Flooding in semi-unformal urban areas in North Africa: Environmental and psychosocial drivers

Urban flooding is recognized as a nature-driven disaster shaped by inherent factors such as climate, morphology, and hydrology, affecting vulnerability and flood exposure. While these factors play a paramount role, significant psychosocial intricate drivers are acknowledged, though they are challenging for prediction and assessment. This study delves into these drivers in a specific context, aiming to draw conclusions that extend beyond. It undertakes a comprehensive approach, integrating cloud-based Radar flood detection, analysis of flood causation patterns, and geostatistical analysis of a social survey based on cross-synthesis, contingency analysis, and structural equation modeling. In particular, we characterize the case of the coastal city of Tetouan in Morocco, which is representative in its environmental and socioeconomic settings to most cities in North Africa. It unraveled the nuanced interplay of psychosocial, economic, and territorial dynamics influencing flood exposure. The findings reveal how watershed location molds unique environmental exposures, steering nuanced, emotional, and behavioral responses among residents. Gender and education differentials reveal diverse perceptions and awareness of flood risks. Psychosocial intricacies come to the forefront, portraying education, income, and awareness as crucial mediators influencing cognitive and affective responses. Elevated education, increased income, and heightened awareness correlate with heightened perception and coping strategies. Findings reveal that risk perception significantly and differently influences risk acceptance, coping, and aversion through an array of identified key factors influencing coping strategies, mediating elements in flood damage relationships, and underscoring the pivotal role of perception in shaping responses to risk. Moreover, it found that lower risk acceptance leads to higher coping and aversion, and the latter positively affects coping, indicating that acceptance reduces the motivation to avoid the risk and decreases the willingness to adopt coping strategies to reduce the exposure. The outcomes carry critical implications for comprehending individual and collective social behaviors, informing strategies, and mitigating flood risk that apply at a wider context. It accentuates the inadequacy of relying solely on structural engineering for risk management, citing spatial constraints, misinformation, and lapses in prior-risk memory as compounding exposure challenges. This recognition catalyzes action, advocating tailored awareness campaigns, educational initiatives, and capacity-building programs, spotlighting the need for heightened individual profiles to enhance social understanding, engagement, and resilience. We anticipate profound insights, fostering a richer comprehension of urban flooding complexities and informing adaptive strategies on a broader scale.

WITHDRAWN: A preliminary assessment on spatial prioritization for integrated urban flood management based on source tracking method

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Possibility of using the STORAGE rainfall generator model in the flood analyses in urban areas

In this investigation, we evaluated the applicability of the Stochastic Rainfall Generator (STORAGE) as a data source for deriving design hydrographs in urban catchments. This assessment involved a comparison with design rainfall calculated using Intensity-Duration-Frequency (IDF) curves derived from observed time-series data. The resulting design rainfall values from both methods were incorporated into a hydrodynamic model of the storm sewer network. To simulate peak discharge and flood areas, the Storm Water Management Model (SWMM) program was employed in conjunction with SCALGO. Our findings indicate that design rainfall values obtained from the STORAGE model exceeded those derived from the observed time-series, with a more pronounced difference for shorter rainfall durations. Simulations further revealed that peak runoff disparities between the two approaches were most evident at a 0.10 probability of exceedance compared to a 0.01 probability. Hydrodynamic simulations demonstrated that the flooding volume induced by design rainfall based on the STORAGE model surpassed that resulting from observed rainfall. Across all events, both the flooding volume and area from STORAGE were consistently greater than those derived from IDF curves. The integration of the SWMM model with the SCALGO application introduced a novel functionality for dynamic visualization of flooding, offering valuable insights for effective flood management in urban areas.

