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Low impact development (LID) strategies are being encouraged in many communities as an approach to reduce potential adverse impacts of development on receiving streams. Many questions exist regarding how well various LID strategies perform in different settings, just as similar questions have been raised regarding performance of traditional stormwater best management practices (BMPs). Whereas historical focus on BMP performance has been water quality concentrations or loads, characterization of volume reduction benefits for both conventional and LID practices is increasingly an objective of researchers and stormwater managers. More than a decade ago, Urban Water Resources Research Council (UWRRC) members worked to develop a set of standardized monitoring and reporting protocols for traditional BMPs and to establish a master database for the purpose of evaluating BMP performance and the factors affecting performance. This effort culminated in the International Stormwater BMP Database (www.bmpdatabase.org),...

This report and the accompanying manual summarize the research and resulting guidance developed for achieving surface runoff volume reduction of highway runoff in urban areas. Literature review, synthesis, and focused new analyses were conducted to inform and develop guidance for identifying, evaluating, selecting, and applying volume reduction approaches (VRAs). A stepwise approach was developed and included in the Guidance Manual that guides the user through the evaluation and selection of VRAs. Regulatory and physical considerations were evaluated related to volume reduction in the urban highway environment, and key factors influencing the ability to achieve volume reduction were identified. Recommendations for project-specific site assessment efforts to support volume reduction planning and design were researched and included. Nine primary VRAs were identified that are specifically applicable to the urban highway environment, as well as accompanying site planning and evaluation approaches. A framework for evaluating the applicability, feasibility, and desirability of these VRAs based on site-specific factors was developed. Processes were also developed for prioritizing VRAs and developing conceptual designs. In support of the Guidance Manual, a spreadsheet-based Volume Performance Tool was developed to provide planning level estimates the performance of VRAs, and four white papers on key technical topics were developed. The findings of this research suggest that site-specific conditions are critical in determining the applicability, feasibility, desirability, and effectiveness of VRAs. Additionally, maintaining VRAs is critical for long term effectiveness.

This guidance manual provides practical, research-based evaluation and implementation practices for the reduction of stormwater volumes in urban highway environments. The manual outlines a five-step process for the identification, evaluation, and design of feasible solutions for runoff volume reduction based on site-specific conditions. It is accompanied by a CD-ROM (CRP-CD-162) containing a Volume Performance Tool to assist the user in efficiently estimating the performance of volume reduction approaches and understanding the effects and sensitivity of local climate patterns, design attributes, and site conditions. The manual also includes a set of volume reduction approach fact sheets and a user guide for the Volume Performance Tool. This guidance manual will be useful to DOT managers, project staff and design engineers, permit writers, consultants, and planners.

A wide range of treatment controls (commonly known as “best management practices” or BMPs) have been developed to manage stormwater. Treatment BMPs use various processes to mitigate the impacts of pollutants and altered hydrology; for example, by attenuating the flow or reducing the volume of stormwater or by reducing pollutants with physical, biological, or chemical processes. This report provides information on long-term performance and life-cycle costs for highway-related stormwater BMPs. Information was gathered via literature review, survey of departments of transportation (DOTs), and interviews with practitioners. Information for treatment BMPs was derived from DOT studies and the International BMP Database. The International BMP Database contains performance records for treatment BMP evaluations throughout the United States. The research included conducting a large number of computer simulations using the U.S. EPA’s Stormwater Management Model (SWMM) to quantify runoff volume reduction through the BMP. The report is accompanied by a CD-ROM (CRP-CD-153) containing a BMP evaluation tool in a spreadsheet format as a computational aid that provides average annual performance and whole life costs for treatment BMPs. A guide (Planning Tool Handbook) is provided as Appendix F to quickly orient the user to the basic functions of the tool provided on the CD-ROM. The report will be of interest to state DOT highway design and environmental practitioners.

The goal of this U.S. EPA-funded cooperative research program with the ASCE is to develop a more useful set of data on the effectiveness of storm water best management practices (BMPs) used to reduce pollutant discharges from urban development. BMP monitoring data gathered at a particular site should not only be useful for that site, but also need to be useful for comparing the effectiveness of similar and different types of BMPs at other locations. Most BMP effectiveness studies in the past have provided data that is difficult, if not impossible, to use in comparing BMP design effectiveness and in the selection among individual BMP types to meet desired goals. This paper describes some of the comparability problems encountered between different BMP effectiveness studies. Also discussed are considerations that affect data transferability, such as methods used for determining efficiency and statistical significance. It outlines the efforts used to establish and analyze the currently available data and prop...

