LOCATION: Long Preston Deeps, the River Ribble floodplain between Settle, Long Preston and Wigglesworth.
PERIOD: June 2015 – March 2016
FUNDER(S): Environment Agency
During the months of September, October and November of 2014 the Environment Agency undertook a project to move a 150m long by 2m high earth-filled flood defence embankment from near the River Ribble side, to a location some 80m into the adjacent pastureland field. They also undertook some geotextile works to the River side to combat erosion. Both were grass seeded to stabilise the earthworks undertaken.
On the 2nd. January 2015 very heavy rainfall created high river levels that totally flooded the area creating a massive lake behind the new embankment. The very strong winds created wave-action that attacked the rear of the earth-filled embankment, causing the sloping face to slump into the field.
The geotextile strengthening works to the river side was badly damaged by the high flows, with the top section being completely washed away and the bank eroded.
Ribble Rivers Trust were asked by the Environment Agency to tender for the repairs to the embankment and the river side. We formulated various possible methods to carry out repairs and came up with the following solutions…
At the embankment site the sheep netting fencing would to be taken down, leaving the fence posts in situ. An excavator standing on top of the embankment would then pull the slumped earth back into the embankment to form the sloping embankment side. To make sure the field side of the embankment was never erode by wave-action again it was decided that the sloping surface would be turfed. The turf would be pegged in position to increase contact with the soil and to help the roots quickly stabilise the embankment.
With regard to the geotextile repairs it was decided that a more environmentally friendly product would be used than the original repair. Following some research it was decided that Hy-Tex CoirMesh 900 woven coconut fibre geotextile would be an ideal product. It would provide erosion resistance to the river side whilst the grass seed germinated and grew, and after 5 years when the vegetation was well established it would biodegrade.
The order of repair would consist of taking off the turf from the top of the river side and storing safely on site. The bare earth would then be pushed down the slope to marry with the top edge of the remaining geotextile. A 0.5 m trench would then be excavated at the top of the river side and the bare earth of the sloping face seeded with grass. The top edge of the geotextile was then to be buried in the trench and rolled down the slope and securely pinned with 300mm long ‘U shaped’ pins. All joints in the geotextiles to be overlapped at least 200mm. Finally more grass seed would be scattered over the top of the geotextile and the saved turf pinned over the top of the backfilled trench. Some Goat Willows would then be planted on the lower slopes to help stabilise the embankment.
Ribble Rivers Trust asked three contractors to submit tenders for the above works to our specification. Dinsdale Moorland Specialist a local contractor with their head office only 5 miles from the site, were successful. Under close supervision from the Ribble Rivers Trust the contractor completed the above works within 2 weeks.
These works are just a small part of the restoration of the Long Preston floodplains. Visit longprestonfloodplainproject.org for further information.