Assessing the Co-Benefits of green-blue-grey infrastructure for sustainable urban flood risk management

Blue-green infrastructure in view of Integrated Urban Water Management: A novel assessment of an effectiveness index

Addressing urban water management challenges requires a holistic view. Sustainable approaches such as blue-green infrastructure (BGI) provide several benefits, but assessing their effectiveness demands a systemic approach. Challenges are magnified in informal areas, leading to the combination of integrated urban water management (IUWM) with BGI as a proposed solution by this research. We employed the Urban Water Use (UWU) model to assess the effectiveness index (EI) of BGI measures in view of IUWM after stakeholder consultation. The procedure in this novel assessment includes expert meetings for scenario building and resident interviews to capture the community's vision. To assess the impact of IUWM on the effectiveness of BGI measures, we proposed a simulation with BGI only and then three simulations with improvements to the water and sewage systems. The results of the EI analysis reveal a substantial improvement in the effectiveness of BGI measures through IUWM combination. Moreover, we offer insights into developing strategies for UWU model application in informal settlements, transferrable to diverse urban areas. The findings hold relevance for policymakers and urban planners, aiding informed decisions in urban water management.

Exploring the contribution of nature-based solutions for environmental challenges in the Netherlands

Nature-based solutions (NbS) offer a promising and sustainable approach to addressing multiple environmental challenges, including climate change, pollution, and biodiversity loss. Despite the potential of NbS, their actual effectiveness in solving these challenges remains uncertain. Therefore, this study evaluates the contribution of NbS implemented in a nature-inclusive scenario for six environmental challenges and associated policy targets in the Netherlands. Fifteen different NbS were applied in the scenario in urban, agricultural, aquatic, and protected nature areas, with measures like flower field margins, green roofs, groundwater level management, and river restoration. The spatially-explicit Natural Capital Model was used to quantify the effectiveness of all applied NbS at a national-scale. Results show NbS significantly contribute to simultaneously solving all six assessed environmental challenges. The most significant impact was seen in improving the quality of water bodies (+34 %), making agriculture more sustainable (+24 %), and protecting and restoring biodiversity (+22 %). The contribution of NbS to address the quality of the living environment (+13 %), climate change (+10 %), and the energy transition was less effective (+2 %). Furthermore, NbS can help to achieve sectoral policy targets at the global, EU, and national levels, including those related to the Birds Habitats Directives, carbon emission, and pesticide reduction targets. This study highlights the potential of NbS to effectively address multiple environmental challenges, although they do not provide a complete solution, and suggests that future research could focus on identifying even more effective ways to implement NbS, and to mainstream their use in policy and practice.

Nature-based solution enhances resilience to flooding and catalyzes multi-benefits in coastal cities in the Global South

Coastal cities are facing a rise in groundwater levels induced by sea level rise, further triggering saturation excess flooding where groundwater levels reach the topographic surface or reduce the storage capacity of the soil, thus stressing the existing infrastructure. Lowering groundwater levels is a priority for sustaining the long-term livelihood of coastal cities. In the absence of studies assessing the possibility of using tree-planting as a measure of alleviating saturation excess flooding in the context of rising groundwater levels, the multi-benefit nature of tree-planting programs as sustainable Nature-based solutions (NBSs) in coastal cities in the Global South is discussed. In environments where groundwater is shallow, trees uptake groundwater or reduce groundwater recharge, thereby contributing to lower groundwater levels and increasing the unsaturated zone thickness, further reducing the risk of saturation excess flooding. Tree-planting programs represent long-term solutions sustained by environmental factors that are complementary to conventional engineering solutions. The multi-benefit nature of such NBSs and the expected positive environmental, economic, and social outcomes make them particularly promising. Wide social acceptance was identified as crucial for the long-term success of any tree-planting program, as the social factor plays a major role in addressing most weaknesses and threats of the solution. In the case of Nouakchott City (Mauritania), where a rise in groundwater levels has led to permanent saturation excess flooding, a tree-planting program has the potential to lower the groundwater levels, thereby reducing flooding during the rainy season.

A quantitative review of nature-based solutions for urban sustainability (2016-2022): From science to implementation

Nature-based solutions (NBS) have great potential for achieving urban sustainability. While several reviews have comprehensively examined NBS, few have focused on its role in addressing urban sustainability challenges. Here we present a systematic review of 142 case studies selected from English papers published in SCI journals (i.e., indexed by Web of Science) during 2016-2022, whose titles, abstracts or keywords contain both urban-related terms and NBS-related terms. Using multiple methods, including statistical analysis, deductive content analysis, and inductive content analysis, we found that: (1) NBS have primarily been utilized to address urban flooding (43 %) and heat stress (21 %), with green roofs (24 %) and urban forests (16 %) being the most extensively studied NBS for tackling these challenges. (2) The ecosystem services (ES) capacity of NBS has been heavily researched (57 %), while studies addressing ES flows (7 %) and ES demand (18 %) are limited. (3) Most studies involved at least one NBS implementation process (83 %), but primarily focused on selecting and assessing NBS and related actions (66 %), with fewer studies on designing and implementing NBS and transferring & upscale NBS. We suggest that future research should contribute to the establishment of a checklist to assist in identifying which NBS types are effective in addressing specific urban sustainability challenges in varying contexts. Integrating the science and practice of NBS for urban sustainability is also crucial for advancing this field.

Flood risk transfer analysis based on the "Source-Sink" theory and its impact on ecological environment: A case study of the Poyang Lake Basin, China

Driven by climate change, the frequent occurrence of regional destructive floods poses a grave threat to socio-economic systems and ecological environments. Previous flood risk studies have disregarded risk transfer within a region, resulting in inadequate flood risk assessment and ineffective disaster prevention and mitigation outcomes. Therefore, this study introduced the "Source-Sink" theory into flood disaster field to constructing flood risk transfer model. Flood risk assessment and transfer was conducted in the Poyang Lake Basin, China, where the impacts of the initial and transfer statuses on ecosystem service values were quantified. The results showed that the flood risk in the Poyang Lake Basin was relatively low, with high spatial distribution characteristics in the central-north areas but low in the surrounding areas. High-risk zones were mainly distributed southwest of the Poyang Lake. The lower-risk zones exhibited a contiguous distribution and were surrounded by higher-risk zones. Following the completion of the flood risk transfer, high-risk zones increased significantly; but there were a few zones where the risk was transferred to other zones, thereby lowering their risks. Flood risk transfer occurs primarily in low- and medium-risk zones, with high-risk zones being the most important growth targets. The change in risk transfer was most evident in the area surrounding Poyang Lake, while that in the Upper Gan River Basin was lower and less sensitive to the transfer effect. Accounting for flood disaster risk, the ecosystem service values of the Poyang Lake Basin decreased by 8.18 %, with the most significant impacts observed in the surrounding environment and southwest Poyang Lake. After the completion of the flood risk transfer, the ecosystem service value in the Poyang Lake Basin declined by 24.66 %. This study provides a reference point for flood risk management and sustainable regional development that account for risk transfer.

Nature-based solutions potential for flood risk reduction under extreme rainfall events

Climate change will substantially increase extreme rainfall events, especially in the Tropics, enhancing flood risks. Such imminent risks require climate adaptation strategies to endure extreme rainfall and increase drainage systems. Here, we evaluate the potential of nature-based solutions by using an ecosystem service modeling approach, evaluating the impact of extreme rainfall events on flood risks in a large urban area and with a real-world land recovery plan. We evaluate the cost-effectiveness of four different land recovery scenarios and associated co-benefits, based on a gradient increase in area recovered and cost of implementation. Although the scenarios reveal increasing flood risk reduction and co-benefits along with greater proportion of land recovery, the most cost-effective scenario was the one with an intermediate land recovery where 30% of the study area would be reforested. We emphasize the striking benefits of nature-based solutions for flood risk reduction in cities, considering landscape scale and stakeholders' needs.

