Flow Height Research Articles

Debris flow impact on flexible barrier: effects of debris-barrier stiffness and flow aspect ratio

Conventionally, flexible barriers are rated based on their ability to resist a free-falling boulder with a particular input energy. However, there is still no well-accepted approach for evaluating performance of flexible barrier under debris flow impact. In this study, a large-nonlinear finite-element model was used to back-analyze centrifuge tests to discern the effects of impact material type, barrier stiffness, and flow aspect ratio (flow height to flow length) on the reaction force between the impacting medium and flexible barrier. Results show that, in contrast to flexible barriers for resisting rockfall, the normal impact force induced by the highly frictional and viscous debris is insensitive to barrier stiffness. This is because the elongated distributions of kinetic energy are mainly dissipated by the internal and boundary shearing, and only a small portion is forwarded to the barrier. Furthermore, a new stiffness number is proposed to characterize the equivalent stiffness between a debris flow or a boulder, and a flexible barrier. Under the circumstance of an extremely elongated debris flow event, i.e., low aspect ratio, the load on a barrier is dominated by the static component and thus not sensitive to the barrier stiffness.

Analysis of Factors Affecting Heat Transfer and Ablation of Glass Wool in Silo Launching

For rocket silo launching, the heat transfer and ablation problem of glass wool, the main material of silencer layer. The glass wool with severely ablated area was selected as research object, and a unidirectional fluid-structure coupled heat transfer ablation model was established. Through the secondary development of abaqus subroutine, the influence of silo diameter and initial impact height of jets flow on the ablation of glass wool was studied using factor decoupling control method. The simulation results show that reducing diameter of silo can aggravate the ablation damage of glass wool in the process of rocketing out of silo. During the tempering phase of jets flow field, the smaller silo diameter is, the higher the peak value of average ablation rate is, and the sooner the peak value appears; Increasing initial impact height of jets flow can mitigate the ablation damage of glass wool in the process of rocketing out of silo. During the tempering phase of jets flow field, the greater initial impact height of jets flow is, the smaller the peak value of average ablation rate is, and the later the peak appears. Changing silo diameter and initial impact height of jets flow have less effect on the ablation damage of glass wool during the drainage stage and subsequent stages of jets flow field.

Experimental observations of tsunami induced scour at onshore structures

Tsunami inundation of the coastal environment can induce scour at structure foundations leading to failure. A series of experiments are made using a unique Pneumatic Long Wave Generator to generate tsunami wave periods of 25–147 s equating to 3–17.3 min at 1:50 Froude scale. The waves propagate over a sloping bathymetry and impinge upon a square structure founded onshore in a flat sediment bed. Flow velocity, height and scour depth are recorded as a function of time during tsunami inundation. The rate of scour is observed to be time dependent. Equilibrium, which is not attained, is argued to be an inappropriate measure for time-dependent transient flows such as tsunami in which the flow velocity, depth and direction are variable. The maximum scour depth is recorded and critically is observed not to be equal to the final depth due to significant sediment slumping when flow velocities reduce in the latter stages of inundation. Current and wave scour predictor equations over predict the scour, while the ASCE 7–16 method under predicts. Comparisons with available data in the literature show longer inundation durations increase the amount of scour.

Modeling impact pressure on the surface of porous structure by macroscopic mesh-free method

In this study, the impacting force on the face of porous structure is investigated by using the mesh-free macroscopic model. To achieve this, the mesh-free macroscopic approach of Moving Particle Semi-implicit method (MPS) is redeveloped. Firstly, a stabilization technique is used in the method to eliminate numerical noise. Secondly, the spatial discretization operators employ a porosity-based coefficient to account for the representative volume to model fluid flows in the porous media. More importantly, an improved numerical technique is proposed to overcome abrupt porosity change at the boundary between the porous and non-porous regions. The numerical model is validated by the dam-breaking waves through porous blocks and flows over porous weirs, in which the simulated free-surface profiles are in good agreement with experimental measurements. By applying the method, how the porosity and mean diameter of porous materials play a role in impacting pressure and wave height for dam-breaking flow through porous block is investigated. It is found that the porous materials with a larger porosity or larger mean diameter are able to easily permit water penetration through the porous media while the impacting pressure at the interface and reflected wave height can be reduced.

