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<br />102
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<br />D7't1roa. PLOW
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<br />clAY. loam. or oiIt, !..be mu-din~ etrect ill much 1_ than "bere the ma&erial
<br />ia OO&l'lle. IUch as pvela or boulders. When tbe material is fine the
<br />value of n is 10... and rmatinly un&lfected by chanr;e in 80'" star;e. When
<br />the material coneists of sra velo and boulders. the value of n is r;enerally
<br />~. particularly at low or hiP au,p. Larser boulders lI8I1ally collect
<br />at the boUom nf the ~. makinr; the channel bottom rour;her than the
<br />~ and incflllllin& the value of n at 10... sta<<ea. At hish ata<<es. a
<br />portion of the 0IlerrJ olllow .. 1I8lld in rollin<< the boulders d01fIllltream
<br />t1au. increMinr; the value of n. A theoretical d.iacuaion of lUriace ro""h~
<br />_ will be r;iven in Art. 8-2.
<br />B. VOfCMtiDll. Veptation JD&Y be J1ISlIl'ded &II a kind of lIIlriace
<br />roucJm-, but it aI80 markedly redu_ the C&pllCity of the channel and
<br />rNrds the 80.... This effect depends mainly on hei&ht. denaity. diatri..
<br />bution, &ad type of TO&\ltation, aDd it ia very important in deaip.ing
<br />....u draina<<e obaunela.
<br />At the UDivenity of IUinoia an invem,;ation baa beeD made to deter-
<br />.me the elfect of veptation on the ooellicient of ro""hn_122J; On one
<br />01 the cfraiDa<<e ditches in central IUinoia under inveetiKation. an averaee
<br />"value of 0.033 w.. mMWred in March. 1925, when the chAnnel w.. in
<br />pod OODdition. In April. IV26. there were buahy wi1Io...a and dry ...eeds
<br />OG the IIitj., IIopes, &Ad II W.. fOWld to be 0.055. This inCfeMe in n
<br />......~KtI the n.ult of ODe y...... p-owth of vesetation. During the
<br />..-...... of 1925 &ad IV26 there .... a thick IfOwth of cattaila on the
<br />Wtom of the c"..n....!. The n value at medium IUrllllb1" st.,,<<ee .....
<br />about 0.115. and at a _Iy bankfull atase it w.. 0.099. The 'cattaila
<br />, in the channel...ere wuhed out by the hish water in September, 1926; the
<br />.?Wllle value of" foUDd alter thia OCOWTence w.. 0.072. The concluaiona
<br />cirawn from thia invtllltiption were, in part, .. follon:
<br />1. The Jllinimum value of " that Ihould be UIed for deai<<nins draina<<e
<br />~ in oentral nHnnie ia O.lHO. This value ia obtainable at bi"h
<br />..... duriDtl; the _ mODtha in the moat carefully maintained .~~-
<br />... ...here the bottom of the channel ia cie&r of v.ution and the aide
<br />aIopee are oovtnd with If&II or low ...eeds, but no bushee. This 10'"
<br />vallie of II lIhould not be II8ed unl_ the channel is to be cleared annually
<br />of III weeds and bUlhee.
<br />2. A value of II - 0.050 ahould be uaed if the channel ia to be cleared
<br />in alternate years only. Larse...eeds and bu.ahy wi1IOWll from :l to 4 ft
<br />IU&h on the aide slopes will produce thia value of n.
<br />3. In channela that are not cleared for a number of years, the growth
<br />...y become IlO ahundant that valuee of 11 > 0.100 may he found.
<br />. 4. -rr- from 6 to 8 in. in diameter growinK on the aide slopes do not
<br />~pede the flow IlO much u do IIDaII humy Il"owtha, provided overhan,-
<br />mg branchee are cut 011.
