LOW-LYING AREA

Nelson Evening Mail, Volume LXIV, 31 January 1930, Page 6

LOW-LYING AREA

STORMWATER DRAINAGE

REPORT BY MR H. R. CLIMIE

The City Council last evening had submitted for its consideration the totlowing report on stormwater drainage by Mr H, 11. Climie: "I have the honour, in accordance with your instructions, to submit the following,report on the best means of preventing the flooding of the low-lying area of your City between Trafalgar and St. Vincent streets.

CATCHMENT AREAS

The flood water is contributed by the following! catchment areas: — (1) An.area of about 1,200 acres extending from St. Vincent street westwards to the Britannia Heights and southwards across the YVaimea road to the hills at the back of Bishopdale. (2) An area of about 400 ftcres extending from St. Vincent street, eastwards to Mount street and southwards across the Waimea road to the east end of Tukuka street.

(3) An area of about 340 acres extending westwards from Trafalgar street to Mount street and southwards to Grampian road at the back of the Bovs' College. (4) An area of about 160 acres extending eastwards from Trafalgar street to Alton street and southwards across Van Dieman street to Grampian road. These areas will be referred to as the St. Vincent, Vanguard, Waimea and Trafalgar catchment areas respectively.

EXISTING DRAINS

Originally the drainage from the two gullies at the head of the St. Vincent and Vanguard catchment areas junetioned near the west end of Motueka street and formed a stream that meandered along the flat between St. Vincent and Vanguard streets until it joined the mudfiat near Gloucester street. Many years ago the catchment areas were separated and ditches cut along St. Vincent and Vanguard street. .The depression between the two however, remains and receives the overflow from the ditches during heavy rain. The St. Vincent street ditch terminates in a culvert about six chains long which discharges into the tidal channel immediately above Halifax street. This culvert* is capable of discharging about .65 inch of rainfall per hour off the catchment area. Above the .railway station the capacity of the ditch is much less.

The Vanguard street ditch at Gloucester street passes into a culvert about ten chains long of varying waterway, then traverses "the Railway Reserve in an open drain, and thence discharges into the tidal channel through a culvert about fifty feet long known as the Saltwater bridge. Under the best, tidal conditions the capacity of the Gloucester Street culvert is limited to l-6th inch and the Saltwater bridge to .60 inch of rainfall per hour off the catchment area. - The drainage off the Waimea catchment area is collected in an egg-shaped drain that runs down Waimea street as far as Halifax street, where it is joined by the drainage off the Trafalgar catchment area, in n similar drain; and both are discharged through the same brick drain emptying into the tidal basin at the mouth of the Maitai river. The Waimea drain is capable of discharging, even " when under pressure, only about .20 inch of rainfall per hour off; its catchment area. The Trafalgar street drain is capable of discharging about .50 inch of ; rainfall per hour. (Only- a small amount of sewage, which is gradually being reduced is now drained into the stormwater drains.)

EXISTING FLOOD CONDITIONS

The sequence of events during heavy vain, is fairly evident from the above study.., Flooding first occurs in -Vanguard street.owing to the small capacity of .the drain across Gloucester street. The drain in. Waimea street gets under pressure and discharges flood water out of -the gullies and traps onto the lowlying area adjacent to Hardy street, the drain then blows out near Hardy street and the discharge rushes down to Vanguard street. The St. Vincent street ditch next overflows above the Railway Station into Vanguard street. A lake forms from St. Vincent street to Hardy street, extending south from Haven Road almost to Parere street. It should be noted that had this basin a' free and deep outlet through Haven and Halifax roads the water could not collect there to any extent. RAINFALL During the last twenty years the daily rainfall recorded in Nelson has exceeded four inches on only three occasions, the greatest being 4.83 inches on the 23rd July, 1916. The flooding of the areas in question, however,' depends not so much on a heavy daily rainfall, as upon rainstorms of gTeat intensity of from thirty to sixty minutes duration.- The actual maximum rainfall intensity experienced in your City cannot be ascertained owing to the absence of autographic rainfall records. Fortunately, during the rainstorm on the 29th April last, observations were made at the Cawthron Institute that enable an aproximate value to be adduced. On that date 1.20 inches,of rain fell between 9 a.m. and 4.30 p.m. and 3.06 inches fell between 4.30 p.m. and 9.30 p.m. Other observers- report, however, that most of the three' inches fell in two hours between 6 and 8 p.m. It can be assumed therefore, that on rare occasions rainfalls of 2£ inches in two hours, Q inches ia one hour, and l!j inches in half an hour will probably be experienced in your City. This assumed variation is in accordance with the well-known character of rainstorms, namely, the shorter the duration the greater the intensity. (If should be recorded that the flood on the 29th April reached its maximum level in Vanguard street on 49.45 above datum at least half an hour after the heavy rain ceased, and five hours before high-tide.) RASIS OF DESIGN The last flood is reported to he the greatest since 1907 when one of about /similar severity is said to have occurred. It ; is considered that immunity from floods following rainstorms of this intensity should and can be given to the low-lying business area between St. Vincent and Trafalgar streets. The following scheme is therefore tit*-

signed to deal with a rainfall of two inches per hour, it being assumed that on the average fifty per rent, of the rainfall immediately runs off as stormwater.

