Improving the fidelity of morphological impact predictions in Arctic Alaska barrier Islands

Abstract:

In light of the concerns due to climate change and sea-level rise, there is a need to predict future stability and evolution of barrier islands in Arctic Alaska. For such dynamic areas, in terms of the computational cost, reduced complexity models are preferred to carry out long term simulations. The new free-form coastline model ShorelineS that is capable to describe large coastal transformations (Roelvink et al., 2020), was selected to simulate the changes in the barrier islands. It was hypothesized that the main drivers to the changes are alongshore transport gradient, overwash and wave refraction. Such processes are schematized in the model.

Four barrier islands were chosen to replicate their geomorphic behavior. These are: Reindeer, Cross, Narwhal, and Stockton islands. The simulations were performed using the historical shorelines, hindcast waves and bathymetry. Also, a routine was added into the model to update the bathymetry when needed, a scale factor was introduced to control the barrier overwash process, and a 2D wave solver was used to transmit and refract the waves. In order to calibrate the model and determine the influential parameters, based on the available data and the user experience, a range of values of the model parameters were defined. First, the values were tested with the four barriers in the period from the 2000s to 2010s. Then additional tests were performed for Cross island in the period from 2006 to 2009 to evaluate the sensitivity of the mode the parameters.

The modelled and the observed shorelines were compared. Modelling results show that the 2D wave solver, overwash and bathymetry updates routines leads to more realistic results. The most influential parameters are the ones controlling the main processes such as closure depth, spit width, and surf width (where the model obtains the wave conditions for the longshore transport formula). The model was capable to capture the barrier rollover and spit growing processes.

Reference

Roelvink, D., Huisman, B., Elghandour, A., Ghonim, M., & Reyns, J. (2020). Efficient modeling of complex sandy coastal evolution at monthly to century time scales. Frontiers in Marine Science, 7, 535. https://doi.org/10.3389/fmars.2020.00535

Funded by USGSDeltares cooperative research project (11204520), and Project: ALG-01-0145-FEDER-28949 "ENLACE”.