Initiation and runout characteristics of debris flow surges in Ohya landslide scar, Japan

The characteristics of debris flows (e.g., velocity, discharge, kinematic energy) are highly dependent on surges incurring abrupt changes to flow height, velocity, and boulder concentration. Therefore, understanding the initiation and runout characteristics of surges is essential when planning debris flow mitigation. Monitoring performed using 10 time-lapse cameras (TLCs) in Ohya landslide, central Japan, where debris flows occur frequently due to mobilization of storage (i.e., talus cone and channel deposits), allowed us to obtain data on a series of surge processes, from initiation to termination, which occurred during each debris flow event. We also analyzed temporal changes in the spatial distribution of storage in the debris flow initiation zone, associated with sediment supply from hillslopes and evacuation of sediment by the occurrence of debris flows, through periodic measurements of topography using unmanned aerial vehicles (UAVs). Debris flow surges were mainly induced by repetitive mass movement of storage through the erosion of channel deposits by overland flow, sliding of channel deposits, and sediment and water supply from channel banks and tributaries. Development of a spontaneous wave on the flow surface was not an important formation process of surges in the Ohya landslide. Many debris flow surges initiated at channel sections with deep storage (>2 m in depth), located <30 m below a junction with a tributary, when the maximum 10-min rainfall intensity exceeded 5 mm. Partly saturated flow, which has an unsaturated layer in its upper part, was the predominant flow type in the steep initiation zone, while fully saturated flow was predominant in the gentle transportation and deposition zones. Flow type often changed as the surges descended. Partly saturated flow was predominant when the volume of storage in the initiation zone was large, whereas fully saturated flow was predominant when the volume of storage was small. When we compared debris flows with similar total sediment volume, travel distance was long when fully saturated flow with high flow mobility was predominant because of the small volume of storage in the initiation zone. The volume of storage also affected flow path avulsion on the debris flow fan by controlling the flow mobility of surges. Consequently, the spatial distribution and total volume of storage are important factors controlling the initiation location, predominant flow type, and termination location of surges.

Climatology of coastal wind regimes in Benin

Using surface and lower-tropospheric wind observations at Cotonou (Benin), a comprehensive ten-year (2006–2015) climatology of coastal wind regimes at the West African Guinea Coast is presented for the first time. Four wind regimes are objectively identified: the monsoon and Harmattan winds, the nocturnal low-level jet (NLLJ) and the land-sea breeze (LSB) system. Amongst other things, these regimes are important for the occurrence of coastal squall line systems and for the redistribution of pollutants emitted from coastal cities. The seasonal cycle is split into the long (December–February) and short (July–September) dry seasons, the long (May–June) and short rainy (October–November) seasons, and the beginning of the long rainy season (March–April). In terms of the LSB, the lack of a diurnal reversal in wind direction obscures sea breeze activity at the height of the southwesterly monsoon flow in boreal summer and during a period of stronger winds in March-April related to large land-sea thermal contrasts. During these periods, the sea breeze manifests itself in a relative increase of the monsoon background flow during daytime, while early morning land breezes are infrequent. The latter expectedly, peak during the long dry season in December with a secondary peak during the long rainy season in May–June. The large–scale monsoon and Harmattan flows, mutually exclusive by definition, occur respectively, 88.2–98.4 % and 1.6–11.8 % per year with Harmattan days restricted to the long dry season and deepest monsoon flows of about 2.5 km confined to the long rainy season. The NLLJ is most prominent during the little dry season with, however, a similar and hitherto less documented peak in occurrence frequency during the beginning of the long rainy season. At this time of the year, the African Easterly Jet passes northward over Cotonou, causing the largest 3000–600 m vertical wind shear.

Physical environmental conditions determine ubiquitous spatial differentiation of standing plants and seedbanks in Neotropical riparian dry forests.

