Unit
hydrograph is an effective hydrograph that tells us how a catchment would
respond to a unit rainfall excess under a storm with duration of d-hour.
When deriving unit hydrograph, we may use hydrographs from a few isolated
storms with approximately the same duration, preferably from 0.9d to 1.1d.
In our
worked example, we have two hydrographs obtained from storm with duration of
6.5 and 6 hours. The catchment area is 378km2. First, we implement straight
line method to conduct base flow separation. Then, we produce direct runoff
hydrograph by isolating the hydrograph from base flow we determined earlier.
Afterwards, we need to find the effective rainfall depth. This value can help
us to scale down the hydrograph to unit hydrograph at their own timeframe. To
produce the final unit hydrograph for storm duration of 6 hours, we can take
the average of the ordinates of the unit hydrograph we developed from the input
hydrograph.
The following summarizes the steps in deriving the unit hydrograph. Watch the video above for full details.
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