Current site scale stormwater management designs typically include multiple distributed stormwater best management practices (BMPs), necessary to meet regulatory objectives for nutrient removal and groundwater recharge. Selection of the appropriate BMPs for a particular site requires consideration of contributing drainage area characteristics, such as soil type, area, and land cover. Other physical constraints such as karst topography, areas of highly concentrated pollutant runoff, etc. as well as economics, such as installation and operation and maintenance cost must be considered. Due to these multiple competing selection criteria and regulatory requirements, selection of optimal configurations of BMPs by manual iteration using conventional design tools is not tenable, and the resulting sub-optimal solutions are often biased. This dissertation addresses the need for an objective BMP selection optimization tool through definition of an objective function, selection of an optimization algorithm based on defined selection criteria, development of cost functions related to installation cost and operation and maintenance cost, and ultimately creation and evaluation of a new software tool that enables multi-objective user weighted selection of optimal BMP configurations. A software tool is developed using the nutrient and pollutant removal logic found in the Virginia Runoff Reduction Method (VRRM) spreadsheets. The resulting tool is tested by a group of stormwater professionals from the Commonwealth of Virginia for two case studies. Responses from case study participants indicate that use of the tool has a significant impact on the current engineering design process for selection of stormwater BMPs. They further indicate that resulting selection of stormwater BMPs through use of the optimization tool is more objective than conventional methods of design, and allows designers to spend more time evaluating solutions, rather than attempting to meet regulatory objectives. / Ph. D.

Urban stormwater runoff is both an environmental threat and a valuable water resource. This dissertation explores the use of two stormwater management strategies, namely green stormwater infrastructure and stormwater real-time control (RTC), for capturing and treating urban stormwater runoff. Chapter 2 focuses on clean bed filtration theory and its application to fecal indicator bacteria removal in experimental laboratory-scale biofilters. This analysis is a significant step forward in our understanding of how physicochemical theories can be melded with hydrology, engineering design, and ecology to improve the water quality benefits of green infrastructure. Chapter 3 focuses on the novel application of unsteady transit time distribution (TTD) theory to solute transport in a field-scale biofilter. TTD theory closely reproduces experimental bromide breakthrough concentrations, provided that lateral exchange with the surrounding soil is accounted for. TTD theory also provides insight into how changing distributions of water age in biofilter storage and outflow affect key stormwater management endpoints, such as biofilter pollutant treatment credit. Chapter 4 focuses on stormwater RTC and its potential for improving runoff capture and water supply in areas with Mediterranean climates. We find that the addition of RTC increases the percent of runoff captured, but does not increase the percent of water demand satisfied. Our results suggest that stormwater RTC systems need to be implemented in conjunction with context-specific solutions (such as spreading basins for groundwater recharge) to reliably augment urban water supply in areas with uneven precipitation. Through a combination of modeling and experimental studies at a range of scales, this dissertation lays the foundation for future integration of TTD theory with RTC to improve regional stormwater management. / Doctor of Philosophy / Urban stormwater runoff contains a variety of pollutants. Conventional storm drain systems are designed to move stormwater as quickly as possible away from cities, delivering polluted runoff to local streams, rivers, and the coastal ocean – and discarding a valuable freshwater resource. By contrast, green stormwater infrastructure captures and retains stormwater as close as possible to where the rain falls. Green stormwater infrastructure can also help remove pollutants from stormwater through physical, chemical, and biological treatment processes. This dissertation describes two modeling approaches for understanding and predicting pollutant removal processes in green stormwater infrastructure (Chapters 2 and 3). Chapter 4 explores the implementation of smart stormwater systems, which use automated controllers and sensors to adaptively address stormwater management challenges. Through a combination of modeling and experimental studies at a range of scales, this dissertation lays the foundation for future improvements to regional stormwater management.

The world is faced with uncertain and dramatic changes in water movement, availability, and quality are due to human-induced stressors such as population growth, climatic variability, and land use changes. At the apex of this problem is the need to understand and predict the complex forces that control the movement and life-cycle of water, a critical component of which is stormwater runoff. Success in addressing these issues is also dependent upon educating hydrology professionals who understand the physical processes that produce stormflow and the effects that these stressors have on stormwater runoff and water quality. This dissertation addresses these challenges through methodologies that can improve the way we measure stormflow and educate future hydrology professionals. A methodology is presented to (i) evaluate the uncertainty due to inadequate temporal sampling of stormflow data, and (ii) develop equations using regional regression analysis that can be used to select a stormflow sampling frequency of a watershed. A case study demonstrates how the proposed methodology has been applied to 25 stream gages with watershed areas ranging between 30 and 11,865 km2 within the Valley and Ridge geomorphologic region of Virginia. Results indicate that autocorrelation of stormflow hydrographs, drainage area of the catchment, and time of concentration are statistically significant predictor variables in single-variable regional regression analysis for estimating the site-specific stormflow sampling frequency under a specific magnitude of uncertainty. Methods and resources are also presented that utilize high-frequency continuous stormwater runoff data in hydrology education to improve student learning. Data from a real-time continuous watershed monitoring station (flow, water quality, and weather) were integrated into a senior level hydrology course at Virginia Tech (30 students) and two freshman level introductory engineering courses at Virginia Western Community College (70 students) over a period of 3 years using student-centered modules. The goal was to assess student learning through active and collaborative learning modules that provide students with field and virtual laboratory experiences. A mixed methods assessment revealed that student learning improved through modules that incorporated watershed data, and that students most valued working with real-world data and the ability to observe real-time environmental conditions. / Ph. D.