Mapping flood vulnerability using an analytical hierarchy process (AHP) in the Metropolis of Mumbai

The burgeoning significance of urban floods in the context of evolving climate dynamics and shifting rainfall patterns underscores the exigency for comprehensive investigation and mitigation strategies. The study employs a multi-criteria assessment (MCE) approach and the analytical hierarchy process (AHP) to evaluate flood-vulnerable zones, wards, and sub-category-wise flood locations in Greater Mumbai. The AHP technique is used to evaluate flood-vulnerable impacting parameters such as rainfall (29.42%), slope (20.96%), land use/land cover (17.52%), vicinity to sewers and storm-water drainage (13.99%), vicinity to natural drainage (8.97%), vegetation (5.58%), and soil (3.56%). The study area is classified under different vulnerable categories as severe vulnerable (46.72%), high to very high (18.74%), and slight to moderate (34.54%). Researchers analysed 234 waterlogged locations, revealing that 85.46% (200 locations) were in the severe to very high vulnerability category, and only 14.52% (34 locations) were in the other three categories. Flood locations are more affected by slope (under the categories of < 5 m and 5.01-10 m), built-up land, sewers and storm water drainage (< 125 m), natural drainage (< 250 m), rainfall (< 2000 to 2200 mm), lowest dense vegetation, and coastal alluvium in soils. These model-based flood vulnerability maps are crucial for planning flood conservation and mitigation measures.

From flood paths to floodways, an efficient method to map, identify and evaluate suitable floodways: A case study from Trondheim, Norway

Due to climate change and rapid urbanisation, many Norwegian cities and urban areas suffer from pluvial flooding caused by intense rainfall exceeding the capacity of the stormwater management system. This results in increased runoff rates, volumes and peak flows in the drainage network. In response to these challenges, the authors explore the potential of utilising the urban surface's ability to transport floodwater as an integral component of the stormwater infrastructure. When the capacity of the stormwater drainage system is exceeded, the overland flow paths transporting floodwater are considered a part of the stormwater management system, as floodways. The study proposes a spatial GIS method to map existing drainage lines and identify existing surface areas that function as floodways, combined with an automated process to identify which drainage lines could be implemented as stormwater management measures. Critical points are introduced to assess the floodways' potential hazards, combined with a classification method to evaluate and sort floodways. A case study from Trondheim, Norway, was used to demonstrate how drainage lines can be identified as floodways using the proposed method. The case study is also used to illustrate how a GIS-based analysis can be extended from identifying to evaluating floodways and whether GIS is sufficient for floodway evaluation. The method enables urban planners and municipalities to identify which areas of the urban surface already function as floodways during extreme events, and to prioritise measures to secure such areas and increase the city's flood resilience. The results highlight the need to assess existing areas that function as floodways, and to implement and design needed areas as floodways. GIS-based methods combined with an evaluation scheme can be an adequate tool to map and evaluate floodways in urban areas. When using GIS-based methods, however, the corresponding hazard potential, and also the uncertainty of the floodway's spatial placement, should be considered.

Monsoon flood risks in urban areas of Pakistan: A way forward for risk reduction and adaptation planning

Climate change poses a significant threat to sustainable urban development. Heavy rainfall has led to severe urban flooding, disrupting human life and causing widespread damage. This study aims to examine the impacts, preparedness, and adaptation strategies related to monsoon flooding in Lahore, Pakistan's second-most populous metropolitan area. Using Yamane's sampling method, a total of 370 samples were surveyed and analyzed using descriptive analysis and chi-square tests. The results indicate that houses and parks were the most commonly damaged properties, with common impacts including roof collapse, house fires, seepage, and wall dampness. These impacts not only caused physical damage but also disrupted basic amenities and damaged roads, resulting in significant socioeconomic costs. Despite these challenges, residents adopted a variety of adaptation strategies such as the use of temporary tarps, moving household appliances to upper floors, and shifting to tiled floors and wall paneling to mitigate damage. However, the study highlights the need for further measures to reduce flood risks and promote adaptation planning in order to effectively address the ongoing challenges posed by climate change and urban flooding.

