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river modelingTRANSCRIPT
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A Modeling Approach to Restoring Pool Riffle Structure in an Incised, Straightened
Channel of an Urban Stream
University of TennesseeCivil & Environmental Engineering
Keil J. NeffJohn Schwartz
Knox County, Tennessee
Andrew B. DodsonMichael S. HamrickRoy Arthur
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 h t b d t i l t i i heterogeneous bed materials, containing gravel, in the size range of 2 to 256 mm.
In general, riffle-pool sequences occur with bed slopes < 2%.
Pool-riffle structure, capable of supporting diverse biological ecosystems, is frequently degraded in urban streams b f h l d h l
Knighton 1988
because of channel incision and the loss of channel-scale helical flow patterns, which are responsible for initiating pool-riffle sequences.
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Study Reach on Beaver Creek
Historically re-located ChannelizedChannelized Lack of pool-riffle sequences Velocity homogeniety Urbanization Impacts Undersized channel 40 square km 270 m length Channel evolution stage III
Beaver Creek, TN, 2010.
Stream Impairment in Beaver Creek
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 Water withdrawals; Pollutant Discharges
Urbanization impacts watershed hydrology resulting in hydromodification of in-stream hydraulics and rapid adjustment of channel morphology thereby disturbing adjustment of channel morphology thereby disturbing natural geomorphic and ecological processes in stream systems.
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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: Channelizationp
Other studies: Dworak, Mallison, Cantrell
Beaver Creek Stream Rehabilitation
Objectives Design and construct stable pool-riffle sequences Diversify hydraulic regime (self-maintaining e s y yd au c eg e (se a ta g
velocity acceleration/deceleration flow fields) Enhance habitat and biotic diversity Evaluate utility of River2D hydrodynamic model
and triangulated irregular network (TIN) editor in AutoCAD Civil 3D in design
Stabilize failing banks Test cost-effective method for stream
rehabilitation
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Limitations
Laterally confined Laterally confined Undersized channel Very mild slope Sediment starved Monetary budget
Beaver Creek, TN, 2010.
Design Framework
Work with existing channel Minor expansion (bank erosion/failure; absence of
trees) Minor constriction (large trees on banks; minor scour)
Integrate cross-sectional area into bed design at riffles
Low flow conceptA l i /d l i
Minor constriction (large trees on banks; minor scour) Bank stabilization Substrate placement
Acceleration/deceleration bed slope controlled
Riffle crest Minor sinuosity
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Design Framework
High flow concept Acceleration/deceleration 1 2 1
Geometry controlled Submerged riffles Hydraulic refugia Scour/deposition Conservation of mass Q=VA; V1*A1=V2*A2;
A2>A1 V2A1 V2
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Initial Assessment Trimble Total Station Survey
Dominant breaks in slope Approximately 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 Rapid Geomorphic Assessment Bed and bank shear tests 3D Acoustic Doppler Velocity Measurements
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 StepsModeling 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 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.
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
Green Side Darter- low flow
Northern Hogsucker- low flow
Poor/fair combined (depth, velocity, channel index) suitability.
Northern Hogsucker- high flow
Green Side Darter- high flow
River2D: Current Condition Habitat at High Flow
Combined (depth, velocity, channel index) suitability.
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Creating Design Channel utilizing Civil 3D
Interactive TIN Editing to create riffles, riffle/runs, pools, bank stabilization features and 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 designsdesigns.
Longitudinal Profile of Design Reach
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Visualizing Design in Google Sketchup Design Features
Removal of trees Excavation of the banks Addition of hydraulic structures 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
Northern Hogsucker- low flow
Green Side Darter- low flow
River2D: Design Condition Habitat at Low Flow
Improved combined (depth, velocity, channel index) suitability.
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Northern Hogsucker- high flow
Green Side Darter- high flow
River2D: Design Condition Habitat at High Flow
Improved combined (depth, velocity, channel index) suitability.
Low Flow Existing Design
Greenside Darter 7 101
River2D: Weighted Usable Area
Greenside Darter 7 101Northern Hogsucker 207 446
High Flow Existing Design
Greenside Darter 11 196Northern Hogsucker 771 1023
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Construction
Winter 2010 (weather dependent) 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 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
Beaver Creek Task Force Knox County Stormwater Division
Project Partners
y University of Tennessee CEE Tim Gangaware Water Resource Research Center Greg Babbit EcoFlow Consulting Americorps CAC Water Quality Team Ecological Engineering for Stream Rehabilitation Class Knox County Parks and Recreation