Sizing Stormwater Infiltration Structures

A design aid is presented for sizing stormwater infiltration structures. The proposed procedure is based on the hydrological storage equation for an infiltration structure coupled with the Green and Ampt infiltration equation. For the filling process, the two equations are solved simultaneously using a numerical method, and the results are presented in chart form. These charts are useful to determine the maximum water depth in an infiltration structure. For the emptying process, the governing equations are integrated analytically resulting in an algebraic equation that can be solved for the emptying time explicitly. A practical application is included to demonstrate the ease of the suggested procedure.     

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.     

Hydraulic Model for Sedimentation in Storm-Water Detention Basins

Treatment of storm-water runoff may be necessary before discharge to surface waters. In urban areas, space constraints limit selection of conventional treatment systems, and alternative systems are needed. This research program involves design and laboratory testing of a small footprint nonproprietary detention basin which consists of pipes and box culvert sections with a specialized inlet and outlet system. This system can be placed below grade near the roadway section as part of the conventional drainage system and does not require additional right-of-way. A mathematical model, based entirely on hydraulic principles, is developed to estimate particle removal efficiency of the rectangular detention basin for the treatment of storm-water runoff by extending ideal horizontal tank theory under the condition in which water level is varied. A physical model was built in 1/5 scale to measure particle removal performance and validates the conceptual model. Experiments were performed for steady inflow conditions with different inflow rates, durations, and suspended sediment concentrations. Measured time series outflow suspended sediment concentrations and particle removal efficiency compare well with calculated results from the conceptual model. The outflow particle-size distribution can also be estimated using the conceptual model.     

Graphical Sizing of Small Single-Outlet Detention Basins in the Semiarid Southwest

A graphical procedure is presented for sizing single-outlet detention basins to control stormwater runoff from small catchments in the semiarid southwestern United States. The approach relies on the modified rational method to provide the catchment runoff hydrograph, a linearized relation between basin water depth and storage volume, and a rainfall-intensity-duration equation for which coefficients can be found easily for any location covered by NOAA Atlas 14, Volume 1 (Arizona, Southeast California, Nevada, New Mexico, and Utah). Numerical solutions of a dimensionless form of the differential detention storage continuity equation are carried out, and solution values are related to a dimensionless discharge coefficient that provides a comprehensive characterization of catchment hydrology, outlet structure type and size, and regional precipitation. Graphs that supply dimensionless values of the maximum peak outflow rate from a basin, the maximum storage volume, and the critical storm duration (that is, the storm duration that produces the peak outflow and maximum storage volume) are prepared from the numerical solutions. The method avoids repetitive calculations and provides accurate solutions rapidly in an uncomplicated way for small catchments of about 12 ha or less with times of concentration less than about 30 min.     

Modeling Flow and Pollutant Removal of Wet Detention Pond Treating Stormwater Runoff

A mathematical model of pollutant removal by wet ponds was developed based on the mass balance principle and the release–storage equation. The release–storage equation can be linear or nonlinear depending on the wet pond shape and the spillway crest features. If the exponent index of the storage relationship (d) is equal to the index of the outflow equation (b), the wet pond hydrological routing model is linear. Otherwise, the model is nonlinear. Substituting the release–storage equation into the continuity equation produces a nonlinear ordinary differential equation (ODE). A Runge–Kutta method was used to solve the resulting nonlinear ODE. When the ratio of indexes d/b = 2/3, the hydrologic wet pond model reduces to a special case that leads to an implicit analytical solution. The pollutant removal and flow routing models were tested with data obtained from an actual wet pond for treating highway runoff. The predicted flow discharges and pollutant concentrations compared well with the observed data.     

