, models of "optimal" bioretention cells sized to meet the RRM for each sub-watershed are evaluated. A direct relationship is determined between the size of the cell required to meet the RRM and the sub-watershed's Natural Resources Conservation Service (NRCS) curve number. However, the required size for some of the cells is much larger than those typically seen. With the RRM applied for all of the sub-watersheds, the resulting hydrograph at the watershed outlet has a lower peak than the pre-development condition. / Master of Science

Despite the runoff reduction efficiencies recommended by various regulatory agencies, minimal research exists regarding the ability of vegetated swales to simultaneously convey and reduce runoff. This study assessed the effect water quality swales distributed among upstream sub-watersheds had on watershed hydrology. The study was also posed to determine how certain design parameters can be dimensioned to increase runoff reduction according to the following modeling scenarios: base, base check dam height, minimum check dam height, maximum check dam height, minimum infiltration rate, maximum infiltration rate, minimum Manning's n, maximum Manning's n, minimum longitudinal slope, and maximum longitudinal slope. Peak flow rate, volume, and time to peak for each scenario were compared to the watershed's existing and predevelopment conditions. With respect to the existing condition, peak flow rate and volume decreased for all scenarios, and the time to peak decreased for most scenarios; the counterintuitive nature of this result was attributed to software error. Overall, the sensitivity analysis produced results contrary to the hypotheses in most cases. The cause of this result can likely be attributed to the vegetated swale design and modeling approaches producing an over designed, under constrained, and/or over discretized stormwater management practice. / Master of Science

This study uses the Natural Resources Conservation Service (NRCS) TR-55 method to quantify the increase in stormwater runoff volume from infill residential redevelopment, or mansionization, in a 34-acre residential subwatershed of Fairfax County, Virginia. Analysis of 10 redeveloped lots in the subwatershed showed an average increase in impervious cover from 8% to 28% after redevelopment, resulting in an average increase in runoff volume of 18% for the 10-year, 24-hour storm. From 1997 to 2009, the total impervious cover in the subwatershed increased from 18% to 25%, resulting in a calculated 6% increase in runoff volume. Low Impact Development (LID) techniques were modeled as retrofits in the subwatershed to mitigate the increase in runoff volume. Measures modeled include bioretention basins, infiltration trenches, amended soils, permeable pavement, and cisterns. Results indicate that placing bioretention basins or infiltration trenches on 0.5% of the subwatershed or amending 20% of the open space with soil composts would reduce the runoff volume back to the 1997 quantity for the 1-year, 24-hour storm. / Master of Science

While many studies have evaluated the hydrologic effects of bioretention at the site level, few have investigated the role bioretention plays when distributed throughout a watershed. This study aims to assess bioretention's effects on an urbanized watershed using two modeled scenarios: one where runoff from many land uses was routed through the practice, and another in which only runoff from large impervious areas was routed. Peak flows, volumes, and lag times from these models were compared to the watershed's current and predeveloped conditions. Both scenarios provided reductions in peak flows with respect to existing conditions for modeled storm events, sometimes to levels below the predeveloped condition. Neither case was able to reduce volumes to predevelopment levels; the option to treat impervious areas had a negligible effect on runoff volume. Both cases were able to extend lag times from the existing development condition. Based on these results, bioretention appears to have the capability to improve watershed hydrologic characteristics. Furthermore, only treating impervious areas could be a viable alternative when funds or space are limiting factors. / Master of Science

This paper serves as a “kicking-the-tires” analysis of low impact developments as a method of stormwater management. Specifically, this paper examines the feasibility, benefit, and current practice of low impact developments in Austin, Texas. Merits, strengths, and weakness are comparatively determined primarily on the basis of the impact and efficiency of design, particularly relating to ability to handle water volume and potential to improve water quality. By examining case studies and “applied” examples the potential of low impact development application is considered for the expected, potential, and/or alleged benefits of low impact implementation. / text

Low impact development (LID), the practice of preserving and restoring natural water cycles in urban development, is considered the next step in stormwater management. Policy and governance play a strong role in the adoption of LID. There has been progress in implementing LID in the Greater Toronto Area but less progress in Southern Ontario in general. This research identifies barriers and opportunities to implementing LID in the context of policy and governance in Southern Ontario. The barriers, opportunities, and policy are identified using a focused literature review, then verified and further explored through key informant interviews. Data are synthesised to produce an Enhanced Governance Model (EGM) for implementing LID. The EGM is evaluated by key informants and further refined. Findings show that public education and provincial-level standards are fundamental to widespread adoption of LID. Five opportunities for jurisdictional integration in stormwater governance are identified and their implications discussed.

