Surface water drainage in hill roads

How surface water is collected and disposed of

Rain falling on the hill above the road and on the carriageway is collected before it can enter the road structure and then carried across or away from it.

1. Catch water drain (uphill, 5-10 m above the cut) intercepts the water from the upper hillside and leads it to a natural stream or cross-drain.
2. Side drain (V or trapezoidal, lined) along the hill-side edge of the road collects water from the cut slope and the carriageway (camber 2.5-3% towards the hill, or crown with cross-fall).
3. Cross drainage - culverts and small bridges take the water of the side and catch drains across the road at close intervals (about 50-100 m or less, and at all natural nallahs).
4. Outlet - the culvert outlet is connected to a chute/cascade, drop wall, or stilling basin to discharge into the natural gully without scouring the fill slope.
5. Valley side - berms and kerbs on high fills guide water to outlets; the road surface is kept sealed.

 UPHILL ~~~~~
   catch drain =====>--+
        \              |
   cut slope           v
   ----side drain--> culvert ---> chute
   =======road=======      \
        fill slope          \__ stream

Special problems of surface-water drainage in hill roads

• Steep slopes give high velocity and scouring of drains, fill slopes and outlets; lining and drop structures are needed.
• Concentrated flow and heavy rainfall in short time; many natural streams and gullies to cross.
• Water entering cracks and cut faces causes landslides, slips and erosion.
• Large sediment/debris load blocks drains and culverts; frequent cleaning is needed.
• Limited space for drains on narrow benches; difficult disposal because no place to discharge safely.
• Snow melt and springs add flow in cold areas.
• The cross-drainage works are many, costly and difficult to build on steep terrain.