SOURCE OF TURBIDITY.
Much turbidity originates in plowed fields of clayey soil, or in fields upon which crops are growing. If it has not rained for some days, and the surface-soil is comparatively dry, the first rain that falls upon such land is absorbed by the pores of the soil until they are filled. If the rain is not heavy, but little runs off over the surface. If, however, the rain continues rapidly after the surface-soil is saturated, the excess runs off over the surface to the nearest watercourse. The impact of the rain-drops upon the soil loosens the particles, and the water flowing off carries some of them in suspension, and the water is said to be muddy.
The particles carried off in this way are extremely small. Mr. George W. Fuller, in his report upon water purification at Louisville, estimates that many of them are not more than a hundred thousandth of an inch in diameter, and not more than a tenth as large as common water bacteria.
The turbidity of the water flowing from a field of loose soil may be 2.00 or more; that is to say, the wire is hidden by a depth of half an inch of water or less. When the water reaches the nearest watercourse it meets with water from other kinds of land, such as woodlands and grassed fields, and these waters are less turbid. The water in the first little watercourse is thus a mixture and has a turbidity of perhaps 1.00.
The conditions which control the turbidity of any brook are numerous and complicated. The turbidity of a stream receiving various brooks depends upon the turbidities of all the waters coming into it. Generally speaking, the turbidity of a river depends directly upon the turbidities of its feeders, and is not affected materially by erosion of its bed or by sedimentation in it. There are, of course, some streams which in times of great floods cut their banks, and all streams pick up and move about from place to place more or less of the sand and other coarse materials upon their bottoms. The materials thus moved, however, have but little influence upon the turbidity.
After the rain is over some of the water held by the soil will find its way to the watercourses by underground channels, and will prevent the stream from drying up between rains, but the average volume of the stream-flows between rains will be much less than the volumes during the rains when the water is most turbid.
Fig. 19.—Fluctuations in Turbidity of the Water of the Allegheny River at Pittsburg during 1898.
These conditions are well illustrated by a few data upon the turbidity of three Pennsylvania streams, recently collected by the author. One of these streams is a small brook having a drainage area of less than three square miles. The observations extended over a period of 47 days. During this time there were five floods, or an average of one flood in ten days. The duration of floods was less than twenty-four hours in each case. Selecting the days when the turbidity was the highest, to the number of one tenth of the whole number of days, the sum of the turbidities for these days was 67 per cent of the aggregate turbidities for the whole period. That is to say, 67 per cent of the whole amount of mud was in the water of only a tenth of the days; the water of the other nine tenths of the days contained only 33 per cent of the whole amount of turbidity. The average turbidity of the water for the flood days was eighteen times as great as the average turbidity for the remaining days.
The next stream is a considerable creek having a drainage area of 350 square miles. The observations extended over 117 days, during which time there were seven floods, or an average of one flood in 19 days. The floods lasted in each case one or two days, and the sum of the turbidities for the one tenth of the whole number of days when the water was muddiest was 55 per cent of the aggregate of all the turbidities for the period.
The last case is that of a large river, with a drainage area of over 11,000 square miles. The observations extended over a full year. In this period there were sixteen floods, each lasting from one to six days, and the sum of the turbidities for the one tenth of the whole number of days when the water was muddiest is 45 per cent of the aggregate turbidities for the year. The floods occurred on an average of once in 22 days, and the average duration was two and one half days.
The results are very striking as showing that a very large proportion of the mud is carried by the water in flood flows of comparatively short duration. They also show that in small streams the proportion of mud in the flood-flows is greater, and the average duration of floods is shorter, than in larger streams. In other words, the differences between flood- and low-water flows are greatest in small streams, and gradually become less as the size of the stream increases.
When a stream is used for water-works purposes in the usual way, a certain quantity of water is taken from the stream each day, which quantity is nearly constant, and is not dependent upon the condition of the stream, or the volume of its flow. The proportions of the total flows taken at high- and low-water stages are very different, and it thus happens that the average quality of the water taken for water-works purposes is different from the average quality of all the water flowing in the stream.
