Hernandez‐Moreira, Ricardo1 ; Jafarinik, Sadegh1 ; Sanders, Sydney1 ; Kendall, Christopher G. St. C.1 ; Parker, Gary2 ; Viparelli, Enrica1

Research facility: University of South Carolina

Published Apr 04, 2022 on Dryad. https://doi.org/10.5061/dryad.c59zw3r9b

Abstract

Relatively common to the submarine setting are depositional sequences that begin with a lower erosional boundary, followed in ascending order by a either a graded or an inherently massive basal unit, a relatively coarse parallel laminated unit, a cross laminated unit, a relatively fine parallel laminated unit, and a capping layer of massive mud. These sequences are present in turbidity current, coastal storm and tsunami deposits, and their study is the key to understanding the physical processes governing sediment transport, erosion and deposition in submarine settings, to the reconstruction of global paleoclimate and paleoflow, to the exploration of hydrocarbon reservoirs and to the prediction of natural hazards associated with earthquake and landslide induced tsunamis. A common feature of these deposits is a basal erosional layer underlying a sandy massive unit, i.e. a unit lacking internal structure. Previous studies have shown that these sequences were deposited from waning energy flows, and the mechanism for the emplacement of the basal massive unit is thought to be associated with rapid deposition of suspended sediment. Here we present the results of laboratory experiments specifically designed to test the hypothesis that these massive units can also be emplaced by very intense bedload transport conditions corresponding to what is commonly called sheet flow in the engineering literature.

Methods

Introduction

Two groups of laboratory experiments were performed in the Hydraulic Laboratory of the University of South Carolina at Columbia. The experiments were performed in open-channel mode, under conditions that are specifically designed to model intense, net-depositional subaqueous flows.

The first group of experiments was performed with uniform sand to study the transition from upper regime plane bed with bedload transport in sheet flow mode to upper regime plane bed with long wavelength and small amplitude bedforms.

The second group of experiments was done with a mixture of sediments differing in grain size to observe the internal fabric upper regime deposits.

Experimental setup

The experiments were performed in the Hydraulics Laboratory of the Department of Civil and Environmental Engineering of the University of South Carolina, in a 13 m long, 0.50 m wide and 0.70 m deep sediment feed flume with glass sidewalls. A 6.9 m long and 0.19 m wide test reach was built with plywood. The flume cross section was gradually reduced from 0.5 m to 0.19 m in the upstream most 2.0 m of the flume. This width reduction was necessary to obtain upper regime conditions with high, but still feasible, feed rates. The test reach started 2.0 m downstream of the flume entrance and ended at the sediment trap.

The constant head tank of the laboratory supplied the flow to the flume. Flow rates were measured with a calibrated orifice plate and a Dwyer 490-1 wet/wet digital differential manometer. The water surface elevation downstream of the sediment trap was controlled with a tailgate. Dry sediment was fed at a constant rate with a Schenck Accurate 600 feeder. Water surface and bed surface elevation profiles were captured with ruler readings through the glass flume wall. Bed configurations were characterized by means of still photographs, time lapses and videos taken with a Nikon d3000 camera. Details on the experimental setup are available through the wiki page of the Sediment Experimentalist Network (SEN) at http://sedexp.net/experiment/setting-hydraulics- laboratory-limited-resources.

First group of experiments: Experiments with uniform sediment

We chose uniform sand with density equal to 2632 kg/m3, geometric mean diameter, Dg, =1.11 mm and geometric standard deviation of 1.44 mm. We performed 13 experimental runs with a flow rate of 30 l/s and sediment feed rates varying between 0.5 kg/min and 20 kg/min to study the transition from upper regime plane bed with standard bedload transport (Gs ≤ 4 kg/min) to upper regime plane bed with bedload transport in sheet flow mode. To further investigate the effect of the flow depth on the bed configuration we performed three runs with a flow rate of 20 l/s and feed rates equal to 1.5 kg/min, 8 kg/min and 16 kg/min.

The study of the upper regime bed configurations for increasing values of the bedload transport capacity presented herein is performed at mobile bed equilibrium, i.e. when the flow hydrodynamics, the bed configuration and the mode of bed material transport did not change in space and time over time scales that are long compared to those that characterize bedform migration. The measurements of water depth, bed slope and bedform geometry reported below have to be interpreted as temporal averages over a series of bedforms.

