•Six nitrogen removal modules (NRMs) were developed and validated with lab results.•NRMs accurately predict N removal rates under various soil moisture conditions.•Best NRM predicted accurate EMCs ...for 20% more events than SWMM.•NRMs simulated reaction rates fall in ranges reported in the literature.•NRMs assist the design of saturated layer or RTC rules for bioretention.
Bioretention systems have the potential of simultaneous runoff volume reduction and nitrogen removal. Internal water storage (IWS) layers and real-time control (RTC) strategies may further improve performance of bioretention systems. However, optimizing the design of these systems is limited by the lack of effective models to simulate nitrogen transformations under the influences of IWS design and environment conditions including soil moisture and temperature. In this study, nitrogen removal models (NRMs) are developed with two complexity levels of nitrogen cycling: the Single Nitrogen Pool (SP) models and the more complex 3 Nitrogen Pool (3P) models. The 0-order kinetics, 1st order kinetics, and the Michaelis-Menten equations are applied to both SP and 3P models, creating six different NRMs. The Storm Water Management Model (SWMM), in combination with each NRM, is calibrated and validated with a lab dataset. Results show that 0-order kinetics are not suitable in simulating nitrogen removal or transformations in bioretention systems, while 1st order kinetics and Michaelis-Menten equation models have similar performances. The best performing NRM (referred to as 3P-m) can accurately predict nitrogen event mean concentrations in bioretention effluent for 20% more events when compared to SWMM. When only calibrated with soil moisture conditions in bioretention systems without internal storage layers, 3P-m was sufficiently adaptable to predict cumulative nitrogen mass removal rates from systems with IWS or RTC rules with less than ±7% absolute error, while the absolute error from SWMM prediction can reach -23%. In general, 3P models provide higher prediction accuracy and improved time series of biochemical reaction rates, while SP models improve prediction accuracy with less required user input for initial conditions.
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Many bioretention models still incorporate simplifications and lumped parameters that do not fully account for fundamental physical processes. This review summarizes the representation of hydrologic ...pathways, notable features, and applications of bioretention models with the goals of recommending models well suited to bioretention modeling and identifying key research needs. As a result, HYDRUS and GIFMod were identified as the only models that use Richards’ equation for determining infiltration under variably saturated conditions. Secondly, this study identified limited drainage configurations by most models except DRAINMOD-Urban. Thirdly, most models were inadequate for considering vegetation and plant water use, an area for improvement in future research. Finally, more calibration and validation studies are needed to build confidence in model results. This review intends to educate modelers of the processing equations for each water balance component, the input requirements in each model, and other model characteristics that should be considered in model selection.
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•17 bioretention models were evaluated for hydrologic and bioretention features.•Hydrologic processes were assessed for six models well-suited to bioretention modeling.•Major needs in the field of bioretention modeling were identified.•Recommendations were given for model selection based on bioretention application.
•Decades old data used in stormwater modeling may not represent current conditions.•Investigated temporal and step trends in four decades of stormwater quality data.•Significant, decreasing monotonic ...trends observed for nutrients, sediments, metals.•Step trend declines aligned with environmental regulations unrelated to stormwater.•Reductions were most substantial in metals followed by sediments then nutrients.
Identifying sources of pollutants in watersheds is critical to accurately predicting stormwater quality. Many existing software used to model stormwater quality rely on decades-old data sets which may not represent current runoff quality in the United States. Because of environmental regulations promulgated at the federal level over previous decades, there is a need to understand long-term trends (and potential shifts) in runoff quality to better parameterize models. Pollutant event mean concentrations (EMCs) from the National Stormwater Quality Database (NSQD) were combined with those from recent sources to understand if untreated stormwater quality has changed over the past 40 years. A significant decreasing monotonic trend (i.e., continually decreasing in a nonuniform fashion) was observed for total suspended solids (TSS), total phosphorus (TP), total Kjeldahl nitrogen (TKN), total copper (Cu), total lead (Pb), and total zinc (Zn) in the resultant database, suggesting that runoff quality has become less polluted with time. Median EMCs decreased from 99.2 to 42 mg/L, 0.34 to 0.26 mg/L, 1.27 to 1.03 mg/L, 40 to 6.8 µg/L, 110 to 3.7 µg/L, and 375 to 53.3 µg/L for TSS, TP, TN, Cu, Pb, and Zn, respectively, from the 1980s to the 2010s. These significant reductions often aligned temporally with advancements in clean manufacturing, amendments of the Clean Air Act, and other source control efforts which impact pollutant bioavailability and atmospheric deposition. Results suggest environmental regulations not specifically targeting stormwater management have had a positive impact on stormwater quality and that temporal fluctuations should be considered in modeling.
