PROPOSALS FOR FLOOD PREVENTION

Bay of Plenty Beacon, Volume 12, Issue 58, 21 June 1948, Page 5

PROPOSALS FOR FLOOD PREVENTION

Borough Foreman-Engineer Says Catchment Areas Need Trees

Mr Henderson dealt separately with the three streams concerned and the area comprising the Catchment that so adversely affects the Borough in the following terms:— • WAIRERE STREAM: The deluge which occurred on Saturday night and Sunday morning, April 17 and 18 was of such intensity that this stream again broke its banks and caused the worst flooding and damage in the business area that has so far been recorded. Practically all the business premises were invaded with water and substantial losses must haye occurred. Approximately '360 cubic yards of silt was deposited in the streets and on footpaths. The approach to Quay Street concrete bridge was washed out and some 180 cubic yards of rock filling was necessary to restore it. The. County Council Chambers foundations were seriously washed out and the building itself was in extreme danger of partial demolition. Wairere Stream for a distance of four chains below the falls has practically ceased to exist with serious inconvenience to business in that area. ( On the east side of the Wairere Stream a house was undermined to such an extent that it is unsafe for occupation. The stone-packed retaining walls between the two bridges have also been destroyed.

Now for the remedies. I would say that the first proposal should be to acquire the Wairere Catchment area which is approximately 740 acres and allow it to revert to second growth and carry out a planned tree planting scheme. It is a serious and drastic step to suggest that 700 acres of developed country should "be taken for tree planting, but the circumstances demand drastic measures. Heavy erosion is taking place in this locality and is all aggravating the position. Assuming that this area is acquired there is still the provision to be made to deal with the stream from'the base of the fall to the outlet,. Recently authority was received from the Highways Board and the Soil Conservation and .Rivers Control Council approving of a scheme with concrete floor and sides 20ft. x Bft. from the fall to box culvert, with a 4’ 6” dia r meter reinforced concrete pipe set in on each side of the box culvert. In this scheme no provision was considered necessary for any work from the outlet of the box culvert to the Whakatane river. However, so much damage was done to this section in the recent flood that it must now be considered part of the total scheme and, the retaining wall and floor will "have to 'be extended to the river. In making an inspection of the damage with Mr Ronayne and Mr Thompson, District P.W.D. Engineers, on the day after the flood, their advice (and I am in agreement) was that the above scheme was inadequate and should be scrapped. However, if the Catchment area is taken and converted from grazing land to afforestation a scheme, increasing the diameter of the pipes to 6ft. would be suitable. This proposal would prowide the necessary extra culvert capacity without interfering with the existing box culvert bridge and would avoid the raising of the street level at this point. The plans for this scheme were thoroughly examined by Government Departmental Engineers and were approved by ■them as suitable before the last flood.

It may be found impossible to secure the Wairere Catchment area for afforestation and in that event provision has to be made to handle the huge volume of water from the base of the fall to the river. During the flood peak of the 18th I carefully observed and marked the point at which the stream broke over the banks and I am convinced that -adequately to deal with that volume a 24ft. x Bft. unobstructed waterway is necessary from the fall to the Whakatane river: y This means that the existing box culvert bridge will haye to be scrapped and replaced by' a flew bridge giving 24ft by Bft clearance. The street level at the bridge site would have to be raised by 3ft and Wairere Street east past Bank of New Zealand would be closed to vehicular traffic.

Now, this is a formidable and costly project but is the minimum re-

Re-afforestation of the headwaters of the streams that cause the Whakatane Borough’s flood problem occupied a major place in the recommendations of the foreman-engineer, Mr W. S. Henderson, in his report on flood damage and flood control, which was released for publication last week. Schemes estimated to cost £38,000 have the Borough Council’s backing and are at present under conisderation ’ by Public Works engineers.

