Presentation on theme: "DESIGN OF ERODIBLE AND NON-ERODIBLE CHANNELS. According to Kennedy the critical velocity ratio Vc in a channel may be defined as the mean velocity of."— Presentation transcript:
DESIGN OF ERODIBLE AND NON-ERODIBLE CHANNELS
According to Kennedy the critical velocity ratio Vc in a channel may be defined as the mean velocity of flow which will just keep the channel free from silting or scouring. His investigations pertain to Upper bari Doab canal in UP. m = Critical velocity ratio = 1.1 to 1.2 for coarse sand = 0.8 to 0.9 for fine sand
KENNEDYS METHOD OF CHANNEL DESIGN PROCEDURE Q = A x V
Assume a depth of flow = d, m Compute the critical velocity from kennadys formula Compute are of c/s of flow = Q/Vc Assuming a side slope of channel, say 0.5:1 compute the bed width Compute the wetted perimeter for the assumed depth abd computed bed width Calculate C from Kutters formula and then the velocity of flow by Chezys equation If the Velocity computed now is same as found by kennadys method the design depth is correct Otherwise repeat the above steps by assuming different depth of flow
CWPC PRACTICE FOR n Type of soilCanal discharge (cumecs)Value of n 1. Soil other than rock Up to to to 14 Above Rocky cuts1. When rock portion at least 15 cm above the excavated bed level is left out in working out cross sectional area to When no portion above bed level is left out 0.05 to 0.080
Channel of conditionValue of n 1. Very good Good Indifferent Poor0.03
He also defined critical velocity as non-silting –non-scouring velocity and gave a relation between critical velocities to the depth of flowing water.
The relation is, V0 = 0.55 D 0.64 (OR V0 = 0.84 D0.64 in F.P.S Units In general V 0 = CDn V 0 = Critical velocity, in (m/s) D = Depth of water over bed portions of a channel in m n = any index number
The equation has been derived on the basis of observations on one canal only, it is applicable to only those channels, which are flowing, in sandy silt of the same quality or grade as that of Upper Bari Doab system.
Kennedy later realized the importance of silt grade on critical velocity and introduced a factor m known as critical velocity ratio (C.V.R) in his equation. The equation is then written as V 0 = 0.55 m D 0.64 Where, m = C.V.R = V/ V 0
Sand coarser than the standard was assigned value of m from 1.1 to 1.2 and those finer than the standard from 0.9 to 0.8. Generally, in a system of canal, higher C.V.R. is assumed in head reaches and lower value of C.V.R is assumed towards its tail end.
The value of constant C in equation for various grades of material may be assumed as follows: Types of materialValue of C Light sandy silt 0.53 Coarser light soil 0.59 Sandy loam 0.65 Coarse silt 0.70
Type of silt Value of m Light sandy silt in the rivers of northern India 1.00 Somewhat coarser light sandy silt 1.1 Sandy loamy silt 1.2 Rather coarser silt or debris of hard soil 1.3 Silt of river Indus in sindhu 0.7 Value of m
Drawbacks in Kennedys theory Kennedy did not notice importance of B/D ratio. He aimed to find out only the average regime conditions for the design of a channel. No account was taken of silt concentration and bed load, and the complex silt-carrying phenomenon was incorporated in a single factor m. Silt grade and silt charge were not defined. Kennedy did not give any slope equation. Kennedy use kutters equation for the determination of mean velocity and therefore the limitations kutters equation got incorporated in Kennedys theory of channel design