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Lacey's Canal Design Theory | Design Steps

This is an improved version of Kennedy’s theory. Lacey proposed that there were 3 types of regimes - True, Initial and final regimes.



As per Lacey’s theory a true regime channel can be achieved only if the below conditions are satisfied (Ideal conditions)-

  • Channel has a constant discharge

  • The channel is on coherent alluvial soil which is a loose soil that can be easily scoured as it is deposited.

The 2 other regimes are described below-

  1. Initial regime: it is the type of regime that appears having ideal conditions but not so because the width of the canal keeps on varying.

  2. Final Regime: As water keeps flowing, depth, perimeter, slope keeps varying which forms a final stable regime.


Lacey’s theory would be applicable to the final regime/true regime only. Similar to Kennedy, Lacey also introduced another term called silt factor.


Silt factor


The silt supporting power depends upon the wetted perimeter which implies the eddies from the sides are also considered. Thus, Lacey introduced a term called silt factor, f that depends upon the size of the grains.


Silt factor f = 1.76 x d^0.5, where d - the average size of particles in mm.


Now, let's see how to design a channel using Lacey's theory.


Design procedure


Step1: Calculate the velocity v using the equation, v = (Qf²/140)^(1/6).

Here Q - discharge, f - silt factor and v - velocity


Step 2: Calculate the Hydraulic depth (R) using the equation, R = 5/2(v²/f)


Step 3: Calculate the area (A) and wetted perimeter (P) from the equations, A = Q/v and P = 4.75 x Q^(0.5)


Based on the Area (A) and Perimeter (P) assume a side slope.


Step 4: Calculate the bed slope (S) from the equation below

S = f^(5/3)/(3340 x Q^(1/6))


Even though it was an improved version of Kennedy's theory, there were some drawbacks to this theory.


Drawbacks of Lacey's theory

  • Equations are empirical.

  • A single factor, f is not adequate to define the silt characteristics.

  • Silt grade and silt charge are not differentiated.

  • The concentration of silt is not included.

Now, Try this question out!

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