In the picturesque Kent fishing town of Rye, the Environment Agency is tackling a double dilemma with a single solution. Rye, known for its scallops, is close to the mouth of the River Rother.
Surge tides race inland up the narrow waterway from the English Channel with growing frequency.
Exceptionally high tides have been inundating parts of the town: 623 homes are affected by floods, on whose waters are borne rubbish and the occasional dead animal.
Meanwhile, fishermen berthing to unload their catches have increasingly found the 23 timber jetties that jut into the Rother submerged by high tides. There were financial as well as safety implications as boats waited for the waters to ebb, and as crews splashed and slithered from boat to river bank.
As the body responsible for flood protection, the Environment Agency wanted to provide protection against a 1 in 200 year flood event. It was also charged with rebuilding landing facilities so that vessels could moor and unload easily.
The £4.7M solution designed by Agency framework consultant Atkins combines berthing and flood protection by partially relocating the quay and replacing the timber jetties with a 200m long sheet piled quay wall.
'There was the option to build new jetties and then build the whole quay up a few metres to prevent inundation, but that would have given only 1 in 10 year protection, ' says Agency project manager Kevin Talbot.
'A sheet piled wall is easier to maintain and it lasts longer, with a guaranteed design life of 50 years.' The sheet piled wall stands 5.8m above ordnance datum, adding an extra 2m to the current defences. To allow boats to dock alongside the wall at all states of the tide, the river bed is dredged down to about 10m below chart datum in some areas.
The wall is made up of 30m long sheet piles. Contractor Jackson has driven them through alluvium into competent rock some 15m down using a vibro-hammer. To resist earth pressure the wall is supported by 67 ground anchors. These are some 30m long and are stressed to 150% of the load bearing onto the wall.
The sh quay's sheet pile wall is integral to a wider, £2.9M flood protection scheme being implemented for the whole town.
This involves building up 8km of flood defences around Rye Bay from Scots Float sluice on the River Rother to the west of the town near the mouth of the river.
Framework consultant for the overarching scheme Halcrow quickly ruled out a barrage gate near the mouth of the Rother because of high maintenance costs and the risk of a costly and time consuming public inquiry. Nor was there enough land to consider managed retreat, or even to allow the building up existing embankment to 5.8m above ordnance datum:
Land take required to achieve slope stability would have encroached on the salt marsh, which is a protected habitat.
Accordingly, a reinforced concrete wall is being built on top of the existing embankment, increasing height without adding to the width of the bund. This is being built along 2.3km of the 3.5km of phase one in areas where the earth embankment cannot be built up.
At points where bends in the river lead to quicker bank erosion brushwood mattresses have been used. Brushwood is tied together and attached to the bank between wooden stakes. The brushwood gradually becomes coated with sediment from the bank creating a protective anoxic barrier, which prevents the wood from rotting. Just to prove its effectiveness, there are parts of the river bank where 100 year old brushwood mattresses are still going strong.
Under the two-year second phase, scheduled to start just after completion of phase one in April 2006, a new concrete wall will be built up around the main part of the town known as Strand Quay. The wall will be clad in bricks matching those in the conservation area of the town.
Working at Strand Quay, Jackson will need all the experience it has gained working on the fishing quay.
'The scheme has not had universal local support: The last major flood in Rye was in 1989 and that wasn't too severe, ' notes Halcrow assistant site supervisor Miles Pickering. 'We will have to go through people's back gardens and logistically it can be very difficult to gain access.' Back at the fish quay, the sheet pile wall is protected against accelerated low water corrosion, which is capable of eating through a sheet pile wall in just a decade, with aluminium/zinc sacrificial anodes at 1.2m centres. These are expected to keep attack at bay for 20 years. Two coats of epoxy paint applied to the wall provide a second line of defence, expected to give another 25 years protection.