Fluvial Design Guide - Chapter 1

Design of works in the fluvial environment

 

1.4 Basic concepts

This section sets out some of the basic information that a designer needs to know and understand when designing for the fluvial environment. In line with the character of this chapter, it covers issues relevant to fluvial design as a whole. Subsequent chapters describe crucial aspects of the basic concepts in further detail.

It is important to recognise that in fluvial design in general – and flood risk management in particular – a full understanding of the historical context greatly assists the development of appropriate solutions. This not only relates to the history of flooding problems (past events, mechanism of flooding, records of flows and levels, significance of blockages, loss of floodplain, for example) but also to the morphological, environmental and anthropomorphic history associated with the river system.


1.4.1 Roles and responsibilities

Legal framework

The legal basis for flood risk management in England and Wales is set out principally in the Water Resources Act 1991, the Environment Act 1995 and the Land Drainage Act 1991. Further information on these and other acts, and those organisations with responsibilities or powers under them, is given in Land drainage and flood defence responsibilities (ICE, 1996). These acts describe the roles and responsibilities of the operating authorities and form the basis for their operational, supervisory, regulatory and executive powers to do work in the fluvial environment. With respect to flood risk management, the operating authorities are primarily the Environment Agency for all main rivers, the Internal Drainage Boards (IDBs) for their respective internal drainage districts, and local authorities for non-main rivers.

The new European Union Directive on the assessment and management of flood risks (EU Floods Directive) will require the production of flood risk management plans and maps as part of a strategic planning framework for fluvial flood risk management (see ‘Strategic planning framework’). In the context of flood risk management in England, Defra’s policy document Making space for water (2005) sets out the aims and objectives, constraints and opportunities for the future.

The environment and human use of the river system can be both driver and constraint for fluvial works. Legislation of particular importance to the fluvial environment includes:

  • The Conservation (Natural Habitats, &c.) Regulations 1994 – otherwise known as the Habitats Regulations – which implement the EU Habitats Directive in the UK;
  • Water Framework Directive at the European level;
  • various other acts and regulations at the national level including those relating to strategic environmental assessment (SEA) and environmental impact assessment (EIA).

The EIA screening process is particularly important, as it often highlights other legislative requirements. Further information on these requirements and their implications for fluvial design is provided in Chapters 4, 5 and 6.

It is a fundamental requirement of any works in the fluvial environment that the rights and responsibilities of the riparian owner and all other involved parties are understood and accepted prior to, or during, the design process. In particular, it is essential that those parties who will be responsible for the operation and maintenance of any asset or intervention are fully aware of these responsibilities and accept them throughout the life of the asset.


Stakeholders and consultation

Designing in the fluvial environment involves the development of management interventions to alter or support the workings of the fluvial system. As a result, it is necessary to ensure effective consultation with the stakeholders in the design process, primarily for two purposes:

  • early input to the planning and design process to identify all relevant information, issues and constraints and to exploit any ‘win–win’ opportunities;
  • feedback of emerging plans to the various stakeholders, explaining how their inputs have affected the outcomes, identifying any outstanding issues and encouraging wider acceptance and ownership.

Three main types of organisation are particularly relevant in the context of carrying out works in the fluvial environment. These are:

  • those with operational responsibilities or powers to manage or carry out works to maintain the functional performance of the fluvial systems;
  • those that regulate aspects of the systems;
  • those that are directly affected by management activities.

The main organisations relevant to fluvial design works in England and Wales include:

  • Natural England, Countryside Council for Wales, English Heritage and Cadw (the historic environment service of the Welsh Assembly Government) – in relation to the regulation and protection of the natural and historic environment;
  • regulatory functions of the Environment Agency – for issues affecting fisheries, ecology, recreation, pollution prevention, waste, water resources and flood risk management;
  • local planning authorities – in relation to requirements for planning or other consents;
  • navigation authorities such as British Waterways and the Environment Agency;
  • landowners, occupiers and local population potentially affected by the design works, including by access to works;
  • managers or regulators of other utilities or major infrastructure networks such as county councils (highways and public rights of way), Network Rail (railway infrastructure), Transco (gas pipelines) and the relevant electricity and telecommunications companies;
  • local conservation and recreation interests such as wildlife trusts, angling and boating clubs.

