How distinctive are flood-triggered turbidity currents?

ABSTRACT Turbidity currents triggered at river mouths form an important highway for sediment, organic carbon, and nutrients to the deep sea. Consequently, it has been proposed that the deposits of these flood-triggered turbidity currents provide important long-term records of past river floods, continental erosion, and climate. Various depositional models have been suggested to identify river-flood-triggered turbidite deposits, which are largely based on the assumption that a characteristic velocity structure of the flood-triggered turbidity current is preserved as a recognizable vertical grain size trend in their deposits. Four criteria have been proposed for the velocity structure of flood-triggered turbidity currents: prolonged flow duration; a gradual increase in velocity; cyclicity of velocity magnitude; and a low peak velocity. However, very few direct observations of flood-triggered turbidity currents exist to test these proposed velocity structures. Here we present direct measurements from the Var Canyon, offshore Nice in the Mediterranean Sea. An acoustic Doppler current profiler was located 6 km offshore from the river mouth, and provided detailed velocity measurements that can be directly linked to the state of the river. Another mooring, positioned 16 km offshore, showed how this velocity structure evolved down-canyon. Three turbidity currents were measured at these moorings, two of which are associated with river floods. The third event was not linked to a river flood and was most likely triggered by a seabed slope failure. The multi-pulsed and prolonged velocity structure of all three (flood- and landslide-triggered) events is similar at the first mooring, suggesting that it may not be diagnostic of flood triggering. Indeed, the event that was most likely triggered by a slope failure matched the four flood-triggered criteria best, as it had prolonged duration, cyclicity, low velocity, and a gradual onset. Hence, previously assumed velocity-structure criteria used to identify flood-triggered turbidity currents may be produced by other triggers. Next, this study shows how the proximal multi-pulsed velocity structure reorganizes down-canyon to produce a single velocity pulse. Such rapid-onset, single-pulse velocity structure has previously been linked to landslide-triggered events. Flows recorded in this study show amalgamation of multiple velocity pulses leading to shredding of the flood signal, so that the original initiation mechanism is no longer discernible at just 16 km from the river mouth. Recognizing flood-triggered turbidity currents and their deposits may thus be challenging, as similar velocity structures can be formed by different triggers, and this proximal velocity structure can rapidly be lost due to self-organization of the turbidity current.

Flood and tides trigger longest measured sediment flow that accelerates for thousand kilometers into deep-sea

Here we document for the first time how major rivers connect directly to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1-2 km3 of sediment from just one submarine canyon, equivalent to 14-28% of the annual global-flux from rivers. It was known earthquakes trigger canyon-flushing flows. We show major river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, but triggered by spring tides weeks to months after the flood. This is also the first field-confirmation that turbidity currents which erode can self-accelerate, thereby travelling much further. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or how terrestrial climate change impacts the deep-sea.

Hydraulic Planning in Insular Urban Territories: The Case of Madeira Island—São Vicente

This study aims to examine the flood propensity of the main watercourse of São Vicente drainage basin and, if relevant, to propose two methodologies to alleviate the impacts, i.e., detention basin sizing and riverbed roughness coefficient adjustment. Geomorphological data were obtained from the watershed characterization process and used through the SIG ArcGIS software for the flood propensity assessment and then for the calculation of the expected peak flow rate for a return period of 100 years through the Gumbel Distribution. Subsequently, the drainage capacity of the river mouth was verified using the Manning-Strickler equation, in order to establish whether the river mouth of the watershed has the capacity to drain the entire volume of rainwater in a severe flood event. In summary, it was possible to conclude that São Vicente’s watershed river mouth is not able to completely drain the rain flow for the established return period. Thus, its drainage capacity was guaranteed by modifying the walls and streambed roughness coefficient and by sizing the detention basin using the Dutch and the Simplified Triangular Hydrograph methods.

Isotopic Evidence for Sources of Dissolved Carbon and the Role of Organic Matter Respiration in the Fraser River Basin, Canada

Sources of dissolved and particulate carbon to the Fraser River system vary significantly in space and time. Tributaries in the northern interior of the basin consistently deliver higher concentrations of dissolved organic carbon (DOC) to the main stem than other tributaries. Based on samples collected near the Fraser River mouth throughout 2013, the radiocarbon age of DOC exported from the Fraser River does not change significantly across seasons despite a spike in DOC concentration during the freshet, suggesting modulation of heterogeneous upstream signals during transit through the river basin. Dissolved inorganic carbon (DIC) concentrations are highest in the Rocky Mountain headwater region where carbonate weathering is evident, but also in tributaries with high DOC concentrations, suggesting that DOC respiration may be responsible for a significant portion of DIC in this basin. Using an isotope and major ion mass balance approach to constrain the contributions of carbonate and silicate weathering and DOC respiration, we estimate that up to 29% of DIC is derived from DOC respiration in some parts of the Fraser River basin. Overall, these results indicate close coupling between the cycling of DOC and DIC, and that carbon is actively processed and transformed during transport through the river network.

Blood cells and some hematological parameters of red drum (Linnaeus, 1766) in Vietnam

This research focuses on hematological characteristics, erythrocyte morphology and some biochemical parameters of red drum Sciaenops ocellatus (Perciformes: Sciaenidae), cultured in natural water environment in areas near river mouth (L1), estuaries (L2) in Ha Tinh province and coastal areas (L3) in Nha Trang city, Khanh Hoa province of Vietnam. A total of 18 speciments were examined in research, six in each location. Blood was drawn from the tail vein, using a microscope to research morphology and automated gauges to determine blood biochemical parameters. Analysis of blood samples showed that the rate of red drum’s erythrocyte morphology disorders in all three locations was quite high. The two main types of disorders were nuclear deformity and nuclear-matter distribution. Changes in erythrocyte size, shape and nuclear were related to salt concentration at culture locations. Blood hemoglobin content was stable in all three regions. Other hematological parameters such as the number of erythrocytes, blood biochemical parameters (glucose, SGOT, SGPT, urea, creatine, plasma iron, albumin, and protein) have differences among the locations, which showed the different reactions of the same species with different environmental conditions.

Impact of River Dams on Littoral Cells Located Adjacent to the River Mouth

Terrestrial sediment is a major source of sediment to all coasts. Suspended sediment is carried away by the rivers and supplied to the coast to maintain sediment budget. The construction of dams across the rivers arrest sediment behind it and affect the sediment budget of littoral cells along the coast. Reduction in sediment supply induces ecological as well as geomorphological changes along the shoreline. Coastal erosion may accelerate due to reduced sediment influx. With the growing number of cross-river dams and water diversion projects, it has become a major concern before the scientific community to measure, understand and find solutions to multi-fold geo-environmental problems that are arising out of river damming. The present study aims to find out the impact of dams on the coast. It examines how the changes in the suspended sediment supply of an Indian river impact the coast in terms of loss of area due to erosion. Temporal analysis of geomorphological changes along the shoreline in relation to sediment influx holds immense importance to coastal management essential for the sustainable life and livelihood of coastal communities. Scientific investigation into the impact of river dams on the coastal environment is likely to provide a strong ground to reconsider the way present basin development projects function. Areal changes in littoral sediment cells adjacent to the river mouth have been quantified and correlated with changes in sediment influx. Changes along the shorelines have been detected through multispectral satellite images of Landsat belonging to different dates. Image processing and quantification of changes have been performed in QGIS 3.14 “Pi” platform. Virtual raster, raster calculator, field calculator and other required tools in QGIS were used during image processing.