Modeling benefits and tradeoffs of green infrastructure: Evaluating and extending parsimonious models for neighborhood stormwater planning

Green infrastructure is often proposed to complement conventional urban stormwater management systems that are stressed by extreme storms and expanding impervious surfaces. Established hydrological and hydraulic models inform stormwater engineering but are time- and data-intensive or aspatial, rendering them inadequate for rapid exploration of solutions. Simple spreadsheet models support quick site plan assessments but cannot adequately represent spatial interactions beyond a site. The present study builds on the Landscape Green Infrastructure Design (L-GrID) Model, a process-based spatial model that enables rapid development and exploration of green infrastructure scenarios to mitigate neighborhood flooding. We first explored how well L-GrID could replicate flooding reports in a neighborhood in Chicago, Illinois, USA, to evaluate its potential for green infrastructure planning. Although not meant for prediction, L-GrID was able to replicate the flooding reported and helped identify strategies for flood control. Once evaluated for this neighborhood, we extended the model to include water quality through the representation of dispersion and settling mechanisms for two pollutant surrogates-total nitrogen and total suspended solids. With the extended model, Landscape Green Infrastructure Design Model-Water Quality (L-GrID-WQ), we examined benefits, costs, and tradeoffs for different green infrastructure strategies. Bioswales were slightly more effective than other green infrastructure types in reducing flooding extent and downstream runoff and pollution, through increased infiltration and settling capacity. Permeable pavers followed in effectiveness and are suggested where spatial constraints may limit the installation of bioswales. Although green infrastructure supports both flooding and pollution control, small tradeoffs between these functions emerged across spatial layouts: strategies based on only curb-cuts better controlled pollution, while layouts that followed the path of water flow better controlled flooding. By illuminating such tradeoffs, L-GrID-WQ can support green infrastructure planning that prioritizes unique concerns in different areas of a landscape.

Economic assessment of nature-based solutions to reduce flood risk and enhance co-benefits

Flooding is expected to increase due to climate change, urbanisation, and land use change. To address this issue, Nature-Based Solutions (NBSs) are often adopted as innovative and sustainable flood risk management methods. Besides the flood risk reduction benefits, NBSs offer co-benefits for the environment and society. However, these co-benefits are rarely considered in flood risk management due to the inherent complexities of incorporating them into economic assessments. This research addresses this gap by developing a comprehensive methodology that integrates the monetary analysis of co-benefits with flood risk reduction in economic assessments. In doing so, it aspires to provide a more holistic view of the impact of NBS in flood risk management. The assessment employs a framework based on life-cycle cost-benefit analysis, offering a systematic and transparent assessment of both costs and benefits over time supported by key indicators like net present value and benefit cost ratio. The methodology has been applied to the Tamnava basin in Serbia, where significant flooding occurred in 2014 and 2020. The methodology offers valuable insights for practitioners, researchers, and planners seeking to assess the co-benefits of NBS and integrate them into economic assessments. The results show that when considering flood risk reduction alone, all considered measures have higher costs than the benefits derived from avoiding flood damage. However, when incorporating co-benefits, several NBS have a net positive economic impact, including afforestation/reforestation and retention ponds with cost-benefit ratios of 3.5 and 5.6 respectively. This suggests that incorporating co-benefits into economic assessments can significantly increase the overall economic efficiency and viability of NBS.

Urban waterlogging structure risk assessment and enhancement

Urban waterlogging patches reflect spatial patterns indicative of drainage system limitations and management challenges, and help pinpoint potential waterlogging impacts and spread risks. Therefore, by constructing an urban waterlogging model to simulate the extent and depth of waterlogging, the Number of Patches index (NP) is used to reflect the number of waterlogging patches, the Related Circumscribing Circle index (Circle) is used to evaluate the potential impact range of waterlogging, the Euclidean Nearest-Neighbor Distance index (ENN) is used to assess the potential connectivity of waterlogging, and the Interspersion and Juxtaposition Index (IJI) is used to assess the difficulty of retrofitting vulnerable points. Finally, the improvement of waterlogging structure is achieved by utilizing Vehicle-mounted Drainage Pump (VDPs). The research results demonstrate that as the return period increases, the waterlogging area (TA) and NP index show an upward trend, while the ENN index shows a downward trend. The Circle index initially decreases and then increases, reaching its lowest point at a one-year return period (1yr). The IJI index is related to the growth of TA, and in the two-year return period (2yr) and fifty-year return period (50yr) design scenarios, both TA and IJI indexshow significant growth. After the deployment of VDPs, the maximum area of waterlogging elimination reaches 0.46 km2 at a five-year return period (5yr). The drainage system reaches its drainage limit at 2yr, and the VDPs achieves its drainage limit at 5yr. The NP index does not decrease significantly, but in the case of a 5yr, the high-density area decreases by 1.66 km2. The Circle index values decrease across the board, and in the case of a 5yr, the potential impact range decreases by 1.92 km2, with 134 roads restored for traffic. The change in the ENN index decreased from 23.35 to 0.82, indicating that the spread of waterlogging can be more effectively controlled at lower return periods. The changes in the IJI index are more complex, with negative adjustments between 5 and 20yr, reducing the degree of mixing of different levels of waterlogging in the remaining return periods. Overall, with the increase of rainfall return period, the waterlogging area increases, the number of patches increases, the shape becomes irregular, the distance between patches decreases, and the potential connectivity increases. After the deployment of VDPs, the system integrity is improved, the waterlogging impact range is reduced, the impact on pedestrians and facilities is mitigated, and the risk of pollutant propagation and expansion of waterlogging area is reduced. This study contributes to reducing the potential risk of waterlogging, improving urban drainage effectiveness, and enhancing the resilience and emergency response capability of cities.

Optimized strategies of green and grey infrastructures for integrated control objectives of runoff, waterlogging and WWDP in old storm drainages

Frequent waterlogging occurs in old high-density urban areas where the sewage is inappropriately connected to storm drainages, resulting in serious wet weather discharge pollution (WWDP). To address urban waterlogging and runoff, the optimization of green infrastructures (GIs) and grey infrastructures (GRs) has been proposed to improve rainwater management efficiency. However, most strategies neglect WWDP and fail to achieve integrated control of runoff, waterlogging, and discharge pollution. In the present study, a new optimization method was introduced to identify optimal solutions for renovating outdated storm drainage systems, considering the management of discharge pollution in wet weather. A case study in Shanghai, China was conducted to demonstrate the application of the method. The cost-benefit index (CBI) of optimized GIs (0.06) was lower than that of optimized GRs (2.78) under 22.2 mm rainfall (no runoff and WWDP), but the costs of the former were only half those of the latter. In a 5-year return period storm (no waterlogging), optimized GIs had a significantly higher CBI (2.85 times) compared to optimized GRs, costing only 44 % of the latter. When WWDP reached the control objective (COD≤70 mg/L), the optimized GIs needed to be further optimized with GRs. The CBI of optimized GI-GRs was higher than GRs by 2.50, and the cost was 58% of the latter. In areas with frequent low-intensity rainfall, optimized GIs and GRs should be selected based on local cost or benefit requirements for drainage reconstruction. In high-intensity storm-prone areas, the optimized GI-GR combination should be selected for drainage reconstruction. The proposed method can compensate for the shortcomings of existing optimization methods in controlling WWDP for the reconstruction of old storm drainages.

Green infrastructure optimization considering spatial functional zoning in urban stormwater management

Green infrastructure (GI) is used as an alternative and complement to traditional urban drainage system for mitigating urban stormwater issues mainly caused by climate change and urbanization. The combination of hydrological model and optimization algorithm can automatically find the optimal solution under multiple objectives. Given the multi-functional characteristics of GI, choosing the optimization objectives of GI are critical for multiple stakeholders. This study proposes a GI optimization method considering spatial functional zoning. Based on the basic conditions, the study area is divided into the flood risk control zone (FRCZ) and the total runoff control zone (TRCZ). The integrated model coupling hydrological model and optimization algorithm is applied to obtain the Pareto fronts and corresponding non-dominated solutions. The Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method is used to support the decision-making process. The optimal solution obtained for the FRCZ achieves a flood risk reduction rate of 60.49% with an average life cycle cost per year of 0.20 × 108 Chinese Yuan (CNY); The optimal solution obtained for the TRCZ achieves a total runoff reduction rate of 22.83% with an average life cycle cost per year of 0.17 × 108 CNY. This study provides a reference for stakeholders in GI planning and design.

