The Evolution of a River Plume Front
EAS Doctoral Dissertation Defense by Agata PifferBraga
Date: Wednesday, April 9, 2025
Time: 2pm
Topic: The Evolution of River Plume Front
Location: SMAST West, Room 204
Zoom Link: https://umassd.zoom.us/j/98837177014
Meeting ID: 988 3717 7014
Passcode: 258088
Abstract:
River plumes form when freshwater discharges from rivers enter the denser, saltier ocean, creating buoyant water masses that play a critical role in coastal ecosystems and oceanic stratification. These outflows not only transport land-derived materials—such as sediments, nutrients, and pollutants—but also generate strong density gradients that drive a range of dynamical processes. This dissertation focuses on the horizontal boundaries of river plumes, where sharp density fronts govern distinct dynamical processes. While significant progress has been made in understanding plume dynamics over the past several decades, fundamental gaps remain in our knowledge of frontal evolution and its role in shaping the dynamics within the interior of the plume.
To investigate this fast-moving and complex region, we developed a novel sampling strategy integrating an Uncrewed Underwater Vehicle (UUV, T-REMUS), an aerial drone, and boat-based measurements to dynamically track and three-dimensionally sample the evolution of plume fronts. Through 32 frontal crossings performed at the Merrimack River plume, we captured the front’s evolution over half a tidal cycle, enabling a detailed examination of its kinematics and key dynamical mechanisms. By measuring density, velocity, acoustic backscatter, and turbulent dissipation rates, we identified variations in frontal shape and kinematics that create a feedback loop, where changes in frontal morphology influence overall dynamics and vice versa.
Changes in frontal depth trigger the generation and release of two trapped-core solitary waves, which play a crucial role in the front’s evolution. These waves effectively detach and transport a portion of the frontal mass, modifying the frontal structure and progression. Our momentum balance analysis revealed that this mass loss significantly contributes to halting the front’s propagation, fundamentally altering its dynamics. Additionally, we found that the along-front momentum balance is strongly influenced by wind forcing, which interacts with Coriolis and tidal forcing to modulate frontal evolution. This interplay between frontal structure, solitary wave generation, and external forcing underscores the complexity of plume front dynamics and their sensitivity to both internal and external physical drivers.
River plumes significantly impact coastal environments and, at times, extend into the open ocean. Studying the evolution of plume fronts provides an ideal framework for advancing our general understanding of oceanic frontal dynamics by allowing sampling and modeling at accessible temporal and spatial scales. Understanding the dynamical processes governing their evolution is essential for advancing both scientific knowledge and effective coastal management. Our findings offer new insights into the complex interplay of physical processes at river plume fronts, contributing to a broader understanding of coastal dynamics and gravity currents.
Advisor:
Dr. Daniel G. MacDonald, Department of Civil Engineering
Committee members:
Dr. Amit Tandon, Department of Mechanical Engineering
Dr. Kimberly Huguenard, University of Maine, Civil and Environmental Engineering
Dr. Geoffrey Cowles, Department of Fisheries Oceanography/SMAST
NOTE: All EAS Students are ENCOURAGED to attend.
SMAST West, Room 204