The Anatomy of an Historic Flood

January 5, 2016

The Anatomy of an Historic Flood

By now, most of you know that the St. Louis, MO area had historic flooding last month. There are many factors that go into producing historic floods, including previously existing conditions as well as current conditions. We also have to look at the ground as well as the atmosphere.  Let’s take a look at why St. Louis had historic flooding last month.

To begin with, urban areas, such as St. Louis, are more susceptible to flooding than rural areas. Why? Urban areas have more asphalt and concrete as opposed to the rural areas which are mainly soils. Concrete and asphalt do not absorb water as does soil, so any water in an urban area needs to be taken away by drainage systems. Record amounts of rain over the past year had already taken their toll on the St. Louis area.  Drainage systems were already overloaded and soil moisture levels in the rural areas were sky-high as well: any new water just had nowhere to go.

To make things worse, St. Louis is wedged between two major rivers, the Mississippi River and the Missouri River. Numerous tributaries to these rivers flow through its boundaries as well. River levels were already elevated to near flood stage from previous rains. So when 9-10” of rain fell in 48 hours, and it couldn’t be absorbed into the ground or taken away by drainage systems, all the water ran straight into the rivers. This caused massive rises in the rivers, which caused them to overflow their banks, and in some cases breech dams.

Take a look at how high the river levels were in the days following the storm and see how they compare to their baseline, minor flood stage. If a record was set, the previous record river stages are also shown.

Onto the atmosphere. The first and foremost atmospheric trait that led to historic floods was overly-abundant moisture. One way we measure moisture in the atmosphere is through precipitable water.  Precipitable water is the amount of total moisture that is available to be completely “rained-out”. A way to understand this is by comparing it to a soaked towel. It would be like if you were to completely wring-out the soaked towel and then measure the water that came from the towel. Values upstream from St. Louis at the time were measured at 1.48”, which is 400% greater than what is normally seen in the month of December. That’s a lot of moisture that can then be turned into a lot of rain. Here is a visual representation of how much larger values were compared to normal.  The star indicates the precipitable water values during the storm. “Normal” is shaded in red.

Another circumstantial advantage this storm had was that there was a front that had stalled directly over St. Louis. As the storms were directly tied to that front, the storms stalled too, causing it to rain, and rain…and rain some more.

All these factors combined are what led to the historic flooding of St. Louis at the end of 2015.

Take a look at these records that were set in December 2015 for St. Louis! (Measured at St. Louis Lambert International Airport)

  • For December 26th, 2015: a daily record of 4.87” of precipitation fell in 24 hours, surpassing the previous record for that day of 1.06” set in 1916.
  • For December 28th, 2015: a daily record of 2.59” of precipitation fell in 24 hours, surpassing the previous record for that day of 1.45” set in 1884.
  • For the month of December: a monthly record of 11.74” of precipitation fell in the month of December, surpassing the previous record of 7.82” set in December, 1922.
  • For the year of 2015: a yearly record of 61.24” of precipitation fell in the year of 2015, surpassing the previous record of 57.96” set in 2008.

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