FLOOD DANGER IN HAWKE’S BAY

Hawke's Bay Tribune, Volume XIII, Issue 285, 17 November 1923, Page 9

FLOOD DANGER IN HAWKE’S BAY

Folly of the “ Wait and-See ” Policy

Prevention Better than Devastation

It may be remembered that at the time of the disastrous floods iu the {South Island early in May last, the “Tribune” drew attention to the good luck of the dwellers on the low-lying plains of Hawke’s Bay in having escaped the deluge that visited the South Island, and the devastation of the country by the floods that followed in its train. At that time there appeared to be an awakening of the Hawke’s Bay people to the grave danger of a similar visitation causing them widespread ruin, if not loss of life, but since then normal rain falls and climatic conditions have lulled the majority into a false sense of security, and active prosecution of safeguarding measures have been allowed to -fall into abeyance. While time ami again the people have been warned that they are living in a fool’s paradise, yet no really serious work of river control has been put in hand that will prevent the rich district being submerged and devastated by a disastrous flood. If such a tragedy does come, besides bringing ruin and grief to its victims, it will give the district a set-back it will not recover from for at least a decade—the ruinous loss it will occasion will be far greater than the money required to be expended now for river control and flood prevention. Again if disaster does come the people will have themselves to blame only. Mr. George Nelson, who has given so much thought and study to river control and the flood problems of Hawke’s Bay, at great personal expense both in time and money, has perfected a scheme for the control of the shingle-bearing river? whose courses divide the Heretaungu plains and whose beds have silted up to a dangerous level and are now higher than the surrounding country. He submitted the plans of this scheme many months ago to the Rivers’ Board and has delivered addresses on the subject, endeavouring to stir up interest in a matter which is of vital importance to everyone residing or owning property in areas that are subject to flood. And all without avail—at least as far as tangible result is concerned. Is it not time that those who live in the danger zones took active steps to safeguard their properties and the lives of their families! An old man flood such as inundated the countryside in 1896 would be a far greater disaster to-day because the areas are now more densely populated, the river courses are silted up, and the estuaries are less free. Recently Mr. George Nelson, at the invitation of the Canterbury Progress League read a paper in Christchurch on river control. The “Tribune” at the time published from it a few extracts having local application, and as the subject is one of considerable interest to a large section of our readers, wc take the opportunity now afforded to republish it in full detail. The Hawke’s Bay Rivers Board elections take place early in January, ami the study now of the natural* trend of shingle carrying rivers and the scientific method for their control should be useful alike to electors and prospective members of the Board.

