Management of the designed risk level of urban drainage system in the future: Evidence from haining city, China.

The design of urban drainage infrastructure is mainly based on historical conditions. Under global warming, more intense precipitation extremes will pose severe risk to current infrastructure. The evaluation of where and by how much design standards need to change, is urgently needed to help maintain well-functioning drainage systems. In this study, we used climate projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and InfoWorks Integrated Catchment Modeling (ICM) to simulate urban flooding. According to the latest design standard of urban drainage infrastructure, we assess the risk of future urban flooding, and evaluate the effect and benefit of drainage infrastructure adaptation measures. The results showed that, under the shared socioeconomic pathway (SSP) 5-8.5 scenario, a 35% increase in extreme rainfall would be expected. Under a 1-in-30-year precipitation event, the maximum depth would increase by 5.59%, and the withdrawal time would rise by 2.94% in the future period, relative to the baseline level. After the enlargement of drainage infrastructure in local areas, 10% pipe enlargement has a better effect to reduce risk and higher benefits than 5% pipe enlargement. These findings provide valuable insights for policymakers in enhancing the drainage system and adapting to climate change.

Real-time control of urban drainage systems using neuro-evolution.

With climate change and urbanization, existing urban drainage systems are being stressed beyond their design capacity in many parts of the world. Real-time control (RTC) can improve the performance of these systems and reduce the need for system upgrades. However, developing optimal control policies for RTC is a challenging research area due to computational demands, high uncertainties and system dynamics. This study presents a new RTC method using neuro-evolution for controlling combined sewer overflow (CSO) in urban drainage systems. Neuro-evolution is an approach to neural network research by evolutionary algorithms. Neuro-evolution realizes RTC by training the control policy in advance, thus avoiding the online optimization process in the application period. The simulation results of the benchmark Astlingen network indicate that the trained control policy outperforms the equal filling degree strategy in terms of CSO volume reduction and robustness in the face of tank level uncertainty. The performance analysis of the typical CSO events shows that the control policy mainly makes positive contributions during 'small' CSO events rather than 'large' ones. In particular, the effectiveness of the control policy in 'small' CSO events is more prominent in the initial phase of the events compared with the final phase. This work stands to support a foundation for future studies in the control of urban water systems based on neuro-evolution.

A dynamic one-dimensional model for simulating unsteady air-water stratified flow in sewer pipes.

Ventilation is paramount in sanitary and stormwater sewer systems to mitigate odor problems and avert pressure surges. Existing numerical models have constraints in practical applications in actual sewer systems due to insufficient airflow modeling or suitability only for steady-state conditions. This research endeavors to formulate a mathematical model capable of accurately simulating various operational conditions of sewer systems under the natural ventilation condition. The dynamic water flow is modeled using a shock-capturing MacCormack scheme. The dynamic airflow model amalgamates energy and momentum equations, circumventing laborious pressure iteration computations. This model utilizes friction coefficients at interfaces to enhance the description of the momentum exchange in the airflow and provide a logical explanation for air pressure. A systematic analysis indicates that this model can be easily adapted to include complex boundary conditions, facilitating its use for modeling airflow in real sewer networks. Furthermore, this research uncovers a direct correlation between the air-to-water flow rate ratio and the filling ratio under natural ventilation conditions, and an empirical formula encapsulating this relationship is derived. This finding offers insights for practical engineering applications.

Tunable linear feedback control of urban drainage systems using models defined purely from data.

