Urban Rainwater Utilization and its Role in Mitigating Urban Waterlogging Problems—A Case Study in Nanjing, China

With the acceleration of the urbanization process, urban waterlogging problems are becoming more and more serious in Nanjing, China. In order to mitigate the urban waterlogging problems, it is necessary to reduce surface runoff from the source by rainwater harvesting and utilization. An urban residential district with an area of 0.58?km² in Nanjing was selected as the study area. Based on a large-scale topographic map data and the long term rainfall data (1951?C2008), the types of underlying surfaces were classified. The potentiality of collectable rainwater and the possibility of runoff volume reduction were calculated. The results showed that exploitation of rainwater harvesting from rooftops and other underlying surfaces has high potential. The annual collectable rainwater is approximately 372,284?m³, 314,034?m³ and 275,180?m³ under different cumulative frequency of rainfall at 20?%, 50?% and 75?%, respectively. The total capacity of cisterns under assumptions of return period of rainfall and rainfall duration with 5?years and 20?min is 11,022?m³. The cistern??s capacity which is used for roof rainwater harvesting is 4,083?m³, the cistern capacity for per unit roof area (1?m²) is 0.0267?m³. The results of the feasibility analysis of setting up above-ground cisterns showed that 55?% of the total roof areas in the study area are available for setting up cisterns. In the three building types, 16?% of the commercial building??s roof areas and 77?% of that of the residential and the ??others?? buildings are available for setting up cisterns. Urban waterlogging problems can be effectively reduced through rainwater harvesting by 13.9?%, 30.2?% and 57.7?% of runoff volume reduction in three cases of the maximum daily rainfall (207.2?mm), the average annual maximum daily rainfall (95.5?mm) and the critical rainfall of rainstorm (50?mm).     

山地城市屋面雨水可利用量分析——以重庆市沙坪坝某片区为例

为定量评估山地城市屋面雨水可利用量,以重庆市沙坪坝某片区为例,分析了重庆市的降雨特征,利用改进的径流系数法定量评估了代表年屋面雨水可利用量,并分析了降雨特征对可利用量的影响;运用设计降雨量和设计暴雨强度两个指标相结合的方法确定了蓄水池规模,使其满足径流控制率和防洪减涝的要求。研究表明:在代表年降雨量为1 198.9、1 104.4和962.7 mm时,研究区域屋面雨水可利用量分别为267 951.8、247 421.3和198 917.9 m~3,雨水利用潜力较大,降雨量对可利用量的影响较大,而降雨强度对其影响很小;研究区域蓄水池总规模为9 312 m~3,屋面径流控制率达到86.4%,为山地城市雨水利用系统中蓄水池的规模设计提供了参考依据。     

Assessing the Potential for Rainwater Harvesting

Rainwater harvesting is one of the promising ways of supplementing the surface and underground scarce water resources in areas where existing water supply system is inadequate to meet demand. Rainwater harvesting is one of the measure for reducing impact of climate change on water supplies. Abeokuta has a mean annual rainfall of 1,156 mm which makes rainwater harvesting ideal. Intra annual variability lies betwen 0.7 and 1.0 while the inter annual variability was 0.2. Annually 74.0 m³ of rainwater can be harvested per household. Estimated annual water demand for flushing, laundry and flushing were 21.6 and 29.4 m³ respectively. Harvested rainwater in Abeokuta can satisfy household monthly water demand for WC flushing and laundry except for November, December, January and February. The excess rainwater stored in September and October is sufficient to supplement the short fall in the dry months provided there is adequate storage.Water savings potential is highest in June and September which is the two rainfall peak period in Southwest Nigeria.     

