Managed flooding

Flood risk can be managed, with mapping and planning.

Normal rainfall maintains the flow in streams and rivers. Exceptional rainfall causes floods of different sizes. Engineers classify the size of floods by the probable period in which they are likely to return. A 1-year event is the largest flood that is likely to return every year and a 100-year event is the largest flood that is likely to return in a century. Contours can be drawn to show the geographical flood extent for each return period. In a natural landscape, larger volumes of water will cause more land to be flooded [Fig 9.12]. 9.12 Existing and proposed flood contours should be surveyed and mapped. In a developed landscape, there will be less variation. Normally, anything up to a 5-year flood will be confined to the river channel or to a wider flood-channel. When a 10-year or 100-year flood event occurs, buildings and roads will be flooded. Users and residents do not like this but their inconvenience has to be set against other considerations. Not only are flood defense works extremely expensive, financially and environmentally, they tend merely to exacerbate the downstream flood problem. One solution is to incorporate managed flooding into the landuse planning system. Each town can draw existing and proposed contours for each flood event: 2-year:nature reserves, playing fields, parks, gardens 1-year:non-essential car parks, lightly trafficked roads, flood-tolerant buildings 25-year:most roads and car-parks; the ground-floors of non-essential buildings 100-year:sizable urban areas, but not hospitals or other essential services In order to lessen flood damage to valuable property, as large an area as possible should be included within the 2-year and 1-year flood zones. Some buildings can be placed within the frequent-flood zone, with measures to make them flood-proof or flood-tolerant [Fig 9.13]: electrical services can be sealed or routed above the flood-level; water-sealed doors can be made, as on ships; furnishings can be resistant to water-damage; pedestrian access routes can be provided above the flood-level; drainage flaps and 'bilge-pumps' can be fitted; buildings can be raised on stilts above the flood level; water-proof concrete can be used for walls; extra foundations can be used to resist erosion by floodwaters. If landowners do not wish to incur the cost or inconvenience of these measures, they should not build in flood-plains and they should have no claim on public expenditure for flood defense.