Thesis Defense: Brandee Carlson, Ph.D. Candidate
Defense Date: Friday, April 17th, 2020
Time: 2:00 p.m.
Thesis Title: The geomorphic evolution of abandoned deltaic lobes and distributary channels
Due to an abundance of natural resources, many of the world’s large deltas are densely populated. However, these populations are at risk due to land submergence caused by rising sea-level rates, anthropogenically-enhanced sediment compaction driven by ground fluid extraction, and reduced sediment supply caused by river engineering practices (e.g. damming). This dissertation is motivated by a need to promote resiliency of highly coupled human-natural delta systems. Naturally, distributary channels on deltas relocate through a process called avulsion, and this disperses sediment and water along coastlines. Mimicking avulsions with engineered diversions of sediment and water is a tool to nourish drowning deltaic landscape and combat landloss. Land-building as a result of sediment and water diversions has been extensively studied. However, the factors that control the stability of land abandoned by avulsion or diversion are unclear. This thesis evaluates the sediment transport processes that control the growth and decay of deltaic land in order to optimize land-building. The Yellow River (Huanghe) delta of China is used as a case study throughout this thesis. Owing to its high sediment load, the Huanghe delta has rapid delta dynamics, demonstrated by a decadal avulsion timescale. Presently, river diversions are also engineered on a decadal timescale. The result is that the Huanghe delta is comprised of many abandoned delta lobes, which are the focus of three dissertation chapters. These chapters are as follows: 1) how do abandoned deltaic distributary channels evolve in response to complete cutoff of upstream sediment and water, 2) how is the shoreline stability of abandoned delta lobes impacted by topography, vegetation abundance and distribution, and fresh water supply 3) how do persistent surface water connections between active and abandoned channels mediate the delivery of sediment and water to deltaic landscape. First, a study of marine-sediment delivery to a channel abandoned by an engineered diversion reveals that modern sediment accumulation, driven by tides and ocean storms, records stratigraphic indicators of marine vs. fluvial mechanisms for filling. Second, remote sensing and field observation shows that the shorelines of lobes abandoned by engineered diversions retreat at a rate four times faster than that of a lobe abandoned by a natural avulsion, and the discrepancy in these retreat rates is driven by topography, fresh water supply, and vegetation distribution. Third, sediment that is routed by a surface water connection from the active and to an abandoned distributary channel persistently deposits sediment within the connecting channel. Eventually, flow is severed between the active and abandoned channels. Broadly, this dissertation shows that the resiliency of abandoned deltaic land is critical for overall shoreline stability.