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 results in an increase in the river’s cross sectional area (and hence its volume), as well as a reduction in the roughness of the channel. Where dredging is used to straighten the river, it reduces its length and increases channel gradient. These effects can increase the efficiency of moving water in the channel. Dredging can therefore lead to a reduction in water levels in the immediate area.
However, it is important to note that dredging may provide no benefit at all during 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 extreme flood events the storage provided by the river channel is typically insignificant when compared to that held in the floodplain and the peak flow of the river may exceed bank full discharges. It is not practical to contemplate dredging to the extent that would be required to confine such large flood flows from the wider floodplain, since the storage and conveyance capacity of the channel is a small fraction of the wider floodplain. It should be noted that the majority of Manx rivers are short and have a relatively steep gradient, catchment to estuary, and the delivery of flood water from catchment to estuary can occur in a very short timeframe. 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?
Manx Utilities have a presumption against dredging however in some individual situations site specific conditions may necessitate sediment manipulation and this can play an important role in flood risk management.
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, for example at Rushen Abbey to ensure that river flow is maintained.
Topographical surveying of gravel bars and river channel cross sections is often carried out and used alongside hydraulic 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.