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<br />v <br />" ' ~: <br />" <br /> <br />70 <br /> <br />001870 <br /> <br />DESIGN OF SMAll <br /> <br />Flood <br /> <br />I <br />I <br />~ <br />r <br /> <br />DAMS i <br />NG 3 ~", .,...~'" <br /> <br />Purpose: A time of concentration from which 0 log time con be computed <br />must be obtained for hydrogroph construction representing runoff from <br />a walershed. Various methods of estimating time of concentration,Tc,are os fallows, <br /> <br />"O,ot: <br /> <br />A. EST IMATING Te FROM STREAM HYDRAULICS (S C 5 GUIDE) <br /> <br />r. Obtain stream reaches and channel cross-sections from field surveys. <br />2. Find oppro.imate channel bankfull discharge for each reach. <br />3. Compute overage velocity for the bankfull discharge of each reach. <br />4. Use the average velocity and the volley length of the reach to compute <br />travel time through each reach. <br />5. Add travel times of reaches to get Tc. <br />Note: Appendix B "Hydraulic Computations" presents methods of computing <br />flows in natural channels. <br /> <br />30pe <br /> <br />20,oC <br /> <br />10,00'" <br />a,DC <br /> <br />~ 6,00 <br />'" <br /> <br />.f: 4.oC <br />...J <br />3,00 <br /> <br />2,OC <br /> <br />B. ESTIMATING Te FROM VELOCITY ESTIMATES AND WATERCOURSE LENGTHS <br /> <br />I,oe <br />8C <br /> <br />Velocity Estimate Guide <br /> <br />U.S.Novy - Technical Publication <br />Navdacks TP-PW-5 <br /> <br />Table 89. March 1953 <br /> <br />Average slope of channel <br />from farthest point to <br />outlet, in percent <br /> <br />Average <br />velocity, <br />eet per second <br /> <br />I to 2 <br /> <br />2.0 <br /> <br />2 to 4 <br /> <br />3.0 <br /> <br />4 to 6 <br /> <br />4.0 <br /> <br />6 to 10 <br /> <br />5.0 <br /> <br />6C <br /> <br /> Te1.os Highway De part me nt <br /> Rational Design of Culverts <br /> and Bridges, October 1946 <br /> Average velocitY,feet per second <br />Slope Woodla nds Pas tures Natural <br /> in (upper portion upper portion channel not <br />percent '- watershed) well defined <br />watershed) <br />0'-3 1.0 1.5 1.0 <br />4 - 7 2.0 3.0 3.0 <br />8 - II 3.0 4.0 5.0 <br />12 -15 3.5 4.5 8.0 <br /> <br />I <br />, <br />, <br />,. <br />,. <br /> <br />Figur" 30, Time of concentration estimate$. (Sheel 1 of 2.1 288-0-2461. <br /> <br />--. <br /> <br />- <br /> <br />:~ -...- i;' 7-_ <br /> <br />+ <br />