Real-time and model-predictive control promises to make urban drainage systems (UDS) adaptive, coordinated, and dynamically optimal. Though early implementations are promising, existing control algorithms have drawbacks in computational expense, trust, system-level coordination, and labor cost. Linear feedback control has distinct advantages in computational expense, interpretation, and coordination. However, current methods for building linear feedback controllers require calibrated software models. Here we present an automated method for generating tunable linear feedback controllers that require only system response data. The controller design consists of three main steps: (1) estimating the network connectivity using tools for causal inference, (2) identifying a linear, time-invariant (LTI) dynamical system which approximates the network, and (3) designing and tuning a feedback controller based on the LTI urban drainage system approximation. The flooding safety, erosion prevention, and water treatment performance of the method are evaluated across 190 design storms on a separated sewer model. Strong results suggest that the system knowledge required for generating effective, safe, and tunable controllers for UDS is surprisingly basic. This method allows near-turnkey synthesis of controllers solely from sensor data or reduction of process-based models.

Classification and description of the drainage state of manholes in urban drainage systems.

Manholes are important structures in urban storm drainage systems connecting roads and underground drainage networks, and they are also an important part of the research on improving urban resistance to storm flooding. Due to cost and space constraints, most of the existing experimental data on manholes come from scale model experiments obtained by scaling according to Froude's similarity criterion, and there is a lack of validation based on full-size experimental data. This also leads to inconsistencies in the form and parameter values of the manhole flow exchange equations derived from different experiments. To remedy this deficiency, a full-scale urban drainage engineering physics model was developed in this study with the aim of investigating the flow exchange of surface water as it flows through manholes into the sewer system. Experiments were conducted under steady flow conditions and compared with predictions from the existing models. The results show that the predictions of the existing model deviate significantly from the measured values when the flow is between free weir flow and submerged orifice flow. Therefore, we constructed a weighting equation based on weir and orifice flows and found that the weighting coefficients decayed exponentially during the transition from weir to orifice flow.

Urban flood modeling with a novel coupling method of surface and sewer hydrodynamic processes.

Drainage modeling that accurately captures urban storm inundation serves as the foundation for flood warning and drainage scheduling. In this paper, we proposed a novel coupling ideology that, by integrating 2D-1D and 1D-2D unidirectional processes, overcomes the drawback of the conventional unidirectional coupling approach that fails to properly represent the rainfall surface catchment dynamics, and provides more coherent hydrological implications compared to the bidirectional coupling concept. This paper first referred to a laboratory experimental case from the literature, applied and analyzed the coupling scheme proposed in this paper and the bidirectional coupling scheme that has been widely studied in recent years, compared the two coupling solutions in terms of the resulting accuracy and applicability, and discussed their respective strengths and weaknesses to validate the reliability of the proposed method. The verified proposed coupling scheme was then applied to the modeling of a real drainage system in a region of Nanjing, China, and the results proved that the coupling mechanism proposed in this study is of practical application value.

Green infrastructure layout based on a dynamic operation feature of drainage systems.

Increasingly frequent urban floods strain the traditional grey infrastructure, overwhelming the capacity of drainage networks and causing challenges in managing stormwater. The heavy precipitation leads to flooding and damage to drainage systems. Consequently, efficient mitigation strategies for flooding have been researched deeply. Green infrastructure (GI) has proved to be effective in responding the increasing risk of flood and alleviate pressure on drainage systems. However, as the primary infrastructure of stormwater management, there is still a lack of attention to the dynamic operation feature of urban sewer systems during precipitation events. To fill this gap, we proposed a novel approach that integrates hydraulic characteristics and the topological structure of a sewer network system. This approach aims to identify influential nodes, which contribute to the connectivity of the sewer network amidst dynamic changes in inflow during precipitation events. Furthermore, we adopted rain barrels to serve as exemplars of GI, and 14 GI layout schemes are produced based on the different ranks of influential nodes. Implementing GI measures on both poorly performing and well-performing nodes can yield distinct benefits in mitigating node flooding. This approach provides a new perspective for stormwater management, establishing effective synergy between GI and the drainage system.

Prediction of the roughness coefficient for drainage pipelines with sediments using GA-BPNN.

