1. Dr. Phil Bedient Rice University
Hydrologic Analysis
Dr. Phil Bedient
Rice University

2. Mansfield Dam - Lake Travis
Unit Hydrographs
S Curve Method
Synthetic UH
Multi-period storms
UH Applications Kinematic Wave

3. Hydrograph computed at outlet of each subarea

4. Hydrographs routed to the outlet of the watershed

5. Hydrographs routed thru one reach of the watershed
I – Q = dS/dt

6. Distribution of Rainfall
Infiltration
Detention    Storage
Depression    Storage
Direct Runoff

7. Equilibrium Hydrograph
Uniform Rain
Inflow = Outflow
Equiv to summing n hydrographs in T
Produces S curve

8. Hydrograph Relations Net Rainfall Rising Limb Crest Falling Limb
Recession
Base Flow
Direct Runoff DRO

9. Base Flow Methods Straight Line ABC continue the recession - concave
Empirical method

10. Infiltration Loss Curves
Horton Method
F Method
Initial and Constant Loss

11. Typical Rainfall and
Hydrograph Response

12. Time-Area Method Watershed Isochrone Hyetographs Time Area Histograms
Add and Lag Method
Resulting Hydrograph

13. Time Area Hydrograph
P1 * A1
P2*A2 + P1*A2
P3*A1 + P2*A    P1*A3

14. Unit Hydrograph Method
Measured Hydrograph
Adjusted for Net RF
Adjusted for
Net Rainfall
Unit Hydrograph Method

15. Base Flow and Infiltration
Subtract BF
Subtract Infiltration
Determine Net Rain
Integrate hydrograph
Adjust hydrograph ordinates to match net rainfall Vol
Determine UH
Net rainfall
Infiltration
Peak Flow

16. 2-Hour Unit Hydrograph
Net Rainfall Qp

17. 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

18. S-Curves for UH

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

20. Lag S-Curves in Time
And Subtract
Lag by D’ and subtract both curves

21. 3 hr. Unit Hydrograph
from a 2 hr. UH (D/D’)
3 HR UH

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

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

24. Development Effects on UH
Natural
Partial
Fully Developed

25. Actual Brays Bayou UH Faster response with Development Ultimate
Higher peak flow with
Need to provide down-
Stream capacity
Ultimate
Present
1959

26. Synthetic UH Methods Methods to characterize ungaged basins
Use data and relationships developed from gages
Variety of approaches but most based on Tp and Qp
England and U.S. led the efforts to standardize UH

27. Synthetic UH Methods Snyder’s Method (1938) Clark Method (1945)
Nash (1958)
SCS (1964, 1975)
Espey-Winslow (1968)
Kinematic Wave (1970s)

28. Snyder’s Method
5 to 7 points

29. SCS Methods
Triangular UH
Dimensionless UH

30. SCS Triangular UH Example
1 inch of RF over 1.6 hr duration

31. 2-hr Duration UH Example
Qp at 400 cfs at 10 hr with 24 hr Tb
Six Hours of Uniform Rainfall = in/hr
400
1.5 Inches Total
300
200
100
Time in Hours

32. SCS RAINFALL RUNOFF CURVES

33. Unit Hydrograph Applications

34. Land Use will alter UH Peak and Timing Land Use 1 Land Use 2
Unit Hydrograph Changes

35. Kinematic Wave
Overland
Collector Channels
Main Stream
Manning’s Eqn

36. Kinematic Wave

37. Unit Hydrograph Methods
Linear Methods
Tested in numerous WS
Stood the test of time
Regional calibration
Still most used approach
Used to compute a storm hydrograph for a subbasin

38. Basin hydrographs- routed & combined  to produce storm hydrograph