Incentive Index Developed to Evaluate Storm-Water Low-Impact Designs

In practice, the challenge of storm-water low-impact-development (LID) design is often related to how to quantify the effectiveness of a LID layout. In this study, the watershed imperviousness was chosen as a basis to evaluate the performances of various LID designs. Often, LID designs apply cascading planes to drain the runoff flow from the upstream impervious area to the downstream pervious area. In this study, the conventional area-weighting method is revised with a pavement-area-reduction factor (PARF) to produce the effective imperviousness. PARF is employed as an incentive index to quantify the on-site runoff volume reduction and cost savings from downsized sewers. Two sets of PARF are derived: conveyance-based and storage-based LID designs. The conveyance-based LID approach is to drain runoff flows on various porous surfaces while the storage-based LID approach is to temporarily store runoff flows in an on-site basin. For a specified LID layout, the PARF provides a consistent basis to translate the infiltration and storage effects into the reduction on the area-weighted imperviousness. The nondimensional governing equation derived in this paper indicates that the PARF depends on the ratio of the soil infiltration rate to rainfall intensity, the ratio of receiving pervious area to upstream impervious area, and the on-site storm-water storage capacity. The PARF serves as a basis for the engineers, planners, and/or developers to select a LID design and also for regulatory agencies to assess meritorious credits for cost savings.     

Storage Volume and Overflow Risk for Infiltration Basin Design

This study presents a risk-based approach for the design of infiltration basins. The design parameters include basin storage volume, drain time, and overflow risk. At a basin site, the storm-water detention storage volume is determined by design runoff capture volume, tributary watershed area, and runoff coefficient. The basin geometry is dictated by the water volume balance between the surface storage volume in the basin and the subsurface storage capacity in soil pores. The drain time at a basin site is found to be a function of initial soil water content, soil porosity, infiltration rate, and distance to the ground-water table. After knowing the basin geometry and size, the basin's performance can be evaluated by the overflow risk analysis using the local average event rainfall depth and interevent time. In practice, a sensitivity test on overflow risk can be conducted with a range of basin storage volumes. The risk-based approach presented in this study provides an algorithm to calculate the long-term runoff capture percentage for a basin size. The diminishing return on runoff capture percentage can serve as a basis to select the proper basin storage volume at the site.     

Simulation of the Performance of a Storm-Water BMP

Vegetated storage-infiltration best management practices (BMPs) have become an increasingly popular means of attenuating and treating runoff from developed land. However, the hydrologic and pollutant removal performances of these facilities can be highly variable. A mathematical model of an idealized BMP was developed in order to quantify the impact of variable hydrologic and pollutant concentration input on BMP performance by simulating the treatment performance of the model system during 1,250 non-steady-state storm events generated based on historic Maryland rainfall data. The model BMP was effective in attenuating volume (42% total volume reduction) and peak flow (median peak output to peak input flow ratio was 0.058). The simulated mean effluent pollutant event mean concentration was much less than the influent (0.284 compared with 1.51 mg/L) and the overall mass load reduction was 92%. However, the performance parameters demonstrated significant variability. Consequently, the results suggest a need to incorporate into BMP performance guidelines the impact of the variable influent hydrologic and pollutant concentration characteristics. Emphasis should be placed on discharge water quality and statistical distributions rather than on single-percent removal values.     

Hydrologic Performance Monitoring of an Underdrained Low-Impact Development Storm-Water Management System

The use of low-impact development (LID) storm-water management facilities will grow with gains in understanding of their performance based on field tests. An innovative flow measurement system was designed and tested for monitoring of an interconnected system of underdrained LID components forming a site management strategy. Pressure transducers housed in stilling wells provided in-line flow measurements in pipes connecting the LID components. A combination of laboratory experiments, field measurements, and computer simulations was used to calibrate the flow measurement system to translate depth measurements to estimates of flow. The monitoring system is well suited for high-resolution temporal monitoring and provides important information for evaluating LID component performance. The measurement system is limited to open-channel flow, but calculations indicate that surcharge conditions are expected to occur at the case study site only under conditions more extreme than the 100-year storm event.     

