Fluvial design guide
This guide examines all the elements of river systems and catchments that need to be considered when designing engineering works to manage flood risk.
Documents
Fluvial design user guide
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The Fluvial Design Guide
The Fluvial Design Guide provides clear and consistent guidance to professionals responsible for planning, designing, maintaining and adapting engineering works aimed at managing flood risk from rivers. These include flood defences and works involved in draining land.
It’s intended to be used by both designers and asset managers. It also supports the training and professional development of practitioners in these fields.
The guide is intended for situations where flood risk management or land drainage is an important issue and a driver for action. It aims to support delivery of fluvial design in line with government policy.
The impact of engineering works involving rivers extends beyond the rivers themselves to cover a wide range of uses such as:
- navigation
- angling
- walking
- water supply
- wildlife
Aim and focus of the guide
The main focus of the guide is managing flood risk inland. It also covers the full river system and catchment which have a direct impact on the hydrology, geomorphology and ecology of the river and need to be considered in the design process.
It does not cover managing land use or surface water drainage or issues to do with salt water or waves although the impact of tides at the downstream end of river systems are discussed.
The guide aims to give enough information to understand the fundamentals of a problem and recognise when professionals can solve it themselves and when they should seek expert advice.
The guide is generally limited to what is termed ‘interventions’ in the fluvial environment including:
- ‘hard engineering’
- ‘soft engineering’
- maintenance interventions such as desilting, vegetation control and the repair of structures
Many of the detailed references included in the guide have been prepared on behalf of Defra or the Environment Agency, and represent good current practice. However, it should be emphasised that it is the designer’s responsibility to make sure that any designs are appropriate for the conditions to which the works will be subjected to during their life. In this context, the word ‘appropriate’ refers not only to the hydraulic performance and engineering stability, but also in the context of ecology, landscape, amenity, health and safety, and public acceptability. If in doubt, seek the advice of an expert in the relevant field.
The process of design in the fluvial environment is both complex and iterative.
The fluvial environment
An understanding of the fluvial environment is needed in order to design appropriate engineering in rivers and watercourses.
In the past, success was often judged on the hydraulic function alone, for example, in terms of ability to carry ‘the design flood’ without damage. Nowadays however, hydraulic performance is acknowledged as only one of the criteria on which success should be judged.
Other success criteria include:
- ecology
- geomorphology
- landscape
- amenity
- social acceptability
In the context of flood risk management, it’s helpful to examine flooding mechanisms in terms of a source–pathway–receptor model.
In this model: Source is rainfall pathways - the routes taken by flowing water receptors - people, buildings, infrastructure and environment at risk from flooding.
This guide deals mostly with the source and pathway elements of the model.
Working within the fluvial environment
The design of any engineering works, including the fluvial environment, is an iterative process.
A series of related activities run alongside the development of the design and feed into it. These include:
- the consideration of risks and uncertainties
- the collection of data
- consultation with stakeholders
- understanding the approvals process and the securing of funds for the works
Although it’s not fundamental to the design process it’s essential to recognise these related activities.
Risk and uncertainties
The design of successful engineering interventions in rivers depend on a thorough understanding of risk and uncertainty.
It’s important for the designer to adopt a systems approach which looks at the whole system and not just one particular element. For example, raising a weir crest to improve navigation can have impacts upstream, including an increased risk of flooding. It may also restrict fish migration and change the aquatic environment. Similarly, desilting a reach of channel in a flood defence system may well solve a local problem by lowering the flood level, but downstream communities may suffer increased flooding and there could be adverse environmental consequences.
Risks
Risks are many and varied and include underestimating a flood level and encountering strong local opposition to a scheme.
The designer must identify all the potential risks well in advance in order to manage them and to reduce or eliminate the adverse consequences. This proactive approach also allows the designer to identify opportunities for enhancements that may provide benefits beyond the original flood defence or land drainage objectives of the scheme.
Uncertainties
Uncertainties can be reduced by better data, more detailed analysis, and wider consultation with stakeholders.
There will always be uncertainties but these should be identified and evaluated as much as possible. For example, the estimate of the 1% (annual exceedance probability) flood flow may have an error margin of ±20%. Therefore, the designer should at least examine the impact of a flow 20% greater than the best estimate in order to understand and address potential consequences.
The human dimension
Wherever fluvial works are carried out there will be a human dimension to consider. This will depend on the type and location of the scheme.
Schemes that are developed and managed with the full involvement of the local community, stakeholders and riparian owners have a better chance of success. It’s also important to consider the amenity of a river (walking, canoeing, angling) to gain support from all interested parties.
Health and safety
Health and safety must be embedded in the design process. This includes people who will come into contact with the works during construction as part of the operation and maintenance and/or members of the public.
Sustainable solutions
Flood risk management works and other fluvial interventions must be sustainable. This means designing solutions that do not place an unacceptable burden on future generations.
It’s impossible to completely plan sustainable choices because changing climate, tougher environmental legislation and the growing world population constrain designers’ options.
These constraints increase incentives to look at more natural solutions such as reducing runoff at source and reconnecting rivers to their floodplains. The incentives however must be addressed in the light of pressures to build more houses, factories and infrastructure. Catchment flood management plans help to inform this process.
It’s clear that we cannot go on building ever higher flood defences. In some locations, the more sustainable answer will be managed realignment, even if this means the loss of some buildings.
The project was started in 2005 and completed in 2010.