WK11: Hazard Science: Part 3c: Meteorological Hazards - Floods and Droughts

Meteorological Hazards: Floods and Droughts

Floods

  • Floods are defined as an excess of water that cannot be accommodated by a waterway or absorbed by the ground.
  • A cross-sectional diagram illustrates:
    • A normal river builds natural levees to contain water during smaller storms.
    • When levees are breached, the floodplain is inundated. The floodplain is the area in the valley that could be flooded.
    • Over time, rivers cut down, creating old floodplains.
  • Causes of Floods:
    • Large storms and heavy rainfall.
    • Tidal floods, especially king tides (e.g., Waikiki).
    • Dam breaks (e.g., Oso landslide).
    • Rapid snow melt.
  • Hazard Intensity Metrics:
    • Water depth.
    • Water velocity (faster velocities indicate more energy).
  • Floods generally occur near waterways, but flash floods can happen anywhere with heavy precipitation.

Types of Rivers and Floodplains

  • Meandering Stream:
    • A channelized river with a floodplain extending to the edge of the valley.
    • The river may eventually meander and cut across the floodplain.
  • Braided River:
    • Multiple channels create a wider river.
    • The floodplain is smaller, confined to the width of the channel.
    • Less likely to have construction in the middle of the braiding due to the width of the channel.
    • Braided rivers can be very fertile, leading to channelization by humans.
  • Channelization:
    • Humans put braided rivers into channels to create usable land.
    • Increases the likelihood of overtopping built levees and causing floods, as channelized rivers cannot accommodate as much water.

Droughts

  • Droughts are extended periods of lacking precipitation.
  • Considerations for defining a drought:
    • Duration since last precipitation.
    • Temperature (influences fire risk).
    • Frequency of lightning storms without rain.
    • Consistency of rain (whether rain is sufficient).
  • Graph Example:
    • A graph displays droughts west of Cooktown in Queensland from 1986 to 2006.
    • Four distinct droughts with varying durations and intensities are shown.
      • Intensity is indicated by how far the water level deviates from zero (more deviation means more water missing).
      • Duration is indicated by the number of consecutive red lines.