Turbid River Plumes Research Articles

Do hyperpycnal-flow deposits record river-flood dynamics?

Research Article| December 01, 2009 Do hyperpycnal-flow deposits record river-flood dynamics? Michael P. Lamb; Michael P. Lamb 1Jackson School of Geosciences, University of Texas, 1 University Station C1100, Austin, Texas 78712-0254, USA *Current address: Division of Geological and Planetary Sciences, California Institute of Technology, MC 170-25, 1200 E. California Blvd., Pasadena, California 91125, USA. Search for other works by this author on: GSW Google Scholar David Mohrig David Mohrig 1Jackson School of Geosciences, University of Texas, 1 University Station C1100, Austin, Texas 78712-0254, USA Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (12): 1067–1070. https://doi.org/10.1130/G30286A.1 Article history received: 01 Apr 2009 rev-recd: 18 Jun 2009 accepted: 01 Jul 2009 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Michael P. Lamb, David Mohrig; Do hyperpycnal-flow deposits record river-flood dynamics?. Geology 2009;; 37 (12): 1067–1070. doi: https://doi.org/10.1130/G30286A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Hyperpycnal flows are turbid river plumes that can plunge to form turbidity currents where they enter a water body of lesser density. Because these flows provide one of the most direct connections between terrestrial sediment sources and marine depositional sinks, their deposits might preserve an important record across a variety of climatic and tectonic settings. A leading hypothesis assumes that hyperpycnal-flow velocity scales directly with river discharge, such that individual turbidites record the rising and falling discharge of a flooding river. We tested this hypothesis using a one-dimensional flow model and found that turbid river flow must move through a backwater zone, depth-limited plume, and plunging zone before becoming a turbidity current. These zones can extend tens of kilometers offshore and significantly affect the transfer of momentum from river to turbidity current. Counter to the proposed hypothesis, our results suggest that expected bed forms and sediment grading patterns in hyperpycnal-flow deposits can record multiple flow accelerations and decelerations even during a simple single-peaked flood. This occurs because of spatial acceleration and deceleration within the three transitional zones and because their boundaries move in response to flow discharge and suspended sediment concentration. Results also suggest that the criterion used to identify plunging hyperpycnal flows (a flow density in excess of the ambient fluid) is a necessary, but not sufficient condition. The basin also must be deep enough, in some cases greater than tens of meters, in order for the plume to collapse and form a turbidity current. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Size and composition of inorganic colloids in a peri-alpine, glacial flour-rich lake

From June 2004 to October 2005, particle size distributions (PSD) and the mineralogy of inorganic colloids in Lake Brienz, a glacial flour-rich Swiss lake, and in its tributaries (the Aare and Lütschine rivers), were quantified by single particle counting based on light scattering (0.1–2 μm) combined with transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction. The two lake tributaries differ in terms of the geology of their catchment area and in their hydrology: the River Lütschine’s glacial regime is unperturbed, while the River Aare has been significantly modified by hydroelectric installations. The dependence of the colloid concentration numbers on colloid diameters can be described by a power-law distribution in all cases. The power-law slopes in the River Aare were consistently smaller, and the total colloid number concentrations consistently higher, (mean values: −3.89 and 6.2 × 10 8 mL −1, respectively) than in the River Lütschine (−3.61 and 1.7 × 10 8 mL −1). The measured PSD in the lake were dependent on season and depth: homogenous in the whole epilimnion in winter and modified by river intrusions in summer. In this season, the presence of a river turbid plume above the thermocline is a significant source of colloids in the epilimnion, where their concentration was high (mean value: 5.1 × 10 7 mL −1). The same main mineral types were found in the different systems: 2:1 minerals, feldspars, chlorite and quartz; with a few particles of oxides. Differences in their relative concentrations in the rivers, essentially reflecting the geological differences of the respective catchment areas, made it possible to trace the origin of the mineral colloids present in lake surface waters throughout the year. The relationship between colloid size and mineralogy was assessed, showing that different minerals have distinct size distributions. In the rivers, where almost no aggregates were observed, PSD would essentially be the result of the nature (particle formation and mineralogy) of the colloids, whereas in the lake, aggregation and sedimentation processes would also play a role, particularly in summer.

