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  • 1

    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.

  • 2

    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.

  • 3

    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

  • 4

    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

  • 5

    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

  • 6

    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

  • 7

    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).

  • 8

    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.

  • 9

    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

  • 10

    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

  • 11

    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.

  • 12

    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

  • 13

    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

  • 14

    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