The importance of gully flow modelling to urban flood simulation

ABSTRACT

Urban flood simulation often ignores or simplifies the function of underground gully systems due to data and computational limitations. To discover the influence of gullies on urban flooding, a novel approach is proposed in this study, by fully-coupling a 1D gully flow model (GFM), a 1D sewer flow model (SFM), and a 2D overland flow model (OFM) to simultaneously simulate the flow exchanges between surface, gullies and sewer pipes. This fully-coupled approach is compared with a simplified approach which directly introduces surface water into sewer pipes without being via gullies. The validation results show that the fully-coupled approach considerably reduces the underestimation of flood extent by 27% compared with the simplified approach. Without considering the capacity of lateral tubes between gullies and sewer pipes, the simplified approach over-drains the surface water into sewer pipes. The modelling of gully flow is crucial for correctly evaluating the efficiency of drainage systems.     

Hydrological assessment of flow dynamic changes by storm sewer overflows and combined sewer overflows on an event-scale in an urban river

In addition to assessing the impacts of water quality changes in urban rivers caused by storm water sewer overflows (SWO) and combined sewer overflows (CSO), the extent to which flow dynamics are changed by these structures must be understood in order to define hydrological assessment criteria to guide sustainable water management strategies as required by the European Community (EC) Water Framework Directive. In this study, the quantitative impacts of SWOs and CSOs on the flow dynamics of an urban river and their variability are investigated. For four single runoff events, hydrological measurements were accomplished in the River Dreisam, upstream and downstream of the city of Freiburg, in southwest Germany. As the catchment is widely free of urban areas upstream of the city, comparison with downstream locations allowed quantification of Freiburg's effects on the changes in the hydrograph on an event scale. The proposed hydrological parameter—flow acceleration, peak discharge, and discharge dosage—were shown to be appropriate to assess the impacts of SWOs and CSOs on flood hydrographs in urban rivers.     

Quantification of groundwater infiltration and surface water inflows in urban sewer networks based on a multiple model approach

The management of sewer systems requires information about discharge and variability of typical wastewater sources in urban catchments. Especially the infiltration of groundwater and the inflow of surface water (I/I) are important for making decisions about the rehabilitation and operation of sewer networks. This paper presents a methodology to identify I/I and estimate its quantity. For each flow fraction in sewer networks, an individual model approach is formulated whose parameters are optimised by the method of least squares. This method was applied to estimate the contributions to the wastewater flow in the sewer system of the City of Dresden (Germany), where data availability is good. Absolute flows of I/I and their temporal variations are estimated. Further information on the characteristics of infiltration is gained by clustering and grouping sewer pipes according to the attributes construction year and groundwater influence and relating these resulting classes to infiltration behaviour. Further, it is shown that condition classes based on CCTV-data can be used to estimate the infiltration potential of sewer pipes.     

Microbial risks associated with exposure to pathogens in contaminated urban flood water

Urban flood incidents induced by heavy rainfall in many cases entail flooding of combined sewer systems. These flood waters are likely to be contaminated and may pose potential health risks to citizens exposed to pathogens in these waters. The purpose of this study was to evaluate the microbial risk associated with sewer flooding incidents. Concentrations of Escherichia coli, intestinal enterococci and Campylobacter were measured in samples from 3 sewer flooding incidents. The results indicate faecal contamination: faecal indicator organism concentrations were similar to those found in crude sewage under high-flow conditions and Campylobacter was detected in all samples. Due to infrequent occurrence of such incidents only a small number of samples could be collected; additional data were collected from controlled flooding experiments and analyses of samples from combined sewers. The results were used for a screening-level quantitative microbial risk assessment (QMRA). Calculated annual risks values vary from 5 × 10⁻⁶ for Cryptosporidium assuming a low exposure scenario to 0.03 for Giardia assuming a high exposure scenario. The results of this screening-level risk assessment justify further research and data collection to allow more reliable quantitative assessment of health risks related to contaminated urban flood waters.     