CONTROL OF SHINGLEBEARING RIVERS. ADDRESS BY MR. GEORGE NELSON The control of a shingle bearing river involves, as I conceive, the taxing of steps to confine it, under all circumstances, within the limit assigned to iti So our main problem is “To carry the run-off of a watershed from the mountains to the sea without spilling any.’* And collateral with this is the problem of so training our shingle rivers that they may thereby be enabled to create for themselves permanent courses along which their waters may flow, for all time, without injury to man or his works. A shingle river may be regarded as a shallow trough, of irregular outline and having rough sides and bottom. The volume of water which the trough will carry depends upon the area of its crosssection and upon the velocity with which it travels. MOVEMENT OF SHINGLE. All rivers bear a burden of some sort, our shingle rivers a particularly coarse one. Part of this, composed of sand and silt, is carried in suspension; the heavier shingle is dragged, or rolled along the bed. Between these modes there is a stage where the particles levitated by the upward eddies,'proceed by leaps and bounds. Now the bearing of this burden to the sea obviously depends upon tn« maintenance of a sufficient current to carry it along. If the waters in their course flow’ through a deep lake they will there deposit the whole of their burden. Similarly if, upon emerging from the hills a river is allowed to sprawd over the plain its current will no longer suffice to carry the whole of its burden, so the bed load or the heaw ier part of it will be deposited. It is of course in this way that the Canter, bury Plain has been built up until it has a slope seawards varying from say 20 to 50 feet per mile. WHY RIVERS CHANGE THEIR COURSE. In the case of the rivers Rakaia, Rangitata and Waitaki, this process of building up has proceeded to tire point where a sufficient current has been established to enable them to carry heavy shingle throughout their courses; they have established for themselves shingle-carrying gradients. On the other hand the Waimakariri, to which circumstances have given a relatively larger task, has not yet completed its job. Flowing water takes what is, for the time being, the line of least resistance, and this river if left to itself will undoubtedly change its course many times before it has raised the land surface sufficiently to permit it to build up its bed to a shingle carrying gradient throughout. NATURE’S METHODS. Tho data is available so let us consider what the level of its floods will be when, like the other* rivers mentioned, the Waimakarriri has reached maturity’. Wc must of course at the same time assume that man retires from the scene and leaves nature to itself. Haast gives the average fall of the ■Waimakariri over a distance of twentytwo miles extending down to the then limit of tidal influence as 28 feet per mile. The Rakaia where it is crossed by the railway, about 16 miles from the sea, is 372 feet above sea level, giving a fall of about 23 feet per mile. Let us assume that the Waimakariri will eventually aggrade its bed for the last ten miles to a fall of 20 feet per mile; the flood level at a point 10 miles from its mouth will then be 200 feet above sea level, or about 100 feet above present ground level. Haast pointed out what is going on, in his “Geology of Canterbury and Westland,” p. 214, where he says, referring to this river: “The slope of the plains in these upper and middle portions, being greater than the gradient of the present riverbed, the high terraced banks, which hitherto accompanied the riverbed, disappear about 12 miles from the mouth of the river, and the lower delta of the river is reached, 1

•> which I have already shown in my report on the foundation of the Canter- ? bury Plains in 1864, is still being forml ed by the river, which if not checked ; by artificial means will shift its chan- - nel in course of time as the deposition 1 of alluvial beds advances.” , MAN’S INTERVENTION. 1 But man is not going to retire froih • the scene and 1 draw attention to this i unavoidable conclusion merely’ to show that nature cannot be left to itself in : this matter, that man must rise to the • occasion and fill his rightful place in ' the scheme of things. The question now is ‘ ‘ What is to be • done about it?” ’ Since it determines the height of the - flood water at any point in the river’s ! courses, it will be seen how extremely ’ important the slope or gradient of a 1 river is; but it is important only by i reason of its influence on the velocity ■ of the current. i . Velocity is the supremely important I element in the constitution of a river l and, if the river has an credible bed, t its velocity is determined by the char- • acter of its burden. For the bed of ! such a stream is composed of the heav-. • ier constituents of its load, which read 1 ily respond to the velocity of th© current, now moving on, now coming to : rest, as the velocity may decide. THE PRACTICAL WAY. 1 This brings us to the kernel of the matter, and wc now see that the ultimate problem is ‘ ‘ by what practicable steps we can secure the necessary velocity of flow to enable our rivers to carry | their burdens of shingle to the sea. ’ ’ L'eopie is discussing this question are prone to assume that tho current of a : river depends lor its flow wholly' upon the steepness of its gradient. ! Happily for Christchurch velocity is ! determined not only' by the slope but 10 an even greater extent by’ the depth, ’ and in marked degree by the alignment of the course, and iiie degree of roughness of its sides and bed. “The “force’* of a river, by which I mean its power to abrade its bed, is directly proportioned to its volume and fall; the “resistance” to that force is to be measured by the irregularity of the river’s course and the roughness | and extent of its sides and bed. But for these resistances the velocity’ of the stream flow would be infinite and rivers would cut down their beds until their water surface, far inland, was at sea level. Resistance to flow being proportional to the surface of the Hides and bed of tho stream there is clearly much to be gained by a curtailment of the width and a compensating increase in depth. And, while restricting the width, the force of the river could be employed to form for itself a channel of regular cross section and alignment. To show how greatly tne flow is impeded under present conditions, I have prepared these calculations, which 1 now explain to you. TABLE I. Based on a width of 5000 feet and a fall of 25 feet per mile. Mean depth Mean velocity Discharge (feet) (ft. per sec.) (cusecs.)

