Robert G. Traver

Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design

AbstractBioretention is one of the most commonly used stormwater control measures (SCMs) in North America and Australasia. However, current design is not targeted to regulatory need, often reflecting an outdated understanding of how and why bioretention works. The purpose of this manuscript is to synthesize research to recommend a suite of design standards focused on the purpose of bioretention SCM. Both hydrologic (peak flow mitigation, infiltration, annual hydrology, and stream stability) and water quality [total suspended solids (TSS) and particulates, pathogen-indicator species, metals, hydrocarbons, phosphorus, nitrogen, and temperature] regulatory and stream ecology needs are addressed. Bioretention cells designed to meet a prioritized subset of those measures would be substantially different than cells that are designed for a different subset of needs. Designers have the ability to adjust bowl volume, media composition, media depth, underdrainage configuration, and vegetation type. This study exami...

Hydrologic Performance of Bioretention Storm-Water Control Measures

The transportation and urban infrastructure relies heavily on impervious surfaces. Unmitigated rainfall runoff from impervious surfaces can lead to a myriad of environmental problems in downgradient areas. To address this issue, novel stormwater control measures (SCMs) are being emphasized and implemented widely to mitigate some of the impacts of impervious surface. Bioretention is a soil/media-based SCM that is often used for this purpose, but current design practices are highly empirical. This study compiles work from three research sites in three states to provide some fundamental underpinnings to bioretention design. Although all sites demonstrate different levels of performance, water volumetric performance trends are common to all. These trends are based on the available storage in the bioretention cell, termed herein as the Bioretention Abstraction Volume (BAV). The BAV is directly related to available media porosity and storage in the surface bowl. A finite capacity to completely store all runoff from smaller events is defined by the BAV. Normalization for this storage provides prediction for volumetric performance. Recommendations for bioretention design are provided.

Understanding the Role of Evapotranspiration in Bioretention: Mesocosm Study

AbstractMany regions are turning toward sustainable green infrastructure practices, such as bioretention to mitigate urban stormwater runoff. Currently, the designed volume control in bioretention stormwater control measures (SCMs) is attributed to infiltration, although evapotranspiration (ET) can be considered a viable mechanism. From a water cycle perspective, reduced ET in urban spaces is a primary source of the increased stormwater runoff volume. This research demonstrates that ET is a substantial water balance component of two differently configured bioretention mesocosm systems. Over a two-year span of time, a freely draining bioretention mesocosm (lysimeter) converted 50% of the direct rain falling on the mesocosm to ET, contrasted by a bioretention mesocosm (lysimeter) with an internal water storage (IWS) layer that converted 78% of the direct rainfall to ET. The measured daily average ET was 6.1  mm/d for the IWS lysimeter and 3.1  mm/d for the freely draining lysimeter from April to November in...

Multiyear Performance of a Pervious Concrete Infiltration Basin BMP

The use of infiltration storm-water best management practices (BMPs) has become a more commonly used approach as a means of reducing postdevelopment runoff volumes in many areas throughout the United States. Although studies regarding the performance of infiltration BMPs are emerging, much remains to be learned about their design, construction, and operation. The increase in knowledge will improve the performance and longevity of these BMPs. The performance of one such infiltration basin over a 2-year cycle is presented in this paper. The study site is a pervious concrete infiltration basin BMP built in 2002 in a courtyard common area at Villanova University. The system consists of three linked infiltration beds lined with geotextile filter fabric, filled with coarse aggregate, and overlaid with pervious concrete. The natural soil beneath the infiltration BMP is a silty sand. The BMP is extensively instrumented to facilitate water quantity and quality research. Both water-quantity and -quality results are presented. The water-quantity analysis showed that the performance of the basin was directly related to its infiltration characteristics. The infiltration rate of the silty sand is cyclic, with higher rates during warmer periods and lower rates during colder periods. The water quality analysis investigated the pollutant reduction for chloride, copper, nitrogen, and phosphorus from the inlet to the surface-water outlet of the structure, as well as differences in pollutant concentration levels between the basin, surrounding ground, and varying soil layer elevations beneath the basin. In general, the pollutant reduction to the surface waters was greater than 90% from inlet to outlet, primarily influenced by the infiltration of the storm water into the natural soils below the BMP. The pollutant concentration of the infiltrating runoff was found to be higher than expected in the area adjacent to the bed when compared to concentration levels found at a similar depth beneath the infiltration bed. Comparison of pollutant concentration levels, as the water moved from within the storage bed to the soil beneath the bed, were shown to vary, with statistical differences found for mean concentration levels of both pH and copper levels; and no statistical differences were found for conductivity, total phosphorous, and chloride at each elevation.

Parking Deck's First Flush

The goals of storm-water management have shifted from a flood control to a holistic and sustainable strategy, emphasizing the relationship between rainfall event size and pollutant loadings. The first flush concept is the first part of a rainfall event that contains the largest pollutant loading. Questions have been raised on the concepts validity for storms across different land uses and pollutant types. The existence and magnitude of the first flush impacts sizing of best management practices used to meet pollutant reduction goals, assessment sampling methodologies, and state storm-water management strategies. Current concepts support the use of distributed control measures focused on smaller storms off impervious surfaces. Runoff from a small impervious parking area was sampled incrementally during multiple storm events to measure pollutant concentration with respect to storm depth. This sampling routine established the existence of a first flush for a single use paved parking area. Total suspended solids, nitrate, chloride, dissolved copper, and dissolved cadmium exhibited a first flush up to a rainfall depth of 25.4 mm; total dissolved solids, total nitrogen, total phosphorus, nitrite, phosphate, and dissolved chromium did not exhibit a first flush.