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<br />PII'T"SI.JOP1IIllM'T or '0'".0"- n..ow 1XD ne ._~41.t.A \oa
<br />The U.6. Boil C-Val.ioo1 s...r..o 1oM.......u.. _..... of_
<br />in IIIlllI1 oba\low c~-,I. JlI"o4eo\ed by . ~ Wive ~ (CMp. 7,
<br />Ilea. C). It...... fOllDd that " val_ for ta-..... II. v..w with the
<br />, _pe &ad 0Z'0fI NCtiOG of ., eIlaaael, t.he rIope 01 Ute r'- ---.I bed MIl
<br />the depth of Bow. Coaparillc two +---1&, all oUoer f....1MiDe ~
<br />the ~ nerap depth r;iv.. the IliPer " vaNe, -a.c \0 a J.rpr
<br />proportion of &leeted "....t.~ion. nUl, a triM!&'.I...L ...J...
<br />hiPII' " vahie tban . tn.,u~.1 +--~1. &lid a wide r----<< 11M .
<br />lower 11 value tIIan a ..-row ~-_. . A low of mIic_\ chp&h MBdlI to
<br />beDd over and IIIbmerp tho .....~tioD. &lid \0 )lnIliuee low " vaI-. A
<br />steep IIope Clt.~ ~ ftIoeity, ~ h<<r... 01 . .....ltdioo.,
<br />and 1011' II valuee. ' '
<br />The elfect of vesetatioD 0II1eod pIaiM will be w.a. ..... ill i&.l H.
<br />c. CAtmMllfT.,w.itr. Chane! irncu\arity aompr;.. imIpJrritiee
<br />in wettrd perimeter &lid variaUoaa ill _ MOtion, abe, ud Ihape .me
<br />, the cllumei\en<<th. In aatun1 +---1' . Rdl ~ are --ny
<br />iatraduced by tile". Ie of IUd bin, aod........ ~ llIId ........
<br />siou, and hol.. and IatiJllpe OIl the ~.n""l bed. ,n.- imIpIaritiee
<br />definitely introduce ro""m- in addition to that .-l by wrfaoe
<br />rouKhne. &lid other factara. Genen11y III r "'''1, a srad-l &ad lIIUlonn
<br />ohance in CI1lfl1 1ClCtion, _, &lid Ihape wi1I not &PI'''"'' ~y &I. the
<br />nlue of n, but .brupt +- l'1li or a1\erDa\ion of sma1I .. Iarp ~
<br />aeeellitatee the UMl of a Iarse value of "- In thi8 _, . __ ill 11
<br />_y be 0.005 or more. Qlo.-..... tba\ Clt.UMl ain~ .,., from aide \0,
<br />Iide of the ch.&nne\ will prod_ the _ elfect.
<br />D. CAtmnd Alip-,. Smooth ourvature with \arp 1Ildiu. will am
<br />a relatively low value of 11, wbereu rMrp ourvatue with .....-e _..d...
<br />iBcwi1l incr_ II. On the bMia of lame teetI, Boobq (33J NllMlbd
<br />that the value of " be in~r_d 0.001 fill' llIlCh 21....._ of IlUrYMw'e in
<br />100 It of vhauael. A1thoG1h it . doabUul 1rfaetber OIIl'ftWre ever
<br />m- II more tban 0.002 or 0.003, it..l. IiIoWd .. be ipcJnd, for
<br />~urvature may induClll the _ulation of drift and tIalII itsdirevtIy
<br />mcr_ the value of II. a-aI1y apeal-l"C. the in.- ofl'Olllm-
<br />ia unlined cbannela Cl8I'I'yinc ....ter .\ low velovitiea ia DClicible. An
<br />~ of 0.002 in " value would ooaati.tute an adequa\e alIo..._ for
<br />curve 10BI0lI in moat f111J1l1ll1 _t.a.n.nc pronounced curva\urea, ...hether
<br />built of concrete or other matlliala. The meanderin& ol JI&Wr&\ atreama,
<br />however, may increase the "value all high u 30%.
<br />B. Sillifl/1 Grad 800uring. Generally epea1dnC, 1liIt.inc may ohaace a
<br />very irl'lllUiar channel into . comlW'&tively uniform one aod decr_ ft,
<br />wbereu IC01II'inI may do \be r8'Ill1'IIll e.nd ~ 11. However, th~
<br />d, minant e1rect of IliItinc will depeDd on the natu.re of the m&tem.:
<br />deposited. Ur.evel1.depositot IlUch sa SII:ld ~... G;": [;..:,: I'i".",~ ::r,
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