The Evening Star WEDNESDAY, MARCH 20, 1920. FLOODS AGAIN.

Evening Star, Issue 20129, 20 March 1929, Page 8

The Evening Star WEDNESDAY, MARCH 20, 1920. FLOODS AGAIN.

Six years ago, when Dunedin last experienced severe Hooding (in two instalments with an interval of three weeks), the then city engineer ventured the statement that such a downpour would not occur again within the next hundred years. On that occasion the rainfall was- ganged at under lOin over a period of about three days. During the twenty-four hours ended at S a.m. to-day the readings at the gauges in the watersheds responsible for the city's main sources of reservoir feeders were: Ross Creek, llin; Sullivan’s Dam, d.oin; and Wharc Flat (Silverstream), 11.-iin. Foi.lnnately so tremendous a. fall seems to have exhausted itself, and it is hoped that the Hood waters will disappear far more rapidly than they did in 192 J. Particularly should this bo the case on the South Dunedin flat. That low-lying district still labours under drainage disabilities winch were i brown into strong relief six years ago, but some of the worst of them have since been removed, Moreover, on tin's occasion the Flat has been largely spaicd invasion by torrents from adjacent hills bordering it on opposite sides, in one case from a foreign watershed. There is thus a smaller inflow to deni with, anti there are now better facilities for disposing of Hood waters impounded in an area a considerable part ot which lies slightly below the high-water mark level.

The first intimation many people had of the imminence of a serious flood was the suspension of traffic through the Cavcrsham railway tunnel early yesterday afternoon. To those with lair memories this could only mean that water which should have run into the Kaikorai Stream, and so out to sea, had got a cress to the Burnside entrance to the tunnel, and was pouring along the subterranean down grade, out of tho Cavcrsham end, and so on to the Flat. Unfortunately this once again proved to bo the case. 1 n the big flood of April, 192 J, a great deal of wafer from. Concord Greek missed tho flume designed to carry it across the railway cutting near the Burnside station, and discovered a new and devastating route through the tunnel at an estimated rat© of JSU cubic tcet a second. The Railway Department ultimately blocked that How, and in the May Hood three weeks later the tunnel was kept dry. The. flume was permanently extended at its lower end, and its intake was also altered so as to trap tho crock waters efficiently. A deficient culvert under the Kaikorai Valley road was also remedied, and it was confidently asserted that no more trouble need be expected from these sources ever again, it is disappointing to record that there was a repetition of this trouble yesterday. There is a conflict of evidence as between the Railway Department and the corporation' officers as to the extent of this trouble, hut the hour at which the first train was able to go through the tunnel makes it plain that tho invasion of foreign water was not a trivial matter.

At the opposite end 01. the I'hit damage was far lc*s pronounced than it was in 1923. On tin's occasion Tainui virtually escaped Hooding, and Musselburgh got off much lighter, ilic main reason is that in the -interim a high-level intercepting conduit has been constructed along the TainuT road, and this leads the water from the hills into tidal water in Anderson's Bay inlet instead of allowing it to invade Urn Flat. The reticulation of Taiuui, now almost complete, enables its surplus water to bo conveyed to the hall-tide basin in the southern endowment. Further, in the interior of the Flat a certain amount of permanent work lias been done, especially in the vicinity of Macandrew road, Surrey street, and Bay View road, and at other places the old box sewers have been replaced by pipes as a temporary measure pending completion ot the scheme. But there yet remain two most important things to be remedied. Alter (be Hoods of six years ago we wrote that “until the, capacity of the Forbury road and Wilkie road storm water carriers is increased the menace ol Hood to the Flat cannot be ignored,” Neither of these works has been undertaken. At that time Mr M'Curdie’s first three out of fifteen recommendations to the t ity Council in order of urgency were the reconstruction of the storm water i:.rriers at St. Andrew street, \\ ilkic read (from King Edward street on), and Forbury road, at respective estimated costs of £15,000, £20.000, and £64,000. Of these only the first-named has been carried out. Once again the familiar sight of a geyser Irom the manhole of the Wilkie road sewer where it goes underneath King Edward street greeted the traffic through Kensington. Without such a vent this badly overloaded conduit, instead of blowing off the manhole cover, would probably do infinitely more damage by blowing out its own crown where it is exposed a little further along. The necessity for reconstruction with much enlarged carrying capacity is freely recognised, but lack of funds is urged in excuse.

It is, however, the Leith which has clone the real flood damage on this occasion. Whether completion of the schemes of the Harbour Board and the City Corporation in respect of this stream would have prevented or even minimised floods arising from the precipitation of llin in twenty-four hours is a mere matter of surmise. As Mr Furkert advised some years ago concerning- the Leith canalisation, the weak point is the junction of the steep

with the flat grades. So phenomenal was the discharge that the benefits arising from the construction of the big St. Andrew street sewer were largely concealed. The Leith waters invaded that area and carried with them so much detritus, much of it floating detritus, that the mud tanks became choked, and the value of the sewer was impaired through its carrying capacity not being taxed to the limit from the above cause. It is highly probable that the pumping station at Musselburgh had to deal with a considerable volume of Leith water instead of the latter going direct into the harbour, and but for this the inundation of the Flat might have been less pronounced. Both City Corporation and Harbour Board have a big problem in controlling the final stages of a stream which may in an hour or two bo transformed Irom a trickle to a raging torrent. The sooner they cooperate and solve this problem successfully, the easier will be the minds of a considerable section ot ratepayers and residents. For the most part it may be said that this 1929 flood was nonpreventhble as compared with those of 1923, despite the lessons conveyed by that twin visitation.