高密度沿海地区洪涝风险评价与适应性规划策略 ——以深圳湾地区为例

气候变化背景下，高密度沿海城市受 到风暴潮和极端降雨引起的洪涝灾害冲击。文 章基于韧性理论构建城市空间洪涝风险指标 体系，制定该评价框架的实施路径；基于水文 软件Mike21、GIS平台及其空间网络分析插件 sDNA，复合“天鸽”台风风暴潮与极端降雨情 景，整合深圳湾地区的路网和土地利用进行危 险性、暴露度、脆弱性和适应能力等多源数据； 通过GIS栅格计算得到各要素层分析及洪涝风 险评价可视化地图，结果显示，潮、洪、涝突破 刚性标准加剧了危险性，高密度城市环境增大了 危险区域的暴露度，路网和土地利用布局具有 一定脆弱性，需完善应急疏散和避难场所规划以增强适应能力；根据评价地图识别高风险片区，从路网和土地利用等城市空间物质要素出发， 提出应对洪涝灾害的适应性规划策略。     

基于GIS的洪水灾害危险性评价模型及应用

该文利用模糊数学中的模糊综合评价方法,以湖北省洪水灾害危险性评价为案例结合GIS的空间分析技术和建模技术,建立了基于GIS的洪水灾害危险性综合评价模型。     

Quantification of energy losses at a surcharging manhole

Hydraulic models of sewer systems are commonly used to predict the risk of urban flooding. However, suitable calibration datasets in flood conditions are scarce. The quantification of energy losses within manhole structures is a current source of uncertainty within such models. To address this gap, a scaled physical manhole model is used to quantify hydraulic energy losses during surcharging and non-surcharging conditions. Two different novel configurations were tested; (1) With and without the presence of a manhole lid; (2) With and without the presence of a shallow flow on the surface. Results showed that total head losses were found to increase in surcharging conditions. The presence of the lid also marginally increased total head losses. The datasets are used to assess the performance of a numerical urban flood model (SIPSON) and comparisons highlighted that SIPSON tends to overestimate energy losses in surcharging conditions.     

Application and comparison of two different dual drainage models to assess urban flooding

Two types of dual drainage models were set up to assess urban flooding for a study area in Germany: (1) a static model based on a conventional method in Germany; the overflow volumes of the manholes are gained by the sewer solver HYSTEM-EXTRAN. Using these water volumes and geographic information system (GIS) tools, an overland flow network, composed of flow paths and accumulated water in sinks, is produced, (2) a HYSTEM-EXTRAN 2D model; a two-dimensional (2D) overland flow module is coupled bi-directionally with HYSTEM-EXTRAN. The overland flow and the flow in the sewer system are simulated alternately.

Both models were supplied with a synthetic design rainfall and 25 extreme storms. After comparing the models and the results, a practical approach to assess urban flooding is proposed. In this approach the 2D model will compute the depth, extent, and propagation of floods only in the prone areas specified by the static model.     

山地城市小流域暴雨径流模型的研发及应用

采用ArcEngine二次开发包结合水力学基础算法,建立山地城市暴雨径流模型,以模拟山地城市降雨径流,分析山地城市排水系统的排放能力。应用该模型对重庆市北部新区盘溪河流域排水系统进行了预测和评估,结果表明,模拟结果与实测数据的效率系数Nash-Suttcliffe系数Ens为0.56~0.76,相对误差8.82%~11.8%。模型关键参数率定及敏感性分析表明,最敏感参数为径流宽度、坡度、曼宁系数和不透水面积率。针对排水管道内流量、充满度和溢流检查井个数等方面进行了排水系统排放能力的评估,结果表明,一年一遇暴雨下有0.52%的排水管道长时间处于满流状态,五十年和百年一遇的超载排水管道占排水管道总数5.86%和 8.20%;百年一遇暴雨下有0.72%的检查井发生溢流,一年一遇暴雨下有0.31%的检查井发生溢流。该模型实现了对山地城市暴雨径流的产汇过程模拟以及内涝点位置和数量的识别,为城市雨水管网管理提供决策支持。     

Enteric pathogens in flood-related waters in urban areas of the Vietnamese Mekong Delta: a case study of Ninh Kieu district,Can Tho city

ABSTRACT

This paper investigates the contamination of floodwaters in the urban center of Can Tho city, Vietnam. We sampled water from sewers, surface water bodies, and flood, before, during, and after specific flooding events. Total nucleic acid was extracted from the samples and subjected to a quantitative polymerase chain reaction (qPCR) to detect specific enteric pathogens. The difference between pathogen concentrations in floodwater and sewer water was compared by using the Mann Whitney U test. Correlations between the different pathogens were determined using the non-parametric Spearman test. E. coli and Rotavirus-A were the most prevalent pathogens in floodwater. We observed a weak association between E. coli and Rotavirus in flood-related waters (r < 0.5). Floodwater quality showed no difference to sewer water quality in terms of the E. coli and Rotavirus A concentrations (p > 0.05). Our results indicate that floodwater poses a significant urban public health risk due to the presence of enteric pathogens.     