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Climate stationarity is a traditional assumption in the design of the urban drainage network, including green infrastructure practices such as bioretention cells. Predicted deviations from historic ...climate trends associated with global climate change introduce uncertainty in the ability of these systems to maintain service levels in the future. Climate change projections are made using output from coarse-scale general circulation models (GCMs), which can then be downscaled using regional climate models (RCMs) to provide predictions at a finer spatial resolution. However, all models contain sources of error and uncertainty, and predicted changes in future climate can be contradictory between models, requiring an approach that considers multiple projections. The performance of bioretention cells were modeled using USEPA's Storm Water Management Model (SWMM) to determine how design modifications could add resilience to these systems under future climate conditions projected for Knoxville, Tennessee, USA. Ten downscaled climate projections were acquired from the North American Coordinated Regional Downscaling Experiment program, and model bias was corrected using Kernel Density Distribution Mapping (KDDM). Bias-corrected climate projections were used to assess bioretention hydrologic function in future climate conditions. Several scenarios were evaluated using a probabilistic approach to determine the confidence with which design modifications could be implemented to maintain historic performance for both new and existing (retrofitted) bioretention cells. The largest deviations from current design (i.e., concurrently increasing ponding depths, thickness of media layer, media conductivity rates, and bioretention surface areas by 307%, 200%, 200%, and 300%, respectively, beyond current standards) resulted in the greatest improvements on historic performance with respect to annual volumes of infiltration and surface overflow, with all ten future climate scenarios across various soil types yielding increased infiltration and decreased surface overflow compared to historic conditions. However, lower performance was observed for more conservative design modifications; on average, between 13-82% and 77–100% of models fell below historic annual volumes of infiltration and surface overflow, respectively, when ponding zone depth, media layer thickness, and media conductivity were increased alone. Findings demonstrate that increasing bioretention surface area relative to the contributing catchment provides the greatest overall return on historic performance under future climate conditions and should be prioritized in locations with low in situ soil drainage rates. This study highlights the importance of considering local site conditions and management objectives when incorporating resiliency to climate change uncertainty into bioretention designs.
•Bioretention climate resiliency analyzed using SWMM models and probabilistic approach.•Climate model bias corrected using novel technique (KDDM); used as SWMM model inputs.•Increasing relative bioretention surface area provided greatest performance returns.•Conservative modifications yielded fewer scenarios surpassing historic performance.•Local site conditions, management objectives key to bioretention climate resiliency.
Pollution build-up and wash-off processes are often included in urban stormwater quality models. However, these models are often unreliable and have poor performance at large scales and in ...complicated catchments. This study tried to improve stormwater quality models by adopting the genetic programming (GP) approach to generate new build-up algorithms for three different pollutants (total suspend solids – TSS, total phosphorus – TP and total nitrogen – TN). This was followed by testing of the new models (also traditional build-up and wash-off models as benchmark) using data collected from different catchments in Australia and the USA. The GP approach informed new sets of build-up algorithms with the inclusion of not just the typical antecedent dry weather period (ADWP), but also other less ‘traditional’ variables - previous rainfall depth for TSS and maximum air temperatures for TP and TN simulation. The traditional models had relatively poor performance (Nash-Sutcliffe coefficient, E < 0.0), except for TP at Gilby Road (GR) (E = 0.21 in calibration and 0.43 in validation). Improved performance was observed using the models with new build-up algorithms informed by GP. Taking TP at GR for example, the best performing model had E of 0.46 in calibration and 0.54 in validation. The best performing models for TSS, TP, and TN are often different, suggesting that specific models shall be used for different pollutants. Insights into further improvements possible for stormwater quality models were given. It is recommended that in addition to the typical build-up and wash-off process, new generations of stormwater quality models should be able to account for the non-conventional pollutant sources (e.g. cross-connections, septic tank leakage, illegal discharges) through stochastic approaches. Emission inventories with information like intensity-frequency-duration (IFD) of pollutant loads from each type of non-conventional source are suggested to be built for stochastic modelling.
•GP can generate meaningful build-up algorithms to improve stormwater quality model.•Different stormwater quality models shall be used for different pollutants.•Inclusion of temperature can improve further TP and TN modelling.•Next generation of models shall consider non-conventional pollution sources.
Daily weather conditions for an entire city are usually represented by a single weather station, often located at a nearby airport. This resolution of atmospheric data fails to recognize the ...microscale climatic variability associated with land use decisions across and within urban neighborhoods. This study uses heat index, a measure of the combined effects of temperature and humidity, to assess the variability of heat exposure from ten weather stations across four urban neighborhoods and two control locations (downtown and in a nearby nature center) in Knoxville, Tennessee, USA. Results suggest that trees may negate a portion of excess urban heat, but are also associated with greater humidity. As a result, the heat index of locations with more trees is significantly higher than downtown and areas with fewer trees. Trees may also reduce heat stress by shading individuals from incoming radiation, though this is not considered in this study. Greater amounts of impervious surfaces correspond with reduced evapotranspiration and greater runoff, in terms of overall mass balance, leading to a higher temperature, but lower relative humidity. Heat index and relative humidity were found to significantly vary between locations with different tree cover and neighborhood characteristics for the full study time period as well as for the top 10% of heat index days. This work demonstrates the need for high-resolution climate data and the use of additional measures beyond temperature to understand urban neighborhood exposure to extreme heat, and expresses the importance of considering vulnerability differences among residents when analyzing neighborhood-scale impacts.
