Proposal to prevent flooding

Press, Volume CXV, Issue 33934, 29 August 1975, Page 2

Proposal to prevent flooding

A scheme to prevent the sort of build-up of water in Lake Forsyth that caused flooding at; Little River in the recent storm, has been! devised by Mr A. K.' Quentin-Baxter, a mem-: her of the Wairewa County Council. The recent flooding was relieved by cutting through the shingle bank at Birdling’s. Flat, and residents are asking,! as they often have, why such; a drainage cutting cannot be permanent. Engineers have said this is impossible because of the shingle build-up at the beach and the impossibility of making an outfall strong enough to resist huge seas from the ■ south. But Mr Quentin-Baxter has said that, with a small; amount 01 courage and some 1 real determination, a permanent opening could be achieved at a reasonable price. With a large catchment in the Little River basin, the lake was silting up. he said. It must, in the course of time, become mostly swamp.! During this process, the main highway to Banks Peninsula would ' become more frequently flooded. Little River’s environment i would become more and

[more untenable. The shingle [bank at Birdling’s Flat was i becoming wider and would ; continue to grow. | It was calculated that an average stone on this beach had spent 30 years on its ‘journey from the Rakaia River, so it was little use ’ looking for any change in the beach build-up process. The world’s weather was changing. During recent years, heavy rainfalls, once ■ considered exceptional, were [becoming normal. There was a strong probability that this ichange would continue. Because of these considerations, the cost of the periodic bulldozing of lake outlets must become progressively greater. Each opening, which might be effective for hours, or perhaps some months, gave only temporary benefit, and the money spent was gone forever, Mr iQuentin-Baxter said. The lake openings were paid for from a fund derived from the rents of Crown reserves, set aside last century, i by act of Parliament, for this i purpose. The Wairewa County Council administered these ! accounts, which had a credit lof about 540.000 from ■ accumulated interest and inicome. The fund was not big enough for the capital works proposed and for contingenI cies, _ _

The council needed either a subsidy from the North Canterbury Catchment Board or a Government grant, or both. This had not been' given in the past. There should be no difficulty in reaching a decision on the benefits to be gained from partially draining the lake, as against the small ecological imbalances it might incur. Removing some of the water from the top of the lake was, in effect, what nature was doing by filling the bottom with silt. In either case, a permanent sea outlet was needed to drain the catchment. Two main schemes had been proposed in the past, and both involved tunnels. One was to be a quarter-mile bore through the Ohahoa point into Ohahoa Bay. The point and cliff face are built up of layers of basaltic flows separated by clay. Tunnelling in this unstable rock would be engineering folly. The tunnel would have no fall, and its outlet would face directly into the prevailing south-east sea. It would block first wfth kelp, then with shingle. The extension of Birdling’s Flat had alreadv reached Ohahoa Bay. The tunnel would block when it was most needed.. How could it be cleared? The second scheme proposed a shorter tunnel, open-

ing on the Ohahoa Point. This had more merit, and might be acceptable if a more reliable and cheaper alternative was not available. A continuous sea current, I of between two and three knots, flowed up the east coast. As it turned to follow the south coast of Banks Peninsula, the current deposited its main load of shingle on the spit. On meeting Ohahoa Point, the current accelerated, causing scouring, and diverting lone-third seaward in a counter-current while the rest continued east. This : scouring action off the point should be harnessed. Completing a channel round the cliff face from Lake Forsyth was common to all schemes. It presented no engineering problems. Mr Quentin-Baxter estimated the icost of this channel at $12,000. The channel would turn [South into a 100 ft long, 12ft diameter culvert,, with a fall of one in six. projecting 20ft into the sea. the end being totally immersed at high tide. The scouring action should be enough to keep the outlet clear, but the culvert was big enough to take a bulldozer. The culvert could be of steel, or reinforced concrete. The hard rock face of the cliff bottom would be drilled and pinned. When the pipe

i was put in place, quickI setting cement would be [forced into the surrounding I shingle, rock, and pins, locking the whole lot together. The cost of this culvert, in ■ place, would be about $25,000.

Unfortunately, with any I scheme of this nature, there |was a surcharge of perhaps 20 per cent to pay off the [Ministry of Works and other Government departments to meet their demands for I appraisal drilling, reports, and so on. This money was unproductive.

This scheme would need only minor maintenance costs for occasional dragfining, and should have a life of 100 [.years. If the National Roads Board had to re-align the : highway through Little River' or build it up. the costs' [could be comparable. A bridge could cross the .channel to open an alternative access to the Peninsula by [the Bossu Road. The bridge' [could incorporate a sluice-! [gate to control the water in either direction.

More into the future, the; Little River -Stream itself; would have to be straightened! to promote a better run-off, and if there was progressive dredging of a channel in the upper part of the lake, the spoil could be used for reclamation, ft would give a better flow through the rest of the lake.