This is equivalent on the average, to a flood discharge of our cusce per acre nf catchment area.

It must, be recognised, however, that .no- scheme ran give f »" protection, against every possibility- A cloud burst might occur in any of the four valleys that converge on your City and cause much desti'utcion notwithstanding the greatest precautions. It is considered on such data as is available that onlv once in every thirty or forty years will a flood occur in excess of the capacity of the scheme proposed below.

ST. VINCENT CATCHMENT

The St. Vincent is the most important catchment area being considerably larger than the other three combined. It has been proposed to increase the capacity of the existing ditch by straightening, widening and removal of willows and obstructions. The gradient, however, south of Gloucester street is so steep that the resultting velocity (10 feet per second) would cause rapid erosion and* make a permanent regime impossible. Further the long outlet culvert at the Globe Hotel has only one third of the. required capacity. * This culvert could be deepened and enlarged and the ditch concreted to give the required capacity. The cost, however, will be little less than for a similar concrete-lined canal capable of taking the discharge from the Vanguard area as well. A combined canal of this kind will now be considered.

MAIN CANAL

The starting point in the design of such a canal will be Auckland Point, as ultimately it must be extended to there as the adjacent reclamation is completed. That section of the canal may ultimately receive the drainage from. 2000 acres, and should therefore liave a capacity of 2,000 cusecs. It should have a bottom width of thirty feet and a concrete 'invert. The sides it is assumed, will be of masonry battered 1 to 1. The invert level at Auckland Point should not be more than 39.00 feet above datum. This will permit the depth at Halifax street to be increased by at least two feet to 39.50. The culvert there will require to be replaced by a bridge of 30 feet span. The St. Vincent street culvert which would be retained to deal with the drainage from Washington Valley would join the canal immediately above the new bridge in an easy curve instead of the present abrupt confluence. At this point also the new canal should change direction for a straight line across the tidal flat to a point on the back section lino near Buxton's store. The invert level at that point should not be more than 40 feet above datum. This section could be reduced to a width of 24 feet in the bottom. It would cross Haven road about three feet deeper than the existing culvert, which would be replaced by a bridge of 25 feet span. Prom the rear of Buxton's store the canal could follow the section, line and thence across Gloucester street on the same alignment to the hollow at the back of Grant Bros' store. This section owing to the decreasing volume of water, and the increasing ~ hydraulic gradient (about 1 in 500) could be reduced to 14 feet in width. At its upper end the canal would bifurcate, one branch tapping the St. Vincent and the other the Vanguard ditches. These branch canals owing to the steep gradients could be reduced to a waterway 6 feet by 5 feet deep. The flat gradient proposed for the invert of the main canal is quite usual for a drain of this size. ' The hydraulic gradient will, however, be about twice as steep, its actual value depending on the state of the tide. It is estimated that the proposed main canal when completed to Auckland Point will be capable of discharging one cusec per acre of catchment area against ordinary high tide, and 80 per cent, of this amount against ordinary spring

tides. That (wo such rare events as a severe Hood and an abnormally high spring tide should occur simultaneously is highly improbable and it would not. lio economically sound to. make provision for full protection against such a remote, contingency. If, however, the. two events did coincide a flooding of lower Vanguard street to on.- third .)!' the. depth ■>■. perieneed List April' would result

li. is recommended thai a, cm d i. outlined above, be constructed as tin means best suited to handle such flood.a> occurred last April.

INTERCEPTING DRAIN

The existing Waimea and Trafalgar street storm-water drains can best bo relieved above Hardy street by means of an intercepting drain discharging into the proposed main canal by the most direct route. • This branch drain could take off from tfie main canal near the hack of Buxi ton's store, and thence in a straight line across Vanguard anil Hardy streets to intercept the Waimea. drain about fifty yards up from Hardy street. Thence the intercepting drain could be run in a, straight line across to the north-west corner of Trafalgar .Square and thence to Selwyn I'lacc.

The size, of this intercepting drain up to Waimea street should be 9 feet by 5 feet deep on a gradient of about' 1 in 500. It would require to be constructed of reinforced concrete and be provided with a cover of the same material. The upper section of the drain between Waimea street and Selwyn Place should be four feet diameter spun, concrete. The existing drain in upper Waimea street is quite inadequate to deal with a heavy rainstorm. It is proposed therefore to supplement 11. with a new drain five feet in diameter of spun concrete laid from Examiner street to the intercepting drain..

