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<br />MANNINGS'" VALUE J)ETERMINATlON <br /> <br />8ESIP€S ASSI6NIN& ';' H vALUeS ()/\/ THe BASIS OF <br />P/cL!> INsPtcr,()N OF TIlE Fl..OO/>PLAIN AReAS, THE <br />Ft>t..t.OtVINt; Mt77Io!> CtJ1UNE!J ON mE NE}ff sll-t-t-/j <br />oP nllS' A/>i){;1J1JUI1 tWfS ,t{-tS"c /MPlGMe.JlFb IN THE' <br />!;ErFRMIN4rlOAl OF "/J'" VALU{;3" ,lOA? 7lI1s S"rl//J V'. <br /> <br />-. <br />~ <br /> <br />PI:~PIaNT or 'OlCD'OaK rLOW ~ 1TB I'OUI'l1LAll 101 <br /> <br />for n between 0.011 ADd o.()((). For practical purpoees, the following <br />approximate forma of Eq. (5-9) are genera.llyauggested for uae: <br /> <br />l' - 1.5 Vil for R < 1.0 m (5-10) <br />l' - 1.3 Vil for R > 1.0 m (5-11) <br /> <br />5-T. DetermiDalion of KllDDlDc'c Rouclu>e" ('_.......1. In applying <br />the Manning formula or the G. K. formula, the pw.teet di1IicuIty lies <br />in the determination of the roU&lm_ coel'icient n; for there is no exact <br />method of &electing the n value. At the preeent star;e or Imowledt;e, to <br />oelect a value of n actually me&ll8 to estimate the resiataDoe to /low in a <br />, given channel, which is reaIIy a matter of intanpbles. To veteran <br />engineers, this means tie exercise of sound encineering judgment and <br />experience; for ~eCJ, it C&D be no more than a <<U-, and cWrerent <br />individuals will obtain .tifferent results. <br />In order to give KUid i nee in the proper determination or the ro"lhn_ <br />coeffoient, four r;eneral approaches will be diacUMed; namely, (I) to <br />UDders'ADd the factors that affect the value of n ADd thus to acquire a <br />basic kttowledt;e of the proble'll ADd narrow the wide r!'nore of <<u_work, <br />(2) to coWi...!~ a table of typic..! n values for channels of vanOUb type.;. <br />(3) to .....mine ADd become acquainted with the appe&r&llce of IOme <br />typical cuaIUCodls whooe roucim- coefficients are known, ADd (4) to <br />determine the value of n by AD analytical procedure hued on the theoreti- <br />cal velocity distribution in the channel Cl'08a IICCtion and on the data of <br />either velocity or rour;hneos measurement. The firat three approaches <br />will be given in the next three articles, ADd the fourth approach will be <br />taken up in Art. 8-7. <br />U I'acto:a U~ )(.-'"'I'a JloTw_. ~--"''tDt. It ia not <br />UIlcommon for eD&ineere to think of a channel as haviDla ainc1e value of <br />" for all Q"O..ions. In nality, the value or n ia hichly variable aDd <br />depends on a number or factors. In lIe!ectinc a proper value of n for <br />vario... deaip conditions, a buic knowledce of tbMe factore lIhould be <br />fOUlld very uaefuI. The factora that exert the createat influence upon the <br />coefficient or ron&/>n_ in both artificial ADd J1&tural ~".n....l. are there- <br />fote deecribed below. It ahouId be DOted that tbMe factors are t.o . cer- <br />tain extent interdependent; hence diacUMiOD about one factor may be <br />repeated in O('nnectinn with aDOther. <br />A. Bvrf- Bout"""'. The lI1U'face ronvn'M8 is re~t.ed by the <br />Iliac and ape of the p-aina of the ID&terial fCl1'll1in& the wetted peritneter <br />&lid producinc A ret&rdiD& ellect on the /low. This is orten 00D&idered <br />the only factor in eeIectiDc a 1OIl&hn- _~~nt, but it ia actua.1ly <br />jll8t ODe or aeveral major factora. Generally ,,-1.i"l, tiDe p-aina reault <br />in . relatively low value of " and ~ crama. in a hi&h value of n. <br />In alluYial atre&ma wbere the ma.terial is tiDe in crain. auch as 1I&Dd, <br /> <br />~ <br /> <br />r <br /> <br />~ <br />