Fluvial Design Guide - Chapter 1

Design of works in the fluvial environment

 


1.2 Design and asset management

It is helpful at this stage to define what we mean by design. But first it is necessary to define the term ‘asset’ in the context of flood risk management (the focus of this guide). The term is used here to describe an engineered or natural component of the fluvial system that performs a flood defence or land drainage function (for example, a floodwall, a sluice structure, a river channel or a revetment to prevent erosion). An asset forms a part of an asset system that includes a number of interdependent assets working together to provide a flood protection, drainage or other function. The design of assets therefore needs to be carried out in the context of their associated systems (see Section 1.4.2).

It is also useful to explain the term ‘function’ as it is used in this guide. All assets have a primary function, which defines their main purpose. Thus, the primary function of a culvert is ‘to convey drainage flow under an obstruction without undue restriction’. Although the primary function of many of the assets described in this guide relates to flood risk management, some assets have other primary functions. These include structures such as navigation locks and fishpasses, though these are also likely to have an impact on flood risk management. All such assets require management throughout their lifecycle, as illustrated in Figure 1.2.

Figure 1.2 Asset management lifecycle

This diagram illustrates the role of design in the lifecycle of an asset.

For new assets, the cycle starts with the assessment that an asset or system is not achieving its stated or implied performance objective.

For existing assets, a programme of monitoring, condition assessment or performance assessment often identifies the need for remedial works or change to the standard of service

In both cases, the design has to take account of the functions the asset is intended to perform.

So in the context of this guide, ‘design’ activities address the management of the asset throughout its whole lifecycle. The term design therefore encompasses:

  • New design – often the easiest design activity and one for which the designer starts with a clean sheet.
  • Design for refurbishment or change of performance – design for the adaptation, upgrading, rehabilitation or decommissioning of assets within existing fluvial systems, following an assessment of their condition and performance. The constraints imposed by the existing system and its wider environment will have a strong influence on the achievement of the design objectives. This is the more common context for fluvial design.
  • Design for operation and maintenance – for example, to address cracking in a masonry wall.

All these design activities may require the design of temporary works (for example, cofferdams and stream diversions) as part of the construction process.

The outputs from design activities can be wide-ranging and may include:

  • design notes;
  • calculations (including computer analyses);
  • output from a mathematical model;
  • specifications;
  • drawings;
  • operation and maintenance (O&M) guidance and manuals.

All these are needed in different proportions to define the design concepts and to convey them to those carrying out the physical works on the site during the construction period and throughout the functional life of the asset.

It is essential that designers of works in fluvial systems appreciate the extent to which the works can impact on the wider environment and thereby affect other users of the system. Consideration of these impacts is an essential part of producing sustainable designs – and in identifying opportunities for enhancements to the environment – as well as recognising the constraints they impose.

In the particular context of flood risk management, recent developments have led to the adoption of the ‘source–pathwayreceptor’ conceptual model to improve the understanding of flooding mechanisms. These terms are defined in the glossary and the concept is illustrated in Figure 1.4. This fluvial design guide deals principally with the source and pathway elements. It does not address the design of local measures to protect individual properties against flooding.
 

 

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