Guideline es for the Design of Causeways and SubmeBridges bridge
2008 Edition

IRC SP 82 requires comprehensive hydrological data for designing causeways and submersible bridges, including: 1) Design flood discharge determined from the highest recorded flood or a 50-year return period flood (100 years for major structures), calculated via empirical formulas, slope-area method using Manning's equation, rational method considering rainfall and catchment features, and unit hydrograph analysis; 2) Flood levels such as Highest Flood Level (HFL), Design Flood Level (DFL), Ordinary Flood Level (OFL), and return period-specific flood heights; 3) Characteristics of the river or stream including waterway width, flow velocity through openings, bed slope, sediment and debris characteristics, and scour potential; 4) Catchment area parameters like size, shape, slope, soil permeability, vegetation cover, and rainfall intensity; 5) Site-specific data including flood marks, historical flood records, flood spread extents, subsurface investigations, and foundation suitability. These inputs enable the design of safe, durable, and cost-effective low-level crossings.

For estimating flood discharge and afflux, IRC SP 82 advises: 1) Calculating flood discharge by multiplying the effective waterway area available for flow by the mean velocity, i.e., Q = A × V, where A is the product of total effective waterway width and flow depth; 2) Estimating afflux (the upstream rise in water level due to obstruction) using formulas for broad crested weirs and orifices, such as Q = C × L × √(2g) × h^(3/2), where Q is discharge, C is a coefficient from graphical data (typically 0.75 to 0.88), L is the width of the opening, h is the afflux, and g is gravitational acceleration; 3) Reducing afflux by minimizing obstruction (less than 60-70% flow blockage), providing vent openings with high discharge coefficients (around 0.88 using bell-mouth entries), keeping the deck low, and streamlining upstream edges to facilitate flow. These methods ensure accurate design of hydraulic parameters for causeways and submersible bridges.

To minimize flood damage to approach roads, IRC SP 82 suggests: avoiding deep excavations exceeding 4 meters to prevent slope failures during submergence; adopting safe submerged side slopes based on soil type—such as 1:2.5 vertical to horizontal for soft soils, 1:2 for black cotton soils, 1:1.5 for soft murum, and 1:1 for hard murum; installing lined side drains constructed of stone, brick, or concrete on both sides that discharge at least 10 meters away from the junction with the causeway to prevent erosion; paving the approach road up to the highest flood afflux spread with the same treatment as the main causeway and confining it between anchor walls; beyond the flood spread, providing a full-width metalled surface, preferably with anchor walls and side drains in soft soil areas; and ensuring a minimum pavement thickness composed of 200 mm compacted moorum or gravel, 150 mm water bound macadam, and 200 mm M30 grade concrete slab. Additionally, face or cut-off walls with deep foundations and trapezoidal profiles should be provided to resist scour and streamline flow.

IRC SP 82 specifies that foundations on erodible strata should extend at least 2.0 meters below the expected scour or protected bed level, while hard rock foundations (with unconfined compressive strength ≥ 10 MPa) require a minimum embedment of 0.6 meters below scour, and other rock types at least 1.5 meters. Bed protection includes upstream flexible aprons, cut-off walls, and rigid flooring, with flooring top level set 300 mm below the lowest bed level. Flooring thickness varies depending on flow velocity: standard flooring consists of 150 mm stone or brick laid on edge in 1:3 cement mortar over 300 mm M15 concrete and an additional 150 mm M10 concrete, while abrasive streams with velocities over 4 m/s require 450 mm M20 concrete over 150 mm M15 concrete. Cut-off and curtain walls should be constructed of M15 concrete or brick/stone masonry with 1:3 cement mortar, extend 2 meters upstream and 2.5 meters downstream below the floor level, and be free of horizontal or vertical joints. Concrete grades for leveling courses and piers follow minimum standards (M15 and M30 respectively), and reinforcement uses TMT bars conforming to IS:1786. Masonry mortar should not be leaner than 1:3 to ensure durability.

As per IRC SP 82 (Clauses 8.10 and 8.3.8), safety measures for submersible bridges include: installing two advance warning signs on each approach—one placed 200 meters before the structure displaying 'Slow Down. Submersible Structure 200 m Ahead Speed Limit 15 kmph,' and another at 50 meters reading 'Dead Slow Submersible Structure 50 m Ahead'; an additional sign on the bridge warns 'Do not Cross when Flood Water Overtops the Carriageway'; providing rumble strips 30 meters before the submersible bridge on both approaches, designed in accordance with IRC:99 standards to alert drivers and reduce speed. These features are critical to enhance driver awareness and prevent accidents during flood conditions.