Stormwater Control Research Articles

Numerical simulation study of urban hydrological effects under low impact development with a physical experimental basis

Global climate change and rapid urbanization led to frequent extreme rainstorms and waterlogging. Low impact development (LID) as an essential measure of urban stormwater control, it is necessary to explore the urban hydrological response under low impact development combined with physical experiment and numerical simulation. This paper takes the Beiyangyuan campus of Tianjin University, where numerous LID practices have been implemented, as the study area. The prototype permeable pavement in the campus was scaled down to a laboratory-scale for infiltration experiments. The results show that the Horton infiltration model could describe the infiltration process of permeable concrete and grass-planting brick. The initial infiltration rate of the permeable concrete is large but decays faster. In contrast, the infiltration process of the grass-planting brick decays slower, and the stable infiltration rate and infiltration capacity are higher than that of the permeable concrete. The mean infiltration rates measured for the permeable concrete and the grass-planting brick were 95 mm/hr and 190 mm/hr, respectively. A SWMM-MIKE21 coupled model was constructed based on the LID experimental results to explore the effects of the removal or density reduction of the LID practices on surface hydrological processes. The results show that permeable pavement performs best in the reduction of runoff volume, inundation area, and inundation depth, while the bio-retention cell takes the second place, and the sunken greenbelt has limited effect. Frequent light rainfall is sensitive to the density of LID practices, and the implementation of even a tiny percentage (4% of the catchment area) of LID practices can be effective in controlling runoff. The removal or density reduction of LID practices has little effect on the inundation region but leads to an increased possibility of flood risk due to increasing inundation depth.

Estimating Stormwater Infiltration and Canopy Interception for Street Tree Pits in Manhattan, New York

Estimates of the amount of stormwater captured by urban trees have focused on the rainfall intercepted by leaves and branches, while the amount of stormwater runoff that flows into a tree pit from the surrounding sidewalk has not been well quantified. This study estimated the amount of stormwater that infiltrates into a tree pit by first calculating the tributary drainage area that drains to street tree pits of varying sidewalk widths and slopes. With Manhattan, New York, as a case study, the study used i-Tree software to find that for street trees in Manhattan, stormwater infiltration greatly exceeded canopy interception, by a ratio of 3 to 1: stormwater infiltration averaged 6842 L/yr, while canopy interception averaged 2228 L/yr. The results contradict prior research that asserted that canopy interception is the primary means by which street trees provide stormwater control. The study also provides a method to calculate street tree drainage areas that would improve estimates of the amount of stormwater captured by street trees, by highlighting the dominant role played by sidewalk widths and slopes. Infiltration averaged 4221 L/yr for a sidewalk width of 3 m and 14774 L/yr for a sidewalk width of 10 m. Infiltration also averaged 5607 L/yr for a street slope of 0.5% and 18,383 L/yr for a street slope of 10%.

Influence of climatic parameters on the probabilistic design of green roofs

Green roofs are effective tools for stormwater control in highly urbanized areas since they allow the reduction of peak runoffs and volumes discharged in sewer systems. Their design is quite standardized, except for the thickness of the growing medium layer, which is strictly related to vegetation type and rainfall regime. The paper proposes an analytical probabilistic approach that relates the climatic variables, the growing medium thickness, and the water content in the condition of fulfilled field capacity to the probability that runoff from green roofs exceeds a fixed threshold. The developed equations also consider the possibility of a reduced retention capacity due to previous rainfall events, that strongly influence the performance of these green infrastructures, especially when short dry periods and/or low evapotranspiration rates occur. This feature, neglected by the traditional design storm approach, and only partially considered by previous analytical probabilistic models, represent a great potentiality of the proposed equations that are also more user-friendly and less time-consuming than continuous simulation analysis. The focus of the paper is on the influence of climatic parameters on runoff probability. To this aim to perform the monthly analysis is fundamental, especially when there is a strong variability of the climatic parameters throughout the year.The model was tested in a case study in Milano, Italy. The application presented a good agreement between the results obtained from the proposed equations and those obtained from the continuous simulation of recorded data. The results also highlighted the importance of performing analysis on a monthly scale.

