Plates XVI., XVII.
Few things add more to the conveniences and general health of a town or a district than a good water supply. Happily for the people of Napier, and for those dwelling on the plains known as the Ahuriri, the Karamu, and the Heretaunga, they have a supply of good well-filtered water which is practically unlimited.
The discovery in this district of what are known as artesian wells, dates back a good many years. The first well sunk in Napier was the one in Hastings Street, near Mr. Swan's brewery, Mr. Garry, so long and so well-known in connection with Garry's foundry, being the gentleman who successfully carried out this important and beneficial work. It ought also to be recorded to Mr. Garry's credit, that he was the first to discover artesian water
in the Poverty Bay District, he having sunk the famous well at Makaraka, four miles from Gisborne, in the year 1877, which, in addition to providing a fair water supply, throws off sufficient luminous gas to be employed on special occasions for lighting the hotel near by.
It is estimated that there are in Napier and the surrounding district not fewer than three hundred artesian wells. I am inclined to think that the number is under-estimated, as I find that within the Hastings Borough boundaries there are at least ninety wells; and Mr. J. N. Williams, Mr. Coleman, and Mr. Tanner have fifty others between them. But the estimate is quite sufficient for my present purpose. These wells have a varying flow, dependent in a great measure upon the diameter of the tube or bore, as it is technically termed, which is driven into the ground by means of a heavy weight until the water-bed is reached. The flow of water, however, is not proportional to the diameter of the tube bores. Thus a pipe of 2-inch bore has been known to give a flow of water averaging nearly 60 gallons a minute, whilst a 3-inch pipe never exceeds, under the most favourable conditions, a flow of 100 gallons a minute; the proportion being in these cases as 3 to 5, whilst the area of the bores is in the proportion of 4 to 9. The cause of the diminished flow in the larger pipes is to be accounted for, so I imagine, by the fact that the friction produced by the movement of the water in its underground course through the shingle-bed where it is found retards the rate of supply much more in the larger pipes than in the smaller ones.
The Napier Borough water supply is provided for by means of six artesian wells, four of them having a 3-inch bore, and the others a 2-inch bore. The estimated quantity of water which flows daily from these wells is set down at 420,000 gallons, or an average of nearly 50 gallons a minute for each well. Now, if we take 50 gallons as the average flow per minute for each of the 300 wells which I have supposed as flowing throughout the Heretaunga Plain, the daily water supply will amount to 21,600,000 gallons, and the annual supply to the enormous number of 8,884,000,000 gallons, or say 40,000,000 tons, the greater portion of which is allowed to run to waste, or is employed for irrigation purposes.
The exceeding value of the artesian water supply to the well-being of the people in Hawke's Bay has been well exemplified during the past year or two. As we are all aware, the district has suffered more or less from a diminished rain supply for the past three years, the climax being reached during the year ended in March last. For the twelve months which ended at the close of the first quarter of this year, it appears that only 15 inches of rain fell on the Heretaunga Plain about Hastings, whilst in Napier barely 17 inches fell during the same time.
The average rainfall for Hawke's Bay is given in Dr. Hector's “Handbook” as 36 inches, and for Napier 37.260 inches. The latter is the average of twelve of the years between 1866 and 1881, no rainfall being given for the years 1867–69 and 1881. As far as I can gather from the statistics of rainfall already published, this district has suffered from a diminished rainfall at regularly recurring periods. In 1866 there was a deficiency in the rainfall of 7.260 inches; in 1872, or six years later, the deficiency was 13.320 inches; in 1878, or six years later, the deficiency was 16.160 inches; whilst during the year ended March last, or say eight years later, the deficiency was actually 21 inches on the Heretaunga Plain, and 20.260 inches at Napier! It is curious that in each year succeeding what might appropriately be termed the “drought year,” there was an excess of rainfall, so that, during the present year, we may expect, if not floods, certainly many rainy days to make up for the unusual deficiency in the rainfall of the past year.
Now if we consider the Heretaunga Plain and the slopes of the hills which naturally drain their surplus waters into it as containing 100,000 acres, we may readily compare the quantity of water that fell within this area last year with the quantity that should have fallen to supply the ordinary requirements of the district, had the normal rainfall been maintained.
