Drainage is the most critical factor in road performance and longevity. Water is the enemy of pavements — it weakens subgrade, causes erosion, reduces skid resistance, and leads to pothole formation. A well-designed drainage system can double the life of a pavement. IRC 75:2015 governs the design of drainage for highways in India.
Why Drainage Matters
- Saturated subgrade loses 60–80% of bearing capacity compared to dry state
- Pavement layer saturation accelerates fatigue failure in flexible pavements
- Erosion of embankment slopes and fill material
- Aquaplaning risk (HS film on pavement surface) — safety hazard at speed
- Frost action in cold regions: water + freeze-thaw cycles → frost heave
Components of Highway Drainage
| Component | Function | IRC Reference |
|---|---|---|
| Camber / cross-slope | Drain rainwater from carriageway surface to shoulder | IRC 38 |
| Longitudinal side drains (roadside ditches) | Collect runoff from surface, embankment, and cuttings; carry to outfall | IRC 75 |
| Transverse culverts / cross-drainage structures | Carry stream/nullah flow under road embankment | IRC 5, IRC 78 |
| Catch drains (hill side) | Intercept hillside runoff before it reaches road | IRC 75 |
| Subsurface drain / French drain | Lower groundwater table; remove infiltrated water from pavement layers | IRC 75 |
| Filter drains / permeable base | Allow rapid internal drainage of pavement structure | IRC 75 + MORTH |
Surface Drainage
Camber
Transverse slope on pavement surface (2–2.5% for bituminous; 1.7–2% for concrete). For divided highways, drain from crown toward both edges; for undivided roads, drain from crown to both sides.
Side Drains (Longitudinal Drains)
Design using Manning's equation for open channel flow:
Q = (1/n) × A × R^(2/3) × S^(1/2)
Discharge Q from rational formula: Q = C × i × A / 360
- C = runoff coefficient (0.70–0.90 for highway surfaces, 0.50 for grass slopes)
- i = design rainfall intensity (mm/hr) for 25-year return period (IRC 75)
- A = catchment area (hectares)
Shape: Trapezoidal (SS = 1H:1V to 1.5H:1V for earthen drains); rectangular stone masonry or concrete lining for high-velocity sections.
Minimum velocity to prevent silting: 0.5–0.6 m/s (Manning's velocity ≥ self-cleansing velocity)
Maximum velocity to prevent erosion: 0.9 m/s for sandy soil, 2.0 m/s for hard rock (lined drain)
Transverse Drains (Causeways, Dips)
On flat terrain, short pipes or transverse structures allow water to cross from one side of the road to the other when culverts are uneconomical.
Culvert Design
Types of Culverts
| Type | Span | Use |
|---|---|---|
| Pipe culvert (RCC) | 0.3–1.5 m dia | Small streams, drainage |
| Slab culvert (RCC deck) | Up to 6 m | Medium streams, with side walls |
| Box culvert (RCC box) | Up to 8 m × 4 m | Where scour depth is limited; allows depressed FSL |
| Arch culvert (stone/brick/RCC) | Up to 8 m | Good aesthetics; high fills |
Culverts < 6 m span are designed as per IS 456 + IRC 112 (for vehicular loading); hydrology from rational method or unit hydrograph per IS 5477.
Hydraulic Design of Culverts
Two flow conditions govern:
- Inlet control: Flow limited by inlet opening; head upstream = HW (headwater depth)
- Outlet control: Flow limited by pipe/box capacity; tailwater depth and friction matter
Critical discharge: Q_design from 25-year flood (IRC 75) or 50-year flood for culverts on National Highways.
Freeboard: 0.5–1.0 m above design HFL (High Flood Level) for culvert headwall.
Subsurface Drainage
When Subsurface Drainage Is Needed
- High water table (<1.0 m below pavement surface)
- Seepage from cut slopes on hillsides
- Water infiltrating through pavement cracks or shoulders
- Spring lines in embankments
Types of Subsurface Drains
- Longitudinal subsoil drain (French drain): Perforated pipe (100–200 mm dia) in gravel-filled trench alongside pavement; geotextile wrap prevents fines clogging; laid at ≥1:200 gradient
- Permeable base course: Open-graded aggregate layer (OGAFC or dense graded permeable) below the bituminous layer; rapid drainage of water entering pavement structure
- Edge drains: Collect water from permeable base and discharge to daylight at pavement edge
- Interceptor drain: Uphill of a cut slope to catch groundwater before it reaches the road cutting
Drainage in Hill Roads — Special Considerations
- Catch water drains along upper hill side at 50–100 m intervals above road
- Chutes or cascades to carry concentrated storm water down embankment face safely
- Masonry check dams in gullies to prevent rill erosion
- Scuppers (openings through road edge) to drain water across carriage way quickly
- Special drainage for unstable slopes to reduce pore water pressure (horizontal drains drilled into slope)
Storm Water Management — Urban Roads
Urban roads face different drainage challenges — imperviousness, flat gradients, and complex utility corridors:
- Storm drains: Underground RCC pipes or brick drains; CPHEEO Manual capacity design
- Storm drain sizing: Return period 2–10 years for urban drains (greater for primary drains)
- Inlet spacing: Inlet capacity must match contributing catchment discharge; placed at low points and street intersections
- Green Infrastructure: Bioswales, permeable pavements, retention basins — reduce peak runoff and recharge groundwater
Frequently Asked Questions
What is the design return period for road drainage in India?
IRC 75 recommends: Side drains — 25-year return period for NHs; 10-year for state highways; Cross-drainage culverts — 25–50-year return period depending on NH classification and risk. The return period determines the design storm intensity from the rainfall intensity-duration-frequency (IDF) curves specific to each rain gauge station (IMD data).
Why does water table rise under pavements, even in good drainage conditions?
Under impermeable pavements, rainwater that previously recharged the ground is now shed to drains. However, in old pavements with cracks, through shoulder edges, and from irrigation fields on either side, water enters the subgrade zone. Limited evaporation from under the pavement means this water cannot escape upward — the water table tends to rise progressively until subsurface drainage removes it.