Holistic valuation of Nature-Based Solutions accounting for human perceptions and nature benefits

When assessing strategies for implementing Nature-Based Solutions (NBS), it is paramount to identify and quantify all benefits for securing better, informed decisionmaking. Nevertheless, there appears to be a lack of primary data for linking the valuation of NBS sites with the preferences and attitudes of people interacting with them and their connection to supporting efforts to reduce biodiversity loss. This is a critical gap, as the socio-cultural context of NBS has been proven to play a big role in NBS valuation, especially for their non-tangible benefits (e.g. physical and psychological well-being, habitat enhancements, etc.). Consequently, through cocreation with the local government, we co-designed a contingent valuation (CV) survey to explore how the valuation of NBS sites may be shaped by their relationship with the users and the specific respondent and site characteristics. We applied this method to a case study of two distinct areas located in Aarhus, Denmark, with notable differences related to their attributes (e.g. size, location, time passed since construction). The esults obtained from 607 households in Aarhus Municipality show that the personal preferences of the respondent are the most relevant driver of value, surpassing both the perceptions linked to the physical features of the NBS and the socio-economic characteristics of the respondents. Specifically, the respondents attributing most importance to nature benefits were the ones assigning a higher value to the NBS and being willing to pay more for an improvement of the nature quality in the area. These findings highlight the relevance of applying a method assessing the interconnections between human perceptions and nature benefits to ensure a holistic valuation and purposeful design of NBS.

Multi-objective optimization methodology for green-gray coupled runoff control infrastructure adapting spatial heterogeneity of natural endowment and urban development

Cost-effective runoff control scheme drafting involves localization, multi-sector coordination, and configuration of multifunctional infrastructures. Numerous independent variables, parameters, weights, and objectives make runoff control optimization quantitatively arduous. This study innovatively proposed a multi-objective optimization methodology for green-gray coupled runoff control infrastructure adapting spatial heterogeneity of natural endowment and urban development. The quantitative methods of multi-objective evaluation, hydrological feature partition, and pressure-adapted multi-objective weight assignment were proposed. Remote sensing inversion of water quality, hydrological model simulation (using SWAT and SWMM software), landscape pattern index calculation, life cycle cost (LCC), life cycle assessment (LCA) on ecological impact, and NSGA-II optimization algorithm were applied. Wuhan, the most water-sensitive city in China, was studied as a case. Runoff control function (RCF), capital investment (CI), and ecological return on investment (EROI) served as optimized objectives. High, medium, and low built-up regions in Wuhan urban development planning district were extracted by topographic factors and landscape patterns, which comprised 28, 34, and 38% of the case area, respectively. Three corresponding hydrological models were then built to illustrate distinct runoff control cost-efficiency in each region. Pressure distributions on runoff control, economic constraints, and ecological resource scarcity were quantitatively evaluated. And four pressure zones were clustered, which occupied 36, 29, 16, and 19% of the case area, respectively. Then the zonal weighted optimization decision-making matrix (with 3 hydrological models and 5 wt) was established by overlaying the pressure zone and built-up zone. In high, medium, and low built-up regions, optimized solutions reduced annual runoff volume by 86, 82%, and 77%The average runoff investments per square meter of impervious underlying surface in high, medium, and low built-up regions were 34.2, 18.7, and 7.9 RMB yuan, respectively. Medium and low built-up regions may only need 55 and 23% of the high built-up region for the unitary impervious underlying surface to balance runoff control and ecological benefits. Runoff control and financial utilization efficiency enhance with hydrological differentiation zones. Thus, the optimization solutions are zonal adaptive, refined, comparable, replicable, and implementable.

Evaluating urban green and blue spaces with space-based multi-sensor datasets for sustainable development

Urban green and blue spaces refer to the natural and semi-natural areas within a city or urban area. These spaces can include parks, gardens, rivers, lakes, and other bodies of water. They play a vital role in the sustainability of cities by providing a range of ecosystem services such as air purification, carbon sequestration, water management, and biodiversity conservation. They also provide recreational and social benefits, such as promoting physical activity, mental well-being, and community cohesion. Urban green and blue spaces can also act as buffers against the negative impacts of urbanization, such as reducing the heat island effect and mitigating the effects of stormwater runoff. Therefore, it is important to maintain and enhance these spaces to ensure a healthy and sustainable urban environment. Assessing urban green and blue spaces with space-based multi-sensor datasets can be a valuable tool for sustainable development. These datasets can provide information on the location, size, and condition of green and blue spaces in urban areas, which can be used to inform decisions about land use, conservation, and urban planning. Space-based sensors, such as satellites, can provide high-resolution data that can be used to map and monitor changes in these spaces over time. Additionally, multi-sensor datasets can be used to gather information on a variety of environmental factors, such as air and water quality, that can impact the health and well-being of urban residents. This information can be used to develop sustainable solutions for preserving and enhancing urban green and blue spaces. This study examines how urban green and blue infrastructures might improve sustainable development. Space-based multi-sensor datasets are used to estimate urban green and blue zones for sustainable development. This work can inform sustainable development research at additional spatial and temporal scales.

A place-based risk appraisal model for exploring residents' attitudes toward nature-based solutions to flood risks

Nature-based solutions (NBS) have gained popularity as a sustainable and effective way of dealing with increasing flood risks. One of the key factors that often hinders the successful implementation of NBS is residents' opposition to their implementation. In this study, we argue that the place where a hazard exists should be considered a critical contextual factor alongside flood risk appraisals and perceptions of NBS themselves. We have developed a theoretical framework-the "Place-based Risk Appraisal Model (PRAM)"-that draws on constructs inspired by theories of place and risk perception. A citizen survey (n = 304) was conducted in five municipalities in Saxony-Anhalt, Germany, where dike relocation and floodplain restoration projects have been conducted along the Elbe River. Structural equation modeling was adopted to test the PRAM. Attitudes toward the projects were assessed in terms of "perceived risk-reduction effectiveness" and "supportive attitude." With regard to risk-related constructs, well-communicated information and perceived co-benefits were consistently positive factors for both perceived risk-reduction effectiveness and supportive attitude. Trust in local flood risk management was a positive and threat appraisal a negative predictor of perceived risk-reduction effectiveness affecting "supportive attitude" only through "perceived risk-reduction effectiveness." Regarding place attachment constructs, place identity was a negative predictor of a supportive attitude. The study emphasizes that risk appraisal, pluralities of place contexts to each individual, and their relations are key for determining attitudes toward NBS. Understanding these influencing factors and their interrelationships enables us to provide theory- and evidence-based recommendations for the effective realization of NBS.

Secondary benefits of urban flood protection

The combined effects of urbanization and climate change put a large portion of the population at risk from pluvial, fluvial, and coastal flooding. To continue to strive for sustainable development, cities will need to protect flood-prone areas, but this will require significant investments in both green and grey infrastructure solutions. Yet, a significant financing gap will need to be bridged to increase cities' resilience. The decision as to which flood protection intervention to finance typically includes an analysis of primary costs (construction) and benefits (averted damages). However, an array of potential secondary benefits occur with increased flood protection that are often not assessed, such as increased well-being and ecosystem health. This review provides a timely overview of the secondary benefits of urban flood protection, a brief analysis of whether they have been included in cost-benefit analyses for investments in urban flood protection projects, and a discussion of methodological concerns. Of the twenty projects reviewed, fourteen make mention of secondary benefits, yet only four quantify them in their analysis. Advances in evaluation methodologies may reduce quantification challenges, but a move away from traditional cost-benefit analysis may be necessary to incorporate a full range of secondary benefits. Ultimately, we argue that a more thorough understanding of the secondary benefits of urban flood protection and their quantification methods could unlock additional financing for flood protection infrastructure, especially in urban centers of developing countries.

Determination of Pollution and Environmental Risk Assessment of Stormwater and the Receiving River, Case Study of the Sudół River Catchment, Poland

Changes in the land use of urban catchments and the discharge of stormwater to rivers are causing surface water pollution. Measurements were taken of the quality of discharged stormwater from two areas with different types of development: a residential area and a residential-commercial area, as well as the quality of the Sudół River water below the sewer outlets. The following indicators were studied: TSS, COD, N-NO₃, N-NO₂, TKN, TN, TP, Zn, Cu, Hg, HOI, and PAHs. The influence of land use on the magnitudes of flows in the river was modeled using the SCS-CN method and the Snyder Unit Hydrograph Model. The results showed an increase in sealing and a resulting increase in surface runoff. Concentrations of pollutants in stormwater and analysis of the potential amounts of loadings contributed by the analyzed stormwater outlets indicate that they may be responsible for the failure to meet environmental targets in the Sudół River. Environmental risk assessment shows that the aquatic ecosystem is at risk. A risk factor indicating a high risk of adverse environmental effects was determined for N-NO₃, Zn, and Cu, among others.