The planning and developing of water resources to meet the country's needs for water supply, flood control, hydroelectric power, irrigation, and navigation now needs to take more account of environmental needs and regulations. Water resource development is often beneficial to the environment, but may also be harmful, as in cases involving salmon and various other endangered species. As a national objective, the environment must be preserved and in some cases restored; but how can this be done consistent with other national objectives, relating to life and welfare of human beings? This problem has aroused the concern of many engineers and water scientists. As a result, a national conference on this subject was held in Chicago in June 1998, as an integral part of ASCE's Annual Conference on Water Resources Planning and Management and the Annual Conference on Environmental Engineering. At the conclusion of that conference, a post-conference meeting was held by a group of prominent water resource practitioners. It was concluded at this meeting that action should be taken by our government to establish a new form of interagency approach, involving the states, as a means of coordination in cases of national importance.

The Water Environment Research Foundation (WERF) has funded a project entitled Linking BMP Systems Performance to Receiving Water Protection to Improve BMP Selection and Design. The project includes the development of a modeling tool called the BMP Selection/Receiving Water Protection Toolbox (Toolbox). After selecting an initial representative list of water quality parameters and stormwater BMPs, algorithms for the BMP Module of the Toolbox were evaluated and compared. Three general types of algorithms were considered for BMP performance modeling: (1) hydraulic algorithms - these determine the volumes captured, stored, and bypassed by the BMP, (2) hydrologic algorithms - these determine the volume losses within the BMP due to infiltration and evapotranspiration and/or use (in the case of cisterns), and (3) treatment algorithms - these determine the concentration reductions provided by the BMP. Based on applicable unit treatment processes, available performance data, and desired level of user input requirements, BMP modeling approaches have been recommended.

Although historically the process of selecting stormwater treatment systems has centered on choosing categories of treatment systems from a menu of options, the state of the practice allows for a more fundamental approach that explicitly incorporates the concept of unit operations and processes (UOPs) in a manner analogous to the conceptual design process for wastewater treatment systems. The incorporation of one or more UOPs into specific design elements of a stormwater treatment system, or treatment system components (TSCs), places emphasis on the selection of systems that are intended, by design, to specifically address project goals and objectives. These TSCs include conventional stormwater treatment design elements (e.g., swales, ponds, tanks, etc.) that provide primary and secondary UOP mechanisms (e.g. settling, filtration, adsorption, precipitation, etc.), as well as pre-treatment devices (e.g., hydrodynamic devices, trash racks, catch basin screens, etc.), custom hydraulic controls (e.g., flow splitters, weirs, orifices, etc.), and tertiary enhancements (e.g., soil amendments, selected vegetative species and microorganisms, mixing and aeration devices, and disinfection systems).The purpose of this guidance document is to provide a framework, or conceptual design methodology, for applying fundamental principles of UOPs to aid in the evaluation and selection of runoff management and treatment control systems for urban and urbanizing areas.The steps of the conceptual design process presented herein include: This title belongs to WERF Research Report Series ISBN: 9781843397410 (eBook)

The retention of stormwater on-site with the goal of mimicking pre-development hydrology is increasingly being required or encouraged for new and redevelopment projects. The recently adopted Ventura and Orange County Municipal Separate Storm Sewer System (MS4) NPDES permits require retention on site of runoff from storms up to the water quality design storm via infiltration, evapotranspiration and/or harvest and use. To date, the retention of stormwater on site has primarily been accomplished via infiltration and, to a much more limited extent, evapotranspiration. In only a few cases has rainwater harvesting and non-potable use been employed on a site scale; typically when it has been included it has been part of meeting LEED requirements. The feasibility and desirability of retaining stormwater on site up to a specific design storm has not been vetted technically on a national or regional scale. For example, there has been almost no consideration of the natural water balance in technical guidance. Often infiltrated volumes must be increased over natural conditions in order to match pre-development surface runoff volumes, yet there has been little consideration for whether increasing infiltration over natural conditions may be an issue. There has also been almost no assessment of the circumstances necessary for rainwater harvesting systems to work well for stormwater management. This paper presents some of the considerations for retaining on site and proposes conceptual criteria for determining whether it is feasible and/or desirable to do so. The paper reviews and discusses the general precipitation and runoff patterns for California and the west coast; the natural water balance and changes to that balance under developed and low-impact development conditions; under what conditions infiltration is feasible and desirable with examples of evaluations; what levels of evapotranspiration can be achieved; and finally presents some examples of the use of rainwater harvesting as a means to retain stormwater on site. Example modeling scenarios of rainwater harvesting for irrigation and toilet flushing are presented from Southern California and Portland, Oregon to highlight both the stormwater management results as well as impacts on potable water demand. Implications related to reclaimed water use will also be presented.