Accurate prediction of the roughness coefficient of sediment-containing drainage pipes can help engineers optimize urban drainage systems. In this paper, the variation of the roughness coefficient of circular drainage pipes containing different thicknesses of sediments under different flows and slopes was studied by experimental measurements. Back Propagation Neural Network (BPNN) and Genetic Algorithm-Back Propagation Neural Network (GA-BPNN) were used to predict the roughness coefficient. To explore the potential of artificial neural networks to predict the roughness coefficient, a formula based on drag segmentation was established to calculate the roughness coefficient. The results show that the variation trend of the roughness coefficient with flow, hydraulic radius, and Reynolds number is consistent. With the increase of the three parameters, the roughness coefficient decreases overall. Compared to the traditional empirical formula, the BPNN model and the GA-BPNN model increased the determination factors in the testing stage by 3.47 and 3.99%, respectively, and reduced the mean absolute errors by 41.18 and 47.06%, respectively. The study provides an intelligent method for accurate prediction of sediment-containing drainage pipes roughness coefficient.

Physical modelling of energy losses at surcharged three-way junction manholes in drainage system.

Motivated by the observation that vortex flow structure was evident in the energy loss at the surcharged junction manhole due to changes of hydraulic and geometrical parameters, a physical model was used to calculate energy loss coefficients and investigate the relationship between flow structure and energy loss at the surcharged three-way junction manhole. The effects of the flow discharge ratio, the connected angle between two inflow pipes, the manhole geometry, and the downstream water depth on the energy loss were analyzed based on the quantified energy loss coefficients and the identified flow structure. Moreover, two empirical formulae for head loss coefficients were validated by the experimental data. Results indicate that the effect of flow discharge ratio and connected angle are significant, while the effect of downstream water depth is not obvious. With the increase of the lateral inflow discharge, the flow velocity distribution and vortex structure are both enhanced. It is also found that a circular manhole can reduce local energy loss when compared to a square manhole. In addition, the tested empirical formulae can reproduce the trend of total head loss coefficient.

Characteristics of sewer biofilms in aerobic rural small diameter gravity sewers.

Small diameter gravity sewers (SDGS) are extensively used to collect rural sewage as they are low in cost and quick to construct. However, the characteristics of biofilms in rural SDGS are still not clear. In this study, biofilms characteristics of aerobic rural SDGS were investigated using simulations in a lab under different flow conditions and slopes. Results indicated that the average thickness of aerobic rural SDGS biofilms was in the range of 350-650 µm, decreasing at locations with variable flow and high slopes. Protein was the most abundant substance in extracellular polymeric substance of SDGS biofilms. The most abundant bacteria, Proteobacteria, Actinobacteria, and Bacteroidetes, and functional bacteria showed different distributions when analyzed through Illumina HiSeq sequencing of 16S rRNA. The relative abundances of denitrifying bacteria, nitrite-oxidizing bacteria, and sulfate-reducing bacteria (SRB) were lower during variable flow than during stable flow. High slopes (15‰) decreased SRB presence, which could be used to mitigate H2S accumulation in aerobic SDGS. Overall, this study describes the characteristics of aerobic rural SDGS biofilms and provides valuable suggestions for the optimal design of SDGS based on these characteristics.

Experimental flow characterization in a spiral vortex drop shaft.

Connecting storm-sewers located at rather different elevations may be made with vortex drop shafts in which the energy dissipation is made by the friction between the vertical shaft and the flow and downstream by the impinging jet in a dissipation chamber. Following the first model design in the 1940s, different types of vortex drop shafts have been developed. One of the most used type is the so-called spiral vortex drop shaft developed to work in supercritical flow with good performance in both energy dissipation and space constrains. In this paper, an experimental flow characterization in a spiral vortex drop shaft is conducted covering the three main components of these structures, namely the inlet channel, the vertical shaft and the dissipation chamber. The results include measurement of water depths, pressure and velocity.

Detection of extraneous water ingress into the sewer system using tandem methods - a case study in Trondheim city.

