Definitions & Terms
Soil Moisture Deficit (SMD)
This is a measure of how much water it would take to saturate the soil. When the SMD is high, rainfall may be completely absorbed by the soil. When the SMD is low, most of the rainfall will run off and can rapidly feed into rivers. The SMD is calculated twice a week by the Environment Agency and is used to predict the impact of any heavy rainfall events forecast by the Met office. The Environment Agency can then decide whether to issue flood warnings.
Flood Zone 1
Area outside the extent of a flood with an annual probability of 0.1% or greatest recorded historical event.
Flood Zone 2
The Extent of a flood with an annual probability of between 0.1% or greatest historical event and an annual probability of 1%.
Flood Zone 3
The Extent of a flood with an annual probability of 1%.
Culvert
An enclosed conduit, usually a large pipe for conveying a watercourse or drain or ditch, below the carriageway or adjacent ground.
Grip
A shallow trench, located across the verge of a road, to lead surface water away from the carriageway.
Outfall
The point where one drainage system discharges into a watercourse.
Sustainable Drainage Systems (SUDS)
SUDS are made up of one or more structures to manage surface water run off. They are used in conjunction with good management of the site to prevent flooding and pollution. The general methods of control are as follows:
- Prevention
- Filter strips and swales
- Permeable surfaces and filter drains
- Infiltration devices
- Basins and ponds
These controls should be located as close as possible to where the rainwater falls, providing attenuation for the runoff. They also provide varying degrees of treatment for surface water, using the natural processes of sedimentation, filtration, adsorption and biological degradation.
Riparian Owner
If you own land adjoining a watercourse, you have certain rights and responsibilities. In legal terms you are the riparian owner.
Main river
Main rivers are usually larger streams and rivers. However, they do include smaller watercourses of local significance. A main river is a watercourse marked as such on a main river map which is an official document held by the Department of Environment, Food and Rural Affairs (DEFRA). The Environment Agency (EA) have powers to maintain and improve rivers in order to ensure the efficient passage of flood flow and to manage water levels. These powers allow the EA to do work; they do not oblige the EA to carry out maintenance or construction of new works on main rivers.
Ordinary Watercourse
An ordinary watercourse is every river, stream, ditch or drain which does not form part of a main river. Usually the responsibility for maintenance lies with the riparian owner. The Vale of White Horse District Council will aim to secure co-operation in ensuring appropriate maintenance takes place but will draw on enforcement powers when necessary.
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