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| Ron Allen's soil and water pages |
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FACT: A key test of sustainable development is the extent to which development remains both uncompromised by the water environment and does not itself compromise that environment.
WHICH MEANS THAT: A sustainable development should not be adversely affected by excess water flows and the development should not itself compromise the ability of others to use water resources.
How does flooding occur and what are the mechanisms?
There is no single cause of flooding. Flooding occurs when excess water enters a site or property and only becomes a problem if damage is caused. In some instances, such as on wetland wildlife sites, flooding may be welcomed.
River overbank flooding is the best known and generally the most spectacular and damaging. Rivers are fed by small streams and artificial watercourses that drain a specific area of land known as the catchment. If the catchment is underlain by impermeable rocks such as granite, hard sandstone or clay, any rain in excess of that which can be soaked up by thin layers of permeable soil will pass immediately to the regional drainage system. In the upper parts of rivers, this water passes rapidly along river channels, but in the lowlands where the ground is level, escape along the river channel is reduced and, as a result, excess water passes over the river banks and spreads across the flood plain. This is a perfectly natural process. Unfortunately many of our towns have developed at the junction of roads and rivers and have expanded over the adjacent floodplain. These settlements are liable to flood in wet years and often flood control measures have been put in place, but when (in exceptional years) these are unable to cope, flooding occurs.
Surface water flooding occurs where rain water falling on impermeable substrates can only escape over the surface (overland flow). Any restriction to this flow will lead to ponding back of the excess water, and if properties are in the way, they will be flooded.
Ground water flooding occurs when natural water levels in permeable substrates (surface aquifers) rise above normal ground water levels. Many rivers on the southern Chalklands are fed by water that rises up into the river channel (bed flow). Where the water levels are consistently below valley bottoms, the valleys remain dry but as ground water levels rise, springs become active and formerly dry valleys become flowing rivers. In the Chalklands, ephemeral streams such as these are known as 'winterbournes'. When exceptionally high groundwaters occur, the winterbourne water will spread wider and wider and may then flood properties and roads. This process affected many normally dry chalkland areas this winter.
Prevention
The obvious means of prevention is not to build in any areas of flood risk. Information on flood risk is available, but this is usually generalised and often not sufficiently specific for any one purpose. Information on river flows is also often available for many water courses, but away from rivers, information is scarce.
Away from rivers, you can check ground water levels in trial pits or from boreholes. You can check if a site lies wet (even in the summer when it may be dry) by examining the colour of the soil. Grey and orange mottles indicate seasonal waterlogging and grey, green or blue colours indicate prolonged or permanent waterlogging.
You can also check the nature of the catchment to see if it is underlain by permeable or impermeable strata. Check geological, soil, Ordance Survey and historical maps and plans for evidence of springs, seepages, ponds, lakes and rivers within or near to your site.
Check the vegetation. If the land contains bog-mosses, sedges, rushes, reeds, willows or alders (for example), you may have a wetness or flooding problem.
Finally check road and place names. If nearby roads or places have names that contain watery words, take care. Look out for Water Lane, Flood Road, Alder Way, Willow Copse, Peat Moor or River Way. Names such as these will alert you to a potential problem.
If you do decide to build in a flood risk area, ensure that you have taken precautions. Build the property on higher parts of the site above all known flood levels and add a bit for exceptional circumstances. If on a flood plain, ensure that adequate compensatory water storage measures are in place. If on impermeable ground, ensure that surface flows can be sufficiently diverted around the site.
Finally
When designing for wet areas, ensure that your actions do not adversely affect others. Do not direct water to areas that will affect other properties or prevent properties being constructed in the future. Ensure that water flowing along drains does not carry silt or chemical pollutants to environmentally sensitive areas such as wildlife sites and amenity ponds.
There are many ways to avoid generating flood water and these are called Sustainable Urban Drainage Systems (SUDS). Balancing lagoons, swales, permeable pavements, reed beds and others all help developments respect the water environment and ensure hydrological sustainability.
Remember also that flood risk is a material consideration in a planning application and you will need to demonstrate that your proposals will be neither affected by flooding, nor cause adverse flooding elsewhere.
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The Environmental Project Consulting Group
44A Winchester Road, Petersfield, Hampshire GU32 3PG
email: Ron Allen, tel: 01730 231019,
Copyright April 2005 Ron Allen
Geologist, Soil Scientist, Applied Ecologist, Hydro-ecologist, Chartered Environmentalist
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