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Estimating Streamflow Channel Losses with the Green-Ampt Model

Estimating Streamflow Channel Losses with the Green-Ampt Model. Neil Hutten Ag Eng 558 April 20, 2001. Presentation Outline. Introduction and Motivation Channel Loss Estimation Methods Rawls & Brakensiek (1983) Determinations of Green-Ampt Parameters Application to a stream site

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Estimating Streamflow Channel Losses with the Green-Ampt Model

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  1. Estimating Streamflow Channel Losses with the Green-Ampt Model Neil Hutten Ag Eng 558 April 20, 2001

  2. Presentation Outline • Introduction and Motivation • Channel Loss Estimation Methods • Rawls & Brakensiek (1983) Determinations of Green-Ampt Parameters • Application to a stream site • Uncertainties and Conclusions

  3. Motivation • RCRA requires TSDs to determine whether they are located in 100-year floodplains • Additional engineering studies are required if TSD is located in a floodplain ($$) • Floodplain extent is influenced by losses • Stream “loss” is groundwater “gain” • Stream channel losses can be pathways for subsurface contaminant transport

  4. Commonly-used methods to estimate stream channel losses • Assume infiltration losses are balanced by local precipitation gains • Representative Reach Loss • Stream Gage A minus Stream Gage B • Adjust/extrapolate a “known” rate • Adjust peak flows from regression equations to equal peak flows obtained from flood frequency analyses of gaged data.

  5. Limitations of stream loss methods • No data at ungaged sites • Ephemeral or intermittent streams • Extent of groundwater contributions • Evapotranspiration vs. Infiltration • Channel wetting and drying cycles • Single value for an entire stream reach

  6. Green-Ampt Overview Ponded or unponded infiltration Deep homogeneous soil Water infiltrates as “piston flow” Sharply defined wetting front

  7. Green-Ampt Overview • Rate Form (f) of G-A Equation assumes a ponded surface so the infiltration rate equals infiltration capacity of the media. • Depth of Ponding can be neglected.

  8. Green-Ampt Model

  9. Green-Ampt Parameters • Effective Suction at Wetting Front • Effective Hydraulic Conductivity • Soil Porosity

  10. Green-Ampt Depth to Wetting Front

  11. Green-Ampt Infiltration Rate

  12. Rawls and Brakensiek (1980s) • Determined ranges of values for: • Wetting Front Suction • Hydraulic Conductivity • Soil Porosity • For eleven USDA Soil Textures • 1200 Soils, 5000 Horizons, 34 States • Methods described in standard references

  13. Wetting Front Suction

  14. Wetting Front Suction with Texture

  15. Porosity with Texture

  16. Hydraulic Conductivity with Texture

  17. Green-Ampt Parameters asDetermined by Rawls/Brakensiek

  18. Meanwhile, back at the river… • Streamflow Losses on Big Lost River were determined from stream gage station data by Bennett (1990) • Average annual streamflow: 1965 to 1987 • Sixteen (16) streamflow measuring sites and stations

  19. Big Lost River Losses • 1.5 cfs/mi: west bndry, INEEL to div. dam • 2.5 cfs/mi: div. dam to Hwy 26 • 5 cfs/mi: Hwy 26 to Lincoln Blvd (ICPP) • 1 cfs/mi: Lincoln Blvd (ICPP) to Lincoln Blvd (NRF) • 4 cfs/mi: Lincoln Blvd (NRF) to BLR Sinks • 2 cfs/mi: above BLR Sinks • 18 cfs/mi: in the Big Lost River Sinks.

  20. Measured Channel Loss • Stream gaging station 11 • Bennett’s measured loss = 2 cfs/mile • Channel Width varies from 40 to 60 feet; • 40 feet was used • Measured infiltration rate = 1.04 cm/hr.

  21. Specifics at Station 11 • Coarse pebble to cobble gravel above gaging station 11 • Sediment grades to sand and “sandy silt” below station 11 • Sandy Loam set of Green-Ampt parameters was used for “sandy silt”

  22. Selection of G-A Parameters • Sandy Loam (R-B) Parameters • Porosity (phi) = 0.453cc/cc • Wetting front Suction head Sf = 11.01 cm • Hydraulic conductivity Ks = 1.09 cm/hr • “Modelled” Sandy Loam Infiltration Rate after ten hours was 1.61 cm/hr • Measured rate was 1.04 cm/hr (2 cfs/mi)

  23. Depth to Wetting Front and Infiltration Rate for Sandy Loam

  24. Depth to Wetting Front and Infiltration Rate for Sand

  25. Ten-hour Wetting Depth and Infiltration Rates with Green-Ampt Parameters

  26. Uncertainties • Pre-existing initial moisture contents were not considered • R-B Porosity, Wetting Front Suction, and Hydraulic Conductivities for media larger than sand not available • Infiltration characteristics of ephemeral channel bottoms compared to infiltration of upland soil sites • Layering, textural changes, surface crusts, etc.

  27. Conclusions • Green-Ampt parameters developed by Rawls and Brakensiek may be a useful tool to determine stream channel infiltration loss rates. • Ten-hour Modeled Infiltration Rate (1.6 cm/hr) approximated the measured infiltration rate (1.0 cm/hr) • Time frame of measured infiltration rate was not specified. • Compare field samples with R-K parameters • This is a research area worth further investigation.

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