A Modeling Approach to Restoring Pool – Riffle Structure in an Incised, Straightened Channel of an...
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![Page 1: A Modeling Approach to Restoring Pool – Riffle Structure in an Incised, Straightened Channel of an Urban Stream University of Tennessee Department of Civil.](https://reader036.fdocuments.in/reader036/viewer/2022062407/56649e605503460f94b5aabc/html5/thumbnails/1.jpg)
A Modeling Approach to Restoring Pool – Riffle Structure in an
Incised, Straightened Channel of an Urban Stream
University of TennesseeDepartment of Civil and Environmental EngineeringKeil J. NeffDr. John Schwartz
Knox CountyStormwater ManagementAndrew B. DodsonMichael S. Hamrick
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Pool-Riffle Structure in Natural Channels
Riffle-Pool Sequence: The development of alternating deeps (pools) and shallows (riffles) is characteristic of both straight and meandering channels with heterogeneous bed materials, containing gravel, in the size range of 2 to 256 mm.
In general, riffle-pool sequences occur with bed slopes < 2%.
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Pool-Riffle Structure in Urban Streams
Pool-riffle structure, capable of supporting diverse biological ecosystems, is frequently degraded in urban streams because of channel incision and the loss of channel-scale helical flow patterns, which are responsible for initiating pool-riffle sequences.
Knighton 1988
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Stream Impairment
• Anthropogenic Impacts to the Stream System
• Watershed land use changes (e.g., urbanization, deforestation)
• Channelization reduces habitat complexity and flood refugia for fish
• Habitat loss or modification• Introduction of exotic species• Water withdrawals; Pollutant
Discharges• Over exploitation of fish and wildlife
Urbanization impacts watershed hydrology resulting in hydromodification of in-stream hydraulics and rapid adjustment of channel morphology thereby disturbing natural geomorphic and ecological processes in stream systems.
Embrass River, IL; Schwartz, 2002.
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Beaver Creek, Knox County, Tennessee
303d listed (TDEC)• Habitat loss due to alteration in stream side• Loss of biological integrity due to siltation• One pollutant source: Channelization
Other studies: Dworak, Mallison, Cantrell
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Study Reach on Beaver Creek
Beaver Creek, TN, 2009.
• Historically re-located• Channelized• Lack of pool-riffle sequences• Velocity homogeniety• Urbanization Impacts• Undersized channel• 40 square km• 270 m length• Channel evolution stage III
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Beaver Creek Stream Rehabilitation
• Objectives• Design stable and sustainable self-regulating pool-
riffle sequences using River2D hydrodynamic model and triangulated irregular network (TIN) editor in AutoCAD Civil 3D.
• Support maintenance of velocity acceleration/deceleration sequencing.
• Enhance habitat and biotic diversity.• Stabilize failing banks.• Provide cost-effective method for stream
rehabilitation.
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Limitations
• Laterally confined• Undersized channel• Very mild slope• Sediment starved• Monetary budget
Beaver Creek, TN, 2009.
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• Low flow concept• Acceleration/deceleration• Riffle crest• Minor sinuosity
Design Framework
• Work with existing channel• Minor expansion (bank erosion/failure; absence of trees)• Minor constriction (large trees on banks; minor scour)• Bank stabilization• Substrate placement
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Design Framework
• High flow concept• Acceleration/deceleration• Submerged riffles• Hydraulic refugia• Scour/deposition• Energy losses• Q=VA; V1*A1=V2*A2;
A2>A1 V2<V1
1 2 1
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Additional Design Criteria• No reference reach• Spacing of pool-riffles sequences• Defining riffle and runs• Hydraulic diversity• Substrate
Iterative Design Approach
Survey
AutoCAD Civil 3DRiver2D
Design Channel
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Initial Assessment• Trimble Total Station Survey
• Initial benchmarks set with Real Time Kinematic GPS (0.05 ft accuracy)
• Breaklines – survey the dominant breaks in slope across the cross section
• Approximately at a 2 meter resolution
• Establish control reach• Benthic Macroinvertebrate Survey• Index of Biotic Integrity Survey• Global Water Continuous Level Logger
Installation• Bedload Sediment Collection• Rapid Geomorphic Assessment• 3D Acoustic Doppler Velocity Measurements
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River2D Modeling
• Evaluate hydraulics• High/Low flow regimes• Placement of in-stream structures
• Evaluate bank shear stresses• Placement of bank stabilization structures
• Evaluate bed shear stresses• Size substrate
• Assess available fish habitat
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River2D ModelingR2D Model User Manual (Peter Steffler – University of Alberta)• Two dimensional, Depth Averaged, Finite Element Model• Basic mass conservation equation and 2 (horizontal) components
of momentum conservationModeling Steps• Create a preliminary bed topography file from survey data using
R2D_Bed program. Define boundary polygon of area to be modeled.
