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Thursday, September 19,2013

The flood of the century? Not exactly

By Paul Danish
Photo by Elizabeth Miller
Table Mesa and Stanford in South Boulder

This story is part of Our Road to Recovery, our coverage of the 2013 Boulder County floods.

In Boulder, they say it was a 100-year flood.

Uh, not exactly.

Boulder city officials, U.S. Geological Survey (USGS) officials, and the media have all averred that last week's flood was a 100-year flood, or as the USGS prefers to call such events these days, a “1 percent annual exceedance probability flood.” (The USGS prefers the latter term, despite its clumsiness, because it makes clear that a 100- year flood is really a flood that has a 1 percent chance of happening in any given year, not a flood that shows up like clockwork once a century. If a city got particularly unlucky, it could have hundred-year floods in consecutive years or months, or even a string of them.)

The claim that Boulder experienced a 100-year flood event appears based on the amount of rain that fell in the city between 6 p.m. on Sept. 11 and 6 p.m. on Sept. 12 — 9.08 inches — which made the period the wettest 24 hours in Boulder since official records began being kept in 1897. (The previous 24-hour precipitation record was 4.80 inches on July 31, 1919.) Last week’s flood might also turn out to be a 100-year event in terms of the amount of rain Boulder received in a single storm, the amount of land flooded, the number of streams that flooded at the same time, or similar measures.

But by one important measure it was not a 100-year flood, and that may be the most crucial “not exactly” in Boulder for, say, the next 100 years.

The most important metric for determining whether a flood is a 100-year flood — the metric used by Boulder and virtually every other city in the country in defining flood plains and fashioning flood plain management policies and emergency response schemes — is the rate of stream flow at the height of the flood, measured in cubic feet per second (cfs).

And by that standard last week’s flood was not a 100-year one, not even close.

The definition of a 100-year flood on Boulder Creek that is used by the City of Boulder and assorted federal and state agencies is a flow of roughly 11,000-13,000 cubic feet per second at a flood’s height, depending on where on the creek the flow is calculated. This figure has been determined by multiple studies of the creek’s hydrology over the last 36 years, the most recent of which was done by Anderson Consulting Engineers, Inc. of Fort Collins, and completed July 31. (The Anderson study calculated the stream flow during a 100-year flood event at 18 locations on Boulder Creek. They ranged from 11,650 cfs at the mouth of Boulder Canyon to 13,300 cfs at the point where Boulder Creek crosses 61st Street.)

Boulder Creek’s maximum flows during last week’s flood were reported to be around 5,000 cfs, both early Thursday after a wall of water entered the creek from Fourmile Canyon and again Thursday evening when a 5,300 cfs flow was measured at 75th Street.

The stream flow during last week’s flood was less than half the flow the city has historically calculated would occur during a 100-year flood. According to the Anderson study, a 5,300 cfs flow last Thursday fell a little short of the estimated minimum flow during a 25-year flood (a flood that has a 4 percent chance of happening in any given year) on Boulder Creek.

If you think last week’s damage was bad, think what might have happened if Boulder Creek was running at 11,500 cfs instead of 5,300 cfs.

This matters, because if public officials and the public at large start thinking that the damage inflicted during last week’s storms is what can be expected to occur during a true 100-year storm, the chances of making bad land use decisions, like allowing major new construction in flood hazard areas, or bad emergency management decisions, like failing to order evacuations of low-lying areas in a timely manner, become much greater.

It also matters because Boulder has a long history of allowing major public and private construction along the creek — even though the risks of doing so have been known since 1894, when Boulder experienced a true 100- year flood (with an estimated peak flow of 13,000 cfs). Boulder’s library and city hall, Boulder Community Hospital’s Foothills campus, Boulder High School, CU married student housing, the CU research campus, the city-county court complex, Ball Research and Corden Pharma are all built in flood hazard areas. Some of these were approved after their builders agreed to take steps like putting up berms or raising the elevation of their sites with fill to get them out of the floodplain to mitigate the flood hazard — which may or may not prove adequate when a real 100-year flood (or larger flood) comes along. In the case of some of the older ones, like Boulder High School and CU’s Newton Court married student housing, the builders chose to blow off the risk.

Actually, compared to what could have happened last week, Boulder was incredibly lucky. The 12.2 inches of rain that fell from Monday to Thursday last week was almost identical to the amount of rain that fell in Big Thompson Canyon during the 1976 flood that killed 144 people. The difference is that Boulder’s 12 inches were spread out over four days, whereas the Big Thompson storm dumped 12 inches into the canyon in a single night — including 7.5 inches that fell in an hour. The single-hour downpour produced a flood that was running at 31,200 cubic feet per second at its height; it’s considered to have been a 500-year flood if memory serves.

There’s no hydrological or meteorological reason why a similar downpour couldn’t have occurred in Boulder Canyon or couldn’t occur there in the future — it’s just the luck of the draw.

If Boulder had gotten two-thirds of last week’s precipitation in an 8-inch-an-hour-long gully washer, the resulting flood on Boulder Creek might have exceeded a 500-year flood on Boulder Creek. Anderson Consulting’s study calculated that a 500-year flood on Boulder Creek would produce a peak flow of 21,000 to 27,200 cubic feet per second. It doesn’t take a lot of imagination to predict the sort of havoc that sort of a flood would cause.

