Design of Canals MCQ Quiz - Objective Question with Answer for Design of Canals - Download Free PDF

Concept:

Lacey gave three independent equation after intensive research in alluvium and gave the idea of regime.

The equations given by Lacey are:

\({\bf{v}} = √ {\frac{2}{5}{\bf{Rf}}}\)

\({\bf{A}}{{\bf{f}}^2} = 140{{\bf{v}}^5}\)

\({\bf{v}} = 10.8{{\bf{R}}^{\frac{2}{3}}}{{\bf{S}}^{\frac{1}{3}}}\)

As per lacey, the wetted perimeter P is given as 

P = 4.75 √Q

Hence the wetted perimeter of the alluvial channel increases with an increase in design discharge.

Important Point:

Difference between Lacey and Kennedy theory are:-

Explanation:

Canal:

• A canal is an artificial channel generally trapezoidal in shape constructed on the ground to carry water to the fields (or irrigation fields) either from the river or from the reservoir. 
• Canals have played an important role in creating assured irrigation supplies to agricultural fields and contributed substantially to the green revolution in the country. 

Canal Alignment:

• A canal has to be aligned in such a way that it covers the entire area proposed to be irrigated, with a shortage of possible length, and at the same time, its cost including the cost of drainage works is a minimum. 
• For a detailed topographical investigation of a canal, a canal is aligned along the ridge line.
• Irrigation Canals are generally aligned along the ridge line.
• Irrigation canals are generally aligned along the ridge line as this ensures gravity irrigation on both sides of the ridge.

As per IS 10430 : 2000, the maximum permissible velocity in different types of canals is given below in tabulated form: The velocity is limited to avoid scouring in canals.

Concepts:

Canal linings are classified into two types based on the surface of lining:

1. Earthen type lining
2. Hard surface lining

1. Earth Type Lining:  They are two types: Compacted Earth lining and Soil Cement Lining

A. Compacted Earth Lining: In this earth material available near the construction sites is used and compacted in order to reduce the size of voids in soil by displacing air and water which in turn leads to increase in the density, compressive strength and shear strength of the soil and reduces permeability and seepage losses.

B. Soil Cement Lining:  In this sandy soil and cement are mixed in some definite proportion and mixture used for canal lining. The cement content should be minimum 2-8% of the soil by volume.

2. Hard surface lining:  This is sub-divided into four types:

A. Cement Concrete Lining: Concrete linings are suitable for both small and large channels and both high and low flow velocities.

B. Brick Lining: In case of brick lining, bricks are laid using cement mortar on the sides and bed of the canal. After laying bricks, smooth finish is provided on the surface using cement mortar.

C. Plastic Lining : In this, different types of plastic membranes such as Low density poly ethylene, High molecular high density polythene, Polyvinyl chloride are used for canal lining.

D. Boulder Lining:  The different sizes of stone blocks having irregular gradation are laid with mortar for canal lining.

Explanation:

Canal Outlets or Module:

(i) A canal outlet is a small structure built at the head of the water course so as to connect it with a minor or a distributary channel.

(ii) It is also called sluice.

Types of Canal Oulets:

Canal outlet can be classified mainly into three classes:

(i) Non modular outlets:

• Non modular outlets are those outlets whose discharge depends on the difference of water levels (i.e. head) between the distributary and the water course.
• Such an outlet is controlled by a shutter at its upstream end.
• Non modular outlets are very suitable for low head condition.
• Example: Submerged pipe outlets and masonary sluices

(ii) Semi-modular outlets:

• Semi modular outlets are those outlets whose discharge depends only on the water level in the distributary and is unaffected by the water level in the water course provided that a minimum working head required for working is available.
• Example: Pipe outlet, Venturi flume

(iii) Modular outlets:

• Modular outlets are those whose discharge is independent of the difference of water levels in the distributary and the water course.
• These are called Rigid modules.
• Example: Gibb's module

Based on Alignment Canals are classified into 3 categories. These are:

1. Ridge Canal, 2. Contour Canal and 3.  Side Slope Canal

Their characteristics are given below:

Explanation:

As per IS 10430 : 2000, the maximum permissible velocity in different types of canals is given below in tabulated form: The velocity is limited to avoid scouring in canals.

Explanation:

Canal Outlets or Module:

A canal outlet is a small structure built at the head of the water course so as to connect it with a minor or a distributary channel. It is also called sluice.

