On drainage

For the control of mosquitoes and malaria, land drainage in some form or other is so often required that the medical officer should learn at least the principles of this art, just as he learns the entomology of the disease. To most medical men the subject is a closed book; few engineers have studied it in relation to malaria prevention. In the tropics the engineer has responsibilities in connection with roads, water supplies, electric lighting, bridge building, and other branches of a great and highly specialised profession; and it would be unreasonable to expect of him familiarity with the entomological aspects of drainage until these had been demonstrated to him. Yet the cooperation of the engineer and the medical officer is essential for a successful result; and cooperation cannot take place unless each understands something of the other’s point of view. The engineer should learn something of entomology; the medical officer something about drainage. So a chapter on drainage as an anti-mosquito measure may be helpful; and, as an American engineer tells us, “there is no mystery connected with the theory and practice of land drainage, as some would have us believe; neither is there an instinct born in men which will relieve them from acquiring knowledge of this work in the old-time way.”

The soil [32] consists of particles of the rocks forming the earth’s crust; sometimes it overlies the rocks from which it came; sometimes it is composed of particles which have been carried by water far from their original source, and in ages long gone by. The particles vary in size, weight, and arrangement; between them are air or “pore” spaces; and the weight of the soil depends on all these factors. Sand particles being large and heavy are easily consolidated and contain a smaller amount of “pore” space than clay. Clay particles are relatively small and light, and are not readily consolidated, and have a higher “pore” space than sand. Although a clay soil is commonly called “heavy,” it is not really heavier than a sandy soil; the expression “heavy” having reference to the difficulty of drawing a plough through it. The “pore” space in sand comes down to 25 or 30% of the whole; and in a “stiff clay” rises to over 50%.

In addition to the particles of earth, soil contains water in varying quantity. Some of the water is so closely associated with the particles that it can only be driven off by heating much above the air temperature, and it is reabsorbed from the air, when the temperature is allowed to fall to normal. This is the hygroscopic moisture; it is not available for plant life.

In addition, each particle of soil is usually surrounded by a thin film of water which maintains itself by “surface tension”; that curious condition which allows us to blow soap bubbles, and causes water to climb up the sides of a glass, so that it stands above the general surface level of the fluid. When particles of soil, surrounded by this capillary water, are in touch with each other, the water exercises a powerful binding force. To its presence is due the fact that damp sand can be moulded into shapes which fall to pieces when the sand becomes dry.

The capillary water occupies very little of the “pore” space, for the thickness of the water film is hardly measurable; nor can this water move through the soil. When more water is added to the soil, the film over each particle becomes thicker; this surplus water can move upwards to the surface when there is evaporation, downwards to the subsoil water or “water table” and to drains, if the land be drained.

When still more water is added, the surplus passes downwards until, at a varying depth in the soil, all the “pore” spaces are filled. This is the level of the “water table,” a level that varies according to the amount of water in the soil, which again depends on drainage and rainfall.

Figure 24.1: Diagram showing the direction of pressure and lines of movement of water in drained land

When the “pore” spaces are filled with water, the thin film round each particle of soil no longer exists, the particles are no longer bound to each other by the “surface tension” of the film, and they may easily move on each other. Hence wet sand is almost fluid; and swampy land is soft. It is the control of this surplus water, and the level of the “water table,” which are of interest and importance to the agriculturist and to the sanitarian.

If the question be put to someone for the first time, “How would you drain an acre of land?” the answer will almost certainly be, “Dig drains through it.” The right answer is, “Dig drains round its sides”; for, having done so, it will be found as a rule that the land is dry. The level of the ground water will not be at an even depth throughout the whole acre; it will be lowest next the drain, and highest in the centre of the block. Only if the level of the water in the centre be still too high for the object in view, whether it be agriculture or a mosquito control, is a drain cut through the centre.

When land is drained, the water begins to move towards the drain, as is shown by the arrows in Figure 24.1. The farther the water has to flow in order to reach the drain, the greater is the friction, and the slower it is in getting away, and the longer the water table remains high. If it be necessary to lower the water-level still more, the drains may be deepened; but in a stiff soil this may lead to the land next the drain being over-dry, and the centre of the block still very wet. In that case another drain may be cut through the centre.

