Notice: This blog piece was created prior to the formation of the British Precast Drainage Association.

Posted by & filed under Design.

Reliable estimation of the hydraulic performance of pipeline systems is one of the most important aspects of drainage design. There are many determining factors and many debates arguing the case for different pipe materials and configurations. To achieve hydraulic analysis accuracy, it is imperative that the designer has the correct design information and makes appropriate assumptions regarding the behaviour of the pipeline system over its operating lifetime.

Here are just a few of the key hydraulic drainage design criteria that need to be considered:-

Size, shape and profile.

The nominal size of the conduit assumed for design purposes may vary from the actual dimensions of the manufactured product.  Check that the nominal diameter of the section of pipeline under consideration is actually available at the size being considered in the design.  Be careful not to specify a pipe nominal diameter that is greater than the available pipe size. For example, according to Hydraulics Research Design Tables based on Colebrook-White equation for pipes flowing full and using a hydraulic roughness Ks = 0.6mm for storm water sewers, a DN300 pipe laid at a gradient of 1:60 will have a reduction in capacity from 140 l/s to 136 l/s if the internal diameter is 291mm, i.e 3% below the 300mm nominal internal diameter. For larger diameter pipes the shortfall may become more significant, particularly if compounded with other capacity-reducing factors. Check out the manufacturing tolerances of different pipe materials and product designs. You may be surprised at what you find.

This may seem obvious, but it is usually overlooked. The hydraulic design of circular cross section pipeline systems assumes that the pipe maintains its circular cross section shape over its service lifetime. The ovalisation of flexible pipes (where the vertical diameter reduces compared with the horizontal diameter) will reduce the hydraulic efficiency of the pipeline. For example, in storm water sewers the majority of rainfall events will lead to flows within pipes at less than 50% proportional depth. An ovalised pipe will lead to lower average pipeline velocities at a specific flow rate compared to the original circular profile. This may lead to greater risk of sedimentation and accumulation of detritus within the pipeline and may be of particular relevance in areas where (a) extended dry periods exist, (b) high intensity rainfall at or close to the design value, where self-cleansing velocities are achieved, is encountered on an infrequent basis or (c) where the sediment entering the system is significant. The fact that rigid pipes such as concrete do not deform or lose their shape during their service life also means that the hydraulic performance of the cross section used in the original hydraulic design is preserved without compromise.  Use your design software to determine the reduction in hydraulic performance of ovalised flexible plastic pipes at 5% (which is allowable in many client authorities) and add this effect to any variance in nominal diameter.

It is also important that pipelines maintain their longitudinal straightness to ensure optimum hydraulic efficiency. They should be constructed with a proper understanding of the ground conditions into which they will be buried, the loads applied to them and provided with sufficient support to ensure that their profile does not waver with localized dips and high points throughout their length. If pipelines deviate out of alignment, additional head losses can result within the system. Flexible pipes have low resistance to bending and can be vulnerable to ground movement over their length.

Hydraulic roughness

The hydraulic capacity of a pipeline depends on the gravitational energy lost due to friction as water flows over the surface of the wall of the pipe.  It is therefore unsurprising that much attention is focused on the smoothness of the parent material of the pipe that makes up the interior pipe wall. The smoother the wall, the greater the hydraulic capacity of the pipeline system, assuming

(a) Any comparison between different pipe materials takes into account other factors such as variance against nominal diameter, consistency of cross sectional shape over the operating life of the pipeline and longitudinal profile.

(b) That the walls of the pipe remain unaffected by the build up of slime or accumulation of sediment that would change the effective hydraulic roughness of the pipe surface.

Wastewater pipelines are required to perform over a very long period, typically in excess of 100 years.  Over its service life, the pipeline will only behave in an “as new” state for a short period, even assuming regular and frequent maintenance.  For this reason, it can be misleading to base hydraulic designs on the “as new” condition using the hydraulic roughness value for the pipe material.  It would be far wiser to use a practical hydraulic roughness value based on a degree of sliming and siltation, such as those used in Sewers for Adoption (Ks=1.5mm for foul sewers; Ks=0.6mm for storm water sewers) for all pipe materials.

Additionally, many flexible sewer pipes incorporate a “ribbed” structure to the outer wall of the pipe. As a consequence of the manufacturing process, the outer ribbed surface of the pipe will often be reflected to some degree on the inner wall of the pipe resulting in a corrugated internal surface. Accordingly, the hydraulic roughness of structured wall (ribbed) plastic pipes can be greater than that of smooth bore pipes, leading to reduced capacity of the pipeline system.

When using concrete pipes, the risks associated with size tolerance, consistency of cross sectional shape, longitudinal profile and corrugated internal bore may be avoided. Clients, designers and installers should understand these basic hydraulic design factors so that they can more accurately determine the long term hydraulic performance of the pipeline system.

For more information on Hydraulic Design please visit the CPSA site.

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