RIVERS DRAINAGE.

New Zealand Herald, Volume LXVI, Issue 20385, 14 October 1929, Page 12

RIVERS DRAINAGE.

iWAIHOU AND OHINEMUIU.

GREAT TASK NEARING END.

' TOTAL COST ABOUT £675,000

MAGNITUDE OF OPERATIONS

r [bX TELEGRAPH. —OWN CORRESPONDENT.] PaEROA, Sunday.

? The end is in sight of n vast under•""taking commenced by the Public Works Department in 1910 with the object of '■-. preventing for all time tho flooding of !- ran iiirmenao stretch of country between the eastern hills of tho Thames Valley and (ho Piako Rivor, ranging from To

- A roll a to Puriri, near Thamos, with an area of 840 square miles, under tho na/mo - <4 tho, Waihou and Ohinomuri Rivers irn- . provement scheme. , ..Tho work was initiated by a special Act

cf Parliament in 1910 as tho result of tho

finding of a Royal Commission set up to inquire into the causes of the inundation

of the area in March of that year. On that occasion both rivers flooded to a record height, causing incalculable dam-

ago to tho affected settlements. ~ A scheme of protective works was accordingly planned, estimated to cost £150,000, but as the work proceeded it was of necessity broadened and enlarged .out of ail recognition, until to-day the actual cost of the completed work is not less than £675,000. Main River System.

" Tho main river system of tho area consists of tho two rivers mentioned. The Waihou vises in the ranges near Mamaku •flowing in a northerly direction to tho Firth of Thames. The Ohinemuri rises in

the Waihi basin, cutting its way through

tho main ranges to tho Karangahake Gorge and joining tho Waihou below Paeroa. Threo . important tributaries in the Komata, Hikutaia and Omahu Streams

enter the Waihou River at distance of fine,' nine and 13 miles rcspee(ively boiow Paeroa.

Tho share of the watersheds of the rivers ' played an important part in the economy of tho scheme. Tho Waihou watershed is narrow and long, about CO

miles by 9 miles, whereas tho Ohinomuri watershed is almost circular in shape,

with an area of 140 square miles. 'J lie result is that at flood periods tho Waihou rises and subsides very slowly, while tho Ohinemuri reaches its peak in from 9 to 12 hours, falling very quickly. Tho Ohinomuri is thus past its peak beforo any seripus flooding is occurring in tho Waihou.

The protective works consist generally cf tho killing and subsequent removal of all willows, the straightening of tho river by tho cutting of canals, enlargement of tho channel by dredging and widening, nnd the formation of a complete system of stop-bonks from Te Aroha to Mackaytown to the Omahu Stream. Cutting of Oanala.

In 1916 tho cutting of the Koutio and Ngararahi canals for a length of 110 ''chains shortened the Waihou River by •; three miles, and also eliminated five bad bends which had considerably impeded the flow of water. Other canals were cut at Hikutaia and Komata with the object of straightening the tributaries. Huge suc--ition dredges lifted tho mud and sand ..from the river bed and deposited it on the banks, thus building them to a height "*'of 6ft. upwards, to give at least 2ft. of A-rireobaard above the computed high flood level. The banks or stop-banks not only oonfino the river to its channel, but en-

able it to carry a vastly increased volumo of water. • „>■

All stop-banks built of sand or silt are 4 .given a coat of soil 12in. thick and sown '. ; with grass. »fiT Tho main machinery units used in the work consist of two suction dredges, four , dragline dredges, and a dipper dredge. Tho first suction dredge was designed and " b'bilt by the department in 1916. In 10 . years this machine, which cost £9500 to , -build, dredged 260,000 yards of spoil, at _a cost of lid a yard. Another suction macnine, costing £20,000 to build, and r capable of generating 300 horse-power, in built on the same principle, dredged r 280,000 yards in less than a year, at a ; r.cost of Is Id a yard. Two dragline dredges, each costing £5850, moved 736,000 yards of spoil in three years, at an average "cost of 7d a yard. The dipper ?'.-.clredge cost £3OOO, nnd dredged 250,000 yards of spoil in eight years at a cost of i: 10d a yard. ' ' j Completion by End o! Year, V " Some idea of tho magnitude of the en- . terprise is revealed by statistics which show that to date the plant has lifted nearly six million cubic yards of mud and "'.sand from tho beds of the rivers, to build ; 60 miles of stop-banks. In addition, the machinery has cut drains of a total of

*-''•'296,000 cubic yards, while other drains aggregating 110,000 yard 3 were dug "by ?r liand" or manual labour. Fifty-five floodgates were created. It is hoped to complete tho works beforo tho end of tho ;'-*year, and that the ultimate cost will not b: exceed £675,00.

• ?"' A river board will later bo set up to -.'.'■ Carry on tho scheme and to levy a rate 7* cm tho lands that benefit commensurate o .-with a proportion of tho total cost. The :-r-successful completion of the schomo this £ year is undoubtedly a triumph of engiskill.