Irrigation & Hydraulic Structures – Quick Review

Furrow Irrigation & Sprinklers

• Furrow: narrow parallel channels (depth $8!{-}!30\,\text{cm}$, length up to $400\,\text{m}$) conveying water between crop rows.
• Typical row crops: potato, maize, cotton, soybean, sugar-beet.
• Sprinkler vs. surface irrigation – key advantages:
  • Adapts to varied topography/soils/crops.
  • Controls erosion; uniform distribution; application efficiency ≈ $80\%$.
  • Precise fertilizer scheduling, low labour, no land shaping, light irrigations for seedlings, more cultivable area.

Methods of Irrigation Water Application

• Free/ordinary flooding – close growing crops, rolling/sloping land.
• Border strips – land divided by levees; narrow/short strips ↑ efficiency.
• Check flooding – permeable & impervious soils; reduces deep percolation.
• Basin flooding – orchards.
• Furrow – only \tfrac15$–$\tfrac12 surface wetted → ↓ evaporation.
• Sprinkler – steep/erodible land, high W.T., very permeable soils.

Soil Moisture & Evapotranspiration

• Field Capacity (FC): soil water retained against gravity; tension $0.1{-}0.33\,\text{atm}$.
• Permanent Wilting Point (PWP): moisture where roots cannot withdraw water; tension $7{-}32\,\text{atm}$.
• Average Soil Moisture = actual content between FC & PWP.
• Potential ET: ET with unlimited water; climate-controlled.
• Actual ET: real ET under existing moisture.
• Measurement: lysimeter – weigh tank, collect drainage;
  $ET = \frac{\Delta W - W_{drain}}{A}$.
• Frequency of irrigation: supply when moisture falls to optimum (readily available) level;
  $\text{Frequency} = \frac{\text{Depth applied}}{\text{Consumptive use rate}}$.

Duty–Delta–Base Period

• Duty $D$: ha irrigated per $1\,\text{cumec}$ throughout base period.
• Delta $\Delta$: total depth applied (m).
• Base period $B$: sowing to last watering (days).
• Relation: $D = \frac{8.64\,B}{\Delta}\;(\text{ha}/\text{cumec})$.

Irrigation Efficiencies

• Water application $\eta<em>a = \dfrac{W</em>s}{W_d}\times100$.
• Water storage $\eta<em>s = \dfrac{W</em>s}{W_n}\times100$.
• Water distribution $\eta_d = \bigl(1-\tfrac{y}{d}\bigr)\times100$.
• Field Irrigation Requirement $FIR=\dfrac{NIR}{\eta_a}$.
• Consumptive use = ET depth over time.

Water-Logging Effects

Inhibits soil bacteria, reduces capillary water, lowers temperature, hampers aeration, accumulates salts (pH > $9$ harmful), delays operations, encourages weeds, health hazards.

Diversion Headworks

Components: weir/barrage, under-sluices, divide wall, fish ladder, canal head regulator, guide banks & marginal bunds, silt excluders/ejectors.
Functions: level raising & diversion, silt control, flood handling, river training, fish migration.

Lacey Regime Theory

• Initial regime: bed slope & depth adjust (width fixed) until shear equilibrium.
• Final regime: width, depth, slope all adjust to semi-elliptical section suited to silt size.

River Training – Groynes & Meandering

• Purposes: bank protection, channel contraction, navigation depth.
• Types: impermeable/permeable; submerged/non-submerged; attracting ((30^{\circ}-60^{\circ}) d/s), repelling ((60^{\circ}-80^{\circ}) u/s), deflecting (≈$90^{\circ}$), T-headed, hockey.
• Design thumb rules: length ≤ $0.2\,B<em>{river}$ & > $1.5{-}2.0\,d</em>{flow}$; spacing $\approx 1{-}2.5$×length.
• Meandering caused by excess silt & aggradation; governed by valley slope, discharge, silt charge/grade, bank erodibility.

Cross-Drainage Works

1. Canal over drain: Aqueduct, Syphon Aqueduct (drain bed depressed).
2. Drain over canal: Super-passage, Syphon (canal in barrels).
3. Intermixing: Level crossing, Inlet & outlet.
   Purpose: maintain uninterrupted canal supply across natural drains.

Gravity Dam – Elementary Profile & Stresses

• Triangular profile (water side vertical).
• Base width for no-tension:
  $B = H\,\sqrt{\frac{\gamma<em>w}{\gamma</em>c S - C\,\gamma_w}}$
  ((S=)specific gravity of concrete, $C$=uplift factor; if no uplift $C=0$).
• Sliding safety: $\mu(\gamma<em>c S - C\gamma</em>w)\,B \ge \tfrac{\gamma_w H^2}{2}$.
• Base normal stresses:
  $\sigma<em>{max}=\frac{6P}{B H}\bigl(1+\tfrac{6e}{B}\bigr),\;\sigma</em>{min}=0\;(e=\tfrac{B}{6})$.
• Max principal near toe & shear:
  $\sigma = \gamma<em>w H\frac{S - C +1}{B},\; \tau = \gamma</em>w H\frac{\sqrt{S-C}}{B}$.

Forces on Gravity Dam & Uplift Diagrams

External forces: water, uplift, silt, wave, ice, earthquake, self-weight.

• Water force $P = \tfrac12\gamma_w H^2$ acting at $H/3$.
• Uplift (no drain): linear from $\gamma<em>w H$ at heel to $\gamma</em>w H'$ at toe.
• With drainage gallery: pressure reduced to intermediate ordinate at gallery.

Galleries & Shafts in Dams

• Galleries: horizontal/inclined openings for drainage, grouting, instrumentation, cooling pipes, access.
• Shafts: vertical openings linking galleries; plumb-shaft for deflection monitoring.

Spillway Energy Dissipation (TWC vs. JHC)

Relative position of Tail-Water Curve (TWC) & Jump-Height Curve (JHC $y_2$) dictates device:

1. $TWC \equiv y<em>2$: floor apron length $\approx 5(y</em>2-y_1)$.
2. TWC > y_2 (all Q): sloping apron above bed or roller bucket.
3. TWC < y_2 (all Q): ski-jump bucket; sloping apron below bed; subsidiary dam/baffle wall.
4. $TWC$ above at low Q & below at high Q: composite sloping apron (partly above & below bed).
5. Reverse of 4: same composite apron used; jump shifts with discharge.

Miscellaneous

• Crop calendar: schedule of sowing–harvesting periods aiding climate-adaptive planning.
• Seepage hazards for weirs: piping (remedied by longer floor/sheet-piles) & uplift (thicker floor/piles).