A new framework for distributed storage tanks placement based on a resilience characteristic metric and reduced modelling

In recent years, urban flooding has been a frequent occurrence, and seriously threatens the safety of lives and properties. Rational placement of distributed storage tanks is one of the effective ways to solve urban flooding, addressing stormwater management and rainwater reuse. However, existing optimization methods (such as genetic algorithm (GA) and other evolutionary algorithms) for determining the placement of storage tanks typically have a high computational burden; as such, they can be very time-consuming, and are not conducive to energy saving, carbon reduction and work efficiency improvements. In this study, a new approach and framework based on a resilience characteristic metric (RCM) and reduced modelling requirements are proposed. In this framework, the resilience characteristic metric, which is based on the linear superposition principle of system resilience metadata, is introduced, and a small number of simulations based on a coupling of MATLAB with SWMM are used to obtain the final placement scheme of storage tanks. The framework is demonstrated and verified with two cases in Beijing and Chizhou, China, and compared with a GA. The GA requires 2000 simulations for two cases (considering the placement of 2 and 6 tanks respectively), while the proposed method needs 44 simulations for the Beijing case and 89 simulations for the Chizhou case. The results show that the proposed approach is feasible and effective, and cannot only obtain a relative better placement scheme, but also considerably reduce computational time and energy consumption. It significantly improves the efficiency of determining the placement scheme of storage tanks. This method provides a new approach for the determining better storage tank placement schemes, and is useful for informing device placement in sustainable drainage systems.

GIS-based fuzzy comprehensive evaluation of urban flooding risk with socioeconomic index system development

Due to the climate change-induced extreme rainfall, urban flooding risk is one of the major concerning risks in the near future with accelerating occurrence frequency and intensity. To systematically evaluate the socioeconomic impacts induced by urban flooding, this paper proposed a GIS-based spatial fuzzy comprehensive evaluation (FCE) framework for local government to efficiently take contingency measures especially under urgent rescue conditions. The risk-assessing procedure could be investigated in 4 aspects: 1) application of the hydrodynamic model to simulate the depth and extent of inundation; 2) quantification of the impact of flooding with 6 methodically picked evaluation indexes concerning the transportation attenuation, residential security, and tangible and intangible monetary losses according to depth-damage functions; 3) implementing FCE method: comprehensive evaluation of urban flooding risk with the diverse socioeconomic indexes by fuzzy theory; and 4) presenting the risk maps of single and multiple impact factors intuitively in ArcGIS platform. The detailed case study in SA city validates the effectiveness of the adopted multiple index evaluation framework, which could help detect higher risk areas with low transport efficiency, high economic loss, high social impact, and high intangible damage. The results of single-factor analysis can also provide feasible suggestions for decision-makers and other stakeholders. Theoretically, the proposed method tends to improve the evaluation accuracy as the inundation distribution can be simulated by hydrodynamic model rather than subjective prediction with hazard factors, while the impact quantification with flood-loss models can also directly reflect the vulnerability of involved factors instead of empirical weight analysis of traditional methods. Besides, the results illustrate that the areas with higher risk levels reasonably coincide with severe inundation situations and dense hazard-bearing bodies. This systematic evaluation framework can support applicable references for further extension to other similar cities.

Evaluation of climate change, urbanization, and low-impact development practices on urban flooding

The Personal Computer Storm Water Management Model was used in this study to evaluate the potential impacts of climate change, urbanization, and low-impact developments (LIDs) on urban flooding in Robe town, Ethiopia. To achieve the objective, four scenarios were developed in order to simulate changes in peak runoff, inundated volume, and the performance of existing drainage systems. The findings revealed that as urbanization increased from 10% to 70%, the inundated volume of nodes and peak runoff increased from 35,418 to 52,118 × 10³ m³ and 89.4-111.96 m³/s, respectively. Furthermore, the peak runoff in response to climate change is increased by 46.9%, 34.8%, and 37.5%, respectively, as a result of the Rossby Centre Regional Climate Model version 4 (RCA4), Regional Atmospheric Climate Model (RACMO22T), and the hydrostatic version of the regional model (REMO2009). Overall, the findings showed that existing drainage systems were unable to collect and convey the amplified inundation from different simulated scenarios, and the Welmel sub-city to roundabout was threatened by increased flooding, causing significant damage to properties and infrastructure. The implemented LIDs are capable of reducing the expected peak runoff, flooding magnitude, and flooded junctions in climate change and urbanization scenarios; however, combining both mitigation measures can further reduce the study area. The implementation of a mitigation strategy with adequate drainage systems will be required to mitigate the flooding risks in Robe town.