Impact of El Niño and Climate Change on Rainwater Harvesting in a Caribbean State

The water situation of the Pusey district in St Catherine parish of Jamaica is acute because of the district’s hilly terrains which made connections to centralised public water supply difficult. Residents depend on rainwater harvesting (RWH) systems to meet potable needs, like many other catchments across Jamaica. Rainwater collecting practices and water use habits of the residents were surveyed and the present and future RWH capacity was evaluated using the available 18 years (1996 to 2013) rainfall data and downscaled PRECIS model A2 and B2 climate change scenarios. In addition, the effect of El Niño episodes on rainfall patterns was evaluated. The coefficients of variations for annual rainfall were found to be higher for the El Niño years than in normal years. In two of the El Niño years (1997 and 2009), rainwater harvesting capacity is negatively impacted as rainfall annual total is (42 % and 34 %) lesser than the average annual rainfall. The ability of RWH to meet potable needs in 2030s and 2050s will be reduced based on predicted shorter intense showers and frequent dry spells. A storage tank of 2.5 to 4.0 m³ per household (4 persons) is proposed to meet water demand during the maximum consecutive dry days, and January and February water shortage periods. Design of efficient RWH systems and provision of government subsidy on storage tanks will enable the residents to capture more rainwater to meet their daily domestic needs.     

Analysis and Modelling of Stormwater Volume Control Performance of Rainwater Harvesting Systems in Four Climatic Zones of China

Rainwater harvesting has been widely used to alleviate urban water scarcity and waterlogging problems. In this study, a water balance model is developed to continuously simulate the long-term (57 to 65 years) stormwater capture efficiency of rainwater harvesting systems for three water demand scenarios at four cities across four climatic zones of China. The impacts of the “yield after spillage” (YAS) and “yield before spillage” (YBS) operating algorithms, climatic conditions, and storage and demand fractions on stormwater capture efficiency of rainwater harvesting systems are analyzed. The YAS algorithm, compared with the YBS, results in more conservative estimations of stormwater capture efficiency of rainwater harvesting systems with relatively small storage tanks (e.g., ≤50 m³). The difference between stormwater capture efficiency calculated using the YBS and YAS algorithms can be remedied by increasing storage capacity and reduced by decreasing water demand rates. Higher stormwater capture efficiency can be achieved for rainwater harvesting systems with higher storage and demand fractions and located in regions with less rainfall. However, the lager variations in annual rainfall in arid zones may lead to unstable stormwater management performance of rainwater harvesting systems. The impacts of storage and demand fractions on stormwater capture efficiency of rainwater harvesting systems are interactive and dependent on climatic conditions. Based on the relationships among storage capacity, contributing area, water demand, and stormwater capture efficiency of rainwater harvesting systems, easy-to-use equations are proposed for the hydrologic design of rainwater harvesting systems to meet specific stormwater control requirements at the four cities.     

奥林匹克公园中心区不同下垫面降雨初期地表径流水质分析

奥林匹克公园中心区雨洪利用示范工程的建立,将雨洪控制与利用纳入到实际建设,采用规模化的雨洪利用系统,建设雨洪利用示范工程,以展示城市雨水排放新概念,实现雨水资源化。对于中心区地表径流水质进行检测,深入研究和分析,科学的展示了雨洪利用示范工程的科学性和合理性,让人们对雨水利用的意义有直观认识,了解简单易行的雨水利用技术,从而把雨洪利用技术措施推广到今后的城市建设中。     

雨水回用对城市水循环和下游生态环境的影响

基于水量平衡理论和城市径流污染控制理论,从水量和水质2个方面探讨城市雨水回用对地表水和地下水原有自然径流以及对下游生态环境的影响。结果表明:雨水回用不仅不会减少雨水入渗量,还可以控制雨水径流污染,削减汛期江河下游区域的洪峰流量,是改善下游区域生态环境的有效途径。最后对城市雨水回用的合理规划提出了建议。     

Storm Water Harvesting in Saudi Arabia: a Multipurpose Water Management Alternative

