Presentation on theme: "Sediment Transport & Geomorphology"— Presentation transcript:
1 Sediment Transport & Geomorphology ObjectivesLearn basic concepts of sediment transport and fluvial geomorphologyUnderstand sediment budgets (sources, sinks, pathways for sediment)Discuss infrastructure and ecosystem response
2 Sediment Transport and Geomorphology in Planning Steps Characterize Physical Attributes ofExisting conditionsReference conditionsFuture w/o project conditionsAlternatives70% of ecosystem restoration efforts are linked to sediment and geomorphology1. Geomorphic analysis doesn’t give you a precise answer, but it is a fact that much of the work we do these days is linked to the effects of sediment transport and geomorphology.
3 Sediment Transport & Geomorphology mechanics of sediment erosion, transport, and deposition by waterGeomorphologygeologic science of landscape formationH&H is the big driver affecting sediment transport and geomorphology
4 Too Much/Too Little Sediment Problems with too much sedimentRaised flood profilesReduced underwater lightDecreased capacity of hydraulic structuresProblems with too little sedimentIncision (channel lowering)Delta lossScour at hydraulic structuresHuman’s and other biota have adapted to a river’s historical condition, and a change in that condition (more or less sediment) invariably creates problems.
5 Sediment Size Clay < 0.004 mm Silt 0.004 - 0.0625 mm Sand mmGravel > 2mmThe larger the sediment particle, the less readily it is picked up by flow at a given speed, and if picked up, the shorter the distance it is likely to travel before re-settlingClay and silt are considered fine sedimentsSand and gravel are considered coarse sedimentNote: Protocols for cobble bed mountain streams may differ, where fines may be considered anything less than 5.6 mm.Sediment can be described by size
6 Descriptions of Sediment Load Ability to MeasureMode of TransportLocation in RiverMeasured Load-Sediment thatcan be measured with a samplerSuspended Load-Sediment thatcan be found at any depth. Includesfine and coarse sedimentWash Load – sediment thatpasses over bed without deposition.exchanges with banks/floodplainSediment can be described using load descriptions, and keeping the terminology straight is important.Bed Load is difficult to measure in sand bed streams and is often assumed to equal 5 to 15% of the total sediment load, though much higher values have been measured. On the Chippewa River in west central Wisconsin, USGS measurement indicate that bed load is 50% of the total sediment load.Un-Measured Load-Sediment in thelowest portion of the water columnthat cannot be measured with mostsamplersBed Load-Sediment thatcreeps or hops along bed(coarse material)Bed Material Load – sediment thatexchanges with and is found inmeasureable quantities in bed
7 Suspended Load Curves (based on measured SS) Suspended sedimentcan be measured usingsamplersA plot of suspendedsediment load versuswater discharge. Noteorder of magnitudevariation.1. Suspended sediment load data on the Missouri River.2. Note logarithmic scales. At a flow of about 50,000 cfs, the measured sediment load varies from 10,000 to 100,000 tons/day3. Research in small “natural” basins has shown that sediment concentration doesn’t increase with water discharge, suggesting that the sediment rating curves that we are used to looking at are a result of anthropogenic effects (John Gray, USGS)Suspended Sediment Load (tons/day) = Sediment Concentration (mg/L) * Water Discharge (cfs) * .0027
8 Bed Load Bed load moves in waves at a certain Sketch & Bathymetry from Dvd Abrahamand Thad Pratt, ERDCBed load moves in waves at a certainspeed (or celerity). The celerity (c) is givenby dividing the distance ∆x by time ∆t.Problem is that measuring c is difficult andexpensive, and calculating it is uncertain evenwith good models and data.Often we just assume that bed load is 5% to15% of the total sediment load.1. Helley Smith samplers are placed on the bottom of rivers and streams to measure bed load. They work OK in gravel bed streams usually hand-held. They don’t work very well in sand bed rivers and streams.
9 Calculating Sediment Transport There are dozens of sediment transport functions that predict sediment transport based on:sediment size, weight, and fall velocitywater velocity and depthchannel widthchannel slope and roughnesswater temperatureMany assumptions are madeChoose functions appropriate for your conditions250 KHz Geoswath echo sounder. The boat is RTK GPS positioned and compensated for pitch,heave and roll. Horizontal accuracy is stated as +/- 2 cm and vertical resolution of bathymetricelevations is approximately 3 cm in 50 meter of water.
