The effect of raindrop impacted flow on sediment composition

Rain Forms of Erosion on a Hillslope Splash Erosion River

Detachment Transport Fall Loose predetached particle Detachment is the plucking of soil particles from within the soil surface where the particles are held by cohesion and inter-particle friction Uplift Raindrop impact is one of the two agents causing detachment in eroding areas Forms of Erosion on a Hillslope detachment is the initializing process

On sloping surfaces more splashed down slope than up so more erosion as slope gradient increases Raindrop Detachment & Splash Transport (RD-ST) Splash Erosion Relatively inefficient erosion system Transport process limits erosion particularly on low gradient slopes Relatively inefficient erosion system Forms of Erosion on a Hillslope

Rain Splash Erosion Surface Runoff Rain-impacted Flow River

1. Raindrop Detachment & Raindrop Induced Saltation (RD-RIS) Uplift caused by raindrop impacting flow Flow Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope 3 common detachment and transport systems

Move downstream during fall Flow Wait for a subsequent impact before moving again Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope 1. Raindrop Detachment & Raindrop Induced Saltation (RD-RIS)

2. Raindrop Induced Rolling (RIR) Move downstream by rolling Flow Wait for a subsequent impact before moving again Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope

Raindrops cause uplift in flow Flow 3. Raindrop Detachment with Flow Suspension (RD-FS) Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope

Small particles remain suspended and Flow Large particles wait move without further stimulation Acts at the same time as RD – RIS/RIR Erosion by Rain-impacted Flow Forms of Erosion on a Hillslope 3. Raindrop Detachment with Flow Suspension (RD-FS)

Splash Erosion and Erosion by Rain-impacted Flow cause SHEET EROSION Loss of soil in a relatively uniform sheet over the area Forms of Erosion on a Hillslope

Rain Splash Erosion Surface Runoff Rain-impacted Flow Rill & Interrill Erosion River

Flow Rill Erosion and Interrill Erosion Interrill area Rill Rills are channels that can be removed by cultivation Close up of a piece of a field Forms of Erosion on a Hillslope

Flow Detachment & Flow Driven Transport Detachment and uplifted using flow energy Flow Rill Erosion Forms of Erosion on a Hillslope

Fine - Transported by flow as suspended load - fast moving Coarse - Transported by flow as bed load – fast moving Rill Erosion Flow Transport Raindrop impact not involved in any way Forms of Erosion on a Hillslope Flow Detachment & Flow Driven Transport

Critical conditions for detachment and transport modes for silt and sand and fine particles Flow Energy Flow detachment (FD) only occurs when the shear stress needed to cause detachment is exceeded - RILL EROSION Raindrop detachment (RD) only occurs when the raindrop energy exceeds that need to cause detachment Raindrop impacted flow Dominates sheet and interrill erosion

Most of the time, material leaving this bare area has been detached from the soil surface by raindrops NOT the flow. Flow detachment is dominant in channels only when channels are developing. Rain- impacted flow (RD-FS/RIS/RIR) is responsible for loss of nutrient rich soil material from the land that may end up in water supplies etc Forms of Erosion on a Hillslope

Flow Erosion by Rain-impacted Flow Raindrop Detachment & Raindrop Induced Saltation (RD-RIS) Particles transported by raindrop induced saltation move horizontally at velocities that depend on their size, density, and the velocity of the flow because these factors control the distance particles travel after a drop impact (x) x

Drop impact Distance particle travel after a drop impact Only impacts within the distance X cause particles to pass over the boundary Looking down on an area of soil covered by rain-impacted flow Positions of drop impacts over some period of time Erosion by Rain-impacted Flow

Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density Drop impact Distance particle travel after a drop impact Only impacts within the distance X cause particles to pass over the boundary Positions of drop impacts over some period of time Erosion by Rain-impacted Flow

Distance particle travel after a drop impact Drop impact Sediment discharge varies with particle travel distance (X) - varies with flow velocity and particle size and density 3 times faster Experiments with coal and sand indicate that coal particles move about 2.75 times faster than sand particles of the same size Only impacts within the distance X cause particles to pass over the boundary Erosion by Rain-impacted Flow

Mechanistic model of raindrop induced saltation 2.7 mm raindrops impacting a 7 mm deep flow 0.46 mm sand0.46 mm coal Drop impacts generated randomly in space as with natural rain Erosion by Rain-impacted Flow - time in flow 0.2 s - time in flow 0.55 s

Particle travel rates Non erodible 2980 mm Flow Erodible : 20 mm long Rain : 2.7 mm drops at 60 mm/h over 3 m length Simulation result Flow velocity = 150 mm/s 7 mm Sand takes 2.75 times as long to reach the end

Particle travel rates Particles of sand can be considered to have times of concentrations that are 2.75 times longer than particles of coal of the same size The concept of time of concentration is useful in looking at the effect of rainfall on runoff

