Impact of single and combined local air pollution mitigation measures in an urban environment

Urban air pollution is one of the most important environmental problems for human health and several strategies have been developed for its mitigation. The objective of this study is to assess the impact of single and combined mitigation measures on concentrations of air pollutants emitted by traffic at pedestrian level in the same urban environment. The effectiveness of different scenarios of green infrastructure (GI), the implementation of photocatalytic materials and traffic low emission zones (LEZ) are investigated, as well as several combinations of LEZ and GI. A wide set of scenarios is simulated through Computational Fluid Dynamics (CFD) modelling for two different wind directions (perpendicular (0°) and 45° wind directions). Wind flow for the BASE scenario without any measure implemented was previously evaluated using wind-tunnel measurements. Air pollutant concentrations for this scenario are compared with the results obtained from the different mitigation scenarios. Reduction of traffic emissions through LEZ is found to be the most effective single measure to improve local air quality. However, GI enhances the effects of LEZ, which makes the combination of LEZ + GI a very effective measure. The effectiveness of this combination depends on the GI layout, the intensity of emission reduction in the LEZ and the traffic diversion in streets surrounding the LEZ. These findings, in line with previous literature, suggest that the implementation of GI may increase air pollutant concentrations at pedestrian level for some cases. However, this study highlights that this negative effect on air quality can turn into positive when used in combination with reductions of local traffic emissions.

A comparative analysis of methods and tools for low impact development (LID) site selection

The site selection for Low Impact Development (LID) practices is a significant process. It affects the effectiveness of LID in controlling stormwater surface runoff, volume, flow rate, and infiltration. This research paper presents a comprehensive review of various methods used for LID site selection. It starts by introducing different methods and tools. Three main methods: index-based methods, GIS-based multi-criteria decision analysis (MCDA), and multi-criteria models and tools, are discussed in detail. A comparative analysis of these methods is then conducted based on ten different criteria. These criteria include the number of variables, data properties, the scale of analysis, benefits maximization approach, multi-attribute decision analysis, user-friendliness, community and stakeholder participation, and the validation methods. This comparison reveals limitations in each method. These include inadequate data availability and quality, lack of evaluation methods, comprehensive assessment criteria and spatial explicitness. These challenges underscore the need for future research to prioritize spatial clarity, broaden criteria, improve data quality through standardization, incorporate field visits and remote sensing for robust results, integrate big data, and develop web-based, open-source tools for enhanced accessibility. These key strategies provide valuable insights for advancing LID site selection methods.

Optimal siting of rainwater harvesting systems for reducing combined sewer overflows at city scale

The installation of green infrastructure (GI) is an effective approach to manage urban stormwater and combined sewer overflow (CSO) by restoring pre-development conditions in urban areas. Research on simulation-optimization techniques to aid with GI planning decision-making is expanding. However, due to high computational expense, the simulation-optimization methods are often based on design storm events, and it is unclear how much different rainfall scenarios (i.e., design storm events vs. long-term historical rainfall data) impact the optimal siting of GI. The Parallel Pareto Archived Dynamically Dimensioned Search (ParaPADDS) algorithm in a novel simulation-optimization tool OSTRICH-SWMM was used to leverage distributed computing resources. A case study was conducted to optimally site rainwater harvesting cisterns within 897 potential subcatchments throughout the City of Buffalo, New York. Seven design storm events with different return periods and rainfall durations and a one-month historical rainfall time series were considered. The results showed that the optimal solutions of siting cisterns using event-based scenarios, though less computationally expensive, may not perform well under continuous rainfall scenarios, suggesting design rainfall scenarios should be carefully considered for optimizing GI planning. The impact of rainfall scenarios was particularly significant in the middle region of the Pareto front of multi-objective optimization. Utilizing high-performance parallel computing, OSTRICH-SWMM is a promising tool to optimize GI at large spatial and temporal scales.

