Tidal defences for Clevedon, North Somerset look after We are t\ your env ENVIRONMENT AGENCY B - for you, and for fi information Services Unit iter you Your envi- Please return or renew this item by the due date ti business, drink anc- Due Date Governm iking your environrr The Envir environm Published by: Environment Ag Manley House Kestrel Way Exeter EX2 7LQ _ Tel: 08708 506 506 Email: [email protected] www.environment-agency.gov.uk © Environment Agency All rights reserved. This document may be reproduced with prior permission of the Environment Agency. Blind Yeo sluice Clevedon Pill Marshall’s Bank Land Yeo outfall Marshall's field River intake structure Three structures - the Land Yeo outfall, Blind Yeo sluice and Marshall’s Bank - reduce the risk of flooding for more than 4,700 properties in Clevedon, North Somerset. All three - located on the edge of the Severn Estuary - have been significantly improved by the Environment Agency. This leaflet describes our work on the Land Yeo outfall and Marshall’s Bank, carried out in 2005/2006, and the Blind Yeo sluice, carriedouLin 2004 ENVIRONMENT AGENCY 1 II! 1 II J for Clevedon. North Somerset 3 102436 background The Land Yeo drains mainly rural land Marshall’s Bank - a 500-metre long upstream of Clevedon - including the embankment that stretches between low-lying Tickenham, Nailsea and the Land Yeo outfall and the Blind Yeo Kenn Moors Site of Special Scientific sluice - also dates back to medieval Interest. times. Work took place in 1958 and 1984 to raise and strengthen the When the M5 motorway was built in bank. the 1970s the River Yeo was divided into two watercourses upstream of The Blind Yeo sluice is not as historic Clevedon - the Land Yeo and the man- as the other two tidal structures, but made Blind Yeo. They are connected its purpose is no less significant. by the Yearling Ditch, which takes flow south from the Land Yeo to the Blind The sluice lies at the tidal outlet of the Yeo via a sluice at Cook’s Clyse. Blind Yeo, where it meets Clevedon Pill and the Severn Estuary. The Land Yeo outfall - where the river discharges into the Severn Estuary - The sluice comprises two gates, was an old masonry structure, known as the north and south eyes. thought to date back to the 1880s. The south eye dates from around Records show there has been an 1952 and the north eye was added in outfall structure at this location since the early 1970s to handle the extra medieval times. water run-off from the M5. 4 Environment Agency Tidal defences for Clevedon. North Somerset The problem The three tidal defence structures are low, narrow bank and cause further located next to the Severn Estuary. erosion and structural damage, and The estuary has the second highest could eventually cause a breach of the tidal range in the world. defence. The most recent flooding happened in The Blind Yeo’s north and south eyes - December 1981, when a large surge constructed of reinforced concrete - on a modest spring tide flooded 305 were in need of structural, mechanical properties and land up to two and electrical refurbishment. kilometres from the sea. Since that time many hundreds of properties The three tidal defence structures were have been built in the tidal floodplain. only capable of preventing flooding from an extreme tidal event with a 1 in Engineers found the previous Land Yeo 25 (4 per cent) chance of happening in outfall was in poor structural condition any one year. and nearing the end of its useful life. The outfall was also no longer The Environment Agency’s national watertight, did not control upstream target is to provide protection against water levels as required and did not a flood with a 1 in 100 to 1 in 300 meet health and safety requirements (1 per cent - 0.33 per cent) chance of for operation and maintenance. happening in any one year. Although Marshall’s Bank was In 2004 we carried out a £1 million structurally stable, erosion and project to refurbish the Blind Yeo settlement had significantly reduced sluice, and in 2005/2006 a its height and width. In an extreme £3.2 million scheme was completed tidal event waves would overtop the on the Land Yeo outfall and Marshall’s Bank. Environment Agency Tidal defences for Clevedon, North Somerset 5 construction work At the Land Yeo outfall we installed: • tide flaps that prevent tidal flooding • water level monitoring equipment to of the Land Yeo and Clevedon, but allow automatic and remote control allow the river to discharge once the of the sluice gate (penstock) and tide has receded tilting weir • powered automatic sluice gate • a trash screen to collect debris and (penstock) in case the tide flaps prevent blockages to the new should block or fail equipment • a tilting weirto maintain water • vehicular and pedestrian access for levels in the Land Yeo through maintenance purposes. Clevedon. This can be lowered to allow