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.
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.