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<br />, <br /> <br />. <br /> <br />. <br /> <br />i <br /> <br />An Analysis of Flow VelocilY Profiles. <br />StreaJIl Bed Roughness. and Resis\aDCe to Flow <br />in NaluraJ Gravel Bed StteamS <br /> <br />Normand B. Bergeron' <br /> <br />Ab,UDil:l <br />The char:u:terisUcs of 82 now velocilY profiles were analyzed from nine differenl <br />gravel bed slrC8lll reaches. A sllltistical1eS1 of !he linearilY of now velocily profIles <br />indicates thai conlr8ry to whal is frequenlly assumed. velocily profiles are often not <br />semilogarilhmic but segmented -- thai is. !hey can be divided in Iwo (or more) <br />scmilog-linear segmen'" The resul.. show thai scgmented velocilY proliles are 1\0\ <br />confined to streamS with very coarse bed material. but that they also occur in streams <br />with relatively small bed particles. Segmented velocily profiles were modeled usiag <br />a technique called spline modeling. This technique allowed !he objective identificalion <br />of scmilog-linear velocily profile scgmen.. and of !he corresponding kaoIs where they <br />joia. An analysis of !he paltern of flow over natural bed obslllCles revea1ed that <br />velocilY profile segmen.. correspond to distincllayers of !he now adjusted to different <br />scales of roughness: a grain toUghness ocaJe conltolliag the shape of near-bonom <br />velocily scgmenl and a bedfonn roughness scale controlling !he shape of !he scgmenl <br />located immediately above. <br /> <br />Inlroduction <br />MooI kaOwledge concerning velocily profiles and resistanee 10 now has been <br />obtained from carefully designed laboratory invcstigalioas of now over a nal boundary <br />or a boundary covered with closely packed small unifonn roughness elemen... In <br />such cireumSIanCCS. a semilogarilhmic velocily profile haa been sbown to develop ill <br />!he 10wermOSl IS 10 20% of !he flow prolile. In nalural open channel nows. il is also <br />frequenOy assumed thai !he scmilogarithmic model provides a good approximation of <br />!he velocilY profile over !he entire now depth (Middlelon and Snuthard. 1984). <br /> <br />'professeur sous octroi. Universiu! du Quo!bee. INRS-GOOressourees. Complelt <br />scientilique. 2100. rue E1aslcin.Casc postale 1500. Sainte-Foy. Qutbec. Canada. GIV <br />4C1. B-mail: nbergeron@gsc.emr.ca <br /> <br />692 <br /> <br />. <br /> <br />A.OW VELOCITY-ANALYSIS <br /> <br />693 <br /> <br />Although the assumption thai velocily proliles are semilogarilhmic is still be' idel <br />used. recenllield dalll have demonstrated that veloc,'IY liiles . IDg w Y <br />sire f ... pro.. naluraJ gravel bed <br />RO=s ::O~ ~I :"","o~anlhm'c (e_&- Jarrell, 1999; Marchand ., al.. 1984; <br />occur ~nly in' slreamso ;f;a:.o;; l'::ss:":'::: n:-:;:m\';'garilhmic. velocily profiles <br />vel~ilY p?,fiI,:" of gravel bed slreams are scare::. it is'ditr:::' Since '::,Ill on !he <br />semlloganthmlc profilc.li occur in such streams TIle oals f ~sess w ther non. <br />assess whe!her segmented velocily profiles are ;""'li~ to v 0 this paper are (I) .'0 <br />streams or occur more widely in gravel bed streams and (2) 10 ~:e ~ malenal <br />now structure in gravel bed streams. pal&em of <br /> <br />Mclhodnlory <br />Flow velocily profiles were obtained for nine gravel bed stream I ted' <br />~:a:f(~::1e Vlo)rk. svermonl. and New Hampshire and in the prov~::f Q~:': <br />. treanl reaches were selected so as 10 co . de <br />relative submergeace (now deplhlbed malerial size) Al h .veter ,a WI ran8e of <br />1'1 lil . eac 51 . lrom seven to len <br />YO oc, ~ pro I es were recorded aloag a 10ngilOOinal straighlline COJTCS 00' . <br />~':~'=i;:::~~':l~bed ~:rofile consis~ of ten mean velociJ: s.:::,sp:;~ <br />(EMCM) model 523 Each el ~ a Marsh McB.rney electromagnetic currenl melcl <br />. v ocuy measuremenl was made al a fi f 5 H <br />~:P~i::: ranging. fro~oo60 10 300 s. Grain size dislributioas w=:':';'~ned b; <br />also measuna~ 10 I~O bed panicles. The slope of the water surface was <br />this :.:;u:::.:f::::I: ::~~~0~1~: rleld si1eS and of !he data collected for <br /> <br /> Table 1 <br />GenetaI DescrIption of Selectecl Slnlltn ReachM <br />Site Bed Maleria! Relative <br /> II of Non-Lincnt Profiles <br /> Size (DSOb. coo) Submergence Entire u-st 15% <br />New York <br />I. Boquel River 11.0 2_08 4 <br />2. Callaraugus Creck 1 <br />8.8 5_16 9 6 <br />3. Ehoa Creck 8.1 5.51 4 <br />Ver,"onl 5 <br />4. umoille River A 24.0 1.63 6 <br />5. Lamoille River B 13.2 3 <br />New Ha".pshire 3.00 6 6 <br />6. Pemigamiset 26.0 2.29 1 2 <br />1. Swifl River A 22.0 <br />g. Swifl River \I 1.65 3 3 <br />Quebec, Canada 5.6 8.5g 4 3 <br />9. Rivi~re au Saumon 10.0 3.95 g 6 <br /> <br />- <br />