Fluvial Design Guide - Chapter 10

Flood storage works

10.2 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, designed to act in conjunction. Table 10.1 gives a more detailed classification of flood storage works.

Table 10.1 Classification of flood storage areas

Type of reservoir



Dry-weather flows pass through the flood storage area


Dry-weather flows bypass the flood storage area


Flood storage area is kept essentially empty under dry-weather flow conditions


Most of flood storage area contains water under dry-weather flow conditions


Part of flood storage area contains water and part is essentially empty under dry-weather flow conditions

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.

10.2.1 Online flood storage

The components of online storage works normally include:

  • an impounding structure – generally comprises an earth or concrete structure across the river and floodplain, behind which the water is stored;
  • a flow control structure – normally located within the impounding structure to control the outflow from the storage area;
  • a spillway – to pass floods that are more extreme than those that the reservoir is designed to attenuate.

Figure 10.1 shows the basic components of an online flood storage area. The flow control structure can be a fixed throttle (such as an orifice, pipe or flume) sized to have little effect on normal flows, but requiring a significant rise in upstream water level to pass larger flows. Sometimes, the control structure incorporates gates, which are normally left open, but are operated during floods to ensure that downstream flows do not overtop the downstream flood defences.

Figure 10.1 Online flood storage area

As noted above, an online flood storage reservoir must also be designed to deal safely with more extreme floods than it is designed to attenuate; these could otherwise lead to the water level upstream of the impounding structure rising above safe levels or unacceptable damage to the impounding structure or surrounding land. This normally involves the provision of an overflow weir or spillway to convey the extreme floods. Sometimes the entire impounding embankment – or a major part of it – provides the overflow route for floods, having been designed to withstand the flow velocities involved. In other cases, the flow control structure incorporates the necessary arrangements for passing flood flows, either through gates or over fixed weirs and other structures.

10.2.2 Offline flood storage

Offline flood storage works generally consist of:

  • an intake structure to divert water to the storage area when the river flow or level exceeds a pre-determined value;
  • a storage area that comprises a reservoir separated from the river, formed either by low ground levels (natural or excavated) or by retaining structures (embankments, walls, or a combination of the two);
  • an outlet structure that returns water from the storage area to the river after the flood peak has passed;
  • a spillway to pass floods that are more extreme than those that the reservoir is designed to attenuate.

As illustrated in Figure 10.2, the intake structure (which is of course an ‘offtake’ from the perspective of the river) often includes two components that may be located separately:

  • a device designed to control flows passed downriver;
  • an inflow structure to the storage area.

The device controlling downstream flow can take most of the forms used for the control of online flood storage works. Rather than building a new control structure, a ‘natural’ control may be used on the river in some cases, or a pre-existing control such as an existing weir in others. In these cases, the offline storage works are designed to make use of the existing stage/discharge characteristics of the river.

Weirs (Rickard et al, 2003) are often favoured for the inflow structure, sometimes in the form of a sideweir (May et al, 2003) along the riverbank. They have the advantage of beginning to operate whenever river levels rise above a given value, but need to be long to limit the rise in river level needed to pass greater flows into the storage area. For this reason, a gated arrangement is often used.

Figure 10.2 Offline flood storage area

The offline flood storage area generally lies within the floodplain, but isolated by purpose-built walls or embankments (see Chapter 9). The volume available for storage in the reservoir depends on the water depth that can be obtained, which is controlled by existing ground levels, and the peak flood level that can be accommodated. This depth is often limited, making it necessary for the reservoir to cover a large area. Choosing a site where the ground is low – either naturally or as a result of excavations (for example, for gravel pits) – increases the depth available, but may mean that pumps are needed to empty the reservoir after the flood has passed.

The outlet may be by gravity (generally using gates), pumped or by a combination of the two (with gravity flow initially, then pumping to drain the lowest sections). The outlet capacity required depends on the volume stored and the time allowed for the system to be fully drained. Whenever the reservoir contains water – and so has less capacity to accommodate another flood – the standard of protection provided will be temporarily compromised. If the outfall is to a tidal river, it may only be possible to complete the emptying the reservoir at low tide, perhaps via a flapgate.

As for online flood storage works, arrangements for the safe passage of extreme floods are again needed to protect the reservoir and its associated structures. These may form a major part of the facility, particularly if the inlet to the storage area is uncontrolled.

10.2.3 Washland development

On some of the larger rivers in the UK, flood alleviation schemes have been developed which build on the natural behaviour of floodplains in alleviating the magnitude of floods that are passed downriver. They do this in a manner that goes beyond the simple concept of a single online or offline flood storage reservoir.

Such washland development schemes normally include most of the following features:

  • raised banks that separate the river from most of the floodplain;
  • subdivision of the floodplain into ‘cells’ separated from each other by dividing embankments;
  • weirs (which normally form parts of the riverbank) that admit flows into the washlands at a defined river stage, often when the river has virtually reached its maximum in-bank capacity;
  • high-level weirs that allow some flow between washland cells as they fill;
  • gates (sometimes flapped) to allow flows back from the washland to the river when the flood recedes.

Washland development schemes typically include no direct means of controlling river flows. Instead they rely on passing sufficient flows into the washland when the river level reaches a particular threshold.

An important feature of most such schemes is the presence of the banks between the washland cells. This maximises the retention of water in the floodplain, thereby reducing the degree to which the floodplain provides a supplementary flood conveyance route parallel to the river.

On many rivers, the natural processes of sediment deposition during floods have often resulted in that part of the floodplain adjacent to the river being a little higher than the swathe of floodplain lying further from the river. In addition, the farmers working the land have often traditionally raised the riverbanks to protect the farmland behind from such frequent inundation. Further raising and strengthening of these banks is usually part of a washland development scheme.

Examples of washland development include the ‘ings’ on the Yorkshire Ouse and its tributaries and many of the Fen river system washlands.


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