The weight of an inch of rainfall over an acre of ground may be stated roughly as 100 tons. Actually it is 101 ¼ tons: for 36 inches of rainfall, the weight on an acre, amounts to 3,600 tons. This is the quantity of rain that should have fallen last year on every acre of land on the Heretaunga Plain. The quantity that actually fell was 1,500 tons. On the 100,000 acres which I have assumed to constitute the drainage area of the plains, 150,000,000 tons of water fell last year, whilst 360,000,000 tons ought to have fallen had the average rainfall for the year been maintained. Thus the year's deficiency was 210,000,000 tons, or a quantity represented by no fewer than 47,060,000,000 gallons. I estimate that, under ordinary conditions of soil and temperature, an inch of rainfall would moisten the ground for a depth of 6 inches, that is supposing no portion of it was allowed to pass away either by evaporation or by drainage in the way of surplus soakage. Consequently, 15 inches of rainfall would moisten the ground to a depth of 90 inches, or 7 feet 6 inches. At this depth it may be said that the ground is generally moist and damp. It is evident, therefore, that, without considering the question of bedding, the district under notice had no surplus soakage last year to maintain the artesian supply, which I have already pointed out amounts, at the lowest possible estimate, to 8,884,000,000 gallons per annum; and either the supply was the accumulation of previous years of soakage, or it is obtained from some outside source. I
am well aware that water percolating through rocks of greater or of less porosity may take a long time to reach what is termed an impervious bed, from which, by means of fissures, it may again be forced to the surface as natural springs. The best spring-water in England is considered by those who are supposed to be capable of giving an opinion on the matter to percolate through rocks for many years, and the “more aged” the water the better in quality it is supposed to be—after the manner of old wine. We have it on high authority that the water used for the supply of the Town of Aylesbury, in Buckinghamshire, England, is obtained from the chalk rocks of the Chiltern Hills, and that it takes two centuries for the rain to percolate through the chalk into the wells from which the Aylesbury town authorities obtain their supplies. Thus it may happen that the rain which fell a century or so ago upon the Napier Hills, and upon the hills surrounding the Heretaunga Plain, and extending from the Kidnappers, past Havelock and Pakipaki, thence rounding to the north-west to Maraekakaho, then north-east and north towards Taradale, is just reaching the impervious beds underneath the plain, and is now coming to the surface by means of artesian wells to supply the requirements of Napier and the surrounding district.
Let us see what light geology throws upon this interesting subject.
Most of those who have resided in this district for any length of time are aware what great modifications have taken place in the surface of what is commonly known as the Napier Swamp, in consequence of the heavy floods which have occurred from time to time.
Only a few years ago the Township of Meanee was inundated, and the land thereabouts was raised in height varying from 2 to 5 feet. One can hardly realise that, within the memory of many now living here, the tide used to rise 2 feet 6 inches in the swamp midway between Awatoto and Waitangi, or that a respected Napier citizen used to ply a 5-ton boat between Napier and Pakowhai; and yet such are the facts. The flooding of the plain has been going on at intervals for many centuries, and it is chiefly by this means that the waters of the bay came to be divided from the land. The whole plain is an alluvial deposit of comparatively recent formation; indeed, as the Napier Swamp now testifies, it is an unfinished work, which the physical agencies, air, water, and climate, were doing so well when civilized man entered upon the scene to hurry on the work, and to adapt the land for the supply of his needs. And here I would digress for a moment to say how much I hope that our President will succeed in obliterating, by means of his proposed reclamation scheme, the last remnant of the swamp plain, which, though interesting from a geological standpoint,
will be much more interesting when made suitable for the abode of man.
As showing the vast and comparatively rapid changes that have taken place in the facies of the plain within the memory of some of those who are amongst our oldest and most honoured members, I quote the following interesting description by the Rev. W. Colenso, F.R.S., taken from his paper describing his first, and to me, remarkable journey over the Ruahine Mountain Range, so long ago as the year 1845, or forty-two years since. Mr. Colenso says: “I have mentioned the trackless mountain forests of the Ruahine, but, if anything, some of the open swampy plains near the sea in Hawke's Bay were worse. I may particularly notice the present well-known extensive grassy plain lying between Farndon and Pakowhai—then a long peninsula bounded by water on three sides. Words would fail me to show the original state of that land. In those days there was no communication overland between the villages (Maori)… I have often, of late years, asked myself which of the two wonderful alterations or changes—the building of the town of Napier, or the great transformation of those swamps, I considered the most surprising, and I have always given it in favour of the plains.”