Mexican tropical dry forests are remarkably extensive and floristically diverse despite manifesting alarming rates of deforestation. Riparian habitats within dry forests provide critical ecological benefits that may mitigate negative impacts, but processes underlying riparian functions are still not well understood. We identified physical environmental conditions affecting the composition and abundance of standing vegetation assemblages and woody and herbaceous components in soil seedbank assemblages of riparian corridors in central Mexico using mainly NMDS ordination techniques, permuted analysis of variance (PERMANOVA), permuted analysis of multivariate dispersions and constrained ordination (CAP). We then determined representative species associated with particular environmental conditions using an indicator species analysis and assessed the effects of physical environmental variables/factors on total seed abundances by fitting a mixed-effect model. For the standing vegetation study, we assessed the effects of the type of the river condition (differing in surface flow permanence), location, and height above river level on the community composition based on three species importance criteria (abundance, coverage and DBH). For the soil seedbank study, we assessed the effects of these variables/factors plus season and land use. Spatial heterogeneity was a prevailing feature in riparian vegetation, in the standing vegetation and soil seedbank of both woody and herbaceous components. Height above river level had a significant effect on the three species importance criteria of standing vegetation and so did the interaction between surface flow permanence and height on coverage. The soil seedbank of woody and herbaceous plants showed significant differences between seasons; Taxodium mucronatum was an indicator tree species in dry seasons. Land use, height, surface flow permanence and the interaction between land use and surface flow permanence had significant effects on the soil seedbank of herbaceous plants. Total seed abundances in the soil varied between years and were higher at lower height values, during the dry seasons, and when rivers were permanent. Tree communities, commonly the most important elements in riparian ecosystems, were preserved in the soil seedbank of cultivated areas for >30 years. Seeds of herbaceous communities were predominant and ecologically relevant as indicator species because of their high sensitivity to several key environmental factors, constituting a critical component of Mexican tropical dry forests riparian corridors.

Assessing the tsunami mitigation effectiveness of the planned Banda Aceh Outer Ring Road (BORR), Indonesia

Abstract. This research aimed to assess the tsunami flow velocity and height reduction produced by a planned elevated road parallel to the coast of Banda Aceh, called the Banda Aceh Outer Ring Road (BORR). The road will transect several lagoons, settlements, and bare land around the coast of Banda Aceh. Beside its main function to reduce traffic congestion in the city, the BORR is also proposed to reduce the impacts of future tsunamis. The Cornell Multi-grid Coupled Tsunami Model (COMCOT) was used to simulate eight scenarios of the tsunami. One of them was based on the 2004 Indian Ocean tsunami. Two magnitudes of earthquake were used, that is, 8.5 and 9.15 Mw. Both the earthquakes were generated from the same source location as in the 2004 case, around the Andaman Sea. Land use data of the innermost layer of the simulation area were adopted based on the 2004 condition and the land use planning of the city for 2029. The results of this study reveal that the tsunami inundation area can be reduced by about 9 % by using the elevated road for the earthquake of magnitude 9.15 Mw and about 22 % for the earthquake of magnitude 8.5 Mw. Combined with the land use planning 2029, the elevated road could reduce the maximum flow velocities behind the road by about 72 %. Notably, the proposed land use for 2029 will not be sufficient to deliver any effects on the tsunami mitigation without the elevated road structures. We recommend the city to construct the elevated road as this could be part of the co-benefit structures for tsunami mitigation. The proposed BORR appears to deliver a significant reduction of impacts of the smaller intensity tsunamis compared to the 2004 Indian Ocean tsunami.

Shock Waves: From Gas Dynamics to Granular Flows

This short article briefly discusses some aspects in shock wave studies in recent years, in particular on the difference between gas dynamics and granular flow problems. It compares the relations of oblique shock waves, where weak, strong and detached shock waves can be observed in both gas dynamic and granular conditions. If the upstream Froude number of granular flow becomes infinitely large a granular shock wave would still remain attached and oblique around a wedge angle near 90°, however an attached gas dynamic shock wave is limited by a maximum wedge angle, say, of 30°. On the other hand, the shock standoff distance for a detached granular shock wave tends to become infinitely small with the increase of the upstream Froude number since it is associated with the flow height ratio across the shock wave.

Compressible Pipe Flow with Friction and Gravity

A viscous one-dimensional compressible pipe flow under gravity effect is studied analytically. The compressible one-dimensional pipe flow with friction is called Fanno flow and the solution is given by analytical formula. In gas dynamics, the gravity effect is minimal and it is not included in the equations. However, it was shown by the present author that the elevation of a pipe could change the flow conditions in a one-dimensional compressible potential flow under gravity. The sonic condition is reached at the maximum height for an inviscid pipe flow. In this paper, the gravity effect is extended to the viscous one- dimensional pipe flow. Subsonic–supersonic transition is also possible by up and down of the pipe as in the inviscid flow, and it is found that the sonic condition deviates from the peak position of the pipe.

