Jonas Eliasson

Large hazardous floods as translatory waves

The theory for non-stationary flow in translatory waves is developed for an inclined plane in a prismatic channel and a funneling channel. The existence of translatory waves traveling over dry land or superimposed on constant flow is established, and the dependance on the initial flow value is discussed. Inherent instabilities of the wave tail are discussed. Data from a CFD simulation of a jokulhlaup (volcanic glacial burst) down the Markarfljot valley in Iceland are shown, and the similarities to the translatory wave established. Geological evidence of such large floods exists, and it is concluded that some historical floods like Katla 1918 have most likely been of this type. It is concluded that in simulation and hazard assessment of great floods, the translatory flow theory has an advantage over estimates based on Manning and alike methods, since the often subjective determination of the Manning n is avoided.

A STATISTICAL MODEL FOR EXTREME PRECIPITATION

The statistical distribution of 1-day (one reading in 24 hours) and 24-hour (several readings in 24 hours) annual maxima is considered and a transformed extreme value type 1 distribution function (TDF) that includes a probable maximum (PM) value is suggested. The distribution function fits standardized annual maximum station values from Iceland and Washington State very well. A generalized distribution function, derived from the TDF, is suggested. To use it, two local parameters have to be known; the 5-year event, M5, that must be picked from a map and a slope factor, Ci, that is a function of the coefficient of variation. The variation of Ci between independent observation stations is assumed to be random, and guidelines on how a Ci may be selected are discussed. The generalized distribution function is used to calculate quantile estimates and a local probable maximum precipitation (PMP). Regional PMPs can be calculated by maximizing this value. Two independent sets of Cis compare favorably: (1) a statistical set compiled from the British Natural Environment Resource Council (NERC) PMP envelope and )2) a meteorological set calculated from U.S. National Weather Service estimates of PMP for Washington State. The regional PMP estimates calculated from the generalized distribution also compare favorably with the NERC PMPs, except that the estimates for low M5 produce up to 33% lower PMPs. This difference may be explained by a number of factors that are also discussed.

Monitoring ash clouds for aviation

To minimize airport closures caused by volcanic ash, aviation authorities should make more use of methods that are now available for measuring airborne particles. Optical particle-counter (OPC) instruments were used to measure dust pollution during Iceland’s Grímsvötn volcano eruption in May this year, after a trial last year in Eyjafjallajökull’s ash plume. The data were used by Isavia, the country’s airnavigation services provider, to help in decision-making. OPC instruments measure airborne concentrations of three different size classes of particulate matter components and total suspended particles, every 6 seconds. The instruments were mounted on a light, slowflying, piston-engine aircraft after calibration in wind-tunnel experiments with volcanic ash. As Grímsvötn erupted, OPC results were quickly dispatched to the local meteorological office and to Isavia. This helped to keep Keflavik International Airport Some more biofuel lessons from Brazil

Assessment of flood hazard in a combined sewer system in Reykjavik city centre

Short-duration precipitation bursts can cause substantial property damage and pose operational risks for wastewater managers. The objective of this study was to assess the present and possible future flood hazard in the combined sewer system in Reykjavik city centre. The catchment is characterised by two hills separated by a plain. A large portion of the pipes in the aging network are smaller than the current minimum diameter of 250 mm. Runoff and sewer flows were modelled using the MIKE URBAN software package incorporating both historical precipitation and synthetic storms derived from annual maximum rainfall data. Results suggest that 3% of public network manholes were vulnerable to flooding during an 11-year long rainfall sequence. A Chicago Design Storm (CDS) incorporating a 10-minute rainfall burst with a 5-year return period predicted twice as many flooded manholes at similar locations. A 20% increase in CDS intensity increased the number of flooded manholes and surface flood volume by 70% and 80%, respectively. The flood volume tripled if rainfall increase were combined with urban re-development, leading to a 20% increase in the runoff coefficient. Results highlight the need for reducing network vulnerabilities, which include decreased pipe diameters and low or drastically varying pipe grades.

Airborne Measurement in the Ash Plume from Mount Sakurajima: Analysis of Gravitational Effects on Dispersion and Fallout

Volcanic ash concentrations in the plume from Sakurajima volcano in Japan are observed from airplanes equipped with optical particle counters and GPS tracking devices. The volcano emits several puffs a day. The puffs are also recorded by the Sakurajima Volcanological Observatory. High concentrations are observed in the puffs and fallout driven by vertical air current, called streak fallout. Puffs dispersion is analyzed by the classical diffusion-advection method and a new gravitational dispersion method. The fluid mechanic of the gravitational dispersion, streak fallout, and classical diffusion-advection theory is described in three separate appendices together with methods to find the time gravitational dispersion constant and the diffusion coefficient from satellite photos. The diffusion-advection equation may be used to scale volcanic eruptions so the same eruption plumes can be scaled to constant flux and wind conditions or two eruptions can be scaled to each other. The dispersion analyses show that dispersion of volcanic plumes does not follow either theories completely. It is most likely diffusion in the interface of the plume and the ambient air, together with gravitational flattening of the plumes core. This means larger boundary concentration gradients and smaller diffusion coefficients than state of the art methods can predict.

