Canal Irrigation & Canal Design

Classification of Irrigation canal

Irrigation canal can be classified in different ways on the following basis

(1)    Based on nature of source of supply

                (a) Perennial canal / बारहमासी नहर

                (b)  Non-Perennial canal / गैर-बारहमासी नहर

                (c) Inundation Canal or Flood Canal / बाढ़ नहर

(2) Based on discharge and relative importance in given network

                (a) Main Canals – Principal canal of canal network

                (b) Branch canal – off takes from main canal on either sides, Q > 5 cumecs

                (c) Major Distributaries – off take from branch canal, Q = 0.25 to 5 cumecs

                (d) Minor Distributaries – off take from major distributaries, Q < 0.25 cumecs

                (e) Water Course or field channels – channels which carries water from outlets of major or minor distributaries to the filed

(3) Based on soil through which it is constructed

                (a) Alluvial Canal – Excavated in alluvial soil

                (b) Non- Alluvial canal – Excavated in non- alluvial soil.

(4) Based on the lining

                (a) Lined Canal

                (b) Unlined canal

(5) Based on Canal alignment

(a)    Ridge Canal / रिज नहर

• Also known as Watershed canal.  वाटरशेड नहर के रूप में भी जाना जाता है।

• Aligned along the ridge or natural watershed line.

        रिज या प्राकृतिक वाटरशेड लाइन के साथ संरेखित होती हैi

• Can irrigate lands of both sides of ridge.

                रिज के दोनों किनारों की भूमि की सिंचाई कर सकते हैं।

• No cross drainage work required

(b)   Contour canal / कंटूर नहर

• Aligned parallel to contours of the land
• Can irrigate areas only on one side of ridg.भूमि के समोच्च के समानांतर संरेखित। रिज के केवल एक तरफ के क्षेत्रों की सिंचाई कर सकते हैं।

• Cross drainage works are required

(c)    Side slope canal / साइड ढलान नहर

• It is aligned right angles to the contour of land
• It also irrigate land on one side

Design of Unlined alluvial canal/channel

• In India Alluvial canal carry a certain amount of sediments.
• These channels are designed on the basis of hypothetical theories  given by Kennedy and Lacey.
• Main problems of alluvial canal is silting(settling of particles) and scouring (erosion of surface).
• Excess s silting reduces the discharge carrying capacity.
• Excessive scouring – bed eroded so that cross section increases and irrigate less command area.
• A channel is said to be stable or in regime condition if there is no problem of silting and scouring.
• Non –silting & Non – Scouring channels are known as channels in regime conditions.

(1)    Kennedys Theory (1895)

• Carried extensive investigation  on some of canal in upper Bari Doab canal system in Punjab. पंजाब में ऊपरी बारी दोआब नहर प्रणाली में कुछ नहरों पर व्यापक जांच की गई।
• He concluded that

a)      Silt supporting power of channel depends upon generation of eddies from base of the channel.

b)      These eddies are generated due to the friction of flowing water with channel surface.

c)       Generation of eddies are responsible for keeping silt in suspension.

d)      Kennedy defined critical velocity (V₀) in a channel as the mean velocity which will just keep the channel free from silting or scouring, and related to the depth of flow

                                                V₀ = 0.55 my^0.64 

                                where V₀  = Critical velocity in the channel in m/sec

                                              y  =  depth of water in channel in m

                                             m  = critical velocity ratio. (CVR) ( depends upon the silt grade)

Design steps for Keneddys theoy –

1. Assuming a trial depth of ‘y’ find the critical velocity V₀  which keep sediments in suspension.         V₀ = 0.55 x my^0.64
2. Find the cross – sec area of channel for given discharge    A = Q/ V₀
3.   find the channel dimensions by assuming it to be trapezoidal channel having side slope of 1V : 0.5H
4. Calculate hydraulic radius     R = P/A         ( P = perimeter of channel)
5. Using value of R find actual velocity by using chazey’s 0r Manning’s formula but Kennedy recommends Kutter’s formula.

