Opinion: Right choice made in face of pressure on flood control


On Tuesday night, the Boulder city council chose the best approach to mitigating the risk of future flooding on South Boulder Creek. But, given our situation, there are no perfect solutions.

First, Boulder is one of the most flood-prone cities in the country. We have numerous creeks that run through the city. And being up against the foothills allows sudden storms to dump rain into the drainages that flow directly into town with almost no chance to dissipate.
Second, some of Boulder’s drainages are huge. For example, the South Boulder Creek drainage above Colo. 93 is about 100 square miles. One inch of rain throughout that area would drop over 5,000 acre-feet of water, about five times the storage capacity of the measures under consideration Tuesday night. (An acre-foot is one acre covered with one foot of water, or 43,560 cubic feet.)
Third, much of Boulder was built without serious consideration of the flood risk. South Boulder is not the only area subject to severe impacts, though it is one of the worst. Even tiny King Gulch, which runs next to where I live, flooded almost every house in my immediate neighborhood in 2013.
Fourth, because of climate change and global warming, storms are getting more severe and occurring at more varied times of the year. This means that even the “500-year design storms” that are used to plan infrastructure improvements are no longer realistic portrayals of what might happen. As we now know, severe storms can occur in the spring, when the creeks are already running high and so have little or no extra capacity, and as part of saturation events when the ground cannot absorb more, so all the rain runs into the creeks.
South Boulder Creek has its own particular issues. It can easily spread out and flood large areas, so containing it is difficult. The channel itself doesn’t have anywhere near the capacity to carry the potential floods. And there is the multi-jurisdictional problem, with the University of Colorado having bought 308 acres of land, much of which is in the flood-prone area.
Given all the above, the only realistic approach to mitigation seems to be to try to detain the flood waters upstream, south of U.S. 36, so that they can be released slowly after the storm has passed. Making such an approach work depends totally on having sufficient volume in which to store the water. And we’re talking a lot of water: The “ponds” for Variant 1 and Variant 2 — the alternatives under consideration on Tuesday — were designed to hold 1,100 acre-feet and 1,000 acre-feet respectively.
The other difference between the two variants is that, in an attempt to allow CU to develop its land while still storing something close to the same amount of water, Variant 2 relies on building the flood walls about six feet higher but covering only about 105 acres versus the 140 acres of Variant 1. In order to fill this smaller area to the greater depth required to store even the lesser amount of water, Variant 2 would have to constrict the flow under U.S. 36 by lowering the top of the opening under the bridge to below the flood water level and also constricting the widths of the passages. This would force the water level higher (but still below the roadway) and push water into Variant 2’s smaller “pond” to a significantly greater depth.
All of this sounds fine in theory, but the big issue with Variant 2 is the bridge constriction. Because water would pile up at the bridge, debris could accumulate and potentially block the opening. Then water would flow much faster into Variant 2’s smaller “pond,” filling it much more rapidly. Once the pond was filled, the water would overflow into the developed areas of South Boulder. And since less water would be going down South Boulder Creek than if nothing had been done at all, the flooding of the neighborhoods would be much, much worse. This unacceptable risk is apparently why six of nine council members supported Variant 1.
I applaud these council members for doing the right thing, in spite of a lot of pressure to support Variant 2. They also recognized that more needs to be done, so they are looking at possible expansion of containment areas upstream, improvements to the pond’s drains to handle more containment, early warning systems to provide flood alerts, downstream channel improvements, etc. And CU still has plenty of land to build their necessary housing.


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