quired to cope with a repetition of the conditions experienced on April 18. It would be possible to reduce expense by substituting , concrete grouted stone for the steel reinforced concrete line of the stream sides and floor, but am not prepared to recommend this. There is an enormous weight of water to be carried. The weight of water in a 24ft by Bft water-way from the base of the falls to the box culvert bridge, a distance of seven chains exceeds 2,000 tons, excluding frictional pressures. Briefly the proposals and estimates are.'— •' (a) Purchase of Catchment area 740 acres at £lO an acre £7400, Planting the area in pinus insignis £7400, Set in 6’ diameter R.C. pipgs each side of existing culvert £IOSO, Concrete retaining walls and floor from falls to box culvert (distance seven chains 20’ x 8’) £3300, Extend concrete retaining walls and floor from outlet* box culvert (to river four chains) £I9OO, Extend Quay Street, bridge wing walls £l2O, a total of £21,170. (b) Dismantle and remove existing box culvert bridge £250, Build new bridge 25ft span x 98ft at level giving 8’ clearance £4OOO, Concrete retaining walls and floor 24’ x 8’ from falls, to bridge seven chains £4600, Extend concrete retaining walls and floor from bridge to Whakatane river distance of four chains £2700, Extend wing walls on Quay Street, bridge £l2O, a total of £11,670. WAIEWE STREAM: For the second time in three years this stream brought down deposits of silt and metal and the 24” culvert which runs under Hamill’s building across the Strand and under business premises on the north side of the Strand was unable to take the water, with the result that this water was diverted into the Strand and aggravated the flooding of streets and business premises. On this occasion the stream brought down 400 . cubic yards of metal etc. depositing it at the side and in front of Hamill’s garage, and this had to be moved by tractor, high loading shovel and truck. The majority of the metal came from a point, between the culvert invert at Hamill’s and three chains upstream. The 18” diameter pipe which is under the highway for four chains from the top of the concrete steps was unable to stand the pressure and a. section, collapsed, with damage to the highway and additional flooding in the Strand. The area of the Waiewe Catchment is approximately 130 acres. Almost half of this catchment is in the Borough, much of it is built on, and it can be expected that many more sections will be occupied and built on. The samfe applies to a big proportion of the balance of the area outside the Borough. It is all easy rolling and easily accessible and is a potential subdivision area. From the point of view of protection' of the town I would say secure the upper 70 to 80 acres and carry out the tree planting scheme. If this is hot practicable, then secure the low drainage or water course area, a width of five chains and approximately the full length of the total area, and introduce tree planting with trees suitable for swampy and wet ground that would retard the run-off. This water course could be bulldosed at intervals to form a series of low ponding areas, the embankm'ents to be heavily planted with willow. The culvert under the highway could be replaced with a 2’ 6” reinforced concrete pipe. The District Engineer has advised me verbally that he would make allowance for this cost, £250 on the Highways estimates for the current year. From the outfall of this culvert to the invert at Hamill’s garage a distance of three chains, constant erosion of the water-way has to be stopped. Unless this is stopped the culvert under the Strand would always be in danger of being blocked or at .least, partly blocked by metal and silt deposits. This can only be done by concreting the floor and sides of the waterway to form a 4’ x 4’ open water course and the provision of two gravel traps. The stream passes under Hamill’s garage and we experience constant trouble at the point of invert. I suggest that the strearn be diverted at a point 35ft above the present invert and that it be brought

through at the side of Hamill’s building, the existing 24” culvert to be duplicated with a 36” diameter reinforced concrete pipe. To lay the new pipes under the building on the Strand north is not an easy task but is not impossible. The proposals and estimates are:— Purchase of 40 acres of catchment £BOO, Tree planting of 40 acres £4OO, Alternatively purchase of five chains wide strip following line of water course approximately twenty chains £3OO, Blading to form series of ponding areas £IOO, Tree planting of water course £llO, Replace four chains of earthware 18” pipe under highway with 30” diameter reinforced concrete pipe (provided for) £250, Reinforced concrete open channel 4’ x 4’ below highway culvert outlet a distance of three chains £IBO, Build two gravel traps £3O, Divert stream to by pass Hamill’s garage £6O, Build new invert and duplicate existing pipe lines with 36” diameter reinforced concrete pipe, distance of 6.62 chains £660, (This is estimate for a new 36” pipe line to extend only to Strand reclamation and not to the main Apanui scour channel. Clean out present pipeline £BO, a total of £2870. WAINUI TE WHARA: On April 17 and 18 this stream was again in heavy flood, the main damage being caused to the Maraetotara gorge road, the Whakatane-Gisbornej State, Highway bridge approaches, the residential area immediately below the State Highway and flooding the residential area in Alexandria Avenue, Anzac Avenue, Douglas Street etc. In company with the Works Committee Chairman and Officers of the Soil Conservation and Rivers Control Council an inspection was made of this Catchment area immediately "following the flooding on April 18, and the first and foremost recommendation that I make is that' the watershed should be secured and any further development or denuding the bush, second growth and scrub should be prevented. Of the total Catchment area of approximately 1400 acres some 800 acres is in bush, scrub etc. and there is- marked evidence that this has a strong retarding effect on the runoff of water. lam convinced, and this will be borne out by Mr Healey of the Soil Conservation Council that were it not for the big proportion that is undeveloped the flooding of the Borough would, in addition to Valley Road, Anzac Avenue, Alexandria Avenue etc., extend to Douglas Street, Goulstone Road, Crete Street and Salonika Street. The Housing Department’s new Garaway, Campbell, Stewart Block would be endangered in parts. This all emphasises the necessity to secure and protect this catchment. The District Engineer, P.W.D., stated in September of last year that his Department would take steps to guard the State Highway and bridge. Unfortunately, this work was not carried out, and some 400 to 500 cubic yards of -metal that had been bulldosed by the P.W.D. was swept down in the recent flood and deposited on the flat grade in Allandale, this further decreasing the capacity of the water-way. Groyne and protection work is essential at the approaches to the WhakataneGisborne State Highway and bridge. A gravel trap should be established at approximately 8 chains above, the State Highway bridge. Immediately below the highway there is a closely built-on area and these sections are all 5 chains in depth with a stream cutting through the centre of each section. The existing watercourse should be dredged, deepened and widened to give at least double the present capacity right through to the proposed new outlet to the Whakatane river below the Whakatane Hospital. The proposals and estimates are: Purchase of Catchment area 1,400 acres £4500, Tree planting 400 acres at £lO per acre £4OOO, Establish gravel trap 8 chains above Whaka-tane-Gisborne State Highway bridge £125, Groyne work and protection to road and State Highway above bridge £2OO, Dredging to widen and deepen existing cut giving 25’ x 8’ and including the new cut from below Whakatane Hospital to river distance of 89 chains £IBSO, Land and compensations £2OO. Douglas Street Bridge: Increase capacity by setting in 5’ diameter reinforced concrete pipes on each side of bridge with suitable wing walls £2BO. King Street Bridge: Set in 5’ diamerer reinforced concrete pipes on each side of bridge with suitable wii.ig walis £340, totalling £11,495.