Difference in depositional behavior of hypopycnal and hyperpycnal flows in the context of continental margins

Hypopycnal;Hyperpycnal;Continental Margins;Continental Shelves;Experimental

Rehn, Andrew Vincent ; Parker ; Gary

关键词: Hypopycnal;    Hyperpycnal;    Continental Margins;    Continental Shelves;    Experimental;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/89088/REHN-THESIS-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The conditions necessary for the development of continuous shelf systems, such as those on the continental margin, are not fully understood. Motivated by the observation that continuous shelf-like benches are found on the continental margins in oceans, but not in most lakes, it is proposed that salt is necessary for the creation of continental shelves. This may be characterized in terms of the difference between hyperpycnal and hypopycnal conditions. The former prevails when the incoming flow is denser than the water of the receiving basin; the flow plunges to the bottom of the lake and forms a bottom density flow. The latter prevails when the incoming flow is lighter than that of the receiving basin; the flow rides on the surface water of the receiving basin to form a surface plume. Sediment-laden rivers which enter a saline receiving body, such as the ocean, usually are not dense enough to plunge. Instead, they create buoyant hypopycnal plumes, which lose momentum so that the sediment falls out of suspension locally. Sediment-laden rivers that enter lakes typically can plunge to create hyperpycnal flows, i.e. turbidity currents, which can run out to the bottom of the lake driven by gravity and sediment re-entrainment. Experiments were run in the Ven Te Chow Hydrosystems Laboratory at the University of Illinois in Urbana-Champaign in order determine the role of dissolved salt in the receiving water in controlling the behavior of the incoming flow. A small flume was constructed, with an upstream experimental region of dimensions 61.0 cm (2 feet) in height, 15.2 cm (6 inches) in width, and 243.8 cm (8 feet) in length, and a deeper and wider downstream tank to dampen reflection of any current that runs into it. Twelve experimental runs were performed with matching input conditions; six hyperpycnal with fresh water in the tank, and six hypopycnal with saline water in the tank. The runs were divided into sets of similar total duration, and after each set was completed sediment deposits on the platform were measured. The results of the experiments show that given the same input conditions it is possible for hypopycnal flows to deposit more sediment proximal to the inflow point, whereas hyperpycnal flows carry more sediment downslope. Results show that in the area of deposition outside of a small region near the diffuser, hypopycnal flows deposit twice as much sediment as hyperpycnal flows. Near the diffuser, hyperpycnal flows deposited slightly more sediment, however over the entire measurement area the hypopycnal flows deposited more. One caveat for these results is that sediment deposition inside the diffuser, which guided the flow into the flume, was not measured. Inclusion of measured values of deposition in the diffuser and could change the results. The particle size distributions of the deposit on the bed were also measured. For hyperpycnal flows, the median particle size of the deposit exhibited steady fining in the downstream direction, while for hypopycnal flows median particle size followed no particular pattern. The results indicate that it is possible that salinity is a driving force for the development of continental shelves. That is, salinity tends to force hypopycnal conditions which creates a tendency for sediment to deposit in the nearshore zone instead of going into deep water. This deposit could eventually build up to wave base to form a shelf-like feature, which could then be elongated along shoreline by alongshelf sediment dispersal processes.