OPENING LAKE OUTLET

Ellesmere Guardian, Volume LI, Issue 66, 19 August 1930, Page 3

OPENING LAKE OUTLET

FURTHER PLAN SUBMITTED,

OPEN CONCRETE CHANNEL.

WEIR, PORTS AND SLUICES,

A new scheme for the draining of Lake Ellesmere was proposed to the Ellesmere Land Drainage Board, at its monthly meeting, by Mr C. H. Mason, of Timaru. ' 'Last Easter Monday I paid a brief visit to the outlet at Taumutu," Mr Mason wrote. "The lake at this point seems most sheltered from winds and best suited for a permanent outlet. I enclose a rough sketch of my idea, which consists of a weir about -120 feet in diameter, of concrete, partly re-inforced, and an open channel of 30 feet wide, also of concrete, sides and bottom, and about 160 feet long. As far as I was able |to judge, the best site for the outlet works seemed to be about five or six chains west of the existing piles and timbering, and as there is no choice of any other than a shingle foundation, the bank here seems fairly consolidated and quite suitable for the scheme. "I propose to end the concrete outlet at a point about two-thirds of the ! distance through the shingle bank, seawards, thus leaving about onethird of the bank as a protection from the sea and lake during ,the course of construction of weir and channel. An important preliminary will be to sink two or more wells—one at the proposed site of weir and the other just beyond the concrete outlet, seawards—to a depth just below the proposed foundations to obtain an estimate of the seepage and probable quantity of water to be dealt with while the foundations are being laid. The floor of the concrete channel at the outlet should be two ifeet below low tide mark, at least provided the seepage difficulties are not too great for advantages to be ! gained.

BRIDGE FOR PUBLIC.

"A wooden bridge with ironbark piles will be necessary to be constructed immediately beyond the end of the outlet. Its chief object will be to provide a means of tipping rubble or angular blocks of stone of sizes varying from Jcwt. to two tons. The bridge will be about 60ft. long and about 100 tons of rubble, more or less, must be on hand and tipped on the seaward side of the bridge immediately after the discharge, helped by the sea, has scoured the channel to a few feet below the mouth of the concrete channel. This rubble will form a bar, or barrier, level with low water mark, to protect the concrete work from being undermined by range during heavy seas. "The shingle at this point is small and seems much finer in the aggregate than the beach in South Canterbury, consequently it is very transitory and easily moved by a strong current. Rubble provides the cheapest and best means for counteracting its vagaries and, used as directed, will limit the depth of water where otherwise there would be danger of undermining. A hundred tons of I stone may prove ample for a time, seeing that the discharge end of the concrete channel is fully two chains inland, but as it is impossible to interfere with nature without experiencing some unlooked-for developments, a further supply of rubble should be on hand and a good margin of protection maintained by this means at this, the only vulnerable spot. The tendency will be to form an estuary one or two chains wide at its mouth with a shoaling bar of shingle and changing often according to direction and strength of the sea running; but with a fairly constant discharge from the lake,' and except, for aji hour or two during high tide, there is no probability of the outlet being completely blocked by shingle.

ACCUMULATION OP SHINGLE.

"If, at the end of a year's test, it be. considered safe to lower the lake further without danger of a blockage, this can easily be done by low-

ering the lip of the weir any number of inches not exceeding, say, twelveprovision being made for this purpose in its construction—but, obviously, considerable extra shoaling would occur at times during the added hour or two which would be required for the tide to recede below the reduced level of the lake before the gradual renewal of the discharge. There is a very narrow limit to which the lake can be safely lowered without considerably increased expenditure, as any reduction in the number of hours of discharge must be compensated by an increased capacity in means of discharge. Some shingle is bound to accumulate at the outlet at high tide during heavy seas, and if the discharge be unduly long suspended, trouble will ensue.

SLUICES AND FLOODGATES,

"I propose to divide the lower lip of weir into 32 ports, each seven feet wide by eight inches deep—a total length of 224 feet—also eight sluices, each six feet wide by 20 inches deep. Each port and sluice is to be fitted with floodgates to keep the salt water out of the lake during high tides. The bottom of the sluice parts will be about four feet below the lip of the weir. The concrete wall will be continued four feet higher than the top of the inlet and finished with coping 20 inches wide and suitable hand-rail. "Mr Bray's estimate, made some 55 J years ago, of the lake's discharge, viz., 625 feet per second, is probably above the actual discharge of to-day. It is equal to about 6.8 inches, or onefourth of the annual rainfall over the whole district. Evaporation, representing three-fourths of the rainfall, usually increases with intensified cultivation, especially any increase in growing timber, therefore it is probable that less than 600 ft. per second is the maximum annual discharge today. I estimate that the weir with a full head will deliver a maximum of 1,000 to 1,100 cubic feet per second during low tide, or an average of about 700 cubic feet per second con- | tinuously.

MINIMUM LEVEL,

"Before the Board commits itself to any scheme for draining the lake, it should possess an approximate estimate of all the land which would be reclaimed between zero and high-tide levels, if it does not already have the information, because it is possible ! that by the expenditure of an extra £1,000 in the cost of construction, the capacity of the outlet may be doubled. This, of itself, would not reduce the level of the lake because, as already indicated, there will be a minimum level to which the lake can be reduced without causing the discharge to be so intermittent as to permit the shingle menace to get the upper hand. This minimum level can only be found by careful and lengthy tests. "The point is: If the outlay seems justified, a 50 h.p. electric 'pump would lift 100 cubic feet of water per second, the necessary two or three feet to increase or start a flow through the discharge channel whenever it appeared desirable to do so. Its use would prevent or remove ah accumulation of shingle and permit of the lake being probably twelve inches lower than is possible with an unassisted discharge. Its use would be for emergencies, primarily. Some days it would be run for a few hours and for some consecutive days, during calm weather, its help would be uncalled for unless the Board decided that its extended use would be good policy."