What is Drainage?
Drainage is defined as the removal of excess water from the ground surface or from the underground rootzone. Hence, it is an artificial removal of water from both the surface and sub-surface. The need for drainage is essential in civil and construction projects to prevent the issues of flooding and related damages.
The adequate system that is used to guide these excess water to a collecting system or a treatment plant is called as a drainage system. The water that need to be removed can be either:
1. Surface Water
2. Subsoil Water
3. Foul and Soil Water
As mentioned above, based on the type of water that need to be drained, the drainage system used will vary. The water that is collected from the surfaces of roofs or paved areas are called as surface water. The water that is collected from the earth that is below the subsoil's water table is called as subsoil water. The water is contaminated by trade or domestic waste is called as an effluent. When this effluent is collected from the sinks and basins of a household, it is called as foul or waste water ( It does not include excreta). The effluent collected from toilets, urinals and water closets are called as soil water.
Surface Water Drainage
A surface water drainage system collects the water from the roofs, pavements, and highways and collect to a final planned point.
1. Surface Water Drainage for Roof Water
As per the Building Regulation's Approved Document H, every building must provide adequate provision to collect water from their roofs without causing any drainage issues. For this, a surface water collection channel or a gutter is used to collect the surface water to the vertical rainwater pipe that is guided to the drainage system.
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| Gully Arrangement |
The roof provided for such an arrangement must require sufficient slope or fall to facilitate easy water movement. The minimum recommended fall for aluminum roofs are 1:60, lead is 1:120, roofing felts and copper requires 1:60 and mastic asphalt required 1:80.
A slope of 1:40 is demanded for a flat roof of standard specification so that an actual finished fall of 1:80 is obtained. This allows for any correction.
2. Surface Water Drainage From Pavement
A yard gully collection or a channel connection is provided for paved areas in the form of drainage. The paver is provided with a slope of 1:60 to let the running water to fall into the gully. The size and the number of gullies provided are dependent on the shape of the paved area and the type of area to be drained.
A channel is laid near to the pavers to collect the water. A paver laid with adequate fall can drain water to the channel system that is connected to the drainage system. The channel can be half-rounded glazed clayware, precast concrete channel blocks or precast or in situ concrete box channel.
3. Surface Water Drainage for Highways
The surface and subsurface drainage pattern of watersheds or hill-slopes is adversely affected by the construction of roads. Hence, it is necessary to provide adequate drainage facility to prevent the accumulation of excess water and moisture within the road construction. Any discrepancies in this would affect the material properties, stability and overall performance of the vehicles on the roads.
Subsurface Drainage
These subsurface drainage becomes necessary to achieve ground stability for a particular construction project, landscaping or to improve horticultural properties etc. It can also installed to avoid the penetration of the ground moisture into the building's interior to prevent damage to the building.
Generally, porous pipes are used to allow the subsoil water to pass through the pipe body. These pipes are laid dry jointed on a trench that is filled with rubbles. A filter mat made out of rubble, over which a pervious membrane is placed above which the pipe is placed. This arrangement is backfilled normally with a topsoil of 150mm. This is a simple subsoil drainage facility achievable up to 1.5 m above which we need to go go for higher groundwater control measures.
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