AN UNRULY RIVER.

Hawera Star, Volume XLVIII, 24 September 1924, Page 7

AN UNRULY RIVER.

PRANKS OF MANGAHAO

DAM FOUNDATION DIFFICULTIES

An interesting paper dealing with the making of the- foundation of the Mangahao dam was read before the technological section of the Wellington Philosophical Society by Mr F. W. Furkert. The address was illustrated by a. number of admirable photographs, and conveyed a graphic impression of the difficulties of the work. The Mangahao dam is not a very big one, as it is built on a narrow gorge; but it bars the wav of a river of exceptionally irregular habits. When it is low the Mangahao flows at the rate of about 32 cubic feet per second./But in full flood the flow' is as much as 18,000 cubic feet per second—nearly GOO times as much; and the river will rise from very low to very liigh in a few hours. It has been known to jump from 1000 cubic feet per second to 15,000 in twenty minutes. Mr Furkert said that such performances were almost inconceivable, but, phenomenal though the river is in this respect, it manages to repeat the performance at quite short intervals. The high fl'.ods averaged about three weeks apart, with minor ones more ferquently. These sudden and drastic floodings, as Mr Furkert showed, controlled the whole method o,f building the dam, and especially for preparing the foundation. It was necessary to put in temporary dams up and down stream ’to enable the foundation proper to be excavated in the dry, and to divert the stream as a whole through a by-pass tunnel, as it would have been impossible to make an open by-pass channel. So far as the temporary dams were concerned, they were covered by the water at every fresh, and time after time work,that had been patiently and carefully done was carried away. The liability to- such accidents was easily realised from photographs of the stream, thirty feet deep, tearing chrough the gorge. The lecturer described how the floodvolumes entered into the calculation of the size of the by-pass tunnel, and how this was made to take all the water except during the comparatively rare very high floods, so that work could proceed with reasonable freedom. When he floods were on, however, gear had to be safeguarded : and interesting details were given of the mounting of the pumps on moveable platforms on rails fixed to the sides of the gorge so that- they 1 could be pulled out of harm’s way, as well as lowered as . they reduced the level of the water while at woyk. . In addition to the mass concrete dam in the visible gorge, there is a flanking dam in an old stream-bed, filled with rock and earth. This was in effect a natural earth-dam, and the engineers, in order to make it watertight, decided- to cut a slit in it- and put in a ‘‘core-wall” of concrete. This meant a narrow ditch 70 feet deep, and the lecturer explained liow this was made by beginning with a tunnel, so. that the spoil did not have to be hoisted, but was moved by its own weight and was thrown into the river to be washed awav.

At the conclusion of the lecture, a number of questions was asked and answered.