Eearthen channel design

Eearthen channel design

• 1.
  Channel Plan Forms/Pattern •Straight Channel – Water moves parallel to the channel banks – Sinuosity varies from 1 – 1.05 • Meandering Channel – Follow the sinuous path • Braided Channel – Channels flows in more than one sub-channels, because the natural topography does not match the hydraulics conditions of a river.
• 2.
  Sinuosity • The meanderratio or sinuosity index is the ratio of actual length along a meandering river (Lm) to the straight distance S between the end points (AB). • It is an indication of quantification of meandering. • For a straight river course this ratio is equal to unity. A ratio varying from 1 to 1.5 defines the river course as sinuous and from 1.5 to 4 as meandering.
• 3.
  Geometric features ofmeander Meande r Belt
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  Straight Channel
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  Meandering Channel
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  Braided Channel
• 7.
  Earthen Channel Design
• 8.
  Introduction • The efficiencyof the canal design has a bearing on its working • The canal efficiency suffers from silting and scouring of the canal due to faulty design necessitating heavy maintenance and at time even remodeling if the original design is past redemption. • The other worse problems whose origin lies in faulty design are; weed growth infection, heavy seepage losses entitling development of water-logging alongside the canal.
• 9.
  Introduction • Canal designalso depend on: soil formation, sediment transport characteristics, operational needs and desired standards of maintenance • Usually irrigation canals are constructed in alluvial soils and the supplies are essentially from river and carry silt. • The sediment passed on into the off-taking channel of low velocity, deposits in the canal bed resulting in silting up and consequently causing loss of discharge carrying capacity necessitating frequent silt clearing. • On the other hand a high velocity in channel cause erosion/scouring thereby lowering Full Supply Level resulting in loss of command.
• 10.
  Types of CanalSections • There are two types of canals sections – Unlined channels, most of our main irrigation canals are unlined/earthen canals which cause seepage and result in losses and raising in the W/T of the adjoining area. – Lined channels, lined with RC, PCC, Bricks, Stones etc, to minimize the seepage losses and increase the efficiency of the system. • Recently irrigation canals are built with lining
• 11.
  Factors affecting theDesign of Canals • Main factors: – Water discharge, Q – Sediment Discharge, Qs – Sediment size, d – Slope of canal – Bed width, depth and velocity
• 12.
  Factors affecting theDesign of Canals • Secondary factors: – Acceleration due to gravity, g – Shear stress – Viscosity – Temperature – Sediment density etc.
• 13.
  Design of irrigationcanals • It is the process to obtain a cross-section, slope and geometry of channel/canal which should not have objectionable siltation and scouring. – Determining (1) depth, (2) bed width, (3) side slope and (4) longitudinal slope of the channel so as to produce a non-silting and non-scouring velocity for the given discharge and sediment load.
• 14.
  Approaches used forDesign of Earthen Canals • Empirical • Semi empirical • Rational
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  Empirical Approaches Empirical Approaches(Regime Theories): • These methods are based on those canals which were working reasonably well and they were not having any objectionable silting and scouring and having stable cross-sections. These channels were fulfilling the requirements to carry designed water and silt discharge. • These theories are not dimensionally homogenous and do not follow any laws or theories.
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  Concept of Channelin Regime • Channel in Regime (Stable Channel) – Lindley (1919): When an artificial channel is constructed in alluvium to carry silty water, its bed and banks would silt or scour until the depth, slope and width attain a state of balance, to which he designated as channel in regime. – Lane (1953) defined stable channel as • Which carries water • The banks and bed of which are not scoured objectionably by moving water and • In which objectionable deposit of sediment do not occur.