Experimental Assessment of Stormwater Infiltration Basin Evolution

Infiltration basins are frequently used for stormwater drainage. They can operate for periods over 20 years but long-term evolution is not well understood or controlled. The two main problems encountered are clogging, which compromises the hydraulic capacity of the basin, and possible contamination of underlying soil and groundwater. This paper focuses on studying long-term evolution of clogging and soil pollution of infiltration basins. Basins of different ages are compared. Also, clogging and soil pollutant concentrations are explored for four infiltration basins in Lyon, France. Ages of the sites range from 10 to 21 years old. Clogging is characterized by the hydraulic resistance. Soil samples were collected at different depths in each basin and analyzed for different pollution parameters (metals, hydrocarbons, pH, and particle size distribution). All four basins have good infiltration capacities. Their hydraulic resistance is low. Such uniformity is surprising because of the age difference between the basins. Pollutant concentrations decrease rapidly with depth whereas pH and grain size increase. Concentrations reach an acceptable value at a 30 cm depth, even after 21 years of operation. Multivariate data analysis does not show significant relation between age, hydraulic resistance, and pollution.     

Predictions of Resuspension of Highway Detention Pond Deposits in Interrain Event Periods due to Wind-Induced Currents and Waves

The paper presents a numerical study of resuspension of deposits from highway detention ponds based on a previous experimental study. The resuspension process is evaluated in dry weather periods with baseflow/infiltration flow through the ponds only. The resuspension is caused by the bed-shear stress induced by the return flow near the bed and waves both generated by the wind. Wind statistics for 30 years have been applied for prediction of the annual discharged bulk of suspended solids and associated pollutants; fluoranthene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene (PAHs) and the heavy metals of cadmium, chromium, copper, lead, nickel, and zinc. The current and wave-generated bed-shear stresses entail a discharged bulk of pollutants corresponding to approximately 10% of the annual accumulation of pollutants in the present pond due to the baseflow in the pond. The mean outlet concentration of suspended solids is well correlated with the wind speed. To reduce the resuspension of deposited materials, two mechanisms are prevailing; either by increase of the water depth of the pond to minimize the effect of the wind in the near-bed region or by reduction of the wind to some degree. The most efficient action for reducing the wind impact on the shallow waters is the establishment of shelterbelts as known from the agriculture. Just a 20% reduction of the yearly wind speeds will reduce the outlet mass with 70% and a 50% reduction with almost 100%. A 50% reduction of the wind speed is far from impossible to achieve with relatively small investments.     

Interactive Effects of Controlled Drainage and Riparian Buffers on Shallow Groundwater Quality

As a result of recent surface water quality problems in North Carolina, riparian buffers and controlled drainage are being used to reduce the loss of nonpoint source nitrogen from agricultural fields. The effect of controlled drainage and riparian buffers as best management practices to reduce the loss of agricultural nonpoint source nitrogen from the middle coastal plain has not been well documented. The middle coastal plain is characterized by intensive agriculture on sandy soils with deeply incised or channelized streams. A 2-year study was conducted to determine the effectiveness of controlled drainage, riparian buffers, and a combination of both in the middle coastal plain of North Carolina. It was hypothesized that raising the water table near the ditch would enhance nitrate-nitrogen reduction through denitrification. On the sandy soils studied, controlled drainage did not effectively raise the water table near the ditch to a greater degree than observed on the free drainage treatment. Due to random treatment location, the free drainage treatment was installed along a ditch with a shallower impermeable layer compared to the impermeable layer on the controlled drainage treatments (2 m versus 3- to 4-m deep). This resulted in a perched or higher water table on the free drainage treatment. Over 17 storm events, the riparian buffer (free drainage) treatment had an average groundwater table depth of 0.92 m compared to 0.96 and 1.45 m for the combination (riparian buffer and controlled drainage) and controlled drainage treatments, respectively. Nitrate concentration decrease between the field wells and ditch edge wells averaged 29% (buffer only), 63% (buffer and controlled drainage), and 73% (controlled drainage only). Although apparently more nitrate was removed from the groundwater on the controlled drainage treatments, the controlled drainage treatment water table near the ditch was not raised closer to the ground surface compared to the free drainage treatment. Nitrate removal effectiveness was attributed to local soil and landscape properties, such as denitrification in deeper reduced zones of the soil profile.     

