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Titre du document / Document title

Air blasts generated by rockfall impacts : Analysis of the 1996 Happy Isles event in Yosemite National Park

Auteur(s) / Author(s)

MORRISSEY M. M. (1) ; SAVAGE W. Z. (1) ; WIECZOREK G. F. (2) ;

Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)

(1) U.S. Geological Survey, Denver, Colorado, ETATS-UNIS
(2) U.S. Geological Survey, Reston, Virginia, ETATS-UNIS

Résumé / Abstract

The July 10, 1996, Happy Isles rockfall in Yosemite National Park, California, released 23,000 to 38,000 m3 of granite in four separate events. The impacts of the first two events which involved a 550-m free fall, generated seismic waves and atmospheric pressure waves (air blasts). We focus on the dynamic behavior of the second air blast that downed over 1000 trees, destroyed a bridge, demolished a snack bar, and caused one fatality and several injuries. Calculated velocities for the air blast from a two-phase, finite difference model are compared to velocities estimated from tree damage. From tornadic studies of tree damage, the air blast is estimated to have traveled <108-120 m/s within 50 m from the impact and decreased to <10-20 m/s within 500 m from the impact. The numerical model simulates the two-dimensional propagation of an air blast through a dusty atmosphere with initial conditions defined by the impact velocity and pressure. The impact velocity (105-107 m/s) is estimated from the Colorado Rockfall Simulation Program that simulates rockfall trajectories. The impact pressure (0.5 MPa) is constrained by the kinetic energy of the impact (1010-1012 J) estimated from the seismic energy generated by the impact. Results from the air blast simulations indicate that the second Happy Isles air blast (weak shock wave) traveled with an initial velocity above the local sound speed. The size and location of the first impact are thought to have injected <50 wt % dust into the atmosphere. This amount of dust lowered the local atmospheric sound speed to ∼220 m/s. The discrepancy between calculated velocity data and field estimated velocity data (∼220 m/s versus ∼110 m/s) is attributed to energy dissipated by the downing of trees and additional entrainment of debris into the atmosphere not included in the calculations.

Revue / Journal Title

Journal of geophysical research    ISSN  0148-0227 

Source / Source

1999, vol. 104, noB10, pp. 23189-23198 (16 ref.)

Langue / Language

Anglais

Editeur / Publisher

American Geophysical Union, Washington, DC, ETATS-UNIS  (1949-2012) (Revue)

Mots-clés anglais / English Keywords

rockfalls

;

Yosemite National Park

;

granites

;

seismic waves

;

dynamics

;

finite difference analysis

;

damage

;

vegetation

;

trees

;

simulation

;

numerical models

;

propagation

;

atmosphere

;

seismic energy

;

shock waves

;

velocity

;

aerial photography

;

California

;

United States

;

North America

;

plutonic rocks

;

igneous rocks

;

Mots-clés français / French Keywords

Eboulement

;

Parc National Yosemite

;

Granite

;

Onde sismique

;

Dynamique

;

Méthode différence finie

;

Dommage

;

Végétation

;

Arbre

;

Simulation

;

Modèle numérique

;

Propagation

;

Atmosphère

;

Energie sismique

;

Onde choc

;

Vitesse

;

Photographie aérienne

;

Effet souffle

;

Onde atmosphérique

;

Energie cinétique

;

Californie

;

Etats Unis

;

Amérique du Nord

;

Roche plutonique

;

Roche ignée

;

Mots-clés espagnols / Spanish Keywords

Derrumbe

;

Parque Nacional Yosemite

;

Granito

;

Onda sísmica

;

Dinámica

;

Vegetación

;

Simulación

;

Propagación

;

Atmósfera

;

Energía sísmica

;

Onda choque

;

Velocidad

;

Fotografía aérea

;

California

;

Estados Unidos

;

America del norte

;

Roca granuda

;

Roca ignea

;

Localisation / Location

INIST-CNRS, Cote INIST : 3144, 35400008795263.0220

Nº notice refdoc (ud4) : 1987361



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