The height of the plains, which, for simplicity, I shall call the Heretaunga Plain, varies considerably, although between Awatoto and Pakipaki it appears to be almost a dead level. Through the kindness of Mr. Rochfort, C.E., our townsman, and engineer of the Hastings Borough drainage scheme, I have been put in possession of data showing the various heights of the plain between Maraekakaho and the sea—Mr. Rochfort having taken many hundreds of observations, with the object of providing against the alteration in the bed of the fickle Ngaruroro. At a point on the plain a mile or so to the west of Roy's Hill, near Maraekakaho, and about 15 miles south-west of Napier, where some years ago important embankment works were carried out on the right bank of the Ngaruroro, the height of the plain is 166.40 feet. For our purpose this may be set down as the highest point on the plain. From this place to the sea-beach near Awatoto the distance is about 14 miles. Between Pakipaki and Awatoto the distance is about 13 miles. Pakipaki is 32.39 feet above sea-level. Havelock, at the bridge over the old Ngaruroro river-bed, is 34 feet above the sea. East Hastings, at the junction of the Karamu and Havelock Roads, is 39.90 feet. West Hastings is 44 feet. Omahu, at the bridge, is 73 feet, at the pa 63 feet, and at the mill 48.2 feet. At the junction of the Clive-Havelock and Clive-Hastings Road the height of the plain is 15.36 feet. At the Clive Bridge Hotel the height is 11.54 feet; near the bridge at the Waitangi the height is 7.29 feet. Pakowhai is 18.47 feet above sea-level; Papakura
is 36 feet; Redcliffe, near Mr. Williams's blue gums, is 25.17 feet. Taradale, at the junction of the Meanee and New Taradale Roads, is 16.63 feet; and Meanee, near the hotel, is 5.31 feet; whilst Awatoto is 7 feet. From these data we are able to form an estimate as to the general slope of the plain, starting from the given point to the west of Roy's Hill. Thus between the starting point and Pakipaki, a distance of 8 miles, the slope is 134 feet, or 16.8 feet per mile. Between Roy's Hill and Havelock, a distance of about 10 miles, the slope is 132.4 feet, or 13.24 feet per mile. Between Roy's Hill and Awatoto the distance is about 14 miles, and the slope is about 11.3 feet per mile; whilst between Pakipaki and Awatoto the slope is little more than 2 feet per mile. From these results we gather:—
First. That from Maraekakaho to the sea there is a gradual slope amounting altogether to 166.4 feet.
Second. That the slope is much greater between Roy's Hill and Pakipaki, that is to the east, and between Roy's Hill and Havelock, that is to the east by north, than it is between Roy's Hill and Awatoto, that is to the north of north-west.
Third. That the lowest portion of the plain is between Pakipaki and Awatoto, by way of the old Ngaruroro river-bed, the slope being only about 25 feet; whilst between Hastings and Awatoto the slope is 35 feet, and between Omahu and Awatoto the slope is 66 feet.
