Improved Stormwater Management Research Articles

Quantitative fecal source characterization of urban municipal storm sewer system outfall ‘wet’ and ‘dry’ weather discharges

Urban areas are built environments containing substantial amounts of impervious surfaces (e.g., streets, sidewalks, roof tops). These areas often include elaborately engineered drainage networks designed to collect, transport, and discharge untreated stormwater into local surface waters. When left uncontrolled, these discharges may contain unsafe levels of fecal waste from sources such as sanitary sewage and wildlife even under dry weather conditions. This study evaluates paired measurements of host-associated genetic markers (log10 copies per reaction) indicative of human (HF183/BacR287 and HumM2), ruminant (Rum2Bac), canine (DG3), and avian (GFD) fecal sources, 12-hour cumulative precipitation (mm), four catchment land use metrics determined by global information system (GIS) mapping, and Escherichia coli (MPN/100 ml) from seven municipal separate storm sewer system outfall locations situated at the southern portion of the Anacostia River Watershed (District of Columbia, U.S.A.). A total of 231 discharge samples were collected twice per month (n = 24 sampling days) and after rain events (n = 9) over a 13-month period. Approximately 50 % of samples (n = 116) were impaired, exceeding the local E. coli single sample maximum of 2.613 log10 MPN/100 ml. Genetic quality controls indicated the absence of amplification inhibition in 97.8 % of samples, however 14.7 % (n = 34) samples showed bias in DNA recovery. Of eligible samples, quantifiable levels were observed for avian (84.1 %), human (57.4 % for HF183/BacR287 and 40 % for HumM2), canine (46.7 %), and ruminant (15.9 %) host-associated genetic markers. Potential links between paired measurements are explored with a recently developed Bayesian qPCR censored data analysis approach. Findings indicate that human, pet, and urban wildlife all contribute to storm outfall discharge water quality in the District of Columbia, but pollutant source contributions vary based on ‘wet’ and ‘dry’ conditions and catchment land use, demonstrating that genetic-based fecal source identification methods combined with GIS land use mapping can complement routine E. coli monitoring to improve stormwater management in urban areas.

Biochar and vegetation effects on discharge water quality from organic-substrate green roofs

Green roofs have been increasingly used to improve stormwater management, but poor vegetation performance on roof systems, varying with vegetation type, can degrade discharge quality. Biochar has been suggested as an effective substrate additive for green roofs to improve plant performance and discharge quality. However, research on the effects of biochar and vegetation on discharge quality in the long term is lacking and the underlying mechanisms involved are unclear. We examined the effects of biochar amendment and vegetation on discharge quality on organic-substrate green roofs with pre-grown sedum mats and direct-seeded native plants for three years and investigated the key factors influencing discharge quality. Sedum mats reduced the leaching of nutrients and particulate matter by 6–64% relative to native plants, largely due to the higher initial vegetation cover of the former. Biochar addition to sedum mat green roofs resulted in the best integrated water quality due to enhanced plant cover and sorption effects. Structural equation modeling revealed that nutrient leaching was primarily influenced by rainfall depth, time, vegetation cover, and substrate pH. Although biochar-amended sedum mats showed better discharge quality from organic-substrate green roofs, additional ecosystem services may be provided by native plants, suggesting future research to optimize plant composition and cover and biochar properties for sustainable green roofs.

GREENING THE CITY: CRITERIA AND INDICATORS FOR EVALUATING THE EFFECTIVENESS OF SMALL URBAN PARKS IN PROMOTING URBAN RESILIENCE TO CLIMATE CHANGE

As cities face the challenges of climate change, small urban parks offer a promising solution for promoting urban resilience. These parks can mitigate the impacts of climate change by providing carbon sequestration, reducing the urban heat island effect, and improving stormwater management. They can also serve as a key adaptation strategy by providing shade and cooling, reducing air pollution, and enhancing social cohesion and community engagement. However, there is a need for criteria and indicators that can guide the design, implementation, and management of small urban parks for climate change mitigation and adaptation. This paper proposes a set of criteria and indicators that can be used to evaluate the effectiveness of small urban parks in promoting urban resilience. The criteria include ecological, social, economic, governance dimensions, and the indicators are measurable, relevant, and practical. The proposed framework can support decision-making processes for planners, policymakers, and practitioners to prioritise and allocate resources for small urban parks that can contribute to climate change mitigation and adaptation in cities.

