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Getting to Grips with... hydraulic drainage design

To achieve an accurate hydraulic design, it is essential to make correct assumptions about the behaviour of pipes over their lifetime

by Stuart Crisp, Business Development Director, Concrete Pipeline Systems Association (CPSA)

Accurate 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.

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

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