Costs and benefits of combined sewer overflow management strategies at the European scale

Combined sewer overflows (CSOs) may represent a significant source of pollution, but they are difficult to quantify at a large scale (e.g. regional or national), due to a lack of accessible data. In the present study, we use a large scale, 6-parameter, lumped hydrological model to perform a screening level assessment of different CSO management scenarios for the European Union and United Kingdom, considering prevention and treatment strategies. For each scenario we quantify the potential reduction of CSO volumes and duration, and estimate costs and benefits. A comparison of scenarios shows that treating CSOs before discharge in the receiving water body (e.g. by constructed wetlands) is more cost-effective than preventing CSOs. Among prevention strategies, urban greening has a benefit/cost ratio one order of magnitude higher than grey solutions, due to the several additional benefits it entails. We also estimate that real time control may bring on average a CSO volume reduction of just above 20%. In general, the design of appropriate CSO management strategies requires consideration of context-specific conditions, and is best made in the context of an integrated urban water management plan taking into account factors such as other ongoing initiatives in urban greening, the possibility to disconnect impervious surfaces from combined drainage systems, and the availability of space for grey or nature-based solutions

Optimising a Fuzzy Logic Real-Time Control System for Sewer Flooding Reduction using a Genetic Algorithm

CENTAUR aims to provide an innovative, cost effective, local autonomous data driven in-sewer flow control system whose operation will reduce urban flood risk. The system comprises of a specially designed flow control device and a wireless local water level monitoring and control system. A data driven algorithm has been developed that is able to analyse the water level data and issue instructions to the flow control device to reduce flood risk at the downstream flooding location. This Fuzzy Logic control algorithm has been linked to a SWMM model to allow virtual testing to take place and provide the basis for a Genetic Algorithm to optimise the Fuzzy Logic membership functions. Methods for generating the initial starting membership functions for input to the Genetic Algorithm have also been investigated. Results confirm that the best Genetic Algorithm optimised Fuzzy Logic controllers reduce flood volume by up to 25% depending on the timestep at which the algorithm is run and the membership function initialisation method. CENTAUR is an Innovation action funded by European Union’s (EU) Horizon 2020 research and innovation programme under grant agreement No 641931

Urban pluvial flood modelling with real time rainfall information – UK case studies

FRMRC2 (Flood Risk Management Research Consortium: Phase 2) Work package 3.6. Review paperIn order to effectively prevent, mitigate and manage urban pluvial flooding, it is necessary to accurately model and predict the spatial and temporal distribution of both rainfall and surface flooding. A number of different modelling and prediction techniques have been applied to three UK case studies. The case studies illustrated potential improvements in the duration of model simulations as well as localised rainfall estimation (downscaling). A method of describing uncertainty in flow forecasts has been illustrated. The provision of urban pluvial flood forecasts, however, remains a challenging issue and it is anticipated that a combination of several techniques may be necessary, depending on catchment size and required forecast accuracy and lead time.Defra; Environment Agency; EPSRC; Irish Office of Public Works; Scottish Rivers Agency; SNIFFE

Improving understanding of the underlying physical process of sediment wash-off from urban road surfaces

Among the urban aquatic pollutants, the most common is sediment which also acts as a transport medium for many contaminants. Hence there is an increasing interest in being able to better predict the sediment wash–off from urban surfaces. The exponential wash-off model is the most widely used method to predict the sediment wash-off. Although a number of studies proposed various modifications to the original exponential wash-off equation, these studies mostly looked into one parameter in isolation thereby ignoring the interactions between the parameters corresponding to rainfall, catchment and sediment characteristics. Hence in this study we aim (a) to investigate the effect of rainfall intensity, surface slope and initial load on wash-off load in an integrated and systematic way and (b) to subsequently improve the exponential wash-off equation focusing on the effect of the aforementioned three parameters. A series of laboratory experiments were carried out in a full-scale setup, comprising of a rainfall simulator, a 1 m 2 bituminous road surface, and a continuous wash-off measuring system. Five rainfall intensities ranging from 33 to 155 mm/h, four slopes ranging from 2 to 16% and three initial loads ranging from 50 to 200 g/m 2 were selected based on values obtained from the literature. Fine sediment with a size range of 300–600 µm was used for all of the tests. Each test was carried out for one hour with at least 9 wash-off samples per test collected. Mass balance checks were carried out for all the tests as a quality control measure to make sure that there is no significant loss of sand during the tests. Results show that the washed off sediment load at any given time is proportional to initial load for a given combination of rainfall intensity and surface slope. This indicates the importance of dedicated modelling of build-up so as to subsequently predict wash-off load. It was also observed that the maximum fraction that is washed off from the surface increases with both rainfall intensity and the surface slope. This observation leads to the second part of the study where the existing wash-off model is modified by introducing a capacity factor which defines this maximum fraction. This capacity factor is derived as a function of wash-off coefficient, making use of the correlation between the maximum fraction and the wash-off rate. Values of the modified wash-off coefficient are presented for all combinations of rainfall intensities and surface slopes, which can be transferred to other urban catchments with similar conditions

