MUNICIPAL ENGINEERING IN THE TROPICS

By Henry Welles Durham, C. E., General Manager, Rene Keilhauer & R. W. Hebard & Co., Inc., Managua, Nic.

Among the various subdivisions which the engineering profession has either given to itself or has had conferred by brilliant advertising intellects there has recently been noted that of the "Tropical Engineer." The usual idea in the North Temperate Zone of the areas known as the Tropics, is that within a belt extending around the world at equal distances on either side of the equator, there is a constant and torrid climate with general uniformity of conditions. As a matter of fact within the countries lying between 232° North and South latitude, there can be encountered as many variations of climate as in any other grand division of the world.

Actually the theoretical conditions are modified to such an extent by ocean currents, winds and topographical features that the climatic torrid zone (leaving out of account the extreme altitudes within this area) is considerably distorted in area from the tropical belt. There are portions of South America for example extending to within 12° of the equator where conditions are very similar to those of milder portions of the temperate zones.

The purpose of these comments, however, is not to produce an explicit definition of what is meant by the term "tropical," but merely to note a few items of experience in the working out of municipal engineering problems in Spanish American countries. bordering on the equator. Along with a theoretically, unique and uniform tropical region there must, of course, come the idea of distinctive professional practice, an idea capitalized at times by so-called "Tropical Engineers." It is my idea that good engineering practice in its fundamentals does not vary geographically, but that in the tropics, as elsewhere, the good engineer is he who produces satisfactory work at least cost.

What are the governing conditions encountered which cause distinctive practice and what are some of the questions to be considered in municipal engineering problems is the subject here.

There is one condition that can almost always be counted on. That is the absence of frost. Actually there are towns near the equator, such as Cerro de Pasco and Oroya, in Peru, where frost, snow and hail are of frequent occurrence, but even here, no frost conditions are met that have any influence on engineering designs.

The engineer who works in the tropics can eliminate from his problems all the troubles of his comrades in the North as regards expansion provisions or danger from frozen water mains.

His problems arising from precipitations are more varied than anywhere else, ranging from rates of rainfall in excess of 4 inches per hour for several hours down to a fraction of an inch per year.

As commonly understood the problems of the municipal engineer include water supply and its elimination by sewerage, also drainage of all other undesired water flows (the main responsibilities of the sanitary engineer) and the construction of streets, roads and pavements with the frequent addition of question of local rapid transit, such as street railways or subways.

WATER SUPPLY:

Taking these items in the order which they have arisen and their present importance, water supply is as always the prime essential. All communities of necessity have water supplies. In the region of which we are treating they are hardly ever adequate or sanitary by modern standards, hence the provision of a new installation in part at least is almost always presented to the engineer and contractor undertaking work in the Latin-American tropics.

The first condition to be considered in studying a special situation is the changed standard of unit requirements as compared with the United States. It is never necessary or desirable to plan on the extravagant basis to which all North American communities are accustomed. The engineer with a basis of European experience will find himself more in sympathy with the local requirements and with less to unlearn. Of the large provision which we make for manufacturing and fire protection it will be found that nearly all can be eliminated. In an experience of many years and extending from Bolivia to Guatemala there has been met no town that would not be adequately served with 50 gallons per inhabitant per day. Among other cities on which this experience is based are Lima, Peru, with 200,000 inhabitants, Panama and various Central American capitals.

The only notable exception is found in the Canal Zone where an excessive per capita consumption is due to waste and a large amount of irrigation on account of the generous free supply of government water. In the city of Panama, where all water is metered and paid for, consumption runs as low as 30 gallons, although originally designed for 50.

Panama City, too, is the site of the only destructive widespread North American type fire which I have seen. Two such fires were successfully extinguished during the early days of canal construction, using the new 50-gallon per head water supply.

Both fires were attributable to fumigating operations undertaken in the work of suppressing yellow fever and would not ordinarily be encountered in the Spanish American city with its stone or adobe walled buildings.

Similarly more moderate pressures than are usual in the United States are entirely adequate. From 50 to 75 lbs. and sometimes less will give a very satisfactory service.

In the selection of a source for a new water supply there arises the choice between an augmentation of whatever is the existing supply or the selection of a new one. Frequently the desire for personal aggrandizement by the creation of a large work leads to unnecessarily elaborate plans. A case may be cited where a consulting engineer planned a new water works with the inclusion of a large dam on an unexplored foundation and across a torrential river to form an impounding reservoir for water which would have to be filtered-and entirely neglected to take into account the existing available large supply of sterile ground water which could be developed to any necessary amount.

The most frequent source of potable water supplies will be from streams, either directly or using storage basins. A variant of the ordinary ground water supply taken by galleries or wells, and encountered along the earthquake or volcanic belt is the crater lake, usually formed by the seepage of ground water into an extinct. crater. Lake water supplies from large bodies of water are very infrequent and usually insanitary.

While it may be necessary to filter after settlement, the river and lake water-as in the Canal Zone and on the Magdalena River for example the crater lake or ground water will, at most, need chlorination to render it ordinarily safe for consumption.

In the arid parts of South America the ultimate source must be a river, draining the western slopes of the Cordillera from regions where precipitation of snow and rain (absent on the West Coast) are encountered. No other water is available in the coastal plain between the Pacific Ocean and the mountains, and extending the entire north and south length of Peru and part of Chile, on the west.

