Volume III - No. 1
INSURANCE FOR THE EARTH DAM
By Tyler B. Kiener,
S CHILDREN many of us have delved into the banks of some small stream with our bare hands, transferred the soil to such a pile of sticks and stones as had been found close by, and rudely molded the mixture into what may be described -- by some stretch of the imagination -- as an earth dam. Working feverishly to keep ahead of the rapidly rising water behind it and slapping clods of clay into those breaches through which the relentless flood seemed most likely to undermine our structure, we produced, after a considerable struggle, a solid barrier between the banks of the stream.
Our momentary advantage was soon lost, however, and the small basin which was formed behind the barrier began to fill with water, the surface of which approached uncomfortably close to the top of the dam. By building it a few inches higher the tide again was stemmed, but we discovered shortly that the contest was unequal, and so resigned ourselves to the inevitable but somewhat fascinating climax of seeing an earth dam overtopped. The water crept toward the crest, began to trickle over in several places, and finally the full flow of the stream poured across the top. Signs of distress promptly appeared in the structure. Under the flowing water its earth portions were dissolving like sugar, and great gaps were cut through the top which permitted larger torrents to gush through. The rate of dissolution became progressively faster and at last our dam gave up the ghost in far less time than had been required for its creators to put it together.
Our childish minds, however, did not register great concern over the disaster. Having a vague purpose, probably confined to the pleasure of playing in the water, we did not realize how much more simple our construction job would have been if we had been able to isolate the running water and not have it build up a pool behind us as we worked. Neither did we recognize that our dam, once built, would have remained intact had we routed the water over a bed more resistant to its action than the soil contained in the dam.
It is quite possible to provide these construction advantages, and today every earth dam built by the National Park Service's cooperation is protected, during its construction, from the stream which it will impound, and after its completion by a permanent passageway for the same stream. The earth embankment either partially or wholly complete may represent an investment of many thousands of dollars in labor and equipment rental and it is therefore proper to insure against its loss by paying a premium in terms of special construction. In a good many instances this premium insures not only against loss of the embankment itself, but against loss of life and property damage in the valley below the dam, for a very large volume of water is often stored behind our dams and its sudden release would sweep to destruction all that stood in its way.
To deal with such potentially dangerous conditions, one might ask at this point why not build a dam of concrete, or stone masonry, or some other equally durable material and avoid the risks which appear to go hand in hand with earth dam construction. Apparently, this question does not seem to have occurred to agencies making use of the Civilian Conservation Corps to build their dams, for we find the following statement in a recent press release from the office of Mr. Fechner, Director of the Corps, relative to some 350 dams built by his enrollees:
Consideration has been given to this question, however, for a definite reason lies behind the popularity of the earth dam in spite of its accompanying hazards. The answer is to be found in the financing of CCC construction programs. Each CCC camp is allotted but a small sum for the purchase of building materials. The earth dam meets the resulting financial situation admirably therefore, because building an embankment out of local soils across a valley requires only small expenditures for construction materials. Compared with a concrete or stone masonry barrier its economic advantage is outstanding and, if properly constructed, it is equally safe.
ROTECTION DURING CONSTRUCTION: When a small stream is to be dammed by an earth embankment, the simplest method of segregating the stream flow consists of laying a pipe or conduit in the valley at the same general level of the stream and diverting the water through it. The embankment may then be built around the pipe without inconvenience. If a valve is set at the upper end of the pipe, the resulting arrangement will serve as a drain for the lake after the dam is completed and our insurance premium which consists of expenditures for the pipe and valve has not only afforded us protection during construction, but it also is converted into a useful operating part of the completed dam. The designer, it should be noted, is encumbered with the problem of choosing the correct size of pipe for this purpose, and a mistake may invite serious trouble. Heavy rains occuring while the embankment is under way will swell the stream beyond its normal flow, and if the pipe is not large enough to carry away the flood flow, water will rise behind the embankment and possibly overtop it.
A larger stream would naturally require a larger pipe, probably far in excess of the size needed to serve later as a drain so that our insurance premium is only partially compensated by subsequent conversion in the manner described above. It follows, as we assume larger and larger streams, that insurance costs will increase to such proportions that other methods of protection must be adopted upon which the premium is cheaper.
Generally speaking, stream flow varies with rainfall, and east of the Mississippi the precipitation is much heavier in the winter and spring months than in the summer. It is therefore possible to schedule a construction program so that an earth dam may be built across a stream in a period of least potential rainfall. In other words, instead of spending an exorbitant sum for a large outlet pipe, a smaller pipe is selected and the dam is built during those months when the danger of heavy runoffs is at a minimum. Obviously, a certain risk is assumed with this procedure because a flood may occur during the time when the records show it most unlikely, but the odds are in our favor, and if we have not tempted fate too strongly, our job can be successfully completed.