Hydrologic and Pollutant Removal Performance of a Bio-Infiltration BMP

Infiltration Best Management Practices (BMPs) have become a major component of stormwater designs (Traver 2003). These BMPs are designed to address runoff volume and quality criteria’s, augmenting and in some cases replacing traditional detention based methods. While the function of the Bio-Infiltration BMP’s may be understood, many questions remain on their design, maintenance, and how to best utilize these devices as part of a watershed plan. To address these issues, Villanova University has constructed and is monitoring a BioInfiltration “RainGarden” through the Villanova Urban Stormwater Partnership (VUSP). Flow data from this effort has been used to create a representative computer model of the watershed, allowing analysis of its performance (Heasom et al 2006). Results indicate that the site outperforms the original design specifications, and that maintenance may be required in the future to continue its high level of performance. Further information on the site is available through the Villanova Urban Stormwater Partnership website - www.villanova.edu/vusp.

Feasibility Study of As-Received and Modified (Dried/Baked) Water Treatment Plant Residuals for Use in Storm-Water Control Measures

AbstractUse of water treatment plant residuals (WTRs) in storm-water control measures (SCMs) is a sustainable alternative to landfill disposal of WTRs. However, research is needed to determine how effective WTR-amended SCMs would be in field-scale applications and what modifications can be implemented to improve performance. The modifications examined in this study were oven-drying (105°C) and baking (1,000°C) of the WTRs. Results showed that both modifications increase the hydraulic conductivity by two orders of magnitude. Dried WTRs showed no loss of phosphate removal potential compared to the as-received WTRs. Baking the WTRs lowered the phosphate removal potential but prevented manganese resuspension. The as-received WTRs, as well as both modifications, removed copper, lead, and zinc from storm-water runoff to below detection. Taken together, these results suggest that amending SCMs with modified WTRs has the potential to enhance the water quality improvement processes of SCMs while maintaining the in...

Continuous Modeling of Bioinfiltration Storm-Water Control Measures Using Green and Ampt

AbstractContinuous simulation of storm-water control measures (SCM) requires consideration of the variable environmental and site factors that affect infiltration. This article reviews the theory of the Green and Ampt infiltration model and presents a unit process approach to how it can be applied to a bioinfiltration SCM. The paper discusses how to vary the soil moisture parameters using the soil water characteristic curve, how to modify the Green and Ampt equation for different basin cross-sections, and how to model or bound the infiltration rate when soil media layers are added. Both the standard Green and Ampt parameters and alternative moisture conditions were compared with data from a bioinfiltration SCM; these results confirm that the standard parameter tables often used to select Green and Ampt parameters represent conservative, wet conditions, and that a single value for the parameters will not be able to reproduce the range of infiltration rates observed in the field. When using variable soil mo...

Evaluation of Soil Class Proxies for Hydrologic Performance of In Situ Bioinfiltration Systems

AbstractThe hydrologic performance of in situ bioinfiltration systems (bioretention systems with no fill media or underdrain) is quantified and soil classes are evaluated as proxies for design requirements. A one-dimensional (1D) Richard’s equation model of a bioinfiltration system is used in combination with a dataset of soil hydraulic properties to conduct a Monte Carlo analysis of the effect of soil hydraulic properties; the results are summarized both by soil textural class and by hydrologic soil group (HSG), showing that textural class is generally a poor proxy for estimating the infiltration performance of in situ bioinfiltration cells (R2=0.40). Because infiltration measurements are required to estimate the HSG, they are a better proxy for bioinfiltration performance (R2=0.89). It is found that soil proxies do provide certain reliable guidelines: HSG-D soils always require engineered fill media with an underdrain; whereas underdrains are not necessary for sand, loamy sand, HSG-A, and HSG-B native s...

Evaluating the Role of Evapotranspiration in the Hydrology of a Bioinfiltration Basin Using a Weighing Lysimeter

Bioinfiltration basins require substantial land area in order to reduce peak outflows and improve the water quality of stormwater runoff. An increased understanding of the water budgets occurring within these basins could result in more efficient land usage. For example, the role of water losses through groundwater recharge, and specifically evapotranspiration (ET), are not well understood. However, research on the entire water budget may yield design standards resulting in substantial volume reduction in bioinfiltration basins when the contribution of ET is viewed through an appropriate time horizon. In order to measure the ET and groundwater recharge occurring in a bioinfiltration basin, a weighing lysimeter was designed and constructed. Storms were simulated representing varying field conditions. These simulations mimicked two and a half hour storm events of 13, 19, and 25 mm producing runoff occurring from both 5:1 and 10:1 impervious area to lysimeter area loading ratios. The simulations were conducted during the morning and evening and in different seasons. Results indicate that the ET measured 24 hours after the storm simulation was found to range from 2.6 to 31.4 mm of water. The measured ET was found to be strongly correlated (R 2 = 0.63) to the climatological parameters that govern the Penman-Monteith equation. The measured ET was compared to the Penman-Monteith reference ET 0 and a mean crop coefficient (K c ) of 1.85 was determined for the summer months. The measured ET was also found to be dependent on the soil saturation of the lysimeter prior to the storm simulation. A soil-water characteristic curve was developed for the soil in the lysimeter. It was determined that the greater the soil suction throughout the storm simulation, the more water that was available for ET, and ET increased relative to groundwater recharge.