Saturday, August 12, 2017

The Fairbanks Flood of 1967: The Rainfall

Hi, Rick T. here. You don't have to live in Fairbanks very long to hear about the great flood of August 1967. Even for a community long accustomed to significant flooding, this was extreme. Something like 95% of the city was flooded, causing millions of dollars in damage. Rasmussen Library at UAF has posted a number of videos from the flood on Alaska Film Archives YouTube channel. The NWS Fairbanks Forecast Office and the Alaska-Pacific River Forecaster Center have also put together a very nice online storybook with a short description of the meteorology and hydrology and lots of photos. There has also been work by the NWS, the City of Fairbanks and FNSB Borough to survey and put up high water mark signs around town, and many of these have been installed in the past couple weeks, such as this one downtown on the north bank of the Chena River.


In this post, I want to look at the rainfall that lead to this flooding from a climate perspective. The rainfall August 11-13, 1967 stands out as the highest of record for daily and multi-day totals. The only rainfall records this event does not hold are short duration records, which are all thunderstorm related. To set the stage, here is the background: the second half of July 1967 saw well normal rainfall: 3.07" fell between July 16 and 31. That's still the fifth highest "second half of July" total. However, that was followed by a dry start to August: only 0.02" of rain fell during the first week of the month. But then the skies opened. Below is a plot of the hourly and cumulative rainfall for the week of August 8-15, 1967 (data extracted from Fairbanks August 1967 Local Climatological Data). More than half an inch of rain fell on the 9th. This was followed by about 36 hours with very little rain. Starting late in the afternoon on the 11th, moderate rain fell without much of a break until the morning of the 13th, though light rain continued to dribble on into the 15th before the fire hose finally shut down.
The following totals were recorded at the Airport, all of which still stand as the highest of record:
  • 24 hour: 3.44" 11pm AKST August 11 to 11pm AKST August 12
  • 36 hour: 4.40" 4pm AKST August 11 to 4am AKST August 13 
  • 48 hour: 4.76" 3pm AKST August 11 to 3pm AKST August 13
  • Single calendar day: 3.42" August 12  
  • Two consecutive calendar days: 4.29" August 11-12
  • Three consecutive  calendar days: 4.98" August 11-13
So beyond "highest of record", what's the climatological context? Was this a one-in-a-million event, or is there some reasonable likelihood it will be broken?  To answer this I've compiled annual extremes of a few of these parameters and then fitted a generalized extreme value (GEV) distribution. If that's greek, no worries: GEV is a standard technique for analyzing extreme event frequency and generating estimates of return periods.

Maximum 24-hour precipitation (not necessarily calendar day) has been recorded in Fairbanks since the Weather Bureau office opened in the summer of 1929. For two and three day totals, I've included the cooperative data from the Ag Experiment Station starting with 1915, when daily precipitation began to be regularly recorded. In the graphic below I show an example of the annual times series, in this case the maximum 24 hour precipitation (upper left) and then the GEV analysis for the annual  maximum 24 hour precip (upper right) and annual maximum two and three day consecutive days (bottom row). The red line shows the fitted return period, while the open circles are the observations (which are plotted in rank order).  I should point out that there is no significant trend in any of the annual values.
So what are the return periods for the precipitation amounts that occurred in August 1967?

With  87 years of data:
  • 3.44" in 24 hours is expected to occur on average once in 203 years
With 102 years of data:
  • 4.29" in two consecutive days is expected to occur on average once in 269 years
  • 4.98"  in three consecutive days is expected to occur on average once in 352 years
I'm actually not a fan of return periods. It's not technically wrong, but many people would look at those numbers and say "gee, I'll never see that." In fact, the return periods are just an alternate way of expressing the probability of occurrence. So if I tell you that there is 13% chance that Fairbanks Airport will receive precipitation totaling 3.44" or more in 24 hours hours once in the next 30 years, that's equivalent to saying the return period is 203 years, but the take-home message is different. Now 13% in 30 years is not high (and that makes the dubious assumption that there is no change in extreme precipitation events in a warming world), but it is hardly unthinkably rare.

Postscript:
The GEV analysis I've presented here differs somewhat from that published in the 2012 NOAA Atlas 14 primarily in that I used a longer period of record (for extremes analysis, the longer the better), and, as near as I can tell, the maximum 24 hour precipitation (as distinct from calendar day) was not used in compiling that work.

The maximum 24 hour precipitation for August 1967 is listed as 3.42" in the August 1967 Local Climatlogical Data publication and has been carried through ever since. In fact the hourly precipitation data in the same publication shows that the correct amount is 3.44", 11pm on the 11th to 11pm on the 12th.



4 comments:

  1. Hi Rick here's some comments and some questions please.

    When they built the Pipeline engineers used a "Q" value for potential flooding...like Q-100 (years) for probability of exceedance...to construct protective spur dikes and whatever. How did they come up with that value?

    Obs from co-workers indicated the Little Chena River was a major source of flooding in addition to the Chena and Tanana. That drainage is downstream of the current flood control project.

    They said when the flood water finally came up out of their toilets it was time to abandon housing.

    What weather event caused the heavy rains? What was the fetch...SW>NE?

    When I arrived here from Sitka two weeks later to attend UAF the heroic Fairbanks folks were drying out and preparing for a cold winter. That was the real challenge as it was a cold winter that followed.

    Gary

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    1. I first missed but then read the "online storybook" you linked...explains much of my questions. Thanks.

      Gary

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  2. Very nice, Rick. Thanks for posting.

    Another aspect of return period that (it seems to me) can be misinterpreted is that it's typically valid only for a single point. For convective storms in which totals can vary a lot over a small distance, we would expect a larger area (e.g. a city or drainage basin) to have events of at least X inches more frequently than each individual location.

    For example, the return period for 2" of rain in 3 hours at any single point in the Fairbanks area may be 500 years, but it's bound to occur more frequently "somewhere within the city". Presumably the storm water hydrologists and civil engineers take account of this.

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    1. Excellent point Richard concerning the point vs. areal return frequency. Important to keep in mind that the above analysis is valid only for the Airport. There's a reason we often jokingly refer to the Fairbanks Airport as the Death Valley of Fairbanks-land: except in the occasional convective situation, the Airport is usually the driest place around. And Aug 1967 was surely no exception: based on the amount of water that came down the Little Chena River along, there must have been far more rain (as is usually the case) north and east of town.

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