AN ALTERNATIVE PLAN FOR DRAINING LAKE ELLESMERE.

Star (Christchurch), Volume LXIV, Issue 829, 1 August 1933, Page 6

AN ALTERNATIVE PLAN FOR DRAINING LAKE ELLESMERE.

To-day’s Special Article.

Canal from Taumutu to Rakaia River at Much Lower; Cost of Construction. “A more efficient and far cheaper method of maintaining a permanently low level in Lake Ellesmere than the proposal to do so by means of an outlet to the sea at T aumutu and the construction there of ah<expensive mole, would be to construct a canal from Taumutu, parallel with the coast, to the Rakaia River.” This is the contention a Southbridge farmer put . before a “Star” representative yesterday.

'J'HE SPONSORS of the outlet-mole scheme, he said, had overlooked two serious obstacles to success—first, that the tide sweeps two ways on the coast, which would result In a mole, if constructed, merely acting as a collector for shingle which would bank up on both sides, rapidly closing any outlet made, and ; secondly, the depth of the water only a short distance from shore is so great at only two chains from the shore as to make the construction of a mole (taking into account, also, the frequency and violence of southerly swells on that coast) an engineering feat of greater magnitude and cost than the scheme provides for. He also pointed out that past experience at Taumatu had proved conclusively that, once the lake had reached a low level, the action of even the mildest of rising tides was sufficient to raise the bed of any outlet and thus effectively close it. None of the schemes so far tried had overcome this difficulty, and the one under contemplation offered nothing fresh in this connection to give promise of any success greater than had attended other attempts. The Canal Idea. Dealing with his suggestion of a canal from Taumutu to the Rakaia, he pointed out the following as factors favouring a canal. A comparison of levels shows that there would be sufficient fall from Taumutu to the Rakaia to ensure a constant flow of water at all stages of the tide. The canal could be dug on the land side of the coastal shingle bank, and within an area under the jurisdiction of the Marine Department, by means of the appliances already available, and at a total cost of, at most, one-eighth of the estimated cost of the proposed mole, etc. As the existing shingle bank between Taumutu and the Rakaia is not subject to change, it would protect the seaward side of the canal. In addition, the spoil from the canal could be added to the shingle

bank, thus adding to its protective value. One small lagoon and a few creeks that would be encountered along the route of the canal could be drained into the canal and help on the outflow from the lake. As the mouth of the Rakaia River is always open to the sea and, as the volume of water flowing down that river i.s at all times sufficient to counteract the silting-up influences of incoming tides, there is no danger of the canal outlet into the Rakaia (the lagoon on the northern side of its mouth) becoming blocked up. Even should that happen at any time, however, it would be a simple and inexpensive matter to remove any blockage. What the Map Shows. An examination of a map of the area shows: The distance from the deep end of Lake Ellesmere (at Taumutu) to the mouth of the Rakaia is 6i miles. At almost exactly half way on the route there is a small lagoon. About a quarter of a mile south of this lagoon, four creeks junction and flow for nearly three miles southward, just inside the shingle bank, to the mouth of the Rakaia. This fact is a clear indication that there is an appreciable fall towards the Rakaia throughout a distance of nearly three miles from the north—which would be another factor in favour of the suggested canal. There are not, it is understood, any private properties along the route of the proposed canal whose acquisition would be necessary. The actual work would comprise the excavation by machinery already available and at a low cost in comparison with the £BO,OOO mole scheme. The main factor in favour of the canal is that the outlet—if it should ever become blocked—could be cleared at small cost; whereas, if the moleoutlet became blocked (as is probable) the cost of re-establishing an outlet would be very heavy.