Modelling urban flooding integrated with flow and sediment transport in drainage networks

Drainage networks play an essential role in mitigating urban flooding, which, nevertheless, are prone to suffer sediment deposits. To date, however, the effects of sediments in drainage networks on urban flooding remain poorly understood. Here an integrated model is proposed for urban flooding. It is composed of a hydrological module for surface runoff integrated with a one-dimensional hydro-sediment-morphodynamic module for coupled open-channel or pressurized flow and sediment transport in drainage networks. The governing equations are solved synchronously using a well-balanced finite volume method. The model is tested against two laboratory cases involving mixed flow and sediment transport in pipes, and the results agree well with observed data. A new residential area with virtually pervious surface and an established urban area with essentially impervious surfaces are studied using the present model to unravel how sediments in drainage networks affect urban flooding under different extreme rainfall and sediment scenarios. The results reveal that sediments alter the discharge hydrographs in the drainage networks to distinct extents under different storm return periods. As far as the present computational cases are concerned, when a third of the pipe diameter is occupied by sediment deposits, the peak pipeline flow discharge decreases by up to 25 %. Accordingly, the surface inundation depth increases by up to 18 %, and the inundation area expands by up to 12 %, characterizing a considerably higher flooding risk. The present findings provide insight into the influences of sediment transport in drainage networks on urban flooding.

Natural hazard insurance outcomes at national, regional and local scales: A comparison between Sweden and Portugal

This study addresses the role of natural hazard insurance in two European countries with different insurance markets and socioeconomic conditions: Sweden and Portugal. The analyses were conducted at the national, regional (Southern Sweden and Lisbon Metropolitan Area - LMA), and local (Malmö and Lisbon cities) scales. Most damage caused by weather and climate-related (WCR) hazards during the 1980-2019 period was not covered by insurance companies in Sweden (71%) and Portugal (91%). An insurance affordability analysis was performed using income for the national and regional scales. Unaffordability is higher in Southern Sweden than in LMA, implying that better socioeconomic conditions do not necessarily mean a higher average capacity to pay for insurance. At the local scale, urban flooding was analysed for Malmö (1996-2019) and Lisbon (2000-2011) using insurance databases, in which the most relevant 21st century rainfall events for each city are included (2014 and 2008, respectively). The influence of terrain features on flooding claims and payouts was determined using Geographic Information Systems (GIS) spatial analyses. The flat Malmö favours ponding and extensive flooding, while the distance to the drainage network and flow accumulation are key factors to promote flooding along valley bottoms in the hilly Lisbon. Flooding hotspots tend to result from a combination of higher depths/lower velocities (accumulation of floodwaters and ponding) and not from a pattern of lower depths/higher velocities (shallow overland flow). More detailed data on insurance, flooding, and socioeconomic conditions, at regional and mainly local scales, is needed to improve affordability and urban flooding risk assessments.

Integrated modeling of 2D urban surface and 1D sewer hydrodynamic processes and flood risk assessment of people and vehicles

