Basics of Flooding and Flood Protection
A "floodplain" is the lowland adjacent to a river, lake or sea. Floodplains are designated by the frequency of the flood that is large enough to cover them. For example, the 10-year floodplain will be covered by the 10-year flood and the 100-year floodplain by the 100-year flood.
Flood frequencies, such as the "100-year flood," are determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size occur. Another way of expressing the flood frequency is the chance of occurrence in a given year, which is the percentage of the probability of flooding each year. For example, the 100-year flood has a 1% chance of occurring in any given year.
Dams, embankments, channels and other protective works are designed to provide protection against some specific level of flooding. The "level of protection" is selected based on cost, desire of the community, potential damage, environmental impact, and other factors. Engineers can design and construct embankments, dams and other measures providing a very high level of protection.
Communities tend to choose lower levels of protection because of the initial financial cost rather than overall costs and benefits.
Embankments may be either "engineered" or not. Engineered embankments are those in which professional consideration has been given to the underlying soil conditions, the kind of earth used in building the embankment, proper compaction of the embankment materials, armouring of the embankment face if needed and other factors. Non-engineered embankments are basically long piles of earth pushed up along a river. Engineered embankments have a far lower rate of failure than non-engineered embankments.
Embankment failures are usually due to either:
- a flow greater than their design flow;
- poor maintenance; and/or
- erosion or undercutting of the embankment by high flow.
There is a wide range of measures that can be used to protect against flooding. They may be grouped in various ways, such as -
"Structural" and "non-structural" measures
Whether they are most suitable for protecting:
a) individual structures or
b) areas containing multiple structures and communities
Whether their purpose is to:
a) modify the flood;
b) reduce susceptibility to flooding;
and/or
c) reduce the impact of flooding
Multiple measures are usually needed to provide protection to an area.
Most of the known floodplains in the UK have been mapped by the Environment Agency. These identified areas account for about 60% of flood insurance claims. The remaining 40% of the claims occur in areas not previously recognized as being vulnerable to flooding, and are generally not located near a river or other water body.
The Environment Agency is responsible for warning the public of the possibility of flooding. There are several different warning messages that may be issued, based upon the conditions and/or probability of flooding
See the web site www.environment-agency.gov.uk