Cooling load and noise characterization modeling for photovoltaic driven building integrated thermoelectric cooling devices

Photovoltaic driven thermoelectric cooling devices are investigated for installation in a modular outdoor test-room. Because of Peltier effect in a thermoelectric cooling (TEC), heating and cooling is achieved by applying a voltage difference across the thermoelectric module. Theoretical design modeling of cooling load and noise characterization of building integrated Thermoelectric (TEC) Devices is analyzed. System design of photovoltaic driven TEC devices is investigated with varying fresh outdoor ventilation rates. Building integrated design of TEC devices inside ceiling suspended duct along with TEC devices mounted on wall driven by rooftop and active façade photovoltaic devices is considered in the analysis. In this way, two-stage dehumidification is achieved by two different sets of TEC devices. The investigation is conducted for effect of voltage, air flow rate and height of fin heat transfer surface. Expressions along with results for noise characterization in photovoltaic driven building integrated TEC devices are also provided.

Analysis of physical mechanisms of human body instability for the definition of hazard zones present in emergency action plans of dams. Case study: Santa Helena Dam, Bahia

ABSTRACT The impacts caused by flood waves due to dam ruptures usually cause irreversible damages to the resident population, and, the loss of body equilibrium in floods contributes to aggravate this scenario. In this context, this work aimed to analyse the influence of consideration of physical mechanisms that cause instability in the human body on the definition of hazard zones. Therefore, it was developed simulation of the propagation of the flood wave due to the hypothetical rupture of Santa Helena Dam in Bahia, using the hydrodynamic model HEC-RAS. The results of flow velocities and heights were related and compared to different criteria of hazard zonings and mechanisms that cause body instability. It was verified that the consideration of instability mechanisms of the human body can contribute to hazard management, through the knowledge of areas in which different individuals may topple or slide. It was confirmed that in supercritical flow regimes is more likely for the individual to slide and that in subcritical regimes the individual will topple. Moreover, the consideration of parameters such as buoyancy force and the angle related to the human body's adaptive ability in a flooding influence on the definition of zones.

A VORTEX DROP STRUCTURE AS DEAERATION SYSTEM FOR A SUBMARINE OUTFALL PIPELINE

Use of submarine outfall pipelines became more common since World Bank Group issued a new guideline for maximum emissions levels for thermal power plants in 1998 (van Dijk, 2005). The more restrictive levels for temperature increase at the receiving water, requires outfall systems to conduct the water down to greater depths to achieve the required dilution standard. However, air entrainment control into outfall pipes could be challenging, especially for discharges with high flowrates for which conventional deaeration chambers become too large. The problem could turn more difficult in coastal shelf areas at seismic zones, where the hydraulic height of the incoming flow must be effectively controlled and the design not only has to pursue hydraulic objectives but also stability requirements for these massive structures subjected to relevant seismic thrusts. A vortex drop structure was designed for the cooling water discharge system of a thermal power plant in Mejillones Bay, Chile. The structure addresses the elevation difference between the return flow pipe and the ocean outfall pipelines while adhering to the spatial restrictions at the project site. Energy dissipation as well as limitation of air entrainment into the outfall pipelines were critical design considerations. Tests where done on a 1:12.5 scale (Froude) physical model. Prototype structure is under construction. Operation is planned to start on mid-2018.

Inflation of Ponded, Particulate Laden Density Currents

Field-based, physical modeling and analytical research approaches currently suggest that topographically confined particle-laden density currents commonly inflate to produce suspension clouds that generate tabular and texturally homogeneous sedimentary deposits. Here, a novel three-dimensional theoretical model details a phase space of the criteria for inflation as a function of flow duration, basin size and geometry, total mass transport, sediment concentration, and particle grain size. It shows that under most circumstances cloud inflation is unlikely at real-world scales. Even where inflation is possible, inflation relative to initial flow height is small except for suspensions of silt or finer-grained sediment. Tabular deposits therefore either arise from processes other than flow ponding, or deposits in confined settings may be significantly more complex than are currently understood, due to processes of autogenic compensation and channelization, with associated implications for reservoir characterization in applied contexts. This study illustrates the potential of analytical flow modeling as a powerful complement to other research approaches.

