Cross drainage structures are civil engineering constructions built where a canal intersects a natural drainage system, such as a river, stream, or nallah. These structures are crucial for ensuring the uninterrupted flow of both the natural drain and the canal, preventing unwanted mixing of waters, and minimizing risks like flooding, erosion, or waterlogging. They are generally expensive and are provided only when such an intersection is unavoidable.
Here are the essential design considerations and calculations for different types of aqueducts and culverts:
Aqueducts
An aqueduct is a cross drainage work that carries an irrigation canal over a drainage channel. Historically, aqueducts were used to transport water across valleys and uneven terrain. In modern civil engineering, they are used to support flood control, irrigation systems, and large water supply systems.
Types of Aqueducts:
- Aqueduct (Standard): In a standard aqueduct, the canal bed level is above the drainage bed level, allowing canal water to flow freely under gravity in a trough supported by piers. The high flood level (HFL) of the drainage remains below the canal bed level, and the drainage water flows at atmospheric pressure. An inspection road is typically provided along the trough.
- Type I: The entire canal section, including earthen banks, is carried across the drainage. This type has the maximum width across the canal and is suitable for small streams as it saves on canal wings and bank connections.
- Type II: The outer slopes of the earthen canal banks are replaced by retaining walls, reducing the culvert length while inner slopes remain earthen. This type is used for intermediate-sized streams.
- Type III: The normal earthen banks of the canal are replaced by a flumed concrete or masonry trough. This provides the shortest barrel length for drainage and is suited for large streams, though the cost of the flumed canal trough is higher. This type typically involves fluming the canal section.
- Trough Type Aqueduct: The canal section is reduced to a rectangular section and carried as a bridge resting on piers and foundations.
- Barrel Type Aqueduct: The natural stream section is flumed to pass through barrels or rectangular passages, while the normal canal section continues above.
- Syphon Aqueduct: This type is similar to a standard aqueduct, but the drainage bed is depressed locally beneath the canal trough, and the stream flow is discharged under pressure via siphonic action. This is preferred when the drain's high flood level (HFL) is above the canal trough or canal bed level. A sloping apron is often provided on both sides to depress the canal level, and cut-off walls are used to prevent scouring. Syphon aqueducts are generally more expensive but often preferred over simple aqueducts.
Essential Design Considerations for Aqueducts:
- Relative Bed and Water Levels: The choice of aqueduct type heavily depends on the relative bed levels and water levels (Full Supply Level - FSL for canal, High Flood Level - HFL for drainage) of the canal and the natural drain.
- Discharge: The discharge of both the canal and drainage are primary factors. The structure must handle the maximum flood discharge without disruption.
- Foundation Capacity: A suitable and strong foundation is crucial for the stability of the structure. Boring tests are conducted to assess foundation suitability.
- Economical Considerations: The cost of construction should be justified relative to the project's overall cost and benefits.
- Canal Alignment: An ideal condition is for the canal to not intersect drainage, but when unavoidable, the alignment may need to be shifted to minimize the need for cross-drainage works.
- Construction Problems: Site-specific challenges like sub-soil water, material availability, and accessibility must be considered.
- Hydraulic Design Principles: Ensuring sufficient capacity for design discharge, minimizing energy losses and turbulence, and preventing erosion and sedimentation. Hydraulic design involves the continuity equation, energy equation, and momentum equation to determine the required size and shape.
- Structural Design Requirements: The structure must be strong, stable, and durable, capable of withstanding various loads (its own weight, water pressure, traffic loads, soil loads) and environmental factors. Adequate foundation and support are essential.
- Fluming of Canal: Contracting the canal's waterway to reduce the length and cost of barrels or the aqueduct's width. Maximum fluming is governed by maintaining subcritical flow to avoid hydraulic jumps and undue head loss. Transitions (e.g., 3:1 upstream, 5:1 downstream) are designed for smooth changes in section.
- Uplift Pressure: For trough-type and siphon aqueducts, designers must consider upward thrust from high floods in the natural stream, especially when the canal is dry. The slab thickness should withstand this maximum uplift, and the dead weight of the trough may need to be greater than the upward thrust or anchored to piers. Uplift pressure can be evaluated by drawing the hydraulic gradient line.
- Bank Connections: Two sets of wings are needed: canal wings (land wings) to connect masonry/concrete trough sides to earthen canal banks, and drainage wings (water/river wings) to retain earthen slopes, guide drainage water, and provide a vertical cut-off from seepage.
Key Calculations for Aqueducts:
- Discharge (Q): Can be calculated using empirical formulas like Dickens' formula: Q = C * A^(3/4), where A is catchment area and C is a coefficient. Rational method (Q = 0.028 * P * f * I * A) is also used, where P is runoff coefficient, f is spread factor, I is rainfall intensity, and A is catchment area.
- Velocity (V): Calculated as Q/A (cross-sectional area). Manning's equation can also be used: V = (1/n) * R^(2/3) * S^(1/2).
- Head Loss (HL): Calculated for flow inside the barrel using formulas like Unwin's formula: HL = (1 + f1 + f2 * L/R) * V^2 / (2g), where f1 is coefficient loss at entry, f2 is related to rugosity, L is length, R is hydraulic mean radius, and V is velocity. Eddy loss and friction loss also contribute to total energy head loss.
- Scour Depth: Maximum scour depth needs to be determined to ensure the structure's safety against erosion. This is based on factors like discharge per meter run and silt factor. For a specific example, normal scour depth (R) = 1.34 * (q^2/f)^(1/3), where q is discharge per meter run and f is silt factor, with max scour depth on U/S side as 1.5R and D/S side as 2R.
- Structural Design of Trough/Barrel: Involves calculating bending moments and shear forces due to various loads (soil load, self-weight, water pressure, traffic loads). Reinforcement steel area is determined based on these moments and permissible stresses.
Culverts
A culvert is a structure that channels water past an obstacle or to a subterranean waterway, typically embedded within soil. They are commonly used as cross-drains for roadside ditches and to pass water under roads at natural drainage and stream crossings.
Types of Culverts: Culverts come in many sizes and shapes, including:
- Circular
- Rectangular (Box-like constructions)
- Elliptical
- Flat-bottomed
- Open-bottomed
- Pear-shaped
- Hume pipe culverts
- Slab culverts
Essential Design Considerations for Culverts:
- Hydraulic Performance: Design considers requirements for hydraulic performance, limitations on upstream water surface elevation, and roadway embankment height.
- Materials: Culverts can be constructed from various materials, including cast-in-place or precast concrete (reinforced or non-reinforced), galvanized steel, aluminum, or plastic (typically high-density polyethylene). Composite structures combining two or more materials are also used.
- Sizing and Installation: Culverts must be properly sized and installed to prevent failures. Undersized culverts can lead to scour, erosion, and sudden failure during flood events.
- Protection from Erosion and Scour: Measures must be taken to protect culverts and surrounding soil from erosion and scour.
- Load Capacities: Culverts are classified by standards for their load capacities (e.g., traffic loads).
Post a Comment