The success of the consultation process relies on:

  • having a clear consultation plan;
  • ensuring adequate records are kept;
  • making genuine efforts to take on board feedback while maintaining the primary objective of the design.

Advice on appropriate consultation approaches is available in Sustainable flood and coastal erosion risk management (Wade et al, 2007).
 


Strategic planning framework

Within flood risk management, this guide supports the implementation of design and management solutions developed through delivery plans, which are typically strategy plans or system asset management plans (SAMPs). Where these delivery plans exist, they are in turn based on policy as set by catchment flood management plans (CFMPs) and shoreline management plans (SMPs).

The strategic planning framework is illustrated in Figure 1.3. In particular, it shows how this guide (represented by the two boxes closest to the bottom right corner) fits into the framework, as well as how flood risk management planning fits into the wider development planning framework.

Figure 1.3 Strategic planning framework for flood risk management

The starting point of any intervention within the fluvial environment is ensuring a clear understanding of the higher level policies, strategies and plans that relate to the associated system. It is then important to develop appropriate management solutions in line with the wider strategic approach.
 


1.4.2 The fluvial system

Overview

In theory, the fluvial system is made up of all land (whether or not it is formally recognised) that conveys or manages water, or where water is naturally stored, runs off, or infiltrates to the strata beneath. Water enters the fluvial system:

  • following rainfall;
  • from coastal inundation;
  • from springs and rising groundwater;
  • from infrastructure systems such as dams and piped drainage networks.

Unless prevented by a mechanical means such as pumping, once water collects on a surface in sufficient quantities, it flows downhill from its source and is transported overland or by natural or man-made conveyance systems through natural and built areas en route to the sea. Water may also spread out as it makes its way downstream, depending on the volume of flow, the conveyance capacity and the topography. The extent to which water spreads out during its passage downstream depends on the topography and constrictions along its length such as dams and culverts, and across its width such as embankments, walls and buildings within the floodplain.

The effect of the water once it leaves the normal waterway through a flood pathway depends on the type of receptors in the inundated area. This is illustrated by the source–pathwayreceptor model in Figure 1.4.

Figure 1.4 Sources, pathways and receptors of flood risk

The source–pathwayreceptor model is a useful tool for understanding flood risk and flooding mechanisms.

 

System interactions

The hydraulic behaviour of a fluvial system is interactive, with the conditions or characteristics of one part of the system having effects on the flows and levels within other parts. For example, a constricted part of the system reduces the conveyance through it, thereby affecting the water levels upstream. These changes may also affect the transport of sediments and debris within the system. Such changes could lead to sediment deposition and associated loss of conveyance in some areas and sediment starvation or riverbank scour in others.

The effect of constrictions becomes marked at larger flows and even more significant if the constriction is surcharged or restricted by blockage. The extent of the impact on upstream levels depends on the backwater effect, while its significance depends on the effect of the raised water levels on the potential pathways into the receptor area. These effects can range from the direct effect of water level and waves leading to overtopping of the riverbank or flood embankment, to increased water forces leading to piping, structural damage or a breach of the defences (see Section 1.4.3).

The interactive nature of a fluvial system demands the assessment of the performance interventions and management interventions applied to it at a system scale, with the extent of the system or subsystem being determined by the relevant area of hydraulic or other influence. See Chapter 7 for more detailed information about hydraulic analysis.
 


1.4.3 Flood risk management

Flood risk management within the fluvial environment often requires management of the engineering performance of fluvial systems such as the conveyance capacity. This can take the form of occasional major improvement works to achieve a significant step change in engineering performance or increased maintenance activities, or sometimes both.