10 Lane’s Balance says that sediment discharge and sediment Sediment discharge and sediment grain size tend to balance against water discharge and slopeAt a watershed scale, sediment balance doesn’t exist for entire watersheds.At a river reach scale, sediment balance can occur.Lane’s Balance says that sediment discharge and sedimentgrain size tend to balance against water discharge and slope
11 Sources, Sinks, Pathways Rio Puerco, NMSourcesBedBanks (Bluffs)Ravines & GullysWatershedWatershed Sources depend on:geology and topography of the watershedmagnitude, intensity, duration, and distribution of rainfallvegetative cover; and the extent of cultivation and grazing.Regression methods are used to develop soil loss relationshipsSourceSinkErosion on the outside of a bend is a source of sedimentDeposition on the inside of this bend is sink for sediment
12 Sources, Sinks, Pathways FloodplainsValley side slopesDeltasOff - channel areasUSACE DredgesMost rivers cannot transport all of the sediment that is eroded within its channels and watersheds, so every river system has sinks for sediment.This city park became a sink for sediment.
13 Sources, Sinks, Pathways The capacity of a stream to transport sediment depends onhydraulic properties of the stream channel and sediment propertiesSediment Transport = F (hydraulic properties & sediment properties)slopevelocity grain size distributionchannel geometry cohesivenessroughnessNote that the hydraulic properties are the same as those that we discussed in hydraulics.Temperature can have an effect on sediment transportIf the grain size is large (cobbles) or if the material has cohesive properties, sediment transport may be limitedpathwaysink
14 Sediment Budgets, Watershed Scale Whitewater River Sediment Budget,NRCS, 1965 to 1994Sources of sediment estimated by AGNPs modelSinks determined by historic survey comparisonSources (1000’s tons/year)Sheet & RillErosion555Ephemeral & ClassicGully Erosion72StreambankErosion86HeadwatersSediment LoadTo Mississippi River24Colluvium553Sandbars &StreambankDeposits17TributaryValleyDeposits36MainValleyDeposits63Whitewater DeltaDeposits20Every river is different, some because they have been altered or because of natural geological events, may pick up more sediment near the mouth.The point here is that sources and sinks are rarely in balance and the transport capacity of the “pathway” (the river channel) varies significantly from reach to reach.Sinks (1000’s tons/year)Only 24,000 tons/year of the total 713,000 tons/year fromsheet and rill erosion, ephemeral & classic gully erosion, andstreambank erosion is transported to the mouth of the river.
15 Sediment Effects on Water Quality The majority of sediment transport in a given year occurs during seasonal high water eventsSediment transport during other times can have a significant effect on underwater light, nutrient loads, substrate.
16 Suspended Sediment Concentration (SSC) is Different than Total Suspended Solids (TSS) SSC sampling: Iso-kinetic sampling where the velocity and concentration in thesampler intake is equal to the velocity and concentration in the surrounding water is required toensure sediment samples that represent the true sediment load.See USGS protocols by Edwards and Glyssen (1999)and Davis (2005)TSS sampling (e.g. automatic water samplers (or pump samples)Often are not iso-kinetic, however this is not a problem for finesediments.The SSC analytical method uses the entire water-sedimentmixture in the analysis. (ASTM D-3977)A TSS analysis entails withdrawal of an aliquot (or part) of the original sample for subsequentanalysis. (SM D). It is OK to use if the sediments are fine sediments, but don’t use if coarsesediments are in the sample.For more information seeTSS sampling is cheaper than SSC, and is used if coarse sediments are not a concern (e.g. for water quality sampling).Need to be aware of sampling protocols.
17 SSC median bias = -1.8%; 25th percentile = -4.4%; 75th percentile = 0.0% TSS median bias = -16%; 25th percentile = -32%; 75th percentile = 8%Same lab, TSS results of 3 QA test sets (35 samples); SSC results of 2 QA test sets (18 SSC)Data from USGS
18 Geomorphology Geologic science of landscape formation High EnergyChannelGeologic science of landscape formationFluvial Geomorphologylandscape formation by streams1. The sediment depositing in this delta was eroded from some upstream site, transported down the river to this site, and deposited where the channel enters the backwater because of the low energy environment in the backwater.Low EnergyBackwater
19 Watershed and Channel Alteration will change H&H Causing Geomorphic Responses Including: Channel incisionChannel pluggingLand loss along channelsGullyingFloodplain depositionIn other words, many of the USACE missions are affected by sediment transport
20 Geomorphic Responses may Affect: Infrastructure: Bridges, FRMHuman uses: Drinking water, recreation, agricultureAquatic HabitatCommercial NavigationWater Quality: Underwater light, nutrients, contaminantsIn other words, many of the USACE missions are affected by sediment transport
21 Spatial Scales Changes in: Channel geometry Slope Roughness Cause changes inSediment transportGeomorphic processesChannel Capacity1. Remember Lane’s relationship: Sediment load and sediment size are proportional to water discharge and slope.2. The water discharge at Hendrum (river mile 5) is about 2,880 cfs, while the water discharge at Twin Valley (river mile 60 is about 1480.3. The lower reaches of the Wild Rice River don’t have the capacity to transport all of the sediment from upstream sources.
