THE CUTTING OF A CANA.

Gisborne Times, Volume XXV, Issue 2079, 14 May 1907, Page 3

THE CUTTING OF A CANA.

EFFECT OX ARTESIAN SLJ'I’JA Tlio effect of the cutting of tlio proposed ship ciuinl from Sumner to Christchurch, upon the artesian water supply, was discussed in n report presented by .Mr. Cyrus 'Williams to tlio Lyttelton Harbor Board recently : Tho report was ns follows: —“Tho Christchurch • artesian wolls are merely artificial outlots to an underground stream of water, flowing through a gravel bed, from a higher or a lower level, between impervious or watertight strata. Such streams usually llow out under tho sea, and tho pros-,-mro which causos the wator to riso in tho wolls is duo to tho weight of water column, of difference of level between the point whore tho wator enters the porous bed, and the top of the well, while tho friction of the wafer- flowing in tho porous stratum prevents its free escape at tho lower mtlot. The fact that tho water does not escape upwards over the whole area above the stream, and that the underground stream remains underground, in spite of its pressure, would, oven in tho absence of positive proof from borings, prove the existence of a water-tight stratum above the stream, hut wo have such direct proof from borings that tho porous ■itratum of gravel which feeds the Christchurch wells, is overlaid by the thick water-tight beds of clayey mud. [’lie Christchurch shallow wolls, usudly. spoken of as first stratum wells, ire led by a stream in'- which the water is struck at about 80ft-. below bo ground surface, or about 60ft. boow water in the city, and somewhat leeper as it aproaches the sea. As ho cutting for the canal would be inly 30ft. deep below low water,, tho mpervious stratum would not bo inorfered with, tho whole of the mnt-or-al to that depth being sand and gruel, bolow tho few feet of surface soil. There would certainly bo a slight difi'ereneo in the weight of tho material over the area of the canal, tlio original sand there being replaced by wator, with somewhat less than twothirds of its weight, but it is highly improbable that tho slight difference u suposod weight would have any effect in tlio shape of allowing the underground stream to force up the bottom of the canal, and unless this happened, there could be no effect m the supply from the wolls from what is called tho first stratum. Those remarks apply with still more fffirco to wells sunk into the second uid lower strata, from which Cliristihurcli is principally supplied. I therefore consider that tlio cutting of such i canal would not have any effect oil die Christchurch water-supply. Its mly effect would be to drain the ad•aeent saturated land by lowering be saturation level in the neighbourhood.” The report was approved of