Let us assume, for example, a stream having a watershed of such a size that in times of moderate floods water from the most distant points reaches the water-works intake in twenty-four hours. Let us assume further that rainfalls of sufficient intensity to cause floods and muddy water occur, on an average, once in ten days, and that the turbidity of the water at these times reaches 1.00, and that for the rest of the time the turbidity averages 0.10. Let us assume further that at times of storms the average flow of the stream is 100 units of volume, and for the nine days between storms the average flow is 10 units of volume. We shall then have in a ten days’ period, for one day, 100 volumes of water with a turbidity of 1.00, and nine days with 10 volumes each, or a total of 90 volumes of water with a turbidity of 0.10. The total discharge of the stream will then be 190 volumes, and the average turbidity 0.57. The turbidity of 0.57 represents the average turbidity all the water flowing in the stream, or, in other words, the turbidity which would be found in a lake if all the water for ten days should flow into it and become thoroughly mixed without other change.
Now let us compute the average turbidity of the water taken from the stream for water-works purposes. The water-works require, let us say, one volume each day, and we have for the first day water with a turbidity of 1.00, and then for nine days water with a turbidity of 0.10. The average turbidity of the water taken by the water-works for the period is thus only 0.19 in place of 0.57, the average turbidity of the whole run-off.
The average turbidity of all the water flowing in the stream is thus three times as great as that of the water taken from the stream for water-works purposes.
It is often noted that with long streams the water becomes muddier farther down, and it may naturally be thought that it is because of the added erosion of the stream upon its bed in its longer course. This, of course, may be a cause, or the lower tributaries may be muddier than the upper ones, but the fact that the water taken at the lower point is more muddy than farther up is not an indication of this.
Let us take, for example, a watershed of twice the size of that assumed above, that is, so long that 48 hours will be required for the water from the most remote feeders to reach the water-works intake. Let us divide this shed into two parts, which we will assume to be equal, one of which furnishes water reaching the intake within 24 hours, and the other water reaching the intake between 24 and 48 hours. Now suppose a storm upon the watershed producing turbidities equal to those just assumed for the smaller stream. On the first day the water from the lower half of the shed, namely, 100 volumes having a turbidity of 1.00, passes the intake, but this is mixed with 10 volumes of water from the upper half of the watershed, having a turbidity of 0.10, and the total flow is thus 110 volumes of water having a turbidity of 0.92. On the second day the water from the lower half of the watershed has returned to its normal condition, and the flood-flow of the upper half of the watershed, 100 volumes with a turbidity of 1.00, is passing, and mingles with the 10 volumes from the lower half with a turbidity of 0.10, and the total flow is again 110 volumes having a turbidity of 0.92. The following eight days, until the next rain, will have flows of 20 volumes each, with turbidities of 0.10. The average turbidity of all of the water flowing off is 0.57 as before, but the water taken for water-works purposes will consist of 2 volumes of water with turbidities of 0.92, and 8 volumes with turbidities of 0.10 making 10 volumes with an average turbidity of 0.26.
By doubling the length of the watershed we have thus doubled the length of time during which the water is turbid, and have increased the average turbidity of the water taken for water-works purposes from 0.19 to 0.26, although the average turbidity of all the water running off remains exactly the same.
If now we assume a watershed so long that three days are required for the water from the most remote points to reach the intake, with computations as above, water taken for water-works purposes will have an average turbidity of 0.32; and with still longer watersheds this amount will increase, until with a watershed so long that ten days, or the interval between rains, are required for the water from the upper portions to reach the intake, the average turbidity of the water taken for water-works purposes will reach the average turbidity of the run-off, namely, 0.57.
In the above computations the numbers taken are round ones, and of course do not represent closely actual conditions. They do serve, however, to illustrate clearly the principle that the larger the watershed, other things being equal, the more muddy will be the water obtained from it for water-works purposes, and the longer will be the periods of muddy water, and the shorter the periods of clear water between them.
It cannot be too strongly emphasized that the period of duration of muddy water is, in general, dependent upon the length of time necessary for the muddy water to run out of the stream system after it is once in it, and be replaced by clear water; and that the settling out of the mud in the river has very little to do with it.
Muddy waters result principally from the action of rains upon the surface of ground capable of being washed, and the turbidities of the stream at any point below will occur at the times when the muddy waters reach it in the natural course of flow, and will disappear again when the muddy waters present in the stream system at the end of the rain have run out, and have been replaced with clear water from underground sources, or from clearer surface sources.