Event and Continuous Hydrologic Modeling with HEC-HMS

Event hydrologic modeling reveals how a basin responds to an individual rainfall event (e.g., quantity of surface runoff, peak, timing of the peak, detention). In contrast, continuous hydrologic modeling synthesizes hydrologic processes and phenomena (i.e., synthetic responses of the basin to a number of rain events and their cumulative effects) over a longer time period that includes both wet and dry conditions. Thus, fine-scale event hydrologic modeling is particularly useful for understanding detailed hydrologic processes and identifying the relevant parameters that can be further used for coarse-scale continuous modeling, especially when long-term intensive monitoring data are not available or the data are incomplete. Joint event and continuous hydrologic modeling with the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) is discussed in this technical note and an application to the Mona Lake watershed in west Michigan is presented. Specifically, four rainfall events were selected for calibrating/verifying the event model and identifying model parameters. The calibrated parameters were then used in the continuous hydrologic model. The Soil Conservation Service curve number and soil moisture accounting methods in HEC-HMS were used for simulating surface runoff in the event and continuous models, respectively, and the relationship between the two rainfall-runoff models was analyzed. The simulations provided hydrologic details about quantity, variability, and sources of runoff in the watershed. The model output suggests that the fine-scale (5?min time step) event hydrologic modeling, supported by intensive field data, is useful for improving the coarse-scale (hourly time step) continuous modeling by providing more accurate and well-calibrated parameters.     

Integration of Low-Impact Development into the International Stormwater BMP Database

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), which contains data for more than 360 BMPs and continues to operate as a clearinghouse for stormwater BMP data and performance analyses. During 2008–2009, the International Stormwater BMP Database project expanded to better integrate LID into the database and develop a set of metrics that can be used to characterize BMP performance with regard to surface runoff volume reduction. 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 International Stormwater BMP Database to better accommodate LID studies, (3)?a summary of LID studies currently included in the database, and (4)?a proposed approach for evaluating performance of LID studies with regard to reducing surface runoff volumes.     

Evaluating Urban Pollutant Buildup/Wash-Off Models Using a Madison, Wisconsin Catchment

Buildup/wash-off (BUWO) models are widely used to estimate pollutant export from urban and suburban watersheds. Here, we propose that the mass of washed-off particulate during a storm event is insensitive to the time between storm events (the traditional predictor of particulate accumulation in BUWO models). Our analysis employed USGS data of total suspended solids and discharge data for nonsnow events in a 9.4-km2 suburban catchment in Madison, Wis. Kinetic energy of rainfall was calculated using National Weather Service NEXRAD radar reflectivity. A regression analysis found that storm event runoff volume and rainfall kinetic energy explained 81% of the variability in event particulate load; volume alone explained 69% of the variability in event loads. Time between storm events was not significant. Additionally, we simulated storm event particulate loads using a BUWO model and a model assuming a constant mass available for wash-off. Both models produced very similar predictions over a range of parameterizations, suggesting that buildup models could perhaps be simplified under many circumstances.     

Seasonal Performance Variations for Storm-Water Management Systems in Cold Climate Conditions

Lack of widespread adoption of low-impact development (LID) designs in northern climates is in large part due to concerns about poor winter performance relating to (1) frozen filter media; and (2) dormant biological functions. An examination of six varied LID designs, in contrast with conventional best-management practices (BMPs) and manufactured systems illustrated that seasonal functionality was evident for many systems; however, the LID designs were consistently top storm water management performers. The designs were tested and monitored for cold climate performance from 2004–2006 to assess: filter media frost penetration, hydraulic efficiency, and seasonal variations of contaminant removal efficiency. LID systems evaluated included: two types of bioretention systems, a surface sand filter, a subsurface gravel wetland, a street tree, and porous asphalt. The LID performance data were contrasted with conventional structural BMPs (swales, retention pond) and some select manufactured storm-water systems (hydrodynamic separators); (3) a filtration system, and a subsurface infiltration system. Seasonal performance evaluations indicate that LID filtration designs differ minimally from summer to winter, while smaller systems dependent largely on particle settling time demonstrated a marked winter performance decline.     

Graphical Calculation of First-Flush Flow Rates for Storm-Water Quality Control

Regulations for mitigating nonpoint source pollution from small catchments often include requirements for treating a first-flush depth of runoff, either by storing the storm water until it can be treated and released, or by passing it through a filtering device. In either case, the structural measure used to improve water quality needs to be designed or selected to accommodate a flow rate that corresponds to the first-flush runoff depth. An uncomplicated graphical procedure for calculating first-flush design flow rates is presented that is based on standard National Resource Conservation Service rainfall–runoff computation methods in which excess precipitation obtained by applying the runoff curve-number approach to 24-h design storm storms is transformed to runoff using triangular unit hydrographs. The solution is made dimensionless by grouping parameters, and, as a result, can be condensed into a single graph that provides highly accurate flow rate estimates.     

