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' DESIGN OF CHANNELS FOR UNIFORM FLOW 1fiS of Reclamation and is tentatively recommended for design of erodible ~annefs. It should be noted that either method at the present stage Rill ve onl}• as a guide and will not supplant experience and sound engi- neering judgment. T-9. The Maximum Permissible Velocity. The maximum permissible Loci[y, or the nanerodible uelaci[y, is the greatest mean velocity that will t cause erosion of the channel body. This velocity is very uncertain and variable, and can be estimated only with experience and judgment. general, old and a•e11-seasoned channels will stand much higher veloci- TABLE i$. Af.tCl][UM PERMIHeIBLE VELOCITIES RECOMME\DED BY FORTIER AI:D SCOHEY AND THE CORRESPONDING LNIT-TRACTR'E-FORCE VALI'ES ' GON\'ERTED BY SHE L.S. BUREAU OF RECLAMATION' (For straight channels of small slope, after aging) ' 9leterial Fine sand, colloidal ... ... .. ......... ~ O.U20 Sandy loam, nonrolloidal .. .. .. ! 0.020 . ' Silt loam, noncolloidal ........... ...... I 0 0?0 .411uciel silts, noncolloidal .. .. .. .. .j 0.020 OrdinnrY Firm loam .. ........ ..... .. I 0 0'20 4'olcanic ash .. .. ... .. .. 0 0?0 ' Still clap, cen' colloidal .. ......... ... . ~ 0.0'-'S Alluaial silts. colloidal .......... ........ .! 0 025 Shales and hardpans . .............. .~ 0.025 Fine grarel.... .. .. .. .. .. .~ 0.0'20 Graded loam to cobbles when nonrolloidal. . i 0 030 Graded silts to cobbles when colloidal ... .' 0 030 Coarse gre ccl. nonrolloidal .. .I 0 0'-5 Cobbles and shingles.. .. .. .... . ~ 0 035 IJ R-ater trans- Clear aster porting col- IDiaal gate V, fps ' ro, lb/[t• ti•, fps - re, Ib/fJ [ 50 0 0?i ? 50 O.OiS 1.75 0.03% '- 50 I 0 0;5 1 3.00 0 048 3 00 1 0.11 ^_ .00 1 0.048 3 .50 ~ 0. I S '? . 50 1 0.0; 5 1 3.50 0.15 2..50 i 0 0;5 3.oU 0.15 3.75 0.26 5.00 0.46 3.i5 0.?6 5.00 0 46 6.00 0.67 8.00 O.6i I n_.50 0.075 5.00 10.32 3.75 i0.3S S.W 0.06 I ~ 00 0 43 5 . SU 10.80 400 1 O30 ! 600 ' O.6i IS00 i ', 0.01 11550 i ~~ 1.10 'The Fortier and S'cobe}' values x~ere recommended for use in 1926 b}' the Special Committee on Irrigation Research n[ the .tmerican Societ}• of Ci\•il Engineers. ties than new ones, because the old channel bed is usually better stabi- lized, particularly with the deposition o[ colloidal matter. Rrhen other conditions are the same, a deeper channel will convey water at a higher mean velocity without erosion than a shallower one. This is probably because the scouring is caused primarily by the bottom velocities and, for the same mean velocity, the bottom velocities are greater in the shal- lower channel.