Density Current Model Research Articles

Application of a simple analytical model to severe winds produced by a bow echo like storm in northeast Italy

AbstractA strong mesoscale convective system affected northeastern Italy on August 8, 2008. Notable damage and two casualties resulted, mainly due to the strong wind gusts. The event is analysed using observations, including surface data from a meso‐network of meteorological stations, radar reflectivity and velocity data from a C‐band Doppler radar, polar satellite images, lightning measurements from a lightning detection network and the ambient thermodynamic conditions derived from local radiosoundings. The role of the cold front is investigated; in particular, the associated strong wind descending from the Alps, which interacted with the preexisting convection in the plain, is analysed to understand the way it affected the storm development. A simple density current model is applied to describe the flow characteristics and to identify the mechanisms that could support the development of such a high wind speed. Lastly, observed wind speeds are compared with the theoretical estimates from the proposed model. The results show that different factors contribute to the very strong wind gusts registered by surface stations. The main contributor to the windstorm is a density current (a) driven by the cold front, (b) maintained by the interaction between the cold air impinging on the Friuli Venezia Giulia plain from the north and the strong regional density gradient and (c) eventually enhanced by the storm cold pool. This study highlights the complex evolution of severe storms in a region on the lee side of the Alpine chain, and emphasizes the role of the orography in the enhancement of storm.

Interdisciplinary application of numerical and machine-learning-based models to predict half-hourly suspended sediment concentrations during typhoons

Accurate forecasting of hourly suspended sediment concentration is a critical issue for reservoir management, especially during typhoon periods. This research proposes a two-stage forecasting approach integrating numerical and machine-learning-based models to provide accurate real-time forecasts of half-hourly suspended sediment concentration. In the first stage, the density current model, SRH2D, was verified and applied by the historical typhoon events due to the lack of measured suspended sediment concentration. Next, in the second stage, the calculated results from SRH2D based on the spatial-temporal relation were employed as the input of the MGSVM-based model. Finally, an application in the Shi-Men reservoir was conducted to demonstrate the forecasting performance of the proposed approach. The results indicated that the calculated data from the SRH2D model corresponded to the trend of measured data in spatial-temporal variation. Moreover, the proposed forecasting approach enables reasonably acceptable 0.5- to 3-h ahead forecasts of suspended sediment concentration by using the simulated data of half-hourly suspended sediment concentration. The accurate forecast conducted by the proposed approach is expected to be properly applied to reservoir management.

Analysis of a southerly buster event and associated solitary waves

This paper is a detailed case study of the southerly buster of 6–7 October 2015, along the New South Wales coast. It takes advantage of recently available Himawari-8 high temporal- and spatial-resolution satellite data, and other observational data. The data analyses support the widespread view that the southerly buster is a density current, coastally trapped by the Great Dividing Range. In addition, it appeared that solitary waves developed in this event because the prefrontal boundary layer was shallow and stable. A simplified density current model produced speeds matching well with observational southerly buster data, at both Nowra and Sydney airports. Extending the density current theory, to include inertia-gravity effects, suggested that the solitary waves travel at a speed of ~20% faster than the density current. This speed difference was consistent with the high-resolution satellite data, which shows the solitary waves moving increasingly ahead of the leading edge of the density current.

Application of the adjoint approach to optimise the initial conditions of a turbidity current with the AdjointTurbidity 1.0 model

Abstract. Turbidity currents are one of the main drivers of sediment transport from the continental shelf to the deep ocean. The resulting sediment deposits can reach hundreds of kilometres into the ocean. Computer models that simulate turbidity currents and the resulting sediment deposit can help us to understand their general behaviour. However, in order to recreate real-world scenarios, the challenge is to find the turbidity current parameters that reproduce the observations of sediment deposits. This paper demonstrates a solution to the inverse sediment transportation problem: for a known sedimentary deposit, the developed model reconstructs details about the turbidity current that produced the deposit. The reconstruction is constrained here by a shallow water sediment-laden density current model, which is discretised by the finite-element method and an adaptive time-stepping scheme. The model is differentiated using the adjoint approach, and an efficient gradient-based optimisation method is applied to identify the turbidity parameters which minimise the misfit between the modelled and the observed field sediment deposits. The capabilities of this approach are demonstrated using measurements taken in the Miocene Marnoso-arenacea Formation (Italy). We find that whilst the model cannot match the deposit exactly due to limitations in the physical processes simulated, it provides valuable insights into the depositional processes and represents a significant advance in our toolset for interpreting turbidity current deposits.