Modeling Storm Water Mass Emissions to the Southern California Bight

Storm water runoff is perceived as a major source of pollutants that results in adverse environmental effects, but large-scale assessments are rarely conducted. The problem is particularly pronounced in southern California where 17 million people have rapidly developed coastal watersheds. The goal of this study was to make regionwide estimates of mass emissions, assess the relative contribution from urbanized watersheds, and compare pollutant flux from different land uses. A geographic information system-based storm water runoff model was used to estimate pollutant mass emissions based on land use, rainfall, runoff volume, and local water-quality information. Local monitoring data were used to derive runoff coefficients; over 1,700 storm water sampling events were used to calibrate and validate annual loadings. An average rainfall year produced 1,073×109?L of runoff, 118,000 metric tons (MT) of suspended solids, 1,940 MT of nitrate-N, 108 MT of zinc, and 15 kg of diazinon. The majority of mass emissions were from urbanized watersheds except for suspended solids, total DDT, and chlorpyrifos. Agricultural areas had the greatest fluxes for pesticides, including total DDT and chlorpyrifos while open areas typically had the smallest.     

Retrofitting Detention Basin with Water Quality Control Pool

Many detention basins built before 1990 are not equipped with storm-water quality control device. With the latest developments in low impact development for storm-water management, these existing detention basins need modifications on their outlet structures to increase on-site runoff treatment and disposal. An outlet shall be designed to have, at least, three levels of release, including water quality release over 12–48 h, low flow release for 10-year event, and 100-year high flow release. All these efforts are to aim at the full spectrum runoff treatment that is not only to capture the minor and major events but also to store microevents. Over the years, the empirical methods under different assumptions have been developed for determining the design storm-water quality control volume (WQCV). To improve the consistency in storm-water detention designs, this paper presents a mathematical model that produces the synthetic runoff-volume capture curves normalized by the local average rainfall event-depth. A runoff-volume capture curve defines the relationship between WQCV and runoff capture ratio on a long-term basis. A higher runoff capture ratio requires a larger storage volume. Using the runoff capture curve as the basis, the WQCV can be consistently determined for the preselected runoff capture target such as the 80% recommended by the U.S. Environmental Protection Agency in 1986. A case study illustrates how to retrofit an existing outfall concrete vault with a perforated plate and a micropool for WQCV. With a three-level release control, the outfall box can have a slow release for microevents and a fast release for extreme events. This procedure has been recommended for designing a new basin and retrofitting an existing one for the metro Denver area. Details can be found in the UD-DETENTION computer model available at www.udfcd.org at no cost to download.     

Case Study: Hydraulic Modeling of Runoff Processes in Ghanaian Inland Valleys

The inland valleys of West Africa are strategic in terms of food security and poverty alleviation, but scientific studies on hydrologic processes happening in these environments have not been well documented. Modeling approaches presented in this paper are an attempt to better comprehend hydraulic phenomena occurring in inland valleys. An inland valley situated in the Northern Region of Ghana is set as the study site. The inland valley comprises well-drained uplands and hydromorphic valley bottoms. There are several earthen dams across the valley bottoms, which are at the same time seasonal wetlands cultivated to rice during the rainy season. A finite volume model for the shallow water equations is developed to numerically simulate surface runoff flows in the valley bottoms during flood events. Innovation is necessitated to handle a series of different hydraulic phenomena. Flux-splitting and data reconstruction techniques are used to achieve stable computation in the complex topography of the valley bottoms. Standard problems of oblique hydraulic jump and dam break flows are used to test the accuracy of the numerical model. The Manning’s roughness coefficient is determined from calibration in another Ghanaian watershed located in the Eastern Region. Using actually observed time series data of rainfall intensity, surface flows during the rainfall events are simulated in the computational domain representing the valley bottoms of the study area. Observed data of water levels in the dams are compared to predictions, and discrepancies between them are examined from the hydrological point of view. In the case of a hypothetical flood event, cascading collapses of the dams and flooding of cultivated fields are reproduced.     

Enhanced Surface Cooling as an Alternative for Thermal Discharges

Excess heat is an unavoidable by-product of electricity generation from fossil and nuclear fuels. In most cases, excess heat is transferred to a cooling water stream and discharged to a local receiving water body, or processed through on-site cooling towers. In many cases existing discharges are potentially responsible for significant ecological impacts, and regulatory authorities are mandating the construction of cooling towers, often at significant expense. Most existing cooling water discharges are designed to reduce excess temperatures through rapid dilution. Enhanced surface cooling is an alternative approach which involves the development of a thin surface plume, while limiting mixing of the discharge with ambient waters. This process encourages rapid transfer of heat to the atmosphere while limiting impacts to sensitive benthic environments and most of the volume of the receiving water body. This discharge approach may be particularly effective for receiving water bodies which have limited natural flushing, such as enclosed bays, estuaries, reservoirs and some river environments. A preliminary case study of a thermal discharge into Mt. Hope Bay (Massachusetts/Rhode Island) is discussed.     

