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.     

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.     

Modelling sewer discharge via displacement of manhole covers during flood events using 1D/2D SIPSON/P-DWave dual drainage simulations

In urban areas, overloaded sewers may result in surcharge that causes surface flooding. The overflow from sewer systems mainly starts at the inlets until the pressure head in the manhole is high enough to lift up its cover, at which stage the surcharged flow may be discharged via the gap between the bottom of the manhole cover and the ground surface. In this paper, we propose a new approach to simulate such a dynamic between the sewer and the surface flow in coupled surface and sewer flow modelling. Two case studies are employed to demonstrate the differences between the new linking model and the traditional model that simplifies the process. The results show that the new approach is capable of describing the physical phenomena when manhole covers restrict the drainage flow from the surface to the sewer network and reduce the surcharge flow and vice versa.     

Validation of 2D shock capturing flood models around a surcharging manhole

This work offers a detailed validation of finite volume (FV) flood models in the case where horizontal floodplain flow is affected by sewer surcharge flow via a manhole. The FV numerical solution of the 2D shallow water equations is considered based on two approximate Riemann solvers, HLLC and Roe, on both quadrilateral structured and triangular unstructured mesh-types. The models are validated against a high resolution experimental data-set obtained using a physical model of a sewer system linked to a floodplain via a manhole. It was verified that the sensitivity of the models is inversely proportional to the surcharged flow/surface inflow ratio, and therefore requires more calibration from the user especially when concerned with localised modelling of sewer-to-floodplain flow. Our findings provide novel evidence that shock capturing FV-based flood models are applicable to simulate localised sewer-to-floodplain flow interaction.     

蜗形滞流器应用条件试验与模拟

为明确蜗形滞流器在排水管网中的应用条件,采用物理模型进行试验,探究蜗形滞流器发挥有效滞流作用的最小压力水头,并建立SWMM雨洪模型,模拟研究加载蜗形滞流器后在不同重现期(3、5、10、20、50年)以及不同降雨历时(1、2和3 h)下研究区域的滞水改善情况。结果表明:蜗形滞流器的滞流效果与相对水头有关,型号为D200R300的蜗形滞流器产生滞流作用的最小相对水头在15 cm左右;总滞水量和滞水时间削减率随着重现期的增大有所减少,且蜗形滞流器对重现期≤10年的降雨具有更好的雨水调控作用,但受降雨历时的影响较小;蜗形滞流器更适宜安装在汇流后下游过水能力不足的管段,在不同降雨条件下削减率能保持在60%以上,最大可达到100%。     

Multi-objective evolutionary optimization for greywater reuse in municipal sewer systems

Sustainable design and implementation of greywater reuse (GWR) has to achieve an optimum compromise between costs and potable water demand reduction. Studies show that GWR is an efficient tool for reducing potable water demand. This study presents a multi-objective optimization model for estimating the optimal distribution of different types of GWR homes in an existing municipal sewer system. Six types of GWR homes were examined. The model constrains the momentary wastewater (WW) velocity in the sewer pipes (which is responsible for solids movement). The objective functions in the optimization model are the total WW flow at the outlet of the neighborhoods sewer system and the cost of the on-site GWR treatment system. The optimization routing was achieved by an evolutionary multi-objective optimization coupled with hydrodynamic simulations of a representative sewer system of a neighborhood located at the coast of Israel. The two non-dominated best solutions selected were the ones having either the smallest WW flow discharged at the outlet of the neighborhood sewer system or the lowest daily cost. In both solutions most of the GWR types chosen were the types resulting with the smallest water usage. This lead to only a small difference between the two best solutions, regarding the diurnal patterns of the WW flows at the outlet of the neighborhood sewer system. However, in the upstream link a substantial difference was depicted between the diurnal patterns. This difference occurred since to the upstream links only few homes, implementing the same type of GWR, discharge their WW, and in each solution a different type of GWR was implemented in these upstream homes. To the best of our knowledge this is the first multi-objective optimization model aimed at quantitatively trading off the cost of local/onsite GW spatially distributed reuse treatments, and the total amount of WW flow discharged into the municipal sewer system under unsteady flow conditions.     