FIVE-DIMENSIONAL ASSESSMENT MODEL FOR OPERATION AND MAINTENANCE OF STORMWATER CONTROL MEASURES - TOOL FOR STRATEGIC PLANNING AND CRISIS MANAGEMENT

Most stormwater infrastructures are aging and deteriorating in the United States. The American Society of Civil Engineers (ASCE) announced in its 2021 Report Card for America's Infrastructure that stormwater infrastructure has received a 'D' grade. The primary study objective is to help decision-makers deal effectively with the control measures of the limited-budgeted, ambiguous and inconsistently applied operation and maintenance of stormwater infrastructures. A five-dimensional assessment model for operation and maintenance of stormwater control measures (5D-SAM) was developed, including location, quality, time/quantity, cost, and environmental aspects. The model is very effective in helping decision-makers identify the current stormwater infrastructure conditions, predict the future state, manage the quantity and improve the quality of stormwater runoff in the most cost-effective manner. It helps determine whether a distressed stormwater system is beneficial to be demolished or it would be cost-effective to either repair, rehabilitate or retrofit. Moreover, the model can be utilized for fast and accurate assessment and better resource allocation for strategic planning of stormwater infrastructures.

Probabilistic modeling of sustainable urban drainage systems

Sustainable Urban Drainage Systems (SUDS) include strategies and solutions for distributed stormwater management and control. They are strongly encouraged, especially in highly urbanized areas that suffer the combined effect of impervious surfaces and the increase in extreme rainfall events due to urbanization growth and climate change. Their integration into traditional urban drainage systems can mitigate flood risk and pollution of receiving water bodies. The main goal of SUDS is to restore the natural water balance by increasing infiltration and evapotranspiration processes and promoting rainwater harvesting and reuse. This paper proposes an analytical-probabilistic approach for SUDS modeling applicable to different systems. Developed equations allow estimating the runoff and residual storage probability for evaluating the efficiency of the storage volume both in terms of flood control and, depending on SUDS type, in terms of emptying time or water needs supply. The modeling considers the possibility of consecutive chained rainfalls; this feature is relevant for SUDS, often characterized by low outflow rates. Relating characteristic parameters to a probabilistic level (the Average Return Interval, ARI) makes the formulas interesting to be used in the design practice. An application to two case studies confirmed the goodness of the proposed method.

Dispersed Urban‐Stormwater Control Improved Stream Water Quality in a Catchment‐Scale Experiment

AbstractTraditional urban drainage degrades receiving waters. Alternative approaches have potential to protect downstream waters, but widespread adoption requires robust demonstration of their feasibility and effectiveness. We conducted a catchment‐scale experiment over 19 years to assess the effect of dispersed stormwater control measures (SCMs), measured as a reduction in effective imperviousness (EI) on stream water quality in six sites on two streams. We compared changes in those sites over 7 years as EI decreased, to changes in the 12 preceding years, and in three reference and two control streams. SCMs reduced phosphorus concentrations and summer temperature to reference levels in dry weather where EI was sufficiently reduced, but effects were smaller with increased antecedent rain. SCMs also reduced nitrogen concentrations which were influenced by septic tank seepage in all sites. SCMs had no effect on suspended solids concentrations, which were lower in urban than in reference streams. SCMs increased electrical conductivity: along with reduced temperature this is evidence of increased contribution of groundwater to baseflows. This experiment strengthens inference that urban stormwater drainage increases contaminant concentrations in streams, and demonstrates that such impacts are reversible and likely preventable. Variation in degree of water quality improvement among experimental sites suggests that achieving reference water quality would require SCMs with large retention capacity intercepting runoff from nearly all impervious surfaces, thus requiring more downslope space and water demand. EI is a useful metric for predicting stream water quality responses to SCMs, allowing better catchment prioritization and SCM design standards for stream protection.

Constructing an inventory for fast screening of hydraulic and hydrologic performance of stormwater control measures

Abstract Stormwater control measures (SCMs) are effective and sustainable complementary means of managing stormwater in cities. Unlike underground drainage systems, they require space on the city surface, and therefore must be included in initial sketches of urban planning and design. These initial sketches are often made by architects and urban planners, who are usually not trained in hydrology, and therefore require simple and robust tools to inform their initial plans with respect to stormwater management. There may be local guidelines for dimensioning SCMs, but their applicability is often limited with regard to the range of SCMs, and the methodology behind them may be oversimplified, including a lack of assessment of benefits on the urban hydrological cycle. We developed a methodology for estimating multiple performance indicators of a wide range of SCMs and applied it to Danish meteorological conditions. The methodology includes consulting expected end users, configuring an SWMM model for each SCM type and choosing applicable parameter ranges, running multiple simulations for each type covering the parameter space, and post-processing the results using python and PySWMM. The outputs can be used to draw general recommendations regarding effective application ranges for different SCMs, and to quickly assess the performance of case-specific configurations.