Knowledge gaps and future research needs for assessing the non-market benefits of Nature-Based Solutions and Nature-Based Solution-like strategies

Nature-Based Solutions (NBS) can be defined as solutions based on natural processes that meet societal challenges and simultaneously provide human well-being and biodiversity benefits. These solutions are envisioned to contribute to operationalizing sustainable development strategies, especially in the context of adaptation to climate change (e.g. flood risk reduction). In order to quantify NBS performance, ease their uptake and advocate for them as alternatives to "business-as-usual" infrastructures, a comprehensive, holistic valuation of their multiple benefits (multiple advantages and disadvantages) is needed. This entails quantifying non-market benefits for people and nature in addition to determining the (direct) cost-benefit of the risk-reduction measure. Despite the importance given to the assessment of non-tangible benefits for people and nature in the literature, systematic data collection on these dimensions seems to be missing. This study reviews publications that used stated preference methods to assess non-market human benefits of NBS and NBS-like strategies. Its aim is to highlight any biases or knowledge gaps in this kind of evaluation. Our results show that the valuation of non-tangible benefits of NBS (e.g. increased recreation and well-being, enhanced biodiversity) still suffers from a lack of common framing. Despite some steps being taken on enabling interconnected benefit assessments, unexploited opportunities concerning the integrated assessment of non-market human and nature benefits predominate. Moreover, the research to-date appears based on a case-to-case approach, and thus a shared holistic method does not emerge from the present literature, potentially delaying the uptake of NBS. We argue that future research could minimize missed opportunities by focusing on and systematically applying holistic benefits assessments. Methods based on stated preference surveys may help to ensure holistic approaches are taken, as well as contributing to their replicability and application when upscaling NBS.

Incorporating receiving waters responses into the framework of spatial optimization of LID-BMPs in plain river network region

Deep insights into the receiving waters responses to optimal spatial allocation of LID-BMPs are considered extremely important. This study addressed the urgent need to incorporate receiving waters responses into the spatial allocation optimization of LID-BMPs and demonstrated the efficiency of the approach to guide watershed management. The integration of an overland-river coupling model and the NSGA-III algorithm resulted in the proposal of a general simulation-optimization framework for the optimal layout of LID-BMPs. The coupled model was swapped out for the surrogates to increase computational efficiency. When 40.71%, 36.06%, and 61.80% reductions in runoff volume, flood volume, and TP concentration are achieved, the newly proposed framework can save 34.44% and 16.31% cost compared to the approach that does not consider receiving waters responses and refined spatial allocation, respectively. Results indicate that the incorporation of receiving waters responses and refined spatial allocation are essential for the optimal design of LID-BMPs. This new framework offers the potential for more cost-effective high-cost solutions. The results of spatial optimization are significantly influenced by imperviousness.

Adaptive pressure-driven multi-criteria spatial decision-making for a targeted placement of green and grey runoff control infrastructures

Traditional runoff control measures ignore the spatial imbalance of regional pressures, thereby failing to achieve a site-specific placement for green and grey infrastructure simultaneously. A multi-criterion decision-making framework for runoff control infrastructure spatial planning was therefore developed in this study. The pressure-state-response framework was applied to creatively match the pressure induced adjustment demands with the infrastructure effectiveness. The pressures were quantified from the perspective of environment, economy, and ecology on a grid scale. States were considered as the relative priority of regional pressure adjustment demand in multiple perspectives. Responses were presented as state-targeted green and grey infrastructure placement. Multi-perspective effectiveness of different green and grey infrastructure was simultaneously evaluated at an effective scale of controlling 1 m³/s runoff for comparison. Methods such as data mining, hydrological model simulation, and remote sensing inversion were combined to quantify the regional pressures. The capital investment and ecological impact of infrastructures were quantified from a life cycle perspective. A case study was carried out in Wuhan, China. The study area was clustered by gridded pressure into three regions. In region Ⅰ, ecological and environmental pressure were of higher weight. In region Ⅱ, the environmental pressure was dominant. In region Ⅲ, the ecological pressure took precedence over the environmental and economic constraints. The area ratios of the region Ⅰ, Ⅱ, and Ⅲ were 43%, 36%, and 21% respectively. The result indicated a synergy and spatial heterogeneity of multi-perspective pressures, and further demonstrating that expert experience tends to fail to weigh the multi-function of green and grey infrastructures for coping with the pressures. Results also stated that green infrastructures were more acceptable in areas that aspire to achieve simultaneous runoff control and ecological improvement. The decision-making framework developed in this study can maximize the overall performance by providing targeted infrastructure placement solutions.

How do we best synergize climate mitigation actions to co-benefit biodiversity?

A multitude of actions to protect, sustainably manage and restore natural and modified ecosystems can have co-benefits for both climate mitigation and biodiversity conservation. Reducing greenhouse emissions to limit warming to less than 1.5 or 2°C above preindustrial levels, as outlined in the Paris Agreement, can yield strong co-benefits for land, freshwater and marine biodiversity and reduce amplifying climate feedbacks from ecosystem changes. Not all climate mitigation strategies are equally effective at producing biodiversity co-benefits, some in fact are counterproductive. Moreover, social implications are often overlooked within the climate-biodiversity nexus. Protecting biodiverse and carbon-rich natural environments, ecological restoration of potentially biodiverse and carbon-rich habitats, the deliberate creation of novel habitats, taking into consideration a locally adapted and meaningful (i.e. full consequences considered) mix of these measures, can result in the most robust win-win solutions. These can be further enhanced by avoidance of narrow goals, taking long-term views and minimizing further losses of intact ecosystems. In this review paper, we first discuss various climate mitigation actions that evidence demonstrates can negatively impact biodiversity, resulting in unseen and unintended negative consequences. We then examine climate mitigation actions that co-deliver biodiversity and societal benefits. We give examples of these win-win solutions, categorized as 'protect, restore, manage and create', in different regions of the world that could be expanded, upscaled and used for further innovation.

Developing an Approach for Assessing Urban Blue-Green Spaces Towards Sustainable Urban Growth Through Retrospective Cyber Metrics Analysis of Operational Estimations Approaches

Urban blue-green spaces provide us abundant social, environmental, and economic benefits, but the disparities often exist in their distribution and accessibility. Traditionally urban blue-green spaces are a consolidation of “blue-green infrastructure” within urban areas. Several urban features like parks, forests, gardens, visible water, such as parks, rivers, canals, reservoirs, ponds, lakes, fountains, etc. are categorized or considered under the blue-green spaces and these are very much crucial for various urban ecosystem services. These play a significant role for all stakeholders of the urban community. Thus, everyone must ensure the equitable number of blue-green spaces for all. Recently, several rules and regulations towards the safeguarding of urban blue-green spaces have been outlined. The work presents a methodological framework to develop an approach towards sustainable urban growth with the help of urban blue-green spaces assessments. The current work has attempted to examine the linkage between issues of the urban blue-green spaces for restoring the required infrastructures. It can be utilised for all sustainable urban development for urban planning and design projects to play a pivotal role. The work emphasizes more to develop a methodological framework to analyze the urban blue-green spaces for augmentation with a theoretical framework. It is expected that the advancement of a problem cum objectives-driven approach will help to design an impact-driven approach for planned and concrete action.

A comprehensive review on the long-term performance of stormwater biofiltration systems (SBS): Operational challenges and future directions

Stormwater biofiltration systems (SBS) are a popular technology for mitigating the negative effects of urbanization on the hydrological processes and water quality in urban areas. However, little is known about SBS's long-term performance in actual field conditions. The findings of a review of the scientific literature on the long-term performance of SBS are presented in this paper. The findings show that only a few studies have investigated the performance of SBS and its change over time, and that the results of laboratory and field experiments differed due to the presence of plants, regular maintenance, and some uncertain environmental factors. Based on the existing knowledge gaps in this field, the main challenges observed was the lack of long-term field data series, and the existing mathematical models are not able to accurately forecast the long-term performance of SBS. This could be owing to the difficulties in monitoring activities, the high costs involved and the unpredictability around the operational timeframe. Future study should concentrate on the implementation of simulation and modeling-based research in pilot and full-scale SBS, and the inclusion of new performance indicators should be considered as a priority.

Assessment of Blue and Green Infrastructure Solutions in Shaping Urban Public Spaces—Spatial and Functional, Environmental, and Social Aspects

Blue and Green Infrastructure (BGI) provide one of the key Nature Based Solution (NBS) approaches for sustainable stormwater management in cities, in conjunction with extending the scope of Ecosystem Services (ES). In both the process of planning and designing highly urbanized areas, the implementation of BGI is important for the improvement of living conditions and counteracting the negative effects of climate change. Based on the literature review, 19 BGI solutions were identified and then valorized in relation to the following three key aspects: spatial and functional, environmental, and social. The results of the assessment were derived using the scoring method and allowed for the identification of BGI solutions with a high, medium or low value for shaping sustainable urban public spaces. Using the potential of analyzed BGI solutions to improve the functioning and attractiveness of urban areas requires a comprehensive approach. Conscious planning and designing should use the knowledge presented to make the implementation of BGI solutions as effective as possible in relation to the above-mentioned aspects of shaping urban public spaces.

