Design of Canals

Lined and Unlined Canal Design

Important Terms

Alluvial Soil

Non-Alluvial Soils

Silt Factor

Silt Factor

Coefficient of Rugosity (n)

Coefficient of Rugosity (n)

Mean Velocity

Critical Velocity

Critical Velocity Ratio (C.V.R)

Regime Channel

Hydraulic mean depth/ratio

Full Supply Discharge

Economical Section

Irrigation Canals Irrigation canals are of two types Lined canals Unlined canals In Pakistan many of the canals are unlined In Pakistan many of the canals are unlined

in nature. In designing of both of the canals there are

some concepts common to both and willbe discussed first.

Irrigation canals Both the types of canals are designed for

uniform steady flow. Mannings or Chezys equation describe

such a flow i.e.such a flow i.e. Mannings Equation V = (1.49/n)R2/3S1/2 (fps units) V = (1/n)R2/3S1/2 (S.I. units)

Irrigation Canals The Chezys formula is given as

V= C(RS)1/2 Where V is velocity R is Hydraulic mean radius (A/P) R is Hydraulic mean radius (A/P) A is Area of cross section P is wetted perimeter S is the longitudinal energy line slope, in case of

uniform flow it is equal to bed slope n is Mannings constant C is Chezys coefficient

Irrigation canals The equation describes that the discharge

Q=(AV) in uniform flow is a function of

Section factor AR2/3 in case of Mannings Section factor AR2/3 in case of Manningsequation and

Section factor AR1/2 in case of Chezysformula

Longitudinal Slope (S)

Best Hydraulic Cross Section The best hydraulic cross section is one

having minimum parameter for a givenarea i.e. this will give maximum dischargefor a given area.for a given area.

Among all the cross sections, the besthydraulic cross section is a semi circle.

The proportions of best hydraulic sectionin case of rectangular, trapezoidal andtriangular channel are given

Best Hydraulic SectionCross

SectionArea (A) Wetted

Perimeter (P)

H.M.R (R) Top Width Depth

T D

Rectangular 2.0y2 4y 0.40y 2y yRectangular 2.0y 4y 0.40y 2y yTrapezoidal 1.73y2 3.46y 0.50y 2.31y 0.75yTriangular y2 2.83y 2.345y 2y 0.50y

Non Silting-Non Scouring Velocity and Permissible Velocity

The term minimum permissible velocitymeans a lowest velocity which will preventsilting and vegetation growth. Normallyaverage of 2-3 ft/sec (0.61-0.91 m/s) willaverage of 2-3 ft/sec (0.61-0.91 m/s) willprevent silting.

The term non silting - non scouringvelocity refers to a velocity which will behigh enough to prevent silting and lowenough to prevent scouring in case ofunlined channels.

Free Board A minimum free board for unlined canals

varies between 1 ft or 0.3 m for smalldistributaries to 4 ft or 1.2 m for maincanals carrying 3000cfs (85 m3/sec)canals carrying 3000cfs (85 m3/sec)discharge.

For irrigation canals with discharge of10,000cfs (283 m3/sec) or more it is 5.5 ft(1.65 m)

Free Board For determination of free board the increase in

the discharge is on log scale and the increasein the free board is on arithmetic scale. Theequation is

F= (Cy)1/2 F= (Cy) where F is free board in ft y is the design depth of flow in ft and C is coefficient which varies from 1.5 at

Q=20cfs (0.57m3/s) to 2.5 for Q=3000cfs ormore.

Free Board The curvature is important in lined canals as the

rise in water levels on outer side of the curve affects the free board.

h = v2b/gR Where h is change in water surface elevation across h is change in water surface elevation across

channel b is the width of channel R is the distance from the centre of the curve to

centre of channel v is the subcritical average velocity

Design of Lined Canals The design of lined canals is very simple

as long as there are no constraints ofselecting higher or lower values ofvelocities.velocities.

As long as Mannings n or Chezys C isestimated correctly. For the given liningmaterial, the canal would work as perdesign.

Line Canal Design Procedure 1. Estimate C or n for given lining

material 2. Compute as per Mannings., section

factor AR2/3=[nQ/(S)1/2] is 1.49 in fps and 1 in metric units. 3. Assume the shape of the channel to be

trapezoidal section with suitable side slopes of 1:2 (vertical : horizontal) and bed width b.

Line Canal Design Procedure Using dimensionless curves Fig 4.1 (book

Iqbal Ali) between AR2/3 / b8/3 and y/b, thevalue of y can be determined.

4. For the best hydraulic cross-section, 4. For the best hydraulic cross-section,channel parameters can be used as givenin table 4.1. otherwise use normalrelations for A etc to compute the channelparameters using y from step 3.

Curves for Determining Normal Depth of Flow

Figure 4.1 from Iqbal Ali Book (Page 120)

Line Canal Design Procedure 5. Check for

Minimum permissible velocity if water carriessediments

Froude number to be less than 1 Froude number to be less than 1 6. Estimate free board, and the height of

lining to be 50% of the free board, abovethe maximum water level.

7. Make a sketch providing all thedimensions.

Example 4.1 A lined canal is to be designed to carry

1000cfs. The concrete lining is smoothand the general ground slope is 0.001.Design the canal.Design the canal.

Assignment A lined canal is to be designed to carry

1000+(Regd. #)cfs (e.g. Regd. Number 40will get the value of discharge as 1000+40= 1040cfs). The concrete lining is smoothand the general ground slope isand the general ground slope is0.001+(Regd. # / 10000), (e.g. Regd. # 40will use slope as (40/10000) = 0.004,hence final answer will be (0.001+0.004) =0.005. Design the canal.

Assignment Also with the above information given in

example 4.1 repeat the design for thetriangular shaped, circular shaped andrectangular shaped canal