                Chezy’s  Equation             V = C. R^1/2 S^1/2       

                   Manning’s Equation         V = (1/n). R^2/3 S^1/2

                    Kutter’s  formula           

       

                                                                                                              ^{where n = roughsity coefficient (0.002 to 0.003)}

6. If  V₀  = V, then our assumed depth of water is satisfactory. Otherwise change the depth and repeat the above procedure till we have V₀  ≈  V
7. Recommended ranges of slope 1 in 3500 to 1 in 5000.
8. Dimensions of channel using Kennedy’s theory is also determined using Garret’s Diagram.

Limitations of Kennedy’s theory

1)      According to Kennedy eddies are generated from bottom but in actual practice it is generated from sides and bottom. केनेडी के अनुसार एड्डीज़ नीचे से उत्पन्न होते हैं लेकिन वास्तविक व्यवहार में यह पक्षों और नीचे से उत्पन्न होते हैं।

2)      Value of ‘m’ was decided arbitrarily.

                ‘m' का मान मनमाने ढंग से तय किया गया था।

3)    Not given his own equation to find actual velocity so limitations of Kutter’s theory becomes                its limitation. वास्तविक वेग ज्ञात करने के लिए उसका अपना समीकरण नहीं दिया गया है,                 इसलिए कुटर के सिद्धांत की कमियां इसका दोष बन जाती हैं।

4)    Not given formula to find channel bottom slope. नहर के निचले ढलान के लिए सूत्र नहीं          दिया गया है।

5)    Did not noticed the importance of b/d ratio.  बी / डी अनुपात के महत्व पर ध्यान नहीं दिया।

6)    Did not account concentration the silt concentration and bed load.गाद की सांद्रता और बेड         लोड पर ध्यान नहीं दिया।

(2) Lacey’s  Theory (1939)

• Lacey found many drawbacks in Kennedy’s theory and gave his own theory.
• Lacey gave more general applicable theory.
• According to him , even a channel showing no silting or no scouring may actually not be in regime.
• According to him perfect regime conditions will not exist and there are 3 regime conditions in general.

                (a) Initial Regime              (b) Final Regime               (c) True Regime

(a)    Initial Regime

• After in service 1^st regime condition obtained by channel.
• When only bed slope & depth of channel varies than a condition of non – silting and non – scouring may exists which is called initial regime condition.
• Constant quantities are discharge, silt grade & charge, width of the channel.
• Lacey’s design theory is not valid for initial regime condition. ( for subsequent flow conditions regime condition may changed if site conditions change)

(b)   Final Regime

• Ultimate stage of regime attained by channel.
• If all the variables such as perimeter, depth, slope, etc. are equally free to vary and finally get adjusted according to discharge and silt grade, then the channel is said to have achieved permanent stability, called final regime.
  यदि परिधि, गहराई, ढलान आदि जैसे सभी चर समान रूप से भिन्न होते हैं और अंत में निर्वहन और गाद ग्रेड के अनुसार समायोजित हो जाते हैं, तो चैनल को स्थायी स्थिरता प्राप्त final regime कहा जाता है।

• Considered Semi- elliptical section.

(c)    True Regime ( ideal condition)

• There can be only single channel section and single bed slope at which a channel carrying a given discharge, silt charge and silt grade would be in regime.
  केवल एकल चैनल अनुभाग और एकल बेड ढलान हो सकता है, जिस पर दिए गए डिस्चार्ज, गाद चार्ज और गाद ग्रेड का एक चैनल स्थिरता में होगा।

• Hence , an artificial constructed channel having a fixed cross section and a fixed slope can behave in true regime if it satisfies the conditions of constant discharge, uniform flow, constant silt grade & Charge and canal is flowing through incoherent alluvium.

 एक कृत्रिम क्रॉस चैनल जिसमें एक निश्चित क्रॉस सेक्शन और एक निश्चित ढलान होता है, स्थिर चैनल के रूप में व्यवहार करता है। यदि यह निरंतर निर्वहन, एक समान प्रवाह, निरंतर गाद ग्रेड और चार्ज की स्थितियों को संतुष्ट करता है और नहर असंगत जलोढ़ के माध्यम से बह रही है।