Evaluation of nitrogen removal, functional gene abundance and microbial community structure in a stormwater detention basin

Stormwater control measures such as detention basins are used to mitigate the negative effects of urban stormwater resulting from watershed development. In this study, the performance of a detention basin in mitigating nitrogen pollution was examined and the abundance of N-cycling genes (amoA, nirK, nosZ, hzsB and Ntsp-amoA) present in the soil media of the basin was measured using quantitative PCR. Results showed a net export of nitrogen from the basin, however, differences between in- and outflow concentrations were not significant. Furthermore, the quantitative PCR showed that nirK (denitrification gene) was more abundant in the winter season, whereas amoA (nitrification gene) was more abundant in the summer season. The abundance of nirK, Ntsp-amoA and hzsB genes also varied with the sampling depth of soil and based on 16S rRNA gene sequencing of soil samples, Actinobacteria and Proteobacteria were the most dominant phyla. Species diversity appeared higher in summer, while the top and bottom layer of soil clustered separately based on the bacterial community structure. These results underline the importance of understanding nitrogen dynamics and microbial processes within stormwater control measures to enhance their design and performance.

Rapid mobilization of old water during urban stormflow

AbstractUrban landscapes do not easily fit into common conceptual models of streamflow generation because extensive impervious surfaces, artificial drainage via sewers, stormwater control measures, and the removal of vegetation substantially modify the pathways rainfall and meltwater take to streams. Hydrologic responses are well characterized for urban streams, however, the relative sources and flow pathways of water within urban landscapes are relatively understudied compared with undisturbed areas. Different water sources (e.g. groundwater, or ‘old’ water, rainfall, or ‘new’ water) can have distinct chemical characteristics whose mixing determines the quality of streamwater. In this study, we investigated the relative contribution of different sources of water (pre‐event water/groundwater, rainfall, wastewater, and tap water) to urban stormflow. Water samples were collected from three highly urban streams (65%–89% impervious cover) during 11 storm events and analysed for stable isotopes of oxygen and hydrogen in water (δ18O and δ2H). Precipitation samples were collected from a nearby precipitation collector to characterize the isotopic signature of new water inputs. Isotopic hydrograph separation (IHS) was used to estimate the relative proportion of new and old water in each event. The IHS results indicated that 25%–63% (δ18O) of the storm hydrograph was old water. For 9 of the 11 storms, the peak in old water contribution coincided with peak flow. These results are similar to findings from IHS studies in undisturbed catchments which suggests that under certain conditions (i.e. low intensity, long duration rainfall) the contribution of old water to stormflow in urban catchments is of a similar magnitude to undisturbed catchments. The use of tracer data is important for further exploring the conceptual model of streamflow generation that suggests that new water dominates stormflow in flashy and heavily urbanized catchments, and can be useful for characterizing the role of greenspaces and storm characteristics on water partitioning.

Utilizing In Situ Ultraviolet-Visual Spectroscopy to Measure Nutrients and Sediment Concentrations in Stormwater Runoff

