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2 Basic Fluvial Processess OverviewOverview ofof fluvialfluvial andand geotechnicalgeotechnical processesprocesses forfor TMDLTMDL assessmentassessment ChristianChristian FF LenhartLenhart ,, Assistant Prof, MSU Research Assoc., U of M Biosystems Engineering Fluvial processes in a glaciated landscape Martin County, MN Rocky Mountains (Rosgen) 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 0.40 Sdr = 63 Sm Rosgen (WARSSS pg. 2 – 3) Sed delivery by slope Sediment delivery by slope 2.5 2 1.5 1 Slope 0.5 0 0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% -0.5 Sediment delivery ratio 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 HillslopeHillslope processes:processes: MassMass soilsoil movementmovement GravityGravity --drivendriven movements:movements: Falls,Falls, slides,slides, flowsflows ,, soilsoil creepcreep BluffsBluffs areare aa majormajor sourcesource ofof sedimentsediment byby massmass --wastingwasting inin MinnesotaMinnesota RiverRiver BasinBasin 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 gully inset within larger ravine CS2 Riffle 25 20 15 10 Elevation(ft) Ravine 5 0 0 20 40 60 80 100 120 140 160 Gully Width from River Left to Right (ft) 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 =( ρρρ*g* A* ∆∆∆x* SIN ααα S0) + (Fp1- Fp2) – (τττb * 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 Particle size of suspended sediment on the Minnesota River at Jordan, MN betweeen 1981 and 2006 levels >70% is 35 silt / clay 30 At high flows 25 fines are <30% 20 15 frequency 10 5 0 90-100 80-90 70-80 60-70 50-60 40-50 30-40 20-30 10 to 20 % silt% of and particles clay finer than sand (0.063mm) Bedload sediment Moves by bouncing, rolling In MN River basin, comprised mostly of sand Smaller component of total load bed sediment easily Threshold sediment size vs. Median Particle mobilized at high Size in Elm Creek flows 40 35 30 25 20 Threshold = D50 Mobilization 15 10 Deposition 5 Threshold sediment size (mm) sizesediment Threshold 0 0 5 10 15 Median Bed Particle size - D 50 (mm) Channel-forming flows Dave Rosgen Hydrologic-watershed processes More generally, Lane’s sediment balance qs D50 ∞∞∞ qS qs = sediment discharge D50 = average diameter of bed particle size q = stream flow S = 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 Schumm Channel 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 Hydraulic erosion Hydrologic role – less mass wasting by lowering soil moisture Grazing effects on roots Bank Erosion Hazard Index quantifies root influence Grass vs. trees Headwaters 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 Creek 1.1 feet (n = 360) Little studied recently Historically by SCS after Dust Bowl years Current Research Ravine, Bluff, Streambank Erosion study in Minnesota River Basin Bioproducts & 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.
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