Evaluating the effect of building patterns on urban flooding based on a boosted regression tree: A case study of Beijing, China

AbstractRapid urbanization and global climate change are likely to exacerbate urban flooding intensity, frequency and uncertainty. Thus, it is fundamental and crucial to investigate the dominant influencing factors for the mitigation of urban flooding. However, knowledge about the influence of building patterns on urban flooding remains limited. Taking Beijing, a typical megacity, as a case study area, we quantified the importance of building patterns and their interaction effect at the subwatershed scale using the boosted regression tree and geographical detector model. The results indicated that (1) the landscape shape index, slope, green space ratio and waterbody ratio were the most important influencing factors determining urban flooding, with a total relative contribution of 67.23%; (2) building metrics had a certain impact on urban flooding, and the sum of the relative contribution reached 21.03%; (3) the landscape shape index, slope, and green space ratio exhibited a combination of negative and positive correlations with urban flooding density; and (4) an enhancement effect existed between building metrics, especially the building congestion degree and building density. These findings provide quantitative insights on rational urban morphology planning which could be applied to improve stormwater management and promote urban sustainability in megacities.

Modeling stormwater management pond thermal impacts during storm events

Discharges of warm water from shallow stormwater management ponds during summer months into receiving headwater streams pose a risk to aquatic ecosystems in urban watersheds, especially in cool- and cold-water streams. Physically based models such as MINUHET have been used to assess the effects of thermal loading on receiving streams. However, these models require a significant amount of data and are complex to calibrate and use for the optimized designs that ensure protection of vulnerable aquatic life in urban stream ecosystems. This research introduces a user friendly and more accurate machine learning method to predict thermal profiles in stormwater ponds and the pond outlet discharge temperature to the receiving streams during storm events. Monitoring data were collected between 2014 and 2016 from three stormwater ponds in southern Ontario to train and test the new model. The new model is used to demonstrate the degree to which cooling effects can be achieved from remedial actions such as increasing the pond depth by dredging one meter and if a bottom-draw outlet is used instead of a mid-depth draw outlet. The methods presented in this study can be used to support an improved stormwater management pond design guidelines based on the key design objective that the thermal regime of the stormwater pond outlet discharge water must match the thermal regime of the receiving stream. The main objective is to avoid any discernable warming of the stream due to the stormwater pond discharge under a range of summer temperatures and storm event sizes.

Two-scale optimal management of urban runoff by linking LIDs and landscape configuration

Landscape composition and configuration of urban patches impact greatly on hydrological process, of which low impact development practices (LIDs) have been incorporated worldwide to management stormwater runoff quantity- and quality-related issues. Although optimization design for LIDs has been studied extensively to improve stormwater management at catchment scale, a further study of LIDs-based landscape configuration at site scale has not been fully explored. In this study, a two-scale optimization framework was constructed to integrate spatial layout and landscape configuration of LIDs using SWMM, NSGA-II and GIS techniques. The spatial optimization at catchment scale showed that area of sink landscape almost doubled as the distance to outlets decreased. A further site-scale optimization by a new landscape metric, in terms of CRSS (configuration ratio between patches of runoff sources and sink), was conduct to achieve LIDs-based landscape configuration optimization. Runoff flow and pollution were significantly correlated with CRSS in power function, quantifying CRSS thresholds of 0.57–0.69 as the most adaptive strategies for stormwater management. Notably, the CRSS thresholds were higher for ammonia nitrogen (NH3-N) and suspended solid (SS) pollution, especially targeting source patches of roofs. The optimization in this study could improve the reduction of runoff peak flow of heavy rain, peak concentration of chemical oxygen demand and SS, and initial scouring of NH3-N.

