Flocculation and Transport of Mud in Rivers and Deltas

Citation

Nghiem, Justin Ahn-Khoa (2025) Flocculation and Transport of Mud in Rivers and Deltas. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rh7k-x369. https://resolver.caltech.edu/CaltechTHESIS:06022025-211248621

Abstract

Mud (grains < 62.5 μm) dominates the sediment load of rivers from continents to the ocean and contributes to building coastal land and sequestering organic carbon. However, predicting mud transport is challenging because flocculation causes mud grains to aggregate into larger, faster settling particles called flocs, which dynamically respond to local flow, water, and sediment properties. In this thesis, I examined the factors controlling mud flocculation in rivers and deltas and the effects of enhanced floc settling velocity on mud accretion in a river delta using fieldwork and data compilations from the river sediment literature. Flocs have the potential to dictate mud deposition rates and transport patterns by effectively enhancing mud settling velocity. First, I developed a semi-empirical model to predict floc diameter and settling velocity in rivers using a global river data compilation (Chapter 2). Results show that turbulence, sediment concentration and mineralogy, organic matter concentration, and water chemistry are the key flocculation factors in rivers. I conducted fieldwork in the Wax Lake Delta, Louisiana, a river delta in the Mississippi River Delta complex. Based on floc measurements at the Wax Lake Delta, I validated the semi-empirical model and showed that a complementary physics-based floc settling velocity model relies on the permeability and fractal structure of flocs (Chapter 3). To better link floc settling velocity and mud transport, I used the Wax Lake Delta field data to demonstrate that flocculated mud might behave as bed-material load rather than washload (Chapter 4). This result implies that mud concentration and flux might be readily predictable from bed-material entrainment theory using local bed and flow measurements. Connecting mud transport to delta island sedimentation and delta resilience, I analyzed discharge and sediment flux in the Wax Lake Delta to understand how sediment is delivered to and transported in islands (Chapter 5). Field data and backwater modeling results show that tall levees can block flow, but intricate feedbacks between flow depth, velocity, and water surface slope set discharge and sediment flux into the island once primary channels overflow into islands. Suspended mud settles fast enough relative to island flow depth and velocity to settle out within the island rather than bypass. As such, mud can accrete and build up the island over time as evidenced by mud-rich island deposits in Wax Lake Delta. Finally, combining Wax Lake Delta data and a river data compilation on suspended sediment grain size and mineralogy, I showed that most suspended sediment in rivers is flocculated silt (Chapter 6). This silt likely flocculates due to physical trapping mechanisms rather than typically considered interactions between clay minerals and salinity because clay minerals compose a minority of the silt. Overall, this thesis informs how flocculation affects mud transport in rivers and deltas, encompassing the mechanisms of mud flocculation, predictions of floc settling velocity and mud concentration, and the significance of mud flocculation in shaping depositional landscapes.

Thesis Files