WARMER model projections of sea-level rise for eight tidal marsh study areas on coastal Oregon and Washington, 2010-2110

Dates

Publication Date: 2015-07-01
Start Date: 2010-01-01
End Date: 2110-12-31

Citation

Thorne, K. 2015. WARMER model projections of sea-level rise for eight tidal marsh study areas on coastal Oregon and Washington, 2010-2110. US Geological Survey Data Release. 10.5066/F7SJ1HNC

Summary

We used WARMER, a 1-D cohort model of wetland accretion (Swanson et al. 2014), which is based on Callaway et al. (1996), to examine SLR projections across each study site. Each cohort in the model represents the total organic and inorganic matter added to the soil column each year. WARMER calculates elevation changes relative to MSL based on projected changes in relative sea level, subsidence, inorganic sediment accumulation, aboveground and belowground organic matter productivity, compaction, and decay for a representative marsh area. Each cohort provides the mass of inorganic and organic matter accumulated at the surface in a single year as well as any subsequent belowground organic matter productivity (root growth) minus decay. [...]

Summary

We used WARMER, a 1-D cohort model of wetland accretion (Swanson et al. 2014), which is based on Callaway et al. (1996), to examine SLR projections across each study site. Each cohort in the model represents the total organic and inorganic matter added to the soil column each year. WARMER calculates elevation changes relative to MSL based on projected changes in relative sea level, subsidence, inorganic sediment accumulation, aboveground and belowground organic matter productivity, compaction, and decay for a representative marsh area. Each cohort provides the mass of inorganic and organic matter accumulated at the surface in a single year as well as any subsequent belowground organic matter productivity (root growth) minus decay. Cohort density, a function of mineral, organic, and water content, is calculated at each time step to account for the decay of organic material and auto-compaction of the soil column. The change in relative elevation is then calculated as the difference between the change in modeled sea level and the change in height of the soil column, which was estimated as the sum of the volume of all cohorts over the unit area model domain. The total volume of an individual cohort is estimated as the sum of the mass of pore space water, sediment, and organic matter, divided by the cohort bulk density for each annual time step. Elevation is adjusted relative to sea level rise after each year of organic and inorganic input, compaction, and decomposition. We parameterized WARMER from the elevation, vegetation, and water level data collected at each site. We evaluated model outputs between 2010 and 2110 using marsh elevation zones defined above.Model inputs Sea-level rise scenariosIn WARMER, we incorporated a recent forecast for the Pacific coast which projects low, mid, and high SLR scenarios of 12, 64 and 142 cm by 2110, respectively (NRC 2012). We used the average annual SLR curve as the input function for the WARMER model. We assumed the difference between the maximum tidal height and minimum tidal height (tide range) remained constant through time, with only MSL changing annually.Inorganic matterThe annual sediment accretion rate is a function of inundation frequency and the mineral accumulation rates measured from 137Csdating of soil cores sampled across each site. For each site, we developed a continuous model of water level from the major harmonic constituents of a nearby NOAA tide gauge. This allowed a more accurate characterization of the full tidal regime as our water loggers were located above MLLW. Following Swanson et al. (2014), we assumed that inundation frequency was directly related to sediment mass accumulation; this simplifying assumption does not account for the potential feedback between biomass and sediment deposition and holds suspended sediment concentration and settling velocity constant. Sediment accretion, Ms,at a given elevation, z, is equal to, where f(z) is dimensionless inundation frequency as a function of elevation (z), and Sis the annual sediment accumulation rate in g cm-2 y-1.Organic matterWe used a unimodal functional shape to describe the relationship between elevation and organic matter (Morris et al. 2002), based on Atlantic coast work on Spartina alterniflora. Given that Pacific Northwest tidal marshes are dominated by other plant species, we developed site-specific, asymmetric unimodal relationships to characterize elevation-productivity relationships. We used Bezier curves to draw a unimodal parabola, anchored on the low elevation by MTL at the high elevation by the maximum observed water level from a nearby NOAA tide gauge. We determined the elevation of peak productivity by analyzing the Normalized Difference Vegetation Index (NDVI; (NIR - Red)/(NIR + Red)) from 2011 NAIP imagery (4 spectral bands, 1 m resolution; Tucker 1979) and our interpolated DEM. We then calibrated the amplitude of the unimodal function to the organic matter input rates (determined from sediment accumulation rates and the percent organic matter in the surface layer of the core) obtained from sediment cores across an elevation range at each site. The curves were truncated to zero below the lowest observed marsh elevation for each site from our vegetation surveys, reflecting the observed transition to unvegetated mudflat. The root-to-shoot ratio for each site was set to 1.95, the mean value from an inundation experiment conducted at Siletz in 2014 for Juncus balticusand Carex lyngbyei, two common high and low marsh species in the Pacific Northwest (C. Janousek et al., unpublished results). Compaction and decompositionCompaction and decomposition functions of WARMER followed Callaway et al. (1996). We determined sediment compaction by estimating a rate of decrease in porosity from the difference in measured porosity between the top 5 cm and the bottom 5 cm of each sediment core. We estimated the rate of decrease, r, in porosity of a given cohort as a function of the density of all of the material above that cohort.Following Swanson et al. (2014), we modeled decomposition as a three-tiered process where the youngest organic material, less than one year old, decomposed at the fastest rate; organic matter one to two years old decayed at a moderate rate; and organic matter greater than two years old decayed at the slowest rate. Decomposition also decreased exponentially with depth. We determined the percentage of refractory (insoluble) organic material from the organic content measured in the sediment cores. We used constants to parameterize the decomposition functions from Deverel et al. (2008). ImplementationFor each site, we ran WARMER at 37 initial elevations (every 10 cm from 0 to 360 cm, NAVD88). A two hundred year spin-up period for each model run was used to build an initial soil core. A constant rate of sea-level rise was chosen that the modeled elevation after 200 years was equal to the initial elevation. After the spin-up period, sea-level rose according to the scenario (+12, 63, or 142 cm by 2110). Linear interpolation was used to project model results every 10 years onto the continuous DEM developed from the RTK surveys. This raster contains data from Bandon marsh with the projection from the WARMER model for the year 2010 with a 63 cm sea-level rise rate.

