Managing & Maintaining our water channels Historic river
improvements and their relationship
to river restoration
The river that everyone sees today is the result of centuries of endeavour on the river and flood plain. The two most significant changes have been due to the creation of mills, and changes to the drainage of the floodplain.
The physical changes to the river due to milling had ceased by the 20th Century, with the operation of the mills not lasting much beyond the middle of that century.
Physical changes to the river for land drainage really occurred in two phases a century apart: 1850s and 1950s. The first was due to land owners using new technology and techniques; the second due to government policy.
The modifications to the river were driven by the wish to pin down the water table, to allow improved growing conditions for either improved grass or conversion to arable crops. There was also a wish to move the water as quickly as possible so that the water table remained low.
To achieve lowering of the water table the river bed was deepened, and, to manage the resulting bank instability, bank angles were reclined, thus opening up the
cross section. This had the added bonus of helping improve flood flow conveyance and the movement of water down the river. To aid this purpose any variability in cross section and plan form (sharp bends) were also removed.
The result was a river channel that bore no relationship to the flows of water in it. Consequently the movement of the silt in the river was stopped, as water velocities in the new cross section were inadequate to continue to move it. This allowed the more vigorous water plants to colonise those silts near the banks and so start a natural reduction in the un- naturally large cross section.
This process of the return to an equilibrium size, where silt deposition had stopped because water velocities had increased due to the confining vegetation, was interrupted by maintenance dredging every 10 years or so. The dredging being to remove silts and vegetation which had previously moved by themselves or reached a maximum growth and helped re-define the natural channel size, that is, one related to the flows in it.
Like most capital dredging schemes, the Wensum improvement works would be based on increased agricultural production over the 50 year life of the works generating more
benefits than the cost of the initial dredging and the continued maintenance over the same period.
The original capital dredging, and the 10 to 20 year cycle of maintenance dredging has removed the original river bed and many of the natural in-channel features. This has severely impacted on the river ecology and therefore the fish populations. It has also prevented connection of the river with its floodplain, which is increasingly recognised as key to a healthy functioning ecosystem.
The historic improvement schemes focused solely on agricultural improvement and did not consider the impact on other uses, including conservation and fisheries, which are increasingly recognised and valued by today’s society.
What is the need for change in the river management?
These days the prime focus of the Environment Agency’s river maintenance is to protect people and built property from flooding. In order for us to exercise our permissive maintenance powers there has to be a cost to benefit justification. The low level of flood risk in the Wensum valley does not allow the continuation of the post war maintenance regime.
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For further information on the River Wensum Restoration Strategy please visit the Environment Agency website, here:
TEL: 0118 972 4041 FAX: 0118 946 4894
Vines Farm, Crane End, Reading, Berkshire, RG4 9HE 18 www.fadsdirectory.com
or email river.re
Perhaps more importantly, the way the country views the river has changed, with the Wensum becoming an SSSI in 1993 and a European SAC in 2001. These designations were implemented to protect the natural features within the river, which includes both the physical features and the associated wildlife. Historical maintenance practices are incompatible with these designations. There is now a statutory requirement on the Agency to further the
conservation of the site, and to implement river restoration measures to help return the river to favourable condition. There are also opportunities to work with natural processes to minimise flood risk, for example by restoring floodplain connectivity to
attenuate flood risk to downstream communities. This new approach is recognised in strategic documents such as Catchment Flood Management Plans.
Managing the Wensum in the future
Feasibility reports have been written for each of the riverine SSSI units, setting out the actions required to restore the physical habitats in the river. Once river restoration has been implemented the channel size will be more in equilibrium with the flow regime, leading to a more sustainable system which will require less management intervention.
A Targeted Maintenance Protocol (TaMP) is also being developed for the Wensum so that appropriate, targeted
maintenance of the river can be identified and implemented. Importantly, the protocol will help, in a small way, towards the objective of the RWRS. The protocol takes into account the Environment Agency’s responsibility to protect people and property from flooding. It will define key locations for regular and programmed inspection and recommends maintenance activities, if necessary, according to set trigger rules for each activity and site.
One of the changes in management relates to recognising that retaining trees and fallen timber (woody debris) within the channel can be beneficial. It not only helps to vary the flow and shape of the channel but also creates physical habitat for many species of plants, invertebrates and fish. Nationally, we have recently adopted a new policy, whereby we will retain woody debris in rivers when carrying out maintenance provided that flood risk to people and property is not increased.
The aim is to manage the river for today’s needs, based on experiences from the past, to help ensure we can manage it better into the future.
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