Developing Multiple Lines of Evidence to Decrease Drainage-to-Surface Area Ratio for Effective Stormwater Control Sizing Using Bioretention

Bioretention units were constructed at the US Environmental Protection Agency's Edison Environmental Center to evaluate drainage-to-surface runoff ratio for sizing of bioretention stormwater controls. Three sizes of hydraulically isolated bioretention units were tested in duplicate with changes in aspect ratio of length from inlet wall by doubling successive length from smallest (3.7 m) to largest (14.9 m) while width remained the same (7.1 m). The watershed areas were nominally the same, resulting in watershed-to-surface area ratios of 5.5:1 for largest duplicate units, 11:1 for the middle units, and 22:1 for the smallest. Each unit was instrumented for continuous monitoring with water content reflectometers (WCRs) and thermistors with data collected since November 2009. The bioretention units were filled with planting media initially comprising 90% sand and 10% sphagnum peat moss by volume and approximately 99% and 1%, respectively, by weight. These units were then planted between May and November of 2010 with a variety of native grasses, perennials, shrubs, and trees that were tolerant to inundation, drought and salt. In late 2012, a survey of the shrubs planted in these bioretention units was performed. The published results of the combined analyses of moisture content, rainfall, and size of shrubs indicated that the smaller units had superior shrub growth due to the more frequent saturation of the root zone as measured by WCR, while the plants in the largest units, particularly away from front wall where runoff entered, potentially relied on direct rainfall only. Starting in 2017, additional monitoring was performed in these units, including chemistry analysis by loss on ignition and total phosphorus of the engineered planting media and an additional survey of the plants. As in the previous study, plants did better in the medium (11:1) and small (22:1) bioretention units than in the largest units (5.5:1), and there was greater buildup of organic matter and phosphorus in the smaller units. One species of grass that dominated the two largest bioretention units away from the inlet was drought tolerant, which indicated that plants in these units relied on rainfall rather than stormwater runoff. Oversized units did not completely use the stromwater control volume, and many of the other original plantings grew slower or were less widespread in comparison to plantings in that smaller units that flooded more frequently and achieved greater growth.

Practical Applications

Defining the size of stormwater controls can be difficult because there are often multiple objectives imposed on the final design of these structures, including safety and flooding. Results presented here would indicate that if the objective is to create a bioretention area with healthy vegetation, undersized controls may be acceptable because undersized infiltrating controls will have healthier plantings and infiltrate throughout the storm. For municipalities, this means that rights of way previously thought to be too small to use for infiltrative stormwater controls may be converted to such a purpose. This does not free municipalities from stormwater systems that address flooding and safety design objectives, but demonstrates that increasing plantings in the municipal right of way could help to address stormwater as well as other objectives, like greenhouse gas emissions, urban heat island reduction, and clean air. Distributed bioretention controls that capture part or all the runoff of the smaller, most frequent rainfall events should be incorporated throughout municipalities and into their overall stormwater control systems. If clogging by runoff is a concern, roof runoff may be more appropriate for bioretention, or other measures such as sediment capture or increased maintenance may need to be performed.

Sustainable Urban Drainage System (SUDS) modeling supporting decision-making: A systematic quantitative review

Decision Support Systems (DSS) for Sustainable Urban Drainage Systems (SUDS) are a valuable aid for SUDS widespread adoption. These tools systematize the decision-making criteria and eliminate the bias inherent to expert judgment, abridging the technical aspect of SUDS for non-technical users and decision-makers. Through the collection and careful assessment of 120 papers on SUDS models and SUDS-DSS, this review shows how these tools are built, selected, and used to assist decision-makers questions. The manuscript classifies the DSS based on the question they assist in answering, the spatial scale used, the software selected, among other aspects. SUDS-DSS aspects that require more attention are identified, including environmental and social considerations, SUDS trains performance and criteria for selection, stochasticity of rainfall, and future scenarios impact. Suggestions for SUDS-DSS are finally offered to better equip decision-makers in facing emerging stormwater challenges in urban centers.

