Fluvial Design Guide Overview

The Fluvial design guide is aimed at professional staff engaged in the design process from the early stages of looking at alternative solutions through to the delivery of the outputs of design for the construction, maintenance, refurbishment or alteration of flood defence or land drainage assets. The guide is thus intended to be used by both designers and asset managers. In the field of flood risk management, the management of existing assets (including their ultimate replacement or disposal) is probably more important than the design of new works – simply because there are so many of them.

The design of any engineering works is an iterative process and the design of works in the fluvial environment is no exception. It is rarely possible to go through the design process without having to step back and re-examine earlier decisions. In addition, a series of related activities run in parallel with the development of the design and feed into it. These include the consideration of risks and uncertainties, the collection of data and consultation with stakeholders. There is also the need to understand the approvals process and the securing of funds for the works. Recognition of these related activities is the key to successful delivery of the outputs of design, though not fundamental to the design process.

The aim of this guide

The aim of this guide is to give you enough information so that you can understand the fundamentals of the problem you are facing and thus appreciate when you can solve it yourself and when you need to seek advice from an expert. If you have to take the latter course, the guide should give you enough understanding to ask the expert the right questions and to make sense of the advice given.

The scope of this guide

The scope of this guide is generally limited to what can be termed ‘interventions’ in the fluvial environment. This includes both ‘hard engineering’ and ‘soft engineering’ as well as maintenance interventions such as de-silting, vegetation control and the repair of structures.

The guide is intended primarily 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, as set out in Defra’s flood management strategy, Making space for water.

The fluvial environment

It is not possible to design appropriate engineering works in rivers and watercourses without an understanding of the fluvial environment in its widest sense. The need for this approach becomes apparent if we consider what we mean by ‘appropriate engineering works’. 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, hydraulic performance is acknowledged as only one of the criteria on which success should be judged, alongside considerations of ecology, geomorphology, landscape and amenity, as well as social acceptability.

In the context of flood risk management, it is helpful to examine flooding mechanisms in terms of a source–pathwayreceptor model in which the source is rainfall, the pathways are the many routes taken by flowing water, and the receptors are the people, buildings, infrastructure and environment at risk from flooding. This guide deals mostly with the source and pathway elements of the model.

Risk and uncertainty

The design of successful engineering interventions in rivers depends on a thorough understanding of risk and uncertainty. The risks are many and varied, and range from underestimating a flood level to encountering strong local opposition to a scheme. Uncertainties can be reduced by better data, more detailed analysis, and wider consultation with stakeholders.

By identifying all the potential risks well in advance, the designer is in a much better position to take action to manage these risks and to reduce or eliminate the adverse consequences. This proactive approach also enables the designer to identify opportunities for enhancements that may provide benefits beyond the original flood defence or land drainage objectives of the scheme.

Uncertainties will always remain, but they are more readily addressed if they are identified and evaluated as far as possible. For example the estimate of the 1% (annual exceedance probability) flood flow may have an error margin of ±20%; in such cases, the designer should at least examine the impact of a flow 20% greater than the best estimate so that the potential consequences can be understood and addressed.

In assessing risks and uncertainties, it is important to adopt a systems approach in which the designer 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, de-silting 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.

The human dimension

Wherever fluvial works are carried out, there will also be a human dimension to take into account. This takes many forms, depending on the type of scheme and its location.

Schemes that are developed and managed with the full involvement of the local community and all stakeholders have a much better chance of success. Consideration of the amenity use of a river (walking, canoeing, angling, for example) is one step in the process, but the aim should be to gain support from all interested parties – not least of all from riparian owners. Nowhere is this more necessary than in the urban environment, where houses and factories often compete for space with a long-neglected watercourse.

All design work must be carried out with due regard to health and safety. This covers all people who will come into contact with the works whether during construction, as part of their operation and maintenance, or as members of the public. Consideration of health and safety must be embedded into the design process and not tacked on as an afterthought.

Sustainable solutions

The design of flood risk management works and other fluvial interventions must also address the issue of sustainability. Fundamentally, this means looking for solutions that do not place an unacceptable burden on future generations.

Sustainability is not an absolute: changing climate, tougher environmental legislation and the growing world population act to constrain the choices that designers have. This increases the incentives to look at more natural solutions such as reducing runoff at source and reconnecting rivers to their floodplains, but these incentives have to be addressed in the light of pressures to build more houses, factories and infrastructure. Catchment flood management plans help to inform this process.

It is clear that we cannot go on building ever higher flood defences and, in some locations, the more sustainable answer will be managed realignment – even if this means the loss of some buildings.