Fluvial Design Guide - Chapter 1

Design of works in the fluvial environment

 

1.3 Fluvial design process

1.3.1 Overview

The fluvial design process is objective-led. The need for design is usually identified from an inadequate standard of performance, operational inefficiency or an asset reaching the end of its life. The design process commences by:

  • understanding and defining the performance objectives of the asset;
  • identifying an optimum solution for achieving these objectives;
  • presenting these in such a way that the asset can be built and managed over its design life (see Section 1.4.4).

Once the design objectives are clearly defined, the nest step is to assess the current and expected future performance of the system in the light of these objectives. This may involve surveys, investigations and assessment of historic records. If the need for intervention to achieve a new performance standard is identified, options for achieving this are then developed and assessed to identify the preferred solution. The preferred option is then designed in sufficient detail to enable its implementation. Although the design process finishes before the implementation, it must provide information to:

  • allow the asset to be constructed;
  • guide its operation, maintenance, future upgrading and decommissioning.

Designing in the fluvial environment involves interaction with many people who have varied interests concerning the natural, managed and built environments. Understanding their needs and incorporating these into the process is crucial to achieving solutions that are appropriate and acceptable. The supporting consultation, risk and data management processes are discussed further in Section 1.3.3.
 


1.3.2 Achieving the design product

To achieve a successful design within this challenging environment, the following important aspects need to be managed properly:

  • performance objectives;
  • design development;
  • design outputs.

These three aspects are covered in more detail in Box 1.1.

Box 1.1 Important aspects in fluvial design

Performance objectives

Once the need for intervention is identified, it is important to capture this in a set of performance objectives (if they do not exist already) that set the higher level focus of the design decisions and outputs, and the benchmark against which the implemented solutions will be evaluated. Examples of these for a new sluice structure could include the ability to pass a particular flow for a given head and the facility to allow migration of particular fish species.

Design development

This aspect is an iterative process of identification of approaches and the development of the preferred solution. It involves three main processes (see right).

The design development process requires an appropriate balance between fact-finding, analyses, value engineering and associated iterations on the one hand, and moving towards a preferred solution on the other. The defining principle here is obtaining enough information to enable design decisions to be made with an acceptable level of certainty.

  • Option identification and screening. The mindset needs to be open to ensure all the relevant disciplines (hydraulics, engineering, ecology, operational, geomorphology, landscape, etc) are involved. The purpose is to identify realistic options that can achieve the performance objectives.
  • Identification of the preferred option – further investigation to find the optimum solution to meet the performance objectives.
  • Consolidation – development of the preferred solution, with all important aspects defined and the underlying assumptions, principles and outcomes captured in a design note or report.

Design outputs

The focus of the detailed design should be the development of the design in enough detail to facilitate its construction and, as much as possible, to inform and direct the future operation, maintenance, adaptation, rehabilitation or decommissioning works. Design outputs are summarised on the right.

Where performance specifications are used (as is often the case for structures such as sluices and pumps where the final design is left to the supplier of the plant), particular care should be taken to ensure the design concepts and performance objectives are described clearly. This should be supported by an unambiguous testing and approval programme.

  • Design note or report to document the design decisions, assumptions, principles and choices.
  • Design calculations (whether a formal output or not) should be clear and accessible.
  • Construction works information including specifications and drawings.
  • Construction support information such as residual risk information (including health and safety and environment), consent information and conditions.
  • Whole life operation, maintenance and performance monitoring requirements, and residual risk information.
  • Information to inform adaptation, rehabilitation or decommissioning.

1.3.3 Managing the design process

This section deals with the three inputs into the design process outlined in Section 1.3.1. Stakeholder consultation is covered in Section 1.4.1.

Risk identification and assessment

Designing in the fluvial environment has its particular challenges and constraints. Any interventions in the existing system need to take account of – and work with or around – the various other interests. These include existing development, stakeholder interests, and environmental interests and designations. Addressing the needs of all of these interests may involve difficult decision-making, particularly where there is adverse interaction between interests (with regard to meeting performance objectives) which may lead to higher costs, delays or other unwanted outcomes.

Early assessment of the potential impacts and opportunities is an integral part of the design process to enable the avoidance or management of potential negative impacts and the realisation of optimum enhancement opportunities. Risk assessment and management processes relating to these – including stakeholder engagement, health and safety and environmental impact assessment (EIA) processes – are described in Sections 1.4.1, 1.4.5 and 1.4.6 respectively.

Factors that can affect the achievement of design objectives give rise to risks. The complex nature of these risks means that it is good practice to use a well-defined process to support better decision-making through identifying, assessing and responding to the risks. This process is called ‘risk management’. Risk management aims to lessen or remove risks, by reducing the probability of occurrence, or by mitigating the consequences, or a combination of both. The management plan also needs to address any residual risk.

To provide a transparent trail of risk management, it is important that identified risks, together with the actions taken to manage them, are recorded and kept up-to-date in a risk register. This also provides risk information to the next stage of the design process and ultimately to the implementation stages. Risks associated with the design of particular types of fluvial structures and their management are described in Chapters 8 to 11.
 


Data collection and analysis

Design choices and decisions need to be based on proper analyses of relevant information. Getting this right is vital to the quality of design decisions as well as the time taken for the design process.

Supporting information normally includes:

  • what is needed to establish how the system works;
  • historical management and associated performance;
  • potential impacts of options for management;
  • legal or other requirements.

For fluvial design, information on historical performance under various hydraulic loading conditions is particularly important. Analysis of these conditions should take account of expected future trends including climate change effects.

There is likely to be a minimum level of information needed for design below which any design decisions would be based on nothing but ‘heroic assumptions’. On the other hand, it is unlikely that an ideal amount of information would ever be available. The effort and cost of obtaining additional information needs to be balanced against the added value to the decision-making process. The need to obtain further supporting information should therefore be based on an analysis of the information available and comparison with that required to make the design decisions with the requisite level of confidence.

It is also important to record the sources of the information obtained, the data attributes and the assumptions made in their use. These metadata provide a valuable record of the data used and their provenance, allowing anyone revisiting the design to understand fully the quality of the data on which it was based. Further guidance on assessing data requirements, data quality and recording information about data through a metadata system developed for flood and coastal management is available in Improving data and knowledge management for effective integrated flood and coastal erosion risk management – a guide to good practice (Robinson et al, 2007).
 

 

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