Creating urban green infrastructure where it is needed - A spatial ecosystem service-based decision analysis of green roofs in Barcelona

As cities face increasing pressure from densification trends, green roofs represent a valuable source of ecosystem services for residents of compact metropolises where available green space is scarce. However, to date little research has been conducted regarding the holistic benefits of green roofs at a citywide scale, with local policymakers lacking practical guidance to inform expansion of green roofs coverage. The study addresses this issue by developing a spatial multi-criteria screening tool applied in Barcelona, Spain to determine: 1) where green roofs should be prioritized in Barcelona based on expert elicited demand for a wide range of ecosystem services and 2) what type of design of potential green roofs would optimize the ecosystem service provision. As inputs to the model, fifteen spatial indicators were selected as proxies for ecosystem service deficits and demands (thermal regulation, runoff control, habitat and pollination, food production, recreation, and social cohesion) along with five decision alternatives for green roof design (extensive, semi-intensive, intensive, naturalized, and allotment). These indicators and alternatives were analyzed probabilistically and spatially, then weighted according to feedback from local experts. Results of the assessment indicate that there is high demand across Barcelona for the ecosystem services that green roofs potentially might provide, particularly in dense residential neighborhoods and the industrial south. Experts identified habitat, pollination and thermal regulation as the most needed ES with runoff control and food production as the least demanded. Naturalized roofs generated the highest potential ecosystem service provision levels for 87.5% of rooftop area, apart from smaller areas of central Barcelona where intensive rooftops were identified as the preferable green roof design. Overall, the spatial model developed in this study offers a flexible screening based on spatial multi-criteria decision analysis that can be easily adjusted to guide municipal policy in other cities considering the effectiveness of green infrastructure as source of ecosystem services.

A comprehensive review of spatial allocation of LID-BMP-GI practices: Strategies and optimization tools

Low-impact development (LID), best management practice (BMP), and green infrastructure (GI) are semi-engineered stormwater management practices that have been widely studied and implemented worldwide. Implemented in the complex environment of urban areas, LID-BMP-GI practices often intertwine with a very large number of hydro-environmental and socio-economic considerations and constraints. Therefore, they need to be carefully selected, designed, and allocated within an urban area. Both planning and optimization can lead to more systematic and strategic approaches to address this multi-scale, multi-parameter problem of practice allocation. In this review, we first identify the main components of the strategic planning cycle, their scope and inter-relationships, and their corresponding mathematical representations. We then present a comprehensive review of the existing literature on spatial allocation optimization tools (SAOTs) for LID-BMP-GI practices and summarize the generic structure and the systematic typology of the existing SAOTs. We conclude with a discussion of several current research gaps in the spatial allocation of LID-BMP-GI practices. In this review, we aim to summarize the strategies and optimization tools for the spatial allocation of LID-BMP-GI practices that are beneficial to practitioners. The other aim is to provide recommendations for future research on the development of more advanced and comprehensive SAOTs.

A review on effectiveness of best management practices in improving hydrology and water quality: Needs and opportunities

Best management practices (BMPs) have been widely used to address hydrology and water quality issues in both agricultural and urban areas. Increasing numbers of BMPs have been studied in research projects and implemented in watershed management projects, but a gap remains in quantifying their effectiveness through time. In this paper, we review the current knowledge about BMP efficiencies, which indicates that most empirical studies have focused on short-term efficiencies, while few have explored long-term efficiencies. Most simulation efforts that consider BMPs assume constant performance irrespective of ages of the practices, generally based on anticipated maintenance activities or the expected performance over the life of the BMP(s). However, efficiencies of BMPs likely change over time irrespective of maintenance due to factors such as degradation of structures and accumulation of pollutants. Generally, the impacts of BMPs implemented in water quality protection programs at watershed levels have not been as rapid or large as expected, possibly due to overly high expectations for practice long-term efficiency, with BMPs even being sources of pollutants under some conditions and during some time periods. The review of available datasets reveals that current data are limited regarding both short-term and long-term BMP efficiency. Based on this review, this paper provides suggestions regarding needs and opportunities. Existing practice efficiency data need to be compiled. New data on BMP efficiencies that consider important factors, such as maintenance activities, also need to be collected. Then, the existing and new data need to be analyzed. Further research is needed to create a framework, as well as modeling approaches built on the framework, to simulate changes in BMP efficiencies with time. The research community needs to work together in addressing these needs and opportunities, which will assist decision makers in formulating better decisions regarding BMP implementation in watershed management projects.