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Current conventional UK sewerage design is covered by the British Standard 8005 (BSI, 1987). Many points are also usefully summarized in the design and construction guide for developers in England and Wales, entitled Sewers for Adoption (WSA, 1995). The separate system is normally adopted for new development in the UK, unless the extent of existing combined or partially separate sewers determines otherwise. The hydraulic design of a combined system is more closely associated with surface water sewerage, and it is not intended to cover that in detail in this chapter. The main points that relate to the hydraulic design of conventional foul sewers are discussed below. |
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In the absence of specific local water use data, a figure of 200 litres per person per day plus 10 per cent infiltration may be assumed. This is multiplied by the population appropriate to the planning horizon adopted (30 years is recommended) to give the average flow or dry weather flow (DWF). Foul sewers are designed for up to 6 x DWF, to allow for diurnal peaks, other fluctuations and extraneous flow such as infiltration. With the assumption of three persons per dwelling, this approximates to a peak design flow of 4000 litres per dwelling per day or, in more meaningful units for a peak flow, 0.046 litres per second per dwelling. |
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8.3.2
Pipe Size, Depth and Roughness |
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Sewers laid within highways should have a minimum cover of 1.2 m to avoid interference with other services. Elsewhere, a minimum cover of 0.9 m is suggested as sufficient to avoid interference with cultivation of the land. The minimum size of gravity sewer for adoption into the public system is 150 mm internal diameter. House connections are usually 100 mm internal diameter, thus ensuring that solids which enter the system can pass downstream through pipes of increasing size to prevent blockages. A surface roughness height (ks) value of 1.5 mm for |
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