The capacity to collect meaningful data to estimate stormwater runoff water quality and subsequent system removal performance is key to selecting the appropriate solutions to protect water resources. Historically, grab sampling and automated composite sampling approaches have been used with training and comprehensive quality assurance protocols to produce defensible data. Innovative approaches use real-time ultraviolet-visual spectrometry (UV-Vis) can provide a powerful tool for understanding pollutant loading regionally. This study found that real-time UV-Vis sensing is a potential new tool for understanding stormwater effluent pollutant dynamics. Researchers compared data from real-time sensing using UV-Vis spectrometers to develop calibration curves for predicting pollutant concentrations in stormwater flows. Results from paired laboratory data and raw spectral data established calibrations for the stormwater runoff composition and support further investigations of the use of this technology to predict in situ concentrations of total Kjeldahl nitrogen (TKN), dissolved organic carbon (DOC), total phosphorus (TP), total suspended solids (TSS), total nitrogen (TN), and nitrate as nitrogen (NO3−N) resulted in robust models with R2 values in the range 0.99–0.93. Using partial least squares regression (PLSR) methods, the study demonstrated a strong correlation between concentrations generated by the raw absorbance data across the full available spectrum (220 to 730 nm). These results indicate the potential for developing specific stormwater calibration curves for pollutants of interest representative of stormwater runoff. Collectively, these results indicate that real-time UV-Vis spectrometers can redefine stormwater control monitoring by potentially delivering more accurate, more repeatable laboratory quality data instantaneously, with greater efficiency.

Removal of zinc from surface runoff by using recycled mussel shell waste as treatment media, with and without heat treatment

Stormwater control measures (SCMs) are essential to manage runoff in urban areas. Mussel shell waste has been recently proposed as sustainable treatment media in SCM to remove metals from runoff. In this study, a group of laboratory-scale column experiments were conducted to investigate the use of crushed mussel shell waste to remove dissolved zinc from actual roof runoff during different filtration flow rates (1, 3, 5, 10 L/min). Heat-treated mussel shells (TMS) and untreated mussel shells (UTMS) were utilized as treatment media with two column depths (1.0 m and 0.8 m). The microstructures and chemical characteristics of TMS and UTMS were examined by using a group of Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) tests before and after the filtration process, and water samples were analyzed by using an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) instrument. TMS and UTMS showed consistent high removal efficiency for dissolved zinc with (>98%) efficiency during 1 L/min filtration rate. The average removal performance was estimated at >94% and >82% for the 1.0 m and 0.8 m column depths of TMS media, and >92% and >72% for the 1.0 m and 0.8 m depths of UTMS media, respectively. The heat treatment improved the removal of zinc with significant statistical difference (i.e. p < 0.05) during short contact times (0.8 m depth, and high filtration rates). Mussel shell waste showed practical removal performance of zinc even during high filtration rates (>5 L/min). Mussel shell waste showed potential benefits as a sustainable and cost-effective filtration media for removal of dissolved zinc in future stormwater systems. • Mussel shell waste showed high removal efficiency for zinc in runoff (>92%). • Heat treatment improved the ability of shells to remove zinc. • Organic matters attached to mussel shells slowed the removal of zinc. • Coarse crushed shells (>4.75 mm) support higher filtration rates through filtration media. • Mussel shell provided practical removal performance (>72%) during high filtration rates.

On the Effectiveness of Domestic Rainwater Harvesting Systems to Support Urban Flood Resilience

The effectiveness of domestic rainwater harvesting (DRWH) systems to support urban flood resilience is analysed at the sub-catchment scale, according to a specific DRWH conversion scenario, under 4 degrees of urbanization, 3 drainage network configurations, 4 precipitation regimes and 3 return periods of the rainfall event. At this aim, a suitable modelling framework is implemented: the semi-distributed hydrologic-hydraulic model is undertaken using EPASWMM 5.1.007 where specific tools are developed to simulate DRWH systems at high spatial resolution. The effectiveness of the DRWH systems simulated for the 144 different cases, is analysed at the event scale by using the Volume and Peak Reduction indexes to measure the hydrologic performance. The dimensionless variable, namely the event storage fraction, is defined in order to easily describe the DRWH effectiveness. The event storage fraction is defined as the ratio between the event runoff volume resulting from the impervious surface of the urban catchment in the reference scenario and the storage capacity of the DRWH systems. Modelling results confirm that DRWH catchment-scale applications allow to support specific stormwater control requirements based on peak-flow or volume regulations strategies. Findings of the elaboration reveal for a typical residential catchment in the Italy-France cross-border coastal area, that DRWH effectiveness in supporting the urban flood management becomes significant (i.e. Volume and Peak Reduction indexes greater than 0.2) starting from a storage event fraction of 0.4 that means realizing storage tanks able to contain at least the 40% of runoff volume generated by the targeted event at the sub-catchment scale.