Figure 24.2: Diagram of influence of distance between drains on depth of drainage, after King [34]

The principle is illustrated in Figure 24.2. It will be understood that when there is a drain at A and C only, the undrained soil must be highest at B, the point farthest from the drains; but if an additional drain be placed at D, then the highest levels of the water will be found at E and F. The water, as a whole, will then be farther below the surface, and the land better drained.

The results of draining land are numerous, and need be referred to here only very briefly. Drainage removes an excess of water, which is replaced by air; not permanently, however, for each fall of rain refills the “pore” spaces with water that in its turn again gives place to air. This is the aeration of the soil, so important to the agriculturist. Without it, the organisms which convert the oxygen and nitrogen of the “pore” spaces to the use of plants, cease to work; the plant dies, and is replaced by the swamp grasses and plants with special adaptations to make soil air unnecessary. To the anti-mosquito worker, the drying of the soil is no less important; it allows rain to be absorbed, instead of standing in pools, or making the land a swamp.

Drainage sweetens the soil, as the farmer says. In some countries, it removes the excess of alkali, which makes the land entirely unfit for agriculture; in peaty lands, it removes the excess of peaty acids, no less injurious to most plants of value to man. These changes come from the movement of the water; stagnant water allowing excess of acid or alkali to accumulate. Drainage hardens the soil. It allows men, and, it may be, cattle and vehicles to move over the surface without inconvenience or without making deep impressions in the ground, which expose the ground water and form mosquito breeding places. As we have seen, when a soil is saturated with water, there is little cohesion between the particles, and so they are easily moved; when land is drained, the soil particles are bound together by the thin films of capillary water that surround them and are not easily moved.

Many other changes occur in drained soil, such as an increase in the temperature, which are important to the agriculturist, but need not be considered by the sanitarian at the moment.

At first, drainage was probably carried out by shallow surface drains; later, these would be converted into trenches. But as land increased in value, and the number of trenches had to be multiplied, subsoil drainage was invented.

Poles and faggots were laid in the drains, which were then covered over, so that the surface of the land was uninterrupted. Needless to say, the drainage by these was not very permanent, except under special conditions.

Stone Drains were an advance on poles and faggots. Sometimes the drain was simply half filled with small stones; sometimes a flat stone was placed horizontally over the small stones, or larger flat stones were placed on edge in the bottom of the drain, from which was evolved the system of putting down three flat stones to form a triangle with a channel between them. But stone drains were expensive; and in time they were replaced by the tile or pipe drain; for a cartload of pipes went as far as a hundred cartloads of stones.

Figure 24.3: Some stages in the evolution of the subsoil drain pipe

In an Chapter 2, I pointed out how money had been spent needlessly in the town of Klang in “filling in” swamps at the foot of hills, and how these swamps could be dried by intercepting the water coming from the hills by a hill-foot drain. In ravine drainage the same principle is applied. In narrow ravines a pipe along each hill foot may be sufficient, but in the wider ones and longer ones a central line of pipes is also required. The method of laying them so that they are not disturbed by storm water has already been described in Section 13.3.

Roots of rubber trees grow into subsoil pipes, just as the roots of many water-loving trees grow into pipes in England. To guard against this, the ravine should either be cleared of rubber trees, or the pipes opened at once, if there be evidence of blockage, and the roots removed.

It sometimes happens that a long ravine begins beyond the sanitary circle and runs through it; and that only from the point it enters the circle is subsoil drainage required. So far no really satisfactory way of draining such a ravine by pipes has been discovered. Attempts have been made to pass the water from the upper and open part of the ravine into the subsoil pipes of the lower part through rough stone filters; but silt has blocked them so frequently that they have become usually but not always useless. It has generally been found necessary to carry the stream down in a cement channel, or to oil the whole ravine.

