Fluvial Design Guide - Chapter 10

Flood storage works

10.3 Planning for flood storage

This section covers the key issues to be considered early in the design process. More detailed coverage of most of the topics, including a 20-step design procedure illustrated by a flowchart, is given in Chapter 2 of Design of flood storage reservoirs (Hall et al, 1993). Box 10.3 highlights some of the factors that make a good flood storage site.

Box 10.3 What makes a good flood storage site?

  • A suitable location within the catchment for the purpose intended – controlling flows from a large enough proportion of the catchment upstream of the location where protection is needed.
  • Sufficient storage volume.
  • A suitable site for the impoundment structure – for example taking advantage of a narrower part of the valley to allow the dam to be shorter.
  • A wide floodplain that allows a low dam height to be deployed.
  • A relatively impermeable foundation.
  • Suitable foundation conditions for supporting the dam and control structures.
  • Suitable access for construction, operation and maintenance.
  • The availability of suitable construction materials on or near the site.
  • Minimum adverse impacts on landowners, land-use and local residents.
  • Minimum adverse impacts on the environment.
  • Opportunities for environmental enhancement.

10.3.1 Scale and location of storage

Flood storage not only comes in various types, but also in a wide range of sizes. These are primarily related to its strategic purpose or its place in the catchment. This is illustrated in Figure 10.3, which focuses on successively smaller areas.

As noted in Section 10.1.1, the location where flood storage is provided can be crucial. Flood storage works have the potential – in some locations and for some types of storm event – to aggravate downstream flooding, so their effects need to be investigated rigorously at the design stage by appropriate flood modelling studies.

Figure 10.3 Examples of storage scale, location and purpose


Strategic catchment storage (a)

Flood storage reservoir (a) built to protect urban area A from the effects of developing area B within its catchment.

Subcatchment storage (b)

Flood storage pond (b) built to alleviate the effects of urbanisation within a subcatchment and provide protection in the lower reaches of the watercourse in urban area B.

Local on-site storage (c)

A flood storage pond or tank (perhaps of the SUDS type) installed within or close to the source of increased runoff from a new  development at the edge of urban area B.

10.3.2 Design objectives

Before design can begin, it is necessary to understand what it is intended to achieve through the provision of flood storage – either on its own or as part of a portfolio of measures – and the design return period (standard of service) that it is expected to provide. The design objectives may depend on the strategic purpose of the flood storage (see above), but would typically include at least one of the following:

  • controlling the flow hydrograph passed downriver, for example to mimic the natural or pre-development runoff from its catchment up to the design return period;
  • controlling the maximum flow passed downstream to a pre-determined fixed amount related to the estimated ability of the downstream watercourse to accommodate it;
  • controlling the flow released in response to flood levels being experienced a short distance downstream;
  • controlling the downstream flows as part of a strategy (for example, in conjunction with other flood storage reservoirs) to control the flows and water levels at some remote location(s) downstream.

In the last of these cases, there is increasing scope for operating a series of flood storage reservoirs in a manner that is guided by a real-time flood forecasting model.

10.3.3 Storage capacity

An early estimate of the storage capacity needed – together with an estimate of the storage capacity available at the potential site – is the key to discovering whether a scheme based solely on flood storage is likely to be suitable, or whether it needs to be combined with other measures. Figures 10.4 and 10.5 illustrate this for a simple online flood storage pond, where the primary purpose is to limit the flow passed downstream to a particular threshold.

Figure 10.4 Ideal utilisation of storage

In this case, the pond is designed to pass downstream the full flood up to threshold, storing all the water in excess of the threshold. In theory, this ideal performance can be achieved through the use of gates but, in practice, it is impossible to achieve this ideal utilisation of flood storage.

Figure 10.5 Practical utilisation of storage

In a practical scheme, the control device starts to affect the flow passed downstream well before the threshold is reached. A greater volume of flood storage is therefore required.

Examination of the available mapping for the proposed site should give an early indication of the average inundation depth needed over the chosen area to provide the flood storage needed, and therefore whether the flood storage scheme is potentially feasible.

In practice, the site may not yet be selected and it may be necessary to consider alternative sites, or the provision of a number of flood storage areas that are designed to act in conjunction.

10.3.4 Construction issues

Early consideration needs to be given to the practical issues associated with the construction of flood storage works. These include:

  • the need for a site investigation to inform the design of the works and establish the suitability of materials for reuse;
  • the availability of suitable construction materials on site;
  • the potential for balancing cut and fill and thus minimising the earthmoving (especially by public highway) associated with the scheme;
  • sources of imported materials and the disposal of waste materials;
  • whether the site involves contaminated land that might require remediation or special precautions and design features;
  • the presence of services – especially overhead power lines and buried pipelines – or other constraints, and the implications for the types of construction plant that can be safely deployed;
  • access to the site for site investigation and surveys, construction work, and subsequent maintenance and operation;
  • the impacts of the scheme – including construction work – on the environment, landowners, local residents and others, which would normally be the subject of an environmental impact assessment (EIA).