This work evaluates the potential for rainwater harvesting at the household level in the dry climate of Saudi Arabia. The amounts of rainwater that can be harvested in several cities in Saudi Arabia were estimated and it was found that a significant volume, exceeding 7.5 m³/100 m² per year, can be harvested. The per cubic meter cost of harvesting rainwater was compared to that of producing desalinated water. It was found that harvested rainwater is cheaper to capture than desalinated water produced from renewable energy-driven desalination plants, but that is not the case for fossil fuel-powered desalination. The study also considered the effects of rainwater harvesting on mitigating floods and reducing greenhouse gas emissions. A cost-benefit analysis of installing rooftop rainwater harvesting systems in Saudi Arabia was performed. It revealed that it is economically feasible to harvest rain in cities including Hail, Jeddah, Taif, and Riyadh, while it is not recommended in the holy cities of Makkah, Medina, and Buraidah.     

Optimizing the Runoff Estimation with HEC-HMS Model Using Spatial Evapotranspiration by the SEBS Model

Most of the commonly used hydrological models do not account for the actual evapotranspiration (ETa) as a key contributor to water loss in semi-arid/arid regions. In this study, the HEC-HMS (Hydrologic Engineering Center Hydrologic Modeling System) model was calibrated, modified, and its performance in simulating runoff resulting from short-duration rainfall events was evaluated. The model modifications included integrating spatially distributed ETa, calculated using the surface energy balance system (SEBS), into the model. Evaluating the model’s performance in simulating runoff showed that the default HEC-HMS model underestimated the runoff with root mean squared error (RMSE) of 0.14 m³/s (R²?=?0.92) while incorporating SEBS ETa into the model reduced RMSE to 0.01 m³/s (R²?=?0.99). The integration of HECHMS and SEBS resulted in smaller and more realistic latent heat flux estimates translated into a lower water loss rate and a higher magnitude of runoff simulated by the HECHMS model. The difference between runoff simulations using the default and modified model translated into an average of 95,000 m³ runoff per rainfall event (equal to seasonal water requirement of ten-hectare winter wheat) that could be planned and triggered for agricultural purposes, flood harvesting, and groundwater recharge in the region. The effect of ETa on the simulated runoff volume is expected to be more pronounced during high evaporative demand periods, longer rainfall events, and larger catchments. The outcome of this study signifies the importance of implementing accurate estimates of evapotranspiration into a hydrological model.

     

Parameters Influencing the Sizing of Rainwater Tanks for Use in Houses

Rainwater harvesting has been studied in different countries as a way of easing water availability problems and reducing potable water demand in buildings. The most important factor relating to the efficiency of a rainwater system is the correct sizing of the rainwater tank. Therefore, the objective of this article is to assess the influence of rainfall, roof area, number of residents, potable water demand and rainwater demand on rainwater tank sizing. The analysis was performed by using computer simulation and by considering daily rainfall data for three cities located in the state of São Paulo, Brazil. The roof areas considered were 50, 100, 200 and 400 m²; the potable water demands were 50, 100, 150, 200, 250 and 300 l per capita per day; the rainwater demands were taken as a percentage of the potable water demand, i.e., 10% to 100% at increments of 10%; and the number of residents was two and four. Results indicated a wide variation of rainwater tank sizes for each city and also for each parameter. The main conclusion that can be made from the study is that rainwater tank sizing for houses must be performed for each specific situation, i.e., considering local rainfall, roof area, potable water demand, rainwater demand and number of residents. Therefore, sizing rainwater tanks according to local tradition is not recommended as it may incur low efficiency.     