Impact of Microbial Partitioning on Wet Retention Pond Effectiveness

Wet detention basins are among the most common best management practices (BMPs) being implemented as means of complying with United States Phase II storm-water rules and impending Total Maximum Daily Load limits. The effectiveness of these basins for removal of microbial contaminants, one of the most frequent causes of water quality impairment, may be significantly affected by the degree to which microbes associate with particles in storm water. Little is known with regard to where microbial-particle associations are initiated within the storm-water transport chain as flow travels from upland sources (e.g., lawns, parking lots) through storm sewer systems and BMPs and finally on to receiving waters. A similar lack of information exists on the relative concentrations of microbes at each point in the transport chain. Both of these factors have important implications for the location of wet detention basins within a watershed, as well as their anticipated effectiveness. This study tracked the concentrations and partitioning behavior of three indicator organisms (fecal coliform, E. coli, and enterococci) at several locations in the transport chain and also explored the impacts of partitioning on wet pond removal efficiency. Results suggest that microbial-particle association is primarily initiated at upland sources and the degree of microbial partitioning does not vary greatly throughout the transport chain; therefore, treatment ponds will likely be most effective if located near upland contaminant sources. The overall reduction in microbial concentration brought about by the ponds was less than that assumed by most regulatory agencies, but the ponds did show some evidence of preferentially removing particle-associated fecal coliform and E. coli, suggesting that sedimentation is a key removal process. These findings should provide insights useful in the design and implementation of storm-water management strategies.     

Prediction of Effluent Quality from Retention Ponds and Constructed Wetlands for Managing Bacterial Stressors in Storm-Water Runoff

Microbial indicator organisms make up the greatest number of reported receiving water impairments, resulting in many questions on the fate of indicator bacteria passing through storm-water best management practices (BMPs). Storm-water BMPs are often considered effective tools to mitigate the effects of urbanization on receiving waters. The USEPA’s, Office of Research and Development investigated the processes occurring within two commonly used BMPs, constructed wetlands and retention ponds. This research focused on creating pilot-scale systems to determine the environmental mechanisms that affect effluent indicator bacteria concentrations and to provide better information for the prediction of bacterial indicators for models when developing and meeting total maximum daily loads. Research results indicate water temperature, light, and a combination of other environmental factors influence bacteria indicator concentrations. Results from this research suggest that both constructed wetlands and retention ponds lower microbial concentrations in urban storm-water runoff. Bacteria inactivation generally followed the first-order, KC* model, which includes irreducible or background concentrations of a stressor. Sediment analyses indicate bacteria accumulated in sediments which may maintain background concentrations could be reintroduced into the effluent of these BMPs by turbulent flow causing resuspension or by accumulation through lack of maintenance. First-order models that do not consider irreducible concentrations may underestimate actual bacterial concentrations. The relationship between turbidity and bacteria suggests storm-water management practices that substantially reduce turbidity may also provide the greatest improvement in reducing concentrations of bacteria in storm-water runoff.     

Design of Street Sump Inlet

Sump inlets are used to collect storm water on the streets or to release stored water in detention basins. The complication in sump inlet hydraulics arises in the transition from weir to orifice flow regimes. Conventionally, the capacity of a sump inlet has been assumed to be either weir or orifice flow, whichever is smaller for a given water depth. Although this practice might not result in a failure to the storm drain, it has led to randomly oversized or undersized inlets. This paper presents a laboratory investigation of the interception capacities of several different types of sump inlets, including bar and vane grates, and 3- and 5-ft curb opening inlets. The observed data revealed significant differences from the recommended HEC 22 design procedure. In this study, new formulas and procedures are developed with the coefficients calibrated by the laboratory data.     

Storm Water Detention Ponds: Modeling Heavy Metal Removal by Plant Species and Sediments

Laboratory and field studies were conducted to elucidate heavy metal removal by three wetland grasses and sediments in storm water detention pond. The removal of heavy metals including Cd, Cu, Pb, and Zn was mediated by fluid-flow intensity in the reactors. The growth of plants and the removal rates of contaminants were plant species dependent. All three wetland grasses removed contaminants from the spiked nutrient solutions. A first-order kinetic model adequately represented the removal of contaminants by plants. The analyses of undisturbed sediment cores in detention pond revealed strong stratification of heavy metal concentrations at the sediment–water interface. A simple model that integrates heavy metal removal by aquatic plants and sediments in storm water detention ponds is proposed. The model provides an estimate of contaminant residence time which can be related to hydraulic residence time in storm water detention ponds.     

