Presentation on theme: "Estimating Streamflow Channel Losses with the Green-Ampt Model Neil Hutten Ag Eng 558 April 20, 2001."— Presentation transcript:
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 Uncertainties and Conclusions
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
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.
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
Green-Ampt Overview Ponded or unponded infiltration Deep homogeneous soil Water infiltrates as “piston flow” Sharply defined wetting front
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.
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
Green-Ampt Parameters as Determined by Rawls/Brakensiek
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
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.
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.
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”
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)
Depth to Wetting Front and Infiltration Rate for Sandy Loam
Depth to Wetting Front and Infiltration Rate for Sand
Ten-hour Wetting Depth and Infiltration Rates with Green-Ampt Parameters
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.
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.