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September 2013 Flood

For people living in Southeast Boulder the September 2013 flood was more of a heavy rainfall event and not a "big flood" event. It was very similar to the 1969 flood - heavy long term regional rainfall. This isn't the big thunderstorm event that the FEMA maps are based on.

There was extensive basement flooding in Greenbelt and Keewaydin Meadows neighborhoods. For most people in Greenbelt Meadows the flooding was due to the heavy rain and the saturated ground overwhelming sump pumps, failed pumps and drainage problems (clogged gutters, window wells, etc). However, further north along Manhattan and to the west along Seminole and Iroquois the flooding was from both groundwater and, more importantly, sewer backup. On many blocks, including those adjacent to the Hogan-Pancost development, 100% of the houses had severe flooding.

Keewaydin Meadows was developed in the 1960s without storm sewers. The streets are relied on to carry surface flows. Furthermore, the City has stated that there is an underdrain system in the area that is intended to drain the excessive groundwater. This system has not been maintained by the City - they don't even know where it is located. This, along with all of the basement flooding, overwhelmed the sanitary sewers and resulted in major basement flooding. Many folks on Kewanee and Manhattan (and I assume Seminole, Aztec, Blackhawk, etc) had 3-6 feet of water in their basements.

Mainstem Creek Flooding
While this flooding brought an incredible amount of damage we actually ducked a bullet. Its hard to believe but both South Boulder creek as well as Boulder creek barely got to flood stage in this event. Roger Pielke, professor of environmental studies at CU has a blog post How fantasy becomes fact that delves into this topic.

The South Boulder creek stream gauge during the storm shows only a moderate flood stage. Likewise for Boulder Canyon. The creeks picked up further flood waters below the gauges.

southbouldercreek.jpg
bouldercreek.jpg

From a Urban Drainage and Flood Control District (UDFCD) presentation on the flood:

Boulder experienced the most rain ranging between 8-18", south Boulder receiving the most rain. This is reflected in the third image. Clearly the rate of precipitation (the amount of rainfall over time) is distinct from the rate of flow in the waterways. So while southeast Boulder experienced the highest rainfall, that cell was not over the upper south Boulder Ck and thus south Boulder Ck did not experience high flows. In contrast, you'll note that the cell over Boulder extended to the northwest and into the mountains and four mile canyon creek and other drainages to the northwest contributing to the 100-year event on four mile canyon creek.

Local Flood Behavior
In southeast Boulder the Viele flood channel didn't overtop, we didn't get any flood flow over South Boulder road and the Dry Creek ditch didn't overtop. The major difference in outcomes between 1969 and the 2013 event is the Viele flood channel - constructed in 1972 it is designed to route the flood waters coming in from the southwest and the south back east into the creek.

That said, the flooding could have been much worse. Late Thursday night the Viele flood channel was becoming bank full along South Boulder road and there was heavy flow around Greenbelt Meadows. The South Boulder creek never did reach the Greenbelt Meadows park. South Boulder creek behind the Greenbelt Meadows park was just starting to overtop and flow northwest into the Kent's property (it never did).

gbmflood.png

FEMA 100 year and 500 year flood zones

Along the Dry Creek ditch adjacent to the Hogan-Pancost property one resident describe extensive flooding:

The Dry Creek ditch did overrun it's bank and it did flood my back yard. we had 6-8" of water along the back 20 feet or so of my yard. all of this water came from the ditch overtoping the bank along my property line.

Photos of the Hogan-Pancost site taken on Thursday morning attest to the extent of the flooding. Its important to note that the major flood waters impacted the area late Thursday night. These photos do not depict the full extent of flooding on the property and along the ditch.

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Hogan-Pancost and Flooding
In all of these years of study we still don't know answers to very basic questions -
  • In the big 100 and 500 year events how much flood waters will flow across the property?
  • How will routing a 1000 foot wide floodplain into a 40 foot wide channel effect those flood flows?
  • How will local drainage on the site combine with these off-site flood flows?
  • Where does the water go when the flood channel overtops?

The City could easily have these answers. The data and the tools and the knowledge are all there yet the staff has steadfastly refused to act.

Would development on the HP site have made our recent flood worse? I think the answer is yes. Where now, most of the rain falling on the property stays in place and soaks into the 20 acres of pasture after development all local drainage on the site is routed to the northwest towards the existing homes. The Dry Creek ditch flows would be channelized and would have run much quicker towards Kewanee. Furthermore, with up to 5 feet of fill on the site they increase the "head" in that water so there is much more force. Likewise, you have 130 more homes tied into the sanitary sewers - sewers that we just saw do not have the capacity to handle the current load.

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Click to view contents EBCC Development Plans
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2013/11/21
2013/11/21
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2013/11/21
2013/11/21
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