Vincent T. H. CHU

Computation of runoff is a complicated matter which depends on many factors like the ground permeability, rainfall duration, rainfall pattern, catchment area characteristics etc. Basically, Rational Method is a means to find out the maximum discharge suitable for design purpose. In this method, it is assumed that the rainfall duration is the same as the time of concentration and the return period of rainfall intensity is the same as the peak runoff. Time of concentration refers to the time required for the most remote location of stormwater inside the catchment to flow to the outlet. When the time of concentration is equal to the rainfall period, the maximum discharge occurs and rainfall collected inside the catchment comes to the same outlet point.

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If sharp edged inlet to pipes connecting to a pump is adopted, flow separation will occur. Flow separates from sharp edges and a recirculation zone is formed. Moreover, turbulence shall form at downstream when the flow at vena contracta subsequently expands to fill up the unfilled void. Flow separation leads to significant head loss.

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Pumps can be installed as dry well or wet well. The wet well is commonly used because of its simplicity and low cost. However, this type of pump arrangement has the potential problem of maintenance. For instance, it requires the de-watering of sump and removal of pumps out of the sump, which is suitable for stormwater pumping station which does not require pumping for most of the time. For dry well, the pumps could be assessed and maintained all of the time.

In general “turned-down” bellmouth of pump inlet is more popular because of the following two reasons:

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It is not uncommon that manhole explosion occurs nowadays. Manhole covers are dislodged from the frames which is associated with a release of energy. Manhole explosion occurs mostly owing to the ignition and combustion of flammable gas. Sources of flammable gas include the followings:

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Sewer manholes should preferably be designed to be watertight owing to the following reasons:

(i) Water-tightness is important in sewer system. Otherwise, leakage of sewage from manholes would seriously contaminate the environment.

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Wetwells are designed to store temporarily water/sewage before it is pumped out. They are usually provided for sewage and stormwater pumping stations and they serve the following functions:

(i) They assist in attenuating the fluctuations of flow owing to the diurnal variation of sewage discharge.

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The presence of sediment in sewers has adverse effects on the hydraulic performance of sewers. For the case of sewage flow carrying sediment without deposition, the presence of sediment in the flow causes a small increase in energy loss.

In case the sewer invert already contains a bed of sediment deposit, it reduces the cross sectional area of sewers and consequently for a given discharge the velocity increases. As such, the head losses associated with this velocity increase. Moreover, the increase in bed resistance induced by the rough nature of sediment deposit reduces the pipe flow capacity of sewers.

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For solids in large sewers, forces on solids position them at different flow heights depending on their specific gravity.

There are generally two modes of movement of solids in small sewers, namely, floating and sliding dam. The floating mechanism operates when the size of solid is small when compared with the diameter of sewer.

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When excessive pipe deformation occurs, it may impair the joint performance and affect the strain in pipes. Based on the information by PIPA, there is a 5% reduction of flow capacity when the pipe is deflected by 15%. Hence, pipe deflection has impact on flow capacity but its effect is not significant.

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Inverted siphons are designed at locations in which a sewer system is blocked by underground utilities or stormwater drains. They are sometimes called depressed sewers because it is claimed that there is no actual siphon action. They connect the upstream and downstream sewers with U-shaped vertical alignment such that they are always running full.

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