On the regularized modeling of density currents

AbstractWe study numerically four regularization models with deconvolution for density currents, namely, Boussinesq-

Estimation of hazardous area caused by pyroclastic surge using density current model

Estimation of hazardous area caused by pyroclastic surge using density current model

Hydraulic and sediment transport properties of autogenic avulsion cycles on submarine fans with supercritical distributaries

AbstractSubmarine fans, like other distributive systems, are built by repeated avulsion cycles. However, relative to deltas and alluvial fans, much less is known about avulsions in subaqueous settings. In this study, we ran a set of subaqueous fan experiments to investigate the mechanics associated with autogenic avulsion cycles of self‐formed channels and lobe deposits on steep slopes. The experiments used saline density currents with crushed plastic to emulate sustained turbidity currents and bed load transport. We collected detailed hydraulic and bathymetric measurements and made use of a 1‐D laterally expanding density current model to better understand different aspects of the avulsion cycle. Our results reveal three major components of the avulsion cycles: (1) distributary channel incision, extension, and stagnation; (2) mouth bar aggradation and hydraulic jump initiation; and (3) hydraulic jump sedimentation and upstream retreat. Interestingly, in all but one experiment, the avulsion cycles led to fans that remained perched above the basin slope break. Experimental data and hydraulic theory were used to unravel actual mechanics associated with cycles. We found that channels stopped extending into the basin due to a decay in sediment transport capacity relative to sediment supply and that the reduction in capacity was primarily an outcome of expansion‐driven velocity reduction; dilution played a secondary role. Once channel extension ceased, mouth bar deposits aggraded to a thickness approximately equal to the critical step height needed to create a choked flow condition. The choke then initiated a hydraulic jump on the upstream side of the bar. Once formed, the jump detained a majority of the incoming sediment and forced the channel‐to‐lobe transition upstream, filling the channel with steep backset bedding and capping the entire channel with a mounded lobate deposit. These intrinsic processes repeated through multiple avulsion cycles to build the fan.

A numerical study of the triggering mechanism of a lock-release density current

A numerical study on the effects induced by the impulsive vertical removal of a lock-gate at the interface between two fluids of different densities is presented. This configuration represents the typical setup of those experiments commonly employed for investigating density currents in the laboratory. Experimentally induced effects resulting from opening the lock-gate are expected to occur, but the evaluation of these dynamics and their impact on the evolution of the laboratory density current produced in such a manner are not easy to estimate. Despite the fact that numerical studies are often concerned with lock-release density currents, the triggering mechanism which occurs in the early stages of the evolution of the fluid flow is commonly neglected. Here a comparison is established between the case when the triggering mechanism is completely neglected and a series of cases where, in contrast, this effect is taken into account. The withdrawal of the lock-gate is modeled either by employing a zero-thickness lock-gate or by accounting for the volumetric nature of the lock-gate. Subsequently the influence of speed on the withdrawal of the lock-gate is assessed. The numerical results suggest that the density current is mainly affected by the constraining effect of the lock-gate on the flow and by the responses of the submerged fluid and the free surface to the displacement of the lock-gate. These differences lead to improved physical modeling and numerical simulation validation in the case where the physics of the lock-gate is accounted for. Such differences can be very important particularly in particulate-laden flows, where small changes in initial conditions may lead to longer-term divergence as a result of positive feedback effects. The work has significant implications for physical modeling of density currents and a series of recommendations are made for the standardization of experimental protocols. Finally, the approach adopted here for the moving gate is applicable to civil and environmental engineering problems including dam-break flows and sluice gate modeling.

Impactoclastic Density Current Emplacement of Terrestrial Meteorite-Impact Ejecta and the Formation of Dust Pellets and Accretionary Lapilli: Evidence from Stac Fada, Scotland

We present an impactoclastic density current model for terrestrial meteorite impact ejecta emplacement that is based on a hitherto-unreported threefold sequence of lithofacies in the recently identified Stac Fada impact ejecta outflow blanket in Scotland. Massive suevite (division A) grades seamlessly up into massive suevite containing whole and broken accretionary lapilli (division B), and this is overlain by a thin layer of clast-supported dust pellets (division C). The sequence closely resembles sequences in accretionary lapilli-bearing ignimbrites emplaced by pyroclastic density currents at volcanoes, and we deduce similar emplacement mechanisms. We propose that ≥99% of the thickness of the Stac Fada ejecta blanket as seen at outcrop was deposited from a single depletive (spatially decelerating), granular fluid–based density current that rapidly waxed and then waned. A previously unreported thin layer of pellets (division C) records direct fallout from a residual atmospheric dust plume. We propose that the impactoclastic density current was density stratified and comprised a ground-hugging component of granular fluid passing up into less concentrated, more turbulent levels and a buoyant, dilute upper component. Larger rock fragments rolled and saltated within the basal granular fluid. In the buoyant plume, dust agglomerated into pellets, and these dropped into lower, more vigorous levels of the density current, where successive concentric layers of fine dust accreted to them as they circulated and hardened. The resultant accretionary lapilli sedimented through the granular fluid into lower levels of the current and were deposited from its base. Dust pellets continued to form in the residual atmospheric dust plume after passage of the density current and fell out to form the thin layer (division C) of framework-supported pellets at the top of the ejecta blanket. The impactoclastic density current model may have applications to finer-grained parts of some continuous impact ejecta blankets elsewhere on Earth as well as on Mars and Venus, where features indicating flow processes are evident.

