POCKET ESTUARY RESTORATION FEASIBILITY ASSESSMENT

AND TOOLS DEVELOPED

AT TWO SITES IN WHIDBEY BASIN

Tribal Habitat Conference 2010

Aundrea McBride and Eric BeamerSkagit River System Cooperative

Camano Island State Park

Possession Park

We examined two sites in Whidbey Basin to determine the feasibility of restoring pocket estuary habitat. (The 2 were selected from an initial cut of 100 sites)

Restoration Feasibility Questions

If we restore habitat, will salmon and other fish use it?

Will restored habitat be sustainable ? Will a restored site become tidally

inundated? Will the channel be open in ~100 years?

What are the landscape-scale considerations for restoration ?

Fish Prediction Based on Compilation Assemblage from Skagit Bay Pocket Estuaries Juvenile chum and wild

Chinook salmon dominate the assemblage early in the year

followed by staghorn sculpins in late spring,

shiner perch and sticklebacks in summer, and

surf smelt in early fall. (Beamer et al. 2007).

Figure 2. Seasonal fish assemblages for shallow intertidal habitat in lagoon-type pocket estuaries with a salinity greater than 20 ppt. (from Beamer et. al. 2007)

A - Overall Fish Assemblage

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B - Salmon Assemblage

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C - Sculpin Assemblage

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tD - Flatfish Assemblage

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A - S kagit B ay, Lagoon P E S hallow

I n t er t idal, >2 0 ppt

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5 0

10 0

15 0

2 0 0

2 5 0

S. Prickleback

Gunnels

Pipefish

Peamouth Chub

Stickleback

Shiner Perch

Surf Smelt

Sandlance

Herring

Salmon

B - S kagit B ay, Lagoon P E S hallow

I n t er t idal, >2 0 ppt

0 . 0

2 0 . 0

4 0 . 0

6 0 . 0

8 0 . 0

10 0 . 0

12 0 . 0Bull trout

Cutthroat

Chum0+

Pink0+

Coho (H)

Coho (W)

Chin (H)

Chin (W)

C - S kagit B ay, Lagoon P E S hallow

I n t er t idal, >2 0 ppt

0 . 0

10 . 0

2 0 . 0

3 0 . 0

4 0 . 0

5 0 . 0

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Prickly Sculpin

Pacific Staghorn

Other MarineSculpins

D - S kagit B ay, Lagoon P E S hallow I n t er t idal,

>2 0 ppt

0 . 0

0 . 2

0 . 4

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1. 0English Sole

Starry Flounder

Predicting Channel Sustainability

Tools we used:

A hydrodynamic model (a refinement of the Whidbey Basin model incorporating finer scale LiDAR data, a theoretical bathymetry for pocket estuary channels and marshes developed from channel cross section data at other sites, and field maps of the sites)

Sediment grain size distribution data, and

Hydraulic geometry relationships.

We applied the tools to two restoration scenarios at Possession Park, and

Figure 5. Site map. Sediment moves from the bluff to the spit. The spit is an accretion shoreform (net sediment gain). The pink area behind the spit is low (below MHHW) as is the wetland (green area). The possible relict channel is mapped on darker vegetation that stands out against the pervasive vegetation in the low areas. The shape of the vegetation is typical of tidal channels (sinuous). The stream continues up the slope beyond what is shown. The parking area, restroom building, road and some trails are on fill. The north parking lot is uneven in elevation and has low spots.

one scenario at Camano Island

State Park

Answers From The Model

high tide

low tide flood tide

ebb tide

Figure 9. Tidal inundation model for scenario 1. Once connected to tidal inundation at the proposed location, the entire existing marsh surface would be flooded at high tide. Arrow length represents tidal velocity. Arrow direction indicates tidal flow direction across the marsh surface. Colors indicate bed shear stress—maximum occurs at the inlet channel during ebb tide. The diagrams represent water on the marsh surface and do not represent bridges or other built structures.

The existing marsh will get wet if connected to Possession Sound

Even more so if scenario 2 is implemented

high tide ebb tide

low tide flood tide

Figure 10. Tidal inundation model for scenario 2. Once connected to tidal inundation at the proposed location, the entire marsh surface of the existing marsh would be flooded at high tide. Arrow length represents tidal velocity. Arrow direction indicates tidal flow direction. Colors indicate bed shear stress—maximum bed occurs at the inlet channel during ebb tide. The diagrams represent water on the marsh surface and do not represent bridges or other built structures.

Low TideFlood

High Tide Ebb

Tool #2: Sediment Data

To predict the critical shear stress for clearing sediments from the inlet channel deposited by longshore drift

Comparing Measured Grain Size to Modeled Shear Stress

crit = Rg(s – f)Dj

where crit = the critical shear stress value for the Dj measured in Pascals, j =the grain diameter cumulative frequency by weight (dimensionless), s = the sediment density (2.65 kg m-3), f = the density of marine water (1.025 kg m-3), g = the acceleration due to gravity (m/sec2), and R = 0.003 is Shield’s parameter for gravel dominated sediments.

Table 1. Comparing crit to the bed shear stress calculated by the hydrodynamic model.

Scenario crit D50 (PA) crit D75 (PA) bed from model (PA) peak velocity from

model (m/s) 1 4.18 11.47 10 1.77 2 4.18 11.47 10 1.59

Table 1. Comparing crit to the bed shear stress calculated by the hydrodynamic model.

Grain size fraction by weight

Grain diameter (mm)

crit (Pascals)

bed (Pascals)

peak velocity from model (m/s)

D50 5 2.39 10 1.97

D65 9.6 4.59 10 1.97

D80 13.5 6.45 10 1.97

D85 20 9.56 10 1.97

D90 25 11.95 10 1.97

Tool #3: Hydraulic Geometry

To predict if the proposed inlet channel size will be at equilibrium relative to the pocket estuary size based on a regression of width to size ratios for similar, stable pocket estuaries in Whidbey Basin.

Relationship Between Inlet Channel Width and Pocket Estuary Area

y = 9.8837x0.4154

R2 = 0.473p = 0.013

y = 3.8879x0.5152

R2 = 0.9063

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0.1 1 10 100 1000 10000 100000

total intertidal area (ha)

inle

t ch

ann

el w

idth

(m

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Whidbey Basin pocket estuaries

Williams et al. 2002 San Francisco Bay data

scen

ario

2

scen

ario

1

Relationship Between Inlet Channel Width and Pocket Estuary Area

y = 9.8837x0.4154

R2 = 0.473p = 0.013

y = 3.8879x0.5152

R2 = 0.9063

1

10

100

1000

10000

0.1 1 10 100 1000 10000 100000

total intertidal area (ha)

inle

t ch

ann

el w

idth

(m

)

Whidbey Basin pocket estuaries

Williams et al. 2002 San Francisco Bay data

Possession Predictions

Scenario 1 is probably too small to be sustainable based on comparison, but scenario 2 could be sustainable based on size (just barely)

Scenario 2 would require inlet channel clearing or some type of engineering solution to clear the coarsest 25% of sediment

Camano Predictions

This site is on the small end, but it is probably sustainable if restored. Channel clearing could be improved by 2 factors: The inlet could be located down drift from

the boat ramp to divert the coarsest 15% of sediments.

The hydraulic head due to freshwater has not been considered and will improve inlet bed shear stress for clearing sediment, especially in the winter when the coarse sediment will be coming through.

Landscape-Scale Context