THE PROBLEM OF FLOODING

Bay of Plenty Beacon, Volume 10, Issue 27, 20 September 1946, Page 5

THE PROBLEM OF FLOODING

borough council s concern SPECIAL REPORT COMPILED REFERENCE TO RIVER’S CONTROL BOARD The problem of flooding within the Borough, was the subject of a special report by the Fore-man-Engineer (Mr W. S. Henderson) to the Borough Council last Monday evening. A very full survey was made, and the writer went to some pains to point out possible causes and logical preventative measures. The matter was fully discussed by the Council and it was finally decided to refer the whole report to the Soil Conservation and River’s Control Council requesting that body to investigate and to advise before any definite steps were taken. The report which was very comprehensive read as under:— Wainui Te Whara Prior to 1936 this stream from Valley Road to outlet had been lightly cleaned out by manual methods. For the next eight years there was normal silting of the stream bed averaging 3” depth per year and trimming a light cleaning out continued to meet the situation. In the heavy flood of March, 1944, enormous displacement of metal deposits took place up the gorge and was carried down to the flat and this process has been repeated on at least four occasions in the last two years. Some eight months ago a mechanical loader and dredge was used to deepen the stream between King Street Bridge and Taneatua Road. Approximately 4,800 cubic yards being excavated and under reasonable normal conditions this would have been effective for some seven or eight years. However, on the night of the 18th July and again on the 2nd September torrential downpours occurred and the last deluge of a few days ago deposited some 2000 cubic yards of metal and shingle between the King Street Bridge and the Valley Road Bridge. Suggested Control

Purchase and tree planting of the total catchment area would be of great preventative value but would not become effective for some six to eight years. ' A suggestion has been made that a gravel trap or traps should be built above Valley Road Bridge. I do not thing that they could be established economically. To be effective these following factors would have to be considered: (1) The weir or weirs would have to have capacity to hold a minimum of 1,500 to 2,000 cubic yards; (2) They would have to be sited to be accessible by machinery and trucks for frequent cleaning out; (3) Sited so that ponding would not effect the adjacent property and road; (4) The cost of such weirs with spillways would be heavy; (5) The cost of frequent maintenance. This stream .above the Taneatua Road traverses at least 2\ miles of metal bed and rock gorge formation and with any repetition of the recent downpours it is inevitable that the metal deposits will be brought down. Diversion of Stream It seems that we must be prepared with machinery to deal with metal deposit when it has reached the flat grade. Assuming that further dredging is undertaken it would be necessary to provide retaining walls through the built-on area just below the Pound. Persons owning property adjacent to this stream have suggested that the stream could be deviated from a point near S. Brown’s boundary and taken in a direct line discharging into a lagoon in Patterson and Platt’s. From here it would cross the County Road at the present lagoon outlet culvert. The culvert would, of course, have to -be replaced by a bridge. It is not an engineering difficulty to deviate but it seems to me that it would be obvating a liability in one direction and creating it for some other unsuspecting property owners. I think it can be ruled out. An Alternative

Further downstream at a point near the hospital garden the stream is only 16 chains from the Whakatane River and it has been proposed that a cut should be put through here to give flooding relief to the lower places such as Cantrell’s, Rutherford’s etc. On the face of it the proposition looks really good and I cannot understand why in the early days this course was not taken as it is of much shorter to the river than the existing course. The comparative distances from the hospital garden is 16 chains against 53 chains of the present route. Some serious disadvantage must have been

considered against deviating the stream as the cut would cost less than the cost of one of the 3 bridges which were subsequently built to cross the stream. The deterrent, as I see it, is the fact that the stream outlet in the river would be on the sweeping bend of the Whakatane River at which heavy erosion is and has been taking place for some years, a position that the new outlt would not improve. To effect improvements it can be summarised thus: By dredging to double the present capacity of drain stopbanks to be evenly formed both sides of drain. Total distance from the Taneatua Road bridge to the outlets, 138 chains. Estimated cost £2,700. Retaining walls through built on section below Pound 7ft. x 6in. reinforced £l7O per chain. Taneatua Road, Douglas and King Street bridges to be given increased capacity. Estimate for twin 4ft. R.C. pipes set in at Douglas Street and King Street £l9O each bridge. Taneatua Road concrete bridge and eroded section 5 ' chains above bridge. The State Highway is interested in this and I have been in telephonic communication with the District Engineer and requested that he should inspect and confer with the Council re same.

