Opinion: A better way to address flooding in South Boulder

Flooding in Boulder will get worse as climate change increases storm intensity and frequency. Thus, making good decisions about flood protection is critical; they should be based on facts, not speculation or defensiveness about past decisions.

The assertion being made that the dam and floodwall currently proposed for South Boulder Creek will be “lifesaving” is a good sound bite. But it is not supported by the facts: No one died in South Boulder during the 2013 floods; the only deaths in or near the city were two people descending a flooded road in the North Boulder foothills. U.S. 36 is frequently closed by snowstorms and will flood anyway in storms larger than the dam is designed for. Boulder Community Hospital is accessible by many other routes. And the city can purchase a few high-water 4WD vehicles for emergency evacuations by first responders.

The City’s 2014 flood survey shows that the proposed dam and floodwall will only address a fraction of the flooding damage in South Boulder. South Boulder Creek itself caused $27.8 million in damage (less than half the total), and, per the city’s own maps, a large portion of this damage was downstream from the U.S. 36 bridge, on which the dam/floodwall will have no effect. Viele Channel flooding caused $12.2 million in damage, but the dam/floodwall would provide no protection, because that water runs around the dam’s north end. Bear Canyon Creek caused $18.3 million in damages, but, again, that water bypasses the dam/floodwall also. 

The city’s maps showing buildings protected from flooding are, I believe, misleading because they only consider the flooding from a “design storm” that is only over Eldorado Springs and of limited duration. Thus, they do not include flooding from Viele Channel and Bear Canyon Creek. 2013 videos of the standing waves in the floodwaters coming down Table Mesa Drive show how important these other tributaries are. 

Rather than building the proposed 30-foot tall high-hazard dam south of Table Mesa Drive and a 2,300-foot-long flood wall along U.S. 36, it would make a lot more sense to trap flood water from South Boulder Creek in the existing depleted gravel pit on the upstream/south end of the CU South property.

This upstream approach has many advantages. The water would be retained using a series of low berms running parallel to the contour lines, rather than a high dam and almost half-mile-long concrete wall, significantly reducing costs as well as the risk of failure, and will also limit flooding on U.S. 36. 

The detained floodwaters would drain directly back into South Boulder Creek before it flows under U.S. 36, rather than through large pipes tunneled under U.S. 36 as in the current plan. Those pipes would be expensive and likely require extensive trenching across all six lanes of U.S. 36, closing the highway. They also might cause Viele Channel to overflow and flood nearby neighborhoods.

This upstream approach also has the advantage that it does not impact the areas along U.S. 36 that have rare species. It does not require an untested system of pipes and valves under the floodwall to keep groundwater flowing to keep these plants alive, and which allegedly must be shut during a flood to prevent too much water from getting through because deeper floodwaters against the dam create too much pressure.

It does not require an impervious lining for the detention pond. Under the current downstream design, the groundwater level is so high that, without the lining, water would permanently fill the pond, making it useless.

Finally, the upstream approach will almost certainly be considerably less expensive than the current design, because it does not require the high dam and long floodwall, the fancy piping, the detention pond lining, etc. These savings could be spent floodproofing many of the buildings that cannot otherwise be protected or buying them out, as was done along the main stem of Boulder Creek after the 2014 flood.

This upstream approach of detaining floodwaters in a series of ponds in the existing gravel pit was brought up early in the process. But it was rejected apparently because it was alleged to be too expensive. But (per a friend who has studied this extensively) the city staff acknowledged that it never did a serious evaluation. 

I realize that seriously considering this approach would require the City Council to hit the pause button and acknowledge that some earlier decisions might have been made based on inadequate analysis or inaccurate information. And I hope that the new CU chancellor, an engineer with a strong interest in sustainability, would give evaluating this option his full support.