Modelling hydrological response of modular blue roofs for storm water control in a Mediterranean site

ABSTRACT The paper proposes a spreadsheet-based model to simulate the hydrological response of a pilot installation of modular tray-based blue roof (BR). The BR is installed on the roof terrace of a building in the Campus of the University of Catania (Italy). The paper firstly describes the results of preliminary field experiments on the pilot aimed at evaluating hydrological parameters used in the model, i.e. the runoff coefficient and the initial abstraction of the terrace, and the evaporation from the BR. Then, the model was applied to 26 precipitation events recorded in the period from 2018 to 2020. Results of the application show a good ability of the model to reproduce the hydrological behaviour of the BR during precipitation events. Finally, the paper discusses limitations of the results obtained and potential for model improvement.

A Systematic Review of the Scientific Literature on Pollutant Removal from Stormwater Runoff from Vacant Urban Lands

Even though the common acknowledgment that vacant urban lands (VUL) can play a positive role in improving stormwater management, little synthesized literature is focused on understanding how VUL can take advantage of different stormwater control measures (SCMs) to advance urban water quality. The project aims to provide urban planners with information on the remediation of vacant lands using urban runoff pollutant removal techniques. To find the most effective removal method, relevant scholarly papers and case studies are reviewed to see what types of vacant land have many urban runoff pollutants and how to effectively remove contaminants from stormwater runoff in the city by SCMs. The results show that previously developed/used land (but now vacant) has been identified as contaminated sites, including prior residential, commercial, industrial, and parking lot land use from urban areas. SCMs are effective management approaches to reduce nonpoint source pollution problems runoff. It is an umbrella concept that can be used to capture nature-based, cost-effective, and eco-friendly treatment technologies and redevelopment strategies that are socially inclusive, economically viable, and with good public acceptance. Among these removal techniques, a bioretention system tends to be effective for removing dissolved and particulate components of heavy metals and phosphorus. Using different plant species and increasing filter media depth has identified the effectiveness of eliminating nitrate nitrogen (NO3-N). A medium with a high hydraulic conductivity covers an existing medium with low hydraulic conductivity, and the result will be a higher and more effective decrease for phosphorus (P) pollutants. In addition, wet ponds were found to be highly effective at removing polycyclic aromatic hydrocarbons, with removal rates as high as 99%. For the removal of perfluoroalkyl acid (PFAA) pollutants, despite the implementation of SCMs in urban areas to remove PFAAs and particulate-related contaminants in stormwater runoff, the current literature has little information on SCMs’ removal of PFAAs. Studies have also found that VUL’s size, shape, and connectivity are significantly inversely correlated with the reduction in stormwater runoff. This paper will help planners and landscape designers make efficient decisions around removing pollutants from VUL stormwater runoff, leading to better use of these spaces.

Dual isotopes of nitrate reveal varying flow paths to stormwater retention basins

Nitrate (NO3–) isotope ratios have been used to track sources of urban runoff, but their use as a tracer of flow paths has not been explored in detail. Urban flow paths are complex, and there is a gap in understanding how spatial and temporal variability impacts runoff chemistry. NO3– in urban systems can originate from atmospheric deposition, leaching from organic material, leaking from sewers, and fertilizer. Sources can be distinguished by their δ15N and δ18O signatures. We collected time series samples of stormwater runoff entering two stormwater retention basins, one large (0.32 km2 catchment) and one small (0.016 km2 catchment) in suburban Philadelphia. We used a Bayesian mixing model to determine that NO3– sources in the stormwater basin inlets were a mix of soil nitrogen (N) and atmospheric deposition across six storm events. Activation of impervious surfaces which collect atmospheric nitrogen was observed at the beginning of storm events and the portion of atmospheric versus soil N sources varied throughout the storms. However, storms with the same characteristics did not lead to the same isotopic patterns. Further evidence of the importance of flow paths was the contrast between the large and small basin. The large basin had more NO3– source variability between samples (ranging 6 % to 58 % atmospheric NO3–) compared to the small basin (16 % to 43 %). Thus, the NO3– isotopes suggest more distinct flow paths in the basin with the large catchment and more mixing of flow across land uses in the small basin’s catchment. Quantifying flow path variability from storm to storm and between different catchments can improve design and placement of urban stormwater control measures.

