Sediment Transport into the Swinomish Navigation Channel, Puget Sound—Habitat Restoration versus Navigation Maintenance Needs
2. Materials and Methods
2.1. Hydrodynamic Model of the Skagit River Estuary with the Swinomish Channel
2.2. Sediment Transport Model of the Skagit River Estuary with the Swinomish Channel
2.2.1. Skagit River Sediment Load and Deposition Characteristics
2.2.2. Skagit River Sediment Transport Model Setup
2.3. Hydrodynamic and Sediment Model Validation
2.4. Application to Dike Repair and Restoration Scenarios
- SCN-1, South Jetty Repair: Recent surveys by USACE indicate that South Jetty of the Swinomish Channel (see Figure 1, C location) that extends southwest from Goat Island along the southern bank of the Swinomish Channel has structurally degraded (jetty elevations reduced from ≈1.9 m to as low as −1.1 m), allowing leakage flow and transport from adjacent flats to the Swinomish Channel. In SCN-1, the South Jetty is repaired by re-setting the elevations of the dike crest nodes back to 1.9 m (NAVD88), thereby restoring the functionality of the South Jetty. The main objective of this scenario is to evaluate if this repair would result in improvement (reduction) in sediment transport into the Swinomish Channel. SCN-1 South Jetty Repair location is shown in Figure 9a.
- SCN-2, McGlinn Causeway Restoration: This habitat restoration proposal involves breaching of the causeway between McGlinn Island and mainland (see Figure 1, D location) and reduction of the crest elevations of the north section of the McGlinn Island to Goat Island Jetty (see Figure 2b). The restoration design calls for reduction in the crest elevation (from ≈3.0 m) to 1.33 m (NAVD88) corresponding to mean sea level (MSL = 0) at the north section of the Goat Island jetty allowing a direct connection between the North Fork of the Skagit River and the Swinomish Channel during high tides. The causeway breach was incorporated by reducing the elevation of 2.5 m of the causeway to an elevation of −0.3 m (NAVD88). In addition to providing better connectivity and restoring historic pathways for upstream and downstream fish migration, the objective of this scenario is to examine the potential benefits in the form of reduction in salinity and improved brackish water habitat. SCN-2 McGlinn Causeway Restoration locations, consisting of causeway breach and reduction of the Jetty crest elevations, are shown in Figure 9b.
3.1. Assessment of Jetty Repair Scenario, SCN-1
3.2. Assessment of McGlinn Causeway Restoration Scenario, SCN-2
Conflicts of Interest
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|Station||Water Surface Level, m||Velocity, m/s||Salinity, ppt|
|AME 1||RMSE 2||SS 3||AME||RMSE||SS 3||AME||RMSE||SS 3|
|S1||–||–||u 3:0.20; v:0.29||u:0.24; v:0.32||0.85|
|S2||–||–||u:0.32; v:0.34||u:0.37; v:0.42||0.60|
|S5||0.32||0.39||0.95||u:0.34; v:0.22||u:0.42; v:0.26||0.55|
|Source||Description||Study Results||Representative Sediment Accretion Rate Estimate cm/Year|
|Khangaonkar et al. ||Preliminary estimate of sediment accretion rate using annual average sediment load (3,962,084 tons/year based on average of load from 1988 to 2010)||Skagit Bay and Swinomish Channel: Uniform distribution and deposition of sediments in Skagit Bay study area of (1.6 × 108 m2, porosity of 0.4, and density of 2650 kg/m3)||Skagit Bay study domain including Swinomish Channel|
|Rybczyk, J. —EPA STAR Grant project (unpublished data)||Sediment Trap and Feldspar five marker horizons/grids at Skagit Bay Nearshore site: |
N 48°21′25.1′′, W 122°28′33.8′′. Accretion rate was noted over the markers during a 15 day period (11/18/08 to 12/3/08)
|Skagit Bay Marsh:|
Mean accretion in sediment trap = 5.15 g.d.w. ± 2.3 (s.d.) = 334.6 g/m2 in 15 days
Bed elevation change
0.33 ± 0.23 (s.d.) cm/15 days
|Skagit Bay nearshore Station—Accretion from the 15-day sample in 2008,|
|Kairis and Rybczyk ||Rate of elevation change derived from the linear regression of surface elevation changes at multiple sites in Padilla Bay from 2002 to 2010||Padilla Bay: Most sites in Padilla show erosion except the three below|
12 (b)—0.13 cm/year
14 (b) —0.12 cm/year
|Padilla Bay sediment accretion rates at selected sites|
|Coastal Geological Services ||Swinomish Channel Sedimentation Study—Shoaling rate analysis using dredging records||Swinomish Channel:|
Analysis of dredging and survey records from two periods
2001–2003 and 2004–2008
|Swinomish channel average accretion rate based on 2004–2008 records|
Skagit River Flood Risk Management General Investigation
|Sediment Budget and Fluvial Geomorphology.|
Examination of Skagit River bed elevation changes
|Skagit River, North Fork and South Fork:|
Comparison of surveyed cross sections between 1975 and 1999.
|Average accretion rate based on 24 year record, Skagit River (RM 10.1 to RM 18)|
N.F. Skagit River
(RM 4.5 to RM 8.85)
≈2 cm /year
S.F. Skagit River
(RM 5.8 to 9.25)
|Model Parameter||Fine Silt||Silt||Fine Sand|
|Settling velocity, m·s−1||0.1||1.0||10.0|
|Mean diameter, mm||0.014||0.04||0.14|
|Erosion rate, kg·m−2·s−1||1.2 × 10−5||1.2 × 10−4||1.2 × 10−3|
|Critical stress for erosion, N·m−2||0.05||0.08||0.15|
|Fractional composition (river load and Initial sediment bed)||0.15||0.45||0.40|
|Mooring||Salinity||Shear Stress||Bottom TSS||TSS|
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Khangaonkar, T.; Nugraha, A.; Hinton, S.; Michalsen, D.; Brown, S. Sediment Transport into the Swinomish Navigation Channel, Puget Sound—Habitat Restoration versus Navigation Maintenance Needs. J. Mar. Sci. Eng. 2017, 5, 19. https://doi.org/10.3390/jmse5020019
Khangaonkar T, Nugraha A, Hinton S, Michalsen D, Brown S. Sediment Transport into the Swinomish Navigation Channel, Puget Sound—Habitat Restoration versus Navigation Maintenance Needs. Journal of Marine Science and Engineering. 2017; 5(2):19. https://doi.org/10.3390/jmse5020019Chicago/Turabian Style
Khangaonkar, Tarang, Adi Nugraha, Steve Hinton, David Michalsen, and Scott Brown. 2017. "Sediment Transport into the Swinomish Navigation Channel, Puget Sound—Habitat Restoration versus Navigation Maintenance Needs" Journal of Marine Science and Engineering 5, no. 2: 19. https://doi.org/10.3390/jmse5020019