USACE Issues and Research in Sediment Transport

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USACE Issues and Research in Sediment Transport
U.S. Army Corps of Engineers
Engineer Research and Development Center Kevin
Barry
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What are the Sediment Transport Issues?
Infilling behind dams River migration/morphology
change Contaminant isolation/dispersion Coastal
evolution/protection Wetland/habitat creation and
protection Flood protection/levee
stability Beneficial use of dredged material
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What are the Sediment Transport Issues?
Impact of channel modifications Turbidity,
transport and deposition of material released
during dredging and at placement sites Mound
formation and consolidation at placement
site River restoration Cap and CAD cell
stability Channel/Harbor infilling
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What are the Sediment Transport Issues?
Propeller/ship induced turbidity Fluid mud
impacts (including navigability) Water
Quality Degradation/recession of Louisiana
nearshore regions
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What Processes Are we Studying?
Fluid mud evolution and navigable depth Mixed
sediment erosion (Sedflume, ASSET flume, SEAWOLF
flume, PES) Improved field measurement
capabilities Developing a system-side sediment
transport modeling system for RSM
(watershed-riverine-estuarine-coastal) start in
FY05 goal to support regional sediment management
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What Processes Are we Studying?
Planning on using TAMU dredging tank to study
dredge-induced and propeller-induced resuspension
(empirical laboratory data) We have spent massive
amounts of money trying to collect field data on
these two issues with little benefit Goal is to
be able to isolate properties, such as current
speed, sediment type, dredging rate
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What Processes Are we Studying?
USACE dredging activity is impacting coastal
sediment transport Nearshore placement of mixed
dredged material for littoral zone nourishment
Beneficial Use of Dredged Material Must
understand migration of nearshore mounds and
separation/transport of different classes of
sediment in wave/current environments
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Savannah River Entrance / Tybee Island Situation
is a Familiar Picture
18661982
Today

• Federal navigation channel project (jetties and
  deepened channel)
• Evolving ebb shoal complex and beach erosion
  problem downdrift of
• the inlet (on Tybee Island)
• Federal shore protection project constructed to
  address the erosion
• High sand loss rate on north Tybee, adjacent to
  the inlet

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Project SpecificSediment Management Approach
Maintenance of the navigation project requires
handling of large quantity of sediment
Past and present maintenance practice isolates
sediment from the littoral/beach system
Shore protection project needs sand to maintain
it separate borrow source/area
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Regional Sediment Management (RSM) Approach
Maintenance material is generally not beach
quality but has a significant percentage of sand
(gt 70)
RSM considers dredged material as a resource to
benefit the region.
How can we best use the dredged material to
mitigate erosion on north Tybee? In a way that
mimics natural sediment bypassing processes.
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Savannah Nearshore PlacementStudy Issues

• Benefits to Tybee Island littoral system?
• Any negative impacts to Tybee Island shoreline?
• Minimize sediment rehandling
• Nearshore turbidity
• Identify optimal placement locations and
  orientation for nearshore placement.

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Savannah Nearshore PlacementStudy Methods

• Collect and analyze existing data and
  information for Savannah
• Collect mixed sediment erosion data for Savannah
  material for model input
• Model hydrodynamics, waves and sediment
  transport at Savannah to characterize
  environmental conditions at potential placement
  sites using tools produced in Corps-funded RD
  Programs
• Examine benefits and impacts of various
  placement alternatives
• Improve and increase confidence in Savannah
  predictions by validating methods and models at
  Brunswick.
• Collect nearshore mound migration data
• Model same processes at Brunswick

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Sediment transport direction and magnitude for
existing natural conditions
Ebb-dominant transport
Ebb-dominate transport

• Sediment model results shed light on existing
  inlet sediment transport processes and pathways,
  and preferred placement locations
• Results consistent with observed shoaling
  patterns in the channel and north Tybee

Decreasing ebb-dominant transport indicates
shoaling
Flood dominant transport
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Nearshore Placement Alternatives

• Crest Elevation
• Berm 01 2.1 mBerm 02 2.1 mBerm 03 3.0
  mBerm 04 4.0 mBerm 05 4.0 m Berm 06 3.5 m
  Berm 07 2.5 mBerm 08 1.5 mBerm 09 2.5
  mBerm 12 3.0 mBerm 13 2.0 mBerm 14 2.0 m

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Sediment transport direction and magnitude at
various berm locations and configurations
Desire is to maximize benefit to Tybee, minimize
distance from dredging location, minimize
rehandling
Recommended location
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Recommendations

• Place mixed sediment from channel into Berm
  13/14
• Allow natural winnowing to remove fine content
• Longshore transport patterns will move sediment
  into north Tybee littoral zone

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Savannah Nearshore PlacementStudy

• Developed recommendations for placement at Tybee
  Island to
• maximize benefit to littoral zone
• minimize rehandling of material
• However, no hard data to support recommendations
• Monitor nearshore mounds at Brunswick Georgia to
  provide confidence to Tybee recommendations

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Brunswick Data Collection

• Seed nearshore mounds with fluorescent tracer of
  both sand- and silt-size
• Hydrodynamic data
• Bathymetric Surveys
• Modeling similar to Savannah

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Sampling Locations for Tracer Study
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Sand Tracer Movement(Feb June 2003)
Flood-dominant transport at nearshore mound
Ebb-dominant transport with longshore current
influence at offshore mound
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Summary Preliminary analysis of survey and
tracer data

• Surveys and tracer movement indicate net
  migration of mound to the SSW.
• Acoustic backscatter, tracer and cores suggest
  winnowing of sediments.

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Summary Preliminary analysis of survey and
tracer data
FebJul
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Other Tracer Study Observations

• Silt tracer
• disperses rapidly.
• Small amount temporarily found in nearshore
• At end of study, majority of silt tracer
  unaccounted for (deep burial or transport outside
  study area)
• Sand tracer
• Transported rapidly from mound crest
• Majority of sand tracer mass buried in migrating
  mound
• Tracer movement consistent with bathymetric
  surveys

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Corps Efforts in Sediment Transport

• Improved data collection supporting work in
• Density Tuning Fork for fluid mud depth
• Supporting HR Wallingford in using ADCP data to
  measure TSS
• Proposed FY05 efforts to study dredge and
  propeller induced resuspension using the TAMU
  dredging flume
• Supporting DOE to expand Sedflume capabilities
  including combined wave and current
• Protocols for monitoring dredging operations
• Improved Models
• FY05 start on effort for linked watershed,
  riverine, estuarine and coastal
  hydrodynamic/sediment/contaminant modeling
• Improved modeling capabilities for dredging
  processes

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Corps Interest in near-term sediment processes
research

• Fluid mud measurement, generation and transport
• Catastrophic erosion events bed failure
• Sediment settling and hindered settling high
  density processes
• Sediment separation processes including armoring
  and winnowing
• Using historic data in developing conceptual site
  model
• Consolidation of large lifts
• Measuring turbidity/TSS in the field
• Methods for extrapolating a few data points over
  a large domain
• Uncertainty associated with measurements,
  extrapolation, and modeling

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