Low Impact Development (LID) performance monitoring and reporting is currently in its infancy and continues to evolve as LID practices and site designs are implemented in more communities. The majority of the currently completed or published LID studies focus on monitoring individual LID practices, such as a single bioretention cell or a single green roof. However, the primary goal of incorporating LID into developments is to affect the ways in which overall site design and implementation impact hydrology and water quality. It is this level of monitoring for which there is not currently adequate guidance or uniformity that would allow comparisons amongst LID sites. The 2009 version of the Urban Stormwater BMP Performance Monitoring Manual (Geosyntec Consultant and Wright Water Engineers for USEPA, WERF, FHA and EWRI/ASCE) has added chapters to focus on site level monitoring and to suggest approaches to assess the collective effects at this scale. The structure and functionality of the International BMP Database has also been revised to accept practice-level and site-level LID studies. LID site-level monitoring is differentiated from practice-level monitoring, and general site-level monitoring approaches are introduced with key considerations for designing a study to reach meaningful conclusions. Creative approaches to optimize study design are introduced that consider such factors as the length of monitoring period and intensity of instrumentation. Concepts in characterization of composite watershed attributes and site hydrologic characterization are introduced and discussed in the context of LID sites. Finally, methods of interpreting data to evaluate site-level performance and methods of comparing LID site performance between sites and to other BMPs are introduced. A case study is highlighted, demonstrating methods that have been employed for LID site monitoring and data interpretation.

Numerous efforts to improve storm water quality on a watershed-wide basis have lacked a reproducible, transparent, methodical approach to optimize the selection and implementation of structural Best Management Practices (BMPs) to meet receiving water quality goals. For urban stormwater retrofit projects, structural BMPs are primarily selected in an opportunistic, site-specific fashion, focusing on one or a few pollutants, stakeholder inputs, or local funding availability. As geographic information systems (GIS) technologies improve, computing power increases, and pollutant loading and treatment efficiency datasets become more robust there is an opportunity to improve watershed-based implementation planning efforts. The authors have developed methods to prioritize water quality improvement needs and identify structural BMP opportunities on a watershed scale to maximize the return on investment for improved water quality. Opportunity evaluation considers property ownership and the potential for retrofitting to achieve multiple benefits, such as water quality, aesthetics, recreation, and habitat. It is recognized that different types of land parcels typically have different types of opportunities and constraints. BMP opportunities can be regional or distributed in nature depending on the land use, size, and location of the public parcel, among other things. By utilizing GIS-based parcel and land use information in the context of water quality need, potential opportunities, and constraints, as well as BMP unit processes and performance data, watershed-wide benefits of BMP implementation can be assessed. An analysis tool has been developed in the ArcGIS™ environment that automates many of these spatial prioritization and identification processes, while allowing for stakeholders and decisions-makers to provide meaningful input that supports BMP implementation decisions. Once candidate BMPs have been identified for a high priority area, the tool can estimate the potential pollutant load reductions and planning level costs associated with site-specific implementation. Example studies are provided to illustrate the concepts and GIS methods employed.

Selenium is among the pollutants of concern in a growing number of areas within California. TMDLs proposed by USEPA (USEPA, 2002) for the San Diego Creek watershed in Orange County, California (and in Ballona Creek and Los Angeles River in Los Angeles County) calls for an in-stream standard of 5 ppb total selenium. Selenium sources and treatment methods, however, are significantly different than those typically utilized as stormwater BMPs for metals such as copper, lead and zinc. Studies on the sources of selenium in the San Diego Creek Watershed indicated that a major potential source is groundwater seepage, particularly in areas where the groundwater table is shallow. Thus dry-weather concentrations and loads are of the most interest. Within the San Diego Creek Watershed, flows appear to correspond to a location where historically, naturally occurring selenium became sequestered under anaerobic conditions in the peat soils within a swamp. Urbanization and draining of the swamp appears to have created aerobic conditions, releasing selenium from these sediments into groundwater in a soluble phase. Selenium concentrations in the surface water discharge have been observed in the 50 ppb range, and local groundwater selenium concentrations commonly exceed 100 ppb. Several technologies potentially reduce selenium (Se) concentrations to low levels (

