The geomorphic impact of sluice or weir removal

Weir or sluice removal can have complex, unexpected and long-term effects upon a river system. Examples from high-energy fluvial environments have illustrated the role that these structures play in moderating streampower, and the consequences of decommissioning such structures. However, weirs and sluices can act as significant barriers to longitudinal river continuity in terms of sediment, nutrient and organism movement, and will require removal in the long-term if we are to achieve our Water Framework Directive obligations. More broadly, weirs and sluices prevent the natural functioning of our river systems.

Below I outline the likely sequence of events following weir or sluice removal:

  1. Upstream water levels are likely to lower due to increased conveyance and the removal of the in-stream obstruction; downstream levels may be elevated
  2. Fine sediment and material that was previously trapped by the structure will be able to travel downstream. Depending upon the lifetime of the structure these sediments may contain pollutants or be dominated by organic material
  3. The removal of the structure will result in a local increase in streampower (i.e. the energy the river has). Depending upon the morphology of the channel directly downstream, this will lead to vertical incision (i.e. bed scouring), lateral degradation (i.e. undercutting and bank erosion) or the migration of the energy, causing erosion further downstream
  4. The increase in streampower will lead to a net increase in the sediment transport capacity (i.e. total sediment load) and competence (i.e. the maximum particle size that can be transported) of the river. Hence, sediment deposition is likely to occur downstream of structure removal
  5. In the short-term the increase in streampower and the potential increased erosion that may result could be offset by the increased sediment availability and deposition (i.e. the river may not necessarily respond obviously to structure removal)
  6. Knickpoint retreat, by which the ‘step’ created by the structure in the river’s long profile migrates upstream, may begin shortly after the structure has been removed and the sediment balance is restored (i.e. the river is no longer transporting material previously held behind the structure)

The diagram below summaries these points in three phases.

Theoretical geomorphic impact following weir removal

My MS Paint skills are legendary

In low gradient and low-energy systems with relatively erodible bed substrate, such as many of those in East Anglia, knickpoint retreat could continue for several centuries following structure removal. Knickpoint retreat will only stop when either the natural gradient of the river has been restored or the knickpoint reaches an area of hard geology. 

Knickpoint retreat is likely to have a number of long-term consequences including: the lowering of water levels as the river deepens; river bank collapse caused by vertical incision into the bed; disconnection between the floodplain and the main river channel.

Although in many systems the rate of adjustment is likely to be slow and chronic, it should be acknowledged that a large (1 in 50+ year) event could induce rapid change if it coincides with structure removal. In a high-energy and highly coupled catchment in north-Wales a weir removal on the Mochdre Brook coincided with a 1 in 100 year flood event to cause 100m of headward incision over three months. Two years on and the knickpoint had retreated 230m, causing substantial river bank collapse.

My intention with this post is not to worry those people looking to remove weirs and sluices – I strongly believe we should deconstruct these structures. All I recommend is that you consult a geomorphologist before you do it.

About Trevor Bond

A Geomorphology Technical Officer at the Environment Agency. All opinions expressed herein are my own and do not necessary reflect the views of my employer.
This entry was posted in Geomorphology, Hydrology, River restoration, Water Framework Directive and tagged , , , , , , , , , , , , , , , , , , , , . Bookmark the permalink.

6 Responses to The geomorphic impact of sluice or weir removal

  1. Pingback: Ever wondered from a geomorphological POV what happens when you remove a weir? - Fly Fishing Forums

  2. Good post, but how can we meet our “Water Framework Directive obligations” when stupid deluded councils are letting companies install hydro electric generators across the country

  3. Trevor Bond says:

    Thanks. You’ve hit on an important point and that is that there are many arguments for and against weir removal. The installation of hydroelectric power stations on existing weirs to generate sustainable, renewable energy is one such argument for retaining these structures. As a fluvial geomorphologist I don’t agree with this argument as weir structures inhibit natural river functioning. As a river manager I appreciate that the issues are more complex than this and that there are many arguments for weir retention:

    • There may be ecological benefits of weir retention if the structure or surrounding habitat (e.g. wetlands that are inundated due to ponded water) supports particular species, such as water voles (who like 1/3 angled river banks) or grey wagtails nesting in the walls
    • The weir may have cultural, historic or landscape value – the foundations of some structures date back to the Roman period
    • The weir may divide a coarse fishery from a sports fishery, or there may be other fisheries interests – fishermen tend to like fishing downstream of a weir
    • The weir may play an important role in maintaining upstream water levels, either for abstraction, navigation or recreational use

    As I stated in my post above, I’m an advocate of weir removal. But where removing a weir could lead to bank collapse and someone’s house fall into the river, you have to think twice.
    In terms of WFD, the preferred choice should be to remove weirs. I know some people think that installing a fish or eel pass helps achieve good ecological status (and it can do) and thereby solves the WFD issue, but this overlooks the need for wholesale river continuity – for sediment, for nutrients and for non-fish species (particularly invertebrates). A compromise that I have seen work effectively is to reinstate a former channel. Many mill structures are built on artificial channels that now take the majority of the flow. It is possible to retain a low head on the weir and open these relic channels, which often have a planform and character indicative of the system prior to human intervention. Maintaining sufficient flow through the reinstated channel can be challenging, but this approach can help address WFD and may satisfy the greatest number of stakeholders.

  4. I’ve been lowering dams, taking out weirs and flushing mill ponds for 25 years. The first point of scour is at the top of the accumulation of silt, as the water that is no longer impounded now runs in its bed, digging into the silt and causing channeling. Large pieces on silt roll down the channels and smash up as the water quickens up. This mobilisation of organic matter is what does the damage to downstream invert and fish life, so the timing is crucial. I flush with the high water, but before the WBT start to spawn. Rapid failure of weirs and sluices can wipe out downstream life, so timing is crucial. Weirs out. Hydro-power advocates leaving weirs in for at least 50yrs.

  5. Pingback: The geomorphic effects of river dredging | The River Management Blog

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