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:
- Upstream water levels are likely to lower due to increased conveyance and the removal of the in-stream obstruction; downstream levels may be elevated
- 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
- 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
- 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
- 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)
- 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.
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.