How to simulate future scenarios of urban stormwater management? A novel framework coupling climate change, urbanization, and green stormwater infrastructure development

Climate change, urbanization, and green stormwater infrastructure (GSI) planning policies lead to uncertainties in future urban sustainability. Coupling multiple influencing factors such as climate change, urbanization, and GSI development, this study proposes a novel framework for simulating future scenarios of urban stormwater. Subsequently, the changes in annual surface runoff and runoff pollutants in Shanghai's new and old urban areas were compared and analyzed based on 35 typical future and seven baseline scenarios. The following results were obtained: 1) The runoff control rate of the new urban area was significantly higher than that of the old urban area before GSI construction. After GSI construction, both areas could control stormwater runoff and pollutants, while the decline in efficiency in GSI facilities enormously impacted the old area. 2) Surface runoff in the new urban area was mainly affected by urbanization, while climate change was a major factor in the old urban area; runoff pollutants in new and old urban areas were mainly affected by urbanization, and the change in pollutants in new areas was more pronounced. 3) GSI facilities were unlikely to guarantee the quantity and quality of water resources, especially in scenarios where the efficiency of GSI facilities decreases. In old urban areas, the more extreme climate change and urbanization were, the more significant the effect of improving stormwater management facilities. Our findings showed that future studies on stormwater management should specifically consider the different characteristics of new and old urban regions, pay attention to the maintenance and management of GSI facilities, and build adaptive strategies to cope with climate change, urbanization, and GSI facility destruction.

Cost-Benefit Analysis of Urban Stream Restoration

The District of Columbia Department of Energy and Environment (DC DOEE) and National Park Service (NPS) have worked in tandem on multiple stream restorations throughout the Washington DC area. Restored stream ecosystems provide a variety of direct and indirect environmental benefits, such as improved stormwater management, water quality, and recreational value.

Monitoring the effects of urban and forested land uses on runoff quality: Implications for improved stormwater management

Urban stormwater is a substantial source of non-point source pollution. Despite considerable monitoring efforts, little is known about stormwater quality in certain geographic regions. These spatial gaps induce uncertainty when extrapolating data and reduce model calibration capabilities, thereby limiting pollutant load reduction strategies. In this study, stormwater quality was monitored from 15 watersheds to characterize pollutant event mean concentrations (EMCs) and loads as a function of urban and forested (i.e., surrogates for pre-development) land use and land covers (LULCs) and rainfall patterns from a geographic region where these data are sparse. Residential and heavy industrial, heavy industrial, and industrial and commercial LULCs, respectively, were the primary generators of nutrients, total suspended solids (TSS), and heavy metals. Increased rainfall intensities (average and peak) significantly increased the EMCs of all particulate bound pollutants. Pollutant loads increased with rainfall depth and, in general, did not follow the same LULC trends as EMCs, suggesting loads were influenced substantially by watershed hydrologic responses. Mean annual urban loads of total phosphorus, total nitrogen, TSS, and zinc (Zn) ranged from 0.4 (low density residential [LDR]) to 1.5 (heavy industrial), 3.2 (single family residential [SFR]) to 11.5 (heavy industrial), 122.6 (SFR) to 1219.9 (heavy industrial), and 0.1 (LDR) to 0.7 (commercial) kg/ha/yr, respectively. Annual urban loads of TSS were 3.5 to 34 and − 1.5 to 6.8-fold greater than annual loads from forested and agricultural watersheds, respectively. Mean annual loads of heavy metals from urban LULCs were substantially greater than loads produced by forested and agricultural watersheds (e.g., 8.6 to 92 and 6.8 to 73-fold greater, respectively, for Zn), while loads of nutrients were generally similar between urban and agricultural watersheds. Findings herein suggest non-point source pollution will continue to threaten surface water quality as land is developed; results can help guide the development of cost-efficient stormwater management strategies.

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.