In order to accurately simulate the whole urban flooding processes and assess the flood risks to people and vehicles in floodwaters, a 2D-surface and a 1D-sewer integrated hydrodynamic model was proposed in this study, with the module of flood risk assessment of people and vehicles being included. The proposed model was firstly validated by a dual-drainage laboratory experiment on the flood inundation process over a typical urban street, and the relative importance of model parameters and model uncertainties were evaluated using the GSA-GLUE method. Then the model was applied to simulate an actual urban flooding process that occurred in Glasgow, UK, with the influence of the sewer drainage system on flood inundation processes and hazard degree distributions of people and vehicles being comprehensively discussed. The following conclusions are drawn from this study: (i) The proposed model has a high degree of accuracy with the NSE values of key hydraulic variables greater than 0.8 and the GSA indicates that Manning roughness coefficients for surface and sewer flows, inlet weir and orifice discharge coefficients, are the most relevant parameters to influence the simulated results; (ii) vehicles are vulnerable to larger water depths while human stability is significantly influenced by higher flow velocities, with the overall flood risk of people being less than that of vehicles; and (iii) about 88.7% of the total inflow volume was drained to the sewer network, and the sewer drainage system greatly reduced the flood risks to people and vehicles except the local areas with large inundation water depths, where the sewer drainage increased the local flow velocity leading to higher flood risks especially for people.

A novel multi-objective optimization framework for urban green-gray infrastructure implementation under impacts of climate change

Frequent urban flooding disasters can cause severe economic and property losses. Accordingly, the construction of sponge city has become critical to alleviating urban flooding. However, the functional and structural integration of Green Infrastructure (GI) and Gray Drainage Facility (GDF) is still a matter of concern. This study proposed a novel implementation framework for GI and GDF synchronization optimization (G-GSOIF) based on the SWMM and SUSTAIN models, and used data from Beilin District in Xi'an, China to verify the effects. The results show that the spatiotemporal integrated optimization design of GI and GDF proves to be effective in stormwater management. The total investment was reduced by 16.7% and economic benefit was increased by 15.4% based on disaster risk control, and the utilization rate of rainwater resources exceeded 40%. The Staged optimization model (SSOM) based on the SUSTAIN model established in the G-GSOIF was demonstrated to effectively cope with the impact of future climate change by adjusting and optimizing the design scheme dynamically in different simulation scenarios. Integrated LID (I-LID) measures are conducive for simulation of large catchment areas, and have the same implementation effect as distributed LID measures. The results of this study could support decision-making for urban stormwater management and sponge city construction.

Cellular automata based framework for evaluating mitigation strategies of sponge city

Urban surface water flooding is increasing because of climate change and urbanization, and brings great challenges to urban sustainable development. It is, therefore, most important to develop urban flood management approaches to alleviate the consequences of floods. China is implementing a "sponge city" initiative to tackle urban surface water flooding and improve urban water management. There is, however, limited cost-effectiveness evaluation to support the choice of economically efficient mitigation strategies. To address this gap, this study developed an evaluation framework based on cellular automata and cost-benefit analysis to assess the performance of mitigation strategies in sponge city construction. This approach is demonstrated with a case study of Siergou (Dalian, China), which has a total area of 10.1 km². The mitigation measures of green roofs, permeable pavements and bio-retention were used to generate mitigation scenarios. A two-dimensional cellular automata-based model was used to simulate urban surface water flooding. The results obtained from the case study indicate that the framework can achieve cost-effective mitigation strategies for sponge city construction, which can support robust decision making. The distribution of mitigation strategies has great impacts on the effectiveness of alleviating urban flood risk. This study provides new insight into the development of cost-effective mitigation strategies for sponge city construction.

An integrated framework for the comprehensive evaluation of low impact development strategies

The impacts of urbanization on water quality, hydrology, society, and the environment can be minimized through low impact development (LID) practices in urban areas. This study has evaluated the performances of seven different LID scenarios including stand-alone and different combinations of green roof (GR), bioretention cells (BC), permeable pavement (PP), and infiltration trench (IT) in the Ayamama watershed, which is one of the most densely urbanized areas in Istanbul. Stormwater Management Model (SWMM) was used to obtain the performances of LID scenarios in quantitative (i.e., volume reduction and peak runoff reduction) and qualitative (i.e., Total Suspended Sediment, Chemical Oxygen Demand, Total Nitrate, Total Phosphate reductions) manner. To calibrate the SWMM model, the Parameter EStimation Tool (PEST) was integrated for sensitivity analysis and parameter optimization. A focus group discussion (FGD) was performed to identify the criteria and LID scenarios applicable to the study area. 16 criteria were determined as suitable, based on three dimensions of sustainability such as social, economic, and environmental. The criteria were evaluated in compliance with the fuzzy analytical hierarchy process (AHP) method before performing technique for order preference by similarity to ideal solution (TOPSIS) for a comprehensive assessment of LID scenarios. The results showed that community resistance, operation feasibility, and quantitative benefits were the most significant criteria for LID scenario selection in social, economic, and environmental aspects, respectively. The integrated evaluation showed that the impacts of urban flooding can be reduced significantly with the combination of GR and BC. Thus, this study provides an integrated and sustainable solution to the topic based on the PEST-SWMM-fuzzy AHP-TOPSIS framework. Furthermore, the developed framework could assist decision-makers and governmental authorities to designate optimal LID scenarios.