Design of Two-Layered Porous Landscaping Detention Basin

Under the mandate of the Federal Clean Water Act, porous landscaping detention (PLD) has been widely used to increase on-site infiltration. A PLD system consists of a surface storage basin and subsurface filtering layers. The major design parameters for a PLD system are the infiltration rate on the land surface and the seepage rate through the subsurface medium. A low infiltration rate leads to a sizable storage basin while a high infiltration rate results in standing water if the subsurface seepage does not sustain the surface loading. In this study, the design procedure of a PLD basin is revised to take both detention flow hydrology and seepage flow hydraulics into consideration. The design procedure begins with the basin sizing according to the on-site water quality control volume. The ratio of design infiltration rate to sand-mix hydraulic conductivity is the key factor to select the thickness of sand-mix layer underneath a porous bed. The total filtering thickness for both sand-mix and gravel layers is found to be related to the drain time and infiltration rate. The recommended sand-mix and granite gravel layers underneath a PLD basin are reproduced in the laboratory for infiltration tests. The empirical decay curve for sand-mix infiltration rate was derived from the laboratory data and then used to maximize the hydraulic efficiency through the subsurface filtering layers. In this study, it is recommended that a PLD system be designed with the optimal performance to consume the hydraulic head available and then evaluated using the prolonged drain time for potential standing water problems under various clogging conditions.     

Documenting Stormwater Quality on Texas Highways and Adjacent Vegetated Roadsides

The primary objective of this study is the documentation of stormwater quality of vegetated roadsides of two Texas highways (State Highway 6 in College Station and Loop 360 in Austin), both of which had high average daily traffic. Three sites each in Austin and College Station were monitored using passive “first flush” stormwater samplers for 16 months. Results from this study indicate that significant removal of sediment and heavy metals occurred over the width of vegetated roadsides, but no apparent nutrient (nitrogen and phosphorus) removal was observed. The results also show that vegetation density has a direct effect on the performance of vegetated roadsides. When roadsides are densely covered with grasses above 90%, significant sediment removal is expected, often within the first 4?m of the edge of pavement. A stepwise regression analysis identifies the antecedent dry period (ADP) as the most significant predictor to pollutant concentration. The pollutant event mean concentration was found to decrease with increasing ADP for all pollutants at the College Station sites, but not the Austin ones.     

Sediment Discharge from a Storm-Water Retention Pond

A small storm-water retention pond is primarily designed to reduce the peak rate of surface runoff. From a water quality standpoint, that same pond may be irregular in shape and trap suspended sediment carried by the surface runoff generated upstream of the site. Considering different pond inlet locations, the writer’s numerical model is used to investigate the change in peak concentration of sediment discharge at the pond outlet. It has been found that the various hydraulic conditions can have a significant impact on sediment discharge. Three different cases are presented to show the flexibility of modeling changes in boundary conditions. The results may help designers evaluate sediment discharge to determine the most effective pond inlet locations.     

Bayesian Storm-Water Quality Model and Its Application to Water Quality Monitoring

A Bayesian statistical approach for determining the parameter uncertainty of a storm-water treatment model is reported. The storm-water treatment technologies included a sand filter and a subsurface gravel wetland. The two field systems were loaded and monitored in a side-by-side fashion over a two-year period. The loading to each system was storm-water runoff generated by ambient rainfall on a commuter parking lot. Contaminant transport is simulated by using a one-dimensional advection-dispersion model. The unknown parameters of the model are the contaminant deposition rate and the hydrodynamic dispersion. The following contaminants are considered in the study: total suspended solids, total petroleum hydrocarbons–diesel range hydrocarbons, and zinc. Parameter uncertainties are addressed by estimating the posterior probability distributions through a conventional Metropolis-Hastings algorithm. Results indicate that the posterior distributions are unimodal and, in some instances, exhibit some level of skewness. The Bayesian approach allowed the estimation of the 10th, 25th, 50th, 75th, and 95th percentiles of the posterior probability distributions. The prediction capabilities of the model were explored by performing a Monte Carlo simulation using the calculated posterior distributions and two rainfall-runoff events not considered during the calibration phase. The objective is to estimate effluent concentrations from the treatment systems under different scenarios of flow and contaminant loads. In general, estimated effluent concentrations and the total estimated mass fell within the defined uncertainty limits.     