Management of flood risk normally involves the reduction of either the probability of flooding (through management of sources and pathways) or the consequence of flooding (through management of the receptor), or both. This guide is primarily concerned with managing the probability of flooding. Consequence management is addressed in other publications including:

  • Planning Policy Statement 25: Development and flood risk (CLG, 2006) and its practice guide published by Communities and Local Government (CLG);
  • various guides on property-level flood resistance and resilience published by CIRIA, the Environment Agency, CLG and Defra.

In the broadest sense, approaches for reducing the probability of flooding to a desired level of flood risk typically include one or more of the following:

  • Holding back water and releasing it at a reduced rate, thereby reducing peak flows and lowering peak water levels. This includes surface water management measures, for example as provided by:
    • sustainable drainage systems (SUDS; see Woods-Ballard et al, 2007);
    • the provision of flood storage reservoirs (see Chapter 10);
    • bringing previously lost areas of the floodplain back to functional use.
  • Lowering the flood levels within the channel. This typically involves:
    • channel deepening;
    • channel widening;
    • dredging;
    • weed-cutting;
    • removal of blockages and constrictions;
    • removal or lowering of weirs.
  • Provision of raised defences to block the pathways of flooding. This approach includes the use of floodwalls, embankments, and temporary or demountable flood protection systems.
  • Diversion of the water away from areas at risk – including flood relief or diversion channels.

These approaches are illustrated in Figure 1.5. This diagram illustrates local runoff control measures within a defended area, but the same approach is also applicable upstream. Chapters 8 to 11 cover the most relevant types of intervention in more detail.

Figure 1.5 Four ways to manage the probability of flooding


1.4.4 Asset management

Performance and reliability of assets

Asset management is a process that seeks to manage continuously the performance, risk and whole-life cost of the associated infrastructure. As explained in Section 1.3, performance objectives are set at the start of a project. These give rise to performance requirements for the associated assets or components. It is therefore important that the designer takes account of the mechanisms that can lead to failure of the asset to perform as intended.

Performance is the extent to which an asset fulfils its intended function; reliability is the probability that the asset does not fail. Risk, performance and uncertainty in flood and coastal defence – A review (Sayers et al, 2003) is the primary reference and source of definitions and concepts of flood risk and associated performance, reliability and uncertainty. Note that some have been updated in a subsequent report from the joint Defra/Environment Agency flood and coastal erosion risk management R&D programme (Buijs et al, 2007).

The performance requirements and associated failure mechanisms can be different for each asset type. For example, a flood defence or reservoir has to act as a barrier, so the failure mechanisms concern overtopping, breach or seepage. In contrast, a watercourse has to convey water, so the failure mechanisms concern blockage or increased resistance. Chapter 9, for example, identifies the specific failure mechanisms for the main asset types in fluvial design. In addition, the other functions of an asset (for example, improving a habitat) impose their own performance requirements for that function, together with associated failure modes.

The performance of a fluvial system depends on how the individual assets within it perform individually and interact as a system. Fluvial design should always consider performance requirements for a whole range of loadings on the system, including the maintenance of ecological, heritage or social functions where these are defined objectives. For example, a channel may be designed to ensure that average summer flows are contained within a smaller channel to maximise flow velocities and minimise siltation, with further flows being contained within a flood defence up to a specified probability of occurrence.

Aspects of performance requirement during extreme events may include a serviceability requirement (such as limiting overtopping of some footpath or road to a maximum rate of overtopping for a specified probability of occurrence) or a requirement for safe overtopping up to some higher flow.

The continued performance and reliability of assets and their associated systems are affected by uncertainties and deterioration over time. These aspects and how they can be managed or accounted for in design are discussed below.

Uncertainty

Every design process has to deal with uncertainty. It can be helpful to distinguish different types of uncertainty, as this determines the best way to handle it:

  • Uncertainty in nature – caused by the huge complexity of interaction inherent to natural systems. An example of this is climate change and its likely impact on flood risk.
  • Knowledge uncertainty – resulting from limitations in our knowledge of the state of a physical system, and our ability to measure and model it. Two types of knowledge uncertainty can be distinguished:
    • Statistical uncertainty – for example, the uncertainty in determining the severity of an extreme discharge resulting from the extrapolation of a limited dataset and from the selection of the probability distribution.
    • Process model uncertainty – for example, the uncertainty caused by the fact that numerical models are not perfect, including the uncertainty about climate change.