China's sponge city development for urban water resilience and sustainability: A policy discussion

China recently introduced a national policy initiative called sponge city development as a holistic, ecosystem-based approach integrated with urban planning and development to address storm-induced pluvial flooding as well as other urban water and environmental issues. The initiative, while following the U.S. low impact development with a concept also similar to the U.K. sustainable drainage systems and Australian water sensitive cities, is subject to a major design issue in practice with infrastructure projects of similar types adopted unanimously across regions despite spatially diverse and heterogeneous hydrological and biophysical conditions. The ecosystem services framework as applied to the urban setting, particularly its holistic consideration of ecosystem structure and management intervention in relation to services or benefits delivery, can and should guide the planning, design, development, and evaluation of relevant projects or nature-based practices for carrying out the policy initiative, a perspective of practical value with foreseeable transformative impact that has received little recognition in China's current green urban movement toward water resilience and sustainability.

Knowledge, attitudes, intentions, and behavior related to green infrastructure for flood management: A systematic literature review

Green infrastructure (GI), which mimics natural hydrological systems, is a promising solution for flood management at the intersection of urban built infrastructure and natural systems. However, it has not yet achieved widespread uptake, due in part to insufficient understanding of human dimensions of the broader socio-ecological-technical system. We therefore conducted a multidisciplinary systematic literature review to synthesize research on people's existing knowledge about flood risk and GI, and how that shapes their attitudes and motivation to adopt new solutions. We systematically screened 21,207 studies on GI for flood management; 85 met our inclusion criteria. We qualitatively analyzed these studies to extract results on knowledge, attitudes, intentions, and behavior relating to GI for flood management. Overall, knowledge of GI was low across the 44 studies in which it was evaluated. Seventy studies assessed attitudes about GI, including the functional, aesthetic, health and safety, recreational, conservation, financial, and cultural value of GI, albeit their measurement was inconsistent. Willingness to implement or pay for GI varied considerably across 55 studies in which it was measured. Twenty studies measured and documented behavior relating to GI use, and these found low rates of adoption. Few studies systematically assessed the role of demographic, socio-economic, or geographic characteristics that could influence individuals' knowledge, attitudes, intentions or behavior, and thereby the success of GI programs. We recommend that researchers should more systematically capture data on human dimensions of GI (i.e. knowledge, attitudes, intentions, and behavior) across diverse settings to improve program design and uptake, especially among vulnerable populations. Greater attention to the social component of the socio-ecological-technical system will help ensure that GI programs are equitable, inclusive, and sustainable.

The analysis of green roof's runoff volumes and its water quality in an experimental study in Porto Alegre, Southern Brazil

