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Estimating Streamflow Channel Losses Using Green-Ampt Model: Insights and Methods

This presentation discusses methods for estimating streamflow channel losses utilizing the Green-Ampt model. It outlines the motivation behind determining channel losses, particularly in relation to regulatory requirements for treatment, storage, and disposal (TSD) facilities in floodplains. Key methods are detailed, including effective parameters and their applications to real-world stream sites. The presentation also addresses uncertainties tied to various estimation methods and concludes with insights on the potential usefulness of the Green-Ampt parameters in calculating infiltration rates.

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Estimating Streamflow Channel Losses Using Green-Ampt Model: Insights and Methods

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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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