Irrigation Engineering the Lacey’s Theory

Lacey’s Theory

1. Irrigation Engineering Lacey’s Theory

2. Lacey’s Regime Theory Lacey followed Lindley’s hypothesis: “dimensions and slope of a channel to carry a given discharge and silt load in easily erodible soil are uniquely determined by nature”. According to Lacey: “Silt is kept in suspension by the vertical component of eddies generated at all points of forces normal to the wetted perimeter”. Regime Channel “A channel is said to in regime, if there is neither silting nor scouring”. According to Lacey there may be three regime conditions: ( i ) True regime; (ii) Initial regime; and (iii) Final regime.

3. True regime A channel is said to be in true regime condition if it is transporting water and sediment in equilibrium such that there is neither silting nor scouring of the channel. It may be seen that for a channel to achieve regime condition following three conditions have to be fulfilled: i . Channel should flow uniformly in “incoherent unlimited alluvium” of same character as that transported by the water; ii. Silt grade and silt charge should be constant; and iii. Discharge should be constant. Final Regime If the channel parameters such as sides, bed slope, depth etc. are changing according to the flow rate and silt grade then it is said to be in final regime condition. Initial Regime A channel is said to be in initial regime condition when only the bed slope of channel gets affected by silting and scouring and other parameters are independent even in non-silting and non-scouring velocity condition. It may be due to the absence of incoherent alluvium.

4. Limitations of Lacey’s Theory: The Lacey’s work is based on field observations and empirically derived equations and therefore it cannot be said to be theory in strict sense. Regime equations in their derived from cannot be applied universally as they hold good mostly for the regions whose data was taken for study. Like Kennedy’s theory even though perfect definition of silt grade and silt charge is not given most of the equations are based on the silt factor ‘f’. In practice regime condition stated by Lacey is very rarely achieved and that too after a long period. The field observations have shown limited acceptance of the concept of semi-elliptical section of a regime channel. Complex phenomenon of sediment concentration and transport has not been scientifically considered.

5. Fig: Lined Canal and Unlined Alluvial Canal