The concept of time of concentration is useful in looking at the effect of rainfall on runoff. Particle travel rates Compare runoff rates (mm/h) over time for a given rainfall event 300 m impervious, n=0.03 Gradient = 3 % Gradient = 0.5 %

Particle travel rates Rainfall rate 50 mm/h 300 m impervious, n=0.3 Gradient = 3 % Gradient = 0.5 %

Particle travel rates Rainfall rate 50 mm/h Gradient = 3 % Gradient = 0.5 % m impervious, n=0.3

Particle travel rates Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material Flow Rain : 2.7 mm drops at 60 mm/h over 3 m length Simulation result Flow velocity = 150 mm/s Detention storage of sediment - build up of loose sand and coal particles on the surface protects the surface against detachment and causes fine discharge to decrease ```````` 7 mm

Particle travel rates Cohesive erodible 3000 mm surface with sand : coal = 1:1 plus fine material Flow Rain : 2.7 mm drops at 60 mm/h over 3 m length Flow velocity = 150 mm/s Detention storage of sediment - build up of loose sand and coal particles on the surface protects the surface against detachment and causes fine discharge to decrease ```````` Initially much more coal is discharged than sand but over time the two materials tend towards composition in the original erodible surface X pd coal = 2.75 X pd sand 7 mm

Enrichment Ratios s = 3% with s = 0.5% 0.46 mm coal with 0.46 mm sand Enrichment ratios ≠ 1.0 only when erosion is not occurring at the steady state Enrichment ratios = 1.0 at the steady state “E R ” for flow example Detention storage of sediment reduces detachment The ratio of the proportion of the material in the discharge to the proportion of the material in the original

Experimental Evidence Walker, Kinnell, Green m long inclined sand surface 2 slope gradients: 0.5%, 5% Events of 1 hour rainfall with uniform drop size 2 drop sizes : 2.7 mm, 5.1 mm 3 rainfall intensities: 45, 100, 150 mm/h

Experimental Evidence 5% slope 0.5% slope 150 mm/h 45 mm/h 2.7mm drops Rolling 2 mins 60 mins Enrichment at 2 mins and 60 mins for 2.7 mm and 5.1 mm drops Reduction in impact frequency and flow velocity gives slower developement Increase in flow depth + reduction in flow velocity gives slower development Rolling Lowest erosive stress Highest erosive stress

Experimental Evidence Palis et al 1990: Sandy clay loam soil on 0.1 % slope 5.8 m long 100 mm/h using continuous spray 0.6 min 5 min 15 min 35 min

Confounding Factors Effective particle travel velocities vary for near zero to that of the flow Aggregates breakdown may occur during transport of soil material – changes relative travel rates Interactions between particles of different sizes and densities

Confounding Factors Model on 10 m long impervious plot inclined at 9 % Cohesive source has 5 particles sizes equally represented 50 mm/h rain intensity (2.7 mm drops) Flow depth and velocity vary down along the slope Height particles are lifted is restricted by height of water above surface Height particles are lifted is restricted by water absorbing drop energy

Confounding Factors Model on 10 m long impervious plot inclined at 9 % Cohesive source has 5 particles sizes equally represented 50 mm/h rain intensity (2.7 mm drops) Flow depth and velocity vary down along the slope

Confounding Factors Model on 10 m long impervious plot inclined at 9 % Cohesive source has 5 particles sizes equally represented 50 mm/h rain intensity (2.7 mm drops) Flow depth and velocity vary down along the slope Time to reach the steady state controlled by the slowest moving particles Slower particles affect the discharge of faster ones Enrichment Depletion

Confounding Factors Critical shear stress for flow driven saltation Raindrop impacted flow Raindrop detachment + flow driven saltation

Confounding Factors Model on m long plots inclined at 9 % 2 part high intensity rainfall event Cohesive source has 5 particles sizes equally represented Flow depth and velocity vary down along the slope Enhanced loss of coal when L > 15m, 0.11 sand when L > 20m resulting from short term change from RIS to FDS

Fundamentally, sediment enrichment occurs because 1.All particles do NOT travel laterally at the same rate 2.Erosion of the soil is occurring under non-steady conditions Time of concentration approach relevant but need to consider the effect of detention storage of sediment on detachment etc Modelling presented here is only qualitative – need to undertake research to determine more effectively how particles of differing sizes and densities actually travel and interact in rain- impacted flows A% carbon or nutrient in soil SEDIMENT ENRICHMENT ---- more than A% carbon or nutrient in sediment Rain

Complicating Factors Flow depth Flow velocity Need to be known in experiments on rain-impacted flows 2.7 mm drops Previous research on depth effect Interrill erosion experiments ? Depth effect not well known Sprays: complex flow depth effect extrapolation