Dynamic Modeling of Storm Surge and Inland Flooding in a Texas Coastal Floodplain

The purpose of this study is to analyze the combined effects of storm surge and inland rainfall on the floodplain of a coastal bayou in the Houston area by using dynamic hydraulic modeling. Most existing floodplains in the Houston area are defined using only rainfall as an input into steady-state hydraulic models and do not consider the impact of hurricane-induced storm surge on the floodplain. HEC-RAS, a one-dimensional flow model, was run for both steady- and unsteady-states to analyze the additional effect storm surge has on the coastal floodplain. Storm surge and rainfall data from Hurricane Ike were utilized to run an unsteady hydraulic model on Horsepen Bayou near Galveston Bay. The dynamic model generated a good match between the modeled hydrograph and measured data in the watershed. Additionally, a timing sensitivity analysis was completed by shifting the timing of the storm surge both earlier and later in time. The dynamic model revealed that the timing of both rainfall and storm surge play a significant role in the magnitude of inland flooding.     

New Insights into the Quality of Urban Storm Water in South Eastern Australia

Quantifying the quality of urban storm water is an important prerequisite to the effective management of urban runoff, which is recognized as the major nonpoint source of pollution in urban areas. Although data on urban storm-water quality are widely available, they are often based on relatively limited data sets, usually containing few samples per event and/or few events per catchment. This paper reports on a large scale monitoring of the key storm-water pollutants found in urban discharges during both wet and dry weather from seven urban catchments in South Eastern Australia. The catchments are all separately sewered (with wholly piped systems) with varying sizes and land uses. Using the same monitoring technique, between 16 and 52 pollutographs were captured at each site for total suspended solid (TSS), total phosphorus, and total nitrogen (TN), while event mean concentrations (EMCs) of heavy metals and major ions, as well as species of N and P, were recorded at a subset of sites. It was found that EMCs of TSS were around 50% less than have been typically reported in earlier literature. During wet weather, nutrients were similar to previously reported, as were most metals concentrations. However, zinc concentrations were significantly higher than previously reported. EMCs of TSS were higher during storm flows than in baseflow, while TN concentrations were consistently higher during baseflow. EMCs of all pollutants monitored were poor with simple hydrological parameters (e.g., event rainfall depth); however, event pollution loads correlated very well with the rainfall intensity to a power, summed over the event duration. It was not possible to distinguish an impact of land use on pollutant concentrations. The first-flush effect was found not to be significant at all sites except the smallest catchment with the simplest drainage layout (the roof of a large building). All these findings have significant implication for treatment strategies with the significantly lower than previously observed TSS requiring consideration in future modeling and treatment design.     

Retrospective Comparison of Watershed Analysis Risk Management Framework and Hydrologic Simulation Program Fortran Applications to Mica Creek Watershed

As part of an ongoing watershed model comparison program for forested watersheds, Watershed Analysis Risk Management Framework (WARMF V5.18) and Hydrologic Simulation Program Fortran (HSPF V10) were independently applied to the Mica Creek Watershed in Idaho. A comprehensive model comparison was made in terms of watershed delineation, hydrologic formulations, model parameterization, meteorological data, hydrologic calibration, and hydrologic verification. Comparison was not made for water quality, which was not simulated in the HSPF application. It was concluded that WARMF is a mechanistic model structured to simulate the hydrologic processes, whereas HSPF is an empirical water budget model. The WARMF is suitable for application to forested watersheds. It successfully predicted stream flows comparable to measured values. The HSPF results were also good, if one ignores an unrealistic amount of water loss to inactive groundwater and an empirical treatment of rain-on-snow events.     