When the sanitarian begins mosquito control in a town, he will find many difficulties in putting the principles of correct drainage into force. Almost at every step he will be tempted to make some compromise; the engineer may not quite grasp what is required; and before he is aware, the sanitarian may find himself saddled with a system of drainage which he can only regard as something considerably below the standard he intended to attain. At least that has been my experience. So when asked in 1911 to advise the Municipality of Singapore on the control of malaria, I suggested that the officers who would carry out the work in Singapore should come to Klang for a demonstration. The points discussed were subsequently embodied in my report, and are republished now in the hope that they may be helpful.

Memorandum on malaria in the district of Telok Blanga, Singapore.

By Malcolm Watson.

A Preliminary Demonstration at Klang.

1. When invited by your Anti-malaria Committee to proceed to Singapore and advise on the anti-malaria measures to be adopted, I at once suggested the advisability of a preliminary visit to Klang of your own officers, who would eventually be responsible not only for the local investigation of the disease, but also for the necessary engineering works.
2. My object in proposing this visit was that those upon whom would be thrown the duty of recommending and of carrying out remedial measures, should have an opportunity of seeing for themselves the anti-malaria works which have been adopted at Klang and at Port Swettenham, and of becoming acquainted with districts where the malaria conditions have been the subject of study for some years.
3. Malaria in the F.M.S. has been found to disappear from areas of low-lying land where the ground water lies very high, and where there exist what would appear to be dangerous mosquito breeding places; while on the other hand many hill areas are intensely malarious, although the only water to be seen consists of springs and of clear running streams.
4. It occurred to me that a visit to such areas would be of infinitely greater value than many pages of descriptive writing. I could thus, moreover, take this opportunity of demonstrating the details and essentials of land drainage, and of pointing to some of the errors (which were perhaps unavoidable) in the pioneer work done here, the repetition of which in Singapore I was anxious to prevent.
5. The officers selected for this visit were Drs. Middleton and Finlayson and Mr. Ball. They spent three days in examining these areas:
   1. Hill-foot drains. In the town of Klang, the system of hill-foot drains was seen, and special attention was called to the importance of draining at as low a level as possible; the higher up the hill the drain is cut, the greater the excavation, and consequently the greater the expense, while the lower the drain and the nearer it is to the edge of the swamp, the more effective it is, both in cutting off from the swamp all water coming from the hill, and also in withdrawing water from the swamp.

      Where a spring appears on the side of a hill some distance above the hill-foot drain, the most economical way of dealing with it is by cutting a lateral drain from the hill-foot drain up to the spring.

   2. Dangerous swamps drained by earth drains. In Klang the dangerous hill-foot swamps were, and still are, drained only by open earth drains.47 These have been found sufficient, and are, of course, much less expensive than brick or masonry drains.

      Originally the whole anti-malaria drainage system at Klang—except the main outlet, which took sewage—consisted of earth drains, and it was by means of earth drains that Klang town was freed from malaria. Gradually the system of earth drains is being replaced by brick drains, and as these have too often been constructed with more regard to some ideal “grade” than to the requirements of the ground through which the drain passes, I have no hesitation in saying that the actual land or anti-malarial drainage of Klang is less effective now with its brick drains than it was eight years ago with its earth drains. I would even go further and say that, if a close watch be not kept on the level at which brick drains are laid down in Klang, the hill-foot swamps will reappear (as two now threaten to do), and malaria will recur in the town.

      It is important, therefore, constantly to bear in mind that a brick drain is unnecessary in the eradication of malaria on flat land in this country or for draining swamps at the foot of small hills. Not only does a brick drain add greatly to the original cost of the work, but, unless graded with care and due regard to the surrounding land, it may actually hinder effective drainage.

