1. Roy C. Sidle, Professor of Geography
Hydrologic Processes that Contribute to Storm Runoff, Soil Erosion, and Landslides in Managed Catchments
Roy C. Sidle, Professor of Geography

2. Types of flow paths at the hillslope or catchment scale
Subsurface
flow
Matrix flow
saturated
unsaturated
Hortonian
overland
flow
sheet
channelized
Saturation
overland
flow
Return flow
Near-surface
water table
Preferential flow
macropores
pipes
fractures
hydrologic
discontinuities
mesopores
biomat flow

3. The Hortonian overland flow concept
Key points:
Rain intensity > infiltration capacity
Generally occurs in arid or semi-arid landscapes where vegetation cover is sparse and organic horizons are thin or non-existent
Sustained runoff concentrates and causes erosion
Infiltration (I)
Evapotranspiration (ET)
Sheet flow
via HOF
Rill erosion as
surface runoff
coalesces
Precipitation (P)
P > I
Storm runoff to
streams mostly
channelized flow
Savannas, North Queensland, Australia
Direct runoff connections to channels delivers excessive sediment

4. Subsurface stormflow within the soil
Subsurface flow that occurs laterally in hillslope soils is often called throughflow or interflow and is characterized by:
Diffuse flow passing through fine pores in the soil
May be saturated or unsaturated
Driven by an energy gradient [H = pressure head + elevation head] for saturated flow
In steep headwaters, elevation head typically dominates over pressure head for saturated flow
Saturated subsurface flow contributes to storm runoff (mostly during recession limb); unsaturated flow affects antecedent soil moisture distribution but does not directly contribute to storm runoff

5. Preferential flow in hillslope soils
Saturated flow that passes through inter-connected macropores, soil pipes or other preferred flow pathways
Generally the perimeters of macropores must saturated for flow to initiate
Slope-parallel preferential flow paths are more important for stormflow generation compared to vertical pathways
Connectivity is related to antecedent moisture

6. Which flowpaths are most important related to storm runoff and floods?
Increasing velocity, thus increasing contribution to storm runoff in streams
Infiltration-excess overland flow (Hortonian Overland Flow)
Preferential flow in soils & regoliths
Subsurface runoff within the soil matrix
Saturated overland flow (water table rises to the soil surface; rainwater cannot infiltrate)

7. Arid, semi-arid, and seasonally dry areas
Hortonian overland flow and minor subsurface flow feeds headwaters dominates
Return flow is less
important, only cccuring
at seeps near slope breaks
Wide, flat floodplain both Hortonian overland flow and some saturated overland flow
Channel interception occurs
throughout
Arid, semi-arid, and seasonally dry areas

8. To effectively control erosion in managed landscapes, it is important to distinguish between the processes of:
(2) Mass wasting or landslides – gravity driven, but typically triggered by rainfall
(1) Surface erosion – water driven

9. Surface Erosion (by water, sometimes called surface wash) involves:
Detachment
Transport
Deposition
Types of Surface Erosion
Splash Erosion
Sheet Erosion (sheet wash)
Rill Erosion
Gully Erosion
5. Piping/collapse

10. Raindrop Impact / Splash Erosion

11. Primary Natural Controls on Surface Erosion
Topography/microtopography
Slope gradient
Slope breaks
Surface roughness
Soil properties
Infiltration capacity
Tendency for surface sealing
Tendancy for water-repellent conditions
Soil depth (as may affect return flow)
Aggregate stability (cementation, texture)
Soil organic matter
Depth, decomposition rate, type, influence on infiltration capacity: all of these factors dissipate raindrop energy and soil displacement

12. Natural Controls on Surface Erosion (continued): Vegetation
Primary soil cover (grasses, forbs)
Roughness elements
Canopy effects (raindrop energy dissipation)
Maintains infiltration rates
A fundamental objective is to minimize overland flow, which:
Increases infiltration and soil water available to vegetation
Reduces erosion
And attention needs to be paid to the connectivity of overland flow paths – not necessarily just those from one land use; thus, looking at the ‘big picture’ is essential

13. Connectivity Example (erosion) – Unpaved Roads & Trails
Discharge nodes from logging roads and skid trails
Stream 1 2 3 4 6 8 5 9
Skid trails
Logging road
Overland flow
Rills, gullies, and concentrated surface wash 7
Subsurface flow
Connectivity of roads and trail networks is more important than road density in terms of water and sediment delivery to streams
Runoff occurs from compacted surfaces, but also from water intercepted along cutslopes

14. Simple illustration of the important of ‘connectivity’ related to movement of water, sediment, and chemicals to streams
Dispersed land uses v
Connected land uses
Thus, these concerns will likely cross property boundaries and require a cooperate approach

15. Shallow, Rapid Landslides

16. Slower, deep-seated landslides
Slump-earthflow, coastal Oregon

17. Slow flows and deformations
Solifluction
Alaska
Slow flows and deformations
Creep terrain, northern California

18. What rainfall conditions are most important for initiation of various types of landslides?
For shallow, rapid landslides, short-term rainfall intensity is essential, often coupled with large total precipitation and wet antecedent conditions
For slower, deep-seated mass movements, long-term precipitation is more important (cumulative rainfall up to a month or more) and these landslides (slumps and earthflows) typically activate and move slowly

19. What conditions related to evapotranspiration (ET) are most important for slope stability?
Transpiration rates: trees > woody shrubs > grasses
Deep-rooted species extract more water near the potential failure plane than shallow-rooted species
Conifer stands intercept more precipitation (both rain & snow) than deciduous forests
Multi-tiered forests may be most effective in intercepting rainfall – e.g., tropical rain forests

20. The mains ways in which woody roots reinforce soil mantles
Root mats, branching, and interlocking with coarse fragments and rocks enhance lateral root strength in shallow soils
soil
These effects are more important in shallow soils
bedrock
groundwater
table
Anchor roots stabilize shallow soil mantles into underlying bedrock
Potential slip surface

21. Mountain Roads and Trails
The Unpleasant Truth – Roads alter hydrologic pathways (both surface and subsurface) in catchments & destabilize steep hillslopes
Hydrologic changes due to road construction exist as long as the road is in place and generally persists after road closure
Mountain roads/trails are largest producer of landslide sediment per unit area in mountainous terrain; ≈ 30 to >1000 times more landslide erosion than from undisturbed forests
Trails have lesser but potentially significant impacts, although they are typically not ‘cut’ deeply into hillslopes
Impacts are greatest on roads/trails near streams and on roads cut into steep slopes