Adrian J. Saul

Specific relationships for the first flush load in combined sewer flows

Many studies have identified the first flush phenomenon as being a relatively high load of pollutants in the initial phases of combined sewer flow. In systems without storage, this first flush of pollutants may be discharged from the system and result in the heavy pollution of the receiving watercourse. However, by the inclusion of a storage tank, this first flush can be retained and the effluent be discharged in a controlled manner. To optimise the storage volume, both the total pollutant load discharged and the temporal variation in pollutant concentration within an event need to be predicted. Sophisticated models like QSIM and MOUSETRAP to predict the pollutants in urban sewer flows are already available. However, the data requirements for these models are extensive, which usually limit their application to major or environmentally sensitive schemes. This paper describes the development of site specific regressional relationships to predict the first flush load of suspended solids in combined sewer flow and these may be used for storage tank design. Data from two sites at Great Harwood and Clayton-le-Moors in the Northwest of England has been used to develop predictive equations which relate the first flush load of suspended solids and the hydrological parameters most likely to influence sewer flow quality. A multiple stepwise linear regression technique has been utilised for this purpose. The maximum rainfall intensity, maximum inflow, rainfall duration and the antecedent dry weather period were found to be the most important parameters influencing the first flush load of suspended solids. The equations were verified using an independent set of data and gave good predictions of the first flush load for the sites considered. This study has the limitation that the equations are catchment specific. However, as more data for different catchments becomes available, it may be possible to establish standard coefficients for application to a wide range of catchment conditions.

Rehabilitation strategies for water distribution networks: a literature review with a UK perspective

Abstract Primarily, a rehabilitation strategy should aim to satisfy the regulatory requirements set down in respect of water distribution network operation. However, water companies in the UK have come to recognise that the business needs associated with the improvement of the deteriorating fabric of their distribution networks extend beyond these requirements. Extra economy can be gained by operating the networks efficiently based on a rehabilitation strategy which considers the associated costs over an extended period. Economic, hydraulic, reliability and water quality performance criteria must be optimised as part of an effective strategy. Numerous rehabilitation decision making approaches have been presented. However, many have adopted flawed economic approaches and have been based inadequately on one or two selected performance criteria. Few models have considered the extended planning horizons associated with a whole-life costing approach to this problem. However, the multi-objective optimisation approaches which have been developed recently have the potential to be developed into the required whole-life costing model based on the appropriate economic model and performance criteria.

Laboratory studies investigating the processes leading to discolouration in water distribution networks.

Results are reported from laboratory experiments conducted to investigate the processes of discolouration within a water distribution system and specifically the concepts underpinning an empirical model proposed by Boxall et al. [Boxall, J.B., Saul, A.J., Skipworth, P.J., 2001. A novel approach to modelling sediment movement in distribution mains based on particle characteristics. Water Software Systems 1, 263-273.] and field validated by Boxall and Saul [Boxall, J.B., Saul, A.J., 2005. Modelling discolouration in potable water distribution systems. Journal of Environmental Engineering ASCE 131(5).]. The model is based on the hypothesis that discolouration is caused by the erosion and transport of fine particles, typically dominated by iron and manganese in the UK, that are attached to the pipe walls of the system by forces in addition to self-weight. These particles display cohesive-like properties and build up in layers on the pipe wall, conditioned by the usual daily flow patterns within the system. Discolouration events are caused by erosion of these layers due to changes in the system hydraulics and specifically changes in shear stress at the pipe wall, for example due to change in demand, a burst or the opening of a fire hydrant. Once cleaned from the pipe walls the layers re-accumulate under the usual conditions within the system. Experiments to determine cohesive layer behaviour and strength characteristics involved development periods followed by the measurement of the resultant discolouration when accumulated material was eroded by an increase in pipe-wall shear stress. The results support the empirical model concepts and hence its application. The results also suggest that the generation of material layers is influenced by the range of daily flow patterns, with greater variability reducing material accumulation, but not by the magnitude of steady state hydraulic conditions.

Experimental and numerical investigation of interactions between above and below ground drainage systems

This paper presents the results of the experimental and numerical investigation of interactions between surface flood flow in urban areas and the flow in below ground drainage systems (sewer pipes and manholes). An experimental rig has been set up at the Water Engineering Laboratory at the University of Sheffield. It consists of a full scale gully structure with inlet grating, which connects the 8 m(2) surface area with the pipe underneath that can function as an outfall and is also further connected to a tank so that it can come under surcharging conditions and cause outflow from the gully. A three-dimensional CFD (Computational Fluid Dynamics) model has been set up to investigate the hydraulic performance of this type of gully inlet during the interactions between surface flood flow and surcharged pipe flow. Preliminary results show that the numerical model can replicate various complex 3D flow features observed in laboratory conditions. This agreement is overall better in the case of water entering the gully than for the outflow conditions. The influence of the surface transverse slope on flow characteristics has been demonstrated. It is shown that re-circulation zones can form downstream from the gully. The number and size of these zones is influenced by the transverse terrain slope.