TUNNEL A LTERNATIV-E

The route proposed for the intercepting drain is the best from an hydraulic point of view. If difficulties are encountered in locating it on that line, an alternative route is available up Gloucester street, through a short tunnel to Waimea street, thence up Nile street, and across the coiner of the Square to Selwyn Place. Another alternative is to collect water off the Waimea catchment area at the west end of Examiner street and discharge it through a tunnel about twelve chains long under -Mount street, across Vanguard street into the main, canal that would require to be widened by two feet.

An intercepting drain, though considerably smaller would still be refquired to relieve the Trafalgar area. It is estimated that on this account this alternative would show no saving in cost over the scheme first, proposed.

OTHER PROPOSALS

The proposal to collect the stormwater in Vanguard and Waimea streets above the low-lying area, and conduct it past there in a. drain capable of withstanding pressure, is faced with the practical difficulty of capturing a Jlood of storm-water, particularly if the drains feeding it are quite inadequate of carrying the abnormal volume of water. Even if the auxiliary drains were of ample size, the gratings soon become choked under the aonormal conditions and it is impossible to collect all the stormwater in the main drain. Further, during intense rainfalls much of the surface water does not run in the ordinary channels, but rushes to a considerable depth over the whole street and directly down the hillsides into the. valleys.

The scheme recommended in this report will be subject to much the same difficulty, lull the surface water that gets past, the gratings will not collect in the low-lying area- as in the last Hood. It will freely How away in (he wide deep open canal provided for it. Kurt her the scheme will d'jal with stormwater from aI! the contributing catchment ureas.

Sir.T».N<

Considerable trouble harf been experienced and maintenance costs increased owing to silt lodging in the flat section of the Waimea drain below Hardy street. This silting will he greatly reduced by means of the proposed intercepting 'drain, which being at a lower level will trap the silt and prevent it from travelling down the old drain. In the proposed main canal a scouring velocity will be experienced only occasionally, and if in the meantime the silt consolidates it may be necessary to remove it from time to time. As the canal will be a wide open one, the silt can he shovelled out directly without much trouble.

EXTENSIONS

ft should he noted thai the proposed works provide only Ihe foundation, for an extensive system of storm-water drainage. From time to lime as conditions warrant the scheme will require to he extended. For instance the Hooding of Ihe low-lying area between St. Vincent and Vanguard streets a.bove I'arcre street can only he overeoine by extending the main canal up the depression. COMPENSATION

The. proposed drains where, across private properly have been located so that they do not interfere with any buildings of a permanent kind. The betterment of the property due to the proposed works can be set off against any damage done. For instance the const met ion of a concrete lined canal through the Railway Yard will permit the Department to reclaim it at little cost. On, that account no serious objection from the Department is anticipated.

GENERAL DESIGN

The canals and drains should be designed without any abrupt confluences or change in waterway. The bridges at Halifax and Haven roads should be of (lis slab and girder type in reinforced not arched, to give the maximum permissible waterway.

The sides of the open canals should be battered about one in eight and tied across every twenty feet with precast concrete beams in'lieu of heavy reinforcement in the sides.

No gas pipes, water mains, nor sew>rs should be carried through the canals ir drains. On this account several sections of sewer will require to he re-laid and water, gas and drainage connections altered.

ESTIMATES

The estimates provide for the contraction of the works outlined above with Tie exception of the main canal below Halifax street. Allowance is made, however, for the deepening of this channel. The total estimated cost of the works inclusive of sums to cover compensation, alterations to existing pipes and ■ewers and for contingencies is £41,911 us set out in the, attached schedule.

The plans supplied by the City Engineer are returned herewith with the routes of the proposed new canals and drains shown in green ink.

SUMMARY

The scheme of flood prevention that I recommend your City to adopt is briefly as follows : 'Hie construction of wide and deep:et bridges across Halifax street and Haven road. The construction of a wide and deep concrete-lined canal from these bridges to the hollow about a hundred yards below i'arcre street to carry the stormwater from Vanguird and St Vincent This canal to be ultimately extended to Auckland Point. Into this canal the stonmvater from Trafalgar and Waimea streets would be d ! verted above Hardy street by means of a large intercepting drain A supplementary drain would also be constructed up Waimea street, us far as Examiner street.

ESTIMATE OF COST Main Canal £ Deepening Outfall Channel 300 Construction of three bridges ... Construction of 24in canal 6003 Construction, of 14in canal 5461 Construction of branches 2677 Alterations lo sewers ami pipes 750 Contingencies 2036 Compensation, etc 1500 £23.772 Intercepting Drains Construction, of lower section ... 6093 Construction of upper section .. 3204 Construction of Waimea-st. sect'n 3875 Alterations to sewers and pipes 1500 Contingencies, etc 1467 Compensation, etc 2000 £18,139 Total Estimated Cost £41,911 It was resolved to ask Mr Climie to divide the report into sections, and to give an estimate of the cost of each and the benefits likely to- accrue therefrom.