high river flows through the outfall Tidal headwall Winch beam and pullies Gabion mattresses and baskets Wing walls Clevedon Pill Tidal outfall structure 6 Environment Agency Tidal defences for Clevedon, North Somerset Access ladders By pass with safety lines High security double leaf gates Trash screen Tilting weir Handrailing Elver pass Penstock and controls Downstream view of river intake structure Environment Agency Tidal defences for Cievedon, North Somerset 7 construction work continued Floodwalls were constructed around the outfall and linked with the higher ground at Wain’s Hill to the west and Marshall’s Bank to the east. The new walls were raised and faced with natural limestone and cock and hen coping to match the nearby boat store At Marshall’s Bank we: • raised 500 metres of the bank and A footpath runs along the crest of Marshall’s Bank surfaced its crest in crushed stone to minimise erosion and make a new footpath • provided an access ramp - suitable • provided vehicular and pedestrian for disabled access - onto the bank access for maintenance purposes and into Marshall’s Field. Local limestone was used to match existing boat store 8 Environment Agency Tidal defences for Clevedon. North Somerset Flood wall PenstockTelemetry kiosk Flood wall PenstockTelemetry Tilting weir l mm i H' Trash screen By pass chamber and penstock Weir plate Land Yeo river intake structure At the Blind Yeo sluice we: • replaced major sections of the • addressed operational safety issues mechanical and electrical to bring it in line with modern components of both eyes and health and safety regulations refurbished the remaining elements • installed steel stoplogs to assist • installed remote water level future maintenance monitoring equipment to provide • replaced the outfall revetment to better control of the sluice strengthen the bank against erosion • repaired the existing structure and from the sea access road • improved public access across the • newly equipped the control room structure. and gave it an independent power supply Environment Agency Tidal defences for Clevedon, North Somerset 9 the Severn Estuar The Severn Estuary is protected under both international and national law, reflecting the conservation importance of its habitats and species. It is designated as a Special Protection Area (SPA), a Ramsar site - a wetland of international importance - and a Site of Special Scientific Interest (SSSI). Its immense tidal range and its classic funnel shape make the estuary unique in Britain and rare worldwide. Redshank Its intertidal zone of mudflats, Environmental considerations sandbanks, rocky platforms and saltmarsh is one of the largest and The boundary of the Severn Estuary SPA most important in Britain. The saltmarsh and SSSI lies at the base of Marshall’s has a range of plant species such as Bank. The new works were designed glasswort, common cord grass, annual and built to avoid adverse impacts on sea blite, corn parsley, common the saltmarsh habitat. saltmarsh grass, sea club rush, sea Construction activities were timed to clover, sea couch and sea milkwort. reduce disturbance to waders and The mudflats support many important waterfowl and there was continued invertebrates, with high densities of liaison with Natural England throughout lugworm and ragworm - an important the proJect. food source for birds. These include Where possible local materials were internationally important populations of used in construction - hence local waterfowl including Bewick’s swan, teal, limestone was used around the Land ringed plover, dunlin, whimbrel and Yeo outfall to match the local boat redshank. In late winter and early spring store. Numerous trees and shrubs were there are also nationally important planted. populations of Shelduck and wigeon. 1 0 Environment Agency Tidal defences for Clevedon. North Somerset Bewick’s swan The area is very popular with dog The projects to improve Land Yeo walkers and birdwatchers. Throughout outfall, Marshall’s Bank and Blind construction every effort was made to Yeo sluice were funded by the ensure access was maintained. Department for Environment, Food and Rural Affairs. Both the Land and Blind Yeo have healthy coarse fisheries - which include Work on the Land Yeo and bream, roach, carp, gudgeon, perch Marshall’s Bank scheme was and eel. Every summer the Blind Yeo carried out by consultants Black sluice dams up water that maintains and Veatch Ltd and contractors levels essential for livestock and Carillion Regional Civil Engineering. farming. It also benefits coarse and eel fishermen and many types of flora and Consultants for the Blind Yeo sluice project were Jacobs Babtie; fauna. The works to the Land Yeo outfall will further enhance the habitat and the electrical and mechanical parts amenity value of the Land Yeo upstream of the scheme were carried out by through Clevedon.