- surface drainage - the removal and diposal of water from the surface of a pavement
- sub-surface drainage - the use of underground pipes and other fittings to direct the flow of unrequired water.
A drainage question which is often asked is: "At what point in the drainage system of my house does it become someone else's responsibility?"
Unadopted sewers serving more than one property are:
'Private Sewers'
The responsibility is shared by all those using the sewer.
A drain serving only one property is the responsibility of the property owner up to the point immediately after the junction where it joins a public or private sewer.
Gradient
All drainage systems, both above ground and below ground, work by creating a slope and relying on gravity to cause the water (or other liquids) to flow from the high point to the low point. This is known as the 'fall' and can expressed in a number of ways, usually either a percentage or as a ratio.
All gradients are defined as the amount of rise (or fall) in height above a fixed point over a given distance (the run). Gradients are expressed in a number of ways, including Ratios [1 in 60 or 1:60] and Percentages [16%] For 100mm pipes, the type normally used for residential drainage, the minimum acceptable falls are 1:40 for Foul Water and 1:100 for Surface Water. Larger diameter pipes may have gentler gradients in certain circumstances
Definitions
Surface Water
Also referred to as Storm Water. Water that has been drained directly from the ground, a pavement or a roof. Also known as 'run off'
Ground Water
Water held by the soil, subsoil or bedrock. The upper level of the groundwater is known as the 'water table' or 'saturation line'.
Foul Water
Used water from any building or process. This includes water from toilets, baths, showers, washing machines, kitchen sink, dish washers etc. Sometimes referred to as 'grey' water, effluent, soil water or waste water.
Combined System
A drainage system that mixes Foul and Surface water and carries both in a single pipe.
Endfall
is the fall or gradient along a pavement or drain, ie, the water will be drained in the same direction as the pavement/drain is running.
Crossfall
This is the fall across a pavement, ie running at or about 90° to the direction of the pavement. Many footpaths alongside roads have crossfall, draining the footpath onto the road. Crossfall is not found in drains, only on surfaces.
Backfall
This is the term used to describe a gradient that is counter to what would normally be expected. A footpath that sloped away from the roadway would be said to be backfalling. Backfall in a drain is not good - it indicates that water/sewage may find itself trapped at a low point.