Laboratory Investigation on Effects of Flood Intermittency on Fan Delta Dynamics

Abstract

To simplify the complex hydrological variability of flow conditions, experiments on delta evolution are often conducted using a representative channel‐forming flood flow and results are related to field settings using an intermittency factor, defined as the fraction of time in flood. Although this factor provides an approximation of dominant flow conditions and makes modeling deltas easier by turning their complex hydraulics into a single representative value, little is known about how this generalization affects delta processes. We conducted experiments with periodic flow conditions to determine the effects of intermittent discharges on fan deltas. For each run, the magnitude of floods was held constant, but the duration changed, thus varying the intermittency factor, between 1 and 0.2. Floods consisted of higher water and sediment discharge, while base flow periods had lower water discharge and sediment input ceased, causing the system to become erosional during these periods. We find that as the duration of floods decreases, the delta topset is larger in area with a shallower slope due to reworking on the topset during base flow conditions. During base flows, the experimental system adjusts toward a new equilibrium state that in turn acts as the initial condition for subsequent flood periods. These results suggest that the adjustment timescale is a factor in determining the behavior of deltas and their channels. We conclude that both periods of flood when most of the sediment is supplied to the system and periods of base flow when topset sediment is reworked contribute to delta dynamics.

Buffered, Incomplete, and Shredded: The Challenges of Reading an Imperfect Stratigraphic Record

“…For example, Toby et al () recognized that the scale of autogenic volume fluctuations that defines transfer thresholds likely varies as a function of the mean and stochastic components of a system's forcing. Work to define the magnitude of autogenic fluctuations as functions of forcing conditions is starting to accelerate, with studies quantifying autogenic scales as functions of the ratio of sediment to water supply (Powell et al, ; Straub & Wang, ), sediment grain size (Caldwell & Edmonds, ) and cohesion (Edmonds & Slingerland, ; Hoyal & Sheets, ; Li et al, ), vegetation (Lauzon & Murray, ; Piliouras et al, ), flashiness of system hydrographs (Esposito et al, ; Ganti et al, ; Miller et al, ), basin water depth (Carlson et al, ), and wave (Ratliff et al, ) and tidal climate (Kleinhans et al, ; Lentsch et al, ).…”

Mechanisms for avulsion on alluvial fans: insights from high-frequency topographic data

“…Because of the large length scale required to simulate the 3D evolution of fans within a reasonable time-frame and spatial extent, similarity-of-process models ("analogue models") have become the established norm in laboratory studies of fans and fan-deltas (e.g. Bryant et al, 1995;Clarke et al, 2010;Davies and Korup, 2007;Van Dijk et al, 2009;de Haas et al, 2016;de Haas, Kruijt and Densmore, 2018;Hamilton et al, 2013;Hooke, 1967Hooke, , 1968bHooke and Rohrer, 1979;Miller et al, 2019;Piliouras et al, 2017;Reitz and Jerolmack, 2012;Schumm et al, 1987). A similarity-of-process model is one that reproduces key aspects of the morphology of the generic "prototype"; importantly, the processes that shape this morphology in the model can reasonably be assumed to do so in the field.…”

“…In the past decade, the spatial resolution of topographic measurement in fan experiments has increased, but temporal resolution generally remains low. For example, experimentalists have used either photogrammetry (Van Dijk et al, 2012) or laser-scans (Carlson et al, 2018;Miller et al, 2019;Reitz and Jerolmack, 2012) to collect high-resolution topographic data, that covers their entire experimental landscape with a resolution on the order of a few millimeters. However, these survey methods require that flow be stopped during data collection, meaning that the intervals between full topographic scans ranged from 15 minutes to a few times per experiment.…”

The Longitudinal Profile of a Prograding River and Its Response to Sea Level Rise

“…where q s0 (m 2 /s) is the specific sediment load at the upstream boundary, λ is porosity, and I f is the flood intermittency factor (Miller et al, 2019). The A/S ratio thus can be written as…”

“…The potential maximum river channel deposit volume per unit width, V s , equals the sediment supplied at the upstream boundary: where q s 0 (m 2 /s) is the specific sediment load at the upstream boundary, λ is porosity, and I f is the flood intermittency factor (Miller et al, 2019). The A / S ratio thus can be written as …”