Rc201 day 1 jennings 10
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Greg Jennings, PhD, PEProfessor, Biological & Agricultural EngineeringNorth Carolina State [email protected]
Stream Restoration
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… a body of water with a current, confined within a bed and streambanks
Synonyms: bayou, beck, branch, brook, burn, creek, crick, kill, lick, rill, river, rivulet, run, slough, syke
Streams are conduits in the water cycle and also important habitats
What is a Stream?
Photo Credit: Eve Brantley, Auburn University
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• Communities of organisms and their physical, chemical, and biological environments
Streams are Ecosystems
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What Makes a Stream Healthy?• Bed stability & diversity
• Sediment transport balance
• In-stream habitat & flow diversity
• Bank stability (native plant roots)
• Riparian buffer (streamside forest)
• Active floodplain
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Bed Stability & Diversity• Appropriate size sediments
to resist shear stress
• Riffle/Pool sequences in alluvial streams
• Step/Pool sequences in high-gradient streams
Photo Credit: Eve Brantley, Auburn University
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Sediment Transport Balance• Minor erosion & deposition
• Alluvial bars and benches
• Sufficient stream power to avoid aggradation
• No net change in sediment over long time
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PoolRoots Wood
Leaf Pack
RiffleRocks
In-stream Habitat & Flow Diversity
Plants
Overhanging Bank
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Bank Stability• Dense native
plant roots
• Low banks with low stress
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Riparian Buffer (Streamside Forest)• Diverse native plants
• Food and shade
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Active Floodplain• Regular (every year) flooding to relieve stress
• Riparian wetlands
• Stormwater retention & treatment
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Why Restoration?
• Water quality impairments
• Habitat loss
• Ecosystem degradation
• Land loss
• Safety concerns
• Infrastructure damage
• Flooding
• Aesthetics
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Stream Insults
• Straightening & dredging
• Floodplain filling
• Watershed manipulation
• Sedimentation & stormwater
• Pollution discharges
• Utilities & culverts
• Buffer removal
• Disdain & neglect
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Activities that initiate or accelerate the recovery of ecosystem health, integrity, and sustainability (SER, 2004).
Ecosystem Restoration
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1. design of an ecological river restoration project should be based on a specified guiding image of a more dynamic, healthy river
2. river’s ecological condition must be measurably improved
3. river system must be more self-sustaining and resilient to external perturbations so that only minimal follow-up maintenance is needed
4. during the construction phase, no lasting harm should be inflicted on the ecosystem
5. pre- and post-assessment must be completed and data made publicly available
Standards for ecologically successful river restoration
Palmer et al., Journal of Applied Ecology, 2005, 42, 208–217
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Habitats
Water quality
Natural flow regimes
Recreation & aesthetics
Public acceptance
Outcomes of Ecosystem Restoration
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Restoration Components
1. Channel morphology & floodplain connection
2. In-stream structures
3. Streambank bioengineering
4. Riparian buffers & habitat enhancements
5. Stream crossings
6. Stormwater/watershed management
7. Monitoring & maintenance
8. Public access & education
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Healthy “reference”
streams serve as design templates
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Natural Stream Channel Stability(from Leopold)
• River has a stable dimension, pattern and profile• Maintains channel features (riffles, pools, steps)• Does not aggrade (fills) or degrade (erodes)
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• Dimension (bankfull & flood flow)• Pattern (meander)• Profile (bed profile)• Floodplain connection
1. Channel Morphology & Floodplain Connection
2005 NCSU Rocky Branch 2006
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2008 NCSU Rocky Branch
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Bankfull Stage: Water fills the active channel and begins to spread onto the floodplain
Stream Corridor Restoration: Principles, Processes, and Practices. 1998. Federal Interagency Stream Restoration Working Group.
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2006 Town Creek Tributary 2007
Priority 1: Raise channel to existing valley and construct new meandering channel
Rain will come during and immediately following construction!
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2008 Town Creek Tributary
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Priority 1: Raise channel to existing valley and construct new meandering channel
2008 Purlear Creek 2009
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2009 Purlear Creek
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Priority 1
Priority 2
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Priority 2: Excavate lower floodplain and construct new meandering channel
2008 Trib to Saugatchee Creek 2008
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Wfpa
Wbkf
Entrenchment Ratio = Wfpa / Wbkf = 75/15 = 5
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2007 Cary Walnut Creek Tributary 2008
Priority 2: Excavate lower floodplain and construct new meandering channel
Photo Credit: David Bidelspach, Stantec, Inc.
