Removal of in-channel structures

Design Guidance

Since in-channel structure removal is a relatively new technique, there is currently limited established design guidance. Dam and weir structures also differ widely in size, situation and construction and the most appropriate approach to in-channel structure removal largely depends on the individual structure concerned. For many structures, especially small ones, removal is a relatively straightforward demolition project, although care must be taken to protect the surrounding structures and natural environments. However, the decision-making process as to whether in-channel structure removal is the best option, is often complex even for smaller structures.

The Decision-Making Process (see also Remove Obsolete Structure Measure sheet)

Much of the available guidance has been published in North America and focuses on decision making to determine whether the structure is indeed obsolete and whether removal is the best option. A useful series of questions in relation to the ecological, economic, social and technical / engineering issues that require consideration is presented and explored in the publication “Exploring Dam Removal – A Decision-Making Guide“(American Rivers and Trout Unlimited, 2002).  Factors to be considered highlighted within the publication are:

  • The ecological circumstances surrounding the case.
  • The economic circumstances surrounding the case.
  • The complexity of the issues.
  • The legal and political context in which a decision must be made.
  • The impetus for considering dam removal (e.g., fisheries restoration goal, dam safety concern).
  • The identity of the decision makers (e.g., dam owner, state agencies).
  • The amount of controversy surrounding the decision.
  • The number, identities, and strength of various stakeholders.

(Source: American Rivers and Trout Unlimited, 2002)

Practical guidance specifically aimed at project proponents has also been prepared at state level in Oregon in relation to removal of small dams (OWEB, 2008) and Massachusetts (Executive Office of Energy and Environmental Affairs, 2007). Whilst some of the legislative and policy details within these state publications are not applicable in the UK, the frameworks and practical case study information presented may still be useful.

Consideration of potential impacts

Weir removal may have an influence on flood risk management in particular downstream of the structure to be removed. Prior to removal of a structure it may be necessary to demonstrate through hydraulic modelling that the work will not conflict with flood risk management objectives.

Regrading channel bed

The headloss from upstream to downstream of a structure can be significant and consideration will need to be given to on length of water course necessary to ensure that head recession is not initiated.

In some cases a series of small scale weirs have been used. See Remeandering straightened rivers.

In several cases in the UK where obsolete weirs have been removed, they have been replaced with “riffle” features. Such “riffle” features can be used if necessary as a grade control to prevent large changes in the gradient of the river in response to dam or weir removal. Examples where “riffle” features have been created are on the River Welland at Harringworth, on the River Wandle, in South Wimbledon (see right). However, it is recommended that use of these structures is avoided where possible in order to aid re-establishment of a natural bed gradient and associated river habitats.

Another useful example is show in Case Study 85 of Working with Natural Processes Guidance.

In-channel structures are more commonly concrete or steel structures that require heavy machinery to remove. Larger structures may require explosives, which has used in the USA but is likely to be less necessary in the UK.

Construction issues

Methods must consider pollution prevention and suspended sediments is a key environmental risk. CIRIA (2001) advises on best practice for preventing pollution to watercourses from Construction Sites and should be applied. Other techniques specifically used in small rivers are the use of ‘Sedimats’ or straw bails to filter suspended sediments just downstream of the construction area.  Working in the dry is best, often achieved using cofferdams (steel piling) Refer to Section 7 of Piling Handbook 8th Edition (ArcelorMittal, 2008) for design guidance of Cofferdams.

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