Low Impact Development (LID) strategies are being encouraged in many communities as an approach to reduce potential adverse impacts of development on receiving streams. Many questions exist regarding how well various LID strategies perform in different settings, just as similar questions have been raised regarding performance of traditional stormwater best management practices (BMPs). Over a decade ago, Urban Water Resources Research Council (UWRRC) members worked to develop a set of standardized monitoring and reporting protocols for traditional BMPs and establish a master database for the purpose of evaluating BMP performance and the factors affecting performance. This effort culminated in the International Stormwater BMP Database (www.bmpdatabase.org), which contains data for over 350 BMPs and continues to operate as a clearinghouse for stormwater BMP data and performance analyses. During 2008–2009, the Stormwater BMP Database project expanded to better integrate LID into the database. This paper provides a condensed overview and progress report on the LID-focused effort, including the following topics: 1) monitoring guidance for LID at the overall site development level; 2) an overview of recent changes to the stormwater BMP database to better accommodate LID studies, 3) a summary of LID studies currently included in the database; and 4) initial concepts related to evaluating performance of LID studies.

Stormwater modeling in urban areas has risen to a high level of accomplishment, yet many improvements and research needs remain. This paper summarizes a foundationtype conference on the state-of-the-art of urban stormwater modeling, held in July 2007. Conference highlights and research needs are provided.

Over the past several decades, there has been a noticeable decline in the clarity of Lake Tahoe. The subject has been one of much debate and study and the scientific community agrees that this decline is largely due to increased human activities within the watershed tributary to the lake as well as potential atmospheric deposition from regionally adjacent areas. Although BMPs have been implemented to target pollutants in surface runoff, water quality of the Lake has still not improved to desired levels. Therefore, enforceable restrictions on nutrients and sediment being carried to the lake via surface water, Total Maximum Daily Loads (TMDLs), are being established for the Lake's watershed. The purpose of this Stormwater BMP Evaluation and Feasibility Study was to evaluate the cost-effectiveness and other attributes of alternative project level and basin-wide enhanced BMP implementation for reducing urban runoff pollution to Lake Tahoe. As part of this study, the effectiveness and costs of a variety of currently implemented BMPs, viable improvements to these BMPs, and potentially effective new and enhanced technologies were evaluated. This paper describes the approach for gathering and developing this information. The results of this work are intended to ultimately be incorporated into a watershed model to estimate the potential reductions in loading of pollutants to the lake through BMP implementation on a basin-wide scale. The current and potential future performance of BMPs (and their costs) will be considered within the context of TMDL development as well as in future Tahoe Basin plan development efforts. The paper provides a summary of the potential for BMPs and enhanced BMPs to meet potential loading reductions for fine particulates and nutrients. The approach including long-term simulation modeling to ascertain the hydrologic and hydraulic performance of potential BMP sizing and designs, assessment of BMP performance through a combination of statistical assessments of the International BMP Database and unit processes, and local Lake Tahoe BMP performance information. The effort determined that significant reductions in fine particulates and phosphorus are achievable if appropriate BMP types (wet ponds, wetlands, and extended detention ponds with a small pool) are employed.

This paper presents the findings of several critical studies that the project team is conducting to evaluate the feasibility and long-term viability of using decentralized controls for CSO management. The project team has developed a substantial set of analysis tools for use in the development of modeling and monitoring programs, including proposed classification schemes for decentralized controls and descriptions of unit processes. The study also provides guidance on determining the feasibility and effectiveness of controls, including an analysis of optimization, cost, and techniques to manage runoff volume, peak discharge rate, and water quality. Regional case studies on strategies, costs, and maintenance practices are presented.

All cities have two primary impervious elements; rooftops and pavement, with a typical average of about 50% of the surface area of a city at full build out. The results of this imperviousness levels have been well documented and include increased destabilization of streams and increased runoff pollutant loadings and concentrations. This paper will present an overview of the research that is taking place in the United State on the effects of EcoRoofs (green roof) to reduce runoff and improve water quality. The paper includes a summary of published research as well as information obtained from a survey of known researchers. Results to date have shown that EcoRoofs can significantly reduce runoff volumes as well as reduce peak flows. Water quality results have been somewhat mixed with some pollutant concentrations being reduced while others increase. The factors that affect this include soil types, roof maintenance and roofing materials. The reduction in runoff volumes however helps to reduce pollutant loadings even in some cases where concentrations increase. Studies from a wide geographic distribution will be evaluated, including North Carolina, Pennsylvania, Florida, Chicago, New York City, Vancouver, and Portland, Oregon. The paper will also present a more detailed evaluation of monitoring data from Portland EcoRoofs and how such data can be used to validate design and modeling approaches for stormwater management. This will include how the effects can be factored into management efforts for controlling combined sewer overflows as well as separated systems and be considered together with other low impact development approaches.