Characteristics of Denitrifying Phosphorus Removal by A2/O-BAF at Low Temperatures

Low C/N domestic sewage was treated by an A2/O-biological aerated filter (BAF) system at low temperatures (11-14℃). The characteristics of pollutant removal, the ratio of denitrifying phosphorus to nitrogen (ΔPO43-/ΔNO3-N) and effects of aeration flow and effective packing height on nitrification in BAF were studied. The results showed that when the average influent concentrations of COD, NH4+-N, TN and PO43- were 193.1, 58.6, 60.3 and 5.1 mg·L-1 respectively, their effluent concentrations were 46.3, 2.5, 13.4 and 0.3 mg·L-1 respectively, which met the first level A criteria specified in the discharge standard of pollutants for municipal wastewater treatment plant (GB 18918-2002). The linear fitting of ΔPO43-/ΔNO3--N was between 0.47 and 1.75. The normal distribution of mathematical statistics was applied-and the average standard deviation for ΔPO43-/ΔNO3--N were 1.20 and 0.29 respectively. When the aeration flows were 60 L·h-1 and 100 L·h-1, the effluent concentration of NH4+-N was less than 5.0 mg·L-1, corresponding to the effective packing heights in the BAF of 1.8 m and 1.0 m respectively. However, when the aeration flow was increased to 120 L·h-1, the air-water flow led to biofilm detachment, which caused the effluent concentration of NH4+-N to increase beyond 5.0 mg·L-1.

A progressive entrainment runout model for debris flow analysis and its application

A progressive entrainment model has been incorporated into an energy-based runout model to calculate the kinematic characteristics of debris flow. The progressive entrainment model considers both rolling and sliding motions of erodible material. To study the effect of the controlling parameters in the entrainment model, sensitivity analyses have been carried out by varying model parameters such as the internal friction angle of the debris, the basal friction angle of the channel, the turbulent flow coefficient, and the mean value of the probabilistic density function (PDF) for the resting angle. The results are evaluated by calculating the change of the maximum entrainment rate, the maximum frontal velocity, the final total volume, the longitudinal length of deposition, the maximum debris flow height, and the runout distance of the debris. The progressive entrainment runout model is sensitive to the characteristic particle size, basal friction angle and turbulent coefficient. Measurements from the 1990 Tsingshan debris flow, Hong Kong, China, are used to validate the model. Velocities at different channel sections estimated using the super-elevation method and entrainment depth are used to evaluate the simulation results. Comparing the results between simulated and observed maximum velocity and entrainment depth indicate that the progressive entrainment model together with the runout model can capture the erosion characteristics of the granular material. The results are also compared with that using a dynamic entrainment model. The results demonstrate that the progressive entrainment model provides a more realistic calculation of entrainment for this case compared with the dynamic entrainment model.

Computational experiments on the 1962 and 1970 landslide events at Huascarán (Peru) with r.avaflow: Lessons learned for predictive mass flow simulations

Mass flow simulations are considered important tools for hazard analysis. For the simulation of single process mass flows such as debris flows, robust tools and reasonable parameter range estimates are available. However, this is much less the case for more complex mass flows, e.g. involving process chains and flow transformation. We explore the challenges of simulating complex flow-dominated landslides by back-calculating the Huascarán events of 1962 and 1970 with r.avaflow, a two-phase mass flow model (Pudasaini, 2012) in a GIS-based open source simulation framework. Both events started as rock-ice falls on the western slope of the north summit of Nevado Huascarán (Cordillera Blanca, Peru) and entrained large volumes of glacial till at lower elevation, resulting in highly mobile debris avalanches. Whereas the 1962 event badly affected the village of Ranrahirca when spreading over a debris cone, the 1970 event overtopped a ridge and led to the complete destruction of the town of Yungay. Well documented in the literature, these events provide an opportunity as a natural laboratory for testing innovative mass flow simulation tools and their features. In a first step, we consider (i) the 1962 event and (ii) the 1970 event separately, for each of them optimizing the key input parameters in terms of empirical adequacy. In a second step, we apply the optimized parameter set for (i) to the 1970 event and the parameter set derived for (ii) to the 1962 event. In a third step, we explore the sensitivity of the model outcomes to selected key parameters (basal friction angle and entrainment coefficient). The results (a) demonstrate the general ability of r.avaflow to reproduce the spatio-temporal evolution of flow heights and velocities as well as travel times and volumes of these complex mass flow events reasonably well; and (b) highlight the challenges and uncertainties involved in predictive simulations with parameter sets obtained from back-calculations. We suggest a strategy to appropriately deal with uncertain outcomes by superimposing the results of multiple simulations.

