asce nc jennings

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

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


Priority 2: Excavate lower floodplain and construct new meandering channel

2008 White Slough 2010

ER = 6; W/d = 11

White Slough 2010



2008 Trib to Saugatchee Creek 2008

ER = 5; W/d = 11


2004 NCSU Rocky Branch 2005

2006

NCSU Rocky Branch

2006

2008 NCSU Rocky Branch


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


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

asce nc jennings

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