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•Suspended pavement practices can increase opportunities to manage urban stormwater.•Performance of two bioretention suspended pavement systems studied over 27 months.•Exfiltration, ...evapotranspiration were major hydrologic pathways from both systems.•Limited surface ponding, drainage-treatment area ratio led to high volume reduction.•Bioretention suspended pavement systems effectively reduced influent runoff volumes.
Trees supply numerous ecosystem services to the urban environment, including mitigating the urban heat island, improving air quality, and providing habitat for wildlife. However, due to the structural stability requirements of infrastructure such as sidewalks, roadways, and parking areas, urban soils are commonly characterized by high compaction levels, low porosity, and nutrient deficiency, often to the detriment of urban tree health. By transmitting surface loads to a compacted subbase, suspended pavement systems create a matrix of uncompacted soil that promotes tree health through increased root access to oxygen, water, and nutrients. Recently, interest has grown in making suspended pavement systems multi-functional by designing them to act as subsurface bioretention practices in ultra-urban areas where space may be limited due to concentrated development and high land costs. Despite this interest, only one study has tested suspended pavement systems in this application. Further, the study utilized lined installations to allow a controlled water quality assessment; however, this negated the ability to fully quantify runoff reduction. Two unlined suspended pavement systems designed to function as subsurface bioretention practices were installed in Knoxville, Tennessee, USA, in 2015 and monitored over 27-months. During that period, over 99% of runoff volumes were reduced by the free-drained north system, which completely captured runoff from 79% of storms. The underdrained south system reduced influent runoff by over 88% and captured all runoff from 83% of events during the study. Influent TSS concentrations were significantly reduced by the south suspended pavement system, though no other significant differences between influent and effluent pollutant concentrations were observed, presumably due to low influent concentrations. This study demonstrates the viability of field-scale suspended pavement systems in a stormwater management application and illustrates the hydrologic and pollutant removal capabilities of these systems to manage urban stormwater runoff.
"Backyard" green infrastructure programs are an innovative way to manage urban stormwater, with many social and ecologic benefits. In many programs, however, residents with lower incomes are not ...reached, though they could benefit from participation, and though their participation could benefit the socioecological system. We examined awareness of and interest in backyard green infrastructure among lower- and moderate-income residents (N = 234). Awareness among our study population is low to moderate, but interest is moderate to high, with variability by some demographic and other characteristics. A spouse/partner, city agency, and/or neighbor may have influential roles in increasing participation in backyard green infrastructure.
Impervious surfaces, such as roads, parking areas, and buildings, found in cities throughout the world have significant impacts on urban hydrology due to increased volumes and peak flow rates of ...run‐off delivered to receiving waterbodies. Bioretention practices are a common stormwater control measure used to mitigate the impacts of urban run‐off. When coupled with suspended pavement systems, which provide tree roots with an uncompacted soil matrix that enhances root access to oxygen and water, engineers can design subsurface alternatives to manage urban stormwater. Two suspended pavement systems designed to function as subsurface bioretention practices were installed on the campus of the University of Tennessee, Knoxville, Tennessee, USA. Sap flow sensors using the heat ratio method were installed in two bald cypress (Taxodium distichum) trees to characterize the role of transpiration in the suspended pavement systems. Mean transpiration rates were greater when water availability was higher in the bioretention media. Regression models indicated that atmospheric vapour pressure deficit (kPa) was the most influential environmental parameter on tree transpiration and that stomatal regulation of water losses was evident when water was limiting. Findings from this study illustrate how tree transpiration rates can vary, even between individual trees of the same species, on the basis of conditions within the practice, and provide insight to practitioners on how design parameters influence fine‐scale tree–water relations in bioretention systems to maximize the contributions of transpiration on system hydrology.
Across the globe, water quality standards have been implemented to protect receiving waters from stormwater pollution, motivating regulators (and consequently designers) to develop tools to predict ...the performance of stormwater control measures such as constructed stormwater wetlands (CSWs). The goal of this study was to determine how well the relaxed tanks-in-series (P-k-C*) model described the performance of CSWs in North Carolina. Storm events monitored at 10 CSWs in North Carolina were used for calibrating the model, and statistical evaluations concluded the model could adequately predict the performance for all pollutants except organic nitrogen. Nash–Sutcliff calibration/validation values were determined to be 0.72/0.78, 0.78/0.74, 0.91/0.87, 0.72/0.62, 0.88/0.73, and 0.91/0.63 for total nitrogen, total ammoniacal nitrogen, oxidized nitrogen, organic nitrogen, total phosphorus, and total suspended solids, respectively. Sensitivity analysis revealed only one calibration parameter with strong sensitivity, the Arrhenius coefficient (temperature dependent model coefficient). With this model, CSWs can be optimized to treat watershed-specific influent concentrations to meet effluent targets. In general, the current design technique used in North Carolina and many other locations (a first flush volume detention method) oversizes CSWs for water quality vis-à-vis the method herein, suggesting improved designs for water quality may be possible through scientifically-informed methods.
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