Cost-effectiveness analysis of extensive green roofs for urban stormwater control in response to future climate change scenarios

Green roof, as a popular low impact development practice, has become important to mitigate adverse impacts of future climate change on urban stormwater. However, there is limited information regarding assessment of the effectiveness of green roofs in response to uncertain future climate change challenges. In this study, the validated model was used to simulate the reduction performance of green roofs on urban catchment outflow and assess their cost-effectiveness in response to design storms under climate change scenarios. Results showed that the median runoff volume of urban catchments increased by 12.5 %–14.6 % and 15.5 %–18.1 % and the median peak flow rate increased by 14.4 %–17.8 % and 17.9 %–22.1 % under SSP2-4.5 and SSP5-8.5 scenarios, respectively. This indicated the variability of runoff volume and peak flow changes for short return storm events caused by climate change was relatively high. Green roof implementation had reasonable mitigation effects on runoff volume and peak flow amplification in urban catchments caused by climate change. The median runoff volume reduction of green roofs for the 1-year storm was 15.2 % under SSP2-4.5 scenario. As rainfall intensity increased, the median runoff volume reduction of green roofs significantly declined to 5.6 % for the 100-year storm. However, the variations of runoff volume and peak flow reduction of green roofs were relatively smaller for longer return periods under climate change scenarios. Runoff reduction percentages of green roofs increased linearly with their implementation cost. The average value of the cost-effectiveness (C/E) index for green roofs was 91.2 %/million $ under base climate condition, and it decreased to 88.9 %/million $ and 88.4 %/million $ for SSP2-4.5 and SSP5-8.5 scenarios, respectively. The C/E values decreased with increasing storm return period, and the values were relatively lower in SSP5-8.5 scenarios. These results could help to understand the potential role of green roofs to mitigate the impacts of future climate change.

A framework for inland cities to prevent marine debris: A case study from Durham, North Carolina

Land-based sources of litter are increasingly recognized as significant contributors to marine debris, and rivers can carry debris to the coast from far-inland sources. In this paper, we demonstrate the important role inland cities can play in the marine debris crisis by reducing their own marine debris contributions. Given this role, we provide a framework for inland cities to prevent plastic pollution along with the lessons learned from introducing these strategies in Durham, North Carolina, a mid-sized, inland city that drains to the ocean through the Cape Fear and Neuse River watersheds. This framework guides city officials, resource managers, and community partners on how to characterize the plastic pollution problem in their city by collecting baseline data on plastic waste and litter. This framework also provides practical and equitable solutions for inland cities to address plastic pollution. We recommend that inland cities prioritize policy solutions that reduce waste at the source – to the extent that their state constitutions allow – and to also use authorities for stormwater controls to capture and remove debris as long as litter persists. Replicating this framework in other inland cities opens vast opportunities to manage and reduce marine debris from an often-overlooked source.

Hydrological and water quality performance of Waste Tire Permeable Pavements: Field monitoring and numerical analysis

Permeable pavements can reduce the amount of surface runoff and peak flow rate and delay the occurrence of peak flow by allowing water to infiltrate underground similar to natural undeveloped catchments. Such suite of benefits of permeable pavements have made them one of the preferred stormwater control measures in most of the integrated land and water programs. Waste tire permeable pavements (WTPPs), as a relatively new permeable pavement technology, are designed with a surface layer made of up to 50% recycled tire particles. This study aims to investigate the hydrological performance of WTPPs to divert surface runoff and their impact on water quality. A large-scale trial in Australia was constructed and a comprehensive field performance monitoring program including double-ring infiltrometer tests and water quality testing was conducted to evaluate the performance of WTPP in real field conditions. Quality assurance tests on samples of the WTPP surface layer were conducted for permeability in the laboratory, and numerical simulations were done to estimate the surface runoff and investigate the sensitivity of the results to important design parameters. The physically-based models used for numerical simulations were developed in MUSIC X by replicating the layers of the constructed permeable pavement system as well as the impervious part of the trial site. The results indicated that the constructed system is capable of mitigating the surface runoff from the studied site, although only 25% of the discharge area was covered with WTPP. The infiltration rate of the WTPP over nine months with and without maintenance was studied. The results revealed that the infiltration rates even in areas without maintenance after nine months were found to be above the recommended values from ASCE permeable pavements task committee, but lower than the areas that were regularly maintained highlighting the importance of a regular maintenance regime for permeability recovery over time. Water quality tests were done on samples taken over a 17 month-long period indicating that the WTPP system successfully reduced most of the studied pollutants and chemical indicators, including most of the heavy metals, total suspended solids (69%) and turbidity (88%) by physically filtering the water.

