« AnteriorContinuar »
THE WATER SUPPLY OF LONDON.
Ta time when an inquiry, of which it is difficult to forecast either the duration or the outcome, is being made into the question of the London Water Supply, it may be of use to indicate those physical and economical limits which control the case.
And it is the more desirable that this should be done, from the fact that the method of investigation now adopted, though one which is a favourite in practice, is subject to serious disadvantages. The conduct of an inquiry before a Committee depends partly on the energy of character and habitual views of the more influential members of the Committee; partly on the selection of witnesses, partly on the skill and tact of the contending counsel. Truth, no doubt, may be threshed out by conflict of opinion, but there is no certitude that the whole truth will so come out. As a rule, only those statements will be cleared up which it is the interest or object of some one litigant to state or to oppose. Points of controlling interest may thus be overlooked, and so much attention concentrated on certain details, as to leave little or no time for the elucidation of others of equal or even greater importance. In the absence of a clear, full, professional report, laying down the lines on which the Committee should proceed to work, exhaustive regularity can hardly be anticipated.
Nor has any writer or speaker, in or out of Parliament, so far as we are aware, yet called attention to the fact that there is a limit to the sources of our water supply which it is not impossible to reach. That those sources are ample, everyone takes for granted. But there has been no inquiry as to how ample they are, and how far they will suffice for our descendants, without definite forethought being directed to the prevention of waste.
The magnitude of the question of the Water Supply of the Metropolis has perhaps not hitherto been adequately realised. It may be doubted whether it has been so realised by men of science. It is plain that it has not been so realised by the public. The problem is one which-not in its kind, indeed, but in its degree-is without example. Not only have we the question before us of the provision of the largest capital of Europe with a primary necessary of life, but we have the further complications of limit of supply, and of a future unlimited increase of demand. This year we speak of the needs, in round numbers, of four millions of souls. But the population doubles in about forty years; so that by 1920 or 1930 A.D. we may have to provide for the needs of eight millions of souls. And what is imperative, is that no step now taken for the advantage of the smaller number, and for the supplying the needs of the day, shall prejudice the future of London.
As to the sources of supply, indeed, they seem at first sight to
be ample, if not practically unlimited. There is the River Thames, with its affluents. There are springs in the chalk, in the greensand, perhaps in other formations. All these sources may seem to amount to an enormous total. There is, however, a ready method of arriving at the maximum aggregate of the water available to London from its natural sources of supply. The outside maximum is the quantity of rain that falls within the watershed of the Thames basin. And this quantity must be taken-not for a wet, or even for an average year-but for a dry year. As to this we have something approaching definite information.
The area drained by the Thames is estimated by the engineers of the Ordnance Survey of England at 5,163 superficial miles. If we add the catchment area of the Medway, which is an affluent of the Thames (although it enters the river near its mouth), the total area is 6,160 square miles. Over this surface, according to the best observations yet made, those of Mr. G. J. Symons and his active and unpaid coadjutors, the rainfall varies from 32 inches per annum on the hills, to 24 inches in the lower parts of the valley, in an average year. The general average has been calculated at from 25 to 26 inches, which is a little over the mean rainfall at Greenwich. observed, we are not safe in dealing with averages in a matter of this importance. The rainfall of 1872 was 35 per cent. above the average in Middlesex, and 36 per cent. above the average over the whole of the British islands. We should not be justified in making a smaller allowance below the average for a year of drought. This will reduce the minimum rainfall to an average of 16 inches in a dry year over the Thames watershed.
But, as before
This fall yields, it is true, an immense quantity of water. If we state it in milliards, or thousands of millions of metric tons, the figures become so large as almost to cease to affect the imagination. In most of the dryest years yet on record since the measurements of the rainfall were commenced, the volume of water shed on the valley of the Thames exceeded 7 milliards of metric tons. For such a year of drought as we have suggested as possible, we should still have upwards of six milliards of metric tons, from which to supply the wants of the population. If we allow 50 tons in the year to each unit of the population, which is equal to rather more than 30 gallons apiece per day, we only require 250 millions of metric tons per annum for our present wants, and double that volume for the wants of forty or fifty years hence. The proportion borne to the rainfall is thus apparently so small, that it is not surprising that little heed has been hitherto given to any fear of a stint in the supply. That proportion rises from one twenty-fourth, at present, to one-twelfth part forty or fifty years hence.