10.3.5 Reservoirs Act 1975

The Reservoirs Act 1975 applies to all reservoirs ‘designed to hold, or capable of holding’ more than 25 000 m3 of water ‘above the natural level of any part of the land adjoining the reservoir’. The reference in Section 16 of the Act to protecting ‘persons or property against an escape of water’ makes it clear that its primary purpose is public safety. The Act imposes a number of duties concerned with the design, construction and ongoing ownership of such reservoirs. Box 10.4 gives further information. The Institution of Civil Engineers (ICE) has produced a useful guide to the Reservoirs Act (ICE, 2000).

Box 10.4 Statutory control over reservoir and dam safety

Regulations are adopted in many countries to ensure that reservoirs and dams are inspected regularly and constructed or altered only under the charge of properly qualified engineers.

Reservoir safety in the UK is governed primarily by the Reservoirs Act 1975, which was brought into effect in 1986, replacing the earlier Reservoirs (Safety Provisions) Act 1930.

The 1975 Act applies to ‘large raised reservoirs’ which are defined as having a capacity of 25 000 m3 or more above the level of any part of the adjacent land. Specifically excluded from the 1975 Act are:

  • lagoons covered by the Mines and Quarries (Tips) Act 1969 (together with the corresponding 1971 Regulations and the Quarries Regulations 1999);
  • navigation canals.

The Act includes roles for the Secretary of State, for the ‘undertaker’ (the owner and/or operator of the reservoir), for the ‘enforcement authority’ and for ‘qualified civil engineers’. A series of ‘panels’ of such engineers have been set up to perform the particular functions under the Act, namely:

  • construction engineer – responsible for supervising the design and construction or the enlargement of a reservoir;
  • inspecting engineer – carries out periodic inspections, normally at intervals of ten years, supervises and certifies remedial works affecting reservoir safety;
  • supervising engineer – has a continuous appointment to ‘supervise’ the reservoir, watching out for problems and keeping the undertaker informed.

It is the undertaker’s duty to ensure that the relevant engineering appointments are in place and the requisite duties undertaken during the design, construction, operation and decommissioning of a reservoir. There are significant cost implications involved in compliance with the Reservoirs Act 1975.

Three panels have been set up to cover the duties of construction engineer and inspecting engineer. These panels are defined according to the type of reservoir on which the engineer is qualified to exercise their duties and are as follows:

  • all reservoirs panel – all reservoirs covered by the Act;
  • non-impounding reservoirs panel – all reservoirs, except impounding reservoirs;
  • service reservoirs panel – service reservoirs only.

All engineers on these panels are also entitled to act as supervising engineer for all reservoirs covered by the Act. In addition, the supervising engineers panel covers those qualified to act only as supervising engineers. Panel appointments are made for a period of five years and are renewable.

The 1975 Act contains provisions for the registration of reservoirs and for the enforcement of the Act, both of which functions were transferred to the Environment Agency in October 2004.

The enforcement of the Act is facilitated by requirements for the issue of certificates by the qualified civil engineers under specific circumstances, in particular in association with:

  • the construction or enlargement of a reservoir;
  • the issue of inspection reports;
  • the implementation of ‘measures taken in the interests of safety’.

Appointments of qualified civil engineers by the undertakers also have to be notified to the enforcement authority.

The Water Act 2003 contained a number of changes to the Reservoirs Act 1975, including transferring the role of the enforcement authority to the Environment Agency and creating a new power to direct an undertaker to prepare a ‘flood plan’ (see Box 10.5) for a large raised reservoir.

The ICE’s A guide to the Reservoirs Act 1975 (issued in 2000) contains the full text of the Act, together with the relevant regulations up to that date, a commentary on the application of the Act and flowcharts to illustrate the duties of the various parties. Anyone engaged on work involving reservoirs in the UK is advised to refer to this guide and to guidance on the Environment Agency website (http://www.environment-agency.gov.uk/business/sectors/32427.aspx).

Particular points worth noting with regard to the Reservoirs Act 1975 are:

  • responsibility for the construction or enlargement of a reservoir remains with the construction engineer for between three and five years from first filling;
  • a supervising engineer is not required until the end of the construction engineer’s involvement, upon issue of their final certificate;
  • the first inspection under Section 10 of the Reservoirs Act 1975 is required within two years of the issue of the final certificate;
  • the inspecting engineer must be ‘independent’ of the construction engineer and undertaker.