   3. The Grading of Brick Drains:
      1. The explanation of the current practice in putting brick drains down with a steeper gradient than the original earth drain is to be found in the necessity of laying a sewer with a stiffer gradient than a drain carrying only clean water from land drainage.
      2. Brick drains are not as a rule laid down in this country except in towns; and then only in places where a considerable population has so polluted an earth drain that it has become a stinking stagnant mass, more akin to a cesspit than a channel carrying off sewage. On the advantages of substituting for such a stagnant channel a self-cleaning brick drain it is unnecessary for me to dwell.
      3. Experience has shown our engineers that a steep gradient is necessary if such a brick drain is to be satisfactory and self-cleaning, and it has consequently become a rule that such drains 9-inch to 12-inch diameter shall be laid down with a gradient of about 1 in 100, unless they be of abnormal size. There is no question that this is sound practice for brick drains when they carry sewage.
      4. It must not be forgotten that engineers have an additional reason for insisting on a steep grade for sewers in this country. Although we have heavy storms to scour out our sewers, the average composition of sewage in a brick drain in this country is much more concentrated than in England, because the absence of water-closets, of baths, and of house-to-house water supply reduces the quantity of water employed in a tenement, and the domestic waste water or sewage is thus not diluted to the extent to which it usually is in England.
      5. The greater the concentration of sewage, the greater difficulty it has in finding its way along a channel; consequently the gradient of the channel must be the steeper in order to be the more self-cleansing; and all channels carrying sewage should be self-cleansing, whether classed as sewers or brick drains.
      6. There are many swampy places in this country where a gradient of 1%, carried up from the outfall, would bring the top end of the drain at such a level that it would be ineffective as a channel for draining the land through which it passes, the invert of the drain coming above the level of the ground.

         As an anti-malarial measure, it would be a pure waste of money to lay down a brick drain at such a level and with such a gradient, although it might be quite proper to do so, merely from the point of view of a sewage system.

      7. It would appear, therefore, almost as if sewage and land drainage were incompatible. Undoubtedly this is so in many places. It should be the rule wherever possible to combine the two, for it is an advantage to carry off the land drainage water quickly by means of a brick channel, and a further advantage to be able to dilute the sewage by the land water, thereby expediting the flow of the sewage.
      8. It sometimes happens that in constructing a brick drain in place of an earth drain, either for the purpose of carrying off storm water, or as part of a sewage system, the level of the invert of the brick drain is the same as that of the original earth drain, and the swampy condition of the surrounding land is overlooked. If the brick drain has been put down at such a level that the swamp cannot be drained into it, and the brick drain is the only outlet for the drainage of the swamp, as is sometimes the case, then the swamp can only be drained by pulling up the brick drain and relaying it at a lower level.
      9. I would therefore advise that no anti-malaria drain be constructed other than as an earth drain in the first instance. If such an earth drain be found to be inefficient through being insufficiently deep, then it is an easy matter to deepen it. If, on the other hand, a brick drain be put down and the surrounding land be still found swampy, the deepening of the brick drain involves the heavy expense of reconstructing it at a lower level, and at even greater expense than that of the original drain.
      10. Where the sewage must be at a higher level than the land drain, it may be possible, when we have had more experience of underground drainage, to combine the two systems with advantage. A system of underground pipes might very well drain the land at the sides of a brick sewer; the sewer, being laid with a steeper grade than the subsoil drain, would soon reach a level that would enable the subsoil drain to discharge into it. I have mentioned this only as indicating the line in which an advance can be made, and a difficulty overcome. While for anti-malarial drainage a brick drain is not essential, it is nevertheless an advantage to have the land water carried off quickly. And equally is it an advantage in this country to dilute the sewage with land water so that it flows more freely.
      11. There is a further advantage in replacing earth drains by some self-cleansing system, either closed pipes or brick drains, namely, that diseases other than malaria are carried by mosquitoes. Not to mention dengue, the mosquito carriage of which may be open to question, the subject of filaria is one of the gravest moment, and it is one which is certain to engage more attention in the future. I will deal with this below.
      12. The visit to Port Swettenham showed how land could be freed from malaria by drainage even when it was below the level of high tide. The position and level of outlets protected by tide valves was discussed. There appears no reason why such outlets should not be one or even two feet below the level of half-tide.48 The lower the outlet, within limits, the greater the reservoir of dry earth which can absorb rainfall until the outlet is opened by the falling tide.
   4. Filling up a Swamp:
      1. In Klang attempts were made in more than one place to get rid of the water of a hill-foot swamp by filling it in with earth. They were failures. Filling might be a success if the earth were of a very pervious nature, so that the water from the springs which were the cause of the swamp could freely traverse the “filling,” and travelling under it make its way to the nearest drain.
      2. But “filling” in this country usually consists of a very impervious reddish clayey earth taken from a hill. To place such an impervious material on the top of the impervious bed of a swamp is merely to raise the level of the impervious layer, and to enable the water descending from the hill to appear at the resulting surface. The springs accordingly appear on the top of the filling, and spreading out on it continue to form important breeding places for dangerous Anopheles.
      3. Not only is such filling ineffective, but it is very expensive, costing at least ten times as much as the earth drains which would effectively drain the swamp, and often much more.
      4. There is another reason for draining a swamp in preference to “filling” it in. Owing to the expense of “filling,” it is unlikely that more will be applied than is just sufficient to obliterate all traces of water from the surface. The ground water will therefore still be within a few inches of the surface.