There are, unfortunately, limiting factors to the use of the above method. For instance, the bulk of the earth fill may be so large that it is impossible to complete it in one short dry season with the equipment on hand for its construction, and to permit the partly finished embankment to brave a season of heavy rainfall would court disaster. There are two general methods of meeting such a situation.
If a small gap is left in the earth dam, flood waters in excess of that removed by the pipe will flow through this gap. Later, at an opportune moment and after the remainder of the embankment is completed, the gap is closed. Although apparently a simple solution, this method has its disadvantages. The break in the otherwise solid mass of embankment complicates the placing of the balance of the fill, and the possibility of getting inferior workmanship while closing the gap is greatly increased. Thus, our insurance premium takes the form of difficult and slower construction. Nevertheless, this method is widely used.
A second scheme is perhaps the most satisfactory solution of the problem and can be adopted readily when the spillway of the dam is to be a concrete or stone masonry overflow structure founded in the valley floor. Such a spillway usually contains a drain channel which takes the place of our previously mentioned pipe. To conduct flood water which is in excess of the drain's capacity, an opening is left in the masonry. After the earth embankment is completed the opening is filled. One of the illustrations shows construction planning of this type. The spillway shown is part of the large earth dam recently completed at the Laurel Hill Recreational Area in Pennsylvania. Actual flood water may be seen discharging through the gap left for this purpose.
EALING IN FUTURES: We now take the position that our earth dam has been successfully finished. In common with all man-made structures it has been built to render useful service for an estimated period of time, and since it represents a sizeable investment it has been insured in order that this life may be sustained. It must be remembered that when water flows over the top of an earth dam its life may be abruptly ended, and the insurance policy which we have taken out guards against this particular possibility. The premium, which we pay in one lump sum, is represented by the spillway of the dam, and its amount depends upon the length of time for which we require protection, because a spillway must be larger if the dam's life is to be longer. The reason for this will appear further on.
The vagaries of the stream flow which were mastered during construction still remain to trouble us after the dam is built and will continue to be a source of potential danger for as long as the dam is expected to stand. To meet this condition the spillway must not only conduct the stream safely by the dam, but it likewise must conduct in the same maimer every flood which will occur during the dam's life. We are dealing no longer with a brief construction period. We are compelled to predict the size of the greatest flood which will take place in a period of many years. We must presume to foretell the future. How may this be done?
When an English poet said "Coming events cast their shadows before" he probably did not realize that many state and federal agencies of this country later would take him at his word and arrange for an application of his theory to the prediction of flood flows. This is exactly what has been done, however, for these agencies have collected "shadows" in the form of accurate rainfall records and stream flow measurements for long periods of time, and they are available for all who wish to try their hand at forecasting the "coming event".
By studying the records of any particular stream we immediately find that the larger floods do not occur very often, and further examination shows that the larger the flood the less frequently it takes place. If the record is long enough, and after we have performed the necessary mathematical gymnastics upon its vitals, we are shortly able to demonstrate that a certain sized flood will be equalled or exceeded only once in a certain number of years, and that various other sized floods will be equaled or exceeded at various other frequencies. We therefore select the flood which will be equalled or exceeded only once during the required life of our dam and design the spillway to pass it safely, together with some extra allowance for good measure which is usually termed a factor of safety. At this point it should now be clear to the reader why a dam with a longer life requires a larger spillway.
Unfortunately, flow records of the stream which we propose to dam cannot always be obtained. This is generally true of the streams impounded by CCC under the supervision of the National Park Service in Region I for the simple reason that records are available only for the larger and more important rivers. We are obliged in such cases to resort to frequencies and flows which are derived from rainfall characteristics of the vicinity of our dam. The principle has some similarity to the generalized explanation already given, and has proven satisfactory.
In conclusion it may be truthfully stated that no man has ever found a way to forecast the magnitude of future floods with perfect accuracy, and the chances are that no one ever will. Such predictions are made, however, with high practical efficiency. Students of hydrology are continually seeking means to increase the accuracy of their forecasts and we find that failures of earth dams due to insufficient spillway capacity are far less frequent than in the past. It is the engineer's responsibility that if he predicts too small a flood, the earth dam may be washed away, and if his prediction is greatly in excess of the largest flood which occurs during the dam's life, he has purchased insurance at extravagant rates.
|<<< Previous||> Contents <||Next >>>|