The provisions of this Act and the earlier Act of 1930 have worked well, in that they have assisted in preventing any dam failures involving a loss of life in the UK for three-quarters of a century. A particular feature of the 1975 Act is that it imposes a personal responsibility on the engineer who issues a certificate. Such responsibility can only be effectively exercised by an engineer having adequate experience of dam design and construction, and who has access to the specialist services frequently required in making a proper inspection.

Consideration is currently being given to further legislative changes that would lead to a risk-based approach to the statutory registration and control of reservoirs and encompass smaller reservoirs (down to 10 000 m3). For further information see: http://www.defra.gov.uk/environment/water/flooding/flow/index.htm.

Box 10.5 Reservoir flood plans

The Water Act 2003 contained a number of changes to the Reservoirs Act 1975, including creating a new power allowing the enforcement authority to direct an undertaker to prepare a ‘flood plan’ for a large raised reservoir. The flood plan is intended to improve emergency preparedness and covers:

  • any escape of water from the reservoir in terms of the predicted extent and depths of inundation;
  • the risks to life and property;
  • arrangements for issuing warnings.

The proposed arrangements for reservoir flood plans are currently being reviewed by the Environment Agency, Defra and their consultants. The detailed requirements have not yet been fully formulated, but it is expected that a five-year programme of directions to prepare flood plans for selected reservoirs will commence in 2009–2010. In the meantime, it would be prudent to assume that a flood plan will be required for any flood reservoir that is currently being planned. For current guidance, see the Environment Agency website (http://www.environment-agency.gov.uk/business/sectors/32427.aspx).

There has been concern in recent years that many small British reservoirs that do not come within the scope of the Reservoirs Act 1975 may nevertheless represent a significant hazard to some communities by virtue of issues such as their particular location, steepness of the valley, height of the structures and state of repair. The principles embodied in the Reservoirs Act 1975, and the guidance associated with it (ICE, 2000), are also applicable to reservoirs that do not come within its scope. Regardless of whether their reservoir comes under the Reservoirs Act 1975, owners and undertakers also need to be aware that anyone who stores water is responsible under common law (based on the historic case of Rylands v. Fletcher) for any damage that may be caused to any other party arising from the release of that water.

Nothing in this guide should be taken as overriding or diminishing the responsibilities resulting from the Reservoirs Act 1975; nor the responsibilities resulting from the design, construction or ownership of bodies of water which, whether or not they come within the scope of the Act, could represent a significant hazard to people and property situated downstream.

10.3.6 Land acquisition and compensation

Land acquisition can take many years – either in protracted negotiation or need for compulsory purchase. If there is a risk that compulsory purchase will indeed be required, the process should be started as soon as possible. Specialists in land negotiations need to be brought into the team at an early stage to advise on the implications for the feasibility of the scheme and its implementation programme.

In some cases, the entire area of the flood storage works may be acquired. In other cases, it may be possible for the impoundment area to serve other purposes (such as agriculture or playing fields), in which case the promoter may not need to acquire it but would instead acquire a ‘right to flood’. In this case, the owner would receive compensation, either as a commuted sum based on the anticipated detriment to its future use, or upon each occasion that the land floods more deeply than it would have done naturally. The loss of development potential could be a significant element of compensation payable.

10.3.7 Planning consent

Most flood storage works require planning permission. If flood storage is being promoted as an integral part of a development, which requires the flood storage works in order to proceed, then the application may be incorporated within that for the development proposal. If the implementation of flood storage works is imposed as a condition of a development proceeding, a separate application may be made.

For a scheme designed to alleviate an existing flooding problem, the scheme would normally be the subject of a planning application in its own right. Even if planning permission is not strictly required (for example, if the scheme comes within the scope of an organisation’s permitted development rights), it is often considered good policy to seek permission in order to give legitimacy to the scheme.

Appropriate planning advice needs to be obtained at an early stage to ensure major pitfalls are avoided.

10.3.8 Environmental impacts and opportunities

It is often assumed that the provision of a flood storage reservoir will automatically result in environmental benefits. This may be the case, but there are often also disbenefits. One of these is the loss of productive agricultural land. With an increasing population and the demand for crops for both food and energy, this issue needs careful consideration to get the balance right.

In common with most works involving construction, flood storage works are likely to have a number of environmental impacts. These have to be addressed through an assessment that may be required as part of the planning process, or that may be undertaken under the Environment Agency’s role as the ‘competent authority’. The impacts are likely to be associated with construction itself, including access to and from the site, as well as the residual impacts of the works.

