Human impacts on riversWaterSedimentWoodclimatecatchment vegetationchannelmorphologychannelmorphologygeologygeologygradientgradientvalleyformvalleyformhydrologyhydrologyriparianveg.riparianveg.sediment&woodinputssediment&woodinputsflowobstructionsflowobstructionsfastslowrate of changeclimatecatchment vegetationchannelmorphologychannelmorphologygeologygeologygradientgradientvalleyformvalleyformhydrologyhydrologyriparianveg.riparianveg.sediment&woodinputssediment&woodinputsflowobstructionsflowobstructionsfastslowrate of changeChannelization = direct human interventionChannelizationLos Angeles RiverChannelizationIllabot CreekSkagit RiverWatershedHuman impacts on riversWaterSedimentWoodHuman impacts on riversGlobally, we have increasedupland erosion by about tentimes, while simultaneouslycutting sediment delivery tooceans by half…Hydraulic MiningSierra NevadaHydraulic MiningSierra Nevada, Yuba County, 1870Sacramento river bed elevations 1850-1910Rising river bedexacerbates floodingon SacramentoRiver beginning in1860sK Street, looking east,1862Sacramento river bed elevations 1850-1910Farmers get mad and outlaw hydraulic miningHydraulicminingHydraulicminingHalf century lag time to move sediment throughsystem…Sacramento and Yuba river bedelevations 1850-1950Sedimentation in San FranciscoBay delta / estuary• 40% of SF Bay filled in with mud• 1.5 billion yards of sediment (8 times theamount excavated to build the PanamaCanal)Hydraulic Mining and DamConstructionSan Francisco Bay watershedDeclining sedimentation in San FranciscoBay delta / estuary due to passing ofsediment wave and dam constructionSan Pablo Bay / San FranciscoBayPost mining levees and subsidencein Sacramento River delta> 20 ftFlooding on Sacramento River20041997Human impacts on riversWaterSedimentWoodGreen River, WashingtonWaterDam built for downstream “flood control”Extensive floodplain development from1961-2009Howard Hanson DamGreen River “flood protection”• Flood control plans on the Green Riverdepended upon maintenance of the dam;• Dam operations limited the flow in the riverto < 12,000 cfs for most years, equal topre-dam 2 yr recurrence interval flow;• With dam in place the 2 yr, 100 yr, and500 yr floods were all determined to be =12,000 cfs.Green River “flood protection”• Control of overbank flows led to the“promise” of no more valley bottomflooding … and so the floodplain landvalues skyrocketed and commercial andresidential development replaced farms onmost of the valley bottom.• But…Howard Hanson dam started leaking almostas soon as it was finishedTwo depressions formed in right abutmentFlood ControlTowerFish PassageFacility ExcavationSpillwayGreen RiverEagle GorgeReservoirBig DepressionLocationEngineeredFilter BlanketDrainageTunnelLandslideDebrisSmall DepressionLocationAnatomy of Howard Hanson DamInside the damʼs drainage tunnelNormally, dyeplaced in thereservoir seepsthrough in 12 hours;it’s now happeningin < 2 hours…Normally, dyeplaced in thereservoir seepsthrough in 12 hours;it’s now happeningin < 2 hours…Potential Internal Erosion• Grout curtain and additional wells,November 2009• Cutoff wall, FutureHow to fix the dam?Green River “flood protection”• To fix the dam will require time as thereservoir is drawn down and can store lesswater;• With dam unable to store the equivalent ofa several year RI flood, let alone a 500 yrflood, what is the prognosis for the leveesbeing able to contain floodwaters?• Extensive annual flooding along valleybottom• “Pre-development” flood extent in 1965;after the dam was in place!• Natural levees along the river built up tocontain the bankfull flow (12,000 cfs)Green River Valley Flooding1965 Flooding Along East ValleyHighway1965 Flooding Along East ValleyHighway13,900 cfs17,600 cfs25,000 cfs“Supersacks” as flood protectionSupersacks add a bit of height to the levees• Other styles of “flood control” includegetting infrastructure out of harms way,levee setbacks, and designing “floodways”along which flood flows would be routed…• For this year, there are not many optionsother than to warn the public, set up aflood warning system, “enhance” thelevees, and pray it doesnʼ t rain a lot…Green River Valley Flooding• FEMA estimates that flooding from a leveebreach at ≈ 12,000 cfs could displace15,000 to 20,000 people and cause $1.3to 3.7 billion in damage.• How does flood control affect flood risks?Green River Valley FloodingInteractions between land use and"increased flood damage" in a
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