Hydraulic performance

Hydraulic Roughness

In the equation of Colebrook and White, the velocity of flow is dependent on the pipe bore, the kinematic viscosity, the gradient and the surface roughness of the interior of the pipeline. This roughness (ks) is dependent upon in the following factors:

  • Length of pipes
  • Surface texture of pipes
  • Types of joints
  • Deposited grit on the invert
  • Adherent slime and grease on the walls
  • Deviation from nominal circular cross-section
  • Deviation from longitudinal straightness

Each factor has its own effect on the apparent roughness and hence the flow in the pipeline. When a drain or sewer is conveying sewage or a combination of sewage and surface water, slime, mostly microbiological, adheres to and grows on the interior surface. After a short period of time a film of slime coats the surface of the pipeline covered by the flow. This slime blankets most effects of the other factors affecting the hydraulic roughness of rigid pipes. The same case applies where grit is deposited on the invert of a surface water sewer. In pipelines of flexible materials, the cross sectional area of the pipeline can be reduced by the fill and surcharges loads. This will have an effect on the apparent hydraulic roughness and will be additional to the effect of the slime.

Hydraulic Experiments

The Clay Pipe Development Association conducted many experiments to determine the hydraulic roughness of various pipelines; firstly with clean water at Messrs. Wimpey’s Central Hydraulic Research Laboratory at Hayes, Middlesex, and then with sewage at the Water Research Centre (Stevenage Laboratory). Further tests with clean water were carried out by the Hydraulics Research Station, Wallingford and were followed by tests using sewage.

Clean Water Tests

The ks values determined at Wimpey’s and the Hydraulic Research Station for clean vitrified clay pipes of three interior surface finishes, flowing full, over the range of discharges for which tests were carried out, are as shown in the table.

Roughness values from clean water tests

Nominal Bore (mm) Interior Finish Joint ks (mm)
100 Salt Glazed


Ceramic Glazed







150 Ceramic Glazed



‘O’ Ring


‘O’ Ring




300 Unglazed ‘O’ Ring 0.060

Foul Water Tests

When drains and sewers are in service, they carry foul not clean water. Therefore, the hydraulic roughness values derived from experiments with clean pipes are not appropriate for the hydraulic design of foul and surface water drains and sewers.

The Clay Pipe Development Association sponsored research at the Water Research Centre (Stevenage Laboratory) to determine the amount of slime that adheres to the interior surface of pipes made of various materials. The materials tested were: vitrified clay with salt glaze, unglazed and ceramic glazed interior finishes; asbestos cement; pitch fibre and pvc pipes. A continuous flow of domestic sewage was passed through them for six weeks. The pipes were flowing half full.

The results showed that the growth and weight of slime were statistically independent of the pipe material, but dependent upon the velocity of the sewage flow.

In order to determine the hydraulic roughness of unglazed vitrified clay and pvc pipes, a second series of tests was conducted with the pipes flowing full.

It was found that the pvc pipeline, at the commencement of the test, was hydraulically smoother than the vitrified clay pipeline, but, as the test progressed the difference between the two diminished until the time came when the pvc pipeline was hydraulically rougher then the vitrified clay pipeline. Then as some of the microbiological slime was sloughed off, the vitrified clay pipeline became rougher until sloughing again took place. This alternation happened quite often. The velocities of sewage flow in these tests varied from 0.75 m/sec to 2.25 m/sec but there was no significant difference in the hydraulic roughness of the vitrified clay and pvc pipelines.

Therefore, as the first series of tests showed negligible differences in the weight of microbiological slime on any of the materials tested and the second series of tests showed negligible difference in hydraulic roughness between the matured vitrified clay and pvc pipelines, it would be anticipated that there would be negligible significant difference in hydraulic roughness between pvc and clay pipes and hence for design purposes the same hydraulic roughness would apply.

The Hydraulic Research Station continued the research at the Littlemore Sewage Pumping station, Oxford. In these experiments, 225mm nominal bore vertically-cast and spun concrete, asbestos cement, pvc and clay pipes had sewage passing through them. In each year of the work, a different hydrograph of flow was used. Maximum proportional depth was 0.60 (0.78 m/s) and the minimum 0.16 (0.38 m/s). The maximum velocity was 1.18m/s.

Typical variations of roughness with time graphs are available upon request.

Subsequently, the Water Research Engineering Centre carried out a programme of analysis of sewerage systems. It was found that the ks values were larger at low velocities than at higher velocities. A range of 3 values of ks was published in the Sewerage Rehabilitation Manual, dependent of velocity of flow.

For information on design ks values and design tables, please see the publication “The specification, design and construction of drainage and sewerage systems using vitrified clay pipes”, Appendix B.

Glaze and other surface coatings

During the firing process, the clay pipe undergoes a chemical process whereby the alumina-silicate clay particles sinter together under the action of the fluxes, which occur naturally in the clay body. The fine clay processing of the UK Clay Manufacturers combined with this fluxing action, which occurs as the temperature of the kiln approaches 1100ºC ensures that upon cooling, a high-density strong, impervious, smooth clay body is produced, which has no need for glazing or protective coating to enhance durability or hydraulic smoothness.

This is as anticipated in EN 295-1: clause 5.1.4 (Appearance) states: “Pipes and fittings can be unglazed or glazed on the interior or exterior. When glazed, they need not be glazed on the jointing surface of the spigot and/or socket”.