Infiltration and inflow (I/I) of extraneous water in separate sewer systems are serious concerns in urban water management for their environmental, social and economic consequences. Effective reduction of I/I requires knowing where excess water ingress and illicit connections are located. The present study focuses on I/I detection in the foul sewer network of a catchment in Trondheim, Norway, during a period without snowmelt or groundwater infiltration. Fiber-optic distributed temperature sensing (DTS) was used for the first time in Norway to detect I/I sources in tandem with closed-circuit television inspection (CCTV) and smoke testing. DTS was an accurate and feasible method for I/I detection, though it cannot identify exact types of failure and sources of I/I. Therefore, other complementary methods must be used, e.g. CCTV or smoke testing. However, CCTV was not completely useful in confirming the DTS results. This study provides practical insights for the rehabilitation and repair of sewer networks that suffer from the undesirable I/I of extraneous water.

Influence of drainage network and compensatory techniques on urban flooding susceptibility.

Urban flooding due to accelerated urbanization and the resulting drainage problems have become a worldwide issue and the subject of several studies in recent decades. Alternative and holistic approaches such as sustainable drainage systems have been gaining prominence. Compensatory techniques represent one of these promising alternatives for managing flooding risk in the transition to regenerative urban environments. The goal of this study is to assess the effect of a drainage network together with compensatory techniques on the susceptibility to urban flooding in Campeche District. This study applies the analytical hierarchy process together with a consistency analysis, using overlapping influential parameters in three scenarios. The results show that introducing a drainage system decreases the susceptibility to urban flooding in approximately 27% of Campeche District. In general, considering the absence of a drainage network, it is concluded that its implementation together with compensatory structures provides a reduction of approximately 32% in the susceptibility to urban flooding. It should be noted that, although costly, interventions for the implementation of a drainage infrastructure associated with compensatory techniques are extremely important for disaster reduction and sustainable development.

The impact of household connection to public network wastewater systems: regulatory impact assessment.

Brazil faces a severe lack of wastewater coverage. Even in urban areas, wastewater is directly disposed of in watercourses without any treatment for a large part of the population. Although the federal, state, and local governments have invested in water and wastewater services (WWS), the expected results have not been achieved. To overcome this problem, the present paper provides an opportunity to observe an ex-ante regulatory impact assessment (RIA) as a policy tool in Brazil. The regulatory policy options will be appraised through the multiple criteria decision analysis (MCDA) according to the following objectives: (i) protect the customers with respect to social aspects; (ii) safeguard the economic, operational and infrastructure sustainability; and (iii) protect the environment. The results show that by making decisions based on evidence, policy makers should reduce the households not connected to wastewater services by 75% and for that they should incur BRL 33 million to the year 2023. Hence, the extra revenues to be obtained with these new connections are capable of making a surplus estimated as BRL 42 million for the same period. This study promotes the use of RIA as a rational, robust and transparent decision framework by the regulatory agencies worldwide.

Stormwater runoff treatment filtration system and backwashing system.

This study has been carried out to evaluate the applicability of the pilot scale hybrid type of stormwater runoff treatment system for treatment of combined sewer overflow. Also, to determine the optimum operation parameter such as coagulation dosage concentration, effectiveness of coagulant usage, surface loading rate and backwashing conditions. The pilot scale stormwater filtration system (SFS) was installed at the municipal wastewater plant serving the city of Cheongju (CWTP), Korea. CWTP has a capacity of 280,000 m3/day. The SFS consists of a hydrocyclone coagulation/flocculation with polyaluminium chloride silicate (PACS) and an upflow filter to treat combined sewer overflows. There are two modes (without PACS use and with PACS use) of operation for the SFS. In case of no coagulant use, the range of suspended solids (SS) and turbidity removal efficiency were 72.0-86.6% (mean 80.0%) and 30.9-71.1% (mean 49.3%), respectively. And, the recovery rate of filter was 79.2-83.6% (mean 81.2%); the rate of remaining solid loading in filter media was 16.4-20.8% (mean 18.8%) after backwashing. The influent turbidity, SS concentrations were 59.0-90.7 NTU (mean 72.0 NTU), 194.0-320.0 mg/L (mean 246.7 mg/L), respectively. The range of PACS dosage concentration was 6.0-7.1 mg/L (mean 6.7 mg/L). The range of SS and turbidity removal efficiency was 84.9-98.2 (mean 91.4%) and 70.7-96.3 (mean 84.0%), respectively. It was found that removal efficiency was enhanced with PACS dosage. The recovery rate of filter was 92.0-92.5% (mean 92.3%) the rate of remaining solid loading in filter media was 6.1-8.2% (mean 7.2%) after backwashing. In the case of coagulant use, the particle size of the effluent is bigger than influent particle size. The results showed that SFS with PACS use more effective than without PACS use in SS and turbidity removal efficiency and recovery rate of filter.