More detailed guidance on types of uncertainty is provided in Risk, performance and uncertainty in flood and coastal defence – A review (Sayers et al, 2003).

The best way to analyse uncertainty in fluvial design depends on the type of uncertainty under consideration. In a general sense, it is important to make uncertainty explicit: it is good practice to go through the design process using the best estimate of each parameter, while keeping track of all the uncertainties that are encountered along the way. It is also important that the uncertainties are clearly communicated as part of the design outputs.

Where uncertainties are explicitly allowed for (for example, by the inclusion of freeboard in a defence height), the assumptions made should be clearly recorded. This will enable future design and operational decisions to be based on a full understanding of the original design. For example, how the information on uncertainty is used in designing trigger levels for evacuation during a flood warning may differ from how uncertainty is used when determining the design crest level for a flood defence.

A specific way to understand the effect of uncertainty on the robustness of the design solution is by using sensitivity or scenario analyses – as is typically used to take into account the process model uncertainty related to climate change. The end result of this analysis should be, for each relevant design input parameter, a best estimate plus an understanding of the uncertainty.

There are two principal approaches to dealing with uncertainty during fluvial design:

  • the precautionary approach (conservative design);
  • the managed/adaptive approach (flexible design).

The approach in conservative design is to increase the certainty of performance. A typical simple example is to design a defence with a higher crest than the design water level through the addition of a freeboard. This approach is generally suitable for managing uncertainty in nature (natural variability).

The other approach is flexible design. For uncertainties with time components such as climate change, this means ensuring the designs can easily be adapted over time as circumstances change or knowledge improves. Examples include accommodating future raisings of crest level by designing a floodwall with stronger foundations, or a flood embankment with a wider crest than currently required. Where uncertainties can directly impact on performance (such as statistical uncertainty about extreme discharges), flexible design can involve resilience measures such as crest and landward slope protection with a view to reducing the risk of catastrophic failure.

Staged design and construction can also be used where the analyses of the sensitivities or future scenarios show that different solutions or parts of solutions are appropriate for each one. This allows aspects of work to be carried out now that meet the current need, but without preventing the implementation of future approaches when trends become clearer. With such an approach, it is important to understand the points in time at which the next design decisions have to be made in order to allow enough time for scheme development and implementation. These approaches are particularly appropriate for managing knowledge uncertainty.

In reality, the optimum solution is usually a combination of these different methods; a certain level of freeboard to take account of statistical uncertainty, with resilient designs and provisions to make later improvements practicable. The decision about this balance should be based on whole-life considerations, including the feasibility and costs of major improvement.

Deterioration

Design has to take account of the whole life of the assets, including how they deteriorate. Deterioration includes any physical process that the asset undergoes and which impairs its performance.

Deterioration of an asset’s flood risk reduction function is directly related to its failure modes. For example, lowering of the defence crest through settlement can cause overtopping at lower water levels than intended, resulting in larger overtopping flows than expected and perhaps causing a breach. Animal infestation can increase the probability of piping, which can again lead to a breach. Similarly, siltation of a watercourse or blockage of a culvert can reduce conveyance capacity, leading to higher water levels than expected for a given flow.

The consideration of deterioration in design typically leads to two types of design criteria:

  • minimising deterioration by the choice of materials and structure types;
  • taking deterioration into account by considering the expected design life and the need for (and ease of) inspection and repair.

An example of the choice of materials would be the use of imported high quality rock for a revetment rather than locally available poor quality stone that would break down quickly under hydraulic forces. An example of allowing for deterioration would be increasing the thickness of steel in a sheetpile wall to allow for corrosion over the life of the structure (30 to 50 years). Both of these have cost implications, but the savings in future costs and disruption make the extra initial investment worthwhile.
 


1.4.5 Management of health and safety

The health and safety of all users and managers of the fluvial environment should be a vital consideration when designing in the fluvial environment.

