The Problem with Flood Statistics for the Front Range

There have been a lot of confusing statements made about 1 in 500 year floods and 1 in 1,000 year rainfall events after last week's floods. Some examples are here and here. Rainfall totals set records and some people equate rainfall totals with the severity of flooding but flooding severity depends on the rate of rainfall, what hydrologists call the rainfall intensity. I know a little bit about this subject - I started my career as a hydrologist at an engineering firm in Minneapolis that was founded by Adolf Meyer, a pioneer and founding father of modern methods of analysis in hydrology.  The 1965 flood had many locations throughout a week of rain that had rainfall intensities of 14 inches in 4 hours.  Big Thompson Canyon had a similar rainfall intensity in 1976. This past week in the Boulder area, the maximum rainfall intensity was about 12 inches in 19 hours, only about 1/4 the intensity of the most severe floods Colorado has experienced.  A rough analogy for the effect of rainfall intensity on flooding severity is the bathtub in your home. With the drain open, turn on the faucet and the water level might rise slightly over the drain but the drain removes everything that is coming in - about 3 gallons a minute . Add a garden hose and double the amount of water going in, the level in the bathtub will slowly rise and may eventually overtop the tub after awhile. Take a fire hose and pump in 20 gallons per minute and within a few minutes the tub is flowing over and your house floors are underwater. That's the difference between rainfall intensity and rainfall totals. In each case, you can pour 500 gallons (the equivalent of the rainfall total) into the bathtub but the effects are quite different depending on the rate at which you pour it in.  In September, 1938, a flood that had maximum flow rates in Boulder Creek similar to this week was caused by rainfall totals of only 4 to 5 inches.

 It was a bit further south and hit Eldorado Springs and South Boulder Creek quite hard.  You can read about it here.  Most of the rain fell in 4 hours on September 4, after days of rain saturated the ground. 

What's the difference between 1938 and 2013? - there are over 5 million people living in Colorado now compared to 1 million in 1938. Here is an air photo of north Boulder in 1938 where Fourmile Canyon Creek comes out of the foothills. 

Here is the same area today that was badly damaged by flooding from Fourmile Canyon Creek last week.

The weather condition that produces large rainfall and flooding events along the Front Range is called an upslope - when moist tropical air from the south streams up to a stationary cold front and stalls against the Rockies for days at a time. How many times do the upslope conditions setup along the Front Range? - several times a year. How many times have upslopes caused major floods along the Front Range in the past 120 years? - over 30 times. How many times on Boulder Creek? - 10 times including this year.  

Clearly, there is a disconnect between reality and the statistical methods being used, perhaps best summed up by a guest ranch operator in Big Thompson Canyon News Link:

"When we first saw this, you know, you kind of go through this sequence of emotions. First is unbelief. I mean, you just can't imagine that you would have two 500-year floods in the span of 30 years," he said.

So - the bottom line is upslope conditions that can can cause catastrophic flooding anywhere along the Front Range can occur as often as every 2 to 3 years. The longest period without severe, damaging floods was 1948 to 1965, 17 years. The second longest period was the current one, 1997 - 2013.  Topography favors some locations more than others for flooding, like Boulder, which has had severe flooding in 1894, 1906, 1909, 1916, 1929, 1938, 1941, 1969, and of course 2013.