EphGEE: Ephemeral Gully Erosion Estimator - Dabney
-
Upload
soil-and-water-conservation-society -
Category
Environment
-
view
415 -
download
3
Transcript of EphGEE: Ephemeral Gully Erosion Estimator - Dabney
EphGEEEphemeral Gully Erosion Estimator
Seth M. Dabney and Dalmo VieiraUSDA-ARS-National Sedimentation Laboratory
RUSLE2 – Revised Universal Soil Loss Equation (version 2) Estimates Sheet and Rill Erosion not Ephemeral Gully Erosion
In 2014 - NRCS implemented the first update to RUSLE2 since 2006
RUSLE2Science Template
topographic deposition and fine sediment enrichment
RUSLER – RUSLE2 called through API, Slope length calculated as ratio of runoff leaving to runoff generated within cell
Ephemeral gully channels end RUSLE hillslopes (AH 703), Treynor Iowa W11
Watershed 11Treynor, Iowa, USA 1975 – 1991
contour-plowed corn averaging
7.6 Mg ha-1
36 Mg ha-1 y-1
16 t ac-1 y-1
RUSLER - RUSLE2 called through API for each raster; slope length calculated as ratio of runoff leaving to runoff generated within cell
Potential Gully Networks
RUSLER SetupDrainage catchments for each channel cell
Raster Channel Network EphGEE
ComputationalNetwork
Computational Channel Network
Refined network Simpler computations Better spatial distribution User-defined channel densification
factor
Water Surface Profiles
Steady, spatially varying discharges Flow properties using evolving channel geometry Standard Step Method
Derived from energy balance Accounts for backwater, varying channel roughness Channel junctions: equal water depth
EphGEE uses CREAMS conceptual model of George Foster
Gully Evolution MethodFoster & Lane Method “Non-erodible” layer
Determined by depth of last tillage operation
Shallow, wide channels
Gully Channel EvolutionFoster & Lane Method Two-phase erosion
Channel incision at ‘equilibrium width’ until non-erodible layer is reached
Channel widening at varying rate
ChannelWidth Depth to non-erodible layer
Non-erodible layer
Initial terrain elevation (thalweg)
Initial cross-sectional shapeWater level
Gully Channel EvolutionFoster & Lane Method I. Downcutting
Channel incision at constant erosion rate
ChannelWidth
Non-erodible layer
Phase I: DowncuttingChannelWidth
Non-erodible layer
Water level
)( cgp KD 5.1)/01.0( tg n
Potential detachment capacity (kg m-2 s-1) K – soil erodibility (s m-1)
Shear stress available for sediment transport (Pa))/1( cp tgDD Actual detachment: g – total sediment load; tc – transport capacity
Gully Channel EvolutionFoster & Lane Method II. Widening
Channel widening at varying rate Until shear stress at corner
equals critical shear stress
Final channel width
Non-erodible layer
Initial terrain elevation
Phase II: Widening
EphGEESoil Erodibility Critical shear stress Soil Erodibility
Function of soil clay content (percentage) and time since tillage
Relationships derived from data in Watson et al. (1986)*
* Watson, D.A., Laflen, J.M. & Franti, T.G. (1986) Estimating ephemeral gully erosion. ASAE Paper No. 86-2020
bCc ea
ta 0075.0294.0 286 1079.21028.10418.0 ttb
ceK 226.00306.0
daystifbanda 10000252.079.0
EphGEESediment Transport Finite volume mass balance
Incoming sediment load
Sediment transport capacity
Lateral sediment inflow
Potential Sediment Detachment
Outgoing sediment load
EphGEE – Ephemeral Gully Erosion EstimatorCross Section #2
Combined sheet, rill, and ephemeral gully erosion with no grassed waterway
33 Mg ha-1 y-1 delivered from
watershed
Combined sheet, rill, and ephemeral gully erosion with grassed waterway
17 Mg ha-1 y-1 delivered from
watershed
RUSLE2/EphGEE 17-year, 6.3 ha Watershed 11No Grassed Waterway
Characterization RUSLER/EphGEE
Sheet/Rill Sediment Yield
CS#1 (Mg ha-1 y-1) 36.3 46.2CS#2 (Mg ha-1 y-1) 48.5 64.5CS#3 (Mg ha-1 y-1) 39.7 53.4Outlet (Mg ha-1 y-1) 36.4 32.9
Observed (Mg ha-1 y-1)
RUSLE2/EphGEE 17-year, 6.3 ha Watershed 11Grassed Waterway
Characterization RUSLER/EphGEE
Sheet/Rill Sediment Yield
CS#1 (Mg ha-1 y-1) 36.3 46.2CS#2 (Mg ha-1 y-1) 48.5 64.5CS#3 (Mg ha-1 y-1) 39.7 21.6Outlet (Mg ha-1 y-1) 36.4 17.5
Observed (Mg ha-1 y-1) 14.6
EphGEE – Summary
C++ object-oriented program to be implemented as a web-based tool delivered by ARS and commercial providers
Potential gully locations determined through terrain analysis
Gully channel geometry evolves in response to a series of event runoff and sediment inputs
Ref: Dabney et al. 2015 J. Hydrol. Eng. 20(6):C4014009
Questions?
Headcuts advancement upslope from Potential Ephemeral Gully (PEG) mouth points identified using a Compound Topographic Index (CTI)