Investigating recent rill erosion at a site approaching geomorphic equilibrium

Ryan WellsLittle 5 Week 2015

Outline• Hortonian theory of overland flow• Field site• Derivation process for model formulas• Data collection• Results

Ref: Horton (1945)

Hortonian Theory

Past Studies:Disagreement

Ref: W.J. ElliotRef: Gyssels 2005

Opportunity: Geomorphic Reclamation at AML Site

Post Reclamation

Rill incising the cap material … generating an alluvial fan

Derivation Process• What’s controlling rill and gully formation?

Derivation Process: Dependent Variables

• τ

•ω = τ (2)

• τ = shear stress (Pa)• = unit weight of water (N/)• = depth of flow (m)• = hillslope (degrees)• ω = stream power (W/• = specific discharge (m/s)

Derivation Process:Predicting Specific Discharge

• (6)

• (7)• = discharge ()• = Cross sectional area of flow ()• = Mannings’s coefficient (s/)• = hydraulic radius (m)• = Slope (degrees)• = depth of flow (cm)• = width of flow (m)

Derivation Process:Predicting Depth

• (3)

• (4)

• (5)

• = discharge ()• = depth of flow (cm)• = width of flow (m)• = specific discharge (m/s)• = runoff coefficient (unitless)• = Catchment area ()• = Precipitation rate (cm/s)

Constraining Independent Variables

Precip. Rates• Rain gage installed

on site• Literature for

extreme rain events

Ref: Huff and Angel (1992)

www.soilmoisture.com

Runoff Coefficient

Hillslope

*Note 5x vertical exaggeration

Model Results

Shear Stress Model : 4 cm Rain/hr

Stream Power Model : 4 cm Rain/hr

Stream Power Model : 8 cm Rain/hr

Conclusions• Stream power appears to be the better variable

controlling rill/gully formation at Minnehaha

Future Work• Drones!• Identify additional gullies• Make a few more measurements about rill

dimensions• Play with critical resistance values more• Investigate other variables