Glossary Terms E-H


All living things, such as trees, flowering plants, insects, birds and mammals, and their habitats.


A biological community of interacting organism, and their physical environment.

Ecosystem services

Services provided by the natural environment that benefits society.


Both the natural environment (air, land water resources, plant and animal life) and habitats.

Environmental Footprint

A measure of environmental impact based on the distance that resources for a development are transported.

Environmental management

A management agreement for an area or project set up to plan and make sure the declared management objectives for the area or project are met. Environmental Management Plans are often undertaken as part of an environmental impact assessment and are set out in several stages with responsibilities clearly defined and environmental monitoring procedures in place to show compliance with the plan.


The group of natural processes, including weathering, dissolution, abrasion, corrosion, and transportation, by which material is worn away from the Earth’s surface.


The process by which the Earth’s surface or soil loses moisture by evaporation of water and by uptake and then transpiration from plants.

Extended detention basin

A detention basin in which the runoff is stored beyond the time normally required for attenuation.  This provides extra time for natural processes to remove some of the pollutants in the water.  It is important to ensure that these areas are extremely shallow to prevent them from being a hazard to children.


Flood Estimation Handbook (FEH), produced by Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology) to aid calculationof the possible severity of flooding.

Filter drain

A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage, to store and conduct water, but may also be designed to permit infiltration.  Filter drains are often used at the side of roads to collect highway run-off and at the toe of embankments to intercept run-off and groundwater flow.  The sides of the trench can be lined with permeable or impermeable lining membranes depending on specific requirements.  The filtration action helps remove suspended solids from the water.

See also French Drains.

Filter strip

A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and filter out silt and other particulates.


The act of removing sediment or other particles from a fluid by passing it through a filter.

First flush

The initial runoff from a site or catchment following the start of a rainfall event. As runoff travels over a catchment it will pick up or dissolve pollutants and the “first flush” portion of the flow may be the most contaminated as a result. This is especially the case in small or more uniform catchments, however, in larger or more complex catchments pollution wash-off may contaminate runoff throughout a rainfall event.

Flood frequency

The probability of a flow rate being equalled or exceeded in any year.

Flood routeing

Design and consideration of above-ground areas that act as pathways permitting water to run safely over land to minimise the adverse effect of flooding. This is required when the design capacity of the drainage system has been exceeded.  Computer simulation software allows flood routeing to be modelled if the topographical information is available.


Land adjacent to a watercourse that would be subject to repeated flooding under natural conditions.  In Wales, most new development within flood plains is not permitted, subject to the requirements set out in the Welsh Government’s TAN 15 guidance.  However many floodplains in Wales have been built upon as they were convenient locations to develop.  As a consequence many thousands of properties in Wales are at risk of flooding.


The plants found in a particular physical environment.

Flow control device

A device used to manage the movement of surface water into and out of an attenuation facility, for example a weir.


A small basin or pond upstream of the main drainage component with the function of trapping sediment.

French Drain

An alternative name for Filter Drain.

Geocellular structure

A plastic box structure used in the ground to create a void which can hold water; often to attenuate runoff.


A plastic fabric that is permeable.

Green Infrastructure

A strategically planned and delivered network of high quality green spaces and other environmental features (often including water features).

Green roof

A roof with plants growing on its surface, which contributes to local biodiversity. The vegetated surface provides a degree of retention, attenuation and treatment of rainwater, and promotes evapotranspiration. (Sometimes referred to as an alternative roof).

Greenfield runoff

This is the surface water runoff regime from a site before development, or the existing site conditions for brownfield redevelopment sites.

To maintain the natural equilibrium of a site, the surface water discharge from a developed site should not exceed the natural greenfield run-off rate.  Environment Agency Wales will advise on allowable discharge rates to watercourses for sites that are to be developed.  However, where possible it is better that surface water is managed at source.


Wastewater from sinks, baths, showers and domestic appliances this water before it reaches the sewer (or septic tank system).


Water that is below the surface of ground in the saturation zone.


The area of environment where an organism or ecological community normally lives or occurs.

Highway authority

A local authority with responsibility for the maintenance and drainage of highways maintainable at public expense.

Highway drain

A conduit draining the highway. On a highways maintainable at the public expense it is vested in the highway authority.


Hydrology of Soil Types (HOST).  A classification used to indicate the permeability of the soil and the percentage runoff from a particular area.


A graph illustrating changes in the rate of flow from a catchment with time.

Hydrological Cycle

See Water Cycle.

Cyfoeth Naturiol Cymru / Natural Resources Wales Welsh Water / Dwr Cymru Welsh Government / Llywodraeth Cymru Atkins Welsh Local Government Association Consumer Council for Water
Home Builders Federation CIWEM Institute of Civil Engineers