Groundwater dependent ecosystems (GDEs), including wetlands and river baseflow systems, are a topic of substantial scientific study. The degradation of GDEs due to urbanization has been well documented. An altered hydrologic regime, through increased impervious area resulting in a flashier hydrologic regime with lower troughs, higher peaks, and quicker changes, has been recognized as a main factor affecting ecological condition. Yet studies on GDEs rarely include a hydrologic modelling component. In this study, the conjunctive hydrologic model MIKE SHE was used to simulate the Lovers Creek subwatershed near Barrie, ON. The hydrologic regime was simulated for pre-development (natural), current (urbanized), and various low-impact development (LID) land use scenarios. The results were linked to the ecological condition via the TQmean metric, which has been used in the literature to relate the hydrologic and ecological conditions of streams. The highest percentage LID scenario restored, on average, 11% of the reduction in TQmean that occurred from pre-development to urbanized conditions, indicating that LID has the potential to protect GDEs in urbanized watersheds. It is expected that the effect of LID would be amplified if considered on a more local scale within a predominantly high density urban area. Recommendations for future modelling efforts to evaluate GDEs and represent LID are made.

Rapid population growth and urbanization in semi-arid and arid regions has led to alterations in the water, carbon (C), and nitrogen (N) cycles (Gallo et al. 2014), prompting demands for mitigation strategies. Green Infrastructure (GI) is one of the methods used in urban storm water mitigation that delays and attenuates stormwater runoff by storing water in vegetated depressions. In the Southwest these depressions, also called bioswales, have the potential to act as biogeochemical hot spots, encouraging nutrient cycling, infiltration, plant growth, and microbial activity (McClain et al. 2003). An influx of water to GI initiates a combination of physical and microbial processes that result in increased CO2 efflux and N mineralization known as the Birch Effect (Birch, 1958). This study examines GI in Tucson, AZ through inducing an artificial precipitation regime and determining how soil properties, GI design, and biogeochemical characteristics influence the response. In natural systems it has been shown that soil moisture, soil properties, organic matter, length of dry period, nutrients such as carbon and nitrogen, and microbial biomass influence soil respiration and nitrogen mineralization (Borken and Matzner 2009). The purpose of this study is to determine the role that the Birch Effect plays in urban stormwater GI. Additionally we seek to determine how soil and nutrient properties and precipitation regime affect the amplitude of the response. It was found that soils from GI features tend to have higher concentrations of organic matter, total carbon, and total nitrogen, as well as higher water holding capacity and lower bulk density. It was also shown that soils originating from GI features tend to illicit a greater CO2 flux upon rewetting than soils from adjacent areas. The linear relationships found between % clay, pH, bulk density, WHC, SOM, TC, and TN suggest that the reason for the greater response to wetting is due to the altered physiochemical composition. The results of this study can be utilized to increase microbial activity and remediation in urban GI features. This fits into the larger goal of GI to help mitigate many of the issues associated with Urban Stream Syndrome (USS) such as flashier hydrography response, increased nutrient and contaminant concentrations, increased erosion, altered channel morphology and reduced biodiversity (Meyers et al. 2005).

Urbanization has considerably altered natural hydrology of urban watersheds by increasing runoff volume, producing higher and faster peak flows, and reducing water quality. Efforts to minimize or avoid these impacts, for example by implementing low impact development (LID) practices, are gaining momentum. Designing effective and economical stormwater management practices at a watershed scale is challenging; LIDs are commonly designed at site scales, considering local hydrologic conditions (i.e., one LID at a time). A number of empirical studies have documented hydrologic and water quality improvements achieved by LIDs. However, watershed scale effectiveness of LIDs has not been well studied. Considering cost, effort, and practicality, computer modeling is the only viable approach to assess LID performance at a watershed scale. As such, the United States Environmental Protection Agency’s Stormwater Management Model (SWMM) was selected for this study. It is well recognized that model predictions are plagued by uncertainties that arise from the lack of quality data and inadequacy of the model to accurately simulate the watershed. To scrutinize sensitivity of prediction accuracies to spatial resolution, four SWMM models of different spatial detail were developed for the Ballona Creek watershed, a highly urbanized watershed in the Los Angeles Basin, as a case study. Detailed uncertainty analyses were carried out for each model to quantify their prediction uncertainties and to examine if a detailed model improves prediction accuracy. Results show that there is a limit to the prediction accuracy achieved by using detailed models. Three of the four models (i.e., all but the least detailed model) produced comparable prediction accuracy. This implies that devoting substantial resources on collecting very detailed data and building fine resolution watershed models may not be necessary, as models of moderate detail could suffice. If confirmed using other urban watersheds, this result could benefit stormwater managers and modelers. All four SWMM models were then used to evaluate hydrologic effectiveness of implementing bioretention cells at a watershed scale. Event based analyses, 1-year, 2-year, 5-year and 10-year storms of 24-hours were considered, as well as data from October 2005 to March 2010 for a continuous simulation. The runoff volume reductions achieved by implementing bioretention cells were not substantial for the event storms. For the continuous simulation analysis, however, about twenty percent reductions in runoff volume were predicted. These results are in-line with previous studies that have reported ineffectiveness of LIDs to reduce runoff volume and peak for less frequent but high intensity storm events.