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2008 Cary Walnut Creek Tributary
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Priority 3: Excavate narrow floodplain benches in confined systems
2009 Little Shades Creek 2010
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Wfpa
Wbkf
Entrenchment Ratio = Wfpa / Wbkf = 60/38 = 1.6
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Before
After
Seoul, KoreaCheonggye“Extreme
Restoration”
Fac. of Environment & Life Sciences, Seoul Women's University
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Before
After
Fac. of Environment & Life Sciences, Seoul Women's University
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Restoration process of Cheonggye stream
Fac. of Environment & Life Sciences, Seoul Women's University
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Typical cross-section of the restored stream section
Fac. of Environment & Life Sciences, Seoul Women's University Fac. of Environment & Life Sciences, Seoul Women's University
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Fac. of Environment & Life Sciences, Seoul Women's University
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Stream Design Approaches
1. Threshold Channel
2. Alluvial Channel
a. Regime Equations
b. Analogy (Reference Reach)
c. Hydraulic Geometry
d. Analytical Models
3. Combination of Methods
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Threshold Channels1. Rigid boundary systems
2. Simple design approach: select channel configuration where the stress applied during design conditions is below the allowable stress for the channel boundary
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Threshold Channels
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Shear Stress
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Threshold Channels – Shear Stress
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Shear Stress
= Rs
= Shear Stress (lb/ft2)
= Unit Weight of Water = 62.4 lb/ft3
R = Hydraulic Radius (ft) = A/P
s = Energy Slope (water surface) (ft/ft)
A = Riffle Cross-Section Area (ft2)
P = Wetted Perimeter (ft) [P ~ Wbkf + 2 x Dbkf]
http://www.epa.gov/warsss/sedsource/bedload.htm
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Threshold Channels – Shear Stress
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Shear Stress Around Bends
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Shear Stress Distribution• Flow around bends
– creates secondary currents – higher shear stresses on the channel sides and bottom
compared to straight reaches – maximum shear stress in a bend is a function of the ratio of
channel curvature to bottom width
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Shear Stress Distribution
from Chang, 1988
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Shear Stress Distribution
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Alluvial Channels1. Movable boundary systems
2. Complex design approach: assess sediment continuity and channel performance for a range of flows
3. Dependent variables: Width, Depth, Slope, Planform
4. Independent variables: Sediment inflow, Water inflow, Bank composition
5. Empirical & Analytical approaches should be used concurrently
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Steady State Equilibrium
dimension, pattern and profile of the river and its velocity have adjusted to transmit the discharge and sediment load from its catchment under the present climate and land use conditions without any systematic erosion or deposition; namely regime conditions (Hey)
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Assumption: Stream Behavior Is Predictable
• Streams evolve to a state of dynamic equilibrium
• Equilibrium is a function of flow and sediment
• Equilibrium is naturally associated with a main channel
and a flood-prone area
• The main channel is formed by the effective (“bankfull”)
discharge over time
• Alluvial stream meandering
is predictable
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Design Criteria Selection
From Will Harman, Stream Mechanics
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Alluvial Channels – Regime Approach1. Empirical equations
2. Use as a check
3. Hey equations:
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Alluvial Channels – Analogy Approach1. Reference reach: Must have similar bed/bank materials,
sediment inflow, slope, valley type, and hydrograph
2. Upstream/downstream of design reach is best
3. Nearby similar watershed acceptable
4. Use as a starting point or check (BE CAREFUL)
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Alluvial Channels – Hydraulic Geometry
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Alluvial Channels – Hydraulic Geometry
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1
10
100
1000
10000
0.1 1 10 100
Bank
full
Disch
arge
, Q (c
fs)
Drainage Area (sq mi)
Hydraulic Geometry Regional Curves
NC Piedmont
NC Mtn
MD Alleghany
MD
NY
VT
OH 01
OH 05
OK
SW OR
Pacific NW
AZ
AZ & NM
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1
10
100
1000
0.1 1 10 100
Cros
s-se
ction
Are
a (sq
ft)
Drainage Area (sq mi)
Hydraulic Geometry Regional Curves
NC Piedmont
NC Mtn
MD Alleghany
MD
NY
VT
OH 01
OH 05
OK
SW OR
Pacific NW
AZ
AZ & NM
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Hydraulic Geometry
National Center for Earth Dynamics
http://www.nced.umn.edu/Stream_Restoration_Toolbox.html
Single-Thread Gravel-Bed Rivers Have Consistent Bankfull Geometries
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Alluvial Channels – Analytical Methods
1. Choose appropriate model
2. Assess a range of solutions
3. Use as a check
Analytical Design Approach, Shields, 2006
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Combination Approach to Natural Channel Design
1. Existing Conditions – valley, watershed, constraints
2. Design Goals
3. Design Criteria
a. Regime Equations
b. Analogy (Reference Reach)
c. Hydraulic Geometry (Regional Curves)
d. Other Restoration Projects
4. Analytical Models
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Natural Channel Design Approach, from Rosgen, 2006
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Design Criteria Selection
From Will Harman, Stream Mechanics
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Reference Reach Versus Design Reach
Reference reach with mature forest Stream restoration project immediately after construction; floodplain devoid of vegetation
From Will Harman, Stream Mechanics
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Reference Reach Pattern
From Will Harman, Stream Mechanics
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Reference Reach:
• Upstream/downstream• Same watershed• Similar watershed• Historical photos
From Will Harman, Stream Mechanics