城市雨水调蓄利用及效益分析——以南京大学仙林校区为例

随着城市化发展,水资源短缺、水环境污染和城市内涝等问题日益突出,雨水调蓄利用成为缓解以上问题的重要手段。以南京大学仙林校区为研究区,进行可集雨量与室外用水需求的水量平衡计算,结合雨水利用措施分析,开展雨水调蓄利用的方案设计及综合效益分析。结果表明:雨水调蓄利用后,研究区综合径流系数下降为0. 532,重现期2、5、10 a对应的屋面集雨量均能满足室外用水需求,对应的单位面积雨水利用年综合效益分别为8 475. 6、10 506. 3、11 786. 4元/hm2。城市雨水调蓄利用可产生显著的生态、经济和社会效益,在城市新区开发和旧城改造过程中应当得到足够的重视。     

Preliminary Assessment of Rainwater Harvesting Potential in Nigeria: Focus on Flood Mitigation and Domestic Water Supply

This study was accomplished using 26 locations in the major ecological zones of Nigeria as well as different classes of residential buildings and different levels of water consumption. For each location, house dwelling class and level of water consumption, a water balance approach was used to assess the proportion of water demand that can be met by rainwater. Results obtained indicate that for all the locations in the rainforest zone and some parts in the guinea savanna zone, over eighty percent (80 %) of water demand of those living in bungalows can be met by rainwater. Rainwater harvesting potential was found to be a power function of rainfall coefficient of variation, with coefficient α and exponent β. High correlation coefficients (0.881?≤?R ² ?≤?1) were obtained between coefficient α and roof area per capita. Also, high correlation coefficients (0.847?≤?R ² ?≤?0.992) were obtained between exponent β and roof area per capita.     

A physically-based flood volume distribution model

An analytically derived distribution model for flood volume is presented. The model is based on water balance computations during a rainstorm at the soil surface. It is applicable for mountainous watersheds with alluvial channels. Three hydrologic processes are considered: precipitation, infiltration, and runoff generation. Rainfall intensity and duration are presented with exponential distributions. They are assumed to be statistically independent. A linear rainfall-runoff relationship is proposed for the mountainous areas. The mountain runoff is regarded as a uniformly distributed water depth on the alluvial channels. The rainfall excess in the alluvial channels is computed to be equal to this depth plus the rainfall depth minus the infiltration losses. Infiltration in the channels is presented with Philip's expression, coupled with an empirical model for the computation of a long-term average value for the soil moisture content. The distribution model is verified through applications for three gauged watersheds in Saudi Arabia: Wadi Liyyah (174 km²), Wadi Turrabah (3720 km²), and Wadi Khulays (5220 km²). The results are found to be in a good agreement with observations.On leave from the Irrigation and Hydraulics Department, Faculty of Engineering, Cairo University, Giza, Egypt.     

巢湖流域土地利用变化对雨洪调蓄能力的影响

为探究土地利用结构变化对雨洪调蓄能力的影响,以1980—2020年巢湖流域的土地利用数据和土壤数据为基础,采用GIS和SCS模型相结合的方法,分析了不同时期不同降雨尺度下巢湖流域径流产生及分布情况。结果表明：1980—2020年巢湖流域土地利用变化主要表现为建设用地的增加及耕地和林地的减少;流域土地利用综合指数持续增长,其中高强度土地利用区面积从127 km²增长到了1 624 km²。1980—2020年渗透性差的地区从1 706.0 km²增加到了2 398.1 km²,以城市区域扩张为主。2020年相较于1980年,在不同土地利用条件下,相同降雨量时径流量明显增大,且主要在巢湖流域的合肥地区,而在增加降雨量时,径流量的增加幅度逐渐减小,在不同降雨量条件下(日降雨量为50、100、250 mm)径流量分别增大了6.11%、2.61%和0.96%。因此可以得出土地利用变化是导致径流量变化的主要因素,降雨量是次要因素。研究结果可为巢湖流域城市规划及城市雨洪风险管理提供参考。     

