For h = 0, u2/g, > u2/g, ∞,
the denominator becomes −∞, 0, > 0, 1.
| Fig. 122. |
§ 118. Case 1.—Suppose h > u2/g, and also h > H, or the depth greater than that corresponding to uniform motion. In this case dh/ds is positive, and the stream increases in depth in the direction of flow. In fig. 122 let B0B1 be the bed, C0C1 a line parallel to the bed and at a height above it equal to H. By hypothesis, the surface A0A1 of the stream is above C0C1, and it has just been shown that the depth of the stream increases from B0 towards B1. But going up stream h approaches more and more nearly the value H, and therefore dh/ds approaches the limit 0, or the surface of the stream is asymptotic to C0C1. Going down stream h increases and u diminishes, the numerator and denominator of the fraction (1 − ζu2/2gih) / (1 − u2/gh) both tend towards the limit 1, and dh/ds to the limit i. That is, the surface of the stream tends to become asymptotic to a horizontal line D0D1.
The form of water surface here discussed is produced when the flow of a stream originally uniform is altered by the construction of a weir. The raising of the water surface above the level C0C1 is termed the backwater due to the weir.
| Fig. 123. |
| Fig. 124. |
§ 119. Case 2.—Suppose h > u2/g, and also h < H. Then dh/ds is negative, and the stream is diminishing in depth in the direction of flow. In fig. 123 let B0B1 be the stream bed as before; C0C1 a line drawn parallel to B0B1 at a height above it equal to H. By hypothesis the surface A0A1 of the stream is below C0C1, and the depth has just been shown to diminish from B0 towards B1. Going up stream h approaches the limit H, and dh/ds tends to the limit zero. That is, up stream A0A1 is asymptotic to C0C1. Going down stream h diminishes and u increases; the inequality h > u2/g diminishes; the denominator of the fraction (1 − ζu2/2gih) / (1 − u2/gh) tends to the limit zero, and consequently dh/ds tends to ∞. That is, down stream A0A1 tends to a direction perpendicular to the bed. Before, however, this limit was reached the assumptions on which the general equation is based would cease to be even approximately true, and the equation would cease to be applicable. The filaments would have a relative motion, which would make the influence of internal friction in the fluid too important to be neglected. A stream surface of this form may be produced if there is an abrupt fall in the bed of the stream (fig. 124).
On the Ganges canal, as originally constructed, there were abrupt falls precisely of this kind, and it appears that the lowering of the water surface and increase of velocity which such falls occasion, for a distance of some miles up stream, was not foreseen. The result was that, the velocity above the falls being greater than was intended, the bed was scoured and considerable damage was done to the works. “When the canal was first opened the water was allowed to pass freely over the crests of the overfalls, which were laid on the level of the bed of the earthen channel; erosion of bed and sides for some miles up rapidly followed, and it soon became apparent that means must be adopted for raising the surface of the stream at those points (that is, the crests of the falls). Planks were accordingly fixed in the grooves above the bridge arches, or temporary weirs were formed over which the water was allowed to fall; in some cases the surface of the water was thus raised above its normal height, causing a backwater in the channel above” (Crofton’s Report on the Ganges Canal, p. 14). Fig. 125 represents in an exaggerated form what probably occurred, the diagram being intended to represent some miles’ length of the canal bed above the fall. AA parallel to the canal bed is the level corresponding to uniform motion with the intended velocity of the canal. In consequence of the presence of the ogee fall, however, the water surface would take some such form as BB, corresponding to Case 2 above, and the velocity would be greater than the intended velocity, nearly in the inverse ratio of the actual to the intended depth. By constructing a weir on the crest of the fall, as shown by dotted lines, a new water surface CC corresponding to Case 1 would be produced, and by suitably choosing the height of the weir this might be made to agree approximately with the intended level AA.
| Fig. 125. |
§ 120. Case 3.—Suppose a stream flowing uniformly with a depth h < u2/g. For a stream in uniform motion ζu2/2g = mi, or if the stream is of indefinitely great width, so that m = H, then ζu2/2g = iH, and H = ζu2/2gi. Consequently the condition stated above involves that