Decision tree (DT), generalized regression neural network (GR) and multivariate adaptive regression splines (MARS) models for sediment transport in sewer pipes.

Sediment deposition in sewers and urban drainage systems has great effect on the hydraulic capacity of the channel. In this respect, the self-cleansing concept has been widely used for sewers and urban drainage systems design. This study investigates the bed load sediment transport in sewer pipes with particular reference to the non-deposition condition in clean bed channels. Four data sets available in the literature covering wide ranges of pipe size, sediment size and sediment volumetric concentration have been utilized through applying decision tree (DT), generalized regression neural network (GR) and multivariate adaptive regression splines (MARS) techniques for modeling. The developed models have been compared with conventional regression models available in the literature. The model performance indicators, showed that DT, GR and MARS models outperform conventional regression models. Result shows that GR and MARS models are comparable in terms of calculating particle Froude number and performing better than DT. It is concluded that conventional regression models generally overestimate particle Froude number for the non-deposition condition of sediment transport, while DT, GR and MARS outputs are close to their measured counterparts.

The leakage of sewer systems and the impact on the 'black and odorous water bodies' and WWTPs in China.

China has achieved significant progress on wastewater treatment and aquatic environmental protection. However, leakage (in- and exfiltration) of sewer systems is still an issue. By using the statistical data of water and wastewater in 2016 in China, and the person loads (PLs) of water and wastewater in Singapore, the leakage fractions of hydraulic flow, organic carbon (COD), nitrogen (N) and phosphorus (P) mass loading, and in-sewer COD biological removal in the sewer systems of China (except Hong Kong, Macau and Taiwan), Shanghai, Guangzhou and Beijing were reported for the first time. The fractions of hydraulic flow infiltration (13%, Shanghai and Guangzhou) and exfiltration (39%, China) were calculated. Except Beijing, whose sewer networks are under appropriate management with small leakage fractions, the exfiltration fractions of COD (including in-sewer biological COD removal) ranged from 41% (Shanghai) to 66% (China) and averaged 55%; N ranged from 18% (Shanghai) to 48% (China) and averaged 33%; and P ranged from 23% (Shanghai and Guangzhou) to 44% (China) and averaged 30%. The exfiltrated sewage, COD, N and P not only wastes resources, but also contaminates the aquatic environment (especially groundwater) and contributes to 'black and odorous water bodies'. In- and exfiltration in the sewer network leads to low influent COD concentration, C/N ratio and high inorganic solids and inert particulate COD concentrations of many municipal wastewater treatment plants (WWTPs) causing high cost for nutrient removal, poor resource recovery, additional reactor/settler volume requirement and other operational problems. Therefore, tackling sewer leakage is of primary importance to today's environment in China. Recommendations for the inspection of sewer systems and the rehabilitation of damaged sewers as well as the development of design and operation guidelines of municipal WWTPs tailored to the specific local sewage characteristics and other conditions are proposed.

Runoff simulation of two typical urban green land types with the Stormwater Management Model (SWMM): sensitivity analysis and calibration of runoff parameters.

