Dredging covers a range of activities from the excavation of material from the river bed and sides of river channels through to the straightening and/or deepening of watercourses.
Can dredging help with flood risk management?
Dredging may provide no benefit at all during severe or extreme flood flows. This is especially so at areas where the flow rate is impeded by channel constrictions such as bridges or high downstream water levels (tide-locking). During severe or extreme flood events, the storage provided by the river channel is insignificant when compared to that held in the floodplain. It is not practical to carry out dredging to the extent that would be required to confine such large flood flows from the wider floodplain, since the capacity of the channel is a small fraction of the wider floodplain (see diagram below).
Dredging does not reduce the risk of other kinds of flooding either, such as surface water or sewer flooding.
Dredging comes with significant risks that must be understood at a local and catchment scale. Reductions in channel roughness and increases in the gradient and capacity can result in increased erosion and flooding downstream. Dredging can have negative impacts on the environment and wildlife.
The benefits gained from dredging need to be balanced with the consequences and all cases should be considered alongside other options such as flood defences, river restoration schemes and natural flood risk management projects.
What is Manx Utilities doing with regard to dredging?
Routine dredging on the Island’s Northern Trenches is undertaken annually. These artificial watercourses have gentle gradients, causing silt to accumulate quickly and a routine maintenance programme is beneficial to maintain channel conveyance reducing fluvial (river or stream) flood risk in the north of the Island. The Island’s natural rivers are not routinely dredged because these same benefits cannot be achieved. However, we do remove localised accumulations of gravel and debris at known pinch points along the Island’s natural watercourses.
Topographical surveying of gravel bars and river channel cross sections is often carried out and used alongside computer modelling methods to establish whether the area will benefit from gravel removal from a flood risk perspective. Such pilot studies are then used to inform gravel management and monitoring regimes on the island’s rivers – see below.
Pilot study on the Silverburn
The study indicated that allowing shallow gravel bars to establish in the vicinity of Ballasalla is unlikely to increase the flood risk to any properties. Whilst more flow can be contained in channel if gravel is not present, gravel is only a secondary influence of flood extents in this area.Flood levels are controlled by other constrictions such as road bridges.
The figure shows the difference in modelled water levels between scenarios with and without a gravel bar in the river channel.
If gravel deposition develops to significant depths, the resulting loss of channel capacity could have implications for flood risk. We will continue to monitor the situation and will manipulate static gravel bars that are becoming a concern from a flood risk perspective.
Can I remove gravel and silt from my section of river?
If you own part of a watercourse and wish to maintain its condition, including removing silt and gravel, you should contact us first to discuss.
If you plan to put up flood defences or carry out works on or near a watercourse, our written consent is required so please contact us on (01624) 687687.
You should also contact DEFA as you may require their consent before working in or near certain watercourses. Under the Fisheries Act 2012 consent must be sought for dredging Any landowners wishing to dredge ditches should contact DEFA on (01624) 685835 in order to avoid committing offences under the Wildlife Act 1990, Section 32 of the Fisheries Act 2012 or the Water Pollution Act 1993. Additional contact details for DEFA can be found here. Depending on the type of work planned you may also require other consents or planning approval.
Please read Watercourse Management in the Isle of Man for further information.