Ipswich Icons - Holding back the high tide of the River Orwell is much harder than it looks
- Credit: Archant
Stopping a particularly high spring tide from flooding Ipswich is, on paper, relatively simple, writes John Norman, of The Ipswich Society.
All that is required is a dam across the river to hold back the water. An added complication is that the barrier should be moveable and used only if a flood tide is expected. And, of course, both banks downstream of the barrier need protecting to stop the rising water flooding left and right before it reaches the barrier.
As we shall see, in practice the whole is much more complicated than that.
A bank or river wall runs from the high ground under the Orwell Bridge (adjacent to the Strand) to the embankment by the railway bridge in Wherstead Road. On the Cliff Quay side of the river a new bund (bank) runs across the front of the brewery and onto Hog Highland.
The gap in the middle allows water into the wet dock (through the lock gates) and to escape out of the New Cut, the daily flow of the River Gipping. If tidal flooding is to be avoided, these gaps must be closed. The new Prince Philip lock gates were installed in 2000 and have recently been supplemented with a flood gate. The lock gates retain water in the wet dock at a high level when the tide recedes; the new flood gate keeps spring tides out of the dock, ensuring that the water level doesn’t rise above the quayside.
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The flood barrier which is currently being installed across the New Cut is a half-round steel box, which for most of its life will sit on the river bed unseen. When a flood tide is expected, it will revolve a quarter-turn to provide a barrier to the incoming tide. It will also be able to manoeuvre a full half-turn to be clear of the water so it can be inspected and maintained. The Thames Barrier operates in much the same way, although the London river has 10 gates and we need only one to close the width of the New Cut.
The obvious flood prevention measure is to close the barrier before the tide reaches a height at which it would flood into the streets of Ipswich, but it is here we encounter the first difficulty. Raising the barrier not only stops the tide but also stops the fluvial flow down the Gipping. When the barrier is closed, the water levels in the river will rise and Ipswich will flood with fresh water (rather than salt water).
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Careful computer modelling and water flow predictions will determine exactly when, before an expected high tide, the barrier should be raised, immediately stopping the Gipping but allowing sufficient capacity for the river to fill the canalised section under Stoke and Princes Street bridges.
Before any construction work was given the go-ahead, and public money was spent, decisions had to be made on the cost benefit analysis of protecting Ipswich. Planning for the scheme started in 2006 with the design of the embankments, the additional gate required within the lock and the position of the barrier. When the installation is complete, the new barrier will protect 1,660 homes and 400 businesses.
A major obstacle was the 132,000- volt electricity power cable which supplies Felixstowe (and Felixstowe Port). This had to be diverted and buried considerably deeper than it had been to run under the lock pit. Vertical shafts were dug, a tunnel bored through the chalk from one shaft to the next, and the power diverted into the new cable. ABP took advantage of the new tunnel to provide a route for their power and communication cables.
Once installed, the new barrier and river protection work will protect Ipswich from a once-in-300- year flood tide. The occurrence of that tide could of course be during any one of the next 300 years, a scenario that becomes increasingly likely as sea levels rise, so justifying the expenditure of £58million of public money.