The characteristics of surface runoff and the infiltration properties of urban green land are important to determine the effects of runoff reduction by low-impact development (LID) facilities. In this paper, two typical types of urban green land (lawn and shrub) in Shanghai were selected to study the runoff characteristics under eight rainfall events. The sensitivity of the runoff parameters was analyzed, and then, the optimal parameters were determined using the Stormwater Management Model (SWMM). The results showed that the interception and infiltration capacities of shrub were greater than those of lawn. The rainfall intensity and rainfall pattern were the major factors that influenced the interception and infiltration of rainwater. The threshold value that generates runoff varied across the eight rainfall events ranged from 1.6 to 28.5 mm for lawn and 4.5 to 32.0 mm for shrub. The maximum reduction ratios of runoff and peak flow for shrub were 52 and 57% higher than them for lawn, respectively. The parameters for shrub were more sensitive to runoff and peak flow compared with those for lawn. Under light rainfalls with a short duration, the maximum infiltration rate and depression storage were more sensitive than those under heavy rainfalls with a long duration. Antecedent dry weather period was not found to be a sensitive parameter except for the shrub under light rainfalls. The relative errors of runoff and dynamic mean runoff (60 min) for lawn and shrub were within ± 9.5%. The errors of peak flow ranged between - 21 and 16.6%. The dynamic runoff characteristics and the parameters for lawn and shrub determined in this study can provide references for simulating urban runoff and planning LID areas.

Stormwater wetlands can function as ecological traps for urban frogs.

Around cities, natural wetlands are rapidly being destroyed and replaced with wetlands constructed to treat stormwater. Although the intended purpose of these wetlands is to manage urban stormwater, they are inhabited by wildlife that might be exposed to contaminants. These effects will be exacerbated if animals are unable to differentiate between stormwater treatment wetlands of varying quality and some function as "ecological traps" (i.e., habitats that animals prefer despite fitness being lower than in other habitats). To examine if urban stormwater wetlands can be ecological traps for frogs, we tested if survival, metamorphosis-related measures, and predator avoidance behaviors of frogs differed within mesocosms that simulated stormwater wetlands with different contaminant levels, and paired this with a natural oviposition experiment to assess breeding-site preferences. We provide the first empirical evidence that these wetlands can function as ecological traps for frogs. Tadpoles had lower survival and were less responsive to predator olfactory cues when raised in more polluted stormwater wetlands, but also reached metamorphosis earlier and at a larger size. A greater size at metamorphosis was likely a result of increased per capita food availability due to higher mortality combined with eutrophication, although other compensatory effects such as selective-mortality removing smaller individuals from low-quality mesocosms may also explain these results. Breeding adults laid comparable numbers of eggs across wetlands with high and low contaminant levels, indicating no avoidance of the former. Since stormwater treatment wetlands are often the only available aquatic habitat in urban landscapes we need to better understand how they perform as habitats to guide management decisions that mitigate their potential ecological costs. This may include improving wetland quality so that fitness is no longer compromised, preventing colonization by animals, altering the cues animals use when selecting habitats, pretreating contaminated water prior to release, providing off-line wetlands nearby, or simply not constructing stormwater treatment wetlands in sensitive areas. Our study confirms the potential for urban stormwater treatment wetlands to function as ecological traps and highlights the need for greater awareness of their prevalence and impact at landscape scales.

Numerical modeling on sediment capture in catch basins.

Catch basins are designed to convey surface runoff into sewer systems. They are also found to be effective in retaining sediments. A number of factors can influence catch basin sediment capture efficiency, such as sediment size distribution, flow hydraulics and catch basin design. To better understand the influence of these factors, numerical simulations using the Eulerian-Lagrangian method were conducted to provide insights into flow fields and to predict sediment capture rates. The numerical model was validated using previous experimental measurements of flow field and sediment capture rates for sediment sizes larger than 0.25 mm. The influence of catch basin designs, including the bottom sump and inflow arrangements, was also studied, and an equation was developed for predicting the capture rate.