Development and Assessment of a Web-Based National Spatial Data Infrastructure for Nature-Based Solutions and Their Social, Hydrological, Ecological, and Environmental Co-Benefits

Comprehensive datasets for nature-based solutions (NBS), and their diverse relationships have not yet been accumulated into a deployable format. This research describes the development of a novel National Spatial Data Infrastructure (NSDI) system for NBS co-benefits throughout the contiguous United States. Here, we gather and integrate robust geospatial datasets from the social, ecological, environmental, and hydrologic domains using seamless, cloud-based data services to facilitate the trans-disciplinary assessment of NBSs as a function of society and Earth. This research enhances practical decision making and research by assimilating web-based datasets and describing the missing links between national policy and robust adoption of NBSs as a sustainability solution. This NSDI serves to foster participatory planning capabilities and integrate local sustainability goals into decision–support frameworks. Such a platform strengthens the knowledge base necessary for addressing multiple, co-evolving issues of societal relevance, an essential component of fully espousing NBSs within the realm of socio-technological systems and improving policies and implementation regarding sustainable solutions. The efficacy of the proposed platform to serve as a holistic data information system is assessed by exploring important characteristics associated with geospatial NSDI tools, namely, openness, spatial functionality, scalability, and standardization. By placing GIS strengths and weaknesses in the context of transdisciplinary NBSs, we reveal strategic directions toward further co-production of such NSDIs. We conclude with recommendations for facilitating a shared vision of transdisciplinary technologies to strengthen the amalgamation of broad co-benefits and multi-disciplinary influences in sustainability planning.

The climate benefits, co-benefits, and trade-offs of green infrastructure: A systematic literature review

Climate change increases risks to natural and human systems. Green infrastructure (GI) has been increasingly recognized as a promising nature-based solution for climate change adaptation, mitigation, and other societal objectives for sustainable development. Although the climate contribution of GI has been extensively addressed in the literature, the linkages between the climate benefits and associated co-benefits and trade-offs remain unclear. We systematically reviewed the evidence from 141 papers, focusing on their climate benefits, relevant co-benefits and trade-offs, and the GI types that provide such climate (co-)benefits. This study presents a comprehensive overview of the links between climate benefits, co-benefits and types of GI, categorized along a green-grey continuum so that researchers/practitioners can find information according to their topic of interest. We further provide an analysis of trade-offs between various GI benefits. 'Bundles' of major co-benefits and trade-offs for each climate benefit can be identified with recommendations for strategies to maximize benefits and minimize trade-offs. To promote climate-resilient pathways through GI, it is crucial for decision-makers to identify opportunities to deliver multiple ecosystem services and benefits while recognizing disservices and trade-offs that need to be avoided or managed.

Multi-Objective Model-Based Assessment of Green-Grey Infrastructures for Urban Flood Mitigation

This paper presents the performance quantification of different green-grey infrastructures, including rainfall-runoff and infiltration processes, on the overland flow and its connection with a sewer system. The present study suggests three main components to form the structure of the proposed model-based assessment. The first two components provide the optimal number of green infrastructure (GI) practices allocated in an urban catchment and optimal grey infrastructures, such as pipe and storage tank sizing. The third component evaluates selected combined green-grey infrastructures based on rainfall-runoff and infiltration computation in a 2D model domain. This framework was applied in an urban catchment in Dhaka City (Bangladesh) where different green-grey infrastructures were evaluated in relation to flood damage and investment costs. These practices implemented separately have an impact on the reduction of damage and investment costs. However, their combination has been shown to be the best action to follow. Finally, it was proved that including rainfall-runoff and infiltration processes, along with the representation of GI within a 2D model domain, enhances the analysis of the optimal combination of infrastructures, which in turn allows the drainage system to be assessed holistically.

Climate Justice Planning in Global South: Applying a Coupled Nature–Human Flood Risk Assessment Framework in a Case for Ho Chi Minh City, Vietnam

Developing countries in the global south that contribute less to climate change have suffered greater from its impacts, such as extreme climatic events and disasters compared to developed countries, causing climate justice concerns globally. Ho Chi Minh City has experienced increased intensity and frequency of climate change-induced urban floods, causing socio-economic damage that disturbs their livelihoods while urban populations continue to grow. This study aims to establish a citywide flood risk map to inform risk management in the city and address climate justice locally. This study applied a flood risk assessment framework integrating a coupled nature–human approach and examined the spatial distribution of urban flood hazard and urban flood vulnerability. A flood hazard map was generated using selected morphological and hydro-meteorological indicators. A flood vulnerability map was generated based on a literature review and a social survey weighed by experts’ priorities using the Fuzzy Delphi Method and Analytic Network Process. Vulnerability indicators including demographic characteristics, infrastructure, and land use patterns were used to generate a flood vulnerability map. The results illustrate that almost the entire central and northeastern parts of the city are at high flood risk, whereas the western part is at low flood risk. The findings have implications in urban planning via identifying risk hot spots in order to prioritize resources for mitigating hazards and enhancing community resilience to urban floods.

Optimizing Green-Gray Infrastructure for Non-Point Source Pollution Control under Future Uncertainties

Non-Point Source Pollution (NPS) caused by polluted and untreated stormwater runoff discharging into water bodies has become a serious threat to the ecological environment. Green infrastructure and gray infrastructure are considered to be the main stormwater management measures, and the issue of their cost-effectiveness is a widespread concern for decision makers. Multi-objective optimization is one of the most reliable and commonly used approaches in solving cost-effectiveness issues. However, many studies optimized green and gray infrastructure under an invariant condition, and the additional benefits of green infrastructure were neglected. In this study, a simulation-optimization framework was developed by integrated Stormwater Management Model (SWMM) and Non-dominated Sorting Genetic Algorithm (NSGA-II) to optimize green and gray infrastructure for NPS control under future scenarios, and a realistic area of Sponge City in Nanchang, China, was used as a typical case. Different levels of additional benefits of green infrastructure were estimated in the optimizing process. The results demonstrated that green-gray infrastructure can produce a co-benefit if the green infrastructure have appropriate Value of Additional Benefits (VAB), otherwise, gray infrastructure will be a more cost-effectiveness measure. Moreover, gray infrastructure is more sensitive than green infrastructure and green-gray infrastructure under future scenarios. The findings of the study could help decision makers to develop suitable planning for NPS control based on investment cost and water quality objectives.

Hydrological Modeling of Green Infrastructure to Quantify Its Effect on Flood Mitigation and Water Availability in the High School Watershed in Tucson, AZ

Green Infrastructure (GI) practices are being implemented in numerous cities to tackle stormwater management issues and achieve co-benefits such as mitigating heat island effects and air pollution, as well as water augmentation, health, and economic benefits. Tucson, Arizona is a fast-growing city in the semiarid region of the southwest United States and provides a unique landscape in terms of urban hydrology and stormwater management, where stormwater is routed along the streets to the nearest ephemeral washes. Local organizations have implemented various GI practices, such as curb cuts, traffic chicanes, roof runoff harvesting, and retention basins, to capture the excess runoff and utilize it on-site. This study models the 3.31 km2 High School watershed in central Tucson using the Automated Geospatial Watershed Assessment (AGWA) tool and the Kinematic Runoff and Erosion (KINEROS2) model. Each parcel in the watershed was individually represented using the KINEROS2 Urban element to simulate small-scale flow-on/flow-off processes. Seven different configurations of GI implementation were simulated using design storms, and we stochastically generated 20 years of precipitation data to understand the effects of GI implementation on flood mitigation and long-term water availability, respectively. The design storm analysis indicates that the configuration designed to mimic the current level of GI implementation, which includes 175 on-street basins and 37 roof runoff harvesting cisterns, has minimum (<2%) influence on runoff volume. Furthermore, the analysis showed that the current level of GI implementation caused an increase (<1%) in peak flows at the watershed outlet but predicted reduced on-street accumulated volumes (>25%) and increased water availability via GI capture and infiltration. When the GI implementation was increased by a factor of two and five, a larger reduction of peak flow (<8% and <22%, respectively) and volume (<3% and <8%, respectively) was simulated at the watershed outlet. The 20-year analysis showed that parcels with roof runoff harvesting cisterns were able to meet their landscape irrigation demands throughout the year, except for the dry months of May and June. Additionally, stormwater captured and infiltrated by the on-street basins could support xeric vegetation for most of the year, except June, where the water demand exceeded volume of water infiltrated in the basins. The current level of GI implementation in the High School watershed may not have significant large-scale impacts, but it provides numerous benefits at the parcel, street, and small neighborhood scales.

