Hydrologic Analysis

1. Hydrologic Analysis CIVE 6361

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

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

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

10. Typical Rainfall and Hydrograph Response

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

12. Time Area Hydrograph • P1 * A1 • P2*A2 + P1*A2 • P3*A1 + P2*A2 + P1*A3

13. Unit Hydrograph Linear Methods Tested in numerous WS Stood the test of time Regional calibration Still most used approach Basin response from 1 inch of uniform rainfall of given duration, D

14. Measured Hydrograph Adjusted for Net RF 2 inches Adjusted for Net Rainfall Of 1 Inch

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. D = 2-Hr Unit Hydrograph Adjusted Net Rainfall one inch over basin 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. Equilibrium Hydrograph Uniform Rain Inflow = Outflow Equiv to summing n hydrographs in T Produces S curve One Hydrograph

19. S-Curves for UH

20. 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

21. Lag S Curves in time, subtract them, and multiply by D/D’ Lag by D’ and subtract both curves

22. 3 hr. Unit Hydrograph from a 2 hr. UH (D/D’) 3 HR UH

23. 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

24. 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

25. Development Effects on UH Natural Partial Fully Developed

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

27. Synthetic UH Developed for basin   that were ungaged Based on data from   similar gaged basins Most methods are very   similar in nature Revolutionized ability to   predict hydro response Ultimate Present 1959

28. 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

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

30. Snyder’s Method 5 to 7 points

31. SCS Methods Triangular UH Dimensionless UH

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

33. 2-hr Duration UH Example Qp at 400 cfs at 10 hr with 24 hr Tb Six Hours of Uniform Rainfall = 0.25 in/hr 400 1.5 Inches Total Rain 300 200 100 2 4 6 8 10 12 14 16 18 20 22 24 26 30 Time in Hours

34. SCS RAINFALL RUNOFF CURVES

35. Unit Hydrograph Applications

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

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

38. Kinematic Wave

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