Let us now consider the character of the plain which at present forms a portion of the bay known as Hawke's Bay, and situated between Napier and Cape Kidnappers, and which is covered by the sea. If a point is taken, out in the bay, 15 miles east by south of Napier, and about 8 miles to the north of the Kidnappers, it will enclose an area which is almost a counterpart of the Heretaunga Plain. The depth of the sea 15 miles from the Napier beach is 29 fathoms, or 174 feet; at 14 miles from the beach the depth is 28 fathoms; at 12 miles the depth is 24 fathoms; at 10 miles it is 18 fathoms; at 8 miles 15 fathoms; at 6 miles the depth is 12 fathoms; at 4 miles it is 10 fathoms; at 2 miles it is 8 fathoms; and at 1 mile the depth is 4 fathoms. Imagine for a moment the water taken from this portion of the bay, and there will be found an area of land as flat, and with a slope as gradual, as the Heretaunga Plain itself between Roy's Hill and Awatoto. This plain was formed in its greater part at the same time and under similar conditions as the Heretaunga Plain, and, geologically considered, it is a part of that plain. The shingle beach, which now forms such a characteristic feature along the coast between Napier and the Kidnappers, forms no part of the plain under consideration. Its history is limited to the period when the Tukituki broke through
the limestones, marls, shingle, and pumice sands at Te Mata, and subsequent to the separation of Scinde Island from the mainland in the direction of the Quarantine Station. The shingle spit is limited both in breadth and depth, and began to be formed when a large portion of the present plain was above water level. It is not more than from 60 to 70 feet in depth, and its average breadth I estimate at less than 400 yards. If this beach were taken away, just as we have supposed the water in that portion of the bay between Napier and the Kidnappers to be taken away, there would be seen a valley plain, 30 miles long and from 8 to 10 miles broad, so flat, and so gradual in its slope, that the most perfect eye could not distinguish the inclination of the beds. I have pointed out that a mile or so to the west of Roy's Hill the plain is 166.4 feet above sea-level; whilst 15 miles out in the bay, or 30 miles from Roy's Hill, the sea is only 174 feet deep, or a difference of 340.4 feet between the highest and lowest points, with a gradual slope the whole way. This gives a fall of only 11 ⅓ feet to the mile, or little more than a foot in each 480 feet of horizontality. Now these, to me very interesting facts, are of great importance in helping us to arrive at a proper conception with respect to the artesian water supply on the plain. Two or three miles outside the 15-mile limit in the bay the ocean bed is composed of blue clay. This clay is the underlying impervious bed which passes under the plain throughout its full extent. The same bed is met with about 2 ½ miles beyond Maraekakaho, where it is seen coming to the surface in the Maraekakaho Creek. At one time the sea washed over the whole length of this clay bed; but the sea has been slowly receding, in consequence of the débris brought down by rivers from the west, and deposited in what was once an arm of the present bay. All the beds which overlie this impervious clay-bed have been deposited in a constantly diminishing, or rather in a constantly-varying, thickness, as they get further and further from the source of supply, which, at the beginning of the plain formation, was the mouth of a large river then situated at Maraekakaho. It needs no explanation on my part to account for the constant thinning out of beds as they proceed from Maraekakaho outwards. Any observer can see the same thing happening daily, wherever water is removing débris and redepositing it under conditions where the movement is free. As remarked above, the lowest impervious bed is a blue clay of a somewhat irregular surface, having a north-east inclination, and troughed along its north-west and south-east edges. On this clay is to be found an overlying series of irregular beds composed of shingle, sand, impervious blue clays, blue sand, shingle, pumice, lignite, pumice and clays, shingle, pumice, fine mud, and sediment, in ascending order. All these have been brought down by the rivers and creeks which have at different times
discharged themselves into what is now the Heretaunga Plain. These beds are not constant or identical in structure over the whole plain, nor should they be, considering the manner of their deposition; but for all that they are practically and geologically the same. In times of flood a river brings down vast quantities of materials, of various kinds and densities. These are not equally distributed over the area of deposition. Shingle may be in one place, pumice in another, sand or clay in another, and fine sediment in another, the deposits being modified as much by surface irregularities as by difference in the specific gravity of the materials brought down.
Bearing this in mind, let us now proceed to inquire as to the character of the artesian supply-wells in this district. Through the kindness of Mr. Garry, to whom I am much indebted for valuable information upon this point, I find that the character of the beds passed through when sinking in different parts of the district, varies greatly, in the 60 wells which Mr. Garry himself has sunk. Speaking generally, there is comparatively little alteration in the character of the beds between Napier and Pakowhai, but the beds in the direction of Redcliffe and West Clive vary much in character. So also do the wells vary in these places in depth, there being a marked diminution as compared with the depth of the wells in places like Pakowhai and Awatoto. But the change in the character of the beds and in the depth of the wells, whether in the direction of Clive and the Big Bush, or of Redcliffe and Taradale, is only what might have been anticipated at these places. They are at the points of the troughing of the underlying beds, and between which the water flowed when the Heretaunga Plain was an arm of the sea.
