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Sediment Transport & Geomorphology

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Presentation on theme: "Sediment Transport & Geomorphology"— Presentation transcript:

1 Sediment Transport & Geomorphology
Objectives Learn basic concepts of sediment transport and fluvial geomorphology Understand sediment budgets (sources, sinks, pathways for sediment) Discuss infrastructure and ecosystem response

2 Sediment Transport and Geomorphology in Planning Steps
Characterize Physical Attributes of Existing conditions Reference conditions Future w/o project conditions Alternatives 70% of ecosystem restoration efforts are linked to sediment and geomorphology 1. 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 water Geomorphology geologic science of landscape formation H&H is the big driver affecting sediment transport and geomorphology

4 Too Much/Too Little Sediment
Problems with too much sediment Raised flood profiles Reduced underwater light Decreased capacity of hydraulic structures Problems with too little sediment Incision (channel lowering) Delta loss Scour at hydraulic structures Human’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 mm Gravel > 2mm The 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-settling Clay and silt are considered fine sediments Sand and gravel are considered coarse sediment Note: 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 Measure Mode of Transport Location in River Measured Load-Sediment that can be measured with a sampler Suspended Load-Sediment that can be found at any depth. Includes fine and coarse sediment Wash Load – sediment that passes over bed without deposition. exchanges with banks/floodplain Sediment 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 the lowest portion of the water column that cannot be measured with most samplers Bed Load-Sediment that creeps or hops along bed (coarse material) Bed Material Load – sediment that exchanges with and is found in measureable quantities in bed

7 Suspended Load Curves (based on measured SS)
Suspended sediment can be measured using samplers A plot of suspended sediment load versus water discharge. Note order of magnitude variation. 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/day 3. 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 Abraham and Thad Pratt, ERDC Bed load moves in waves at a certain speed (or celerity). The celerity (c) is given by dividing the distance ∆x by time ∆t. Problem is that measuring c is difficult and expensive, and calculating it is uncertain even with good models and data. Often we just assume that bed load is 5% to 15% 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 velocity water velocity and depth channel width channel slope and roughness water temperature Many assumptions are made Choose functions appropriate for your conditions 250 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 bathymetric elevations 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 slope At 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 sediment grain size tend to balance against water discharge and slope

11 Sources, Sinks, Pathways
Rio Puerco, NM Sources Bed Banks (Bluffs) Ravines & Gullys Watershed Watershed Sources depend on: geology and topography of the watershed magnitude, intensity, duration, and distribution of rainfall vegetative cover; and the extent of cultivation and grazing. Regression methods are used to develop soil loss relationships Source Sink Erosion on the outside of a bend is a source of sediment Deposition on the inside of this bend is sink for sediment

12 Sources, Sinks, Pathways
Floodplains Valley side slopes Deltas Off - channel areas USACE Dredges Most 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 on hydraulic properties of the stream channel and sediment properties Sediment Transport = F (hydraulic properties & sediment properties) slope velocity grain size distribution channel geometry cohesiveness roughness Note that the hydraulic properties are the same as those that we discussed in hydraulics. Temperature can have an effect on sediment transport If the grain size is large (cobbles) or if the material has cohesive properties, sediment transport may be limited pathway sink

14 Sediment Budgets, Watershed Scale
Whitewater River Sediment Budget, NRCS, 1965 to 1994 Sources of sediment estimated by AGNPs model Sinks determined by historic survey comparison Sources (1000’s tons/year) Sheet & Rill Erosion 555 Ephemeral & Classic Gully Erosion 72 Streambank Erosion 86 Headwaters Sediment Load To Mississippi River 24 Colluvium 553 Sandbars & Streambank Deposits 17 Tributary Valley Deposits 36 Main Valley Deposits 63 Whitewater Delta Deposits 20 Every 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 from sheet and rill erosion, ephemeral & classic gully erosion, and streambank 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 events Sediment 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 the sampler intake is equal to the velocity and concentration in the surrounding water is required to ensure 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 fine sediments. The SSC analytical method uses the entire water-sediment mixture in the analysis. (ASTM D-3977) A TSS analysis entails withdrawal of an aliquot (or part) of the original sample for subsequent analysis. (SM D). It is OK to use if the sediments are fine sediments, but don’t use if coarse sediments are in the sample. For more information see TSS 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 Energy Channel Geologic science of landscape formation Fluvial Geomorphology landscape formation by streams 1. 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 Energy Backwater

19 Watershed and Channel Alteration will change H&H Causing Geomorphic Responses Including:
Channel incision Channel plugging Land loss along channels Gullying Floodplain deposition In other words, many of the USACE missions are affected by sediment transport

20 Geomorphic Responses may Affect:
Infrastructure: Bridges, FRM Human uses: Drinking water, recreation, agriculture Aquatic Habitat Commercial Navigation Water Quality: Underwater light, nutrients, contaminants In other words, many of the USACE missions are affected by sediment transport

21 Spatial Scales Changes in: Channel geometry Slope Roughness
Cause changes in Sediment transport Geomorphic processes Channel Capacity 1. 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-Scales Annual geomorphic changes like sand bar migration, bank erosion, point bar building occur due to seasonal high flows Long-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 Investigations Existing conditions substrate, bankfull conditions, vegetation, discussion with local experts Surveys: Cross sections, profiles, sediment cores Analytical Techniques Numerical Models: Watershed models River models Aerial photo comparisons Change from Historic conditions Sediment Budgets Specific Stage Discharge Analysis A combination of techniques usually is needed to establish multiple lines of evidence.

24 Erosion of a River Bend Sink Source Sink
Geotechnical Failure From Bend Migration And Toe Erosion Low Flow Cross Section High Flow Cross Section

25 Flood of Record Washed Out Railroad, Undermined Houses
This flood of record probably caused direct erosion of the bank Note 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 Bridge Canadian Pacific RR Bridge Whitewater River Avulsion August 2007 New Mouth of Whitewater River Avulsion is a sudden change in channel position, with abandonment of old channel. Whitewater River shortened By about 6,000’ Old Mouth of Whitewater River

29 Culvert Outlet Failure

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 Levels In Lower Pool 8 Island Loss And Erosion Increased Connectivity Sediment Deposition 1938 1954 1991 Extensive land loss has occurred in the lower ends of navigation pools on the Upper Mississippi River

32 Bridges Usually 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 levels A scour signature may not last. See the next six images. Note that the bottom elevation is about at the start of this event.



35 Summary of Rising Flood



38 Summary of Falling Flood

39 Bridge TH 212 over Minnesota River Overflow Abutment Fill and Approach Panel Lost Q500 = 7,300 cfs Q1997 = 10,000 cfs Excerpts from Inspectors Notes: 4/4 108” below NW, channel has shifted to east 4/5 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

42 ~ The End ~

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