Effects of Exposed Tidal Flats on Water Temperature in the Snohomish Estuary

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This study aims to evaluate the short and long-term impacts of estuary restoration to support development recommendations for restoration and resilience planning. This study will be completed in several phases. As part of Phase 1, water temperature, depth, and salinity data collected since 2012 were used to generate a post-restoration analysis of water conditions within the estuary, and the effects of the Qwuloolt and Smith Island restoration projects on water quality conditions. This included updating the existing Snohomish Estuary Model from the year 2007 effort to reflect current conditions circa 2014 and 2021 using water temperature, depth, and salinity data along with updated bathymetry. The model was used to simulate pre- and post-Qwuloolt and Smith Island restoration and set the stage for subsequent phases of model and scenario development. Phase 2 & 3 of the project will build on Phase 1 by taking the information learned from the Phase 1 modeling effort and developing model runs of the Snohomish estuary including integration of a three-dimensional hydro-heat flux numerical model to capture the effects of exposed tidal flats on water temperature (Phase 2), and modeling full restoration buildout (Phase 3). The final phase (Phase 4) of the project will look at water temperature, salinity, and tidal conditions utilizing the previously developed modeling components under future climate conditions in 2095. This page reports the ongoing effort of Phase 2.

This study is the result of a combined effort by Tulalip Tribes, Salish Sea Modeling Center, Cramer Fish Sciences, NOAA NWFSC, and Snohomish County Surface Water Management Division.

Background

We will utilize the previously developed Snohomish River Estuary High-resolution Model (Phase 1) to examine the hypothesis that large exposed tidal flats areas could, in certain conditions, lead to increases in temperature and salinity distributions. This phase of the project will incorporate data and analysis from Phase I and an integrated sediment heat flux model. The most recent surveys represent post-restoration conditions and will include newly inundated restored regions including Qwuloolt (2015, Ebey Slough), Smith Island (2018, Union Slough), Biringer Farm and Union Slough projects.

Study Question

•        Could restoration projects resulting in large exposed tidal flats, in certain conditions, lead to changes in temperature and salinity distributions in the short-term?

Approach

Acquisition and review of sediment and water column intertidal temperature data
Prior to and during Phase 1, the following data were collected: water level (pressure), temperature, salinity (conductivity), sediment surface temperatures, and Seabird CTD water profiling. They will be reviewed for use in informing and calibrating the integration of the Snohomish model with the sediment heat flux model.

FVCOM (finite volume community ocean model) implementation of sediment temperature module testing
This task will consist of integrating the Snohomish model with the sediment heat flux model, testing, and calibrating the models utilizing data from the CWS network, sediment surface temperatures, and CTD water profiles. Once the model is updated and calibrated using the Phase 1 model and bathymetric and topographic data, we will run several model scenarios. Phase 2 will build on Phase 1 by integrating a sediment temperature model into the FVCOM based model of the Snohomish Estuary. The heat flux from heated sediments in the summer or cooled sediments in the winter to the overlying waters will be properly accounted for through this effort.

Estimate of heat contributions from intertidal sediments and restoration areas
The model grid and the associated FVCOM implementation in Snohomish Estuary Model will be expanded to cover the most recent representation of the present conditions. The heat flux computations associated with wetting and drying treatment in the model will then be improved as part of the model validation. After confirming the post-restoration temperature and salinity response, a series of model simulations will be conducted to better understand the potential cumulative, estuary-wide, impacts.

particles of clay, silt, sand, gravel, or cobble, transported by water, are called sediment.