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