Commencing at Pakipaki, which, as already stated, is 32.39 feet above sea-level, artesian water is found at depths varying from 65 feet to 80 feet, or an average depth of say 70 feet, according to the point selected along the plain. This depth would give about 40 feet below sea-level, and 206 feet below the highest point taken near Roy's Hill. The depth corresponds to a point out in the bay, somewhat less than 2 miles to the south-east from the Napier beach. At the Pakipaki Hotel, water barely comes to the surface; and for this reason wells have been put down some distance further along the plain, so as to obtain a necessary flow. Near the railway-station, and at the Maori pa near by, I am informed that the water rises about 5 feet above the surface. At Pukahu, and at the railway crossing, Longland's Road, about midway between Pakipaki and Hastings, water is obtained at depths varying from 80 to 110 feet, or say 55 feet below sea-level, with a flow rising from 4 to 8 feet above the surface. The depth of the wells in these places corresponds to a point out in the ocean above
4 miles from the Napier beach. Hastings may be put down as being 40 feet above sea-level, and artesian water is obtained at depths varying from 130 feet to 160 feet, or say 100 feet below the sea, and 266 feet below the highest point on the plain. This depth corresponds to the depth of the ocean about 9 miles to the south-east from the Napier beach. Pakowhai is 18.47 feet above the sea, and artesian water is reached at depths varying from 140 feet to 170 feet, or say 137 feet below sea-level, and 303 feet below the highest point on the plain. This depth corresponds to the depth of the ocean between 11 and 12 miles out in the bay. Awatoto is 9.32 feet above sea-level, and water is reached at depths varying from 170 to 200 feet below the sea-level, and 340 feet below the highest point on the plain. This depth corresponds to the depth of the ocean about 15 miles out in the bay. At Napier, artesian water is found at depths varying from 180 feet to 240 feet, according to the proximity of the wells to the hills. This would give a mean of 210 feet below the sea-level, or 381 feet below the highest point on the plain. The flow at Napier in the best wells reaches a height of 30 feet or more. As we proceed from Napier or from Awatoto in the direction of Taradale, from Pakowhai in the direction of Redcliffe, from West Clive in the direction of the Grange and the Big Bush, and from Hastings in the direction of Omahu, the depth of the wells diminishes with the greatest regularity. Thus at Meanee the wells are 160 feet deep; at Roseneath, midway between Meanee and Taradale, they are 100 feet deep; whilst about Taradale they vary from 60 feet to 90 feet. At the Farndon Hotel the artesian well is 135 feet deep, at West Clive water is reached at 130 feet; at East Clive 90 feet; and at the Grange, and in the vicinity of the Big Bush, at depths varying from 60 to 85 feet. The variation at all these places exactly corresponds to what might have been expected from the troughing of the beds in these directions, as already pointed out. It will be observed that the increase in the depth of the beds from which the artesian water supply is obtained, as we proceed from Pakipaki to the sea in a north-east direction, corresponds in a striking manner to the slope of the plain between Roy's Hill and Awatoto, the slope of the plain being 166.4 feet, and the increase in the depth of the wells between Pakipaki and Awatoto being about 160 feet. To me this gradual deepening of the wells, as we proceed along the plain from Pakipaki in a north-northeast direction, is sufficient to prove that the whole of the artesian supply is obtained from the same shingle river-bed which passes underneath the plain, and gradually discharges its surplus waters on the floor of the ocean some 16 to 18 miles to the east of the Napier beach. But there is another important fact connected with the artesian supply which goes to support the theory that there is only a single
water-supply bed. At a short distance to the north-east of Pakipaki the artesian flow in the best wells is about 8 feet above the surface, which at this point is about 375 feet above sea-level. This gives an actual rise for the artesian wells at this place of about 43 feet above the sea. Hastings is about 40 feet above the sea, and the flow varies, so I am told, from 1 to 7 feet. Now the flow at the artesian well recently sunk at the Napier gas-works reaches a height above the surface of more than 30 feet, and I should imagine that the well is at least 6 feet above sea-level. This would give a flow of more than 36 feet above the sea. If we take into consideration the fact that the water, flowing as it does through shingle in its underground course, is much retarded by friction, I think that it will readily be conceded that there is sufficient to account for the small variation in the height of the artesian flow at places so widely apart as Pakipaki, Hastings, and Napier. I have no data as to the water supply between Omahu and Pakipaki, and between Pakipaki and Maraekakaho; but if my theory be correct, the question is not a difficult one to solve. It resolves itself into one of height. For example, the flow a short distance from Pakipaki is about 8 feet above the surface in the best wells, or between 40 and 50 feet above sea-level. Water may be expected and no doubt got anywhere on the plains within a few feet of this height. Of course water will be found on the higher parts of the plain, both in the direction of Omahu and Maraekakaho, but it will be by means of ground wells, which will vary in depth as they proceed along the plain in the direction of Roy's Hill. At the latter place the artesian water-bed ought to be reached or met with at a depth of about 120 feet, and near the Maori pa at Omahu water ought to be reached at a depth not exceeding 25 feet.
