1. ERT 246 Hydrology & Water Resources Engineering
Siti Kamariah Md Sa’at
School of Bioprocess Engineering, UniMAP

2. Unit Hydrograph
Cont.

3. More Example 4:
Two storm each of 6-hr duration and having rainfall excess values of 3.0cm and 2.0 cm respectively occur successively.
The 2-cm ER rain follows the 3-cm rain. The 6-hr UH for the catchment is the same as given in previous example. Calculate the resulting DRH.
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7. Unit Hydrograph of Different Durations
Under condition where lack of adequate data in developement of unit hydrograph
D-hour unit hydrograph is used to develop unit hydrographs of differing durations nD
Two method available:
Method of superposition
The S-Curve
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8. Method of superposition
If a D-h unit hydrograph is available, and its desired to develop unit hydrograph of nD, its is easily accomplished by superposing n unit hydrographs with each graph separated from the previous on by D-h.
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9. D = 2-Hr Unit Hydrograph Adjusted Net Rainfall one inch over basin Qp
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10. Change UH Duration Consider 1 hr UH Add and Lag two UH by one hour
Sum and divide by 2
Results in 2 hr UH
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11. Example 5
Given the ordinates of a 4-hr unit hydrograph as below derive the ordinates of a 12-hr unit hydrograph for the same catchment
Time (hr) 4 8 12 16 20 24 28 32 36 40 44
Ordinates of 4-hr UH 80
130
150 90 52 27 15 5
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14. Equilibrium Hydrograph
Uniform Rain
Inflow = Outflow
Equiv to summing n hydrographs in T
Produces S curve
One Hydrograph
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15. S-Curve Also known as S-hydrograph
Hydrograph produced by continous effective rainfall at a constant rate for infinite period.
Curve obtained by summation of an infinite series of D-h UH spaced D-h apart.
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17. S-Curve Construction
Obtained by adding a string of D-h UH each lagged by D-hours from one another.
If Tb= base period of the UH, addition of only Tb/D UH are suffcient to obtained S-curve
U(t) = S(t)-S(t-D) or S(t) = U(t)+S(t-D)
Where
S(t-D)= S-curve addition at time t
S(t) = Ordinate of D-h UH at time t
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18. S-Curves for UH
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19. S-Curves S-curve Convert 2 hr Unit hydrograph to 3-hr
Lag each 2-hr UH by Duration D
Add to produce S-curve
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20. Example 6 Solve previous example (Example 5) with S-curve method:
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22. Hydrograph Convolution
Add and Lag Method 1 2 3 1 2
STORM
HYDRO 3
Add up the ordinates of all three to produce storm hydrograph
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23. Multi-Period Storm Pg - F = Net Rainfall
NET RF
Multi-Period
Storm
Pg - F = Net Rainfall
RF units = UH duration (i.e. 1 hour)
Adjust UH by Net RF
Add and Lag UH
Produce Storm
   Hydrograph UH
STORM
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24. Development Effects on UH
Natural
Partial
Fully Developed
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