Recent sedimentation and sediment accumulation rates of the Ba Lat prodelta (Red River, Vietnam)

The Ba Lat River is the major distributary of the Red River system in North Vietnam. To assess the recent to subrecent depositional processes in the Ba Lat prodelta, a detailed sediment analysis was conducted. Bottom samples were collected during two field surveys, one in the dry season (winter) and one in the wet season (summer). A steep frontal prodelta slope is characterized by very rapid sedimentation (tens of cm per year) of muddy sediments under influence of the turbid river plume. Beyond direct influence of the river plume the bottom slope decreases and bottom transport by the prevailing southward directed currents becomes important. Coarse-grained tempestites alternate with the dominating muddy sediments. Downcore changes in the 234Th activities indicate that the subaqueous delta progrades to the southwest, with erosion and reworking of older sediments occurring north of the present outlet. The southwestward progradation is also encountered in the trend of 210Pb activities indicating that this process has continued for at least 100 years. Avulsion of the Ba Lat outlet in 1973 has led to a decrease in sedimentation rates north of the Ba Lat outlet.

Sedimentation in mangroves and coral reefs in a wet tropical island, Pohnpei, Micronesia

A six-month-long study was conducted of the fate of turbid river plumes from the Enipein watershed in Pohnpei, Federated States of Micronesia. Pohnpei is one of the wettest places on earth, with a mean annual rainfall exceeding 4 m in the lowlands and 8 m in the highlands. The river waters were clear of sediment except after major storms with rainfall exceeding 5 cm day −1. Following a storm, the river plume spread in the mangrove fringed estuary and in the coral reef lagoon. The waters were highly stratified in temperature, salinity, and suspended sediment concentration. The brackish water was flushed out in four days, while the suspended sediment all settled out in the estuary, in the mangroves, and in the lagoon including on the coral reefs, in less than one day. The mean rate of sedimentation exceeded 35 mg cm −2 d −1 both over the mangroves and on the adjacent coral reefs. While this leads to no detrimental effects on the mangroves, sediment smothers corals and leads to substantial coral mortality in the lagoon. The mud is not flushed out from the lagoon because there are no strong currents from waves or tides. This high sedimentation rate is attributable to poor farming and land-use practices on the upland areas.

Dispersal of suspended sediments in the turbid and highly stratified Red River plume

The Red River, annually transporting 100 million tons of sediment, flows into a shallow shelf sea where it rapidly deposits most of its sediment on a prograding delta front. Oceanographic cruises were carried out in February–March and July–August 2000 to determine the vertical structure of the Ba Lat river plume and sediment transport patterns on the delta front. The surface waters in the coastal zone were strongly stratified with a low density and high sediment concentration during the larger part of the wet season, caused by low mixing rates of river plumes with ambient water. The river plume is advected to the south by a well-developed coastal current which originates from the river plumes that enter the Gulf of Tonkin North of the Ba Lat and are deflected southward by the Coriolis force. Sediment predominantly leaves the surface plume by settling from suspension and less by mixing of fresh and marine water. A one-dimensional model for plume deposition valid for fair weather conditions indicates that most sediment is deposited within 10 km and southward of the river mouth. Of prime importance for this depositional pattern is the phase relation between river outflow and tidal currents, in combination with the southward surface flow; alongshore advection is very low during outflow of the turbid river plume. The agreement of modeled plume sedimentation patterns with long-term bathymetric changes strongly suggests that fair weather depositional processes determine delta front development. This may be related to the fact that reworking of sediment mainly occurs several months after the peak deposition period; in the meantime sediment compaction and consolidation have increased the shear strength of deposited sediments.

Climate Change: Numerical Modelling of Sedimentation and Coastal Processes, Eastern Canadian Arctic

We use three sequential [open quotes]water-sediment transport[close quotes] numerical models to evaluate changes in fluvial sediment outputs for two near-future contrasting climate-change scenarios (warmer summers or warmer and moister winters) for the eastern Canadian Arctic. Our integrated model generates values for deposition into a fiord basin. The effects of climate warming are imposed on 200-yr fluctuations in relative sea level (base level) that in turn have a long-term forcing associated with late Quaternary glacial unloading. Thus some areas are submerging (around the periphery of the former ice sheet) and others are emerging (inland at the heads of fiords). Small, short-term changes in sea level may involve meltwater supply to the world ocean from the melting of glaciers and ice sheets, and sea water expansion with a warming ocean. Alternatively, large ice sheets may also thicken over the short term (200 yr), thereby withdrawing water from the global hydrological cycle. Both climate scenarios result in an increased sediment flux over the next 200 yr. However, warmer summers will have the largest impact by (1) causing ice caps to melt, (2) inducing more expansive and turbid river plumes, (3) increasing progradation of the coastline into the sea, (4) raising relative sea level,more » and by (5) increasing the number and size of turbidity currents generated off river mouths. Warmer and moister winters that may lead to deviations from these impacts include (1) growth of ice caps, (2) fall of relative sea level, (3) stability of coastlines as increased sediment delivery keeps pace with changes in regional relative sea level, and (4) fewer basin turbidity currents, as more of the bedload is trapped on top of sandur surfaces. 55 refs., 11 figs., 2 tabs.« less