Water & Wastewater Treatment Magazine
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In the know Getting to grips with... hydraulic drainage design 30 | MAY 2016 | WWT | www.wwtonline.co.uk SUPPORTED BY EXPERT SPEAKERS INCLUDE: INTEGRATED WATER RESOURCES MANAGEMENT CONFERENCE 14 SEPTEMBER 2016 | BIRMINGHAM DRIVING INTEGRATED THINKING IN WATER The 5th annual WWT Integrated Water Resources Management Conference brings together those tasked with the sustainable management of water, land and natural resources to maximise economic, social and environmental welfare. ● Learn how the water industry is implementing the WFD and managing abstraction reform ● Explore how innovative partnerships are transforming the water sector ● Discover how to build resilience to enhance cost- savings and drive sustainability BOOK NOW www.WWT-environment.net UK Rainwater Management Association David Black Senior director, Water 2020 Ofwat Jo Harrison Director of asset management United Utilities Tony Ballance Director of strategy and regulation Severn Trent Water Jean Spencer Regulation director Anglian Water Henry Leveson-Gower Head of abstraction and upstream Defra Robin Milton Chairman National Farmers' Union Luke de Vial Head of environment and resources Wessex Water WWT-IWRM-HPHad.indd 1 24/03/2016 12:50 If the flow has been predicted correctly, what other factors will impact on a pipe's capacity? Energy is lost by water in contact with a pipe's walls and other components within the pipeline system; the smoother the walls the greater the pipeline's hydraulic capacity. However, over time the build-up of slime and the accumulation of sediment can change the effective hydraulic roughness of a pipeline. This is important when you consider that wastewater pipelines are required to perform for a long time, typically for a century or more. Over this length of service life the pipeline will behave in its new condition for only a fraction of its lifespan; so it is more realistic to use a hydraulic roughness based on the occurrence of some slime and sediment, such as those used in the Sewers for Adoption document, which gives a surface roughness (Ks) of 1.5mm for foul sewers and 0.6mm for storm sewers for all pipe materials. Are there any manufacturing issues that can impact a design? It is worth checking that the internal diameter of the manufactured pipe matches the diameter used in the design calculations – the scale of the variations that can occur in some materials may come as a surprise. The impact a reduced diameter can have on a pipeline's carrying capacity can be significant. For example, if the actual diameter of a 300mm nominal diameter storm water sewer is, say, 3% less, then its carrying capacity will be reduced by 4 litres per second to 136 litres per second. Any capacity shortfall will be even more significant for larger diameter pipes. Can the pipe material impact discharge capacity? The hydraulic design of a pipeline with a circular cross section assumes the pipeline will remain circular over its service lifetime. This is not always the case for flexible pipes where ovalisation, or flattening/buckling, can occur under compaction of embedment or sustained long term loading from backfill, for example, which can reduce a pipeline's hydraulic efficiency. In the majority of rainfall events flow rates are significantly less than the maximum design value and the proportional depth of flow in pipes less than 50%. Under reduced flow conditions ovalisation means the average flow velocity will be less than if the pipe had remained circular. The reduction in flow velocity caused by ovalisation can increase the likelihood of sedimentation and detritus accumulation in a pipeline. This can be significant in areas where long dry periods exist, because this minimises opportunities for a pipeline to self-clean during a higher rainfall intensity design storm event. Ovalisation is not an issue for rigid pipes, such as precast concrete, which do not deform or lose their shape over their service life. For further info on the impact of pipe deflection see the CPSA's factsheet at www.concretepipes.co.uk For insights and show news visit utilityweeklive.co.uk INNOVATION CONTENT STREAM: For insights and show news visit utilityweeklive.co.uk DATA & ANALYTICS CONTENT STREAM: For insights and show news visit utilityweeklive.co.uk RESILIENCE CONTENT STREAM: For insights visit For insights visit For insights visit CUSTOMERS