22 Time-ScalesAnnual geomorphic changes like sand bar migration, bank erosion, point bar building occur due to seasonal high flowsLong-Term geomorphic change like incision or delta building or loss might be natural or anthropogenic.Climate variation, watershed development, channelization, dams, urbanization affect geomorphic change at both scales.Fluvial Geomorphic Analysis must account for time scales. For instance is a shift in conditions due to land use change over the last few decades or longer term change driven by past geologic events or climate variration.
23 Use Multiple Tools for Sediment Transport and Geomorphological Analysis Field InvestigationsExisting conditions substrate, bankfull conditions, vegetation, discussion with local expertsSurveys:Cross sections, profiles, sediment coresAnalytical TechniquesNumerical Models:Watershed modelsRiver modelsAerial photo comparisonsChange from Historic conditionsSediment BudgetsSpecific Stage Discharge AnalysisA combination of techniques usually is needed to establish multiple lines of evidence.
24 Erosion of a River Bend Sink Source Sink Geotechnical Failure From BendMigration And Toe ErosionLow Flow Cross SectionHigh Flow Cross Section
25 Flood of Record Washed Out Railroad, Undermined Houses This flood of record probably caused direct erosion of the bankNote RR tracks hanging
26 River Meandering and Effects of 0.2 % Chance Flood Significant migration occurred in the 37-year period between these two photographs.The flood of record in 2002, caused significant floodplain erosion in this highly altered river reach.
27 Geomorphic Response to Watershed Development Floodplain Deposition, Channel Incision
28 Whitewater River Avulsion August 2007 Highway 61 BridgeCanadian Pacific RR BridgeWhitewater River AvulsionAugust 2007New Mouth ofWhitewater RiverAvulsion is a sudden change in channel position, with abandonment of old channel.Whitewater River shortenedBy about 6,000’Old Mouth ofWhitewater River
30 Channelization Sediment Deposition, Loss of Capacity Many channelized reaches of rivers were oversized resulting in lower velocities and sediment deposition. They essentially became sinks for upstream sediment sources.This agricultural levee break resulted in deposition of sand on the farmers field.Agricultural Levee Break
31 Island Loss And Erosion Increased Connectivity Sediment Deposition Geomorphic Response to Raised Water LevelsIn Lower Pool 8Island Loss And ErosionIncreased ConnectivitySediment Deposition193819541991Extensive land loss has occurred in the lower ends of navigation pools on the Upper Mississippi River
32 BridgesUsually footings are deep enough to handle a certain amount of scour.Changed hydrological conditions (climate variation, land use change) or changed hydraulic conditions (dam removal, debris) can create greater scour levelsA scour signature may not last. See the next six images. Note that the bottom elevation is about at the start of this event.
39 Bridge TH 212 over Minnesota River Overflow Abutment Fill and Approach Panel Lost Q500 = 7,300 cfs Q1997 = 10,000 cfsExcerpts from Inspectors Notes:4/4 108” below NW, channel has shifted to east4/5 69” from SE wingwall, deck vibrating, riprap eroded, looking down through water.4/6 2:00 PM Panel undermined, water flowing under it.4/6 5:00 PM Panel fell in.TH 212 Minnesota River Overflow just west of Granite Falls, MN.In 1997 a roadway failed upstream redirecting main channel flow into the overflow channel. Approximate Q1997 = 10,000cfs, V1997 = 10 fps.(Q500 = 7300 cfs)District Bridge Inspector observed scour making measurements with a sonar device, visible signs of structural distress (deck vibrating and riprap eroded) noted prior to failure.Road was closed to traffic prior to panel falling in.Bridge superstructure and substructure in good condition.
40 Bridge 54002 Bridge is still structurally stable, but from the travelers perspective it failed. Bridge upstream which used to catch debris washed out.Repairs $200,000 replace fill, panels, road, guard rail, riprap.
41 Long Lake Water Control Structure after 2001 Flood