Natural rock catchment and Open water reservoir

Water catchment area on Sullivan Rock, USA.

These are naturally occurring catchments of bare rock that have high runoff coefficients (around 0.9). Water can be stored as an open reservoir behind a retaining structure, with storage capacities ranging from 20 – 4,000 m3, or can be stored directly in a covered storage tank that collects water directly from the catchment.

Suitable conditions

• The rock that makes up the catchment should be bare and free of vegetation/soil. It should have no fractures or cracks that would result in a loss of water through seepage.
• Site the dams for rock catchments to maximize the natural topography – to get the best volume, make dams on the lower side of existing rock pools.

Advantages

Disadvantages/limitations

- High runoff coefficient = similar to roof catchments in that even small showers produce water - Minimal seepage - Maintenance is simple and cheap - Rock catchments do not occupy farmland and often no one owns the land, so it easy to implement

- Not many sites suitable - If building tanks that store water directly, storage capacity is limited compared to an open reservoir - Cost is high – experience from Kenya shows that a 56 m3 dam cost $4,000 including labour (= $71 per m3 of storage) - Vectors can breed in open water - Microbiological and chemical water quality is likely to not be acceptable for direct consumption (see “Natural ground catchment & open water reservoir”)

Reference manuals, videos, and links

• How to Calculate Runoff Coefficient.