erosionWeathering, soil & erosion…Sediment BudgetErosional Processes Erosional Processes ways to move soil:ways to move soil:ways to move soil:Soil CreepErosional Processes Slide Number 11Slide Number 12Slide Number 13Slide Number 14Overland FlowErosional Processes Bedrock landslidesSoil landslideslandslidesLandslidingErosional Processes Glacial ErosionErosional Processes River IncisionRiver IncisionErosional Processes bank erosionerosion controlsclimateslopeerodibilityland-usemountains: the big picturea final thought:erosionmoving away from the source…Weathering, soil & erosion…Sediment regime: amount, size, characteristic sediment transport style in a rivergoverned by (not surprisingly) the amount and size of material delivered from both hillslopes and upstreamthis, in turn, depends on erosional processes (today’s lecture)weathering rates & stylerock properties soil propertiesSediment Budgetalong a stream reach:Soil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputI - O = SSediment inputs from upstream and across channel banks are balanced by either downstream sediment transport or changes in sediment storage.Erosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver incision into bedrockBank ErosionErosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver IncisionBank ErosionSoil creep is the gradual, non- catastrophic downslope movement of weathered material under the influence of gravity (i.e., not by flowing water).ways to move soil:The burrowing activity of animals results in a net downslope transport of material that in some environments can be the dominant sediment transport process.ways to move soil:Tree-throw can uproot rocks and also typically results in a net downslope transport of soil and broken rock.Plowing a hillslope, ca. 1935National Archives: RG083 G 36711ways to move soil:human modifications…Soil CreepSoil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputSlow, steady input of material across channel banks, or delivered to valley bottom.Typical rates of 0.1 to 1 mm yr-1.Erosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver IncisionBank ErosionErosion by overland flow occurs once enough flow accumulates to overcome the erosion resistanceof the ground surface.Precipitation that runs off as overland flow can cause substantial erosion once enough flow accumulates to incise the ground surface. XcXc is the critical distance needed to incise a channel.Badlands environments are an extreme example where Xc may be just centimetersUnchanneled valleys occur where the erosion resistance of the ground surface is high relative to the amount of overland flow Xc is very large.Entrenched channels and gullies can develop in landscapes where overgrazing decreases the erosion resistance of the valley floorXc was large, became smaller…Overland FlowErosion by overland flow is rare in forested mountain landscapes because:• rainfall tends to infiltrate into the ground•the ground has substantial erosion resistance due to vegetationErosion by overland flow is most common in disturbed or semi-arid landscapesErosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver IncisionBank ErosionLandslides involve the downslope movement of soil and/or rock under the influence of gravity and may be either slow and gradual or rapid and catastrophic.Bedrock landslidesearth flows: some internal deformation typically slowrelatively little waterearth flow in NZSoil landslidesdebris flows: lots of internal deformationrapidrelatively high water contentfluid-like flowFailure typically occurs along well-defined shear plane at soil-bedrock interface.debris flows along Tolt R.landslidesBedrock landslides can limit the relief of mountain rangesMt. Cook, New Zealand:top 10 meters of summit fell away in a massive landslide/avalanche on December 14, 1991. ScarpRunoutzoneDepositLandslidingSoil CreepLandslidingBank ErosionUpstreamInputStream ReachDownstreamOutputRapid, infrequent inputs of large volumes of sediment.Rates of delivery set by landslide frequency, which is often centuries to millennia at a point.Erosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver IncisionBank ErosionGlaciers can both entrain loose surface materials and gouge deeply into bedrock.Glacial ErosionRapid erosion of material from above perennial snow line.Rates can exceed 10 mm yr-1.Processes of erosion and rates depend on temperature, glacier size, precipitation rate, etc...Erosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver IncisionBank ErosionRivers can carve deeply into bedrock and such incision provides another source of sediment.In the world there is nothing more submissive and weak than water. Yet for attacking that which is hard and strong nothing can surpass it.- Lao-Tzu, 6th century B.C.River IncisionErosion = f (discharge, channel width, slope)More water in a narrower channel down a steeper slope means faster river incision Rates of bedrock river incision typically range from <0.01 mm yr-1 to 1 mm yr-1, but can exceed 5 mm yr-1 in extreme topography.River IncisionErosional Processes Soil “Creep”Overland FlowLandslidesGlaciersRiver IncisionBank ErosionBank erosion recycles material stored on the valley bottom, typically in the floodplain.bank erosionreworking valley-bottom sedimentsentraining sediments delivered by other erosional processesRangitata R., NZerosion controlsclimate topography/slope vegetationlithology/erodibilityland-useclimateprecip & veg.sediment yield: total amount of sediment generated by a particular landscape00.20.40.60.810 5 10 15 20 25 30 35Erosion Rate (mm yr-1)Mean Slope (degrees)Olympic Mountainsslopeerodibilitybedrock erodibility ranges over at least 5 orders of magnitudeland-usemountains: the big pictureweathering, soil formation & erosion interact ultimately provide sediment for rivers & beaches…rivers and beaches are important sediment transfer (material transfer) systems within larger Earth systemremember this context!a final
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