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e e e MEA8UREMENT OF PEAK DI8CHARGE BY THE SLOPE-AREA METHOD 3 and ex is the velocity-head coefficient. The value of ex is assumed to be 1.0 if the section is not subdivided. The value of ex in sub- divided channels is computed as (T,K.'!a.') ex KT'/AT" where the subscript i refers to the conveyance or area of the individual subsections and T to the area or conveyance of the entire cross section. The energy loss due to contraction or ex- pansion of the channel in the reach is assumed to be equal to the difference in velocity heads at the two sections (tJ.h.) times a coefficient k. The value of k is taken to be zero for con- tracting reaches and 0.5 for expanding reaches. However, both the procedure and the coef- ficient are questionable for expanding reaches and thus major expansions are avoided, if possible, in selecting sites for slope-area meas- urements. The value of A.h. is computed as the up- stream velocity head minus the downstream velocity head; thus, the friction slope to be used in the Manning equation is computed algebraically as s- A.h+f./2) (when A.h, is positive), (7) and S tJ.h+A.h. (h h' ') L w en tJ. ,IS negatIve . Selection of Reach The selection of a suitable reach is probably the most important element of a slope-area measurement. Ideal reaches are difficult to find, and usually it is a matter of selecting the best reach ava.ilable. Good high-water marks are basic to a reliable slope-area computation, so that the presence or quality of high-water marks is an important consideration. A steep-sided rock channel might have near-perfect hydraulic qualities, yet if the walls did not retain high-water marks, it would be useless as a slope-area reach. If heavy rains (6) have followed the peak prior to the survey, marks on clean banks may bave been de- stroyed or washed to a lower elevation, whereas marks within wooded parts of the channel might have been little affected. The selection of a reach is thus first governed by the availability of high-water marks. The geometry of the channel in the reach is also important. Marked changes in the shape of the channel along a reach should be a voided because of the uncertain ties regarding the value of the velocity-head coefficient. The channel should be as uniform as possible, but in any event, the changes in channel conveyance should be fairly uniform from section to section in order to be consistent with the ....'!Sumption that the mean conveyance is equal to the geometric mean of the conveyance at the end sections. It is desirable that flow be confined within a simple trapezoidal channel, because n values have been determined for such con- ditions. However, compound channels ca.n be used if they are properly subdivided. The reach should b8 contracting rather than ex- panding if there is a choice. Straight reaches are preferred, but they are seldom found in nature. The method assumes that the cross-sectional area is.fully effective and is carrying water in accordance with the conveyance for various portions of the section, For this reason it is desirable that the cross section be uniform for some distance abov8 the reach, so that discharge will be distributed in accordance with channel depths, roughness, and shape. Conditions, either upstream or downstream from a reach, which will cause an unbalanced distribution should be avoided. For example, for some distance downstream from a bridge which constricts the width, the effective flow will be contained within the center of the channel; the sides of the channel will not carry water in proportion to the computed con- veyance and may even have negative velocity. Natural channel constrictions or protrusions may have the same effect. A sudden deep- ening of the channel may also represent a non- effective area. Such situations should be warehed for and .avoided as slope-area reaches. Sometimes slope-area reaches must be se- lected in mountainous areas where the channels are very rough and steep and may have free (8)