|
|
|
|
|
|
|
use in the ColebrookWhite equation is usually now assumed for design purposes. This is approximately equivalent to a Manning's n of 0.013, which is close to the value of 0.012 that equates to the traditional Crimp and Bruges formula. |
|
|
|
|
|
|
|
|
Sewers should be laid at gradients that will produce velocities sufficient to prevent permanent deposits of solids. A velocity of 0.75 m/s should be exceeded daily in foul sewers to maintain selfcleansing conditions. The Sewers for Adoption guide requires this velocity at one-third of the design flow, that is at 2 x DWF, which represents a typical diurnal peak DWF. BS8005 includes the comment that this can be achieved by laying the sewers to a gradient that will give a velocity of 1.0 m/s at full bore flow. This allows for the reduction in velocity in pipes flowing less than half full. |
|
|
|
|
|
|
|
|
Sewers for Adoption also includes a 'deemed to satisfy' criterion whereby, provided 10 dwellings are connected, the minimum velocity requirement is deemed to be satisfied by a 150 mm diameter sewer having a gradient not flatter than 1 in 150. This is to limit the extent of pumping required in flat regions, but is not advocated where the topography permits steeper gradients. |
|
|
|
|
|
|
|
|
8.3.4
The Self-Cleansing Criterion |
|
|
|
|
|
|
|
|
It has been shown from sediment transport criteria (Ackers, 1984) that the British Standard recommended self-cleansing velocity appears inadequate in pipes of large diameter. This would apply to the large collector culverts, many several metres in diameter, employed in the Greater Cairo Wastewater Project (Darling and Drake, 1985). In contrast, Novak and Nalluri (1978) concluded, particularly for smaller sewers less than 1000 mm in diameter, that the standard self-cleansing velocity results in greater slopes than would be required to initiate motion even for large grit and sediment particle sizes, when considering flow over a fixed bed. |
|
|
|
|
|
|
|
|
More recent research into the nature of sewer sediments (Crabtree, 1989), has shown that a distinction should be made |
|
|
|
|
|