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Streams and Dams as a swimming pool
The owner whose property includes a stream which can be damned off to make a good swimming hole is fortunate indeed. Somehow swimming in the impounded waters of a flowing stream is more exhilarating than swimming in a formal pool. There is often a greater variety of depths, and the surroundings are usually more in spirit with outdoor recreation. The water is usually free of irritating chemicals. Men whose lives include boyhoods in the country rightfully look back to the old swimming hole as source of fun which cannot be matched by a masonry pool no matter how elaborate. Unfortunately, in many regions, the old swimming hole is not what it used to be. Often its waters have long ago been polluted by factories or even an overabundance of dwellings along its banks. Denuding of the forests has exposed the soil to the ravages of every storm with the result that the waters that flow through the old hole are now more often muddy than clear. Nevertheless, there are still some streams that have been unspoiled or which, because of anti-pollution legislation, have been more or less restored to their original purity. Even the old swimming hole was seldom provided entirely by nature. Usually nature made a shallow hole in the stream bed and the boys of the neighborhood helped her along by clearing it of rocks and building a dam to deepen the water. When a swimming hole is made or deepened now, it is usually necessary to construct a dam. Although most dams require the outlay of some money, few such as are built to deepen swimming holes cost anything like even a small swimming pool. Before the construction of a dam is undertaken, there are several steps which should be followed through. The first is to find out whether it is permissible to build a dam on the stream. Even though both banks of a stream are your property, there may be restrictions which will prohibit the building of a dam or make its construction too costly. In all states there is some unit of the state government which has jurisdiction over streams. Often the authority is vested in the health department, but sometimes it belongs to an engineering board or other unit. In most states there is also a considerable body of common law which has been built up over the centuries in court decisions.
A layman who is not used to digging around in law books usually derives his knowledge of the common law from his lawyer and his neighbors. Actually in most jurisdictions, the legal aspects of dam building are not so complex as to hinder the building of small private dams. We have pointed them out only in order to emphasize the advisability of finding out about them before rather than after building a dam. Another matter to be investigated is whether the water in the stream is safe for bathing. The way to find this out is to have samples analyzed for B. coli. Such analyses are made without cost in many states by the state health department. A letter to the state health officer will usually bring some sterile bottles to be filled with water samples and returned for the analysis. Although the directions which are supplied for handling the samples should be followed strictly, it is usually better to take stream samples when the water is at a high rather than at a low level because it is at high water levels that most of the contamination occurs. Of course, if samples taken when the water level is intermediate or low show the presence of dangerous numbers of B. coli, there is no point in having another analysis made when the water is high. On the other hand if the water samples taken under flood or near flood conditions prove to be impure, it is sometimes advisable to take samples again when the level is more normal.
Types of dams
Dams can be made of any strong material which is watertight or can be laid up in structures which are watertight. The dam at the traditional old swimming hole was usually made of logs held in place with stones and stakes and plastered up with mud. It was taken for granted that frequent repairs would be needed. In fact, filling up the chinks every day or so to stop the larger leaks was part of the fun of using the place. Furthermore, no one worried very much if a new dam had to be built every year. In a few locations it is still permissible to build such temporary dams, but in some states they are prohibited because of their potential dangers. Such dams can cause considerable damage if they should break when the stream is in flood and release all the impounded water in one mass. Small stone masonry dam. Another method of dam construction is to build a massive bank of soil in such a way as to make what is called an earth dam. Small earth dams are rather widely used in the construction of ponds, particularly ponds built on farms for fish propagation or water storage. The features of a good earth dam are as follows: The base is very broad in relation to its height. In fact, the slope of the earth in the exposed portions of the dam should not exceed the natural angle of repose which, for most soils, is between 35° and 37° with the horizontal. Either all of the soil used will be of a kind which is not permeable to water when compacted, or the core of the dam will be built with such soil. The dam is protected from elements that would erode its surface. For example, its top is not used for a walk, and cattle are not allowed near it. Most important from this standpoint is the provision of an outlet which will carry off all the discharge of the stream even at flood times, for an earth dam is quickly destroyed by water flowing over its crest. Earth dams are not well adapted to swimming holes because of the protection that they require and also because their long, underwater slope takes up a considerable portion of the pond area.
