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<br />EM 1110-%-1{08 <br />1Mr.reh1~ ( <br /> 15 <br /> , <br /> Legend No. of K .6.t X <br /> Subreaches <br /> -- 8 1/2 1/2 1/4 <br /> 4 1 1 1/4 <br /> ----- 2 '_ 2 2 1/4 <br /> 10 <br />ILl <br />t!l <br />a: <br /><l: <br />:I: Outflow <br />u <br />C/) 5 <br />Ci <br /> <br /> <br />o <br />o <br /> <br />15 <br /> <br />20 <br /> <br />5 <br /> <br />10 <br />TIME <br /> <br />Fi~re 8 <br /> <br />and the routing is b&-<ed on six subreaches for ,~ach of which the ve.lues of l>t and K were taken as four <br />hours. An X ,'alue of 0.3 gave a closer reproduction of the measured hydrograph. at Trinidad than <br />values of 0.2 and 004. The routing was made with the Use of routing constants of ple.te No.2 e.s the <br />procedure was far more convenient than routing through six subreaches by the commnnly used procedure <br />of the coefficient method. <br />It is desirable to evaluate X from several sets of data. If no de.ta are available, the s,.lection must <br />be based on judgment, derived from determinations on other streams, allowances being made for the <br />effects of re~ch length, flood-wave travel time, and time intervall>t. Also, it is desirable to retain the <br />value of 6t which was used in the determination of X in any other applications of flood routing in the <br />reaeh. If 6t is changed, generally the value of X also must he changed. <br />c. Variable X: Although X generally is taken as a constant for flood routing, it is evident {rom figure <br />2 and plate '"'0.4 that X for a given reach may vary during a flood. Also, the relationship between X <br />and storage or outflow may vary {rom one flood to another. If the variable nature of X must be deter- <br />mined, it generally may bc done by estabLning by trial and error a reasonable curve of 1\ versus 0 which <br />will reduce the loops of the storage discharge curves practically to a single line. It is occasionally neces- <br />sary to derive X from data based on the amount of storage between steady-flow and unstee.dy-f1()w <br />profiles. Flowline profiles in the reach may be determined by computations or field surveys first for a <br />constant flow and secondly for discharge increasing directly with the distance from one end of the reach. <br />The storage computed from cross sections under the steady-flow profile is the prism storage KO. The <br />difference in the stDrages under the two profiles having the same discharge and stage at the downstream <br />end of the reach is the so-called wedge storage, KX(I-O). .A complete determination for a'reach will <br />result in a family of curves relating 0, I and X. Generally X varies more with 0 than with I, in which <br />. . <br />case X, like K, may be represented by a single curve showing X Versus O. <br />The pre,e.ding method is sometimes the only practicable approach for developing X and K values <br />for rouling flo.. in a primary stream affected by t,iblltary inflows or by other independent variables. <br />In the case of tributary influw, curves of X and K venls 0 may be developed first for the primary stream <br />for several values of lributar.r inflow and secondly for the tributary for several values of main"'etream <br />discharges, . <br /> <br />10 <br /> <br />\ <br /> <br />1- <br /> <br />i <br />, <br />\, <br />