That in turn suggests a simple, common sense approach for future floodplain management in Boulder: Don’t build in it.

Respond: letters@boulderweekly.com

This story is part of Our Road to Recovery, our coverage of the 2013 Boulder County floods.

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It was major luck our "1000-year" rain event was so spread out over time that the flood (in the City of Boulder) was only "25-year."  It only took 6" of rain (compared to last week's 15 ") to cause the 1894 Great Flood which severed all Boulder Creek bridges, because snowmelt added greatly to the Great Flood. 

Question: If Boulder is again cut in half by another 100-year flood, will our utilities function? I bet not.

Should we start re-working our sewage, gas, electric systems to function independently in a North Boulder cut off from South Boulder? Are there areas where we should be converting to composting toilets because geography and gravity conspire to make sewers back up through toilets in these areas?

"The white man gets water from a hole in ground; shits in river. Indian gets water from river; shits in a hole in the ground."



This fully validates my assertions - backed with photos and stats - made in comments on several Daily Camera articles mis-attributing "100 year flood" to this recent event.

I observed the Gilbert White gauge at Central Park, and used line-of-sight from the gauge levels and high water marks in the area to determine that we didn't reach a 50 year flood level at that spot.

I don't understand the impulse to over-inflate the event. This miscalibrates expectations and makes the public ignorant about how bad / big an actual 50 year or 100 year flood can be.  We simply have not had one since 1894.

This isn't to diminish the trouble and loss this recent event caused, but rather, to highlight that a 25 year flood caused a great deal of damage and real tragedy.

A true 100 year flood would have swamped everything up to Spruce street in the downtown area alone.  (The Mall included). Boulder Creek didn't even breach Canyon in this event. The water on Canyon was from other catchments.

Given the amount of precipitation, we're lucky the rainfall was less concentrated than it was in the 1894 event, and lucky we didn't get much more  concentrated rain after full soil and infrastructure saturation / capacity was reached.

This is testament to good design and infrastructure on many of Boulder's waterways (active and dormant), as well as storm drainage. I'm often critical of City Council, but kudos to the City in general. This event had real potential for worse consequences. The engineering helped avert the worst.




I agree with most of what you're saying, but the point i'd like to make about relying on just a gauge reading for determining a hundred year flood is the amount the time of year could affect that. 


Like Evan said about the 1894 flood, it was greatly added to by snowmelt. and the difference between a snowmelt and rain flood versus just rain(what we just experienced) is that in the former a larger percentage of the flood waters are actually in the creek systems since the water is coming from higher up because that rain is causing snow to melt even faster. In the latter the source was more spread out so boulder creek is recording low for the amount of water actually coming down. If that rain had happened in mid may during peak run-off i bet that gauge would have spiked towards 10,000, especially since it would have started around 700-900 cfs.... just something to think about


...let's think it through. Boulder Creek flow during the spring runoff is typically in a peak range around 2000 CFS. Even if we add that to the 2013 flood peak of 5000 CFS, you get to 7000 CFS, not 10,000. Even if we subtract the snow melt runoff from the 1894 event, taking it down to 9000-11,500 CFS, the 1894 event would still be substantially higher. ...or, do both: Take the snow melt out of the 1894 event AND add the snow melt to the 2013 event, that still puts the 1894 event 2000 CFS higher. We had 4 consecutive days (96 hours) of rain during the 1894 flood, 5 days of rain for the 2013 flood. The area impacted by the precipitation in the 1894 event was roughly comparable to the area impacted by the 2013 event. "1894: May 29 - June 2. Heavy rains fell over the mountains extending from the Colorado-Wyoming border southward into the Republican and Arkansas river basins. Rainfall over the Boulder and South Boulder Creek basins was particularly heavy. Rainfall records for a 96 hour period ending at 3 AM on June 1894 show that the mountain drainage area received from 4.5 to 6 inches of precipitation. Rainfall amounts over the high plains gradually decreased from west to east varying from 5 inches at Boulder to approximately 2.5 inches at the mouth of Boulder Creek. The mountain rainfall, combined with the snowmelt runoff to produce the greatest flood known at Boulder, which came roaring down the valley during the night of 30 May 1894. Buildings, bridges, and even long sections of roads and railroads were washed away. Although damages were extensive, a dollar amount was not estimated at that time." From US Army Corps of Engineers, August 1969 Floodplain Mapping of Boulder Creek. Information compiled by Elizabeth Black." 1894 was worse, in part, because there was no dam. However, there's no denying that the Boulder Creek flow was a 25-year flood, vs. the 100-year flood of 1894. Given the burn areas in the catchment and the higher level of precipitation (one extra day and several more inches), the 2013 event was well-mitigated by infrastructure - and yes....spread out a bit more over time.



Well, here's an obvious question - if this rainfall was more intense than any in a 24 hour period, and if the rainfall from the storm as a whole was larger than any in the last 100 years, why wasn't the flow of water also greater than any other?  Where did that water go?  Why did it not create a '100 year flood' by those measures?



A link to the source of all this information:



The snowpack in 1894 was very large one.