Unit Hydrograph
Unit Hydrograph
Unit Hydrograph
rainfall excess or direct runoff which is the same as the area of DRH
resulting from unit depth (1 cm) of rainfall excess occurring uniformly over
catchment.
Basic Assumptions of UH
runoff, derive DRH of same time base, regardless of the rainfall intensity
ii. At a given drainage area, the direct runoff caused by a specific rainfall is
• DRH for a given ER in a catchment is always the same irrespective of when it occurs
Principles behind UH Theory
2. Linear response: Principle of proportionality
➢ Direct runoff response to the rainfall
excess is linear
computed
the rainfall excess depth (in cms), to get the ordinates of D-hr
UH.
Example: Rainfall excess of 12 mm and of 1-hr duration resulted in a flood
hydrographs at the outlet of the basin. Find the the 1-hr UH of the basin
Then,
• 𝑄1 = 𝑃1 𝑈1
• 𝑄2 = 𝑃1 𝑈1 + 𝑃2 𝑈2
• …….
• 𝑄𝑛 = 𝑃1 𝑈𝑛 + 𝑃2 𝑈𝑛−1 + 𝑃3 𝑈𝑛−2
Example: The storm hydrograph resulted from an excess rainfall of 2 hr
duration (30 mm/hr in 1st hour anf 7.5 mm/hr in 2nd hour) is given below.
Derive the 1-hr UH of the basin
➢ Method of superposition
➢ S-curve method.
Method of superposition:
➢ Superpose n unit hydrographs with each graph separated from the previous
on by D-h.
S curve
Unit Hydrographs of different duration – S curve method
Example: Ordinates of a 1hr UH is given. Derive a 3-hr UH using
S-Curve method