• Define boundary conditions (discharge and downstream water surface elevation) and define roughness.
• Create, triangulate, and smooth mesh. Define breaklines at toe and top of bank. Add additional nodes at critical positions.
• Run River2D to solve for velocity and depth.• Model outputs: 2 (horizontal) velocity components and a depth at
each node.• The fish habitat module is based on the PHABSIM weighted usable
area approach, adapted for a triangular irregular network geometrical description.
http://www.river2d.ualberta.ca/
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River2D: Current Condition - Hydraulics
Channelized, uniform hydraulic regime, devoid of riffles, 1 minor pool (local scour from in-stream tree).
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River2D: Current Condition – Habitat at
Low Flow
Poor/fair combined (depth, velocity, channel index) suitability.
Green Side Darter - low flow
Northern Hogsucker - low flow
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Combined (depth, velocity, channel index) suitability.
Northern Hogsucker - high flow
Green Side Darter - high flow
River2D: Current Condition – Habitat at
High Flow
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Creating Design Channel utilizing Civil3D
• Interactive TIN Editing to create riffles, riffle/runs, pools, bank stabilization features, and log vanes.• TIN modified by adding hard and soft breaklines, modifying the underlying nodes, and eventually transforming the surface to represent multiple stream rehabilitation channel designs.
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Longitudinal Profile of Design Reach
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Visualizing Design in ArcScene
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Design Features
• Removal of trees • Excavation of the banks• Addition of hydraulic structures• Addition of bank protection• Addition of habitat features/substrate material
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Design Channel River2D Output
• Refining the mesh with the design bed modifications
• Interpreting and using the output
High flow
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Improved combined (depth, velocity, channel index) suitability.
Northern Hogsucker - low flow
Green Side Darter - low flow
River2D: Design Condition – Habitat at
Low Flow
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Improved combined (depth, velocity, channel index) suitability.
Northern Hogsucker - high flow
Green Side Darter - high flow
River2D: Design Condition – Habitat at
High Flow
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Low Flow Existing Design
Greenside Darter 7 101Northern Hogsucker 207 446
High Flow Existing Design
Greenside Darter 11 196Northern Hogsucker 771 1023
River2D: Weighted Usable Area
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Construction
• Spring/Summer 2011• Weather dependent• Construction reports• Project agent on-site or
available at all times• High degree of
accuracy required• Sediment sizing• Paint substrate in each
riffle/run• Invert elevations• Placement of structures
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Measurements of Success
• Stability of bed form and stream bank• Survey (reach and cross-sections)• Visual assessment of structures
• Improved habitat• Benthic macroinvertebrate survey
• Index of biotic integrity
• Increased leaf litter (organic carbon cycling)• Stability of riffle/run substrate
• Bed load sampling
• Diverse hydraulic patterns• Velocity measurements
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• Beaver Creek Task Force• Knox County Stormwater• University of Tennessee – CEE• Roy Arthur• Tim Gangaware• Americorps CAC Water Quality Team• Ecological Engineering for Stream
Rehabilitation Class (Schwartz) • Knox County Parks and Recreation
Project Partners