The US EPA/ASCE National Stormwater BMP Database has grown significantly since the first evaluation of BMP performance data in the database was completed in 2000. The project team has conducted a limited re-evaluation of the data contained in the database to assess the overall performance of BMPs as well as compare BMP design attributes to performance. The evaluations have included the assessment of various BMP types as categorized in the database with regards to their ability to reduce runoff volumes as well as the effluent quality they can achieve. It is apparent that certain BMP types can reduce the volumes of runoff through soil soaking and resulting evapotranspiration and/or infiltration as opposed to BMPs that are more “sealed,” such as wet ponds, wetlands, vaults and hydrodynamic devices. Runoff reductions contribute to pollutant loadings reductions as does improved effluent quality. On average, dry detention basins were found to reduce runoff volumes by an average of 30 percent (comparison of inflow to outflow), while biofilters reduced volumes by almost 40 percent. As expected, wet ponds, wetlands, hydrodynamic devices, and retention ponds show little or no runoff volume reductions. BMP types vary with regards to effluent quality that is achieved. BMPs such as wet ponds and wetlands appear to achieve lower concentrations in effluent quality than other BMPs such as detention ponds (dry) and hydrodynamic devices for some parameters. These differences vary with pollutant type. Finally, analyses of BMP design vs. performance are beginning to result in statistically valid results. For example, a relationship (ratio) between the treatment volume of retention ponds (wet ponds) vs. the average size measured storm event volume monitored has been established, showing that those with a ratio of 1 or more have been observed to achieve better effluent quality. Based upon these findings, this paper discusses potential BMP selection, design, and BMP design standards implications. A set of potential recommendations for how communities develop and specify BMP requirements such that more local goals for improved water quality is discussed.

The US EPA/ASCE National BMP Database has grown significantly since the first evaluation of BMP performance data in the database was completed in 2000. The project team is currently performing a re-evaluation of the data contained in the database to assess the overall performance of BMPs as well as compare BMP design attributes to performance. Although this analysis has not been fully completed, several initial results are presented in this paper. The evaluations have included the assessment of various BMP types as categorized in the database with regards to their ability to reduce runoff volumes as well as the effluent quality they can achieve. It is apparent that certain BMP types can reduce the volumes of runoff through soil soaking and resulting evapotranspiration and/or infiltration as opposed to BMPs that are more “sealed,” such as wet ponds, wetlands, and vaults. Runoff reductions contribute to pollutant loadings reductions as does improved effluent quality. On average, dry detention basins were found to reduce runoff volumes by an average of 30 percent (comparison of inflow to outflow), while biofilters reduced volumes by almost 40 percent. As expected, wet ponds, wetlands, and hydrodynamic devices, and retention ponds show little or no runoff volume reductions. BMP types vary with regards to effluent quality that is achieved. BMPs such as wet ponds and wetlands appear to achieve lower concentrations in effluent quality than other BMPs such as detention ponds (dry) and hydrodynamic devices. These differences vary with pollutant type. Finally, analyses of BMP design vs. performance are beginning to result in statistically valid results. For example, a relationship (ratio) between the treatment volume of retention ponds (with wet pools) vs. the average size storm event volume monitored has been established, showing that those with a ratio of 1 or have been observed to achieve better effluent quality.

It is well documented that urbanization modifies the natural watershed and stream hydrologic and geomorphic processes by altering the landscape, modifying vegetation and soil characteristics, and introducing impervious surfaces and drainage infrastructure. The resulting increases in the volume, frequency, and duration of runoff from development projects are known as hydromodification. Hydromodification of natural runoff processes intensifies sediment transport and often leads to stream bank erosion and channel incision. These types of changes have occurred in San Francisco Bay Area streams. Stormwater management programs in the Bay Area are being required to address the impacts from new and redevelopment projects on the beneficial uses of streams resulting from erosion caused by hydromodification. The Santa Clara Valley Urban Runoff Pollution Prevention Program (SCVURPPP) NPDES permit requires co-permittee agencies to develop a Hydromodification Management Plan (HMP) that describes how they plan to manage increases in runoff volume and peak flow from development projects and protect stream systems. The HMP will establish in-stream stability criteria and runoff control design criteria, and provide guidance on management measures, which include a combination of on-site, in-stream, and regional control strategies. The HMP will also consider water quality and habitat impacts in evaluating and recommending control measures. To date, the project team has conducted a literature review, developed a conceptual model and defined the assessment method. Characterization of streams and future development, field data collection, and hydrologic/hydraulic modeling is underway, and an HMP assessment report is scheduled for completion in June 2003.

