PROPOSED WATER SUPPLY.

Gisborne Times, Volume V, Issue 128, 10 June 1901, Page 3

PROPOSED WATER SUPPLY.

ME MESTAYER FAVORS WAR

HIRERE.

TO COST £55,970,

The report furnished by Mr R- R- Mestaycr on a water supply scheme for Gisborne is to hand, and will be presented to the Borough Council meeting to-morrow night. As with the drainage scheme, Mr Mcstayer deals with the subject exhaustively. He states Before discussing the details of these proposals, it will be advisable to determine the conditions to be fulfilled by a satisfactory town supply. * The first essential is purity of water., J.o ensure this, the gathering ground must be free from any possibility of contamination due to dwellings being erected on it, and, if possible, it should be kept absolutely free from cultivation. The second ossen^ ' tial is that the supply should bo ample, not only for the- present population, but also for such additions as may be reasonably ! anticipated during the next 20 or 25 years. In the third place, the height ' * must bo sufficient to afford a pressure of such force as will command the highest buildings likely to be erected within the' aroa supplied, and this pressure must be available during the period of maximum consumption, in case a fire should occur during the daytiiiie. Given two or more schemes which fulfil these conditions, the least expensive is, generally speaking, the best.” He goes on to show that the sources which have been suggested are as | follows; — , ~ 1. Lako Waikaromoana (about 60 miles from Gisborne). 2. Tiniroto Lakes (about‘42 miles from Gisbordc). 6. Ilangaroa Rivcr*(about 00 miles from •Gisborne). 4. AVharokopae Stream (about 29 miles from Gisborne). 5. Ngatapa Falls (about 29 miles from Gisborne). 6. Caulton’s Creek. , 7. Barker’s Spring. 8. Douglas’ Creek. 9. Patutahi Creek. 10. Lako ltopongacro (about 12 miles from Gisborne). 11. VVaipaoa Rivor. 12. Waimata Itivor. 18, Waikanao Stream. T' 11. Focdors of Waipaoq Rivor (Mr [ill’s proposal). 15. Todd’s Stream. . 16. Mangarueka, Mangaono, and Manganainuku Crocks. - ■ • , 17. Waihiroro Stream. Of these sources, the first five would, ho states, be too costly, • varying from XIOO.OOO to .£200,000 for pipe-fines alone. , The supply at Nos. '6, 7,3, and, 9, is 'inadequate. Lake Repdngacro is net high enough to afford the necessary pressure, even if tho water were good. Nos. 11 and 12 would be unsuitable, as th’oy would , require pumping schemes, and the water would be subject* to pollution in flood time and otherwise.' Mr Mostayor states No. IB is a proposal by Mr Ross to utilise tho spring forming' tho head Waters, of the Waikanao stream by pumping it into a water-tower 65ft in height, from which tho water would bo convoyed to the town i in a 7in main. As this would not afford sufficient pressure for firo extinction, Mr Ross proposes to uso a second ongino for this latter purpose, and to pump direct into tho • mains. The scheme, although ingenious, is not by any means what is ■ required in Gisborno. Apart altogether from tho objection of tho firo pressuro not being always ready for use, the normal prossuro from tho tank would bo quito ' insufficient for the requirements of ] tho town, and it quite fails to tako into account tho probability , of any futuro extension of tho supply ' to North Gisborno and the Kaiti. No. 14 : Mr Hill proposes to collect the ; wator from tho crooks feeding tho Waipaoa rivor behind To Karaka, and to conduct it by. gravitation to storage rcsorvoirs : constnietod on the hills' at somo suitablo sito nour tho Kaitoratalji bridge. .There is ; no doubt that an abundant supply of good ' wator could bo obtained in. this manner, and I quito ofidorso tho claims made by Mr IliH as to tho advantages to be do- , rived from.it, especially as to tho futuro oxtensions of tho supply to Whataupoko and tho Haiti. That tho scheme would be very costly is evident owing to tho dis-. tanco. Tho cost of tho main supply pipo alono would be over ,£60,000, and to this • must bo addtd cost of head-works, storago. reservoir, river crossings, etc. No. 15 i Tho objection as to cost of bringing the wator such a long distanco applies equally to this sehemo as to tho one last considered. This loaves Nos. 16 and 17, the Mangarueka and Waihiroro. Mr Mcstayer describes tho country surrounding oach, and makes a comparison of 'tho two schemes. Mangarueka has a catchment area of 2760 acres,.of which 300 acres arc under cultivation. Provision would bo neoded for storing 60 days’ supply to supplement the ordinary flow in dry soason. Tho ground is nil -of papa formation, and clay or gravel required for tho dam would havo to bo brought from a distanco. Tho site is 175 feet above the B.M. pog at tho intersection of Teclstreot and Road’s Quay,, and is 18 miles from tho town. With regard to Waihiroro, ho points out that the catchment area has been over-ostimated, tho actual area being only 796 acres, of which 65 aro below tho sito available for a dam, leaving a not area of 701 acros to supply tho rcser- ' voir basin. Mr Mcstayer states: “ Tho distanco from town to tho proposed rosorvoir is 9 miles, and tho sito is 264 feet abovo tho B.M. at Peel street, 90 feet higher than tho Mangarueka sito. The - wholo of the drainago area is uncultivated and covered with manuka, fern, and coarso grass. Tho water collected from it would therefore bo perfectly pure, thus satisfying tho first essential condition of a town supply. The rock underlying tho surface appears to belong to the limestones, and as far as could be ascertained without actually opening up tho ground it is free from faults, and would form good holding ground fora storago rcsorvoir. At tho site proposed for a dam tho rock is from 4ft to 18ft below the surface, the latter depth being only in the immediate bottom at the : creek, sc that but little excavation would bo needed to got to a solid foundation. Thoro aro no indications of slips, and the ground being covered with fern, etc., tho water in times of heavy rainfall would carry but little silt into the reservoir. So far as the character of tho watershed and quality of water is concerned, the Wailiircre is practically perfect. Is tho supply to bo obtained from this 731 acres sufficient •to afford an ample supply to your town ? tjpon the answer to this question the decision as to its adoption or rejection must depend. No systematic records have been kopt of the flow of the creek, but the following isolated gaugings have K :en made: Bv Mr Drummond at the end of February, 1832,'56,700 gallons a day. .By Mr Boyian at the end of Juno, 1896, 230,000 gallous a day. In February, 1882, the rainfall in Gisborne was only 28in; the flow was therefore gauged at tho end of an unusually dry month, only two drier months being recorded during the last 23 years, viz., December, 1878, 21in, and October, 18S5, 26m. In June, 1896, the rainfall in Gisborno was B.27iiv. This is much below the average for June, which •is 5.81 in. Isolated observations such as theso arc, however, worthless to determine a question of such importance, and tho probable supply must bo arrived at in somo other manner. As there aro no springs within the watershed, the supply must depend solely upon the rainfall. It is fortunate that complete records of the rainfall in Gisborne for tho last 23 years - . are in existence, and they wero kindly -lent to me by the Rev. Herbert Williams. . From theso” a very fair estimafo of the ' probable supply may bo obtained.' The Waihiroro watershed being at an average ' elevation of 500 ft abovo Gisborne, and ' facing - to the south-west, the quarter whence tho principal wet winds come, ■the rainfall upon it will bo considerably in excess of that in Gisborne, probably to the extent of from 20 to 25 per cent more; - . but iiir order to be on tbo safe side as to | I propose to assume it at] ' ’only 10 per cent more, and in considering certain special cases I shall assume it to bo only the same as in Gisborne.” Ho then goes on to show that, “ taking the mean of the three driest, consecutive years (the usual basis of calculations for * storage supplies) would give 330 million gallons, whilst tho driest year on record (1878) would have yielded 240 million gallons. These figures may he better ap-

