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, . .1 ROUGHNESS COEFFICIENTS FOR HIGII-GRADlENT CIIANNELS IN NEW YORK STATE William F. Coon' BSTRACT __ Manning's roughness coefficients (n values) were compu~d for a range of ~OW5 at six slreams in New York Slate (USA) thai have high energy gr'.Kllcn~ (gl'Ca~r ~1~ d I cd'an bed panicle sizes (betwccn 0 14 and 0.36 meters). c coo IClcn s. O.~2) an ~1e m o~ 100061 for bankfull nows. ~erc computed from dischatgc~.channcl We~~~~rang:nd :er:surfacc profiles measured at each ofthc siles. An invcrse,relauon g IfYthe' aI and flow dcplh (or hydraulic radius) was evident allive SlleS. 1bc . betwccn n v lACS ..' d pIh and roaches an asymptouc u hncss coefficient decreases rapidly wllh ulcreasmg c app :1:': as bankfull now is approached. At individual siles. the stope (w.a~r.surracc sl~.PC ;n: energy gradient) generally incrcasc., with increasing now ~~h :d~ ~;r.:~wre a a~~rc~1 the n value. Among lhc sites. however, me roughnes."i toe I:e a 0008 and 0 014 were relation The near-bankrull n vaJues on chaMels wllll slopes lween . . Ii . Ihc eorOOS 1O 006 whereas lhose on channels wilh lower slopes, bclween ~c~ra ~ I~ 005 ~:rc in rhe 0.03:to-0.04 range. Slrcambank vegelalion has a .measurable frecl on ~ ~ led n values atlwo or the siles. Dense vegclallon on o~ Side or ~he :~~~~;~;:.t.=I~~:.e;~~;:'~~e~lri~~;;2l"~t~~~;:e~:;~r~~~rIO Ihe ~:.:: ;~:t:~::~~c::~c ln~:~SsbY as ?,u~h as 0;012 rrom nong~~in~ l~ nc i s The effect of lIle vegelalion decreases wtlll increasing now dep'.h as a resu I grow ng season. . the ve ctalion becomes submerged and bends wllh the now. ~~~s~==::~~ =: ~~re ~puted around 1950 ror &hrcc of the sludy sites, are reasonably close to those: compulCd in this sludy. INTRg~~~a~i~~ of slream discharge or nood~waler ele~alions require ~va~ualiO~h~e~~C now-rclardin characteristics or stream chaMels and their banks. Manmng ~ roug .. r coefficient (ngv.al~e) is c~monly us Sed ~ tiusan~tat:~eg:~~e::~~~t(~~~~~~~l~~oe~~~:i~n these charactcnsucs. Dunng 1983-8 ,t .. eo d . mpule ~i~~~r:g~e;u~~~~t~~~~:~::~:: ::I:Cr:~~::r:~~~~i~~~~':a~t:~~~i~ ;;7~~~~a~~~ of New York streams. This paper presents the Mannmg s-roughncss-coe. tClen cscribeS Icctcd from si.. high.gradient channels that were Included in !he study. I~ al.50 (I) dw'de It; the relalions between lIle n values and twO sclectcd hyd~uhc c~acte~SlIc~ ~~~ITici~lIS; and slope); (2) discusses the crrect of strcambank Yegetauo~ on t roug PeS d for three or and (3) provides a comparison or the computed n values With &hose compute thc sites during an earlier study. tHydrologist, U.S. Geological Survey, 903 Hanshaw Road, Ithaca. New York 14850. 122 -- . ROUGHNESS COEFFICIENTS-NY ST ATE 123 DATA COLLECTION AND COMPUTATION All siles were near USGS strcamOow~moniloring sial ions and weI'C selected on the b.1sis of crill'ria presented in Dalrymple and Benson (1961) ror indin:cl measurement of discharge by the slope-area method. kcaches wen:: relalively straight: had unifonn shape and minimal overbank Row: and were devoid or major Oow-rctanling ractOf5 other than boundary friclion from bed and bank materials, which unavoidably included sueambank vegeuuion. The reaches were inSlnuncnlcd with USGS crest-stage gages (Rantz and otbers. 1982, p. 77) to rel.'Ord water-surracc clevaHons al two or three cross sections within each reach. Discharges associated with rcconJcd waler-surface proliles were obtained rrom the discharge record of Ihc nearby stream now-monitoring stalion. A compuler program developed by Jarrell and PelSch (1985) was used to compule lhe values of Manning's roughness coefficient. This program is ba.lIed on !he equalion presented in Barnes (1961) and is a modification or the equalion rOt computing discharge by the slope-area method as presented in Dalrymple and Benson (1967). Hydraulic dala and computed roughncs.