The green roofs are structures characterized by the application of vegetation cover in the buildings, using adequate waterproofing and drainage systems. It allows the reduction of surface runoff and delay in peak flow, contributing to the mitigation of flood events in urban areas. Therefore, this study aimed to evaluate the effect of the use of vegetal coverings on the surface runoff, taking into account quantitative and qualitative aspects, using an experimental module installed in the city of Porto Alegre, Brazil. The experimental station consisted of four modules: two horizontal modules with and without vegetation cover and two modules with slopes of 15° with and without vegetation cover. It was evaluated 19 precipitation events, and it was verified the volumes drained in each module after 3, 6 and 12 h from the beginning of precipitation. The water samples were collected in order to analyse the quality of the runoff from the experimental modules. The results have shown that the use of vegetal coverings can provide better distribution of the surface runoff, as well as a decrease of the speed of excess water release with no surface runoff in the first 3 h after the onset of rainfall in the horizontal module. Additionally, it was proved the reduction in drained volumes, with the flat module with vegetation cover being capable of retaining completely precipitations with volumes of approximately 22 mm. The vegetation cover module in roofs was the one that has presented better results regarding the reduction of the flow, presenting an average retention percentage of 91.7% for the first 3 h, indicating that the slope is an important factor. The physical-chemical analysis of the water shows that for all analysed modules, it is possible to use water for non-potable uses, although the water quality of the modules with vegetation cover is lower when compared to the water coming from the module without vegetation cover.

Understanding the synergistic effect between LID facility and drainage network: With a comprehensive perspective

In recent years, low impact development (LID) has been deemed as an effective strategy for better storm water management. Among tremendous literature concerning the effectiveness of LID facilities to improve urban drainage system (UDS) performance, the scale, location, variety, and parameters of LIDs are fully discussed while the role of urban drainage network is rarely considered. Since LIDs and drainage network work together to realize the function of UDS, this research aims to explore their synergistic effects on the social, environmental, and technological performance of UDS. To represent different synergistic effects, a case area in Kunming, China is divided into upstream, midstream, and downstream sections. The hydraulic and topological characteristics of the drainage network in different sections are varied and 4 LID siting strategies are designed to distribute LID facilities in one of the 3 sections or in the total catchment evenly. Uncertainties from rainfall intensity and LID distribution are discussed as well. Based on the modeling results, the existence of synergistic effect is confirmed. Different spatial relationships between LIDs and the drainage network lead to disparate UDS performance. The synergistic effect may appear as the aggravation or mitigation of the conflicts among various functions of UDS, such as the reduction of urban flooding, combined sewer overflow, and shock loadings to wastewater treatment plant. The synergistic effect is sensitive to the rainfall intensity, suggesting the necessity of system performance investigation under non-design conditions. Due to the complicated synergistic effect, even distribution of LIDs in the target area can be a regretless and simplified solution. The discoveries reveal the significance of the synergistic effect and its implications for the LID planning.

Effects of spatial planning on future flood risks in urban environments

Urban development may increase the risk of future floods because of local changes in hydrological conditions and an increase in flood exposure that arises from an increasing population and expanding infrastructure within flood-prone zones. Existing urban land use change models generally consider the expansion process and do not consider the densification of existing urban areas. In this paper, we simulate 24 possible urbanization scenarios in Wallonia region (Belgium) until 2100. These scenarios are generated using an agent-based model that considers urban expansion and densification as well as development restrictions in flood-prone zones. The extents of inundation and water depths for each scenario are determined by the WOLF 2D hydraulic model for steady floods corresponding to return periods of 25, 50, and 100 years. Our results show that future flood damages and their spatial distributions vary remarkably from one urbanization scenario to another. A spatial planning policy oriented towards strict development control in flood-prone zones leads to a substantial mitigation of the increased flood damage. By contrast, a spatial planning policy exclusively oriented to infill development with no development restrictions in flood-prone zones would be the most detrimental in terms of exposure to flood risk. Our study enables the identification of the most sensitive locations for flood damage related to urban development, which can help in the design of more resilient spatial planning strategies and localize zones with high levels of flood risk for each scenario.