Degradation of surface water quality by excess nutrients in stormwater is a substantial environmental and economic problem in the U.S. Phosphorus (P) is often the limiting nutrient for harmful algal blooms and the best target to prevent degradation. Natural treatment strategies such as constructed wetlands (CW) demonstrate effective and economical P management but obstacles exist to implementation. Biological P removal has large land requirements that limit the use of best management practices (BMP) in high land-value areas. Various BMP also utilize sorption processes (SP) for P removal but variations in performance and finite sorption capacity limit SP as a viable long-term removal strategy. However, by understanding variability and making sorption capacity renewable, SP could provide, with shorter retention times, a space-efficient, long-term removal strategy. This multi-study research program developed the urban wetland filter (UWF), a concept intended to overcome the unique limitations of high land-value areas to natural treatment strategies and provide a low-cost, easily implemented BMP to meet P management goals while harvesting sequestered P for use as a fertilizer. Experimental factors included substrate and influent properties pertinent to understanding performance variation and optimizing microbial iron (Fe) reduction for rejuvenation of sorption capacity. Regarding performance, modeling identified major sources of variability including, by order of importance, magnitude of a solution/substrate concentration gradient, length of the "antecedent dry period" between loadings, and pH. Field-scale results confirmed this multifactor dependence of P-removal while also supporting the inclusion of cast-iron filings in substrate to improve P removal. Regarding rejuvenation, results indicated that microbial Fe reduction is capable of releasing previously sequestered P from substrates. A sufficient carbon source was necessary, but microbial inoculation was not necessary to facilitate Fe reduction, which released most of the previously sequestered P, albeit more slowly than P sequestration. Field-scale results indicated that Fe reduction might occur faster under field conditions, possibly due to humic acids, and that inclusion of cast-iron filings enabled additional P removal after rejuvenation by providing a conservative source of Fe for the creation of new sorption sites; however, cast-iron filings may also limit the release of P during rejuvenation. / Ph. D.

The responsibility for mitigation of the ecological effects of urban stormwater runoff has been delegated to local government authorities through the Clean Water Act's National Pollutant Discharge Elimination Systems' Stormwater (NPDES SW), and Total Maximum Daily Load (TMDL) programs. These programs require that regulated entities reduce the discharge of pollutants from their storm drain systems to the "maximum extent practicable" (MEP), using a combination of structural and non-structural stormwater treatment — known as stormwater control measures (SCMs). The MEP regulatory paradigm acknowledges that there is empirical uncertainty regarding SCM pollutant reduction capacity, but that by monitoring, evaluation, and learning, this uncertainty can be reduced with time. The objective of this dissertation is to demonstrate the existing sources and magnitude of variability and uncertainty associated with the use of structural and non-structural SCMs towards the MEP goal, and to examine the extent to which the MEP paradigm of iterative implementation, monitoring, and learning is manifest in the current outcomes of the paradigm in Virginia. To do this, three research objectives were fulfilled. First, the non-structural SCMs employed in Virginia in response to the second phase of the NPDES SW program were catalogued, and the variability in what is considered a "compliant" stormwater program was evaluated. Next, the uncertainty of several commonly used stormwater flow measurement devices were quantified in the laboratory and field, and the importance of this uncertainty for regulatory compliance was discussed. Finally, the third research objective quantified the uncertainty associated with structural SCMs, as a result of measurement error and environmental stochasticity. The impacts of this uncertainty are discussed in the context of the large number of structural SCMs prescribed in TMDL Implementation Plans. The outcomes of this dissertation emphasize the challenge that empirical uncertainty creates for cost-effective spending of local resources on flood control and water quality improvements, while successfully complying with regulatory requirements. The MEP paradigm acknowledged this challenge, and while the findings of this dissertation confirm the flexibility of the MEP paradigm, they suggest that the resulting magnitude of SCM implementation has outpaced the ability to measure and functionally define SCM pollutant removal performance. This gap between implementation, monitoring, and improvement is discussed, and several potential paths forward are suggested. / Ph. D.

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

The following document serves as a design guidebook to assist landscape architects, designers, planners, engineers, and architects in the practice of developing land while preserving water quality. This guidebook outlines methods for maximizing permeable surfaces by providing examples of ways to minimize impervious surfaces. / Master of Landscape Architecture

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

In 1999, Phase II of the National Pollutant Discharge Elimination System engaged operators of small Municipal Separate Storm Sewer Systems (MS4) in the control of runoff from urban areas.  The complex task of urban runoff mitigation has been investigated for several decades, resulting in a large variety of available computing and measurement tools for urban stormwater management.  Unfortunately, these tools may not be available to the MS4 operator in a format that is both concise, and directly applicable.  To address this need, this thesis recommends stormwater model creation and refinement strategies for Phase II MS4s using GIS and Python scripting.  Further recommendations on using a popular discharge measurement technique for model calibration are provided.  This workflow is then demonstrated in a watershed in Blacksburg, Virginia, where a unique MS4 permitting partnership allowed the development of these tools.  Finally, further improvements to the workflow are suggested along with ideas for additional research for stormwater management in Phase II MS4s. / Master of Science