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Panorama of The Grand Tetons From the top of Table Mountain, Wyoming © Alan Holyoak, 2011

Thursday, July 17, 2014

The Rexburg Idaho Flood of July 15 2014 - What happened!?

During the late afternoon of Tuesday July 15, 2014, scattered showers accompanied by hail started to fall in Rexburg, Idaho.  It wasn't long before the showers became a full-blown thunderstorm of Midwestern or Great Plains proportions!

It is unusual for rain showers in this part of the state, even heavy ones by local standards to last more than 5-10 minutes before prevailing winds push them farther up the Snake River Plain.  This time, however, the storm which was extremely localized and extremely heavy stalled out right above the city of Rexburg for about 40 minutes.

Weather radar (see below) showed that the storm was located directly above Rexburg, and areas just a few miles north and south of the city received no measurable precipitation.  Official reports from an automated weather station at the Rexburg Municipal Airport reported a total of 1.14" of precipitation, but unofficial reports from higher elevations at the south end of town suggest that over 2" of rain probably fell during the storm.


The downpour produced flash floods through the city and immersed low-lying intersections and residential areas.  The massive amount of water that fell also produced back pressure in city storm drains and sewer lines that caused water to gush upward through drains and toilets into buildings and homes.

This was by all accounts a once-in-a-generation weather event for the city.  I moved here in 2002 and in all that time we never had a downpour like this one.  Life-long residents say that you have to go back over 30 years before you would come across a comparable rainstorm.

Click on this link to see images of the aftermath:

http://www.buzzfeed.com/davarav/the-12-things-you-saw-at-the-byui-flood-last-night-ulbz

You might be thinking, "Come on, one or two inches of rain doesn't sound like all that much," especially to people who live in areas where this kind of storm is relatively routine.  So why all the flooding?  What happened!?

Here are a few basic facts:
  1. Rexburg is located in eastern Idaho, an area that typically receives only about 15" of rain annually.
  2. Most of Rexburg is built on level ground at an elevation of 4850', while the SE quarter of town is built on a gradual rise topping out at 5150'.
  3. Because Rexburg is built in a semiarid area, it is not engineered to accommodate large amounts of rain runoff from a storm like this one.  It is, however, designed to accommodate significant amounts of water from snow melt runoff in the spring that are released more gradually.  Ironically, the city was in the middle of installing an upgraded storm drainage system in the SE corner town designed to reduce surface runoff when the storm hit.
  4. A significant amount of new residential building in Rexburg has occurred at the higher elevations, thus greatly increasing the proportion of area covered by roads, driveways, sidewalks, parking lots, etc., and reducing the amount of green space that is good at soaking up precipitation.  
OK, back to what happened...why the flooding?

This is a topographical map of Rexburg, Idaho (click on the map for a larger view):


The arrows on the map below show two of the tracks where water flooded most severely - remember, surface water run-off is mostly going to follow roads in a situation like this (click on the map for a larger view):


If you look at the topographical map below, the bottom-right corner has the highest elevation, and the hill slopes off to the NW.  OK, find the circular water tank located just south of the center of the photograph.  The four houses on the south side of the road, just east of that water tank, were among the most heavily damaged in the city.   Why?  (Click on the map for a larger view.)


I admit that I'm no hydrographer, but I watched this storm develop and watched the water flow from my house, which is just around the corner from those houses.  

This is probably what happened, at least in this localized area.

First of all, the storm was extremely intense, dropping a significant amount of water in a very short time - more than this area hardly ever sees in a single storm.  If you follow the topographic map's contour lines you will see that they run generally from the SW to the NE corners of the map.  Unless otherwise constrained, surface runoff water will flow downhill perpendicular to those lines.  

Rain that fell on the upper part of town quickly filled the storm drains so lots of the water moved as surface runoff.  Look at the housing development just north of the "5150" contour line marker.  This housing area slopes downward to the NW, and all roads in in converge at a low spot where the road seems to bulge upward toward the NW.  There is a walking path easement there that passes between that housing area and a neighboring apartment complex.  Water from the housing development was forced through the narrow opening along that path between neighboring houses and joined water that fell on the apartment complex's lot.  I walked that area again last night and saw that grass there was literally abraded down to nearly nothing by flowing water, sand, gravel, etc., as the water swept through in sheets across the parking lot.  The vulnerable houses were located directly in at the path of this flooding water.  Water flooded down into these houses' back yards, into basement window wells, broke those windows inward, and flooded the basements all the way to the ceiling before also breaking out basement windows on the sides and front of the house.

Of course this water was not done moving there...it was subsequently channeled largely along streets and roadways in the northerly and westerly directions.  The next thing all this water hit was the BYUI-Idaho campus where water gushed along sidewalks and walkways, streets, into quads, and even through some buildings.  This closed campus for the rest of that evening, but it reopened the next day.

Water continued to flow downhill to the lower part of town, flooding out virtually all of Main Street and many low-lying areas and intersections as water from the hill joined water that fell over the rest of town.

In the meantime, a second area hit with intense flooding was the "Hidden Valley" housing development.  This development is indicated by the blue arrow along the right side of the image above.  There's a reason it's called "Hidden Valley"...it's built up into the mouth of a valley.  That valley exists because water from fields above drains through there.  Of course this area was hard hit because water from the entire area above that development was channeled right down through the middle of the valley!  A friend who lives there has a home that backs onto a shallow drainage depression designed to channel moderate amounts of water away from homes there.  He said that by the end of the storm that the depression had been eroded into a gully seven feet deep! I heard secondhand reports that many families were displaced from their homes in that development.

Then while all of this was going on many of the student apartments around town also suffered significant flooding.  This stems largely from the fact that for whatever reason, it's been a tradition to build the first floor of student apartments so that their first floors are partially below grade.  This way walkways along these apartments put a person about 3-4' below grade (the level of the ground).  I saw MANY photographs posted showing flooding to the doorknob-level in these apartments.

Suffice it to say that this was a mess.

Interestingly, the overall attitude associated with this flood appeared to be a mixture of partying, fun, horror, and depression, depending on how you were affected.

As soon as it became clear just how bad things were in some areas and for some people, however, the community really pitched in to start to clean up.  The community response to this flood has been an inspiration to me.  As soon as the rain was gone and the floodwaters were passed, people were out all over town checking on one another to make sure everyone was all right and to see if there was any damage and pitching in to help.    

Sadly, people in this area do not have flood insurance.  Sure, if a pipe breaks and you get flooding or water damage that way in your home then homeowners insurance will cover it, but that kind of insurance will not cover things like this freak storm/flooding, since it falls under the category of "Acts of God" for which insurance companies are not accountable. 

There you have it...what happened during the Rexburg storm and flood of July 15, 2014. 

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