The Influence of Thermal Instabilities on the Initial Conditions of the Backdraft Phenomenon

This experimental study aims to better the knowledge of the flow-mixing phenomena involved in the first period of a backdraft, before the potential reignition of fuel gases in the enclosure (step not studied in the study). The authors describe the aerodynamics mechanisms of the evolution of thermal instabilities leading to the formation and propagation of a gravity wave appearing when dense fresh air enters an enclosure rich in hot gases (mixture of combustion products and fuel gases). A specific device and an experimental procedure were developed with flow conditions representative of a backdraft, but the tests were performed with inert gases in which fresh air entering in a enclosure containing heated air. Time-resolved laser tomography and particle image velocimetry measurements were performed to describe the wave displacement. The results show a strongly unsteady flow with formation in a pulsative mode of large scale Kelvin-Helmholtz structures. These obtained experimental results are essential for the calibration and validation of the subgrid turbulence model used by the numerical model.

Enclosure Fires, Gravity Waves, and the Backdraft Problem

The air flow to an underventilated compartment fire often depends on the flow velocities in the gravity wave of cold air that feeds the fire with oxygen. This problem has been studied in laboratory experiments and by CFD simulations. The main problem seems to be whether mixing and entrainment between the two layers of hot and cold air has a profound effect on the flow velocities. In this article, an analytical gravity wave model that can calculate the velocities in a simple gravity wave is presented. This model uses the equations of stratified flow hydraulics and the translatory wave solution of the flow equations. It is found that the velocities of the model compare very well to the velocities reported from laboratory tests and numerical simulations. Numerical simulations of stratified flow in a CFD model are discussed with respect to model construction. It is concluded that the densimetric Froude number is the main parameter for the velocity calculations and the length/height ratio is important for the friction forces.

A Different Approach to Vent Flow Calculations in Fire Compartments using the Critical Flow Condition

In enclosure fires, density-driven vent flow through an opening to the fire compartment is directly dependent on the state of the fire and the evacuation of smoke and hot gases. If a fire is strongly under-ventilated, there may be heavy production of flammable gases. If a sudden opening occurs, e.g., a window breaks or a fireman opens a door to the fire compartment, fresh air enters the compartment and mixes with hot gases, thus creating a flammable mixture that might ignite and create a backdraft. In this article, we consider the critical flow approach to solve the classical hydraulic equations of density-driven flows in order to determine the gravity controlled inflow in a shipping container full of hot unburnt gases. One-third of the container’s height is covered by the horizontal opening. For the initial condition, i.e., just before opening the hatch, zero velocity is prescribed everywhere. When the hatch is opened, the incoming air flows down to the container floor and the hot gas flows out. The interface in between them (the neutral plane) can move up like a free surface in internal flows, making it possible to use the techniques of open channel hydraulics devised by Pedersen [1]. In this article the critical flow condition, known from classical hydraulics, is used providing a new equation for the vent flow problem. Two flow correction coefficients are considered at the opening, taking into account the uneven distribution of velocity (α) and the effect of mixing and entrainment (C). The value of these coefficients is evaluated using computational fluid dynamics simulations and physical model results performed for the same geometry. Together, these two coefficients form the flow correction coefficient used in practical formulas for vent flow in fire protection engineering. These are known to have a little different values for different geometries and flow situations. The resulting flow coefficient varies slowly with the density difference, shows a small variation with geometry and compares well with previously published data.

Stability measurements on rubble-mound breakwaters

Abstract This paper describes methods and equipment for testing the stability of rubble-mound breakwaters. One instrument, called the OBDS (Optical Break-Down Sensor), is intended for global evaluation of the break-down process as a function of time. A second instrument, called the BCOM (Blink Comparator), is for observing and measuring the break-down process in detail. The instruments have been tested and found to give reliable results in the laboratory. Suggestions for improvement are made and it is believed that these will increase the usefulness and make this new technique applicable in the field.

Initial Wave Height and Total Energy of Landslide-Generated Tsunamis from Translatory Wave Theory

The conventional opinion of scientists is that tsunamis are usually caused by earthquakes, but the discovery of the huge submarine landslides on the continental shelf of the North Atlantic Ocean has changed this view considerably. Most tsunamis are triggered by earthquakes, but if it starts a landslide, the tsunami can become enormous, even though the earthquake is small. The energy balance in such tsunamis was studied for the first time using the translatory wave theory by Eliasson and Sigbjornsson. The theory was originally proposed by Eliasson. The mechanical wave energy generated in tsunamis of such origin is estimated for landslides on mountain slopes and submarine landslides. The initial tsunami wave height is estimated and compared to the results of the block slide estimation method. The energy transmission of the water wave can be translatory or by a solitary group of oscillatory waves where the length of the group varies according to water depth. In a case study, the Tohoku tsunami and earthquake in 2011 in Japan is found to be caused by a coseismic slip and a landslide in combination. A discussion of applying the theory to find a hazard curve for a tsunami wave from different sources at a given location is discussed.