A SILTING DAM

Poverty Bay Herald, Volume LXIII, Issue 18938, 13 February 1936, Page 8

A SILTING DAM

SERIOUS PROBLEM BLENHEIM’S POWER There is a small river in Marlborough, the Waihopai, which is giving engineers an unusual amount of trouble. Well up in a rocky gorge, in the foothills above Blenheim, it was dammed 10 years ago to supply storage for Blenheim’s hydroelectric supply, but since that time so much silt and shingle has backed up against the dam that a storage lake no longer exists. The Minister for Public Works, the Hon. R. Semple, inspected the works at Waihopai, and was shown a bridge about a mile above the dam which eight years ago was 12ft. higher above the bed of the river than it is now. The bed has been so raised by silting that in flood the river sweeps over the bridge, and there is even danger of its being washed away unless it is heightened, but the most serious trouble arises from the silting up of the lake above the dam. When the dam was built there was a lake probably a mile long, and 20ft. deep, with enough storage to ensure a continuous supply of electricity for the driest summers. Now there is only a small shallow pool immediately above the dam supplying water to the power-house, and even that pool has to be kept clear of silt by a syphon and other means. What was the bed of the lake is now mud - flats on either side, raised well above the level of the river, which runs through it. The silt has reached the height of the spillway over the dam. REDUCTION IN DEPTH Where the water entered the pipe line at the dam, the water was formerly 80ft. deep. Now it is only 35ft., and the authorities are afraid that a dry spell would cause a shortage which could only be met by the extension of standby facilities. It is estimated by the engineers that more than 5,000,000 cubic feet of silt and shingle coines down the Waihopai every year. The Marlborough Electric Power Board, which constructed the works at Waihopai at a cost of £300,000, is seeking sanction from the ratepayers to raise a loan to erect additions to the standby plant, and extend the powerhouse as a precaution against a breakdown because of shortage of water or other trouble at Waihopai. The Government is interested in this prooosal, and in the present situation at Waihopai, because of the suggestion that a major hydro-electric scheme might be installed at Lake Rotoroa where the Gowan River leaves the lake. This scheme would serve all the northern part of the South Island.