Storm water drainage

Discharge into storm water channels or pipes

Where a storm water pipe or ditch exists within reasonable distance of the property on a building site, the drainage from the roof and from any paved or enclosed areas must be collected and discharged into the storm water pipe or ditch. In many cases, the ditches or channels are laid alongside the road just outside the boundary of the property and are the responsibility of the highway authority, which may have its own connection requirements that should be incorporated into the plumbing code of practice. For piped sewers, any connecting drains will need to comply with requirements similar to those that apply to drains carrying wastes to the soil sewer. The saddle or junction connection must be made under the direction of a licensed plumber or qualified person and must not obstruct the flow of the sewer or the drain. The drain itself must be laid to a selfcleansing gradient and must be properly jointed to prevent the access of tree roots or of the surrounding soil. However, the materials may not need to be of as high a quality and the drain may not need to undergo a test for watertightness. Discharges into an open drainage channel may be through a pipe or through a subsidiary channel. Care must be taken to prevent erosion and damage to the channel lining. Subsidiary channels and pipes will usually be required to discharge in the direction of flow of the main channel at a level above that of the normal drainage flow. If the main channel is unlined and the discharge into it is through a pipe, a protective concrete apron may be required at the point of discharge. Subsidiary channels should be laid to a self-cleansing gradient, but this, together with requirements relating to diameter, may be modified according to soil conditions. Discharges into storm water sewers or channels must not contain any human waste, sullage water or other substances that may cause a nuisance or injury to health. In tropical countries having a long dry season, small discharges, such as a drain from a single tap, may increase the risk of infestation because protozoans or other parasites may breed in shallow pools or waterlogged ground. Where a channel or drain may remain virtually dry for perhaps months at a time small discharges may cause considerable nuisance, especially if they contain deleterious matter such as oil or grease. The authority may make special provisions to avoid this by requiring that a paved area where cars might be washed should be provided with a petrol, sand or oil trap and plate separator. These dangers must be balanced against the desirability of dealing with clean water discharges without requiring their being connected to human waste sewers. Such instances as the drainage from air-conditioning units and of cooling water from a dairy or small industry, or the hosing down of a warehouse floor, should not call for disposal treatment, but the volume of water may be too great to be dealt with by soakaways.

Combined sewers

 Some sewerage authorities operate systems of combined sewers into which both sewage and rainwater may be admitted. These systems were installed in the past but are not currently recommended. Combined sewers are not economical because much greater flows must be provided for in the sewers and in the sewage disposal plant. These systems are also hazardous to health because storm overflows must be provided to handle heavy downpours. Those overflows are necessary to relieve surcharge of the system at peak flows, and they may permit untreated sewage waste to discharge into open watercourses. Combined sewers are rarely installed today, but they are often found in congested areas of older cities where physical and financial constraints may prevent the laying of a second system of pipes to carry off rainwater. Wastewater authorities provide separate sewerage facilities for new developments, as every additional connection to a combined sewer makes its ultimate replacement more difficult and expensive.


Rainwater from sloping roofs must be collected in gutters and carried to ground level by downpipes or downspouts. Flat roofs should be drained by vertical pipes and the drainage should be conveyed by pipe to a surface water sewer or to a suitable soakaway. Except when a roof is thatched, gutters and downpipes or downspouts should always be considered essential because they prevent roof runoff falling from a height in concentrated sheets or streams, which can cause erosion close to the foundations of the building. If guttering cannot be installed a concrete path or apron should be laid immediately under the eaves, and should be sloped to carry the water away from the foundations. The sizes of gutters and downpipes or downspouts will depend on the area of roof to be drained, the slope of the gutter and the intensity of rainfall expected. To insist on guttering capable of dealing with the worst storms would be unreasonably expensive in many areas, and would be of little overall benefit when the entire surrounding ground was being subjected to a downpour. The authority should calculate the average storm intensity expected and fix their standards accordingly. Tables 16.1, 16.2 and 16.3 relate roof area, guttering slope and storm intensity (Taylor & Wood 1982). These tables can be adapted to suit local conditions. The capacity of a range of storm water drains is also shown, and may be applied to the drains connecting the lower ends of the downspouts with the surface water sewer. Whether or not the use of soakaways is a practical option will depend to a great extent on the nature of the soil. Soakaways should be well clear of the building foundations, and should consist of holes deep enough to penetrate the subsoil, filled almost to the surface with hard material such as broken stone, concrete or brick that will not soften when wet. Where the water table is high, it may be preferable to use shallow ditches filled with hard rubble instead of soakaway pits.

Rainwater tanks

When rainwater is being stored for domestic use the tanks should be of watertight construction, covered with material that is weatherproof, insectproof and verminproof, ventilated, and supplied with access for regular inspection and cleaning. There are many standards throughout the world. For more technical detail on this subject, please check your area for its standards. Rainwater storage tanks are a valuable supplement to mains supplied in arid areas and may even substitute for a mains supply. A system of gutters and collector piping must also be watertight, and the contents must be protected against pollution from dust and refuse blown by the wind, entry by birds and vermin, and mosquito breeding. If the rainwater is supplementary to a mains supply it may be lifted from the principal storage tank via a pump, from where it is piped to all fixtures. If there is no mains drinking-water supply then water for all purposes will need to be taken from the rainwater storage tank. Strict precautions should be observed in such cases to maintain the quality of the stored water. A wash-out drain tap or diverter should be included in the collector pipe so that the first washings of the roof at the beginning of the rains can be run to waste (these washings will be contaminated with bird droppings, windblown dust, etc.). It is at this time that the storage tank should be given its annual cleaning, a process that is much easier if the tank is built in two sections that can be emptied and cleaned in turn.

Rainwater intensity and roof drainage

The variable factors in selecting the size of rainwater guttering are:

• the anticipated intensity of the rainfall;

• the slope at which the gutters are to be fixed;

• the area of the roof surface drained by each gutter.

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