Natural Disasters Week 13

Water Shead

• Before Development

  • Water Held in detention storage by pit and mound topography

  • Infiltration into organic-rich forest soil

  • Infiltration slowly fed baseflow during dry periods

  • Flood peaks were smaller and came later

  After Development

  • Infiltration became limited

  • Runoff builds rapidly and enters streams quickly

  • Baseflow limited (need infiltration to get gw)

  • Flood peaks are larger and occur sooner

  • Larger flood magnitudes become more frequent (e.g., once every 10 or 25 years now occurs once every 2 years

  • Channel Degradation!!

Mass Wasting

• The downhill movement of masses of bedrock, rock debris, or soil under gravity

• Slope stability: can be stable or unstable

  • Two stability is a tradeoff between two forces

  • Gravity pulls downward

• Movement occurs when downslope forces are greater than resisting forces

Factors making mass wasting likely:

• Steep slopes (higher stress)

• Large relief (higher stress)

• Weak Rock Layers (low material strength)

• Thick layers) of loose rock, debris, soil (low material strength)

• Presence of water (friction, material strength)

• Lack of vegetation (low material strength)

• Seismic (earthquake) activity

Classification of Mass Wasting

• Rate of movement  

  • Slow as 1 cm a year

  • Fast as 100 km a year