predated if put in a somewhat different form. Assume a population of 15,000, with a supply of 40 gallons per head per day. The foregoing amounts could be . expressed as follows : —Mean of three driest years would give 433 days supply ; mean of three driest consecutive years would give 550 days’ supply ; driest year since 1878 would give 400 days supply'. From this it appears that- in 18(8, the driest year of which there is any record, the Waihirere would have supplied a population of 17,000 with 40 gallons per head per day. Taking the present population of Gisborne, North Gisborne, and the Kaiti at about 5000, it follows that the Waihirere would supply this district until the population had more than trebled itself, which would not be likely to happen in less than 25 years. So far, then, as the annual supply is concerned, the Waihirere fulfils ‘ the second essential condition of affording an ample supply for from 20 to 25 years. It remains to be seen what storage will be required to enable the surplus fall in wet weather to balance the deficiency during dry seasons. He adduces arguments to the effect that a “ reservoir containing 110 million gallons would supply a population of 15,000 with 40 gallons per head per day through a drought of greater severity than has been experienced during the last 23 years. A dam holding the' water, up to the 52ft. contour level would contain 110 million gallons, and would thus supply all storage required for the next 25 years. This, however, is not by any means the limit of the supply which could, be afforded by the Waihirere*; as by raising the darn to the 60ft contour the storage would be increased to 150 million gallons, which would suffice for the supply of a population of over 18,000. About 400 ft below the falls there is a suitable

site for the construction of a sfnall dam about 30ft in height, which would hold an additional 4V million gallons, and it might be found advisable in the future to construct this, in order to provide compensation water for persons in the valley at times when all the water is taken from the Waihirere watershed, instead of supplying them from the town main. Taking the daily consumption at 16,000 gallons, one-half of this would be used in about six hours, and the pipes must therefore be capable of delivering 874 gallons per minute, whilst maintaining a head of not less than 75ft in the town mains, during the period of maximum consumption. A 12in. pipe would bo required to do this from Waihirere, and a ICin. from iMangaruoka.".

The advantages of the Waihirere over Mangaruoka are that at Waihirero the water would be absolutely pure, and would bring little silt into the reservoir during ' heavy rains. The Mangaruoka water is not absolutely pure, owing to the cultivation of part of tho' catchment area, and the papa formation rendering it subject to slips, which would fall into the reservoir. * While at Mangaruoka it would not bo safe to .construct a large reservoir, owing to the papa formation, it would be difficult to find a better site than that of Waihirere for a large reservoir, and there would be no difficulty in storing there .tho wholq yield of a dry year. Waihirero has the advantage of being 90ft higher, only half tho distance, and being quito. free, from any source of pollution. Tho discoloration of tho Manga-l-ueka wator in. heavy fafns would probably necessitate filtration of the water, and this would reduce the available head by fromffift to 10ft. Ho also points out the extra cost of acquiring the land at Mangaruoka, or if only a part is acquired, as suggested by Mr Lysnar, dwellings might bo erected on tho remainder,* and a singlo case of typhoid fever in any dwelling might pollute the whole of tho supply to the. town. •He also shows that tho storjrgo - at Waiiiirerexcouldbo increased to supply a population of 20,000, and that the * of. So doing would only bo one half that cd obtaining an equivalent supply frgm Man'garuok'it.. A dry year’s yield.might‘be supplemented" by the surplus -from the' preceding year. If it is over required to provide for a larger population than 20;000, the solution of -the problem will, he states, probably be found in combining Todd’s crook with tho Waihirero. In assigning 20,000 ns the .limit of population to be served from the Waihirere basin, it must bo noted that this applies only to exeopContinuod on page 4.