Watershed Slope Lower Bounds for Methods EWRI MAY 2012

Acknowledgements Texas Department of Transportation; “ Establish Effective Lower Bounds of Watershed Slope for Traditional Hydrologic Methods”

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.

Why does slope matter? Magnitude

Why does slope matter? Direction

Rainfall Intensity

Why does slope matter? Losses/Storage

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

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

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.

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

Defining Low Slope Literature interpretation

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

Low Slope Alternate Methods Small Plot “Paved”

Low Slope Alternate Methods Small Plot – “Grass”

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

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.

Low Slope Alternate Methods Experimental Data Interpretation – Literature Studies

Low Slope Alternate Methods Experimental Data Interpretation – Literature Studies

Low Slope Alternate Methods Experimental Data Interpretation – Our Experiments

Low Slope Alternate Methods Parametric Study

Low Slope Alternate Methods Parametric Study

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

Low Slope Alternate Methods Impermeable surfaces – Low slope starts at 0.005

Low Slope Alternate Methods Permeable surfaces – Low slope starts at – 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.

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

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

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)