A Procedural Modeling Approach for Ecosystem Services and Geodesign Visualization in Old Town Pocatello, Idaho

City population has been growing rapidly worldwide due to urban expansion, which can bring negative impacts on local ecosystem services (ES). Efficient tools for urban design and visualization are essential for city planners and stakeholders to better understand the valuation impact of plans for future sustainable development. Current urban design methods are mainly based on a 2D perspective and lack vertical visualization. Although conventional 3D modeling was introduced to address these limitations, it still has some challenges, such as requiring powerful computing resources and specialized training. Procedural 3D modeling is a grammar-based set of rules that can effectively generate 3D models and enhance spatial visualization when compared with conventional 2D or 3D methods. This paper describes a framework for developing a geodesign tool in Old Town Pocatello, Idaho, USA using procedural modeling to improve planning and visualization for urban design, including (1) Geospatial data preparation in ArcGIS, (2) 3D cityscape model generation in CityEngine, and (3) interactive visualization applications for multiple platforms developed with the Unity game engine. Pocatello is a mid-sized city in southeast Idaho that faces several challenges towards integrating ecosystem services in urban design. As a case study in ecosystem service modelling, we proposed a green scenario for Old Town to demonstrate a tool where permeable surfaces were increased from 37% to 45% to help mitigate urban land surface temperature and improve stormwater management. This geodesign tool offers city planners and stakeholders an opportunity to visualize and analyze block-level scenarios in real time. The interactive applications can encourage public participation in the design process. More ES measurements can be implemented into this tool in the future. The techniques of 3D procedural modeling and ES modeling in this study are also applicable to other small to mid-sized cities worldwide.

Willingness to pay for the outcomes of improved stormwater management

ABSTRACT Sydney Water owns approximately 450 kilometres of stormwater and waterway infrastructure spread across more than 70 catchments in Sydney, Australia. Improving waterway health and enhancing urban environments is an important part of Sydney Water’s strategy to enhance the liveability of Sydney. This strategy can have a range of costs and benefits to the community. The costs of stormwater management can generally be estimated using market data. The benefits are not marketed directly, so estimation requires the use of nonmarket valuation methods. This paper reports the results of a choice modelling study that examined the willingness to pay of households in the catchments of Georges, Cooks and Parramatta rivers for improved stormwater management. The study found positive household values for improvements in waterway health, additional native vegetation plantings, including wetlands, additional recreation facilities in local open spaces used for stormwater management, and removal of rubbish and litter from the waterways.

Urban Gardens’ Potential to Improve Stormwater Management: A Comparative Analysis among Urban Soils in Sorocaba, São Paulo, Brazil

Permeable surfaces are increasingly rare in urban centers, but they have the utmost importance for stormwater infiltration. In this context, green spaces are key to reducing problems caused by runoff. This work aimed to evaluate the physical characteristics of the soil used for agroecological gardening, in comparison with parks, wasteland, and riparian forest in Sorocaba, São Paulo, Brazil. During the one-year data collection, urban gardens were superior to other areas in hydraulic conductivity (35.8 mm h−1), humidity (25.8%), and soil penetration resistance (1.21 MPa). On the other hand, the riparian forest showed signs of soil degradation, with low water infiltration rates (121.9 mm h−1) and humidity (14.4%). These findings highlight the importance of better soil management solutions to avoid compaction, such as the protection and conservation of riparian forests. Furthermore, the encouragement of urban gardens and parks with multiple uses can be an option for the enhancement of stormwater management in cities, since this practice has the potential to improve the physical characteristics of urban soils and provide several ecosystem services.