Effects of land use on thermal enrichment of urban stormwater and potential mitigation of runoff temperature by watershed-scale stormwater control measures

Stream temperature is critical to the survival and reproduction of fauna in cold-water ecosystems. Urbanization causes abrupt increases in ambient stream temperature via stormwater inputs while concurrently decreasing groundwater recharge and stream baseflow. Relatively little is known about how various urban land uses affect thermal load discharged to streams or whether a network of green infrastructure retrofitted at the watershed scale can ameliorate these impacts. Runoff temperature and hydrology were monitored at 17 urban and two forested watershed outlets to assess 1) how urban land use affects stormwater thermal pollution and 2) if thermal pollution can be mitigated through infiltrating stormwater control measures (SCMs). Significantly and substantially larger runoff temperatures (i.e., event mean temperature, event maximum temperature) and thermal loads were observed from every urban land use (without SCMs) in comparison to forested (i.e., surrogate for pre-development) conditions. Event mean temperatures generally followed the land use trend of parking lot > light industrial > commercial > heavy industrial > multi-family residential, single family residential > low density residential > forest. Retrofitted bioretention cells and permeable pavements were not successful in mitigating runoff temperature in residential watersheds compared to (1) watersheds without SCMs or (2) restoring temperature to surrogate pre-development conditions. SCMs were seldomly able to mitigate enough runoff volume to have a significant thermal load reduction compared to watersheds without SCMs. These results suggest stormwater management and cold-water species management have not yet been successfully integrated. More work is needed to optimize green infrastructure SCMs design for sufficient treatment of runoff to protect cold-water ecosystems from urban development. Beyond SCM implementation, wholistic watershed management practices (e.g., riparian buffers, clustered imperviousness, underground storage/conveyance) are needed to stem thermal enrichment of streams and lakes by stormwater.

Evaluating the effects of stormwater control measures on percolation in semi-arid watersheds using a high-resolution stormwater model

Over the coming decades, global population is expected to grow rapidly and much of that growth will occur in urban areas. Stormwater Control Measures (SCMs) are implemented in urban areas to reduce watershed runoff, increase infiltration, and promote groundwater recharge. Although the effectiveness of SCMs on surface runoff quantity and quality have been well recognized, the influence on percolation has been rarely investigated especially using the combination of various SCMs in arid and semiarid regions and finding the most cost-effective practices. In this study, a spatial simulation-optimization framework (RSWMM-Cost) was developed to evaluate the efficiency of various SCMs on percolation in a highly urbanized semi-arid watershed, Ballona Creek, in southern California. Stormwater Management Model (SWMM) was used to simulated infiltration and percolation in the watershed. A surface stormwater model was utilized to evaluate ten SCM scenarios that were recommended by local agencies and stakeholders. The evaluated SCMs included bioretention, dry pond, vegetated swale, infiltration trench, porous pavements, and regional dry pond. A cost-optimization was performed to determine the required number of SCMs in each scenario based on two objectives: annual percolation and capital costs of the SCMs. The results indicated that Option 1b, which contains vegetated swales and bioretention, was the most cost-effective option for implementation. To obtain the solution for the maximum net infiltration increase (67.8% or 160,947 m3), scenario1b (35,415 bioretention cells and 8231 porous pavement units with an approximate cost of $1.6 × 109) is the most cost-effective solution to meet the required net infiltration increase. We also found that of the ten SCM scenarios, the scenario of combining vegetated swale and dry pond provided the least increase in annual percolation (25.5% or 60,533 m3 increase). The results also suggested that high impervious areas currently exhibit lower percolation rates and that SCMs can be used to offset this effect by enhancing percolation that will ultimately improve aquifer recharge and groundwater availability. The implementation of SCMs such as bioretention and vegetated swale would be ideal for municipalities looking for an inexpensive way to increase percolation and potential groundwater recharge. Study findings guide urban planners aiming to use SCMs for infiltration, groundwater recharge and water supply in semi-arid areas with overstressed groundwater systems, such as in the Los Angeles Region.