Irrigation Engineering and Hydraulic Structures
Irrigation Engineering and Hydraulic Structures covers water requirement, duty and delta, canal design, dams, spillways, weirs and cross-drainage works. The exam tests the relationship between duty, base period and delta, plus hydraulic design principles.
Core rules
- Duty D is area irrigated per unit discharge; delta Δ=8.64B/D with B in days, D in ha/cumec and Δ in m.
- Consumptive use includes transpiration and evaporation; effective rainfall reduces irrigation requirement.
- Canal design uses Kennedy's critical velocity and Lacey's silt theory for regime channels; lining reduces seepage and improves duty.
- Gravity dam stability checks overturning, sliding and crushing; uplift reduces effective weight.
- Spillway capacity uses the ogee crest formula; a stilling basin dissipates energy.
Worked example
A crop has base period 120 days and field duty 1200 ha/cumec. Delta=8.64×120/1200=0.864 m. For 100 ha, volume=100×0.864=86.4 ha m=864,000 m³. If canal loss is 20%, head discharge must be increased; duty at head is lower than field duty. Do not use 8.64 with B in hours. For a trapezoidal canal, Manning's equation uses hydraulic radius, not depth alone.
Traps
- Using base period in hours.
- Mixing duty at head and duty at field.
- Ignoring canal losses.
- Designing a dam only for overturning while ignoring sliding.
- Using gross command area as culturable command area.