Arctic and Sub-Arctic Coastal Processes
UAA has an active research program in the areas of coastal processes, coastal monitoring, geomorphic change, and instrumentation in Arctic and sub-Arctic areas. Our goal is to develop a fundamental understanding of coastal processes in order to be able to predict trends and quantify threats to coastal communities, infrastructure, and ecosystems. We are particularly focused on the impacts of climate change on these processes. In pursuing this goal, we integrate various approaches including: (1) numerical and analytical modeling of circulation (flow velocity and water elevation), sediment transport, and geomorphic change, (2) field measurement of relevant parameters (e.g., water level, velocity, and sediment concentration/character), (3) laboratory studies using flumes, and (4) surveying of bathymetry/topography. Current projects include:
Impact of an Arctic Causeway on Hydrodynamics and Sedimentation (2010-2012)
Tom Ravens and MS graduate student Garrett Yager recently completed a study (funded by the North Pacific Research Board) addressing the hydrodynamic and sediment transport impacts of the Endicott causeway on the North Slope of Alaska. Solid-filled causeways have been used along the Beaufort Sea coastline to support oil and gas drilling operations and to provide maritime access on a shallow coast. However, in some cases, these causeways also alter the nearshore hydrodynamics and sediment transport leading to excessive sedimentation.
The Endicott causeway, located in the Sagavanirktok River delta on Alaska's North Slope, caused increased sedimentation in the lagoon area landward of the causeway within a few years of its construction in the mid 1980's. Past studies raised concern that infilling of the lagoon may eventually alter fish migration.
The main objectives of this project is to determine if the lagoon has continued to infill and if the pattern of infilling has been related to the alterations imposed by the causeway. Key elements of the research include numerical modeling of the hydrodynamics with and without the causeway as well as bathymetry(the measurement of water depth at various places in a body of water) surveying of the lagoon area.
The results indicated that sediment has continued to deposit in the lagoon and the deposition has been related to the placement of the causeway. The depositing sediments were observed to be significantly finer than the native sediments. Our analysis indicates that the rate of sediment deposition in the lagoon has been decreasing and will likely continue to decrease in the future. The figure below shows the geographic setting of the project and the net deposition in the vicinity of the causeway in a 20 year period.
(a)Geographic setting of the Sagavanirktok River Delta and the Endicott Causeway; (b) contour plot of observed change in bathymetry between 1989 and 2009/10.
Modeling of Storm-Induced Inundation, Sediment Transport, and Water Quality Impacts on the Yukon Kuskokwim Delta
Tom Ravens and research assistant Jon Allen are developing a storm surge modeling system to examine the frequency and severity of storm-induced inundation of the YK Delta. In addition, the team is examining sediment transport impacts (in particular sedimentation) and water quality impacts. The storm surge modeling system consists of a course-grid ADCIRC model and a fine-grid Delft3D model. The validation of the storm surge model and some calculations of inundation frequency/intensity will be the subject of a presentation or poster at the Dec. 2012 AGU Fall Meeting. An example of a preliminary calculation of inundation is shown in the figure below. The modeling will soon be extended to examine effects of climate-change-induced sea level rise.
Arctic Coastal Erosion Modeling
Tom Ravens and collaborators are developing process-based coastal erosion models. In contrast to conventional models, these models account for the thermal and mechanical processes that are important in the Arctic. Importantly, these models are well-adapted to forecast future erosion rates in a warming Arctic. There are a number of mechanisms by which Arctic coastal erosion proceeds.One of the most dramatic mechanisms involves the thermal and mechanical cutting of a niche at the base of the coastal bluffs often during storm surges. This mechanism is predominant at Drew Point and other locations on the North Slope coast with high ice content and fine sediments.The niche grows until the overburden exceeds the bluff strength and block collapse results. An image of a fallen block and a conceptual model of the erosion mechanism are provided in the figure below (from Ravens et al. 2012).
Following up on this work, Tom and collaborator Li Erickson (USGS) are working to address other types of coastal erosion mechanisms. At the 2011 AGU Fall Meeting, Tom and co-authors presented preliminary work on a new coastal erosion model applicable at high coastal bluffs with high ice content and coarse sediments (e.g., Barter Island, Figure 8). At these sites, erosion proceeds via two complementary mechanisms. First, throughout the summer there is thaw slumping of the bluff face due to radiativeand convective heat transfer.Also, there are annual storm surges that transport the slumped sediment offshore. The thaw slumping and the annual storm surges together establish the normal beach profile which is much higher at the bluff toe than what is seen at a location like Drew Point. The normal yearly storm does not have sufficient surge to cause niche erosion. Occasionally, about every 10 years, there is a massive storm with a sufficient surge to cause niche erosion and block collapse. Figure 8 shows evidence of niche formation probably from the 2008 storm surge.