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Flood Factors

September 17, 2013

There is nothing like a real-life example to bring classroom lessons to life. Last week we discussed rainfall factors and physical characteristic that influence how water travels within a watershed, and what this means for controlling and storing water.

Rainfall Factors

  • Intensity- A function of  amount (think inches) & duration (think hours) of rain.
  • Distribution of rainfall over a drainage area
  • Direction of storm movement
  • Previous rain events- Impacting level of soil saturation & water height in rivers.
  • Seasonal timing- Winter: cold ground and barren trees means runoff . Summer: photosynthesizing trees absorb more water.

Physical Factors

  • Land use within drainage area- % vegetation cover (pasture vs cropland vs forest), and % developed as impermeable surfaces.
  • Soil type- Fast draining sand, nearly impermeable clay, or a well balanced loam, soil type has a huge impact on the speed of water drainage and aquifer recharge.
  • Basin shape- Topography and slope, elevation and overall spread.
  • Lakes, pond and reservoirs- These features are speed bumps, slowing water at certain point, but can also amplify risk if they are overtopped.

The current flooding in Colorado, while tragic, has also been an instructive development to track, as much of the explanation for the flooding directly reflects the factors mentioned above:

  1. Rainfall on Sept 9th and 10th saturated the ground and increased river flows.
  2. A low pressure center with a high pressure system to the north caused the storm system to stall over the Great Basin.
  3. From Sept 9th -13th, 14.62″ fell in Boulder, with 9.08 on Sept 12th alone.
  4. Exceptionally high volumes were concentrated in several key areas. The above image illustrates the concentration of rainfall geographically. This map shows the massive variation in distribution that can occur in one rain event.

As more rain is potentially forecasted in Colorado later this week, all eyes will be watching to see if the reprieve is enough for communities to regain some stability. These floodwaters continue to course downstream, much of it eventually discharging in the Mississippi River Delta, and as this emergency situation continues to develop, I watch with curious concern to see how geography and weather patterns either amplify or dissipate the forces felt by downstream communities.

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