Sewer system operation into the 21st century, study of selected responses from a UK perspective

The present study examined the future of UK sewer systems at timescales, up to 2020 and to 2080. The most significant drivers for change were identified for different socio-economic and climate scenarios. A wide range of potential responses to these drivers were studied using the analysis of literature, expert workshops and limited modelling. The assessment of effectiveness of a number of these responses are presented and discussed. In the short to medium term it was shown that technological responses were most likely to be effective. In the longer-term controls on urbanisation and land use were seen to be more effective.

Comparing quantitative precipitation forecast methods for prediction of sewer flows in a small urban area

Abstract Due to the relatively small spatial scale, as well as rapid response, of urban drainage systems, the use of quantitative rainfall forecasts for providing quantitative flow and depth predictions is a challenging task. Such predictions are important when consideration is given to urban pluvial flooding and receiving water quality, and it is worthwhile to investigate the potential for improved forecasting. In this study, three quantitative precipitation forecast methods of increasing complexity were compared and used to create quantitative forecasts of sewer flows 0–3 h ahead in the centre of a small town in the north of England. The HyRaTrac radar nowcast model was employed, as well as two different versions of the more complex STEPS model. The STEPS model was used as a deterministic nowcasting system, and was also blended with the Numerical Weather Prediction (NWP) model MM5 to investigate the potential of increasing forecast lead-times (LTs) using high-resolution NWP. Predictive LTs between 15 and 90 min gave acceptable results, but were a function of the event type. It was concluded that higher resolution rainfall estimation as well as nowcasts are needed for prediction of both local pluvial flooding and combined sewer overflow spill events. Editor D. Koutsoyiannis; Guest editor R.J. Moore Citation Schellart, A., Liguori, S., Krämer, S., Saul, A., and Rico-Ramirez, M.A., 2014. Comparing quantitative precipitation forecast methods for prediction of sewer flows in a small urban area. Hydrological Sciences Journal, 59 (7), 1418–1436. http://dx.doi.org/10.1080/02626667.2014.920505

The first foul flush in combined sewers: an investigation of the causes

A previously developed numerical model, able to simulate the erosion from an organic in-pipe deposit, was used to investigate the physical parameters that control the first foul flush in combined sewers. Systematic adjustment of physical parameters indicated that the accurate characterisation of bed properties, i.e. the surface erosional strength and its variation with depth, was more important than the accurate description of the imposed hydraulic conditions.

Determining maintenance requirements of a water distribution network using whole life costing

A whole life costing (WLC) methodology has been developed for determining long term maintenance expenditure requirements for water distribution networks. The methodology utilises an accounting scheme that ties the costs incurred by the operator and other stakeholders to the attributes or performance that drive the costs. It has specifically been derived with the requirements placed by the regulatory regime on the water companies that operate in England and Wales in mind. Expenditure constraints are implied by the regulator through price caps that companies can charge their customers. Appropriate levels of expenditures included as part of the price cap determinations are required by the regulator to be economically robust and tied to the service received by the customers. Therefore, maintenance decisions must reflect more immediate concerns of meeting performance requirements, but must ensure that such levels are sustainable in the long term. The WLC methodology achieves this through an integrated platform that links costs identified within a structured accounting scheme with their performance based drivers commonly modelled based on historical data. Thus, a robust and fully auditable methodology is provided that can address the requirements of all stakeholders. This methodology is the basis for software (WiLCO) that provides decision support in determining appropriate pipe rehabilitation and operational strategy and thus expenditure levels over extended time horizons.

Predicting combined sewer overflows chamber depth using artificial neural networks with rainfall radar data

Combined sewer overflows (CSOs) represent a common feature in combined urban drainage systems and are used to discharge excess water to the environment during heavy storms. To better understand the performance of CSOs, the UK water industry has installed a large number of monitoring systems that provide data for these assets. This paper presents research into the prediction of the hydraulic performance of CSOs using artificial neural networks (ANN) as an alternative to hydraulic models. Previous work has explored using an ANN model for the prediction of chamber depth using time series for depth and rain gauge data. Rainfall intensity data that can be provided by rainfall radar devices can be used to improve on this approach. Results are presented using real data from a CSO for a catchment in the North of England, UK. An ANN model trained with the pseudo-inverse rule was shown to be capable of predicting CSO depth with less than 5% error for predictions more than 1 hour ahead for unseen data. Such predictive approaches are important to the future management of combined sewer systems.

GIS Water-Balance Approach to Support Surface Water Flood-Risk Management

Controversy has arisen as to whether the lack of appropriate consideration to surface water flood risk in urban spatial planning is reducing the capacity to manage urban flood risk. A screening tool is required which would allow spatial planners to identify potential surface water flood risks and explore their management opportunities. An urban water balance approach is presented. The hypothesis is that key hydrological characteristics, storage volume and location, flow paths, and surface water generation, capture the key processes responsible for surface water flooding. The model is assembled and run by using ESRI ArcGIS software. Surface sinks and their catchment areas are identified by using a Lidar DEM. Excess surface water is calculated by using a runoff coefficient that is applied to rainfall volumes, and no other losses are considered. A surface water accumulation module sums the excess surface water from the catchment area of each sink. A sensitivity analysis of model assumptions demonstrates that...