Collection and disposal of surface water

Surface water must be collected from the pavement and the area around it and taken to a safe outlet.

Rural roads

1. Camber and shoulders: the cross slope (about 2.5% for bituminous, 3 to 4% for gravel/earth) sends water from the carriageway to the shoulders, and the shoulder slope (3 to 4%) sends it to the side drain.
2. Side (longitudinal) drains: open earth, stone-pitched or lined drains along both sides in cutting, or on the hill side. In embankment, water flows down the side slopes or into drains at the toe.
3. Catch (intercepting) drains: placed above the cut slope to stop hill water entering the road.
4. Mitre / turnout drains lead water away from the side drain into the natural ground at intervals (every 100 to 250 m or so, depending on gradient).
5. Cross-drainage works: culverts, causeways and minor bridges carry the collected water across the road at natural streams and at fixed spacing.
6. Disposal: at stable natural outlets, with energy dissipation if the fall is steep.

 catch drain   ____ cut slope
 ----\_/-----\      \ side drain
              \______\___ road ___/ shoulder
                 --> culvert --> natural stream

Urban roads

1. Kerb and channel (gutter): the road is bounded by kerbs; water flows along the gutter at the edge of the road.
2. Gully inlets / catch basins: grated inlets set in the gutter at regular spacing take water into underground pipes. A silt trap catches sand and debris.
3. Underground storm sewers / box drains carry the water, with manholes every 30 to 90 m for inspection.
4. Outfall into a river, a larger storm drain or a soak pit. Footpath drains join this system, and open drains are often covered with slabs.

 footpath |kerb| road ---> gutter -> [gully inlet]
                                        |
                              storm sewer (pipe) ===> outfall

In both cases the design uses the rational method for runoff and Manning's equation for the capacity of the drains.