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[SVC31-12] A numerical study of the Influences of aeolian drift of deposited ash on ashfall distribution of built-up areas in cities
Keywords:ashfall simulation, aeolian drift of volcano ash, functional damage, cities
In volcanic ashfall events on windy days, blowing up of ash deposited on the surface of the ground as well as building roofs will take place when wind blows at a certain strength and the weather conditions allow the ash particles to be kept not wetted. In the present study, a parameterization of aeolian drift process of ash particles is introduced in the numerical ashfall simulation to see some fundamental aspects of their effects on the ashfall distribution in the building canopies in a city. The introduction of the aeolian processes into the governing equations of the present numerical model is done in a similar manner taken in previous numerical works on snow and sand drift (e.g., 1)), where local equilibrium between the vertical ash mass fluxes of deposition and erosion in the saltation layer on the surface of the deposited ash are assumed. The wind induced erosion of ash are assumed to occur when aerodynamical friction velocity at the surface exceeds a certain threshold value given as a threshold friction velocity. With the assumptions of the thickness of saltation layer of 0.5m and the threshold friction velocity of 0.2m/s, which are based on 1), ashfall simulations in narrow built-up areas with low to middle rise buildings in a city with horizontal grid interval of 2m, which is capable of crudely but explicitly resolving individual buildings, for various wind conditions and ash diameters.
Comparisons of the results of simulations with and without aeolian drift processes show several features representing the character of the effects of wind-driven redistribution on ashfall in built environment in cities. One of the results to be noted is that the aeolian drift can increase airborne ash in the building canopies and the ashfall rates on the ground including roads by several to more than 10%, although the magnitudes of them vary with wind conditions as well as the size of ash particles. This tendency is considered primarily owing to the wind erosion of the ash sedimented on the roof of buildings. During the development of aeolian erosion of roof ash, the roofs act as the sources providing ash, which should be remaining on roofs if the aeolian erosion is absent, to the airflow from the heights of roofs. The aeolian drift process can be considered to helps the ash ‘trapped’ on building roofs to restart falling toward the ground below. The increase of ash concentration in the building canopies and ashfall rate on the ground are thought to be the results of ash supply from building roofs in the way like this. Since airborne ash concentration are concerned with the ashfall induced damages of building air conditioning and the ashfall rate on the ground affect road traffics, the present results imply the aeolian ash drift has the potential to give non-negligible impacts on the degree of functional damages of cities.
1) Komatsu, A. and K. Nishimura 2022, SOLA, Vol. 18, 71-75.
Comparisons of the results of simulations with and without aeolian drift processes show several features representing the character of the effects of wind-driven redistribution on ashfall in built environment in cities. One of the results to be noted is that the aeolian drift can increase airborne ash in the building canopies and the ashfall rates on the ground including roads by several to more than 10%, although the magnitudes of them vary with wind conditions as well as the size of ash particles. This tendency is considered primarily owing to the wind erosion of the ash sedimented on the roof of buildings. During the development of aeolian erosion of roof ash, the roofs act as the sources providing ash, which should be remaining on roofs if the aeolian erosion is absent, to the airflow from the heights of roofs. The aeolian drift process can be considered to helps the ash ‘trapped’ on building roofs to restart falling toward the ground below. The increase of ash concentration in the building canopies and ashfall rate on the ground are thought to be the results of ash supply from building roofs in the way like this. Since airborne ash concentration are concerned with the ashfall induced damages of building air conditioning and the ashfall rate on the ground affect road traffics, the present results imply the aeolian ash drift has the potential to give non-negligible impacts on the degree of functional damages of cities.
1) Komatsu, A. and K. Nishimura 2022, SOLA, Vol. 18, 71-75.