Doctoral defence in Environmental studies - Morgane C. Priet Mahéo

Ph.D. student: Morgane C. Priet-Mahéo

Dissertation title: Internal dynamics of a medium size subarctic lake: field measurements and numerical modeling

Opponents: Dr. Ben Hodges, Professor at the Department of Civil, Architectural and Environmental Engineering, University of Texas at Austin
Dr. David C. Finger, Assistant Professor at the Department of Engineering, Reykjavík University

Advisor: Dr. Hrund Ó. Andradóttir, Professor at the Faculty of Civil and Environmental Engineering, University of Iceland

Doctoral committee: 
Dr. Francisco Rueda, Professor, Water Resources Engineering, University of Granada
Dr. Jón Ólafsson, Professor Emeritus in Oceanography at the Faculty of Earth Sciences, University of Iceland
Dr. Sigurður M. Garðarsson, Professor at the Faculty of Civil and Environmental Engineering and Dean of the School of Engineering and Natural Sciences at the University of Iceland

Chair of Ceremony: Dr. Guðmundur Freyr Úlfarsson, Professor and the Head of the Faculty of Civil and Environmental Engineering, University of Iceland

Abstract:

As the Arctic Circle opens up due to the effects of global warming, the surrounding regions, and in particular the subarctic zone, experience both climatic and societal changes, which makes it imperative to understand the existing ecosystems. Lake Lagarfljót, an Icelandic fjord lake with glacial inflow, is used to evaluate processes occurring in weakly stratified water bodies in the sub-Arctic. Field observations during the summers of 2010 and 2011 show that the lake experiences a short period of stratification, with overturning occurring when the heat fluxes become negative. The internal-wave regime evolves rapidly as the stratification strengthens, higher vertical modes and Kelvin waves are identified. Glacial inflow to Lake Lagarfljót delivers large volumes of fine sediment. It is expected that most of the sediments remain in suspension and this can alter the density of the water-column, resulting in inverse thermal gradients. Additionally, the glacial river is deflected to the eastern edge of the lake by Coriolis forces, causing the inflow to remain in shallow water before entering the center of the lake. Results from the Münnich model demonstrate that the natural oscillation of the lake basin is close to the oscillation period of the wind. The 3D hydrodynamic model Si3D was adapted to account for suspended sediment, and it successfully reproduced the motions observed in the lake, showing its utility for the study of weakly stratified lakes. Results suggest that wind is the main driver of internal waves in Lake Lagarfljót, but that the river influences stratification, meaning that it cannot be ignored when considering lake circulation. The model confirms the presence of Kelvin waves in the basin. Tracking the pathways of the inflowing glacial water, it appears that the initial intrusion depth is dependent on internal waves. During summertime stratification, the river penetrates the lake as interflows or underflows. Underflows can become trapped, causing them to oscillate and becoming flushed episodically when large glacial discharges enter the lake as underflow. Water entering the lake as interflow mixes more easily and it is flushed quasi-continuously out of the basin. Because the circulation of Lake Lagarfljót is sensitive to the strength of the stratification, the internal dynamics of the lake are expected to change significantly in a warming climate.

About the doctoral candidate:

Morgane is from Rennes, France. She received a B.Sc.degree and a maîtrise in geography from the University Rennes II in 2003 and 2004 and a M.Sc. degree in Geoenvironment from the Blaise Pascal University in Clermont-Ferrand in 2005. After moving to Iceland and working at the Maritime Administration for three years, she started her doctoral studies at the University of Iceland in 2010. Morgane joined the Hydrology Research Group at the Iceland Meteorological Office in December 2015.