a modeling approach to restoring pool – riffle structure in an incised, straightened channel of an...
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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
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%.
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
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.
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
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
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.
Limitations
• Laterally confined• Undersized channel• Very mild slope• Sediment starved• Monetary budget
Beaver Creek, TN, 2009.
• 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
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
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
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
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
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/
River2D: Current Condition - Hydraulics
Channelized, uniform hydraulic regime, devoid of riffles, 1 minor pool (local scour from in-stream tree).
River2D: Current Condition – Habitat at
Low Flow
Poor/fair combined (depth, velocity, channel index) suitability.
Green Side Darter - low flow
Northern Hogsucker - low flow
Combined (depth, velocity, channel index) suitability.
Northern Hogsucker - high flow
Green Side Darter - high flow
River2D: Current Condition – Habitat at
High Flow
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.
Longitudinal Profile of Design Reach
Visualizing Design in ArcScene
Design Features
• Removal of trees • Excavation of the banks• Addition of hydraulic structures• Addition of bank protection• Addition of habitat features/substrate material
Design Channel River2D Output
• Refining the mesh with the design bed modifications
• Interpreting and using the output
High flow
Improved combined (depth, velocity, channel index) suitability.
Northern Hogsucker - low flow
Green Side Darter - low flow
River2D: Design Condition – Habitat at
Low Flow
Improved combined (depth, velocity, channel index) suitability.
Northern Hogsucker - high flow
Green Side Darter - high flow
River2D: Design Condition – Habitat at
High Flow
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
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
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
• 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