Impact of combined sewer overflow on urban river hydrodynamic modelling: a case study of the Chicago waterway

Combined sewer overflow (CSO) can be a critical inflow source for urban rivers during storm events. This paper presents a case study of the Chicago waterway. A three-dimensional (3D) river hydrodynamic model was developed and integrated with an urban rainfall-runoff model using the Open Modelling Interface (OpenMI). Both the effects of CSO discharge on river and river water levels on CSO outlets were considered by the integrated model. A historical storm, which was similar to a 100-year return period rain event, was simulated and compared with field measurements. This study highlights the necessity of quantifying CSO for hydraulic modelling of urban rivers under extreme storm event conditions, and shows that an integrated hydrologic and hydraulic approach can be used to address this challenge. The 3D river hydrodynamic model can deal with the complex hydrodynamics at river confluences and provide better hydrodynamic results for water quality modelling in the future.     

Experimental estimation of the head loss coefficient at surcharged four-way junction manholes

ABSTRACT

The head loss caused by the surcharged flow from four-way junction manholes is the main cause of increased flood damage in urban areas. The flow pattern significantly varies depending on the inflow conditions of the inlet pipes and constitutes the flow conditions of a four-way junction manhole, three-way junction manholes, and middle manholes. Therefore, the head loss changes with various manhole shapes must be analyzed. In this study, physical model study apparatus was prepared. Various flow rate conditions were selected by changing the flow rate ratios of the inlet pipes at 10% intervals. The head loss coefficients were also estimated. A head loss coefficient range diagram was generated based on the results. A head loss coefficient empirical formula that considers all flow conditions for surcharged four-way junction manholes is proposed. The proposed equation should be applicable to the design and assessment of drainage systems with varying inlet pipe flow rates.     

NbS在城市雨洪韧性规划中的应用研究

城市雨洪灾害的产生与气候变化、土地利用、防洪排涝基础设施空间布局等密切相关。大量研究证实，单一依靠市政改造雨水管网的传统做法工程成本高、改造难度大，实际效果不理想。而基于自然的解决方案(Nature-based Solutions,Nb S)倡导对生态系统的最小干预，将灰蓝绿基础设施有机结合，为城市雨洪韧性规划提供了一种新思路。采用“理论研究+应用案例”相结合的方法，首先分析了将Nb S理念应用到城市雨洪韧性规划中的切入点，提出Nb S视角下城市雨洪韧性规划的一些思考；其次，应用MIKE FLOOD模型，对场地雨洪风险进行评估；最后，结合应用案例特点，以堤防岸线、水系网络、水位管理为抓手，阐述了Nb S理念在城市雨洪韧性规划中的具体方案。结果表明，Nb S视角下的城市雨洪韧性规划方案具有可行性，以期共同推动Nb S实施行动。     

Long-term versus Real-time Optimal Operation for Gate Regulation during Flood in Urban Drainage Systems

ABSTRACT

In this study, a simulation-optimization framework is built to present two approaches of long-term and real-time optimal operation for the regulation of gates during flood in urban drainage systems (UDSs). The modeling approaches are applied to a prototype network in a portion of the UDS of Tehran, Iran. A sensitivity analysis is conducted to determine the number of decision variables of the optimization models. Results are then compared with those of an uncontrolled (UC) approach as a baseline business-as-usual scenario. It has been inferred that the two optimization approaches outperform the UC method. Additionally, the real-time optimal (RTOP) operation approach, benefiting from both the current system’s state and the latest storm data, is superior to the long-term optimal (LTOP) operation approach with respect to flood mitigation efficiency. The RTOP approach has the flexibility to adapt to changes in operational conditions when part of the system’s regulation capacity is lost.     

Improved understanding of combined sewer systems using the Illinois Conveyance Analysis Program (ICAP)

Understanding the conveyance of sewer networks is vital, especially in cases of great variability in flow rates, such as in combined sanitary and storm sewer systems. Conventional conveyance studies in sewer systems often have extended computation times due to complexity of the solution, or alternatively make assumptions that ignore the water-surface profile within a pipe. In previous research, the hydraulic performance graph (HPG) was successfully used for open-channel capacity determination. The HPG summarizes the results of many backwater calculations for a reach so that these calculations do not need to be repeated. This article describes algorithms utilized by the Illinois Conveyance Analysis Program that uses the HPGs to describe the conveyance of a system, identify bottlenecks for varying conditions, conserve mass by tracking outflow and overflows under stepwise steady flow conditions. The software is freely available at https://github.com/obergshavefun/icap/wiki.     