Assessing the ecological balance between supply and demand of blue-green infrastructure

Given that improving urban ecological environment requires a clear recognition of the urban ecological elements, investigating the ecosystem service capabilities of urban green-blue infrastructures (UGBIs) becomes ever important. This study aims to reveal and compare the synergistic ecosystem service ability of UGBIs with different characteristics and the relationship with human demand in Wuhan city. It was found that the climate regulation service and water regulation service value of lake-type parks both reached the highest over the other UGBIs. Nature-type parks revealed the most capable cultural service, and green-type parks demonstrated the greatest exercise cultural service value. The analysis showed that the ecosystem services delivered by the UGBIs were influenced by the park area, the total value of the normalized difference vegetation index and normalized water body index, and the distance from the city centre. Furthermore, a significant spatial phenomenon was found that the ecological capacity of lake-type parks in the city centre was higher than that of the other UGBIs at the same location. Regarding the relationship with the human activity intensity, the high-demand and high-supply regions were mainly concentrated in highly developed areas in terms of regulating services. Nevertheless, a severe environmental inequality occurred in small urban centres, which requires urgent attention from the government. This work answered the question of where and how to optimize the green-blue infrastructures in Wuhan, and it contributes to the construction of the existing blue-green space.

Adaptation of urban drainage networks to climate change: A review

The present work reviews the main challenges regarding adaptation of urban drainage networks to climate change by comparing 32 case studies from 29 articles, published between 2003 and 2020. The aim is to: (i) identify the state-of-the-art scientific approaches of adaptation of urban drainage networks to climate change; (ii) assess whether or not these approaches incorporated monetization of the adaptation practices and the associated costs/benefits; and (iii) define a novel approach (Blueprint) for the future development and assessment of urban drainage network adaptation to climate change and other drivers. First, the motivation is provided that makes urban drainage adaptation a globally relevant issue. Second, the main impacts of climate change on precipitation, flooding and urban drainage systems are discussed. Then, current practices are described. Finally, a blueprint for an integrated urban adaptation framework to climate change and other drivers is proposed. Our research indicated that future quantity and quality of urban runoff is not widely addressed in the scientific literature. The Storm Water Management Model is the most widely used software in modeling adaptation options. Solutions such as plans of maintenance and rehabilitation, public awareness, flood forecasting and warning, mobility measures and insurance measures are not widely reflected in the literature. Uncertainties of climate projections and bias correction methods are still significant, and uncertainties of socio-economic scenarios, hydrologic and hydrodynamic models, and adaptation options are not fully addressed. Finally, environmental cost and benefits associated with the ecosystem services provided by the adaptation options are not fully addressed.

International approaches to protecting and retaining trees on private urban land

Most studies of urban forest management look at vegetation on public land. Yet, to meet ambitious urban forest targets, cities must attempt to maintain or increase trees and canopy cover on private urban land too. In this study, we review and evaluate international approaches to protecting and retaining trees on private urban land. Our study combines a systematic academic literature review, two empirical social science studies on the views of urban forest professionals, and a global case study review of innovative regulations and incentives aimed at protecting and retaining trees on private urban land. Case studies were evaluated for the extent they exceeded minimum standards or went beyond 'business-as-usual'. We found that the most innovative mechanisms combine many regulations, instead of relying on a single regulation, and use financial incentives to retain or plant trees in newly developed or re-developed sites, as well as private residences. We did not find any cases where appropriate monitoring was in place to determine the efficacy and efficiency of these mechanisms. We also found no single simple solution that could effectively and efficiently protect and retain trees on private land. Only by combining policies, planning schemes, local laws, and financial incentives with community engagement and stewardship will cities protect and retain trees on private land. Useful and innovative ways to protecting and retaining trees on private land involves providing solutions at multiple governments levels, embedding trees in existing strategic policy and management solutions, incentivising positive behavior, creating regulations that require payment up front, and engaging the broader community in private tree stewardship.

Risk Reduction Framework for Blue-Green Roofs

As climate change in the Nordic region brings an increase in extreme precipitation events, blue-green roofs have emerged as a solution for stormwater management, hereafter referred to as “blue-green roofs”. The addition of blue-green layers on a conventional compact roof represents several multi-disciplinary technical challenges and quality risks that must be managed. This paper aims to list and address the key building technical challenges associated with blue-green roofs and to present a framework for managing these risks. Literature and document studies as well as qualitative interviews and expert meetings have been conducted to collect research data on defects in blue-green roofs and causes thereof. A list of nine key challenges has been extracted along with recommendations on how to address them. The recommendations are structured around a framework developed for practical use in building projects. For ease of use, the nine key challenges are presented on a general level, with references to detailed recommendations. The framework is intended to be used to reduce the building technical risks of blue-green roofs, by addressing the most important quality risk elements.

Earth Observation for Sustainable Infrastructure: A Review

Infrastructure is a fundamental sector for sustainable development and Earth observation has great potentials for sustainable infrastructure development (SID). However, implementations of the timely, large–scale and multi–source Earth observation are still limited in satisfying the huge global requirements of SID. This study presents a systematical literature review to identify trends of Earth observation for sustainable infrastructure (EOSI), investigate the relationship between EOSI and Sustainable Development Goals (SDGs), and explore challenges and future directions of EOSI. Results reveal the close associations of infrastructure, urban development, ecosystems, climate, Earth observation and GIS in EOSI, and indicate their relationships. In addition, from the perspective of EOSI–SDGs relationship, the huge potentials of EOSI are demonstrated from the 70% of the infrastructure influenced targets that can be directly or indirectly derived from Earth observation data, but have not been included in current SDG indicators. Finally, typical EOSI cases are presented to indicate challenges and future research directions. This review emphasizes the contributions and potentials of Earth observation to SID and EOSI is a powerful pathway to deliver on SDGs.

Getting the message right on nature-based solutions to climate change

Nature-based solutions (NbS)-solutions to societal challenges that involve working with nature-have recently gained popularity as an integrated approach that can address climate change and biodiversity loss, while supporting sustainable development. Although well-designed NbS can deliver multiple benefits for people and nature, much of the recent limelight has been on tree planting for carbon sequestration. There are serious concerns that this is distracting from the need to rapidly phase out use of fossil fuels and protect existing intact ecosystems. There are also concerns that the expansion of forestry framed as a climate change mitigation solution is coming at the cost of carbon rich and biodiverse native ecosystems and local resource rights. Here, we discuss the promise and pitfalls of the NbS framing and its current political traction, and we present recommendations on how to get the message right. We urge policymakers, practitioners and researchers to consider the synergies and trade-offs associated with NbS and to follow four guiding principles to enable NbS to provide sustainable benefits to society: (1) NbS are not a substitute for the rapid phase out of fossil fuels; (2) NbS involve a wide range of ecosystems on land and in the sea, not just forests; (3) NbS are implemented with the full engagement and consent of Indigenous Peoples and local communities in a way that respects their cultural and ecological rights; and (4) NbS should be explicitly designed to provide measurable benefits for biodiversity. Only by following these guidelines will we design robust and resilient NbS that address the urgent challenges of climate change and biodiversity loss, sustaining nature and people together, now and into the future.

Influence of Blue-Green and Grey Infrastructure Combinations on Natural and Human-Derived Capital in Urban Drainage Planning

The natural capital and ecosystem services concepts describe the multiple benefits people get from nature. Urbanisation has been identified as one of the key factors influencing the decline of natural capital globally. Urbanisation has also been associated with a recent increase in urban flooding incidents in most cities globally. While the understanding of blue-green infrastructure in urban drainage is well established, little is said about its influence on natural capital. This study utilises the Natural Capital Planning Tool, Benefits Evaluation of Sustainable Drainage Systems tool and expert stakeholder interviews to assess the influence of blue-green and grey infrastructure as adaptation pathways in urban drainage, on natural capital and ecosystem services, and to determine how these contribute to other forms of human-derived capital. Key findings show that blue-green options can enhance natural capital and ecosystem services such as amenity value while also contributing to social and human capital. Although the assessed blue-green options contribute to regulating ecosystem services such as floods regulation, their most significant contribution is in cultural ecosystem services, especially amenity value. It is concluded that incorporating blue-green infrastructure in urban drainage adaptive approaches can mitigate natural capital losses and contribute to other forms of capital crucial for human well-being.

