asce nc jennings
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Greg Jennings, PhD, PE Professor, Biological & Agricultural Engineering North Carolina State University [email protected]
Stream Restoration Innovations and Opportunities
• Channel (bed & banks) • Floodplain • Water • Sediment • Plants & animals
Stream: A system of fluvial forms & habitats
Photo Credit: Eve Brantley, Auburn University
• Communities of organisms and their physical, chemical, and biological environments
Streams are Ecosystems
1. Bed stability & diversity
2. Sediment transport balance
3. In-stream habitat & flow diversity
4. Bank stability (native plant roots)
5. Riparian buffer (streamside forest)
6. Active floodplain
7. Healthy watershed
What makes a stream healthy?
Healthy Streams?
Stream Impairments • Straightening & dredging • Floodplain filling • Watershed manipulation • Sedimentation & stormwater • Pollution discharges • Utilities & culverts • Buffer removal • Disdain & neglect
§ Activities that initiate or accelerate the recovery of ecosystem health, integrity, and sustainability (SER, 2004).
Ecosystem Restoration
Why Restoration?
• Water quality impairments • Habitat loss • Ecosystem degradation • Land loss • Safety concerns • Infrastructure damage • Flooding • Aesthetics
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
• Habitats & water quality • Natural flow regimes • Recreation & aesthetics • Public enthusiasm
Outcomes of Ecosystem Restoration
1. Channel morphology
2. Floodplain structure
3. Hydrologic & hydraulic analysis
4. In-stream structures
5. Habitats & vegetation
6. Site & watershed conditions
7. Monitoring, maintenance, education
Restoration Components
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
Threshold Channels
Alluvial Channels 1. 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
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)
Alluvial Channels – Analogy Approach 1. 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)
Alluvial Channels – Hydraulic Geometry
1
10
100
1000
10000
0.1 1 10 100
Bankfull Discharge, Q (cfs)
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
1
10
100
1000
0.1 1 10 100
Cross-‐section Area (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
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
1. Channel Morphology
• Dimension (baseflow, bankfull, flood flows)
• Pattern (meandering, straight, braided)
• Profile (bedform – riffle, run, pool, glide, step)
2005 South Fork Mitchell River 2006
Photo Credits: Darrell Westmoreland, North State Environmental, Inc.
2011 South Fork Mitchell River
2011 South Fork Mitchell River
High-quality “reference” streams serve as design templates
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)
Dimension (cross-section)
• Area • Width • Depth • Width/Depth Ratio • Entrenchment Ratio • Bank Height Ratio
Bankfull Stage: “incipient flooding” “corresponds to the discharge at which channel maintenance is the most effective, that is, the discharge at which moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work results in the average morphologic characteristics” (Dunne & Leopold,1978)
Stream Corridor Restoration: Principles, Processes, and Practices. 1998. Federal Interagency Stream Restoration Working Group.
Bankfull Terrace
Bankfull Width, Wbkf = 36 ft; Bankfull Area, Abkf = 112 ft2
Mean Depth, dbkf = Abkf / Wbkf = 112 / 36 = 3.1 ft
Width to Depth Ratio, W/d = Wbkf / dbkf = 36 / 3.1 = 11.5
BHR = 5.3 / 2.5 = 2.1
Entrenchment Ratio = Wfpa / Wbkf = 75/15 = 5
Pool Run
Point Bar (deposition)
Glide
Riffle
Meandering Stream: Alluvial Forms
Sinuosity = stream length / valley length K = 1850 / 980 = 1.9
Valley Length
Meander Length Ratio = meander length / width = 78/15 = 5.2 Meander Width Ratio = belt width / width = 57/15 = 3.8
Radius of Curvature Ratio = radius / width = 23/15 = 1.5
Meander Length
Belt Width
Glide Slope
Riffle Slope
Run Slope
Pool Slope
Water Surface
Thalweg
Pool Spacing, Lp-p
Profile (bedform)
Riffle Slope Ratio, Srif / Sav
Pool Slope Ratio, Spool / Sav
Pool-to-Pool Spacing Ratio, Lp-p / Wbkf
2. Floodplain Structure
• Regular (every year) flooding to relieve stress
• Floodwater retention & riparian wetlands
• Stormwater discharge retention & treatment
Priority 1: lift channel
Incised Stream
Priority 2 & 3: lower floodplain
Stream Corridor Restoration: Principles, Processes, and Practices. 1998. Federal Interagency Stream Restoration Working Group.
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!
ER = 15; W/d = 12
2008 Town Creek Tributary
Entrenchment Ratio = Wfpa / Wbkf = 150/10 = 1.6
Priority 1: Raise channel to existing valley and construct new meandering channel
2008 Purlear Creek 2009
ER = 7; W/d = 14
2009 Purlear Creek
Entrenchment Ratio = Wfpa / Wbkf = 100/14 = 7
Priority 2: Excavate lower floodplain and construct new meandering channel
2008 White Slough 2010
ER = 6; W/d = 11
White Slough 2010
Entrenchment Ratio = Wfpa / Wbkf = 72/12 = 6
Priority 2: Excavate lower floodplain and construct new meandering channel
2008 Trib to Saugatchee Creek 2008
ER = 5; W/d = 11
Priority 2: Excavate lower floodplain and construct new meandering channel
2004 NCSU Rocky Branch 2005
2006
NCSU Rocky Branch
2006
2008 NCSU Rocky Branch
Entrenchment Ratio = Wfpa / Wbkf = 48/12 = 4
2005 NCSU Rocky Branch 2006
Priority 3: Excavate narrow floodplain benches in confined systems
ER = 2.2; W/d = 12
2008 NCSU Rocky Branch
Priority 3: Excavate narrow floodplain benches in confined systems
2009 Little Shades Creek 2010
ER = 1.6; W/d = 15
Entrenchment Ratio = Wfpa / Wbkf = 60/38 = 1.6
Little Shades Creek 2010
Priority 3. Excavate floodplain benches and add structures to maintain straight channel
2000 NCSU Rocky Branch 2001
ER = 1.8; W/d = 14
NCSU Rocky Branch
2008
Qbkf: Bankfull discharge (cfs) appropriate for watershed size, sediment transport & valley conditions
Vav = Qbkf / Abkf: Bankfull average velocity (ft/s) appropriate for valley, soils, bed material
τav: Bankfull average applied shear stress (lb/ft2) & local max stresses appropriate for sediment transport conditions & bed/bank restistance
ωav: Bankfull average stream power (lb/ft/s) appropriate for sediment transport conditions
Riffle substrate size distribution appropriate for hydraulic conditions & habitats
Streambank protection to resist erosion (short-term & long-term)
3. Hydrologic & Hydraulic Analysis