But if we take the practical step of endeavouring to trace the course of this ample supply of water, as it rushes from the hills of Middlesex, Surrey, Oxfordshire, Berkshire, and Wiltshire to the sea, we shall become aware of some awkward facts. Of the total rainfall
of a country by far the greater portion never makes its way into the rivers at all. It is removed from the surface of the ground by evaporation. How large the proportion is, is not accurately known. It must be remembered, however, that the greater the drought and the hotter the year, the more active is the process of evaporation. In a communication made to the Institute of Civil Engineers by Mr. C. Greaves in 1876, certain measurements of rainfall, percolation, and evaporation, taken in the valley of the Lea, are recorded. In these the rainfall was 25.8 inches, and the evaporation 189; leaving only 6.9 inches of water to percolate into the soil. In another case brought forward at the same time, while the rainfall over a water surface was 25.7 inches, the evaporation from the same surface was 206 inches. The conclusion arrived at by Mr. Greaves is that the range of evaporation is from 12 to 25 inches per annum; and that 75 per cent. of the total rainfall is removed from the surface of the earth by this silent but unresting agency.
It is obvious that the ample margin between the supply of water and the quantity required for human consumption in this densely peopled district at once disappears when we come to reckon with this great agency of evaporation. Out of the minimum six milliards of tons of a year of drought it is possible that four and a half milliards of tons may be evaporated. Mr. Greaves is not alone in his estimate of the activity of this phenomenon. Mr. Ansted, in his book on Water and Water Supply' (page 94), estimates the evapo ration range in England to be between ten and eighteen inches, 'and even more.' He also tells us that in 1864 the rainfall at Greenwich was only 16.8 inches, which is very close on that previously indicated as a not impossible minimum. Then we have to remember that there are other sources of loss besides evaporation. The rainfall is not evenly spread over the year. In storms much of the excessive fall runs off in floods and freshets. Then organic nature, animal and vegetable, claims its share. In order, therefore, to form any reliable estimates of the quantity of water which may, under unfavourable circumstances, be safely counted upon for the needs of the metropolitan population, we must endeavour to ascertain how much actually escapes to the sea by the channel of the Thames...
As to this, it is better again to refer to the pages of Mr. Ansted than to perplex the reader with details. Evaporation, in the valley of the Thames, as usually estimated, is, he tells us, equal to about eighteen inches of rainfall; in dry seasons going up to twenty, while in wet and cloudy seasons, when there is little wind, it may not amount to twelve inches. It is probable that not more than three and three-quarter inches of rainfall on an average are conveyed by the river to the sea. If we take this rainfall over 6,000 superficial miles, we have an outflow of 1,440,000,000 tons, or close upon our former estimate of one and a half milliards of metric tons of water.
The summer flow of all rivers, however, is something very dif
ferent from the average flow all the year round. In certain cases, indeed, as in that of the Nile, the floods occur in summer. So it is with rivers which, like the Po, are fed by the melting of mountain In the Nile floods nearly thirty-two times the quantity of water comes down per second, as compared with the flow during the lowest state of the river. In the floods of the Po thirty-five times the least flow of the river takes place. In the Brenta the difference is seventyfold. In the Thames the ordinary maximum flow is ten times the minimum, and in an extraordinary flood fourteen times. Mr. Ansted's figures will not allow a daily summer flow of more than 515,000,000 gallons for the united waters of the Thames and Medway, of which 440,000,000 is the volume of the Thames alone at the confluence.
