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, . 1,,, HYDRAULIC ENGlNIiliRlNG '94 Manning roulJhness coefficient In engineering practice the Manning roughness coefficient n is probably the most commonly used coefficient to describe flow resistances. Fig.3 shows an example of the change of measured values of Manning roughness coefficient with hydraulic radius R in river siles Podedvory (PA) a tabuze (CA). Field measuremenls confirmed the change of Manning roughness coefficient a in .some cases even more t~n 100%. Conclusions I. Based on the now resistance analysis in channels with coarse-grained bed it was proved that the longitudinal slope is a proper parameter for expressing those losses thai up to now are not possible to be separated and quantified. 2. The considerable effect on flow resistance has a change of velocity head in the river reach ~ caused by a longitudinal non-uniform decelerated flow. The effect of accelerated now on flow resistance seems to be small. J. The differences between the result resistance equations d"lived us.ing the height of roughness element kt as a hydraulic roughness and those equations using grain size Di are statistically insignificant. By this way the possibility of using height of roughness protrusions for evaluating Row resistances, instead of grain size. was proved. This possibility represents a significant advantage in cases when due to the large depths in the river, the sampling of bed malerial by picking up stones is impossible. 4. horn the evaluation of field data conclusions can be made that for the calculation of rating curves in investigated river sites the Manning equation is not convenient. S. It is necessary to obtain further empirical data, mainly from further river sites having different characlers of channels. Based on these data the extending of validity of the profile equations for a wider range of geometric characteristics of the channel, for a wider range of sediments and of Row characteristics. would be possible. By Ihis way a separalion of further effects mighl be possible, 100. Wilhoul the knowledge obtained from such or similar measurements one cannot manage properly the re-naturalization of previously trained rivers, for instance. References: BATHURST, lC: Flow Resistance Estimation in Mountain Rivers. .I01m",1 of Hydralllic ElIgillcerillg. ASCE, Vol. II, No.4. Apr., 1985 BRAY, D.: Eslimating Average Velocity in Gravel Bed Rivers. JOl/malll1 Hydralllic Divisi"", ASCf~ Vol. 105, No BY 9. Sept., 1919 MARESOV A, I., MARES, K.: The Effect of Longiludinal Slope and Non-Uniform Flow on Resistance Equalion. Proceeding$ t!f lhe XXIV. JAHR CunKn:.",.., Vol. A, Madrid, 1991 WOLMAN. MG.: A Melhod of Sampling Coarse River-Bed Malerial. Tral/s. AUI/. Vol. 35, No.6, 1954 '. . Row Resistance of High-Gradient Gravel Channels R.O.Millar and M.C.Quick, M.ASCEI Abslraet The flow resis~ in terms of the friction faclor for bank-full or high in-bank flows f~r. 120 hIgh-gradient gravel-bed rivers arc examined. The friction faclor f was diVided 1010 gram and fonn cnmponenl8 and il was shown that the flow resistance d~ 10 !he grain roughness can be readily calculated by assuming k, = D,.. The flow teSlS~ due to bars, bedforms, and other channel irregularities can be large in gravel nvers even at bank-full flow. Inlroducllon The data analyzed in !his paper comes from published data ..ts from Charlton ./ al. 1978, Bray 19?9, An.drews 1984, and Hey and Thome 1986. The.. rivers ate generally descnbed as slable, gravel-bed alluvial channels. The channel slopes r~ged belween 0.002 - 0.026, and the relative roughness which is defined as !he ratIo of !he mean channel dep!h to !he median bed grain diameter, Y / D ranged belween 4.2 - 74.7. ..' The analysis is Iimiled 10 fully rough flow, and a wide channel approximalion will be .',-,umed. In the absence of sl~ong secondary currents flow is considered 10 obey the Prandli - von Karman logan!hmic velocilY disltibulion and flow resi.lance in lenDS oflhe darcy-weisbach friction faclorfcan be expressed: I =( 2.0310ge2~~y)r (I) I ~~specti~eIYI. PiI.n. Candidate and Professor, Department of Civil Engineering University or 8nush Columblll, BC. Canada, V6TI7A. . 111 T "..