Remove Obsolete Structure

Flood defence structures can have a number of detrimental impacts to the functioning of aquatic ecosystems including for example:

  • Restricting longitudinal and lateral movement of water, sediment and nutrients.
  • Increasing velocities and localised currents which results in an erosive environment.
  • Preventing or limiting the passage of fish.
  • Reducing available habitat due to changes in the area, frequency and duration of floodplain, saltmarsh and / or weltand inundation.

The removal of obsolete flood defence infrastructure within river or coastal environments mitigates these impacts by restoring or assisting natural recovery of habitats within the water body. Removal of flood defence infrastructure, which results in effective recovering of the system is still an area where more can be learned through practical application.

It should be noted that this measure relates to removal of obsolete structures, rather than those which are still serving a defined function. Where structures are still serving a function it may be possible to alter existing arrangements so that the structure become obsolete, although this is not specifically included within this guidance. The following steps can be applied to guide decision making when seeking to remove a structure. 

Is the structure obsolete?

When considering the removal of a structure it is important to assess the current function of the structure and the users that the structure is currently serving. Key questions to answer at this initial stage are:

  • What service(s) was the structure designed (or later altered) to provide, such as flood risk management, water supply, recreation, irrigation or navigation?
  • What service(s) or benefits does the structure provide today?
  • Are any specific habitats supported by the presence of the structure?
  • Who is benefitting from current services?
  • Who owns the structure?
  • What public agency, if any, has regulatory authority over the structure and its function?
  • Who has an interest in the future of the structure?

Having established the answer to these questions it should be possible to determine whether the structure is still required by any of the interested parties involved. If the structure is found to be obsolete, further prgress can be made towards establishing the feasibility of removal. If the structure is still required by certain parties it may be possible to start a dialogue to agree modification of current arrangements so that the structure is made obsolete.

Is removal of the obsolete structure feasible?

The feasibility of removing a structure, particularly larger structures, is dependent on several factors, including ecological, economic, social, technical / engineering and legal considerations. Key questions to answer are:

  • What is the current condition of the structure - materials, construction, size, design?
  • Is structure removal technically feasible?
  • What is the legal context for removing the structure?
  • Who has the ultimate decision making authority regarding removing the structure?
  • Has an adequate assessment of flood risk been undertaken?
  • Have adequate environmental surveys been undertaken?
  • Is the structure of archaeological importance?
  • Has a risk assessment been undertaken to identify the risk of potential contaminated sediments?
  • Has a geomorphological assessment of potential impacts of removal been undertaken?
  • Have impacts to local cultural resources been considered?

In particular, it is important to consider how the river, estuary or coastline will readjust following removal of the structure. In many cases, particualrly where structures were constructed a long time ago, the system may have adjusted to achieve a state of equilibrium further to the introduction of the structure. Removal of the structure may once again upset the equilibrium causing erosion or deposition problems which did not previously exist. Whilst readjustment of the system is to be expected in the short-term, care is required to avoid long-term instability. Whenever removing a structure, expert advice should therefore always be sought (suitably experienced geomorphologist and / or civil / hydraulic engineer).

In many cases, structures have been constructed in conjunction with other modifications, such as river straightening, bank re-profiling or coastal reclamation. In order to manage instability and promote restoration of natural processes, it may be necessary to combine removal of obsolete structures with techniques from other measures, such as those listed under Manage and Restore Aquatic and Riparian Habitats, Use of soft engineering in bank stabilisation and protection and Managed realignment (bank).

Is the removal of the obsolete structure necessary?

Structures that are deemed obsolete are often towards the end of the design life and likely to fail at some point in the future. Where the consequences of structural failure are considered to be acceptable, it may be possible to leave the structure to fail over time without removing it. Advantages of this approach are that it does not involve active intervention and is effectively an 'exit strategy'. Disadvantages are that failure would be unmanaged, so that conditions following failure may lead to instability and / or may not maximise the ecological benefits that could be realised. In addition, the timescale of failure would be uncertain and the impacted situation could persist for longer prior to failure than if removal was undertaken.

How will removal be managed during / post removal?

If removing an obsolete structure is considered to be an appropriate measure, efforts to ensure minimal impact on the environment during the removal process should be taken. Increased turbidity (suspended solids) in particular, could be a potential issue during removal, if large scale works to the bank of bed are undertaken. Water quality monitoring and appropriate mitigation measures (e.g. silt traps) may be required. Key considerations are:

  • Have construction works been scheduled to avoid fish migration / spawning?
  • Are appropriate Pollution Prevention Guidelines (e.g. PPG5) being followed?
  • How will potential disruption to the local community be managed?
  • Have silt traps been removed to prevent sediment re-suspension?
  • Has post removal monitoring been considered and planned, including monitoring prior to removal?

In-channel structures also differ widely in size, situation and construction and the most appropriate approach to 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 dam or weir removal is the best option, is often complex even for smaller structures. Replacement of the bed with a hard structure may be required as a grade control and examples of hard beds referred to sometimes as ‘riffles’ are shown.

Remove Obsolete Structure