Causal Loop Diagrams for supporting Nature Based Solutions participatory design and performance assessment

The contribution of Nature Based Solutions (NBSs) for supporting climate change adaptation and water-related risks reduction is becoming increasingly relevant for policy and decision-makers, compared to 'grey infrastructures', thanks to their capability to jointly deal with a multiplicity of societal and environmental challenges, producing several co-benefits besides limiting the impacts of water-related risks. Nevertheless, their mainstreaming is still limited by several barriers, which are often related to socio-institutional (e.g. limited cooperation and stakeholders' involvement, limited awareness about NBSs impacts) rather than to technical aspects. In this context, innovative tools for NBSs planning, design, implementation and assessment are required, along with effective processes capable of supporting stakeholders' participation. The present research aims to propose a shift in the approach to NBSs design, based on the early stakeholders' involvement in the identification, modelling and performance assessment in terms of benefits and, particularly, co-benefits production. A multi-step methodology was implemented for the purpose, combining both individual and participatory activities. Reference is made to one of the case studies of the NAIAD project, namely the Balta Potelu Pond Area (Lower Danube, Romania). Causal Loop Diagrams (CLDs) were used to describe the system in terms of causal connections and mutual influences, incorporating stakeholders' views and ideas. Inputs from both institutional (e.g. ministries and municipalities) and non-institutional stakeholders (e.g. NGOs and members of the local communities) were integrated. This allowed a comparative assessment of multiple NBSs, based on the analysis of benefits and co-benefits produced, as well as the identification of trade-offs among different stakeholders (e.g. the increase of agricultural production versus biodiversity conservation) and potential side effects. CLDs were then coupled with a Performance Matrix (a basic feature of Multi-Criteria Decision Analysis) and fuzzy logic to help decision-makers identify the most suitable NBSs for the area. The whole process was aimed at facilitating the process of NBSs selection and analysis, while considering the multiple impacts associated with their implementation.

Integrating Structural Resilience in the Design of Urban Drainage Networks in Flat Areas Using a Simplified Multi-Objective Optimization Framework

Structural resilience describes urban drainage systems’ (UDSs) ability to minimize the frequency and magnitude of failure due to common structural issues such as pipe clogging and cracking or pump failure. Structural resilience is often neglected in the design of UDSs. The current literature supports structural decentralization as a way to introduce structural resilience into UDSs. Although there are promising methods in the literature for generating and optimizing decentralized separate stormwater collection systems, incorporating hydraulic simulations in unsteady flow, these approaches sometimes require high computational effort, especially for flat areas. This may hamper their integration into ordinary commercially designed UDS software due to their predominantly scientific purposes. As a response, this paper introduces simplified cost and structural resilience indices that can be used as heuristic parameters for optimizing the UDS layout. These indices only use graph connectivity information, which is computationally much less expensive than hydraulic simulation. The use of simplified objective functions significantly simplifies the feasible search space and reduces blind searches by optimization. To demonstrate the application and advantages of the proposed model, a real case study in the southwest city of Ahvaz, Iran was explored. The proposed framework was proven to be promising for reducing the computational effort and for delivering realistic cost-wise and resilient UDSs.

Co-benefits and synergies between urban climate change mitigation and adaptation measures: A literature review

Accounting for over 70% of global CO2 emissions, cities are major contributors to climate change. Acknowledging this, urban climate change adaptation and mitigation plans are increasingly developed to make progress toward enhancing climate resilience. While there is consensus that focusing on both adaptation and mitigation is necessary for addressing climate change impacts, better understanding of their interactions is needed to efficiently maximize their potentials. This paper, first, provides a bibliographic analysis to map existing knowledge regarding adaptation-mitigation interactions. This is done using methods such as bibliographic coupling, co-citation analysis, and co-occurrence analysis. Then, drawing on the literature, this study explores two types of interactions between adaptation and mitigation measures, namely co-benefits and synergies. These interactions are explored through analyzing evidence reported in the literature on different measures related to sectors such as energy, transportation, waste, water, green infrastructure, urban planning, and governance. Results of the bibliographic analysis show that there is a lack of research in the Global South. Results of the detailed content analysis show that many measures can provide co-benefits and synergies. Measures related to green infrastructure, buildings, energy systems, and, transportation are particularly capable of providing co-benefits. In addition, it was found that appropriate levels of density, promotion of public transportation, and urban greenery are measures that are more likely to provide synergistic benefits if combined with other adaptation and/or mitigation measures. This study highlights the need for more empirical research to better understand the magnitude of synergistic benefits between different measures.

Hydrological Behaviour of Extensive Green Roofs with Native Plants in the Humid Subtropical Climate Context

Different mitigation measures with vegetation have been proposed to sustainably manage rainwater, among which green roofs have demonstrated to be a valid solution in urbanized areas. Green roofs have gained interest also in Italy, but their spreading is generally based on application of ready-to-use packages, poorly tested in the specific climate conditions. A study was carried out to evaluate the green roof solution most suitable in the humid, subtropical climate context of Veneto Plain (north-eastern Italy) to reduce outflow volumes from building roofs into the urban drainage systems. Twelve different microcosm combinations of extensive green roof (three plant mixtures × two substrates × two storage/drainage layers) were tested and compared with gravel (considered as a conventional flat roof with gravel ballast). The tested drainage/storage layers were a preformed layer in recycled HDPE (PL) and an expanded perlite mineral layer (ML), and the growth medium layers were recycled brick substrate (RS) and volcanic substrate (VS). Three different mixtures of native plant species were transplanted: Sedum (SE), herbaceous perennial (HE), and suffruticose (SF). Results showed that all the green roof systems have a good ability to manage rainwater, with a retention ranging on average from 46.2% (SE-RS-PL microcosms) to 62.9% (SF-RS-ML microcosms) of the precipitation in the two-year period (September 2014–August 2016), against 15.4%, retained by gravel. Over the two-year period, the retained rainfall volumes were about 100% for all the light rainy events (<10 mm) and varied within a range of 48–95% for medium rainy events (≥10 and <25 mm) and 20–88% for heavy rainy events (≥25 mm), depending on rainfall depth and the antecedent weather period. The layer that gave the highest relative contribution to the stormwater retention was the vegetation layer, followed by the drainage/storage layers and then the substrate layer. In particular, SF plants decreased the outflows by 15.2% on average compared to SE, and ML layer retained more than 10% of precipitation compared to PL layer. At last, the analysis of variance showed that, within each layer, the more effective in water retention, able to generate less outflow volumes, was similarly suffruticose and herbaceous mixtures, the crushed bricks substrate, and the mineral drainage/storage layer.

Developing a Reliability Index of Low Impact Development for Urban Areas

A defining characteristic of the urbanization is the transformation of existing pervious areas into impervious areas during development. This leads to numerous hydrologic and environmental problems such as an increase in surface runoff due to excess rainfall, flooding, the deterioration of water quality, and an increase in nonpoint source pollution. Several studies propose supplementary measures on environmental change problems in development areas using the low impact development technique. This study investigated the reduction of nonpoint source pollutant loads and flooding in catchments through urban catchment rainfall–runoff management. For the quantitative assessment of flood disasters and water pollution problems, we propose a reliability evaluation technique. This technique refers to a series of analysis methods that determine the disaster prevention performance of the existing systems. As the two factors involve physical quantities of different dimensions, a reliability evaluation technique was developed using the distance measure method. Using the storm water management model, multiple scenarios based on synthetic rainfall in the catchment of the Daerim 2 rainwater pumping station in Seoul, South Korea, were examined. Our results indicate the need for efficient management of natural disaster risks that may occur in urban catchments. Moreover, this study can be used as a primary reference for setting a significant reduction target and facilitating accurate decision making concerning urban drainage system management.