Improved stormwater management through the combination of the conventional water sensitive urban design and stormwater pipeline network

With rapid urbanization, flooding events become more frequently in daily life, causing enormous economic damage and loss of life. Water Sensitive Urban Design (WSUD) is a common approach for mitigating stormwater runoff. However, it showed limited performance in big catchment areas (>1000 ha). This study proposed an innovative approach by combining conventional WSUD projects with the stormwater pipeline network through linear connections for better stormwater runoff management for a big catchment. The performance of combined WSUD projects and conventional WSUD was evaluated using the urban water system of a catchment (over 1200 ha) in Sydney, Australia, through the water mass balance modelling approach using annual rainfall data of 70 years (from 1950 to 2020). Combined WSUD reduced the stormwater runoff by over 124 ML/yr compared to that of the conventional WSUD model in accommodating future development. Combined WSUD restored the evapotranspiration and infiltration under high, average and low annual rainfall scenarios with an increasing 20–30% increase of evapotranspiration and infiltration in combined WSUD than the conventional WSUD. The results obtained from the study demonstrated that combining WSUDs with the stormwater pipeline network through linear connections is a promising approach in stormwater management and restoring the natural hydrological cycle.

Monitoring the Effects of Urban and Forested Land Uses on Runoff Quality: Implications for Improved Stormwater Management

Monitoring the Effects of Urban and Forested Land Uses on Runoff Quality: Implications for Improved Stormwater Management

Concrete in the city

ABSTRACT Despite the hydrological imperative and engineering capacity for change, concrete storm water infrastructure remains obdurate in the urban waterscape. This obduracy manifests both as an unwillingness to remove existing infrastructure and the continuing construction of new infrastructure in locations previously free of these systems. This paper identifies four critical socio-political values underlying the obduracy of concrete storm water infrastructure and the resultant urban stream syndrome. Following a brief critique of reactive storm water management frameworks to manage this syndrome, this paper articulates four common values of Indigenous science(s) that are well placed can contribute to improve storm water management. Supporting this argument is an example of Indigenous science(s) changing the form and function of a reach of an extant concrete storm water channel in Canberra, Australia. While these interventions will be assessed primarily from water quality perspectives, they contribute to a greater range of environmental processes than purely hydrological.

The motivations, desired outcomes, and visions of partner organizations to Collective Impact tree planting: A transdisciplinary case study of CommuniTree in Northwest Indiana, U.S.

In Northwest Indiana, a new urban forestry effort called CommuniTree has emerged with a goal of helping solve some of the region’s post-industrial social and ecological issues. Though no panacea, tree planting can have significant benefits, including air pollution abatement, temperature moderation, stormwater management, and beautification. CommuniTree is a Collective Impact-style collaborative of over a dozen partner groups (public, nonprofit, private, and partnership organizations) that plants trees on public and industrial properties in post-industrial Northwest Indiana communities where urban forestry governance and resources are relatively lacking. This paper presents the results of the first phase of a transdisciplinary research program that draws insights from urban forest governance scholarship and transdisciplinary sustainability science as well as from urban forest professionals involved in CommuniTree who helped design the research. We used a transdisciplinary case study approach with qualitative interviews with key informants from CommuniTree partner organizations to investigate partner organizations’ motivations for participating, desired outcomes, and long-term visions for CommuniTree. We find that the CommuniTree partnership itself is an intrinsic incentive to partners’ participation: once an influential champion brought the group together, they were motivated to stay involved by the work and structure of CommuniTree. The scant existing urban forest governance in the region also motivated partners to contribute to CommuniTree collective action. For desired outcomes, we find that partners are interested in the potential improvements to ecological quality that may result for the region such as improved stormwater management and air quality, but that more than ecological outcomes, partners are interested in the social outcomes that might come of the CommuniTree, including engaging communities in ecological stewardship activities and the beautification and general improvements to public spaces that result from tree planting. And for its future, partners conveyed a plurality of visions for CommuniTree’s growth including expanding the program to new communities, planting trees on new types of locations, and involving more partners, the general public, and new volunteer groups. However, current CommuniTree structure has vulnerabilities to its sustainability and longevity, including a reliance on predominantly grant funding, shifting organizational dynamics within/among partners, and a lack of systematic integration of the voices of local community members. In conclusion, this research on CommuniTree’s structure and function (i.e., the governance and activities) and on what partners value as desired outcomes and for the future informs an understanding of the vulnerabilities and opportunities for CommuniTree as the program evolves to better meet the needs of current partners, future stakeholders, and, most importantly, community members.