~ coefficients Cor various nows at the liix study sites are given in Table-I. The tabulalcd values for area, stream-top width. hydraulic radius. velocity. and Froude number are aver-oIges or values computed Cor each cmss section within a reach. Roughness coefficients that were compuled in 194K or 1951 for three of the sites-Esopus Cn..-ck at Coldbrook. Beaver Kill al Cooks Falls, and East Branch Ausable River at Au Sable Forks-are included In Table I ror comparison with the recent cOmptllations. DATA ANALYSIS Jarrell (1985) and Sargent (1979) showed lhat the roughness cocfficienl for high- gradicnI channels willl small relative smoolimcss is cxpeCled to decrease rapidly whh increasing now deplll (hydraulic radius) and to approach an a~ymplotic value as bankfull now Is approached. The n values atlhe Sludy siles generally followed this ,rend (fig. I). The roughness coefficient decrea.~es suOOlanlially (by more &han 0.015 rrom low-flow to bankfull condilions at four of Ihe siles) wilh increasing now depth. 11lc n values computed for low- now conditions, when Row depths are insurftclenl for full development of the vclocity proRle, renectthe effcct of energy-loss faclors Dlher lhan boundary-layer friction. lbcsc factors, primarily the combined fonn drags from individual rouglmcss clements (Dathursl, 1978), eonlribute subslanlially more to the roughness coefficient and produce larger n values lI1an would result from higher flows. The roughness coefficient Is related to !he slope (boI.h water.surface slope and energy gradient) in two ways. On individual channels, as now depth increases (and n values decrease). slope generally increases (fig. 2); !his inverse relalion between n and slope was evident at many sites studied by Janelt (1985) and Hicks and Mason (1991). Among the sites, howevcr. slope and Ihc n value generally show a direct rclalion for ncar-bankfull discharges. The rouglu1css coefficients for such nows on study-site channels Wilh slopes between 0.008 aad 0.014 were generally in the range 0[0.05 to 0.06, where.. \hose on channels with lower slopes. between 0.004 and 0.005, were in the 0.03-to..o.04 range (lig. 3). This posilive association between n and slope has been subslatlliatcd by previous research (Bray. 1979; Riggs, 1976: Jarrell, 1985). Streambank vegetation had no measurable cffccl on me roUghncRS coefficients or most of the high-gradient channels. 111c effect or vegetation is evldenl at Beaver Kill at Cooks Fall (fig. 4). however. where vegetation growing on one side of the channel inteRUpts lhe decreasing trend in the roughness coefficienl as water levels risc. Also, the summertime growth of vegetation al Tremper Kill near Andes causes a subslanliaJ increase (as much as 0.012) in the n values (fig. 5). As the water level rises and Inundates the vegetation, higher- than-expected n values arc produced. at least up to the point of vegetation SUbmCtgCllCe. (Note: All curves in the figures are hand drawn.) The roughness coefficients at three or the study siles (nlble I) were previously computed and presented in Bames (1967). If small differences between lite computed n values rrom the two studies can he auribmoo 10 differences in actual site location..lI on Ihe respective sire8ms,the n values compuled for both studies ilIt' in close agreemenl.