The immediate source of water may be direct from the stream (seldom closer than at 50 to 100 mile intervals) or by collection from the subsurface flow furnished by a river passing over a porous bed of sand and gravel deposited in some previous geological age, and through which the stream is cutting a new channel. This source is especially available in the delta areas along the lower valleys. of such rivers, where the ground water flow may be found at a distance of several miles from the river and may, before it reaches the mouth, absorb the entire stream flow leaving its bed dry. The question as to what areas of the coast can be inhabited and cultivated is entirely dependent on the economic possibility of bringing stream or ground water in sufficient quantity to the region inhabited and cultivated. That such possibilities greatly exceed present. provisions can be seen in the ruins of ancient canals where today is a desert waste, and in the fact that in pre-Incaic days a population vastly greater than the present, inhabited the coast of Peru.

Most notably successful of such supplies is that of Lima. Few of the world's great cities are so favored in that regard. Beginning with the original well developed in the time of Pizarro, the wells or

galleries have been periodically extended, and recent studies have indicated a volume of ground water available, adequate for a city of several times the present population. Such defects as have existed have always been due to inadequate development, and such plans as have been made for more complicated and other developments have overlooked the advantages of natural filtration and gravity supply.

Frequently in the smaller coast towns the same conditions exist. At others the situation presents itself, of bringing water a long distance by pumping from a river, as at Payta, or by gravity as at Mollendo. In both these cases the necessity arises from the fact that a town has been located for other reasons than in virtue of its situation in a delta adjacent to one of the transverse drainage streams from the Cordillera. There are extensive desert regions btween these rivers and any development in them must be supplied by water brought from a long distance. In general it may be said that at any point on the arid west coast possessing qualifications for exploitation there may be found, if there does not already exist on record, a practical solution of the water supply problem. In many cases such a solution was arrived at in prehistoric days and on a scale much larger than is demanded for the present population.

A striking example is found at Chimbote, Peru, whose small community is supplied by tank cars, and lives in an arid basin, around whose rim can still be seen the ruins of an ancient irrigation ditch that made fertile many square miles of the present sandy desert confronting one of the world's best harbors.

When we leave the coast and study the cities situated on the slopes of the Sierra or in the valleys and plains between the various Andean chains, the question of a water supply is more easy of solution. In almost all cases there exist within a reasonable distance of the town, springs or a stream having a constant flow from sources on the higher mountains, and capable of furnishing the necessary amount of water in an unpolluted condition. It is desirable wherever possible that the source be at such elevation as to permit of gravity supply. Such cities as Quito, Cuzco and Arequipa, for example, have for centuries possessed sources of potable water of good quality, and such failures of quantity and in sanitary conditions as have from time to time existed, have been due to a lack of knowledge on the part of the authorities as to proper means for safeguarding and distributing the supply.

In few cases will there be found any necessity for expensive filtration plants. Following are the fundamental essentials:

1. Careful protection of the source of supply to prevent any pol lution.

2. A suitable aqueduct to convey the necessary quantity of water to a distributing reservoir. (This may necessitate also a pumping plant.)

3. A protected reservoir at such elevation as to distribute water to the community, under sufficient pressure for domestic use and fire protection.

4. Provision for chlorination, as an additional safeguard.

5. A well designed system of pipes for local distribution.

6. An efficient organization operating under municipal regulation and supervising the collection and distribution of water, and the business management of the water works to derive an adequate income for its maintenance.

Some details of design worthy of attention should be noted, as there exists a tendency on the part of some engineers to design overelaborate or unnecessary features adopted from the practice of countries having different conditions, which has resulted frequently in the abandonment of proposed improvements due to excessive cost.

Local materials should be used wherever possible in order to avoid expense of import duties and freight. Conduits operating under low pressure can be constructed of concrete, or cement pipe, where suitable stone or gravel is available.

Wood stave pipe can be imported and moved at much less cost than cast iron pipe and is equally satisfactory in many cases, but it cannot be used where there is danger from wood borers which flourish in moist hot regions.

Advantage should always be taken of local climatic conditions which frequently permit a more inexpensive type of construction than can be followed in the northern temperate zone.

Simplicity of design is desirable and complicated features should be avoided, because of the difficulty in many cases of securing the necessary expert assistance from the laboring class.

The designing engineer must have constantly in mind the fact that no system is self operating or efficient under a careless management, but that he can by proper attention to details protect his supply to some extent against the possibility of future contamination due to carelessness or wilful neglect, as for instance, in the avoidance of open conduits or reservoirs and the safeguarding of manholes, valves, and similar structures against possible tampering. In one case noted, the already inadequate supply of a Peruvian city was reduced by one-half, due to theft of water by private interests at exposed points in the conduit and at a blow-off where water was diverted for private irrigation. The designer can reduce such possibilities by careful attention to detail, but the final protection of the public must result from an efficient and interested water works management.

SEWERS:

Sanitary sewers exist in few towns outside of foreign colonies in Tropical America. Such sewerage systems as exist provide for only limited portions of large communities. Many have only the most primitive conveniences for large masses of the population and often