城市排水管网明满过渡流模型的研究及应用

通过引入Preissmann窄缝假定,采用统一的控制方程组模拟排水管网无压和有压流.实现了排水管网内明满过渡流的同时模拟.利用Preissmann四点加权隐格式对所建方程组进行离散,并采用"管道-汊点-管道"的三级联解法对其进行求解.通过对环状管网明满过渡流和长沙雅华小区的雨水管网进行模拟计算,应用文献资料或实测资料进行验证分析,可以看出模拟结果和文献的计算值及实测值吻合较好,表明所建模型可以简洁有效地对排水管网明满过渡流进行模拟,为排水管网的设计和管理提供了一定的理论支持,具有实际应用价值.     

A model to study the effects of time-variable pollutant loads on stream quality

A deterministic computer model has been proposed to study the effects of discharges from combined sewer drainage areas on receiving bodies of water. The model provides capability for studying the effects of varying treatment plant capacity, interceptor sewer capacity and storage on stream quality. The operating parameters for the storage vessel are defined in detail. A cost function for all system components is included in the model.

The variation of total mass phosphate discharge, peak instantaneous phosphate concentration and cost with the variation of the various system parameters is reported for a series of model runs on three drainage areas.     

Uncertainty in urban stormwater quality modelling: the effect of acceptability threshold in the GLUE methodology

Uncertainty analysis in integrated urban drainage modelling is of growing importance in the field of water quality. However, only few studies deal with uncertainty quantification in urban drainage modelling; furthermore, the few existing studies mainly focus on quantitative sewer flow modelling rather than uncertainty in water quality aspects. In this context, the generalised likelihood uncertainty estimation (GLUE) methodology was applied for the evaluation of the uncertainty of an integrated urban drainage model and some of its subjective hypotheses have been explored. More specifically, the influence of the subjective choice of the acceptability threshold has been detected in order to gain insights regarding its effect on the model results. The model has been applied to the Savena case study (Bologna, Italy) where water quality and quantity data were available. The model results show a strong influence of the acceptability threshold selection and confirm the importance of modeller's experience in the application of GLUE uncertainty analysis.     

南昌市暴雨积涝仿真系统研究

在南昌市排水管网信息的基础上,研制了城市积涝的仿真模型。模型以平面二维非恒定流的基本方程和无结构不规划网格划分技术为骨架,结合一维非恒定流方程的算法,建立了南昌市城市内涝仿真模拟系统。以2003年6月一次特大暴雨过程的模拟,取得较好效果,模拟的积水网格中,误差基本在20cm以内,预报结果能满足市政排水部门的要求。     

Effect of drainage conditions on porewater pressure distributions and lining stresses in drained tunnels

This study deals with the effect of drainage conditions on porewater pressure distributions and lining stresses in drained tunnels. Firstly, simple closed-form analytical solutions for the steady-state porewater pressure are re-derived within a common theoretical framework for two different boundary conditions (one for zero water pressure and the other for a constant total head) along the tunnel circumference by using the conformal mapping technique. The difference in porewater pressure distributions among the analytical solutions is investigated. The numerical simulation of a drained circular tunnel under the steady-state groundwater flow condition is made to investigate the effect of porewater pressure distributions on lining stresses. Secondly, the case study of the planned South Blue Line Extension subway tunnel under the steady-state groundwater flow condition is performed for four different drainage conditions (sealed, fully-drained with zero water pressure or a constant total head, and invert only-drained) along the tunnel circumference. The effects of different drainage conditions on porewater pressure distributions, flow nets, and lining stresses are investigated.     

