CONSTRUCTION OF UNIT HYDROGRAPH
- 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.
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