The changing pattern of urban flooding in Guangzhou, China

Extensive research has focused on modelling and analysis of urban flooding in relatively small catchments. Findings in small areas tend to be site-specific, and may not be applicable to larger study areas. Larger scale studies can identify general patterns and influential factors; however, few studies have investigated urban flooding on a larger scale such as a metropolitan area. In this study, we explored the spatial-temporal patterns of urban flooding during the period of 2009-2015 in the central area of Guangzhou, China. Under the framework of Pressure (precipitation and impervious surface)-State (urban flooding)-Response (drainage improvement), we evaluated reasons for the State change and effectiveness of the Response. Overall the urban flooding state showed a fluctuating and improving trend. The fluctuation of the flooding state trend is due to precipitation variation, and the improving trend is attributed to drainage improvement. Furthermore, drainage improvement in the upstream area had led to new flooding in the downstream area. It is evident that the mitigation effect of urban flooding in Guangzhou varied significantly across the city. It is further suggested to regularly collect urban flooding records in cities with flood risk, so that more appropriate policies and measures about urban flooding mitigation can be developed.

Street floods in Metro Manila and possible solutions

Urban floods from thunderstorms cause severe problems in Metro Manila due to road traffic. Using Light Detection and Ranging (LiDAR)-derived topography, flood simulations and anecdotal reports, the root of surface flood problems in Metro Manila is identified. Majority of flood-prone areas are along the intersection of creeks and streets located in topographic lows. When creeks overflow or when rapidly accumulated street flood does not drain fast enough to the nearest stream channel, the intersecting road also gets flooded. Possible solutions include the elevation of roads or construction of well-designed drainage structures leading to the creeks. Proposed solutions to the flood problem of Metro Manila may avoid paralyzing traffic problems due to short-lived rain events, which according to Japan International Cooperation Agency (JICA) cost the Philippine economy 2.4billionpesos/day.

Importance of anthropogenic climate impact, sampling error and urban development in sewer system design

Urban drainage design relying on observed precipitation series neglects the uncertainties associated with current and indeed future climate variability. Urban drainage design is further affected by the large stochastic variability of precipitation extremes and sampling errors arising from the short observation periods of extreme precipitation. Stochastic downscaling addresses anthropogenic climate impact by allowing relevant precipitation characteristics to be derived from local observations and an ensemble of climate models. This multi-climate model approach seeks to reflect the uncertainties in the data due to structural errors of the climate models. An ensemble of outcomes from stochastic downscaling allows for addressing the sampling uncertainty. These uncertainties are clearly reflected in the precipitation-runoff predictions of three urban drainage systems. They were mostly due to the sampling uncertainty. The contribution of climate model uncertainty was found to be of minor importance. Under the applied greenhouse gas emission scenario (A1B) and within the period 2036-2065, the potential for urban flooding in our Swiss case study is slightly reduced on average compared to the reference period 1981-2010. Scenario planning was applied to consider urban development associated with future socio-economic factors affecting urban drainage. The impact of scenario uncertainty was to a large extent found to be case-specific, thus emphasizing the need for scenario planning in every individual case. The results represent a valuable basis for discussions of new drainage design standards aiming specifically to include considerations of uncertainty.

The effects of low impact development on urban flooding under different rainfall characteristics

Low impact development (LID) is generally regarded as a more sustainable solution for urban stormwater management than conventional urban drainage systems. However, its effects on urban flooding at a scale of urban drainage systems have not been fully understood particularly when different rainfall characteristics are considered. In this paper, using an urbanizing catchment in China as a case study, the effects of three LID techniques (swale, permeable pavement and green roof) on urban flooding are analyzed and compared with the conventional drainage system design. A range of storm events with different rainfall amounts, durations and locations of peak intensity are considered for holistic assessment of the LID techniques. The effects are measured by the total flood volume reduction during a storm event compared to the conventional drainage system design. The results obtained indicate that all three LID scenarios are more effective in flood reduction during heavier and shorter storm events. Their performance, however, varies significantly according to the location of peak intensity. That is, swales perform best during a storm event with an early peak, permeable pavements perform best with a middle peak, and green roofs perform best with a late peak, respectively. The trends of flood reduction can be explained using a newly proposed water balance method, i.e., by comparing the effective storage depth of the LID designs with the accumulative rainfall amounts at the beginning and end of flooding in the conventional drainage system. This paper provides an insight into the performance of LID designs under different rainfall characteristics, which is essential for effective urban flood management.