The depth of water to be impounded and the total height of wall determine the wall thickness. Modern small dams are usually made of masonry materials, either poured concrete or stone set in mortar. Poured concrete dams are usually reinforced with steel and therefore can be made somewhat thinner than stone masonry dams. They are not necessarily less expensive even though they contain less material because the cost of the reinforcing steel and, also, the cost of the forms must be added to the total cost. A well-designed and constructed reinforced concrete dam is, however, subject to fewer deteriorating influences than a masonry dam containing many joints. The relative cost of these three types of dam construction when applied to small dams depends upon several factors which are local in nature. If earth-moving machinery, such as a bulldozer or power shovel, is available at reasonable rates, an earth dam can be built very reasonably, especially if the soil for the dam can be obtained from the excavation for the pool. If hard sedimentary stone is avail- able in the bed of the stream itself, a stone masonry dam may prove to be the cheapest. On many sites, especially where good stone is not available, a poured concrete dam is the least expensive type when all elements that contribute to the cost are taken into account. Except under very unfavorable conditions, the cost of the typical small dam is not excessive. Fairly substantial ones can be built for $500 or less.
Design of dams
In spite of the number of dams which have been built by boys and other amateurs, a dam should not be designed or constructed by persons without some knowledge of engineering principles. It is not unusual for a dam only 4 or 5 ft. high to be holding back several thousand tons of water, and such a quantity of water is a destructive agency of great force if it should be let loose suddenly by the failure of a weak dam. One method of dam design is to treat the dam as a retaining wall, but even this method is not applicable to a majority of situations. The type of dam which is arched on the upstream side is de- signed on quite different principles. An important part of the engineering is designing the base of the dam so that there is practically no possibility of it being undermined by water at the forward edge, because a dam so undermined is readily turned over by the force of the water bearing against it.
It is not enough to have good water and a strong dam for there is still the problem of mud. A small stream which runs crystal clear in ordinary weather will sometimes turn into a broad, silt-laden river after heavy rains. When water which is carrying silt and flowing rapidly is slowed down, as when it enters the pool behind a dam, it drops a considerable part of the silt. It is not unusual for pools which are as deep as 8 or 10 ft. to fill up completely within four or five years if they are located in a swift stream which flows through open country. Even in heavily wooded regions the silt problem is not entirely absent. On the other hand there are some streams which carry little silt even at high water. There is no perfect solution to the silt problem which is within the reach of individual property owners. A thorough program of reforestation and soil conservation in the watershed of the stream will gradually reduce the amount of silt, but such a program is a community project and, even at best, is not immediately effective. The construction of a settling basin upstream from the dam will get rid of a fair proportion of the silt during periods of normal stream flow, but at flood times the silt in the settling basin is likely to be picked up and deposited in the swimming pool. Settling basins of ample size require a considerable amount of land, and their excavation involves a good deal of labor. Another and somewhat better method is to build into the base of the dam a large cast-iron or concrete pipe which can be used as a means of emptying the pool with fair speed. When the deposited silt becomes a nuisance, the water of the pool is allowed to run out of this bottom drain. Since it runs rapidly, it will carry a fair amount of the silt with it and what remains behind can usually be mixed with additional water and flushed out. One catch to this scheme is that is may lead to trouble with property owners downstream who don't want their own pools or even their portions of the stream bed suddenly swamped with silt. The best and also the most expensive way around the problem is to build a regular swimming pool at some distance from the stream and only to utilize the stream as a water supply for it. On some properties the pool can be located so that stream water can be run into it by gravity. In others a pump is necessary. Whatever the arrangement, water is moved from the stream to the pool only at times when it is clean. Unfortunately there will be a mud problem long before much silt is carried into the pool behind the dam. The bottom and sides are usually of soil, and will turn into mud under slight provocation. After the dam is constructed, but before the pool is filled with water, the bottom can be covered with several inches of clean, round gravel. To supplement this covering, the sides of the pool can be walled up with stone laid up without mortar. As much attention should be given to good workmanship in making such a stone lining as is given to the building of a good stone wall, for if the stone is not laid up well, it will slip into the pool and prove to be a considerable nuisance to swimmers and divers.