Flow Estimation from Flap-Gate Monitoring

The measurement of flows and recording of storm events at combined sewer overflows are a key requirement in effective urban pollution management. Staff of Thames Water Utilities have developed a low-cost system based on monitoring the behaviour of flap valves on numerous major river outfalls. The paper presents the development of a modelling approach based on the principle of conservation of angular momentum and its application to experimental data for circular gates. Flow-prediction accuracies, based on the measurement of gate inclination and water depths, in the range ± 20–30%, are found to be achievable for wider gate openings (<20°) – the recommended calibration offering conservative flow predictions at lower angles.     

Experimental characterization of various scale hydraulic signatures in a flooded branched street network

ABSTRACT

Understanding urban flood flow repartition mechanisms within street network remains challenging. This paper presents an experimental dataset of water depth and velocity profiles in the Icube experimental rig. The spatial variability of discharge along a street is highlighted. Comparing the water depth gradient orientation and the flow repartition promotes for complex repartition mechanisms at small scales. Comparable unit-discharge values are observed in both large and narrow streets. Interestingly, the intensity of the discharge deviation at the crossroad scale is related with the asymmetry of the velocity profile entering a crossroad. Moreover, discharges at the crossroad and the subdistrict scales show that downscaling of the flow repartition relationship is difficult. At real scale, results show that inflow velocity profile asymmetry may persist on a distance of several street widths downstream of a crossroad. Such distances are insufficient to consider disconnected crossroads as done in the literature to establish discharge repartition relationships.     

A Fuzzy Expert System for Prioritizing Rehabilitation of Sewer Networks

Abstract: Rehabilitation of sewer networks is a huge and very costly global problem that has often been treated on a crisis‐based approach. The development of a rehabilitation program requires models and tools for assessing the condition and performance of sewers. The original contribution of this study is the development of a ranking scheme for sewer rehabilitation priorities. A fuzzy expert system was applied with inputs from a combined assessment of hydraulic, structural performance and potential failure consequences. The fuzzy structural system computes the global structural performance index for each pipe using internal condition, surrounding condition, and site vulnerability (SV) as inputs. The fuzzy hydraulic system uses hydraulic performance index (HPI), hydraulic performance impact, and SV to compute the global HPI. Finally, the fuzzy global system uses all these factors to compute the global performance index for each pipe. This methodology was successfully applied to the sewer system of the City of Laval in Canada. The results show how the fuzzy inference system may be used to establish rehabilitation priorities for each pipe section. The fuzzy expert system provides more realistic results than the intuitive approaches that use structural and hydraulic performance maximum and mean.     

Selecting a series of storm events for a model-based assessment of combined sewer overflows

The hydraulic verification of combined sewer systems as well as the assessment of combined sewer overflows (CSOs) can be conducted using a hydrodynamic model. Unfortunately, long-term simulations with hydrodynamic models for the assessment of CSOs can cause unacceptably long computation times. Using only a series of storm events instead of a precipitation continuum can reduce this time and enables parallel simulation of single storm events. We introduce a method to select this series of storm events. The method’s parameters have been optimized to replicate the overflow volume of the continuous simulation and to minimize the overall computation time. This optimization revealed a generally applicable parameter set that results in series of storm events that are shorter than the precipitation continuum by 86.2–95.2% for the investigated cases. Additionally, the deviation of overflow volume between continuous simulation and series simulation ranges between only 0.1% and 4.1%.     

Decision making under deep uncertainty for adapting urban drainage systems to change

ABSTRACT

Urbanisation and climate change are augmenting the uncertainty surrounding the future state of the world’s water resource and are resulting in cities experiencing growing levels of risk of pluvial flooding. Drainage infrastructure is generally built using the paradigm of ‘predict and optimise’; however, this approach fails to account for erroneous predictions. This can result in drainage systems delivering insufficient levels of flood protection. Irrespective of these uncertainties new drainage systems must be built, and existing ones adapted in such a way that they remain reliable. This work presents a critical analysis of the drivers of change of urban pluvial flooding and the uncertainties surrounding urban flood planning; thereby highlighting the shortcomings of current planning methodologies. Different Decision Making Under Deep Uncertainty (DMDU) frameworks are then explored and it is shown that they offer an improved ability to design reliable urban flood systems regardless of highly uncertain future conditions.