Proceedings of the Ninth International Conference on Urban Drainage, held in Portland, Oregon, September 8-13, 2002. Sponsored by Urban Water Resources Research Council of the Environmental Water Resources Institute of ASCE; International Association for Hydraulic Research; Joint Committee on Urban Drainage of the International Water Association. This collection of 329 papers presents state-of-the-art solutions to common urban storm drainage problems, such as flooding, combined sewer overflows, nonpoint-source runoff quality, urban streams, and more. Papers report on practical applications of new and traditional methods of analysis, design, construction, operation, and maintenance of urban storm drainage systems, as well as on institutional and educational approaches for improving the urban water environment. Topics include: best management practices; decision support and modeling; combined sewer overflows and sanitary sewer overflows; stormwater management; urban streams; monitoring; precipitation and hydrology; flooding; education and involvement; sewer processes; hydraulics; real-time control; and Portland, Oregon, wet weather.

A Natural Treatment System (NTS) Master Plan comprised of a network of constructed wetlands was evaluated for treatment effectiveness of dry weather base flows and runoff from smaller more frequent storms in a 120 square mile (311 km 2 ), urban watershed. The goal of the `regional retrofit' wetland network is to serve as an integral component in watershed-wide BMPs for compliance with recent and proposed pollutant loading limits (TMDLs) requiring discharge limits of sediments, nutrients, pathogen indicators, pesticides, toxic organics, heavy metals, and selenium. The NTS Plan was assessed with `planning-level' water quality models that account for the integrated effects of the planned 44 NTS facilities. The NTS Plan is estimated to achieve total nitrogen (TN) TMDL for base flows, and in-stream TN concentrations would be reduced below current standards at most locations. Total phosphorous TMDL targets would be met in all but the wettest years. The fecal coliform TMDL would be met during the dry season, but not all wet season base flow conditions, and not under storm conditions. The NTS Plan is not designed to meet the sediment TMDL, but would capture on average about 1900 tons/yr (1724000 kg/yr) of sediment from urban areas. The wetlands are estimated to remove 11 percent of the total copper and lead, and 18 percent of the total zinc in storm runoff. The NTS Plan provides a cost-effective alternative to routing dry-weather flows to the sanitary treatment system.

Groundwater flow and contaminant transport models have been widely used for planning and design purposes in the past decade. Two of the most significant limitations for application of these models are data availability and parameter estimation. By use of a parameter identification algorithm and synthesized data, it is possible to isolate the effects of data availability and data uncertainty. This approach was implemented using the U.S. Geological Survey's method of characteristics (USGS-MOC) model for a hypothetical aquifer. A parameter identification scheme attached to the USGS-MOC model was used to determine unknown transmissivities and dispersivities. The study results showed that the predictive ability of the USGS-MOC model (and, by implication, similar models) is limited unless relatively extensive and good quality data are available. For the example tested, it was found that extending the length of the observation series was more effective in improving parameter estimates and resolution of the contaminant plume prediction than adding observation wells. Further, when the boundary conditions were known, the contaminant predictions were much more sensitive to accurate estimation of transmissivity than to the estimation of dispersivities. The numerical results also showed that after a relatively short simulation period (less than 4 years), predicted contaminant concentrations could be significantly in error. This suggests the importance of integrating uncertainty analysis into the prediction of long-term contaminant transport.

The purpose of this paper is to answer the question of why it is so difficult to obtain reliable performance data for urban stormwater best management practices (BMPs) such as retention/detention ponds, wetlands, swales, infiltration devices, proprietary devices, and others. Our assessment of this question is based on extensive experience developing, implementing, and conducting stormwater monitoring programs, as well as development and oversight of the U.S. Environmental Protection Agency (USEPA)/American Society of Civil Engineers (ASCE) National Stormwater BMP Performance Database (www.bmpdatabase.org), on behalf of ASCE’s Urban Water Resources Research Council. The paper begins with a list of factors that contribute to the difficulty of BMP monitoring, and concludes with a review of representative problems that we have experienced when reviewing BMP performance data sets that have been submitted to the National BMP Performance Database Clearinghouse for integration into the database.