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Environment / Sustainability

The art and science of planning for heavy rains

April 7, 2010
By Nolan Doesken

Most people don't ever give it a thought. It's totally taken for granted until something bad happens. But anywhere you go in Colorado and across the nation, the infrastructure we rely on -- our streets, highways, communications systems, power transmission lines, railroads, public and commercial buildings, parking lots, sewers, and even the gutters and downspouts on our houses -- are designed and built with intense rainfall in mind.

Designing structures to withstand heavy rain

In rural areas, ditches, culverts, and bridges are the most visible structures designed for specific rainfall standards. Feedlots, fuel, and fertilizer storage areas and other regulated commercial enterprises are also built to withstand certain rainfall rates, as are farm ponds and dry dams designed by the old “Soil Conservation Service” decades ago.

Even our “big box” retail stores have to take heavy rain into account. Have you ever considered how much an inch of rain on the roof of the nearest Wal-Mart store weighs?

Relying on data from 1940-1960

Extensive work was done across the country in the 1940s–1960s processing weather station data back to the late 1800s to estimate the frequency of various rainfall intensities and durations. Engineers need this information to design structures that will safely accommodate heavy rain and the flooding that may result.

Some low-risk structures may only be designed to withstand a 10-year or a 25-year storm, while others may be engineered to withstand 50- or 100-year storms without experiencing significant flooding.

In many areas, building codes are written to take this sort of information and decision making into account. The engineers who designed the Interstate Highway system in the 1950s and 1960s were major users of rainfall frequency information.

Are things changing?

What we design and build today will need to serve for many decades into the future. Are we using the best information to build for a future with potential climate change? This is a tough question.

There is much uncertainty about what Colorado precipitation patterns will be in the future.

  • Will it be wetter?
  • Drier?
  • More extreme?

The current generation of computer models probably isn’t able to confidently tell us this information.

So far, nothing stands out in Colo.

Precipitation is so variable that identifying trends and change will remain extremely difficult. So far, observations of heavy rainfall continue to show large variations with occasional extreme events.

An increase in extreme hydrologic events has apparently been observed in some parts of the country, particularly the northern tier of states, but so far nothing stands out here in Colorado.

Planned precipitation frequency project

The Colorado Water Conservation Board is working now to finalize a contract with the National Weather Service to participate in the Precipitation Frequency Project for the Midwestern states. While we wait for this contract to be finalized, Colorado is included in the early phases of this project, and data processing has begun.

Within five years we should have comprehensive state and region wide updates and enhancements to all of our precipitation frequency statistics, including the addition of 500-year storm estimates.

Just a matter of time until next extreme storm hits

The July 1997 flash floods in Fort Collins and on Pawnee Creek in northeastern Colorado stirred several special hydrologic studies and a year-long controversial reassessment of the Fort Collins rainfall design criteria for stormwater projects.

Fortunately for Colorado, it has now been over 10 years without another extreme precipitation event. But our history tells us that it's only a matter of time before the next extreme storm hits, and it will most likely be somewhere along the Front Range -- where the intersection of mountains and plains tends to focus and exacerbate flood-producing storms. Will we be ready?

What comes next?

That, of course, is the big question. A clear finding from past rain gauge and streamflow data is that Colorado’s largest storms and most extreme flash floods occur at elevations below about 7,500–8,000 feet, primarily east of the Continental Divide.

This is because the bulk of the moisture in our atmosphere is transported in the warmest layers in the nearest few thousand feet above the ground, and the greatest sources of moisture come in summer air masses that originate near the Gulf of Mexico or across the humid Midwestern states.

If the atmosphere continues to warm. . .

But if the atmosphere continues to warm, there could periodically be more water vapor available to fuel extreme storms at higher elevations, both east and west of the Continental Divide. If this were to happen, we might see more extreme rain events at higher elevations that would lead to dramatic flash floods.

Climate scientists and hydrologists are not sure this will happen, but it could. Are we ready?

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Excerpt from article by CSU researcher and State Climatologist Nolan Doesken originally published as, "The Climatology of Heavy Rains in Colorado," in Colorado Water, the newsletter of the Water Center of Colorado State University, March/April 2010.