Port Susan Restoration

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Stillaguamish port susan sunset.JPG

The Nature Conservancy completed a dike setback that restored tidal inundation to 150 acres of abandoned farmland at the current river mouth on the Stillaguamish Delta in 2012. The Delta is the largest tidal wetland complex and freshwater input to the Port Susan Bay Ecosystem. The project site has subsided, and restoration is anticipated to allow sediment accretion, both on site and along the face of Florence Island. Ongoing monitoring of the site supported by the River Delta Adaptive Management Strategy will track recovery, evaluate sediment accretion rates, and how marsh abates wave energy.

Notes on Project Context[edit]

  • As part of their monitoring work WWU developed a Story Map Journal describing site conditions.
  • The project might be more precisely named the TNC Hat Slough Dike Setback, as additional project are anticipated in Port Susan Bay, including Leque Island Restoration and the Matterand Restoration. These activities are being negotiated in part under the Snohomish Sustainable Lands Strategy as they result in the loss of land zoned by the county for agriculture.
  • Aerial View in Nearshore Portal - the restoration project is defined by multiple entries describing acquisition and assessment, design, and implementation phases.
  • The location of the dike removal at the current river mouth is anticipated to increase distribution of river sediment to the north of the river mouth. User:John Rybczyk of WWU and USGS are involved in monitoring sediment transport.
  • Subsidence has been significant within the site, such that the fringing marsh is serving as a dike with two breaches. This may limit channel development to two networks, and may create a velocity barrier to fish passage during some portion of tidal exchange.
  • Fuller & McArdle 2014 describes the historical pattern of marsh gain and loss. Based on the air photo record, between 1990 and 2011 approximately 270 acres of vegetated marsh has been lost. Lost marsh has mostly occurred in the vicinity of South Pass, likely caused by the interaction of multiple factors including a rapid expansion of the snow goose population, winter storm wave erosion, and change in the delivery of sediment due to channel configuration shifts at the mouth of Hatt Slough which direct more flow to the south rather than towards the northern marsh.
  • Barber 2014 documents thesis work that observed vegetation and sediment change following restoration.

Practical Lessons Learned[edit]

The following project management observations were found in final project reports to NOAA:

  • A "constructability review" was valuable in tightening the bid documents for the purpose of controlling costs. TNC hired an on site "owners representative" to catch and resolve problems early, given the quantity and rapidity of earth moving required.
  • Snohomish County charged permit fees based on yards of earth moved as if the project were a housing development cut/fill. This inflated project costs, however solving this problem will require county code changes.
  • Local landowners were included in the Technical Advisory Committee, and ultimately became advocates for the project. Open and transparent interaction with the local community throughout design and construction increased trust. The project ultimately included an approximately $500,000 flood drainage structure anticipated to improve flood drainage when Florence Island dikes are over-topped. The flood control structure has not been used since construction.

Observations of Restoration Dynamics[edit]

five zones used to compare observations in WWU/TNC monitoring

The following ideas were pulled from final grant reports and Dec 2015 monitoring report.

  • Haul routes are still present on the site and may be affecting water flow.
  • Due to subsidence within the dike, and a fringe of higher elevation marsh outside the dike, the site functions as a bathtub. Construction of only two channel outlets has resulted in high velocity during ebb and flow tides. In retrospect, more outlets could have been constructed. Over 2-3 hours of ebb flow each day, the flows may be too fast for fish access.
  • sediment monitoring indicates sediment accretion is greater than sea level rise both on the site, and across a majority of the fringing marsh, except where other factors are causing localized erosion. The subsidence is resulting in accretion rates much higher than found outside the restoration site.
  • Goose abundance has increased 30-fold since restoration, along with invertebrate community diversity. Bird foraging was very high in the restoration site. Snow geese used the site in winter during high tide, but move to agricultural land during low tide, and are replaced by shore birds.
  • Salinity has decreased at monitoring stations, suggesting that freshwater mixing has been improved by the project, however variability makes this claim hard to verify.
  • Accretion is higher at higher elevations, despite lower inundation frequency. Overwintering leaves left by high marsh species are hypothesized to trap sediment.
  • In the northern marsh where there is evidence of erosion, slope along the marsh plain is steeper.
  • The zone between MHW and MHHW is the most productive of marsh vegetation, as found in previous studies.
  • Following the March 22, 2014 Oso landslide there has been a threefold increase in suspended sediment delivered to the site with 60% of it being sand.
  • The now missing 1960 tidal marsh would have the effect of reducing water levels during storm events by 0.4 to 0.5 meters, not considering additional effects of vegetation structure.

Plans and Reports[edit]