Watershed Slope Lower Bounds for Methods

1. Watershed Slope Lower Bounds for Methods EWRI-2012 21 MAY 2012

2. Acknowledgements • Texas Department of Transportation; “0-6384 Establish Effective Lower Bounds of Watershed Slope for Traditional Hydrologic Methods”

3. Project Team • Theodore G. Cleveland - Texas Tech University • Ming-Han Li – Texas A&M University • William H. Asquith – USGS • Xing Fang – Auburn University • David B. Thompson – R.O. Anderson Inc.

4. Why does slope matter? • Magnitude

5. Why does slope matter? • Direction

6. Rainfall Intensity Rainfall Intensity

7. Why does slope matter? • Losses/Storage

8. Why does slope matter? • Drainage channel has capacity – but water “slow” to get there.

9. Why does slope matter? • Simple expression of time in terms of speed and distance • Expression for time in hydrology

10. Why does slope matter? • Consequences of “slope” in denominator • As slope vanishes time becomes huge. • Huge time impacts intensity estimate. • Huge time impacts unit hydrograph behavior.

11. What we did • Defined low slope • Literature interpretation • Experiments (Physical and Numerical) • Developed alternative methods • Parametric study using numerical model • Created an “adjustment” for low-slope hydrology • Developed plan for future monitoring • Emerging technologies needed

12. Defining Low Slope • Literature interpretation

13. Low Slope Alternate Methods • Experiments • Rainfall simulator • Small plot – paved • Small plot – grass

14. Low Slope Alternate Methods • Small Plot “Paved”

15. Low Slope Alternate Methods • Small Plot – “Grass”

16. Low Slope Alternate Methods • Experiment Purpose • Obtain data for validating a computer model • Computer model used for the parametric study to develop guidelines for low-slope hydrologic computations. • Experiment Summary • Simulator slopes range from 0 to 5% • Field slopes are 0.25% (0.0025) • Paved, grass, bare-earth surfaces represented

17. Low Slope Alternate Methods • Experimental Data Interpretation • Diffusion-Hydrodynamic-Model (DHM) to interpret experimental results • Use DHM to examine behavior in a parametric study to develop alternative estimation tools.

18. Low Slope Alternate Methods • Experimental Data Interpretation • Literature Studies

19. Low Slope Alternate Methods • Experimental Data Interpretation • Literature Studies

20. Low Slope Alternate Methods • Experimental Data Interpretation • Our Experiments

21. Low Slope Alternate Methods • Parametric Study

22. Low Slope Alternate Methods • Parametric Study

23. Low Slope Alternate Methods • Additional support for where “low slope” begins – about 0.5% or smaller • Use the DHM to investigate alternate method for time in hydrologic models

24. Low Slope Alternate Methods • Impermeable surfaces • Low slope starts at 0.005

25. Low Slope Alternate Methods • Permeable surfaces • Low slope starts at 0.005 • A unified general equation could not be found, instead three based on dominant soil type • Mathematical structure same, exponents are different by soil, the offset lower bound is the same.

26. Aggregate Observational Database • 100+ watersheds • Low and moderate slopes • Small and large areas • All plot along similar pattern except for two.

27. Monitoring Plan • Discovered that current measurement technologies are not low-slope compatible. • Use existing culverts as gaging stations, should the need arise. • Emergent (low-cost) technologies that can make the measurements in low-slope conditions • Laser and reflective targets • Visible/IR image interpretation • Pressure sondes

28. Conclusions • Defined low slope, about 0.3% and smaller. • Alternative timing model to use in such conditions. • Emerging technology needs to be deployed to measure discharges on low slope. • Guidance in final report on how to use these findings. (available July 2012)