南宁市设计降雨及其总量控制率时空异质性特征

针对当前我国海绵城市建设及成效考核中设计降雨及其控制率指标单一均化、难以有效定量指导不同下垫面LID设施建设并评价其效果等问题,以南宁市市区为例,基于2020年Quick Bird土地利用及子流域精细划分,根据1981-2010年南宁市0.1°CMFD格点日降雨序列资料,采用ArcGIS空间插值法、《指南》中设计降雨量与控制率关系法以及容积法,分析了研究区日降雨量、设计降雨量及其总量控制率的时空异质性特征,并讨论了现状下垫面调蓄及管网排水对控制率指标的定量影响。结果表明：南宁市市区日降雨频次、年分配、设计降雨量及其控制率均具有显著的时空异质性特征,按75%控制率对应各月的日设计降雨量变化范围为9.72（2月）~32.05 mm（7月）;南宁市主城区（面积占比31.99%）现状年降雨总量控制率普遍低于50%,仅靠绿色LID设施难以实现75%控制率目标,要实现该控制率目标,主城区还需增加调蓄容积120~211 m³/hm²,若考虑管网排水能力70~103 m³/hm²,则所需调蓄容积可减少30~108 m³/hm²。综上,为实现南宁市75%控制率目标,应针对现状下垫面水平,建立考虑时空异质性的不同控制率目标与调蓄容积、管网排水之间的定量函数关系,这是有效提升海绵城市调控雨洪径流能力的一个主要途径。     

Water Quality Monitoring and Hydraulic Evaluation of a Household Roof Runoff Harvesting System in France

The quality of harvested rainwater used for toilet flushing in a private house in the south-west of France was assessed over a one-year period. Twenty-one physicochemical parameters were screened using standard analytical techniques. The microbiological quality of stored roof runoff was also investigated and total flora at 22°C and 36°C, total coliforms, Escherichia Coli, enteroccocci, Cryptospridium oocysts, Giardia cysts, Legionella species, Legionella pneumophila, Aeromonas, and Pseudomonas aeruginosa were analysed. Chemical and microbiological parameters fluctuated during the course of the study, with the highest levels of microbiological contamination observed in roof runoffs collected during the summer. Overall, the collected rainwater had a relatively good physicochemical quality but variable, and, did not meet the requirements for drinking water and a microbiological contamination of the water was observed. The water balance of a 4-people standard family rainwater harvesting system was also calculated in this case study. The following parameters were calculated: rainfall, toilets flushing demand, mains water, rainwater used and water saving efficiency. The experimental water saving efficiency was calculated as 87%. The collection of rainwater from roofs, its storage and subsequent use for toilet flushing can save 42 m³ of potable water per year for the studied system.     

福建省山洪灾害风险识别与定量分析

受极端天气、复杂下垫面和人类活动多重因素影响,山洪灾害在我国频发高发,且预测预报难度较大。本文基于全国山洪灾害调查评价成果,着眼于15-20 km~2的小流域,从降雨、下垫面、社会经济、现状防洪能力、涉水工程及监测预警设施配置率6个方面,提取了54个备选指标,运用主成分分析法进行降维处理,筛选了10个核心且独立的指标,构建了风险评估指标体系,建立了风险立方体模型,并运用福建省2306场历史山洪场次数据进行了模型验证。结果表明,基于小流域的山洪灾害风险识别可有效反映灾害对降雨和下垫面的响应关系;沿海台风暴雨高发区和内陆山区短时强降雨高值区是山洪灾害高风险区,涉及小流域1151个,面积近2万km~2,人口608万。     

Modeling the Effect of Cistern Size,Soil Type,and Irrigation Scheduling on Rainwater Harvesting as a Stormwater Control Measure