Contacts

Point of Contact :: Karen M Thorne
Originator :: Karen M Thorne
Distributor :: U.S. Geological Survey, GS ScienceBase

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warmerProjections.fgdc.xml Original FGDC Metadata	View	16.59 KB	application/fgdc+xml
Bandon.zip		10.94 MB	application/zip
Bull Island.zip		10.16 MB	application/zip
Grays Harbor.zip		7.12 MB	application/zip
Nisqually.zip		6.6 MB	application/zip
Padilla.zip		812.05 KB	application/zip
Port Susan.zip		6.01 MB	application/zip
Siletz.zip		12.31 MB	application/zip
Skokomish.zip		3.47 MB	application/zip
Willapa.zip		6.46 MB	application/zip

Extension: marshesToMudflats_warmerProjections.zip
marshesToMudflats_warmerProjections.sd		56.27 MB
thumbnail.png		3.22 KB

Related External Resources

Type: Online Link

Web Link	https://doi.org/10.5066/F7SJ1HNC

Type: webLink

Digital Data	https://doi.org/10.5066/F7SJ1HNC

Purpose

In the Pacific Northwest, coastal wetlands support a wealth of ecosystem services including habitat provision for wildlife and fisheries and flood protection. The tidal marshes, mudflats, and shallow bays of coastal estuaries link marine, freshwater and terrestrial habitats and provide economic and recreational benefits to local communities. Climate change effects such as sea-level rise are currently altering these habitats, but we know little about how these areas will change over the next 50-100 years. Our study examined the effects of sea-level rise on nine tidal marshes in Washington and Oregon, with the goal of providing scientific data to support future coastal planning and conservation. We compiled physical and biological data, including coastal topography, tidal inundation, vegetation structure, and current and historic sediment accretion rates to assess and model how sea-level rise may alter these ecosystems in the future. Multiple factors, including initial elevation, marsh productivity, sediment availability, and rates of sea-level rise affected marsh persistence. Under a low sea-level rise scenario, all marshes remained vegetated with little change in the present configuration of marsh plant communities or gradually increased proportions of mid, high, or transition marsh vegetation zones. However at most sites, mid sea-level rise projections led to loss of middle and high marsh and gain of low marsh habitat. Under a high sea-level rise scenario, marshes at most sites eventually converted to intertidal mudflats. Two sites (Grays Harbor, and Willapa) appeared to have the most resilience to a high sea-level rise rate, persisting as low marsh until at least 2110. Our main model finding is that most tidal marsh study sites have resiliency to sea-level rise over the next 50-70 years, but that sea-level rise will eventually outpace marsh accretion and drown most high and mid marsh habitats by 2110.

WARMER model projections of sea-level rise for eight tidal marsh study areas on coastal Oregon and Washington, 2010-2110

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