If fluvial systems don't have sufficient room for natural detention of floodwater in the floodplain, the development and management of flood storage within and adjacent to the natural floodplain is recommended and described in more detail in this measure. It addresses aspects like the process of selecting where to locate the flood storage, deciding how much storage is needed, how to measure the storage capacity, selecting appropriate flow control structures, analysing how the works will perform and making sure that the flood storage scheme is safe in extreme floods.
Based on "J C Ackers, J M Bartlett (2009): 10 Flood storage works. 28p. In: UK Environmental Agency (2009): Fluvial Design Guide (Contains public sector information licensed under the Open Government Licence v3.0.)"
There are two main reasons for providing temporary detention of floodwater:
- to compensate for the effects of catchment urbanisation;
- to reduce flows passed downriver and mitigate downstream flooding.
Although these may be separate drivers for a flood storage scheme, they are in essence identical. The flood storage works are designed to reduce the peak flood flow passed downstream, spreading the overall volume passed downstream over a longer period. Alternative methods of providing this flood protection would be to:
- enlarge the river channel;
- raise the riverbanks;
- construct floodbanks set back from the river;
- provide specific protection around flood-prone buildings or groups of buildings.
Providing flood storage is thus one of a portfolio of options for managing and controlling the risk of flooding. In some cases it can provide sufficient flood protection on its own; in other cases it may be chosen in conjunction with other measures.
The advantage of flood storage is that the flood alleviation benefit generally extends further down-stream, whereas the other methods benefit only the local area, and may increase the flood risk downstream.
If the primary driver for flood storage is to compensate for the effects of urbanisation (see Box 10.1), the objective is normally to detain the additional and faster runoff that results from an increase in the impermeable area. This is then released downstream at a slower rate, designed to mimic the natural runoff from the non-urbanised catchment, avoiding any increase in flood depths and frequencies being propagated downstream.
According to Design of flood storage reservoirs (Hall et al, 1993), the provision of flood storage: ‘has much in its favour. The capacity of the reservoir both attenuates the incoming flood peak to a flow that can be accepted within banks by the downstream channel and delays the timing of the flood so that its volume is discharged over a longer time interval’.
Hall et al (1993) go on to point out that, where several flood storage reservoirs are deployed in a river basin, their overall effect has to be considered carefully. For example, a flood storage reservoir on a minor tributary could delay the peak of the tributary flood so that it coincides with the peak flood coming down the main stream, thereby making matters worse. It is normally obvious if there is a risk of this occurring, but the application of appropriate hydrological and flood modelling approaches can demonstrate if this is indeed a problem. Such studies need to cover a range of rainstorm and flood scenarios – including the effect of rainstorms moving across the catchment – to ensure that the provision of flood storage at a site is a robust solution that does not have detrimental effects at other locations downstream.
Adaptability
Flood storage works sometimes lend themselves to adaptability to changed circumstances such as:
- increased upstream catchment runoff;
- a change in the vulnerability of downstream communities to flooding.
Increases in catchment runoff may be due to progressive urbanisation or a change in climate, while the downstream vulnerability might change with increased urban development or the implementation of a flood alleviation scheme.
Adaptability of the flood storage works could include such features as:
- the ability to raise the impounding embankment to increase the flood storage capacity;
- adjusting the settings of gates or orifices that control downstream releases;
- if applicable, changing the setting of weirs and gates that admit flows to the reservoir.
At the time when flood storage works are proposed and designed, it is vital to consider:
- whether it may be desirable to adapt the design of the scheme in the future;
- whether any such flexibility should be incorporated into the design.
Types of flood storage
Flood storage works can usually be described as one of the following:
- online – in which the water is temporarily stored within the river channel and its floodplain;
- offline – in which the water is diverted from the river channel, stored in a separate area (which may be part of the floodplain) and subsequently released back to the river or to another watercourse.
In general, online storage works are normally located in the upper catchment (where the catchment area is modest) while offline storage works are more common on larger rivers with wide floodplains. Some complex flood storage schemes include a combination of online and offline components, de-signed to act in conjunction.
Floodplains that are modified to augment their natural flood storage and attenuation characteristics are often described as washlands – a term that can be used in the context of either online or offline flood storage.
To learn more about different type of flood storage, click here.
Hall, M J, Hockin D L and Ellis, J B (1993). Design of flood storage reservoirs, B014. CIRIA and Butterworth-Heinemann.