18-16. Investigating geomorphic evolution of salt marshes using coupled biophysical models

Estuarine marshes provide important and unique ecological habitat, are highly productive, and protect coastlines by attenuating waves. They are also highly vulnerable to sea level rise due to their low elevations.  Sea-level rise and sediment deficits can lead to marsh loss through three mechanisms: lateral retreat, internal expansion of ponds and channels, and loss of elevation relative to mean tide level. All of these mechanisms are active along wave-exposed marsh edges. 

The morphology of wave-exposed marsh edges varies from steep scarps to gentle slopes. Conceptual models indicate clear feedbacks between marsh edge type and wave-driven processes at the marsh edge: steep scarps are created by wave erosion, and are likely to erode further, whereas gentle slopes attenuate waves (especially if vegetated), and may be depositional, or may promote deposition within the marsh. Thus geomorphic type is linked to marsh resilience (Reed et al. 2018). Furthermore, changes in geomorphic type are expected as sea level rises.  The processes governing erosion and accretion at the marsh edge are complex and difficult to predict, involving wave conditions, tidal transport, wetting and drying, vegetation, and sediment properties, and many of these processes are strongly dependent on water depth and bed slope. Idealized models have simulated some of the dominant processes (Mariotti, 2016; Fagherazzi et al., 2012). Currently, process-based models that account for the biogeomorphic complexity of marsh edges are in development. Calibration of these models is challenging because of the range of simulated processes,and lack of suitable data.

Scientists at Woods Hole Coastal and Marine Science Center (WHCMSC) have developed a vegetation-hydrodynamic-sediment transport model which is integrated into the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) system (Beudin et al 2017, Warner et al. 2010), and includes newly developed capabilities to simulate marsh edge erosion as a function of wave thrust. To date, validation of these model capabilities has been limited due to insufficient field data along the estuary-marsh continuum. Scientists at the Pacific Coastal and Marine Science Center (PCMSC) have collected hydrodynamic and sediment transport data spanning mudflats to the marsh platform in San Francisco Bay, and both PCMSC and WHCMSC have collected data from other systems that can serve as useful test cases.  These resources create opportunities to strengthen capabilities for modeling marsh edge dynamics, and to use these models to explore factors influencing marsh resilience, expected trajectories for marsh edge geomorphology as sea level rises and storminess increases, and the consequences for managing marsh habitat conservation and restoration.

The goal of this research opportunity is to investigate the influence of vegetation and geomorphic type on marsh edge dynamics and marsh resilience with numerical modeling.  The successful candidate will have a strong background in estuarine processes and modeling. Applicants have the opportunity to define and focus on a specific question within the broad topic that align with their background and interests. Examples are:

• How does geomorphic type influence lateral marsh erosion and marsh resilience?

• How do differences in marsh vegetation and marsh plain elevation (relative to sea level) between west coast, east coast, and/or gulf coast marshes influence marsh edge dynamics?

• What mechanisms contribute to lateral marsh retreat and vertical accretion?

• How do marsh edge dynamics vary over the course of marsh restoration?

• How do management actions such as sediment placement influence sediment delivery across wave-exposed marsh edges?

• How will sea level rise and increased storminess affect the flood protection services of salt marshes?

Model application can build on existing domains (San Francisco Bay, Plum Island Estuary, Barnegat Bay, Chincoteague Bay), new realistic domains, or idealized contexts. Depending on the applicant’s expertise, further work on the feedbacks between vegetation and geomorphology or biogeochemistry could be explored with the modeling system.  Interested applicants are strongly encouraged to contact the Research Advisors early in the application process to discuss project ideas.

References: 

Beudin, A., T. S. Kalra, N. K. Ganju, and J. C. Warner (2017), Development of a coupled wave‐flow‐vegetation interaction model, Comput. Geosci., 100, 76– 86.

Fagherazzi S., Kirwan M.L., Mudd S.M., Guntenspergen G.R., Temmerman S., D’Alpaos A., van de Koppel J., Rybczyk J.M., Reyes E., Craft C., Clough J., (2012). Numerical Models of Salt Marsh Evolution: Ecological and Climatic Factors, Reviews of Geophysics 50, 1, doi:10.1029/2011RG000359

Mariotti, G., & Carr, J. (2014). Dual role of salt marsh retreat: Long-term loss and short-term resilience. Water Resources Research, 50, 2963-2974. doi: 10.1007/2013WR01476

Reed, D., van Wesenback, B., Herman, P. M. J., & Meselhe, E. (2018). Tidal flat-wetland systems as good defenses: Understanding biogeomorphic controls. Estuarine, Coastal, and Shelf Science, 213, 269-282. doi: 10.1016/j.ecss.2018.08.017

Warner, J.C., He, R., Zambon, J., and Armstrong, B., 2010, Development of a Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System, Ocean Modelling, 35, 230-244.

Proposed Duty Station: Santa Cruz, CA or Woods Hole, MA

Areas of PhD: Coastal engineering, physical oceanography, environmental engineering, coastal geology, or related fields (candidates holding a Ph.D. in other disciplines but with knowledge and skills relevant to the research opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications: Research Oceanographer, Research Hydrologist, Research Geologist, Research Geophysicist, Research Computer Scientist  

(This type of research is performed by those who have backgrounds for the occupations stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Human Resources Office Contact: Audrey Tsujita, 916-278-9395, atsujita@usgs.gov