I need to design a weir for a small earth dam in an arid area with a small (smaller than 200 ha), fairly flat catchment area in southern Africa. The idea behind the dam is to collect not more than 10 000 cubic metres of water for small stock watering purposes for the immediate time after the rainy season, somewhat reducing the dependency on ground water.

The challenge lies in applying the "rational method" (hydrology) where time of concentration is the determining factor to determine the peak flood. This method however was developed for larger catchment areas.

Is there an alternative to the rational method for smaller catchments, or alternatively an adjusted methodology?

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    This question was originally asked on Engineering SE but has been substantially revised there with additional details. – Air Apr 28 '16 at 16:18
  • The rational method is actually recommended for drainage areas smaller than 200 acres (80 ha), although this depends on the complexity of the watershed. – Delforge Aug 8 '17 at 15:45

The rational method is especially usefull for small areas. For larger catchments it is less accurate. Then you should use a rainfall-runoff model but that is only possible when you have sufficient data. A more simple method is the use of runoff coefficients. Maybe something like the SCS method.

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