Urban stormwater runoff could have negative impacts on water resources and the environment. Rainwater Harvesting (RWH) can serve both as a stormwater control and water conservation measure. Cistern size and irrigation scheduling are two of the factors that directly impact the total runoff from a residential unit with a RWH system and the amount of potable water used for irrigation. The effectiveness of RWH was evaluated for four soil types; Sand, Sandy Loam, Loamy Sand, and Silty Clay, with a root zone of 15.2 cm using three irrigation scheduling methods (Evapotranspiration (ET)-based, soil moisture-based, and time-based), and five cistern sizes. Total runoff volumes and total supplemental potable water used were compared among the three irrigation scheduling systems and a control treatment without RWH. A model was developed to simulate the daily water balance for the treatments. Irrigation and runoff volumes were compared for the various scenarios. Silty clay soil resulted with 83 % more runoff than Sandy soil, while Sandy soil required on average 58 % more supplemental water than Silty Clay soil. On average, the 833 L cistern resulted with 41 % savings in water supply and 45 % reduction in total runoff. Results showed that the greatest volumes of runoff predicted were for the silty clay soil Control Treatment using a time-based irrigation scheduling method, while the least volumes calculated were for the sandy loam soil time-based irrigation scheduling treatment with 833 L cistern size. The greatest volumes of total supplemental water predicted were for sandy loam soil Control Treatment, while the least volumes were for silty clay soil ET-based irrigation scheduling treatment with 833 L cistern size. Regression equations were developed to allow for users to select a RWH cistern size based on the amount of water they want to save or runoff to reduce.     

Numerical Simulation of Flood Wave Propagation in Two-Dimensions in Densely Populated Urban Areas due to Dam Break

Dams are important structures having many functions such as water supply, flood control, hydroelectric power and recreation. Although dam break failures are very rare events, dams can fail with little warning and the damage at the downstream of the dam due to the flood wave can be catastrophic. During a dam failure, immense volume of water is mobilized at very high speed in a very short time. The momentum of the flood wave can turn to a very destructive impact force in residential areas. Therefore, from risk point of view, understanding the consequences of a possible dam failure is critically important. This study deals with the methodology utilized for predicting the flood wave occurring after the dam break and analyses the propagation of the flood wave downstream of the dam. The methodology used in this study includes creation of bathymetric, DEM and land use maps; routing of the flood wave along the valley using a 1D model; and two dimensional numerical modeling of the propagation and spreading of flood wave for various dam breaching scenarios in two different urban areas. Such a methodology is a vital tool for decision-making process since it takes into account the spatial heterogeneity of the basin parameters to predict flood wave propagation downstream of the dam. Proposed methodology is applied to two dams; Porsuk Dam located in Eski?ehir and Alibey Dam located in Istanbul, Turkey. Both dams are selected based on the fact that they have dense residential areas downstream and such a failure would be disastrous in both cases. Model simulations based on three different dam breaching scenarios showed that maximum flow depth can reach to 5 m at the border of the residential areas both in Eski?ehir and in Istanbul with a maximum flow velocity of 5 m/s and flood waves having 0.3 m height reach to the boundary of the residential area within 1 to 2 h. Flooded area in Eski?ehir was estimated as 127 km², whereas in Istanbul this area was 8.4 km² in total.     

高速公路雨水径流污染浓度的系统动力学模拟

针对降雨事件中高速公路雨水径流污染浓度变化的随机性复杂特征,建立了雨水径流污染物浓度演化的系统动力学模型,研究交通流特征、高速公路特征、降雨事件特征对雨水径流污染浓度的影响。利用模拟软件Anylogic,基于5条高速公路的21个降雨事件抽样数据,对模型进行了拟合分析。通过对模型中降雨时间、径流深度、降雨量等参数进行敏感性分析,阐明了不同降雨事件情景下雨水径流污染浓度的时变规律。结果表明:提出的系统动力学模型可以很好地模拟降雨事件中雨水径流污染浓度的变化过程;在持续时间短而强度高的降雨事件中,高速公路雨水径流污染浓度较高,对沿线受纳水体的影响较大;雨水径流深度与污染浓度是正相关的,降雨量大的雨水径流中污染物的最大析出浓度反而会比降雨量小的降雨事件提前出现;在降雨事件的前期,如果选择合理的控制措施,可以显著降低雨水径流的平均污染浓度。