Pollutant Transport and Mixing Zone Simulation of Sediment Density Currents

Prediction of water column concentrations of suspended sediment is often necessary for environmental impact assessment of point source industrial discharges. For example, in ''flow lane'' or ''open water'' disposal, suction dredges discharge large volumes of suspended sediment into shallow water disposal locations. A sediment density current mixing model is presented here as part of the D-CORMIX expert system for hydrodynamic simulation of mixing zone behavior. This density current model extends the CORMIX decision support system to simulate continuous negatively buoyant discharges with or without suspended sediment loads on a sloping bottom with loss of suspended particles by sedimentation. Sedimentation is modeled using Stokes settling for five particle size classes. Density current width and depth, trajectory, total solids, tracer concentration, dilution, and particle size concentration are predicted. In addition, location and widths of sediment deposits, accretion rates, including particle size fractions within the spoils deposit, are predicted. The model results are in good overall agreement with available field and laboratory data.

K-ε Turbulence Modeling of Density Currents Developing Two Dimensionally on a Slope

Dense underflows developing two dimensionally on a slope are simulated numerically. The k-e model is used for the turbulence closure. The boundary-layer approximation renders the governing equations in the form of parabolic partial differential equations, which are easier to solve numerically than elliptic equations. Evolution of vertical structures of dense underflows is computed along the streamwise direction. Excellent similarity collapses of the computed vertical structures are obtained. The computed profiles of velocity and concentration are compared with measured data, resulting in good agreement. The impact of a parameter representing the stratification level in the k-e model is investigated. Appropriate values of this parameter, yielding results that are nearly identical to the integral model, are proposed. Water entrainment coefficients are estimated from computed solutions, and are observed to fall within the range of previous measurements. Finally, by using the collapsed vertical structures, pr...

The extension of a density current model of katabatic winds to include the effects of blowing snow and sublimation

A density current model was extended for use in katabatic flow over the steep slopes of Antarctica through the inclusion of the Coriolis effect and weight flux terms corresponding to blowing snow and cooling caused by sublimation. The model was calibrated and tested against data obtained during two flights in Adelie Land, Antarctica, along a trajectory starting about 170 km inland and extending to Dumont d'Urville. The predicted trend in water vapor flux agrees with measurements of this flux, lending support to empirical formulae for both snow flux and sublimation rate. Model predictions of velocity were in good agreement with measured quantities when reasonable estimates of radiation divergence and surface heat exchange were provided as input to the model. The potential temperature gradient above the katabatic layer was found to play a major role in flow stability for high velocity and deep katabatic flows. Velocity predictions were in better agreement with the data when a locally determined value was used for the coefficient in the empirical snow flux expression.

Environmental conditions and structure of the West African and Eastern Tropical Atlantic squall lines

The environmental conditions in advance of three West African disturbance lines and three Eastern Atlantic squall lines are examined as is the modification of the environment during the passage of such storms over a site. A comparison with other tropical squall lines observed over Venezuela is made. A model of the relative airflow within a West African disturbance line is presented and some aspects of storm movement are considered. The hydraulic jump analogy and a density current model are applied to the convective systems and the results obtained are compared with observations. The hydraulic jump analogy gives reasonable predictions for the storms' speeds and the density current model results show that for the West African disturbance lines, as well as the Venezuelan storms, a close relationship exists between the storm speed and the speed of the density current. As this relationship is not observed in the Eastern Atlantic squall lines, it appears that the density current model is not applicable to convective systems occurring over the ocean.

Traveling Convective Storms over Venezuela

Abstract The low-level atmospheric transformation associated with a class of traveling convective storms observed. over Venezuela is described. A strong low-level cooling is observed, confined mostly to the subcloud layer, and associated with a deeper layer of drying and acceleration of the easterly flow. A density current model is used to stratify the storm travel speeds, peak surface gusts and the accelerated flow at low levels behind the storm, and to relate these to the low-level flow ahead of the storm. There is reasonable agreement between these atmospheric data and laboratory observations of density currents. The updraft and down-draft structure is discussed using an interesting sounding cross section and trajectories from a three-dimensional numerical simulation. It appears that two distinct downdrafts exist: one driven by precipitation within the cumulonimbus cell, and a second mesoscale feature which is dynamically driven, and associated with descent over the spreading cold outflow.