The Construction (Design and Management) Regulations 2007 (CDM) detail the procedures and roles required for all construction projects. The aim of the Regulations is to ensure that, as far as is reasonably practicable, all foreseeable health and safety risks are identified, removed if possible or managed to an acceptable level, as part of the design process, with residual risks identified and documented. Further information on the application of the CDM Regulations is available from the Health and Safety Executive (HSE, 2007) together with supporting guides.

It is essential that designers evaluate the risks for:

  • construction workers;
  • operatives carrying out maintenance work;
  • members of the public who make use of, or gain access to, the completed works.

The early involvement of operational staff in this process is vital. For example, the design of the crest width and slopes and the specified frequency of maintenance of a flood embankment should take account of the requirement for safe access for operation and maintenance such as grass cutting and inspection. Similarly, fluvial designs should avoid the need for access to unsafe areas, such as over or within the watercourse. Examples include making sure that facilities for operating mechanical structures are positioned where they can be accessed and operated safely. For structures over or within watercourses, the use of durable and low-maintenance materials and finishes can reduce the need for operational access, and thus reduce the exposure to risk. Where the need for access is unavoidable, the design and specification should ensure safe operation.

The selection of fluvial works that create confined spaces should be avoided wherever possible; otherwise they should be designed to limit the need for operational access. Interventions to existing systems should also consider the removal or improvement of such conditions. Opportunities for this include ‘daylighting’ of culverts (demolishing at least the crown of a culvert, to re-create an open channel), or the provision of adequate ventilation, access and escape facilities.

Designers should ensure that operators of the works do not have contact with contaminated water if this can be reasonably avoided. This should reduce health risks such as leptospirosis (Weil’s disease).

In general, the management of the health and safety risk should be underpinned by the general risk management principles and approaches set out in Section 1.3.3.

See Chapter 8 (Section 8.8) for further information about health and safety aspects of work in river channels.
 


1.4.6 Environmental impacts

In line with the government’s sustainable development strategy, the design of works in the fluvial environment should ensure that we live within certain environmental limits and respect the sensitivity of the planet’s environment to change (Wade et al, 2007).

To achieve these objectives, fluvial design needs to ensure that it works with the natural systems, and that it identifies and takes account of the wide range of interests that could be affected by any proposed intervention, through the use of environmental impact assessments. Within the fluvial system, these interests often include:

  • fish;
  • birds;
  • bats;
  • invertebrates and macrophytes;
  • recreational and social features;
  • cultural heritage including areas of historic or archaeological importance;
  • landscape setting.

Proper consideration of these interests requires an understanding of the baseline conditions, constraints and opportunities, and the development of design solutions with these in mind. This is discussed in more detail in Chapters 3 to 5. The Water Framework Directive sets out important legislation with respect to the ecological status of water bodies, and places strict limits on what are the acceptable impacts of river works. Fluvial design works should always aim to enhance the overall ecological status of the affected watercourses.

Wider impacts such as climate change and energy use should also be considered. It is already clear that anthropogenic carbon dioxide emissions are leading to climate change. The management options we choose, and how they are designed, have a significant impact on the carbon footprint associated with their implementation and whole-life management.

The role of the options appraisal and the design process in reducing such impacts cannot be overemphasised. Subsequent stages, in which the chosen design solutions are being implemented, generally afford less scope for reduction. A good example of this is given in Chapter 9, where the use of compressed tyre bales in a flood embankment reduced the need for imported earth fill and avoided thousands of tyres going to landfill.

Design can be used to:

  • reduce energy use;
  • make operation and maintenance activities more efficient;
  • make better use of materials (including minimising the use of primary materials and aggregates, and the waste generated);
  • facilitate eventual decommissioning.

The planning and design process needs to include an understanding of the local availability of materials and sustainable construction and operational processes, and to design around them as much as practicable. Approaches to realising environmental opportunities, reducing environmental impacts and improving the sustainability of flood risk management, including case studies, can be found in Sustainable flood and coastal erosion risk management (Wade et al, 2007).
 

 

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