Design of a monitoring network and assessment of the pollution on the Lerma river and its tributaries by wastewaters disposal

While the 2005 progress report of the United Nations Millennium Development Goals stresses out the need of a dramatic increase in investment to meet the sanitation target in the third world, it is important to anticipate about some parallel negative impacts that may have this optimistic programme (extension of sewer networks without sufficient treatment works). Research was initiated on Lerma River (Mexico), subjected to many rejects disposal, to design a monitoring network and evaluate the impact of wastewaters on its water quality. The discharges was inventorized, geo-positioned with a GPS and mapped, while the physico-chemical characteristics of the river water, its tributaries and main rejects were evaluated. Microtox system was used as an additional screening tool. Along the 60 km of the High Course of Lerma River (HCLR), 51 discharges, with a diameter or width larger than 0.3 m (including 7 small tributaries) were identified. Based on the inventory, a monitoring network of 21 sampling stations in the river and 13 in the important discharges (>2 m) was proposed. A great similitude was found between the average characteristics of the discharges and the river itself, in both the wet and dry seasons. Oxygen was found exhausted (<0.5 mg/L) almost all along the high course of the river, with COD and TDS average levels of 390 and 1980 mg/L in the dry season, against 150 and 400 mg/L in the wet season. In the dry season, almost all the sites along the river revealed some toxicity to the bacteria test species (2.9 to 150 TU, with an average of 27 TU). Same septic conditions and toxicity levels were observed in many of the discharges. Four of the six evaluated tributaries, as well as the lagoon (origin of the river), were relatively in better conditions (2 to 8 mg/L D.O., TU<1) than for the Lerma, acting as diluents and renewal of the HCLR flow rate. The river was shown to be quite a main sewer collector. The high surface water contamination by untreated wastewaters that is depicted in this research should be taken into account in the Millennium Goals strategies, by promoting treatment plan works simultaneously, when sewer networks in the third world would extend.     

Estimating flow rates through individual outlets of siphonic roof drainage systems

An experimental investigation is used to quantify the flow rates through individual outlets of multi-outlet siphonic roof drainage systems under both full-pipe and partially filled pipe flow conditions. A pressure transducer and a propeller-type current meter were installed within each tailpipe to measure water depth and flow velocity. The study tests the hypothesis that it may be possible to predict accurately the flow rates through individual outlets of multi-outlet siphonic roof drainage systems. A new technique of estimating flow rates is trialled by comparing instantaneous pressure transducer and current meter readings with previously calibrated flow data to find the best data match. The study results were very positive and clearly demonstrate that the underlying methodology was appropriate and that it may be possible to model numerically the individual outlet flow rates in multi-outlet siphonic roof drainage systems.     

Characteristics of combined sewer overflows in Shanghai and selection of drainage systems

The overflow frequencies of combined sewer systems (CSSs) along Suzhou Creek in Shanghai are definitely lower compared with low interception ratio and design expectation. This noteworthy phenomenon was analysed and the results indicate that large sewer storage capacity due to a dense interconnection between sewer systems, flat pipe slope and pump drainage pattern, as well as the spatial–temporal difference of rainfall on a large catchment that is integrated by the connected network, lead to the overflow frequencies of combined sewer overflows (CSOs) along Suzhou Creek that are much less than those of designed and of Japanese counterparts. The drainage hydraulic model was used to simulate the performance of typical CSSs in Shanghai centre area to further explain the phenomenon. Meanwhile, as separate systems have serious illicit connections and are subject to heavy wet weather pollution in Shanghai, separate systems did not show the expected advantage in nonpoint pollution control. Thus, keeping old CSSs in the Shanghai downtown area seems to be of great value with respect to the control of urban non-point pollution.     

监测技术在排水管网运行管理中的应用及分析

针对排水管网溢流造成的城市污染问题,对监测技术在城市排水管网运行管理过程中的应用模式进行了探讨和案例分析,对监测数据的分析方法和监测数据支持下的模型动态模拟技术进行了研究。案例分析表明,通过对监测数据的分析和模拟可全面掌握整个排水管网系统内各个管道的水力负荷和水质状况,利用监测数据与模型的紧密集成使用的方法,可提高模型预测的可靠性和城市排水管网管理的科学性与高效性。     