Types of Canal Oulets:

Canal outlet can be classified mainly into three classes:

(i) Non modular outlets:

• Non modular outlets are those outlets whose discharge depends on the difference of water levels (i.e. head) between the distributary and the water course.
• Such an outlet is controlled by a shutter at its upstream end.
• Non modular outlets are very suitable for low head condition.
• Example: Submerged pipe outlets and masonary sluices

(ii) Semi-modular outlets:

• Semi modular outlets are those outlets whose discharge depends only on the water level in the distributary and is unaffected by the water level in the water course provided that a minimum working head required for working is available.
• Example: Pipe outlet, Venturi flume

(iii) Modular outlets:

• Modular outlets are those whose discharge is independent of the difference of water levels in the distributary and the water course.
• These are called Rigid modules.
• Example: Gibb's module

Concept:

The minimum size of sediment that may remain stable in a channel with hydraulic radius R and bottom slope S, is

d ≥ 11 RS

Where R = hydraulic radius and S = bed slope

Calculation:

Given data,

R = 2 m = 2000 mm

S = 0.0001

The expected smallest size of the sediment particles on the bed of the river is

d ≥ 11 RS

d ≥ 11× 2000 × 0.0001

d ≥ 2.2 mm

Concept:

Lacey's Regime Width and Depth of Alluvial River:

• According to lacey's, for the alluvial river, the regime width = wetted perimeter = 4.75 √Q
• For such rivers lacey's normal scoured depth, R = 0.473 (Q/f)^1/3

This equation of scoured depth will be applicable only when the river width = regime width of 4.75 √Q

• For any other value of actual river width, the normal scoured depth is given by,

R' = 1.35 (q²/f)^1/3

Where, q = Q/B

B = Base width of the channel

f = Sil factor

Calculation:

Given,

f = 1, Q = 96 m³/s, B = 12 m

q = Q/B = 96/12 = 8 m²/sec

Normal scoured depth is given by,

R' = 1.35 (q2/f)1/3

R' = 1.35 (8²/1)^1/3 =5.4 m 

Explanation:

Lacey’s Silt Theory of Canals

Lacey stated that a channel may not be in regime condition even if it is flowing with non-scouring and non-silting velocity. Therefore, he distinguished three regime conditions as follows :

1. True regime
2. Initial Regime
3. Final Regime

1. True regime

A channel is said to be in regime condition if it is transporting water and sediment in equilibrium such that there is neither silting nor scouring of the channel.  But according to Lacey, the channel should satisfy the following conditions to be in regime condition.

• Canal discharge should be constant.
• The channel should flow through incoherent alluvium soil, which can be scoured as easily as it can be deposited and this sediment should be of the same grade as is transported.
• Silt grade should be constant.
• Silt charge, which is the minimum transported load should be constant.

If the above conditions are satisfied, then the channel is said to be in true regime condition. But this is not possible in actual practice. Hence lacey defined two other conditions which are initial and final regime conditions.

2. 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. According to Lacey’s, regime theory is not applicable to initial regime condition.

3. 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. The channel shape may vary according to silt grade as shown in the figure below :

Note:

Lacey’s specified that the regime theory is valid for final regime condition only and he also specified that semi-ellipse is the ideal shape of regime channels.

Explanation:

According to Lacey's theory, following formulae has been given below:

1. Hydraulic mean depth, \(R = \frac{5}{2}\frac{{{V^2}}}{f}\)
2. Area of the channel section, \(A = \frac{Q}{V}\)
3. Wetted perimeter, \(P = 4.75\sqrt Q \)
4. Regime mean velocity, \(V = {\left( {\frac{{Q{f^2}}}{{140}}} \right)^{1/6}}\)
5. Bed Slope, \(S = \frac{{{f^{5/3}}}}{{3340{Q^{1/6}}}}\)

Calculation:

Q = 2500 cumec

\(P = 4.75\sqrt Q =4.75\sqrt {2500 }\) = 237.5 m

Explanation:

The minimum freeboard recommended for a lined canal is as follows:

Hence, The minimum freeboard recommended for lined canal carrying discharge greater than 10 m3/s, is 0.75 m. 

Explanation:

According to Kennedy's theory, the critical velocity V is given by V = 0.55 ×  m × D^0.64

where D= depth of flow

m=critical velocity ratio. The value of m depends upon soil condition

Therefore, V = 0.55 × 1× 1^0.64 = 0.55 m/s

Concept:

Capacity Factor:

a) The capacity factor of a canal is the duty based on the discharge at the canal headworks.

b) The capacity factor is defined as the ratio of mean supply discharge in a canal during a period to its design full capacity. 

Capacity Factor of the canal = \(\frac{Average \ supply \ discharge}{Full \ supply \ discharge}\)

Calculation:

Given Data:

Full supply discharge = 60 cumec

Average supply discharge = 40 cumec

Capacity Factor of the canal = \(\frac{Average \ supply \ discharge}{Full \ supply \ discharge}\)

Capacity Factor of the canal = 40/60

Capacity Factor of the canal = 0.666

Capacity Factor of the canal = 0.67

Additional Information
Time Factor: It is the ratio of the number of days the canal actually runs during a watering period to the total number of days of the watering period.

Outlet Discharge Factor: The outlet discharge factor is the duty of water of a crop at the outlet of the canal.