Saturation overland flow

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

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

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 Evapotranspiration (ET) Precipitation (P) Sheet flow via HOF Infiltration (I) Rill erosion as Storm runoff to P > I surface runoff surface runoff streams mostly coalesces 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 • Slope-parallel preferential flow paths are more important for stormflow generation compared to vertical pathways • Connectivity is related to antecedent moisture • 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

6. Increasing velocity, thus increasing contribution to storm runoff in streams Which flowpaths are most important related to storm runoff and floods? 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. 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. (2) Mass wasting or landslides – gravity driven, but typicallytriggered by rainfall (1) Surface erosion – water driven To effectively control erosion in managed landscapes, it is important to distinguish between the processes of:

9. Detachment Transport Deposition Surface Erosion (by water, sometimes called surface wash) involves: 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. Discharge nodes from logging roads and skid trails Connectivity Example (erosion) – Unpaved Roads & Trails 1 6 2 7 3 4 9 8 5 • 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 Skid trails Logging road Overland flow Subsurface flow Rills, gullies, and concentrated surface wash Stream

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

15. Shallow, Rapid Landslides

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

17. 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 arelargest 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