7.5 Regulating and Cross-Drainage Structures

7.5 Regulating and Cross-Drainage Structures

Introduction to Network Control and Crossing Structures

An irrigation canal network is an artificial drainage system superimposed on the natural landscape. It requires two critical categories of hydraulic structures to function effectively: Regulating Structures to control water flow within the canal system, and Cross-Drainage Structures to resolve conflicts between the canal and natural drainage lines (streams, rivers, gullies). These structures ensure precise water delivery, safe disposal of excess water, protection of the canal from natural drainages, and safe passage of these drainages across or under the canal. Their design integrates principles of open channel flow, seepage, and structural stability.

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1. Functions of Various Types of Regulators

Regulators are gated structures used to measure and control the flow in canals. They are classified based on their location and function.

1.1 Canal Head Regulator

• Location: At the head of the off-taking canal, immediately downstream of the diversion weir/barrage.

• Primary Functions:

  1. Regulate Discharge: Control the quantity of water entering the canal from the parent channel (river or main canal) using adjustable gates.

  2. Prevent Entry of Excess Silt: Often incorporates a silt excluder in its floor.

  3. Stop Water Supply: Completely shut off the canal for maintenance.

1.2 Cross Regulator

• Location: Constructed on the parent (main/branch) canal, downstream of an offtake.

• Primary Functions:

  1. Maintain Water Level: Raises the water level in the parent canal upstream of it to ensure sufficient head for feeding the offtaking canal(s).

  2. Divert Water: Facilitates the diversion of water into the offtake when required.

  3. Control Discharge: Can be used to control the flow downstream for operational flexibility.

1.3 Distributary Head Regulator

• Location: At the head of a distributary channel taking off from a parent canal.

• Primary Functions:

  1. Regulate Flow: Control the discharge entering the distributary.

  2. Measure Discharge: Often incorporates a measuring device (weir, flume).

  3. Protect Distributary: Prevents backflow from the distributary into the parent canal.

1.4 Canal Escape (or Surplus Water Escape)

• Location: At suitable intervals along a canal, typically at the tail end.

• Primary Functions:

  1. Dispose of Excess Water: Releases surplus water from the canal into a natural drain or river to prevent overtopping of banks due to operational errors or sudden reduction in demand.

  2. Empty the Canal: Allows complete dewatering for inspection, repair, or during the non-irrigation season.

  3. Scour Sediment: Located at low points to help flush out deposited silt (scouring escape).

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2. Design of Regulators and Escapes: Crest, Length and Thickness of Impervious Floor

The hydraulic design of regulators and escapes (which are essentially low-head weirs with gates) follows principles similar to weirs, combined with seepage control.

2.1 Crest Design

1. Crest Level:

   • For Head Regulators & Cross Regulators: The crest is kept higher than the parent channel bed. The crest level = Required Full Supply Level (FSL) of the off-taking canal minus the head loss over the regulator.

   • For Escapes: The crest is kept at or slightly below the canal bed level at that point so it can fully dewater the canal. For surplus escapes, the crest is set at the FSL.

2. Crest Shape: Often broad-crested or ogee-shaped for efficient flow.

3. Discharge Equation (for gated flow): $Q = C_d \cdot L \cdot \sqrt{2g} \cdot H^{3/2}$ (For free overflow) or $Q = C_d \cdot L \cdot a \cdot \sqrt{2gH}$ (for submerged/orifice flow under gate). Here, $L$ = clear waterway length, $H$ = head over crest, $a$ = gate opening.

2.2 Length of Impervious Floor

1. Governed by Seepage: Water percolates under the structure due to the difference in water levels upstream and downstream.

2. Application of Seepage Theories (Bligh’s, Lane’s, or Khosla’s) is essential:

   • Calculate the Total Hydraulic Head (H): Difference between upstream FSL and downstream water level (for escape, downstream may be dry).

   • Determine required creep length $L = C \times H$ (Bligh) or weighted creep length $L_w = C_w \times H$ (Lane).

   • Floor Length Must ≥ Required Creep Length. The floor is extended upstream and downstream of the crest/gates.

3. Typical Proportion: Downstream floor length is longer (≈2/3 of total) to safely dissipate seepage energy and uplift pressure.

2.3 Thickness of Impervious Floor

1. Governed by Uplift Pressure: The floor thickness must resist the upward seepage force.

2. Design Check:

   • Calculate residual uplift head ($h_x$) at key points (toe of glacis, end of floor) using creep theory or Khosla’s method.

   • Required thickness $t$: $t = \frac{4}{3} \cdot \frac{h_x}{G-1}$ (where G = specific gravity of concrete ≈ 2.24).

3. Provision: Floor is thickest at the downstream end where uplift is maximum (due to minimum creep length there). A nominal minimum thickness (0.6-0.8m) is maintained.

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3. Design of Pipe Outlet: Free and Submerged

A pipe outlet (or sluice) is a conduit (usually concrete pipe or masonry barrel) built under a canal bank or embankment to deliver water from a canal to a watercourse or field.

3.1 Types Based on Downstream Condition

1. Free (Modular) Outlet: The pipe discharges freely into the atmosphere with the jet surrounded by air. The outlet discharge is independent of the tailwater level.

2. Submerged (Non-Modular) Outlet: The downstream end of the pipe is submerged under tailwater. The discharge depends on both upstream and downstream water levels.

3.2 Design Considerations

1. Diameter (D): Determined from discharge equation.

   • For free flow (pipe flowing full): $Q = A \cdot C_d \cdot \sqrt{2gH}$ Where A = cross-sectional area, H = head causing flow (difference between upstream water level and pipe centerline at outlet), $C_d$ = coefficient of discharge (≈0.6-0.8).