This measurement of the total summer daily outflow of the Thames below London is not for a year of drought, but for an average year. It is therefore liable to a possible diminution of 36 per cent., which would bring it down to 282,000,000 gallons. This figure, considering the daily consumption of London is already upwards of 120,000,000 gallons, is perilously close on the requirements of consumption.
Let it be observed that the above are not sensation figures. They are not estimates which have been prepared with a set purpose. They are the outcome of the best attention that has been given to the hydrography of the valley of the Thames from the time of Telford to the present day. Therefore, without insisting on the absolute accuracy of the figures, it may be confidently stated that they are the most accurate that exist. And there is this further proof of their accuracy that, by different methods, they lead to much the same result. The rainfall is very fairly shown. The two less known items of evaporation and river outfall so nearly balance one another that it is difficult to see where room is left for any very serious correction.
But what does this imply? It should be remembered that, with the exception of any possible subterranean outflow and escape into the sea below high-water mark, the mean outfall represents all that portion of the rainfall on which we can draw for the supply of human need, and of any other artificial applications of water. Out of the 25 per cent. of rainfall that is not evaporated has to be taken all that is fixed by vegetation or by annual growth, as well as all that is required for the consumption of man and beast. The outflow comprises all that volume of water which has passed through the animal system, and made its way into drains and sewers, as well as the much larger volume required for the mechanical purification of such conduits. The trout streams of Bucks, the chalk springs of Kent, the wells of Herts, the reservoirs of Middlesex, all contribute to this outflow. The aggregate contributions which may be obtained from these or other sources for the water supply of the Thames valley are all comprised in the quantity we have given. They are
all included in the average daily flow, which may run as low as 284,000,000 gallons, measured at the mouth of the Medway on a hot day in a dry year. The only sources to be looked to in augmentation of the supply are two, namely, discovery of subterranean flow, which is highly problematical, and such modes of storing the rain water as shall check evaporation from that which may be collected for human use.
It is probable that this view of the case will be taken as affording a basis for some of those gigantic projects for bringing the head waters of the Severn, the Wye, the Dove, or the Eden to the valley of the Thames which have already found advocates. We do not regard the facts in that light. It is true that the density of the population of the Thames valley is raised to an unusual figure by the position of the metropolis. We have, in round numbers, a population of five millions on an area of 6,000 square miles. But in the basin of the Dee and the Mersey, which is one of those from which it has been proposed to abstract the head waters, there is a population of three millions, over an area of 3,000 square miles. Nowhere so dense as in the metropolis for an equal area, the density of the population over the whole watershed is thus even greater. Even the district of the Ribble, the Lune, and the Eden, which is another that it has been proposed to tap, contains a million and a half of souls on an area of 2,500 square miles. It is thus evident that in any proposal to rob Peter to pay Paul, in the matter of water supply, the wants of Peter, actual and prospective, must be duly taken into account.
There are very strong objections to any plan for bringing the bulk of the water supply of London from a very distant source. We have indicated one the local interests affected. Second, may be ranked the cost. Mr. Hassard's estimate for a supply to be drawn from the gathering grounds of the Welsh districts amounted to 20,500,000l., or to an annual expenditure, in interest alone, of 52,500l. for every million gallons of daily supply. In 1878 the total revenue of the water companies, which covered not only interest but working costs, was under 100,000l. for every million gallons of daily supply. More than half of this sum represents actual annual expenditure. Thus the first result, even supposing a rough estimate to be borne out in practice, which would be an unusual bit of good fortune, would be to double the capital cost of the entire water supply of London, and to raise the price of water to the consumer by at least 50 per cent.
Even this objection, however, is less serious than is that which arises from an economic point of view, in the broad sense of public economy. For the supply of the whole, or of any large part, of an enormous and densely peopled district to be made dependent on the maintenance of an aqueduct of 270 miles long, would be to incur the most serious risk. In any case of war or commotion, the application of a few pounds of dynamite at any convenient spot over this great length of line, would possibly produce a water famine in London. It may be added, that the loss by evaporation in summer