Water surface and bed elevation profiles were periodically measured with ruler readings at 15 to 30 minute intervals, with higher feed rate experiments warranting more frequent measurements. When the elevations and slopes of the water surface and of the bed profiles did not change significantly in time, we assumed that the system had reached conditions of mobile bed equilibrium.

When the system reached conditions of mobile bed equilibrium, water temperature was measured as an input for the JSR sonar system, with which instantaneous measurements of bed surface elevations were taken at six locations along the centerline of the flume.

Photographs were taken through the glass side-windows of the flume, sometimes during the experiment and other times after the flow had been stopped. Videos and time lapse images were recorded for some experiment runs through the glass side windows as well.

Second group of experiments: Experiments with sediment mixtures

Five experiments were performed with a flow rate of 30 l/s and sediment feed rates varying between 1.5 kg/min to 20 kg/min corresponding to bed configurations ranging from upper regime plane bed to plane bed with bedload transport in sheet flow mode. The sediment was a mixture of three components, purple fine sand with D = ~0.5 mm, medium white sand with D = 1.11 mm and the coarse brown sand with D = ~1.4 mm. The geometric mean diameter, Dg, of the sediment mixture was 0.95 mm and the geometric standard deviation was 1.65.

Each experiment was characterized by two phases, the equilibrium phase, in which we waited for the system to reach conditions null net erosion and deposition over time scales that are long compared to the time scales of bedform migration, and the aggradation phase, in which the water surface base level was slowly raised to induce channel bed aggradation under the transport conditions observed on the equilibrium bed. At the end of the aggradation run, stratigraphic sections were cut (one dip section in the center of the test reach and several strike sections) and photographed, core samples were taken and to measure the spatial variation of grain sizes.

The experiments started with an empty flume. As the flow and the sediment feed were turned on a deltaic deposited formed with a downstream migrating delta front. When the delta front reached the sediment trap, periodic measurements of bed and water surface elevation were performed with rule readings from the glass flume sidewall. When the bed and the water surface elevation and slopes did not significantly change between consecutive readings, we assumed that the flow and the sediment transport reached conditions of mobile bed equilibrium. Three to five readings of bed and water surface elevation were collected at equilibrium to estimate the equilibrium slopes and water depths used to estimate the bed shear stresses.

Pictures and videos of the equilibrium deposit were taken from the glass flume sidewall to estimate the thickness of the bedload layer, the average bedform height and wavelength and the aggradation phase of the experiments started.

Aggradation of the bed deposit was then induced by slowly raising the tailgate at a constant rate. During the aggradation phase periodic measurements of bed and water surface elevation were collected to monitor that the water surface and the bed slopes did not significantly change from their equilibrium values. At the end of the aggradation phase, we took pictures of the deposit from the glass flume sidewall, one central dip and several strike stratigraphic sections were cut and photographed to characterize the internal fabric. Core samples were then collected and dried in the oven to measure the spatial (vertical and streamwise) variation of the grain size distribution in the emplaced deposit.

Data processing information

Data has been minimally processed, if at all.

Water surface and bed elevation profile data

Water surface and bed elevation profile data are presented as CSV files of the actual measurements, with some profiles ignored for the purposes of computing associated shear stresses, since they deviated from mobile-bed equilibrium conditions. These profile files are presented herein, where needed, for completion.

Bed elevation sonar data

Instantaneous bed elevation sonar data are presented as CSV files of the actual measurements. The tail end of the captured data, captured after the experiments were stopped, were trimmed to preserve only the data relevant to the experiments.

Photograph and video data

Photographs are presented in the Nikon-native NEF raw file format, unprocessed. Videos are presented in MOV files, unprocessed.

Sieve analysis data

Sieve analysis data for the sediment deposit cuts are presented in Excel .XLSX spreadsheets. Most useful information is perhaps in the `Summary of sieve analyses.xlsx` file.

File naming convention as follows: volumetric water flow rate in liters per second followed by sediment mass feed rate in kilograms per minute, followed by the letters gsd, i.e, `30-1.5-gsd.xlsx` refers to the 30 l/s, 1.5 kg/min experiment run of mixed sediment.

Emplacement of massive deposits by sheet flow

Data files