• The unit hydrograph method is use for determination of the maximum flood discharge of a stream and also for developing a flood hydrograph corresponding to any anticipated rainfall.
  • For this a unit hydrograph of suitable unit duration is derive from an observe hydrograph of the drainage basis under consideration.
  • Using the unit hydrograph, the flood hydrograph corresponding to any rainfall of the same unit duration can be obtained.
  • Use following steps to derive a unit hydrograph.

Construction of unit to hydrograph from flood hydrograph

  • Choose a hydrograph resulting from an isolated, intense, short-duration rainfall of nearly uniform distribution over the entire drainage basis as well as within its duration.
  • To obtain direct run-off deduct the base flow from the total run off represent by the hydrograph.
  • For base flow separation any of the methods explain earlier may be use.
  • Draw a series of ordinates at some uniform time interval for the entire hydrograph.
  • From these ordinates of the hydrograph obtain the ordinates of direct run-off by deducting the corresponding ordinates of baseflow.
  • Compute the volume of direct run off which is equal to (ΣR× t × 3600) m3. where ΣR is the sum of the direct run-off ordinates in cumecs and t is the time interval in hours between the successive ordinates.
  • Divide the volume of direct run-off by the area of the drainage basis to obtain the equivalent depth of direct run-off as follows.