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Thursday March 30th 2023

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Ten Lessons for the City of Boulder from the Great Flood of ’13


Fourmile Canyon Creek jumped its banks to the north in several places west of Broadway, wrecking havoc down through light industrial areas (all photos courtesy Elizabeth Black)

  1. The “1000 year flood” is media hype; believe it at your peril.  Some smaller drainages had perhaps up to 500-year floods, but Boulder and South Boulder Creeks through town were at 40-year and 75-year levels by early estimates.  Stay tuned for the city’s release of official flood numbers for your drainage way.  Make sure you understand how big a flood you actually survived, and your future flood risk. Purchase flood insurance and understand its limitations, if you are at risk.
  2. The most common story of flood damage in the city starts with the phrase, “I had a sump pump but…” Test and maintain your sump pump at least annually; quarterly is better.
  3. Twomile Canyon Creek killed again, taking two more lives.  It also killed two in 1909.  Pine Brook Hills, Wonderland and Newlands residents: Remember, your creek is a killer!  To prevent more deaths: close Linden Ave. top and bottom during large storms, educate new residents, and keep telling the stories of the 4 deaths.
  4.  We were lucky: the Great Flood crested at night when folks were asleep, not when folks were out driving.  Most flood fatalities are motorists washed downstream while attempting to cross flooded roadways.  Let’s fix the important access roads and bridges that were flooded this time, so that next time, motorists will not be swept off roads into the creek.
  5. Drainages with established, improved floodways saw less damage than drainages with sketchy, non-existent floodways.  Twomile and Fourmile Canyon Creeks wreaked havoc in North Boulder neighborhoods around their tiny ill-defined channels.  Goose and Bear Canyon Creek neighborhoods, with major channel improvements, saw relatively little damage. Let’s keep establishing floodways for our remaining missing links.

A falling Fourmile Canyon Creek at Broadway crested well over the top of this bridge and deposited the debris in the foreground on the roadway and on the bridge railing

  1. Flood modelling is an inexact science.  Sometimes the modelling accurately predicted where the Great Flood would go, other times not at all.  Computers cannot yet completely model flood chaos: plugged culverts, boulders filling channels, irrigation ditches sending water to new places. We must redo much of our flood modelling because of new information and debris in creeks.
  2. Irrigation ditches can both protect from and exacerbate flood damage in unexpected ways.  The Great Flood overwhelmed closed ditch headgates, sending large flows down previously empty ditches.  Ditches along hillsides caught sheet-flows from above, protecting houses below the ditch.  But then a gully-washer or low spot on the bank overtopped the brimming ditch, sending the collected water down on whatever was below.  Let’s include irrigation ditches in flood modelling, and convene ditch companies with city staff to better understand irrigation-related flood problems and solutions.

The Great Flood overwhelmed the closed headgate of the Boulder and Whiterock Ditch in Central Park

  1. Our bridges, roads, property lines, floodways, sewers, municipal water infrastructure, agricultural irrigation and many recreational amenities are built assuming creeks will NOT move.  Many of us erroneously thought creeks-staying-in-the-same-place was bedrock to build our society on.  But the City of Boulder is built on multiple alluvial fans formed by the 14 drainages transecting it.  Creeks like ours gather lots of water, debris and energy as they rush down confined mountain canyons during floods.  As they exit onto the plains, they tend to whip around like the loose end of a fire-hose.  They fill their current channel with flood debris, and then jump to a new location, forming a new channel.  In future floods, they may jump to a third, fourth or back to the first location, constantly changing and building up the alluvial fan.  Our society cannot tolerate the incessant destruction and rebuilding of infrastructure as would ensue, were we to let our creeks keep changing channels. We have invested too much to change course now, and must finally pay the price of our choices.  We must do the hard and expensive work of keeping city creeks in their primary channels.
  2. The Great Flood was much more than just water.  It included trees, mud, gravel, boulders, trash, floating vehicles and more.  Often debris caused more damage than water. We can’t detain all the water of future floods, but we can limit future property damage by detaining debris.  We now know where debris likes to settle: the Evert Pierson Kids’ Fishing Pond and the Violet Park-site on Fourmile Canyon Creek.  Let’s design and build similar multi-use areas into our floodways, to detain and facilitate debris removal after future floods.

The Great Flood included trees, mud, gravel, boulders, trash, floating vehicles and more. It blew open storage lockers along Broadway and swept their contents down into North Boulder neighborhoods

  1. Some floodways are now so filled with debris that they can no longer carry flows for which they were originally designed.  Structures formerly out of the flood plain may no longer be.  We must remove debris from floodways, returning them to their full carrying capacity, so we will be protected from future floods.  This is expensive, controversial, and means ripping up vegetation along greenways to remove new gravels.  It also means earth-moving on Boulder’s sacred Open Space and masses of boulders/gravels to dispose of.  But riparian vegetation grows back quickly if replanted.  And since city residents have generously paid for its protection, Open Space is now obliged to help city residents in their time of need, to protect us from future flood impacts by re-establishing certain at-risk waterways leading to and from town.

I will gladly pay more on my utility bill to expedite this expensive laundry list. Will you? What are your Great-Flood-Lessons?  Let’s talk about it.

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