Prediction of organic combined sewer sediment release and transport

Accurate predictions of sediment loads released by sewer overflow discharges are important for being able to provide protection to vulnerable receiving waters. These predictions are sensitive to the estimated sediment characteristics and on the site-conditions of in-pipe deposit formation. Their application without a detailed analysis and understanding of the “initial conditions” under which in- sewer deposits were formed normally results in very poor estimations. In this study, in-sewer sediment samples deposited during dry-periods in a combined sewer system were collected, and their properties assessed. Parameters in the sediment transport relationship first proposed by Skipworth for in-pipe deposits were estimated based on simulating the in-pipe deposit formation conditions in laboratory erosion tests. The measured parameters were then used to simulate sediment transport through a small combined sewer network for a number of rain events for which rainfall, hydraulic and water quality data were available. Results showed that the model of Skipworth can provide good predictions of the sediment loads released from such in-sewer deposits. The experimentally-derived calibration parameters used with Skipworth’s model allowed for a realistic simulation of the in-sewer sediment behaviour and so can be used to accurately estimate the sediment load released from combined sewer systems during rainfall events

Analysis of ground-source heat pumps in north-of-England homes

YesThe performance of Ground Source Heat Pump (GSHP) systems for domestic use is an increasing area of study in the UK. This paper examines the thermal performance of three bespoke shallow horizontal GSHP systems installed in newly built residential houses in the North of England against a control house which was fitted with a standard gas boiler. A total of 350 metres of High Density Polyethylene pipe with an external diameter of 40 mm was used for each house as a heat pump loop. The study investigated (i) the performance of a single loop horizontal Ground Heat Exchanger (GHE) against a double loop GHE and (ii) rainfall effects on heat extraction by comparing a system with an infiltration trench connected to roof drainage against a system without an infiltration trench above the ground loops. Parameters monitored for a full year from October 2013 to September 2014. Using the double GHE has shown an enhanced performance of up to 20% compared with single GHE. The infiltration trench is found to improve performance of the heat pumps; the double loop GHE system with an infiltration trench had a COP 5% higher than that of the double loop GHE system without a trench

UNCERTAINTY IN SEWER SEDIMENT DEPOSIT MODELLING: DETAILED VS SIMPLIFIED MODELLING APPROACHES

The paper presents the results of a study in which the uncertainty associated with different sewer sediment modelling approaches: a detailed and simplified/parsimonious approach have been compared. Infoworks CS model coupled with a sediment transport code and a simplified sewer water quality model have been employed for the detailed and simplified modelling approach, respectively. The two approaches have been applied and compared to a single case study. The case study was selected as moderate storm events had occurred during the monitoring period. Flooding had been previously observed and this was thought to be caused by significant solids accumulation in the sewer network. The field campaign and the model simulations lasted a period of six months. The uncertainty of both modelling approaches were based on Monte Carlo based computational methods. This was a limitation for the detailed approach with regards to computational time. The simplified model approach was not affected by such a computational burden and enabled a more straightforward assessment of the uncertainty. The simplified approach, due to its structure, only provided a temporal estimate of uncertainty at the final section of the catchment. The detailed approach enabled an assessment of uncertainty at an individual pipe scale but only at the final time in the simulation period. When possible, comparison of the uncertainty estimations from both methods indicated comparable values of predicted uncertainty. Therefore a complementary use of both approaches would allow estimations of levels of uncertainty at both a spatial and temporal scale. The use of both model approaches may constitute a useful decision making tool for sewer system management

Effects of coolant flow rate, groundwater table fluctuations and infiltration of rainwater on the efficiency of heat recovery from near surface soil layers

NoThis paper aims to investigate experimentally the effects of circulating coolant flow rate, groundwater table fluctuations, infiltration of rainwater, on the amount of thermal energy that can be recovered from the near surface soil layers. A comprehensive experimental investigation was carried out on a fully equipped tank filled with sand. A heat collector panel was embedded horizontally at the mid-height of the tank. Measurements of the temperature at various points on the heat collector panel, adjacent soil, inlet and outlet were continuously monitored and recorded. After reaching a steady state, it was observed that increasing water saturation in the adjacent soil leads to a substantial increase on the amount of heat recovered. A model was proposed for the estimation of temperature along the heat collector panel based on steady state conditions. It accounted for thermal resistance between pipes and the variability of water saturation in the adjacent soils. This model showed good agreement with the data. Whilst increasing the flow rate of the circulating fluid within the panel did not cause noticeable improvement on the amount of heat energy that can be harnessed within the laminar flow regime commonly found in ground source heat panels. Infiltration of rainwater would cause a temporary enhancement on the amount of extracted heat. Measurement of the sand thermal conductivity during a cycle of drying and wetting indicates that the thermal conductivity is primarily dependent upon the degree of water saturation and secondary on the flow path