Performance of pervious concrete under shear and bearing strengths

Pervious concrete is one of the materials with potential applications in the construction industry because it can help reduce environmental problems caused by conventional concrete, improve storm water management, and provide a low-impact development alternative. Pervious concrete has numerous advantages, including assisting with water filtering and lowering pollutants. The ability to reduce storm water runoff is the most important feature that attracts the attention of pervious concrete. The objective of the present investigation is to evaluate the properties of pervious concrete and compare them with normal concrete. On the other hand, the requirement for aggregate to produce concrete is high, while natural resources are reduced. An attempt was made to use Black Marble Stone Waste Aggregate in concrete by replacing natural aggregate at a ratio of 0, 50, and 100%.To obtain the behavior of pervious concrete, compressive, shear and bearing strength are evaluated. A total of 54 specimens were casted and tested in this experimental work to study the behavior of concrete. To assess the shear and bearing strengths, a regression model was developed.

Testing a modified environmental flows framework for a Southern Ontario (Canada) river system: assessing hydrological alteration and management recommendations

To date, environmental flow strategies have predominantly been used in cases where a lack of available water has degraded the ecological quality and natural functioning of a river system. In this study, we used environmental flows on an urbanized watershed where flow volumes and flow rates have increased, and large (e.g. 100-year) event return periods become more frequent (∼10 years). Using an environmental flows strategy to model the amount of hydrological alteration, we assessed whether urban development and land cover changes have impacted the flow regime and the ecological health of this watershed. We reconstructed the flow regime at Lovers Creek (Barrie, Ontario, Canada) using three scenarios of urban development: a pre-settlement reference condition with 100% natural cover, mid-development with 6% urban cover and the current condition with 21% urban cover. We found that, in contrast to many other studies, increased urban cover has coincided with increased baseflow volumes, as well as increased volumes in other components of the flow regime. Of particular concern were the increased flow rate and volume of large events such as channel forming and riparian flows; an increased rate of change in event hydrographs, and a much shorter return period of large event storms (e.g. a 100-year event under reference conditions, is now a 10-year event in the current urbanized condition). In this watershed, restoration of flow to the reference condition is not practical; however, using improved stormwater management such as low-impact development technologies and green infrastructure may offset some of the alteration to the flow regime and mitigate future further alterations due to increased urbanization. As Lovers Creek has been identified as a critical coldwater stream habitat with brook trout, the results of this study are being used to assess changes in the ecological health due to increased urban cover, and to develop management strategies that assist in restoring a more natural flow regime in order to better protect both natural habitats and human infrastructure that are vulnerable to flooding caused by more frequent, increased volume flow events.

Exploring the Optimal Cost-Benefit Solution for a Low Impact Development Layout by Zoning, as Well as Considering the Inundation Duration and Inundation Depth

Urban flooding now occurs frequently and low impact development (LID) has been widely implemented as an effective resilience strategy to improve storm water management. This study constructed the inundation curve to dynamically simulate the disaster, and established an inundation severity indicator (ISI) and cost-effectiveness indicator (CEI) to quantify the severity and cost effectiveness at each site. The study set 10 different density scenarios using a zonal approach. The results showed that LID could reduce the overall ISI value, but as the construction increased, the CEI exhibited a downward trend, showing that there is a marginal utility problem in LID. However, the performance of CEI differed slightly in areas of different severity. In the vulnerable resilience zone, the CEI increased initially and then decreased, and the optimal cost–benefit combination was 60% permeable pavement +20% green roof +50% vegetative swale. The mutual effects of LID measures in different zones led to synergistic or antagonistic effects on LID. This study explored the tradeoff between the resilience enhancement effect and strategy transformation cost, and determined the optimal combination of the LID strategy, thereby providing a new analytical perspective for the sustainable development of sponge cities.