Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | MAY 2016 | 29 Stuart CriSp Business DevelopMenT DirecTor concreTe pipeline sYsTeMs AssociATion (cpsA) Drainage decisions To achieve an accurate hydraulic design, it is essential to make correct assumptions about the behaviour of pipes over their lifetime In the know Getting to grips with... hydraulic drainage design concrete pipes avoid the risk of ovalisation, which can be a risk in drainage systems A ccurate hydraulic analysis is fundamental in determining a drainage pipeline's reliability and safety by ensuring water flows through a pipeline in the manner intended. The analysis will determine the optimum configuration of that system; it will influence the choice of pipe material and installation methods; and, from a business perspective, it will help optimise capital expenditure. If a system is not constructed in line with the hydraulic analysis, then the pipeline could fail to perform as intended, which could result in damage to both systems and property. are hydraulic design considerations the same for storm sewers and foul sewers? Similar criteria must be considered with the design of both storm and foul sewers. The use of Sustainable Drainage Systems (SuDS) is becoming standard practice throughout the UK for managing surface water flows and as such, the trend in the design of pipeline systems for storm water management in new construction is increasingly for underground storage and flow attenuation rather than as a mechanism for moving runoff from one location to another. Design criteria for pipeline systems include: lengths of drainage pipe and connection / outfall positions, average and peak flows and their duration, the depth of the sewer below ground, topographical or structural features, surface features and access to the sewer for inspection and maintenance. How is the rate of flow estimated in storm sewer design? The rate of flow of water can be estimated by applying one of the traditional methods such as the Lloyd-Davies or 'Rational' method, which was modified by Transport and Road Research Laboratory and widely used in the UK for many years. More recently the Wallingford Procedure was introduced by the Hydraulics Research Station, now HR Wallingford. This incorporates sophisticated computer programs that take into account catchment geography, predicted rainfall intensity, return period and duration of storms, nature of the soil and the percentage of impermeable area take up by roads, roofs and pavements. Is the rate of flow in foul sewer designs estimated in a similar way? Traditionally the volume of flow in foul sewers has been calculated using the general rule of thumb equation of four times the dry weather flow for a new sewer with sound joints, or six times the flow when joints are less robust and infiltration might be expected. More recently, domestic flow according to Sewers for Adoption has been based on 4000 litres per dwelling per day. Foul sewage from industrial sources should be assessed taking into account the type and use of the property in discussion with the local authority.