River Management Techniques. Upper Wharfedale Best Practice Project (Information Series No 6)

4.0. Rehabilitating river channels

4.1. Buckden gravel trap rehabilitation scheme

4.1

LOCATION:
Cray Beck confluence, Buckden, SD 933781-936779

LENGTH: 250m

DATE UNDERTAKEN:
Summer 2002

COST: £60,000

DESCRIPTION

This is a scheme to rehabilitate a length of river which had been subjected to river engineering modifications in the 1980s. These included the installation of a gravel trap, removal of gravel shoals and blockstone work on both river banks. This past work, together with the impact of large amounts of gravel moving down the river, has caused serious erosion of land on the right bank of the river and an ‘unnatural’ alignment of the river and an incoming tributary, Cray Beck. The design aims to demonstrate ‘best practice’ in achieving a sustainable solution, which takes account of current environmental, social and economic needs. It is made up of a number of interconnecting components, based on the scientific evaluation of geomorphological data.

Stone revetment has been selected to safeguard the position of the right bank of the Wharfe, necessitated due to the aggressive nature of the river. The aim is to ensure that the position of the right bank is fixed and that erosion of the bank does not take place, because it could allow flood water to flow down the valley. ‘Soft’ revetment has been used in some sections incorporating native willow stakes and plants, biodegradable coir matting and a Yorkshire Dales SSSI grass seed mix. The top of the bank is being planted with native deciduous trees and the bank fenced to stop livestock from damaging planted trees. These trees will be important for the long-term stability of the banks since their roots will bind the bank sediments together. The vegetated banks will, therefore, be able to withstand erosion processes in coming years. The scheme protects the Dales Way footpath, aligned 5m to the south of its current course, due to the stockproof fencing. The bed of the river has been regraded, involving the removal of the two concrete weirs of the gravel trap and infilling the deep scour trench that has formed on the outside of the meander bend. The scour trench has become deeper with the build-up of gravel on the point bar on the left bank.

The point bar gravels have been regraded to remove the excess gravels which have accumulated and the shape of the bar  changed to avoid strong flows of water being formed on the right bank of the river, which may compromise the stone wall revetment. The gravels will need to be maintained by riparian owners to ensure that excess material is removed periodically.

The alignment of Cray Beck has been changed as it flows into the Wharfe. This has enabled the Cray Beck islands to be regraded and their land levels lowered. This will give the Wharfe a large area channel which will be essential to allow flood waters to pass through this section. The broader channel at this point will limit the build up of water upstream, which has contributed to the erosion problems of the right bank.

Some willow and ash trees have been removed from the left bank in order to lower the land levels of Cray Beck. Boulder revetment has also been removed in part from this bank.

DESIGN

Two of the techniques used, showing soft and hard revetment are illustrated in detail below:

Buckden Gravel Trap – Type 4 Willow Mattress Revetment

Buckden Gravel Trap – Type 3 Blockstone Revetment

All the techniques and their positions along Buckden Gravel Trap are featured on pages 14 and 15.

Upper Wharfedale: Buckden Gravel Trap Rehabilitation Scheme

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