Experimental analyses of impact forces on buildings exposed to fluvial hazards

Fluvial processes have a significant impact on buildings and infrastructure and despite their local character they cause annually considerable costs in mountain areas worldwide.Vulnerability studies are based mainly on empirical data from past events whereas laboratory studies investigating the impact forces on buildings are still limited.The paper presents a study on impact forces resulting from complex flow processes on buildings, the influencing factors they depend on and the identification of process parameters they correlate with.A 1:30 physical scale model was built, representing the torrential fan of the Schnannerbach torrent (Austria). Measurement devices, installed on the building structures located on the torrential fan, recorded impact forces induced by a set of fluviatile flood scenarios. The measured impact forces were compared to computation results using recently developed calculation approaches and to measurements resulting from experiments with a simplified, homogeneous scale model set-up with a single wall element in a rectangular flume. The results show a clear correlation between the approaching flow heights and the impact forces on exposed buildings. Compared to clear water conditions, bed-load transport and deposition processes highly influence the impact forces due to sediment accumulations close to the buildings on the torrential fan. The gained insights contribute to a better understanding of potentially damage-causing impacts on buildings resulting from the interaction of flood processes and the exposed built environment.

Characteristics and dynamic runout analyses of 1983 Saleshan landslide

In 1983, a catastrophic landslide occurred in Saleshan, Dongxiang, Gansu, China, which resulted in the death of 237 people and the destruction of 585 houses. Saleshan landslide was a rapid moving landslide in loess area, China, which moved with long run-out distance on a gentle sliding surface. This paper reviews the characteristics of the landslide and its consequence, analyzes the possible triggers and the processes of development, and back-calculates the kinematic characteristics of the landslide. The digital elevation models (DEMs) of the study area before and after the event were used to analyze the geomorphological characteristics before and after the event. The authors inferred the possible deformation and failure processes of the landslide based on available observations. Direct shear tests were carried out to obtain the parameters of the soil involved in the failure and post-failure process, which was adopted in the following simulation using an energy-based runout model. The velocities of moving materials at a different time stage together with the variation of runout distance and maximum flow height are presented. The authors also compared the calculated results with field observations and previous simulation results. Sensitivity analysis was conducted to understand the effect of internal and basal friction angles on the kinematic characteristics. This study indicates that the Saleshan landslide was a fast-moving landslide with long traveling distance. The landslide lasted more than one minute with a maximum velocity of approximately 25 m/s according to the calculation using the energy-based runout model, which was consistent with the estimations based on the observation of eyewitness. This study also gives an insight on the progressive failure mechanism and explains the high mobility of Saleshan landslide in details, which provides a reference for hazard zonation for areas along loess platforms.

Preliminary results of numerical modelling of debris flow ‐ case study Leva reka, Serbia

AbstractLeva reka debris flow (Kraljevo area, Serbia) was triggered by extreme rainfall in May 2014 in Serbia. A Huge amount of weathered Cretaceous flysch material formed debris dam, over 10m high, and made a lake on Leva reka (about 150m long, and 5m deep). Debris flow appears in an active ravine with the occasional stream flow. The debris flow is about 400m long, 100 wide at widest part, and about 30 m deep in deepest main part. Digital Elevation Model and orthophoto images were obtained by UAV survey in very high resolution as a detailed topographic input model, as well to define recomposition of material after displacement. A numerical modeling was made in RAMMS® software, which is based on Voellmy friction law. Main models, gave information about maximum flow height, velocity, and pressure in different parts.