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Watershed Slope Lower Bounds for Methods

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Watershed Slope Lower Bounds for Methods

EWRI-2012

21 MAY 2012

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

- 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.

- Magnitude

- Direction

Rainfall Intensity

Rainfall Intensity

- Losses/Storage

- Drainage channel has capacity – but water “slow” to get there.

- Simple expression of time in terms of speed and distance
- Expression for time in hydrology

- Consequences of “slope” in denominator
- As slope vanishes time becomes huge.
- Huge time impacts intensity estimate.
- Huge time impacts unit hydrograph behavior.

- 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

- Literature interpretation

- Experiments
- Rainfall simulator
- Small plot – paved
- Small plot – grass

- Small Plot “Paved”

- Small Plot – “Grass”

- 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

- 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.

- Experimental Data Interpretation
- Literature Studies

- Experimental Data Interpretation
- Literature Studies

- Experimental Data Interpretation
- Our Experiments

- Parametric Study

- Parametric Study

- Additional support for where “low slope” begins – about 0.5% or smaller
- Use the DHM to investigate alternate method for time in hydrologic models

- Impermeable surfaces
- Low slope starts at 0.005

- 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.

- 100+ watersheds
- Low and moderate slopes
- Small and large areas
- All plot along similar pattern except for two.

- 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

- 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)