This assessment takes account of impacts on the different receptors (that is on people, property, infrastructure and land affected) and attributes a level of significance to each impact. The assessment includes all aspects of the environment from the natural environment, species and habitats through to the community and its health and wellbeing. Every project is screened to determine whether it requires an environmental impact assessment.

Some of the permanent impacts are usually beneficial to various flora and fauna, while some of the adverse impacts can be mitigated through the inclusion of appropriate design features and operational practices. Where opportunities to enhance the environment exist, every effort should be taken to adopt them. But it should not be assumed that all flood storage basins can offer environmental benefits, as their impact could be negative.

Chapter 6 of Design of flood storage reservoirs (Hall et al, 1993) describes the potential for flood storage works to offer environmental benefits, focussing on the impacts of detention on water quality.

10.3.9 Landscaping

Appropriate integration of landscape design (see Chapter 5) during the design development for flood storage works is often an essential element in reducing their impact and making the scheme acceptable. It is important, however, to give proper regard the following issues, some of which are related to reservoir safety.

  • Floodbanks and floodwalls need to be inspected from time to time and should not be hidden from view. The planting design should ensure that adequate inspections can be made.
  • Flood storage works should be designed and maintained so that colonisation by burrowing animals is discouraged.
  • The root systems of trees and bushes can be detrimental to various components of flood storage works, but especially to embankments.
  • Embankments should be covered with grass (or some other form of sympathetic erosion protection) and suitably maintained to aid inspection and prevent colonisation.
  • In planning landscaping and planting schemes within the impoundment area, due account should be taken of the risks that vegetation debris can pose to the proper hydraulic operation of the flood storage works. (Similarly, the design of the flow conveyance structures needs to take into account the potential impacts associated with the agreed landscaping plans, for example in terms of the anticipated debris load on trash screens.)

10.3.10 Multiple uses

Flood storage works can often provide opportunities for multiple uses. Section 7.4 of Design of flood storage reservoirs (Hall et al, 1993) covers this at some length and gives the following examples:

  • amenity – such as public open space and parkland;
  • car parking – though this is generally not to be recommended because of the risks associated with cars remaining when the flood arrives and floating away, adding to the downstream risks;
  • recreation – fishing, sports fields and the like;
  • agriculture – livestock and crops (subject to the availability of a livestock escape route, avoiding livestock damage to the works and crop tolerance to inundation);
  • water quality – sediment control and removal of biological oxygen demand (BOD);
  • nature conservation and ecology – wetland habitat and nature reserves.

These opportunities need to be considered carefully as there can be implications for public safety and the proper functioning of the works in their flood storage role.

In some situations, an existing reservoir may be modified – or its operational practices modified – so that it provides a flood storage and attenuation function.

10.3.11 Safety

The particular issues associated with the safety of flood storage works are listed in Table 10.2. Some of these are common to other fluvial works.

Table 10.2 Safety issues for flood storage works


Nature of hazard

Scope for mitigation

Culverts and operating equipment

Public entry to culverts, risk of drowning or injury by operating mechanical equipment.

Exclude public from hazardous areas.


Tendency to block during floods, requiring clearance in hazardous conditions, sometimes at night.

Provide safe access to remove collected debris (including lighting, safety harness facilities).

Dispense with trash screens where possible.

Otherwise use sound and conservative trash screen design.

Gates (manual or remote operation)

Need to attend during floods, either to operate or deal with failure of gates to operate correctly.

Automatic gate operation.

Adopt non-mechanical control devices where practicable.

Otherwise provide safe access and appropriate operating rules.

Gates fail to operate correctly, for example due to power failure.

Provide back-up power and operating systems.

Danger to public during operation.

Provide warning signs and klaxons, information boards.

Or exclude public access.

Embankment maintenance

Grass cutting on slopes and maintaining access to adjoining structures

Adopt mild slopes, preferably no steeper than 1 in 4, with gentle curves.

Public footpaths etc

Normally negotiable route becomes impassable during operation of flood storage scheme.

Provide clear signage (with lighting where appropriate), including alternative routes.

Display public information about the scheme.

Water body

Falling in and drowning or suffering hypothermia.

Have shallow margins with reeds and use planting to discourage access to the water’s edge.

Large drops

Falling into deep or swiftly flowing water, especially if a drop is obscured below rising water.

Fit handrails and warning signs.

Provide escape ladders.

Provide lighting; lifebelts and rescue lines.

Steep slopes adjoining impoundment

Slipping down waterlogged slope and unable to gain a foot-hold to escape.

Use gentler slopes (no steeper than 1:3).

Restrict water depth at toe of slope to no more than 0.5m where practicable.


Children using the works for various recreational purposes and being subjected to any of these hazards.

Make sure the mitigation measures referred to above are suitable for children and not just for adults.


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