Hydrology

1. UNIT
HYDROGRAPH
REPORTERS
DON ROY TAMPUS
RACHEL TAMUDTAMUD

2. UNIT HYDROGRAPH
• Unit Hydrograph
• Hydrograph usually consists of a fairly regular lower portion that
changes slowly throughout the year and a rapidly fluctuating
component that represents the immediate response to rainfall.
The lower, slowly changing portion is termed base flow. The
rapidly fluctuating component is called direct runoff

4. UNIT HYDROGRAPH
• The amount of run-off resulting from 1 unit (1cm, 1mm,
ift, etc.) of rainfall excess.
• is essentially a tool for determining the direct runoff
response to rainfall.
• Once you know the watershed's response to one storm,
you can predict what its response for another will look
like.

5. BASIC ASSUMPTIONS OF UH
• 1. The effective rainfall is uniformly distributed within its
duration
• 2. The effective rainfall is uniformly distributed over the whole
drainage basin
• 3. The base duration of direct runoff hydrograph due to an
effective rainfall of unit duration is constant.
• 4. The ordinates of DRH are directly proportional to the total
amount of DR of each hydrograph
• 5. For a given basin, the runoff hydrograph due to a given
period of rainfall reflects all the combined physical
characteristics of basin (time-invariant)

7. • Soln:
• Step-1: Compute DRH (Col. 3) DRH=Ordinates of flood
hydrograph – Base flow
• Step-2: Compute 6hr-UH (Col. 4)
• Ordinates of UH = Ordinates of DRH / Excess Rainfall

9. Derivation of T-hr Unit Hydrograph from Given
D-hr unit Hydrograph
• If UH of specified UH is available, then we can derive of any
other duration UH by using superposition techniques.
• But, the use of this technique is limited i.e. if required duration is
integral multiple of given duration UH, then this technique is
easy to use and compute req. UH.
• For ex. Derivation of 16 hr UH from given 4hr UH, then
superposition technique is applicable, But when it is necessary
to derive 4hr UH from given 16hr UH, then S-curve technique
should be

11. • Soln:
• Step-1: See the given duration and req. duration. Given Duration of UH = 3hr
• Req. duration = 9hr
• i.e. 9 is the integral multiple of 3. Thus, superposition technique is applicable.
• Step-2: if we add 3 hr UH by 3 times then we got 9 hr UH i.e. 3hr + 3hr + 3hr = 9hr
• but, at the same when we adding the 3 UH of 1 cm each, resulted hydrograph will be DRH
of 1 + 1 + 1 = 3 cm excess rainfall.
• Thus, in 2nd step, lag ordinates of given UH by 3 hr and then again lag by 3 hr (Col. 3 & 4)
• Step-3: Add these lagged UH i.e. Col.6, this will give us DRH of 3cm ER.
• Step-4: Divide col.6/3cm (bcoz we want to plot UH)

13. THANK YOU FOR
LISTENING