         On the other hand, the cutting of an extra depth of drain will lower the subsoil water at a very small cost, as compared with filling.

      5. There is an advantage in lowering the level of the subsoil water, namely, that the dry soil above the water forms a reservoir for rainfall. Two feet of soil may very well be able to absorb one-third of their volume of water; in other words, will be able to take up eight inches of rain.

         Such drained land will therefore be much less liable to become swampy in very wet weather than the land which has been so “filled in” that the ground water even in dry weather is still only a few inches from the surface.

      6. It is an old observation, made long before anything was known of the part played by the mosquito in the propagation of the malaria parasite, that lowering the level of the subsoil water and the dry cultivation of the soil which then first became practicable, led to a disappearance of malaria. And it is well known that even in non-tropical and non-malarious countries, houses built on land with a low ground water are healthier than those on land where the water stands nearer to the surface of the ground.
      7. If houses are to be built at all on the swampy land, I have no hesitation in saying that they will be healthier in every respect if the swamp has been drained, than if it has simply been “filled in.”
      8. Of course there are times when, and places where, to fill in and raise the level of land will add greatly to its value. And the money spent on such filling may be a sound commercial investment, which may be quickly realised in the market with great profit. Not to “fill in” under these circumstances would be to neglect an opportunity not only of adding to the area of land available for use, but also of making an addition to the revenue which may be employed advantageously in other directions.
      9. Or, as in the case of the Tanjong Pagar reclamation, a great area may be added to a city, the occupation of which will relieve prevalent overcrowding of more central portions. Such a work has a sanitary value which can hardly be overestimated.

         To reclaim a dangerous swamp, to bring a labouring population near to its work, to house that population in buildings constructed and maintained according to the highest sanitary standard, to add to the revenues not only a large annual sum from rent, but also a great asset of ever-increasing value, all at one stroke, is a conception of sanitary statesmanship of the highest order.