Using a system thinking approach to assess the contribution of nature based solutions to sustainable development goals

Climate change and the overexploitation of natural resources increase the need to integrate sustainable development policies at both national and international levels to fit the demands of a growing population. In 2015 the United Nations (UN) established the 2030 Agenda for sustainable development with the aim of eradicating extreme poverty, reducing inequality and protecting the planet. The Agenda 2030 highlights the importance of biodiversity and the functioning of ecosystems to maintain economic activities and the well-being of local communities. Nature Based Solutions (NBS) support biodiversity conservation and the functioning of ecosystems. NBS are increasingly seen as innovative solutions to manage water-related risks while transforming natural capital into a source of green growth and sustainable development. In this context, NBS could potentially contribute to the achievement of several Sustainable Development Goals (SDGs) by promoting the delivery of bundles of ecosystem services together generating various social, economic and environmental co-benefits. However, to achieve the full potential of NBS, it is necessary to recognize the trade-offs and synergies of the co-benefits associated with their implementation. To this aim, we have adopted a system perspective and a multi-sectoral approach to analyse the potential of NBS to deliver co-benefits while at the same time reducing the negative effects of water-related hazards. Using the case study of Copenhagen, we have analysed the relationships between the co-benefits associated with the scenario of the restoration of the Ladegaardsaa urban river. Our hypothesis is that enhancing the understanding of the social, economic and environmental factors of the system, including mutual influences and trade-offs, could improve the decision-making process and thereby enhance the capability of NBS to contribute to the achievement of the SDGs.

Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds

Floods cause considerable damages worldwide and mitigation of their adverse effects through effective protection measures is needed. Along with the commonly applied “grey” infrastructure, “green” measures that can offer additional benefits, such as ecosystem services, are increasingly being considered lately. While the recent research tendencies are focused on the effectiveness and the value of green measures in urban areas, this paper presents a comprehensive financial evaluation of green and grey flood mitigation scenarios for a smaller rural watershed. A micro-scale damage model that builds on the hydrodynamic modeling of hazard, detailed asset identification, and damage assessment is presented and applied for evaluation of benefits from various flood mitigation measures in the Tamnava watershed in Serbia. Four scenarios are considered: (1) existing flood protection system; (2) green scenario involving new detention basins; (3) grey infrastructure enhancement by rising of the existing levees and diverting flood discharges; and (4) green-grey scenario that combines scenarios (2) and (3). The benefits (loss reduction) are the greatest with the green scenario and marginally higher with the combined green-grey scenario. The results suggest that for small rural watersheds, a holistic, integrative approach that includes both types of infrastructure can provide the most effective flood risk mitigation.

Urban stormwater characterization, control, and treatment

This review summarizes over 280 studies published in 2019 related to the characterization, control, and management of urban stormwater runoff. A summary of quantity and quality concerns is provided in the first section of the review, serving as the foundation for the following sections which focus on the control and treatment of stormwater runoff. Finally, the impact of stormwater control devices at the watershed scale is discussed. Each section provides a self-contained overview of the 2019 literature, common themes, and future work. Several themes emerged from the 2019 literature including exploration of substrate amendments for improved water quality effluent from stormwater controls, the continued study of the role of vegetation in green infrastructure practices, and a call to action for the development of new models which generate reliable, computationally efficient results under the physical, chemical, biological, and social complexity of stormwater management. PRACTITIONER POINTS: Over 280 studies were published in 2019 related to the characterization, control, and treatment of urban stormwater. Studies on bioretention and general stormwater characteristics represented the two most common subtopics in 2019. Trends in 2019 included novel substrate amendments, studies on the role of vegetation, and advancements in computational models.

Assessment of Community Vulnerability to Different Types of Urban Floods: A Case for Lishui City, China

Urban flooding is a severe and pervasive hazard caused by climate change, urbanization, and limitations of municipal drainage systems. Cities face risks from different types of floods, depending on various geographical, environmental, and hydrometeorological conditions. In response to the growing threat of urban flooding, a better understanding of urban flood vulnerability is needed. In this study, a comprehensive method was developed to evaluate the vulnerability of different types of urban floods. First, a coupled urban flood model was built to obtain the extent of influence of various flood scenarios caused by rainfall and river levee overtopping. Second, an assessment framework for urban flood vulnerability based on an indicator method was used to evaluate the vulnerability in different flood hazard scenarios. Finally, the method was applied to Lishui City, China, and the distribution and pattern of urban flood vulnerability were studied. The results highlight the spatial variability of flooding and the vulnerability distributions of different types of urban floods. Compound floods were identified to cause more severe effects in the urban areas.

A robust approach for comparing conventional and sustainable flood mitigation measures in urban basins

An integrated methodological framework for assessing different flood mitigation measures in urban catchments is presented. The framework comprises hydrologic, hydraulic and economic indices aiming at quantifying the effect of different alternatives regarding flood hazard mitigation. The alternatives evaluated include both conventional drainage solutions and low impact development measures. The conventional drainage solutions were: (i) off-line detention tanks; and (ii) sewer enlargement. The low impact development measures included: (i) green roofs (GR); and (ii) permeable surfaces (PS). Each solution was modeled using SWMM5 with respect to flood reduction effectiveness, and the results were compared to those of the existing condition (i.e., no flood mitigation measures). All the examined solutions were also compared based on their construction and operation and maintenance costs for a typical lifespan (i.e., 30 years). The results of the simulation revealed that both low impact development measures and conventional drainage solutions were highly effective even for storm events with low probability of occurrence. However, sewer enlargement was found to be the best alternative from an economic perspective. Nevertheless, peak at the sewer exit increased and time to peak remained unchanged; as a result, local flooding problems are resolved but downstream flooding problems may be introduced. If other criteria are considered, i.e., traffic obstruction, noise, construction easiness, co-benefits and downstream impacts, low impact development measures become more attractive compared to conventional drainage solutions.

Urban green infrastructure: A review on valuation toolkits from an urban planning perspective

As a response to increasing urbanization and changing weather and climatic patterns, urban green infrastructure (UGI) emerged as a concept to increase resilience within the urban boundaries. Given that implementing these (semi-) natural solutions in practice requires a clear overview of the costs and benefits, valuation becomes ever important. A range of decision-support tools for green infrastructure and ecosystem services exist, developed for various purposes. This paper reviews the potential of 10 shortlisted and existing valuation tools to support investment decisions of urban green infrastructure. In the assessment, the functionality is regarded specifically from the urban planning and decision-making viewpoint. The toolkits were evaluated on 12 different criteria. After analyzing the toolkits on these criteria, the findings are evaluated on the (mis)match with specific requirements in the urban planning and management context. Secondly, recommendations and guidelines are formulated to support the design of simple valuation tools, tailored to support the development of green infrastructure in urban areas. Approaching the valuation toolkits biophysically and (socio-)economically provides an integral overview of the challenges and opportunities of the capacities of each framework. It was found that most tools are not designed for the peculiarities of the urban context. Several elements contribute to the hampering uptake of GI valuation tools. Firstly, the limited effort in the economic case for green infrastructure remains a burden to use toolkits to compare grey and green alternatives. Secondly, tools are currently seldom designed for the peculiarities of cities: urban ecosystem (dis)services, multi-scalability, life-span assessments of co-benefits and the importance of social benefits. Thirdly, toolkits should be the result of co-development between the scientific community and local authorities in order to create toolkits that are tailor made to the specific needs in the urban planning process. It can be concluded that current tools, are not readily applicable to support decision making as such. However, if applied cautiously, they can have an indicative role to pinpoint further targeted and in-depth analyses.

Performance assessment of coupled green-grey-blue systems for Sponge City construction

In recent years, Sponge City has gained significant interests as a way of urban water management. The kernel of Sponge City is to develop a coupled green-grey-blue system which consists of green infrastructure at the source, grey infrastructure (i.e. drainage system) at the midway and receiving water bodies as the blue part at the terminal. However, the current approaches for assessing the performance of Sponge City construction are confined to green-grey systems and do not adequately reflect the effectiveness in runoff reduction and the impacts on receiving water bodies. This paper proposes an integrated assessment framework of coupled green-grey-blue systems on compliance of water quantity and quality control targets in Sponge City construction. Rainfall runoff and river system models are coupled to provide quantitative simulation evaluations of a number of indicators of land-based and river quality. A multi-criteria decision-making method, i.e., Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is adopted to rank design alternatives and identify the optimal alternative for Sponge City construction. The effectiveness of this framework is demonstrated in a typical plain river network area of Suzhou, China. The results demonstrate that the performance of Sponge City strategies increases with large scale deployment under smaller rainfall events. In addition, though surface runoff has a dilution effect on the river water quality, the control of surface pollutants can play a significant role in the river water quality improvement. This framework can be applied to Sponge City projects to achieve the enhancement of urban water management.