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

3.0. Unpicking hard engineering works


3.1. Redmire river bank erosion scheme

BACKGROUND INFORMATION

During the 1980s river engineering works were undertaken to modify the river channel and improve drainage of meadowland on this part of the river. At the same time, several sections of river bank which had been subject to erosion were strengthened using blockstone revetments. In the main this technique has proved successful and the large size of stone used, although somewhat out of scale, has been colonised by mosses and algae, helping to blend the feature into the surroundings. In one particular instance, a considerable amount of movement occurred in the toe stone foundations of a reveted section. As a consequence, this structure had not stabilised sufficiently to allow vegetation to become established over it and has remained an incongruous feature in the landscape.

DESCRIPTION

The section of river bank involved is on the outside of a slow meander bend. River flow is further directed towards the right bank of the river from a gravel bar directly upstream, which acts as a side weir to impart and increase directional flows. The river bank had started to erode further towards and into the grassed floodbank protecting adjacent meadowland.

DESIGN

The scheme was designed to produce a bank sufficiently robust to withstand the dynamic character of the river whilst achieving a ‘soft’ bank top.

Primary considerations involved the need to reposition insecure bank toe blockstone revetment and securely key these into the river bed. Importance was also given to the need for the re-worked structure to be softened to blend and fit into the adjacent land. To facilitate the retention of topsoil, placed as a seedbed for sown native grass seed species, birch brash faggoting material was used as a short-term protection. This was fixed at right-angles to the stream flow to aid the deposition of sediments and fastened with biodegradable sissal rope.

3.1

LOCATION:
Buckden SD 938771

LENGTH: 45m

DATE UNDERTAKEN:
Autumn 2001

COST: £7,259

SPECIFICATION

 

The existing 1-2 tonne boulders (removing surplus material) were re-positioned to form a bank toe foundation well keyed into the river bed. The top of the bank toe was set at around 200mm above typical summer water levels. About 80m3 of high clay content soil were installed to provide a bank top base for the establishment of vegetation. This was graded to blend the bank toe into the adjacent bank and field contours. About 140m2 of 200mm birch brash faggots were fastened down tight over the clay base soil with biodegradable sissal rope of a minimum diameter of 8mm and wood stakes (each approximately 1m in length at 500mm centres). Approximately 10m3 of topsoil were spread into the birch brash faggots to provide a seed bed, sown with Emorsgate EM19 Yorkshire Dales SSSI grass seed mix.

SUBSEQUENT PERFORMANCE

Toe stone placement has been severely tested by high flow conditions and has remained stable.

ADLib logo Content provided by the Agricultural Document Library
© University of Hertfordshire, 2011