Guidelines for the Design of Small Bridges and Culverts
2004 Edition

IRC SP 13 recommends multiple approaches for flood discharge estimation: (1) Empirical runoff formulas such as the Dickens formula, which estimate peak runoff based on catchment area and rainfall coefficients, suitable for small culverts; (2) Using flood marks from existing structures to calculate discharge with experimental formulas, especially if surveyed promptly after floods; (3) Cross-sectional survey methods involving measurements of area, wetted perimeter, and slope, applying Manning’s formula for velocity; (4) Lacey’s theory for alluvial streams to determine stable channel dimensions and discharge; and (5) Selecting the design discharge as the highest estimated value from these methods, unless it exceeds the second highest by more than 50%, while designing for approximately a 50-year flood return period.

For alluvial streams where the bridge's linear waterway is at least equal to the regime width, normal scour depth is taken as the regime depth derived from Lacey’s regime equations. The maximum scour depth is then determined by applying site-specific multipliers: 1.27 times the normal scour depth for straight reaches with single-span bridges, and up to 2 times for bends, diagonal currents, or multi-span bridges. It is important to verify that the maximum scour depth is not less than the deepest scour hole observed during site inspections. Protective measures such as curtain walls or aprons should be considered based on foundation conditions.

The code provides standard designs for RCC structures including slab bridges and box culverts. Slab bridges have standard clear spans ranging from 2.6 m to 9.6 m with corresponding effective and deck lengths, and skew slab bridges are available for spans of 4, 6, 8, and 10 m with various skew angles. RCC box culverts are offered in single, double, and triple cell configurations with sizes such as 2x2 m up to 8x7 m for single cells, designed for foundation bearing capacities up to 20 t/m². Standard pipe culverts with diameters of 1000 mm and 1200 mm are also included.

IRC SP 13 classifies soil bearing capacities into four categories with corresponding net bearing capacity requirements: Category A requires 5 T/m², B requires 10 T/m², C requires 15 T/m², and D requires 20 T/m². These categories guide foundation design for box cell structures. Soft or loose soil patches must be replaced with compacted granular fill in layers no thicker than 300 mm. In expansive black cotton soils, additional reinforcement with boulder and sand layers 450 to 600 mm thick is recommended. Minimum foundation depth should extend at least 2 meters below the anticipated or protected scour level.

Protection works should include rigid floor protection beneath the bridge extending 3 meters upstream and 5 meters downstream or up to the line of splayed wing walls if longer. The flooring layers consist of 150 mm thick flat stones or bricks laid on edge in 1:3 cement mortar, above which lie 300 mm thick M15 concrete and a 150 mm thick M10 concrete base. The top of this flooring should be placed 300 mm below the lowest bed level to prevent sediment deposition and scour. The design limits post-protection flow velocity to 2 m/s and discharge intensity to 2 m³ per meter width. Excavation must avoid loose pockets and ensure compaction before laying concrete in dry conditions. Regular maintenance includes clearing debris, preventing washouts, and maintaining stable wearing courses and drainage.