It is not possible to speak with any degree of certainty as to the actual character of the underground basin which supplies this district with such vast quantities of pure water, but for my own part I have no doubt whatever that it is an underground stream which moves slowly and steadily through the shingle-bed, which is bottled, as it were, within two impervious beds. This stream has flowed since the formation of the first beds on the plain, and the water has been discharged along the bed of the ocean far from the land. A curious fact in connection with the outflow is to be found in the effect of high tides upon the artesian flow near the sea-beach. I am informed that the difference in the height of the flow in the artesian wells in the vicinity of the sea-beach, as at Napier and Farndon, at high tide and low tide, varies as much as 2 feet. This fact goes to confirm my theory as to the outflow of the artesian supply-stream being far away in the ocean. The enormous amount of extra pressure upon the beds in the bay during high tides, and amounting to
not less than 600 millions of tons, must cause a slight compression of the beds, and this would tend to keep back the fresh water from entering the ocean as freely as under a diminished pressure. This would result in a temporary accumulation in the direction of the point of outflow, which, with the increased pressure, would probably cause a higher flow to take place from the wells. A similar effect might be expected upon the artesian wells by the percolation of river water into the overlying beds on the plain, especially during times of flood. I believe that this increase in the artesian flow has been noticed about Hastings at the time of flooded rivers; and the natural inference has been that the artesian supply comes directly from the rivers Ngaruroro, Tukituki, and Tutaekuri by percolation through the shingle, as the rivers wend their way across the plain.
As for the origin of the water-supply, little can be stated with certainty. We know, however, that it cannot be by soakage through overlying beds, the one immediately overlying the shingle in which the water is found being impervious equally with the one underlying it. From this it follows that the rainfall on the Heretaunga Plain proper cannot affect the supply in any way whatever. It seems to me that the large quantity of water that is constantly passing underneath the plain, is to be accounted for by the presence of numerous under-ground springs at the junction of the limestones and the clays which underlie them, and by the percolation of river water through the shingle and sands at the outcrop of the beds. All the hills surrounding the district under notice are made up mostly of limestones, sands, clay, pumice sands, and shingle; and the same may be said of the rocks to the westward, even as far back as the eastern flanks of the Ruahine Mountain Range. All these rocks are exceedingly porous, and water passes through them most readily.
The limestones about Havelock, and the shingle conglomerates, clays, and pumice-sands at the Kidnappers, dip directly underneath the plain. So do the conglomerates and pumice-sands at Redcliffe, and so do the shingle and pumice deposits to the north-west of Maraekakaho; and we may expect to find large and important underground springs at the junction of these beds and the underlying clays. During my recent visit to the volcanic district of this island, one of the most remarkable sights, among the many remarkable ones to be seen, was the source of the Tokaanu River, which rises not a mile and a half from the southern shore of Lake Taupo, into which lake the said river discharges itself. At the base of one of the spurs of the now dormant volcano, Pihunga, a spring bursts from the ground, just after the manner of an artesian well, and the volume of water is so large that a river of no small size is at once formed as if by magic. The whole country around, both mountain and plain, is made up of porous rocks, and rain
passes through them most readily, and must move in underground courses until forced to the surface by the filling and overflowing of some underground basin. Everything is favourable to the existence of similar springs at the junction of the pervious and impervious rocks in this district; and, if we add the quantity of water likely to percolate through the artesian shingle-bed along its line of outcrop, situated some distance to the west of Maraekakaho, we may easily account for the existence of that wonderful supply of pure well-filtered water which now passes underneath the plain, and which has proved of so much real worth from a sanitary point of view, as well to the people of Napier as to those residing in the surrounding district.
I append sections (Plates XVI. and XVII.) showing the inclination of the plain in various directions, also showing the depth of the ocean for a distance of 16 miles out in the bay. The deepening of the artesian bed is also shown between Pakipaki and Napier.
The sections fully illustrate the points referred to in this paper.