I believe that the velocities shown h ip Table (1) fairly represent those . which at present obtain. If we take the width of the Waimakariri in high J flood as 5000 feet, its gradient as 25 feet per mile and its discharge as 100,000 cusecs (cub feet per second) R it will be seen that its mean depth is 3 no more than 3| feet and its mean 8 velocity only about 6 feet per second. 1 It is true that in places its velocity 3 may be, and doubtless is, much greater, but I am of opinion that the mean & velocity of a number of complete cross--1 sections would be, under the conditions e stated, approximately as shown. Kut--1 ter’s formula was employed in making - the calculations, the value of “n” a being taken as .04. TABLE 11. Based on a fall of 40 feet per mile. » Mean depth Mean velocity Discharge

1 Reference to Table 11. will show that if the fall be increased to 40 feet per 3 mile, the width remaining the same, a the velocity is increased to only 6-| feet 3 per second. (The same value of “n” t has been employed in calculating this ? table.) 1 TABLE 111. ; Fall Mean . per mile. Depth, velocity. Discharge, (feet) (feet) (ft. per sec.) (cusecs)

1 ing the velocities and rates of dis- . charge of a regularly formed channel, J in shingle, restricted to a bed width ■ of 300 feet, at various depths and i gradients, we see that on a fall of 25 r feet per mile the required discharge is 1 obtained when the depth reached 15 » feet, the corresponding velocity being r 21 feet per second. As the ability of a river to tear up its bed is as the square j of its velocity it is evident that such 3 a, fall is far in excess of the requiro- ' ments of a properly trained river. 4 If we take a fall of 10 feet per mile j, the required discharge is obtained on , a depth of 20 feet and a mean velocity l5l feet per second—which is still tin excess of requirements. • \\ ith a fall of 6 feet per mile and a 1 depth of 25 feet we get a velocity of 13J feet per second and a discharge of J 107,000 cusecs. On the same fall we get with a depth, of 20 feet, a velocity t of 12 feet per second, and a discharge - of 75,000 cusecs. (The maximum dis- - charge assumed by the commission ? which reportel two years ago was 1 80,000 cusecs). (Continued on Seventh Column.)

trees; 2 is. per sec-; 3.8 tcusecs.; 38,000 3 5,2 78,000 4 6.5 130,000 5 7.7 192,000 6 8.7 261,000

(feet) (ft. per sec-) (cusecs.) 2 4.8 48,000 3 6.6 99,000 4 8.1 162,000 242,000 5 9.7 6 11.0 + r» TT a 111 £! II w 330,000

25 5 ' 10.6 16,000 10 16.5 50,000 15 21.0 96,000 20 24.8 153,000 20 5 9.5 14,000 10 14.8 45,000 15 18.9 87,000 20 22.3 138,000 15 □ 8.2 12,000 10 12.8 39,000 15 16.3 75,000 20 19.2 119,000 25 21.7 169,000 10 o 6.7 10,000 10 10.4 31,000 15 13.2 60,000 20 15.6 96,000 25 17.7 138,000 8 5 6.0 9,000 10 9.3 28,000 15 11.8 54,000 20 14.0 86,000 15.9 124,000 30 17.4 164,000 6 □ 5.2 7,800 10 8.1 24,000 15 10.2 47,0000 20 12.1 75,000 25 13.7 107,000 30 15.0 141,000 4 o 4.2 6,300 10 6.6 20.000 15 8.4 38,000 20 9.9 61,000 25 11.2 87,000 30 12.3 116,000 If we turn now to Table III. show-