   • For submerged flow: $Q = A \cdot C_d \cdot \sqrt{2g \Delta H}$ Where $\Delta H$ = difference between upstream and downstream water levels.

2. Velocity (V): Should be within permissible limits (1.5-3 m/s for concrete) to prevent erosion/scouring at outlet. $V = Q/A$.

3. Protection Works:

   • Upstream End: Trash rack to prevent debris entry.

   • Downstream End: Energy dissipater (still basin, riprap apron) to protect against scour from the high-velocity jet. Length of apron based on expected scour depth.

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4. Design of Vertical Drop (or Fall)

A vertical drop (or canal fall) is a structure built across a canal to lower its bed level in a controlled manner when the natural ground slope is steeper than the designed canal slope. It prevents excessive erosion by dissipating energy.

4.1 Components of a Vertical Drop (e.g., Trapezoidal Notch Fall, Sarda Type)

1. Crest Wall: The raised portion over which water falls. May be plain or notched.

2. Cistern (Stilling Basin): A depressed basin downstream of the crest where the hydraulic jump forms to dissipate energy.

3. Impervious Floor (Apron): Upstream and downstream floor to protect against seepage and uplift.

4. Upstream & Downstream Curtain Walls (Cut-offs): Deepened walls at ends to control seepage.

4.2 Crest Design

1. Crest Level: Determined by the required drop in canal bed level.

2. Crest Length (L): Equal to the canal bed width at that section.

3. Discharge over Crest: Designed using weir formula. For a rectangular crest: $Q = 1.84 \cdot L \cdot H^{3/2}$.

4.3 Length and Thickness of Impervious Floor

1. Seepage Design: The floor length is determined using Bligh’s or Khosla’s theory for the total head H equal to the difference between upstream FSL and downstream bed level (or water level).

   • High head difference necessitates longer creep length and deeper cut-offs.

2. Thickness: Governed by uplift pressure, calculated similarly to regulators. The floor under the downstream cistern is subject to high uplift and impact, hence made thickest.

4.4 Cistern Design

1. Purpose: To force the formation of a hydraulic jump for energy dissipation.

2. Cistern Length (L_c): $L_c = 5 \times (y_2 - y_1)$ (Empirical), where $y_1$ is pre-jump depth at toe and $y_2$ is sequent depth.

3. Cistern Depth (d): $d = \frac{1}{4} (y_2 - y_1)$.

4. Check for Jump Formation: Ensure tailwater depth ≈ $y_2$. If not, adjust cistern depth/length.

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5. Design of Cross-Drainage Structures

When a canal and a natural drain (stream, river) intersect at the same level, a structure is needed to allow one to cross the other without interruption. The choice depends on their relative water levels and discharges.

5.1 Types of Cross-Drainage Structures

1. Aqueduct:

   • Situation: Canal passes OVER the drainage.

   • When Used: Canal FSL is well above the High Flood Level (HFL) of the drain.

   • Design Focus: The canal is carried in a trough (barrel) supported on piers over the drain. Design involves hydraulic design of the trough (as a canal) and structural design of piers/abutments for drainage opening.

2. Siphon Aqueduct:

   • Situation: Canal passes OVER the drainage, but the canal bed is below the drain bed.

   • When Used: Canal FSL is above drain HFL, but the canal bed is lower than the drain bed. The canal trough is depressed (becomes a siphon) to go under the natural ground.

   • Design Focus: The canal flows under pressure through the siphon barrel. Head loss calculations and provision for air vents are critical.

3. Super Passage:

   • Situation: Drainage passes OVER the canal.

   • When Used: Drain HFL is well above the canal FSL.

   • Design Focus: The natural drain is carried over the canal in a trough. Similar to aqueduct, but roles reversed.

4. Canal Siphon (or Siphon):

   • Situation: Drainage passes OVER the canal, but the canal bed is above the drain bed.

   • When Used: Drain HFL is above canal FSL, but the drain bed is lower than the canal bed. The canal is carried under the drain through a siphon barrel (pressurized flow).

   • Design Focus: Similar to siphon aqueduct, but for the canal.

5. Level Crossing:

   • Situation: Canal and drain cross at approximately the same level.

   • When Used: Their water levels are very close. A common chamber with regulators on both is constructed.

   • Design Focus: Complex operation. Requires careful regulation to avoid mixing or backflow.

6. Inlet and Outlet (for small drains):

   • A simple pipe or box culvert under the canal bank for small, non-flooding drains.

5.2 Key Design Considerations for All Types

1. Hydraulic Efficiency: Minimize head loss for the carried stream (canal or drain). For siphons, calculate head loss $h_f$ using Darcy-Weisbach/Manning’s formula: $h_f = \frac{f L V^2}{2g D}$.

2. Waterway for the Drain: The cross-section provided for the natural drain (in aqueduct/super passage) must be adequate to pass the design flood discharge without causing excessive afflux (rise in upstream HFL).

3. Foundations and Seepage: Often constructed on permeable foundations. Must design impervious floors and cut-offs using Khosla’s theory to prevent piping/failure.

4. Structural Design: Design of piers, abutments, wing walls, and the trough for all loads (water pressure, earth pressure, self-weight, seismic).

5. Protection Works: Upstream and downstream aprons, pitching, and revetments for both the canal and the drain to prevent scour.

Conclusion: Regulating and cross-drainage structures are the essential control nodes and conflict-resolution points in an irrigation network. Their design is a specialized field combining hydraulics, seepage analysis, and structural mechanics. A regulator’s design centers on precise flow control under seepage forces, while a cross-drainage structure’s design revolves around safely passing one stream over or under another with minimal interference, ensuring the uninterrupted and efficient operation of the entire irrigation system.