Influence of Hydrology on Rainfall-Runoff Metal Element Speciation

Examination of speciation for metal elements transported in urban rainfall-runoff events is critical when evaluating the potential fate, bioavailability, and effective control of such constituents. In many urban areas anthropogenic activities result in rainfall pH levels that are acidic and low in alkalinity. As a result, finely abraded metallic components and exposed metal infrastructure can be leached into rainfall-runoff. This study examines the influence of hydrology on storm water metal element speciation at the upper end of a Portland cement concrete small urban watershed. This study focused on Pb, Cd, Cu, and Zn; metal elements commonly found in urban and transportation land uses. For this site partitioning results demonstrated that Cd and Cu partitioned nearly equally between particulate and dissolved phases while Zn was generally particulate-bound and Pb was highly particulate-bound. Utilizing water quality analyses, measured ion balances and speciation modeling, results for Cd and Zn indicated that divalent ionic forms of these metals dominated the dissolved species for all events, while Pb was predominately associated with dissolved organic matter (DOM), and Cu was predominately associated with carbonate species or DOM. Of the three events examined, only the mass-limited events demonstrated a change in speciation during the passage of the hydrograph. Results from this study indicate that effective control of storm water metal elements at the upper end of the urban watershed requires unit operations and processes that account for the ionic, complexed and particulate-bound species and account for the hydrology at the upper end of the urban watershed.     

Hydrologic and Water Quality Integration Tool: HydroWAMIT

A spatially distributed and continuous hydrologic model focusing on total maximum daily load (TMDL) projects was developed. Hydrologic models frequently used for TMDLs such as the hydrologic simulation program—FORTRAN (HSPF), soil and water assessment tool (SWAT), and generalized watershed loading function (GWLF) differ considerably in terms of spatial resolution, simulated processes, and linkage flexibility to external water quality models. The requirement of using an external water quality model for simulating specific processes is not uncommon. In addition, the scale of the watershed and water quality modeling, and the need for a robust and cost-effective modeling framework justify the development of alternative watershed modeling tools for TMDLs. The hydrologic and water quality integration tool (HydroWAMIT) is a spatially distributed and continuous time model that incorporates some of the features of GWLF and HSPF to provide a robust modeling structure for TMDL projects. HydroWAMIT operates within the WAMIT structure, developed by Omni Environmental LLC for the Passaic River TMDL in N. J. HydroWAMIT is divided into some basic components: the hydrologic component, responsible for the simulation of surface flow and baseflow from subwatersheds; the nonpoint-source (NPS) component, responsible for the calculation of the subwatershed NPS loads; and the linkage component, responsible for linking the flows and loads from HydroWAMIT to the water quality analysis simulation program (WASP). HydroWAMIT operates with the diffusion analogy flow model for flow routing. HydroWAMIT provides surface runoff, baseflow and associated loads as outputs for a daily timestep, and is relatively easy to calibrate compared to hydrologic models like HSPF. HydroWAMIT assumes that the soil profile is divided into saturated and unsaturated layers. The water available in the unsaturated layer directly affects the surface runoff from pervious areas. Surface runoff from impervious areas is calculated separately according to precipitation and the impervious fractions of the watershed. Baseflow is given by a linear function of the available water in the saturated zone. The utility of HydroWAMIT is illustrated for the North Branch and South Branch Raritan River Watershed (NSBRW) in New Jersey. The model was calibrated, validated, and linked to the WASP. The NPS component was tested for total dissolved solids. Available weather data and point-source discharges were used to prepare the meteorological and flow inputs for the model. Digital land use, soil type datasets, and digital elevation models were used for determining input data parameters and model segmentation. HydroWAMIT was successfully calibrated and validated for monthly and daily flows for the NSBRW outlet. The model statistics obtained using HydroWAMIT are comparable with statistics of HSPF and SWAT applications for medium and large drainage areas. The results show that HydroWAMIT is a feasible alternative to HSPF and SWAT, especially for large-scale TMDLs that require particular processes for water quality simulation and minor hydrologic model calibration effort.     

Probabilistic Approach to Estimation of Urban Storm-Water Total Maximum Daily Loads: Unregulated Catchment

This work examines the basic processes and functions behind urban storm-water pollution delivery into surface waters and develops a set of tools that allow the estimation of pollutant load dynamics on receiving waters. In particular, the group of expressions developed in this paper allows the calculation of runoff parameters (volume, discharge rate and pollutant load) on an event average basis for an unregulated catchment. Using Monte Carlo simulation techniques, the runoff pollutant concentration probability distribution (as event averages) are obtained. Merging these runoff statistics with the stream parameters allows the receiving water pollutant concentration characteristics to be obtained as well as the probability of exceeding threshold pollutant concentrations in the mixing zone of a stream. The simulation can be performed with allowance for different levels of complexity with respect to catchment hydrologic representation and pollutant load functions. As a result, the magnitude of influence of urban runoff on a surface water body can be determined, pollutants of concern can be identified, and certain remedial measures recommended.