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<br />B,9 <br /> <br /> <br />, <br /> <br />Q = 1,486 a r 2/3 s 1/2 <br />n 0 <br /> <br />(ll,l) <br /> <br />Let Kd = 1.~86 a r 2/3, then <br /> <br />Q = Ko. So 1/2 <br /> <br />(B,2) <br /> <br />Assume the conditions are such that it is desirable to recognize <br />more than one subdivision, each having a different n, Let subscripts <br />1, 2, and 3 refer to the section subdivisions and subscript t to the <br />total section, <br /> <br />From equation B,2 <br /> <br />Q =(Ko.1 + ~ + Ko.3 - <br /> <br />+ Kcm) So 1/2 = EKo. So 1/2. <br /> <br />(B,3) <br /> <br />Also: <br /> <br />Q 1. 486 <br />=EKo.-- <br />1/2. nt <br />So <br />ll.t = 1.486 at rt 2/3 <br />l:Ka. <br /> <br />at rt 2/3; therefore <br /> <br />(B,4) <br /> <br />, <br /> <br />Table B,2 shows the computations for Example 2 and Figure B,3 <br />shows a plot of roughness coefficient for the complete section versus <br />depth. <br /> <br />In natural streams n normally shows a minor decrease as stage in- <br />creases up to, or somewhat above, the bankfull stage, then appreciably <br />increases as overbank stage increases. When n is significantly dif- <br />ferent for different parts of the cross section, subdivision of the <br />cross section, as a basis for making the computations, automatically <br />causes nt to vary with stage above the bankfull stage. This is true <br />although nt is not computed in methods for determining water surface <br />profiles. Note on Figure B,3 that nt, which has been computed in <br />Example 2 for illustrative purposes, shows considerable increase with <br />stage above the 10-foot depth and that this increase is automatically <br />recognized by subdivision of the cross section. <br /> <br />. <br /> <br />The plot of hydraulic radius On Figure B,3 illustrates a typical <br />characteristic of natural streams. Note that the hydraulic radius for <br />the complete section increases up to bankfull depth, then decreases <br />through a limited range of depth, and again increases as depth of over- <br />bank flow increases, <br /> <br />This example also illustrates that recognition of high retardance <br />for flood plain subdivisions by the use of relatively high n values does <br />not cause n for the complete section, nt, to be unreasonably high, In <br />this case, the channel and flood plain are assigned n values of 0,04 <br />and 0,08. The value of ll.t ranges up to 0,072 as shown by' 'Table B.2 and <br />Figure B.3. <br />