Grey-box modelling of pollutant loads from a sewer system

Using a compact measuring unit with on-line meters for UV absorption and turbidity, it is possible to determine concentrations of organic load (chemical oxygen demand (COD) and suspended solids (SS)) anywhere in a sewer system. When measurements of the flow are available as well, the pollutant mass flow at the measuring point can be calculated.The measured data are used to estimate different models describing the load of pollutants in the sewer. A comparison of the models shows that a grey-box model is most informative and best in terms measured by the multiple correlation coefficient. The grey-box model is a state-space model, where the state represents the actual amount of deposition in the sewer, and the output from the model is the pollutant mass flow to the wastewater treatment plant (WWTP). The model is formulated by means of stochastic differential equations. Harmonic functions are used to describe the dry weather diurnal load profiles. It is found that the accumulation of deposits in the sewer depends on previous rain events and flows.By means of on-line use of the grey-box models, it is possible to predict the amount of pollutants in a first flush at any time, and hence from the capacity of the plant to decide if and when the available detention basin is to be used for storage of wastewater. The mass flow models comprise an important improvement of the integrated control of sewer and WWTP including control of equalisation basins in the sewer system. Further improvements are expected by the introduction of an additive model where dry weather situations and storm situations are modelled separately before addition to the resulting model.     

基于暴雨雨型的设计径流量研究及应用

为简化排水模型在城市排水防涝规划中的应用,利用武汉短历时暴雨雨型,采用三角概化法推导出雨型径流量计算公式。通过武汉雨型径流量设计程序在9个排水区的应用,在相同重现期标准下与武汉暴雨强度公式进行对比,同频率雨型设计径流量的平均绝对值相对误差为1.29%,最大误差<5%。采用相关因子拟合公式计算排水区节点雨水汇流时间,可建立雨型径流量节点管控简化模型,无需排水管网建模,可实现城市排水防涝规划水位管控,也可进行雨水溢流污染控制调节池容量的计算。     

A Bayesian network model to assess the public health risk associated with wet weather sewer overflows discharging into waterways

Overflows from sanitary sewers during wet weather, which occur when the hydraulic capacity of the sewer system is exceeded, are considered a potential threat to the ecological and public health of the waterways which receive these overflows. As a result, water retailers in Australia and internationally commit significant resources to manage and abate sewer overflows. However, whilst some studies have contributed to an increased understanding of the impacts and risks associated with these events, they are relatively few in number and there still is a general lack of knowledge in this area. A Bayesian network model to assess the public health risk associated with wet weather sewer overflows is presented in this paper. The Bayesian network approach is shown to provide significant benefits in the assessment of public health risks associated with wet weather sewer overflows. In particular, the ability for the model to account for the uncertainty inherent in sewer overflow events and subsequent impacts through the use of probabilities is a valuable function. In addition, the paper highlights the benefits of the probabilistic inference function of the Bayesian network in prioritising management options to minimise public health risks associated with sewer overflows.     

Production and transport of urban wet weather pollution in combined sewer systems: the “Marais” experimental urban catchment in Paris

An experimental catchment area was set up in the centre of Paris (France) so as to follow up the quality of wet weather flows from the entry to the exit of a combined sewer network. The distinctive characteristic of this site is its location in a town centre and the extent of the equipment used to monitor the water pollution over the whole length of its course through the catchment area. The results obtained show a change in quality between the runoff entering the sewer network and the combined storm water flow at the sewer's outlet, which cannot be explained only by the mixture with domestic wastewater. In particular, an increase was observed in the concentrations of suspended solids (SS), VSS, COD, BOD and Cu, in the proportion of pollutants linked to particles and in the characteristics of the particles. A calculation of the total masses going in and out of the sewer network during a rainfall event shows that the erosion of in-sewer pollution stocks is the main source of particles and of organic matter in wet weather flows, whereas heavy metals loads originated from roof runoff, due to the corrosion of metallic roofs. Particles eroded from the sewer sediments during rain events were found to be quite different from the particles of type A deposits and organic biofilms. Nevertheless, they have mean organic and metallic loads that are of the same order of magnitude as the particles of the organic layer at water sediment interface. A change in the chemical form of heavy metals was noticed during the transport in the sewer and it is suspected that a fraction of the dissolved metals from the runoff is adsorbed on sewer sediments.