Avalanche risk assessment - Ministry of Forests, Lands and Natural ...

More documents

Recommendations

Info

3.8IMPACT PRESSURES Impact pressure is a function of density (ρ) of the flowing speed, multiplied by the square of the speed (v), expressed as units of force per unit area on an object positioned perpendicular to the flow direction. The force is normally averaged through the time of the avalanche to give an average impact pressure. Impact pressure (in kilopascals) = ρ × v2 where 1 kPa = 1000 N/m2 Large, high-speed dry flowing avalanches are likely to exert the greatest impact pressures. Studies from Rogers Pass, B.C. and elsewhere have shown that the maximum impact pressure occurs as the frontal pulse of an avalanche strikes an object perpendicular to the flow direction. Maxima occur within the first second or two of impact (McClung and Schaerer 1993, p. 112). In a dry flowing snow avalanche, peak pressure may be two to five times the average impact pressure. Large dry snow avalanches typically have impact pressures in excess of 100 kPa (≈10 t/m2 ). In recent field measurements in Switzerland, recorded impact pressures averaged around 80 kPa for the first 6 seconds during a large avalanche, with many peaks of 200–400 kPa and a few strong peaks of up to 1200 kPa (Figure 95; Dufour et al. 2000). Keylock and Barbolini (2001) discuss vulnerability relationships for different-sized snow avalanches, as a function of runout distance. Table 22 gives typical relationships between impact pressures and potential damage. table 20 Relation between impact pressures and potential damage (after McClung and Schaerer 1993) Impact pressure (kPa) Potential damage 1 Break windows 5 Push in doors 30 Destroy wood frame structures 100 Uproot mature spruce trees 1000 Move reinforced concrete structures Note: Impact pressures are often expressed as tonnes force per square metre rather than the SI unit of kilopascal (100 kPa is approximately equal to 10 t/m 2 ). 82 Snow Avalanche Management in Forested Terrain
Avalanche speed (m/s) 90 2500 80 2400 70 2300 60 2200 50 2100 2000 40 1900 30 1800 20 1700 10 1600 0 1500 0 200 400 600 800 1000 1200 1400 1600 1800 Avalanche track length (m) figure 95a Radar speed measurements of the frontal pulse of a large avalanche gave a maximum of 80 m/s (290 kph) (dashed lines indicate error ranges associated with the measurements). The path falls 900 m. (Dufour et al. 2000) Impact pressure (kPa) 1200 1000 800 600 400 200 0 Velocity Min-V Max-V Track profile Interpolated velocity (3rd order polynom) Black: 3.0 m above ground Gray: 3.9 m above ground 510 520 525 530 535 540 545 550 Elapsed time (seconds) figure 95b Impact pressures measured at two heights on a tower located in the path shown above. At 3 m above the ground quasi-static pressures were around 500 kPa for 30 seconds, with distinct peaks of up to 1200 kPa. (Dufour et al. 2000) Chapter 3 Avalanche risk assessment 83 Height above sea level (m) C lick for next page
Page 1 and 2: 3.1 PROTECTION FORESTS In Europe, w
Page 3 and 4: Wildfire or logging on steep slopes
Page 5 and 6: dbh (cm) Height (m) 25 20 15 10 5 H
Page 7 and 8: matrices below are constructed on t
Page 9 and 10: table 14 Examples of risk managemen
Page 11 and 12: Risk management typically involves
Page 13 and 14: During the harvest period, forest l
Page 15 and 16: Two criteria are defined in the pro
Page 17 and 18: table 16 Continued Name Example Str
Page 19 and 20: Air Photo Interpretation Oblique co
Page 21 and 22: Ortho-rectified, monochrome air pho
Page 23 and 24: Gruber and Bartelt (2000) describe
Page 25 and 26: The mappers used the hazard index c
Page 27 and 28: The Ministry has responsibility for
Page 29 and 30: Existing snow avalanche paths are m
Page 31 and 32: 3.6 DOCUMENTING EXISTING AVALANCHE
Page 33 and 34: Elevation (m) 1050 950 850 750 650
Page 35 and 36: McClung and Mears (1991) define a r
Page 37: The important probabilistic concept

Avalanche risk assessment - Ministry of Forests, Lands and Natural ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?