Christian F Christian F LenhartLenhart , , Assistant Prof, MSU Assistant Prof, MSU

Research Assoc., U of M Research Assoc., U of M BiosystemsBiosystems EngineeringEngineering

Overview of fluvial and Overview of fluvial and geotechnical processesgeotechnical processesfor TMDL assessmentfor TMDL assessment

Fluvial processes in a glaciated landscape

Rocky Mountains (Rosgen)

Martin County, MN

Hillslope processes

� Surface erosion

� Land-use history & changes to load

� Rates have decreased, but…

� RUSLE and other models- well studied

Sediment delivery

� Sed. delivery poorly understood

� Small % of eroded sediment is carried all the way to river mouth

� Sdr = 63 Sm0.40 Rosgen (WARSSS pg. 2 – 3)

Sed delivery by slope

Sediment delivery by slope

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Sediment delivery ratio

Slo

pe

Case studies

� Driftless area: Coon Creek, WI� 5% of sediment eroded since European

settlement carried out to Mississippi River(Trimble, 1993 - Science article).

� Rush Creek, MN – 4-8 feet on floodplain

� Elm Creek, MN� Sediment measured at gage 8-13% of

estimated annual soil erosion (Lenhart 2008)

Legacy sediment

Where is the excess sediment from the past 150 years stored?� Stream valleys

� Wetlands and lakes� Stream reaches with low velocity and slope;

Overwidened reaches (ditches) low unit stream power ( ω )

Depositional areas

� Ditches have become depositional areas

� Increased width reduces shear force, inducing deposition

� (Landwher, 200x)

Lakes and Wetlands

HillslopeHillslopeprocesses: processes: Mass soil Mass soil movementmovement

GravityGravity--driven movements:driven movements:Falls, slides, flows , soil creepFalls, slides, flows , soil creep

Bluffs are a major source of sediment by Bluffs are a major source of sediment by massmass--wasting in Minnesota River Basin wasting in Minnesota River Basin

Geomorphic categories

� Valley wall = Bluff� Streambank = Active channel boundary� Ravines = steep tributaries flowing over

the valley wall to larger rivers

Ravines

Ravines in Minnesota Basin

Hillslope Erosion: Gullies

•Sheet erosion

•Rill erosion

•Gullies

Gullies within ravines

CS2 Riffle

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Width from River Left to Right (ft)

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ft)

Ravine

Gully

gully inset within larger ravine

Channel Processes

Patterns of erosion and deposition

Equilibrium Theory and Streams

� Idealized stream in equilibrium:� Sediment supply in balance with transport

� Deposition on point bars in balance with erosion on outer bend

� Are Minnesota streams in equilibrium?

Physical forces in streams� Force balance described by equation of motion

For channel with flowing water:

d(mV)/dt = F gravity + F pressure – F shear

expanded out: d(mV)/dt =( ρρρρ*g* A*∆∆∆∆x* SIN αααα S0) + (Fp1- Fp2) – ( ττττb *

wp* ∆∆∆∆x)

[where, ρ = density of water, g= gravitational constant, A= area, ∆x = change in distance over control volume, S0= channel bottom slope, Fp1 = force at point x, Fp2 = force at point x + ∆x.]

� Force balance: streams exist in a dynamic equilibrium

� Lane’s = predicts channel adjustment� Channel dimensions shaped by frequently

occurring floods – bankfull flows

Sediment Transport in channels

� Bedload

� Suspended load� Wash load

Entrainment equations

Shield’s Equation

Suspended Sediment

� Often estimated by TSS (total suspended solids) - organic matter and sediment

� Turbidity is regulated pollutant

Particle size of SS

� At most flows levels >70% is silt / clay

� At high flows fines are <30%

Particle size of suspended sediment on the Minnesot a River at Jordan, MN betweeen 1981 and 2006

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% of particles finer than sand (0.063mm)

freq

uenc

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% silt and clay

Bedload sediment

� Moves by bouncing, rolling� In MN River basin, comprised mostly of

sand� Smaller component of total load

Threshold sediment size vs. Median Particle Size in Elm Creek

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Median Bed Particle size - D 50 (mm)

Thre

shol

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ize

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)

Threshold = D50

Deposition

Mobilization

� bed sediment easily mobilized at high flows

Channel-forming flows

Dave Rosgen

Hydrologic-watershed processes

� More generally, Lane’s sediment balance

qs D50 ∞∞∞∞ qSqs = sediment discharge

D50 = average diameter of bed particle sizeq = stream flowS = slope

Changes to equilibrium

Changes to watershed hydrology and streamflow cause channel adjustment

in Minnesota � Recent drainage increases

� Private tile drainage expansion < 30 years

� Precipitation high in 1990s� Result: increased low and mean flows

(Zhang and Schilling, 2007)

Simon and SchummChannel Evolution Model

� Most southern Minnesota streams are in stages 3-5, especially 4 and 5

Sources of sediment in rivers

� Streambanks� Bluffs� Ravines/gullies� Legacy sediment

Channel erosion: streambank

Photo of Elm Creek by C. Lenhart

Sediment sources: streambanks

Soil Traits of MRB streambanks� Allluvium

� Minnesota River streambanks (high sand%)� Gullies within ravines

� Young glacial till� Des Moines Lobe Till (fine silts and clays)

� Old glacial till� Superior lobe- highly compressed, stable

Role of vegetation

Grazing effects on roots

Bank Erosion Hazard Index quantifies root influence

Hydraulic erosion

Hydrologic role – less mass wasting by lowering soil moisture

Grass vs. treesHeadwaters prairie

Riparian forests on larger rivers

Sediment sources: Bluffs (valley wall erosion)

� Dramatic examples of mass-wasting� High delivery ratio� Stability of denser tills?

Sediment sources: Ravines/gullies

� Hard to capture events from gullies� Active gully only a small % of ravines� Sediment delivery is lower than

streambanks and bluffs – dump out onto MN River floodplain

Sediment sources: Legacy sediment

� Mean depth of fine sediment in Elm Creek1.1 feet (n = 360)

� Little studied recently� Historically by SCS after Dust Bowl years

Current Research

� Ravine, Bluff, Streambank Erosion study in Minnesota River BasinBioproducts & Biosystems Engineering, U of M

Minnesota Pollution Control Agency

� Purpose: to quantify sediment loads from R, B and S sources; contribution to turbidity problem

Methods

� Ravines: runoff, TSS monitoring at gully outlets; geomorphic assessment� Stream classification, CEM assessment

� Physical property measurement: critical shear stress, particle size

Study sites

Bank stability and toe erosion model (BSTM)

� Critical shear stress

� Cohesive strength

Data

� Field-measured rates of bank erosion� Modeled erosion and transport using

CONCEPTS� Sediment loading from gullies/ravines� Historic rates of channel migration

estimated from photos

Preliminary findings

� Bluffs – major sources of sediment; some hard tills are stable (Gupta, Thoma, Mulla)

� Ravines (Mulla GIS work) ?� Gullies� Streambanks:

Conclusions

� Examine processes from watershed headwaters to river mouth using WARSSS framework + extra tools

� Some key processes are different in flat glaciated landscapes versus mountains

� Total sediment erosion from watershed far exceeds amount carried out

Management Issues

� Ag erosion has decreased since mid 1900s

� Channel erosion is increasingly a large % of suspended sediment in rivers

� Legacy sediment largely ignored� Need channel management as well as

watershed management