Alexandria Avenue

The recent flooding in this area was mostly from surface water and was not caused as on previous occasions by the Wainui Te Whara stream breaking its banks. At the same time I admit that the rising of the stream by some 6ft. at the outlet of the Alexandria Avenue open drain checked the surface water in Alexandria Avenue-Doug-las Street from getting away quickly. The Alexandria Avenue-Douglas Street intersection is the lowest part of that country and all surface water from the south east escarpment for -f mile south converges at this point, which, by the way, is actually lower than the floor of the Wainui Te Whara stream at the Douglas Street bridge. The open drain from the intersection, if not obstructed by two small culverts and is kept clean and free running, has done and will serve all normal wet periods, but'naturally, cannot cope with the type of outbursts recently experienced. J Footbridges Obstruct You will recall that the deluge on July 18 broke the banks of the Wainui Te Whara at several points below the Pound. In one instance the water broke over just below S. Brown’s property and flooded the Alexandria Avenue-Douglas Street area. I am convinced that this breakthrough was- accentuated by the three small pedestrian bridges from adjacent houses which were washed down, were all caught and piled up by a cross fence causing an obstruction to the stream flow. The placing of these bridge heights above flood level etc. should be watched.

The only method of getting storm surface water away While the open drain outlet is retarded by high main stream level is to pump with a heavy duty low level pumps with a capacity of 250,000 to 300,000 gallons per hour. The installation of the unit would be in the vicinity of £I4OO Briefly the position appears to be: Unless the Wainui Te Whara capacity is doubled and the pumping system installed to cope with surface water, this area will always be subject to flooding. It then is a question whether building in the locality should be’encouraged.

Wairere Stream

I cannot visualise that this catchment area which, has been brought into productive farm land at so much cost would be allowed to revert to scrub and afforestation. It seems therefore that there is no alternative but to make provision to adequately cope with the water when it reaches the base of the Wairere Falls. The capacity of the box culvert at the Bank of New Zealand is a total of 56ft. Measurements of the rainfall at the catchment area taken on the night of July 16 shows that an 80ft. culvert is required to deal with a deluge of that nature. The most satisfactory method of providing the additional space would be to widen the present box culvert by eight to ten feet on the north-west side. However this would bring the outfall too close to the County Chambers. The alternative is to widen on each side of the existing culvert using reinforced 4ft. diameter concrete pipes. The extra culvert width will necessarily effect the width of the street running past the Bank of New Zealand and on the Wairere Street, the width of carriageway being cut down by approximately 12 feet. When the culvert is widened the retaining walls of the outlet and intake will have to be built and adjusted to the new culvert dimen -

sions. The concrete parapet on the top side of the culvert to be widened and carried upstream on both sides for one chain. Following completion of work at the culvert the retaining walls should be carried upstream to- the bottom of the Falls to prevent erosion to the Wairere Street and prevent the heavy deposits of metal being washed down. The stone faced walls from below the culvert to Quay Street outlet to be cement grouted. If at all financially possible I recomend that a concrete floor be put in with the upstream retaining walls. It will eliminate any accumulation of metal at or near the culvert intake. The total estimated cost of the above undertaking was £3673. Councillors Opinions Cr Shapley said that all Councillors were conversant with the problem and the question was—what were they going to do- about it ? Something definitely had to be done before next winter. The Mayor: We certainly must tackle this thing, and we can’t do it out of revenue. It can only be done by way of loan money. I suggest that the fairest way would be by the creation of special rating areas, whereby it would be carried out at the expense of those who are suffering. It certainly cannot be done

piece-meal any longer. Cr Warren suggested that the Borough was too small to allow for the creation of several special rating areas.

On the Mayor’s motion it was decided to refer the report to the Soil Conservation and River’s Control Board, with the suggestion that consulting officers be sent to Whakatane to investigate and advise. With regard to the Wairere Stream it was decided to notify the P.W.D. that a new box culvert was considered imperative, and enquiring the prospect of subsidies in view of the fact that the road was a main highway.