   5. Mosquito-borne diseases other than malaria:
      1. There is another point of view from which the anti-malarial drainage of the land must be looked at. I refer to the other diseases which are carried by mosquitoes. The most important of these in this country is filaria.49 We know it occurs among Malayans in all parts of the Archipelago. According to Manson, 10% of Amoy’s inhabitants harbour the parasite, and it is present only in a slightly less percentage in the Southern Chinese. Some parts of India are seriously affected, and so are places in Japan. These are countries which send many of their inhabitants to Singapore.
      2. The disease is none the less serious because infection by the parasite may show no symptoms for many years. But the condition of the Fiji Islands, where Filaria is present in 25% of the population, and elephantiasis is so terribly prevalent, is a warning that the ever-increasing immigration attracted to Singapore by its prosperity may be drawing unawares to the island and to the city unrecognised carriers of the disease, and a rude awakening may come to a knowledge of the fact that the disease is silently but none the less tragically spreading among the populace.
      3. Year by year we see more clearly that, along with its benefits, Western civilisation, with its free system of communications, is introducing serious dangers in the form of increased liability to disease; and the greater the populations and the freer the communications with other countries, the greater is the danger.
      4. Plague was carried to India at the end of last century by ships from the West. But for these, India would in all probability now be free from the disease. Indeed, in vast regions of India any benefits ever given by Western civilisation have been more than balanced by this disease.
      5. Sleeping sickness is an example how in a few years a terrible disease can march across a great continent, keeping step with the white man in his progress, and cause such devastation that whole regions have to be abandoned.
      6. Throughout the tropical cities of the world, the security from fire given by the more substantial house built under Western rule, is being paid for in an alarming increase in tuberculosis.
      7. The great increase of malaria in Singapore is undoubtedly due to the conditions produced by its increasing population and prosperity. It would surely be unwise to wait until a dreadful disease like elephantiasis is upon the city before taking steps to check its course.
      8. I would strongly recommend, therefore, that in such time as he can spare from his malaria work, Dr. Finlayson should carry out an investigation into the prevalence of filaria among the people. This can easily be done by having slides of blood taken from the inmates of the hospitals. For, while the drainage of swamps by means of open earth drains does away with probably 95% of the mosquito-breeding area, the very drains, especially if neglected, produce a few mosquitoes capable of carrying this disease.
      9. While urging the use of earth drains for the purpose of eradicating malaria, I confess I regard them only as marking a stage in our progress towards the not far distant day when all breeding places of mosquitoes within a town will be obliterated. In time it may be possible to replace the open earth drains by closed subsoil drains, thereby not only reducing the cost of upkeep, but rendering them impossible as breeding places.
      10. This, however, must be in the future, and only when from experience we can be sure that the subsoil pipe in this country has a reasonable life of usefulness. So far as I know, there do not appear to be any special difficulties in using pipes in this climate. But before launching out on any large scheme, it would be wise to give a small area a thorough trial.
      11. If no unforeseen difficulties arise, the drainage of the future will be brick drains where there is sewage, and a system of subsoil drainage for the land; the latter falling into the brick drains. Storm water will be carried off by shallow drains, under which are the subsoil drains. Should a definite system of closed sewers be adopted for Singapore50 with pumping stations at various places, it would obviously be more economical to deal with storm water separately. The employment of subsoil drainage, connected with the sewers but independent of the shallow storm water drains, would in all probability facilitate the separate treatment of the storm water.
6. I have endeavoured in the foregoing pages to place on record some of the points discussed at Klang and Singapore. It will be seen that the malaria problem, like most other problems, cannot be kept in a water-tight chamber. Measures for its eradication should be considered not only from the malaria aspect alone, but from its bearing on other questions.
7. In dealing with malaria, and especially in spending money on measures for its eradication, every care must be taken that some other problem is not made thereby more acute or more difficult of solution.
8. I would summarise the foregoing demonstration as follows:
9. Do not spend money on filling in a swamp unless—
   1. You cannot drain it—a situation which will rarely be the case—or
   2. You can see your way clearly to realise within a reasonable period the money so invested at a profitable return in either cash or health.
10. Don’t begin by filling up the natural outlet when a swampy area must be filled up. This only aggravates the swamp, and it will be pure luck if an outbreak of malaria does not follow, or if already present, is not increased.
11. An open drain properly placed at the foot of a hill intercepting the water from the hill, costs a mere fraction of the filling which would be necessary to cover the water of a hill-foot swamp, and renders the ground drier, and less liable to revert to a swampy condition in wet weather, and healthier as a building site.
12. Only consider the question of filling in a swamp after trying to drain it. After your attempts it will often be found that the filling is unnecessary. Much waste of money will thus be prevented.
13. If an earth drain, when first cut, does not efficiently drain the swamp through which it runs, it is often possible to dig it deeper, at a very small additional cost.
14. If a brick drain has been constructed, the cost of deepening it will be probably more than the original cost of the drain.
15. No anti-malaria drain should be piped or bricked until it has been proved to be thoroughly capable of draining the swampy land, through which it runs. If it cannot drain the water off as an earth drain, it will certainly be less efficient as a brick or piped drain.
16. An open earth drain will, except in hilly land, eradicate malaria. In the future it may be possible either to brick in or pipe the drain, so as to render it harmless as a breeding place for any class of mosquito. In the meantime, it is doing its work in eradicating malaria at a minimum cost, and it forms no obstacle to any system which may be desirable in the future.