Fractional Kinetic Model for First Flush of Stormwater Pollutants

By generalizing the urban ground as a fractal surface and revising the classical Fick’s formula as a law of dispersion with a fractional-order derivative, a fractional kinetic model is developed for simulation of the first flush phenomenon of urban stormwater pollutants. The model is comprised of (1) a fractional dispersion-advection equation (FADE); (2) the kinematic-wave overland flow equation; and (3) methods for numerical solution of the equations. A split-operator method is proposed for numerical solution of the FADE by means of a newly presented F.3 finite-difference scheme for fractional partial differential equations. The kinematic-wave overland flow equation is solved using the Lax–Wendroff explicit scheme. Under a constant rainstorm the hydrograph displays an initial rising limb followed by a constant flow discharge. The pollutograph exhibits a steep receding limb (the first flush), followed by a long stretched tail (heavy tail process). The agreement between simulated and measured dispersion characteristics is found to be good, demonstrating that the fractional kinetic model is capable of accurately predicting the characteristics of the first flush phenomenon.     

Storm-Water Infiltration and Focused Recharge Modeling with Finite-Volume Two-Dimensional Richards Equation: Application to an Experimental Rain Garden

Rain gardens are infiltration systems that provide volume and water quality control, recharge enhancement, as well as landscape, ecological, and economic benefits. A model for application to rain gardens based on Richards equation coupled to a surface water balance was developed, using a two-dimensional finite-volume code. It allows for alternating upper boundary conditions, including ponding and overflow, and can simulate heterogeneous soil-layering or more complex geometries to estimate infiltration and recharge. The algorithm is conservative, and exhibits good performance compared to standard models for several test cases (less than 0.1% absolute mass balance error); simulations were also performed for an experimental rain garden and comparisons to collected data are presented. The model accurately simulated the matrix flow, soil water distribution, as well as deep percolation (potential recharge) for a natural rainfall event in the controlled experimental setup.     

Trace Metal Pollutant Load in Urban Runoff from a Southern California Watershed

In order to implement efficient and effective management strategies for coastal water quality in Southern California, it is important to consider the relative pollutant contributions from urban dry-weather flow (DWF) and wet-weather flow (WWF). This study uses both historical flow coupled with water quality monitoring data and computer modeling to characterize the annual DWF and WWF discharges from an urban catchment in Los Angeles, Calif. The DWF and WWF pollutant loading of the trace metals copper, lead, nickel, and chromium for 6 water years dating from 1991 to 1996 is predicted. The results indicate that DWF contributes a considerable amount of flow and pollutants. Approximately, 9–25% of the total annual Ballona Creek flow volume is DWF. The simulations indicate DWF accounts for 54, 19, 33, and 44% of the average annual load of total chromium, copper, lead, and nickel, respectively. In the dry season, the simulations indicate DWF accounts for 89, 59, 58, and 90% of the load of total chromium, copper, lead, and nickel, respectively. This research suggests DWF controls may be an important part of pollution mitigation plans for urban stormwater drainage systems in Southern California.     

Water Quality Improvement through Reductions of Pollutant Loads Using Bioretention

As an increasingly adopted storm-water best management practice (BMP) to remedy hydrology and water quality impairment from urban development, bioretention facilities need rigorous investigation to quantify performance benefits and to allow design improvements. This study examines water quality improvements [total arsenic, total cadmium, chloride, total chromium, total and dissolved copper, E. coli, fecal coliform, lead, mercury, nitrogen species, oil and grease, phosphorus, total organic carbon (TOC), total suspended solids, and total zinc] via monitoring for a 15-month period at two bioretention cells in Maryland. Both bioretention cells effectively removed suspended solids, lead, and zinc from runoff through concentration reduction. Runoff volume reduction promotes pollutant mass removal and links BMP water quality benefits with hydrologic performance. From a load perspective (kg/ha?year), all but TOC at one cell showed pollutant reduction. Bioretention effluents exhibited good water quality for all significant pollutants except for nitrate, copper, and phosphorus in one cell, the latter two of which may be attributed to media organic matter dissolution. Copper dissolved/particulate analyses showed that significant changes in copper speciation behavior result from transport through the bioretention media.