View a video of a presentation to the Boulder City Council concerning the unknown but potentially serious flood hazard in South Boulder that is described in a City report.

It has recently come to our attention that the flood hazards that we face in South Boulder are not being properly identified by the South Boulder Creek Flood Study.

The current study is looking at "mainstem" flooding along South Boulder Creek. The "design storm" that is being used is centered over the foothills west of Eldorado Springs. As the study group states:

Our focus was to define the most critical condition for floods along the mainstem of the Creek.

However, a preliminary investigation has been done by the City of the flood impacts from storms located over the urban portions of South Boulder. This work is summarized in the Lower Basin Storm Center report.

As this report states:

However, the team and the community’s technical representatives (the Peer Review Evaluation Panel - PREP) recognized that other types of flood hazards exist. Recognition of these potential hazards due to flooding off the mainstem is the purpose of this special investigation.

The study group came up with a different storm center, one located over South Boulder. The figure on the left shows the current 100 year design thunderstorm in use in the study. The figure on the right shows the "Lower Urbanized Basin Storm". The storm shifts 3-4 miles to the northeast.

Current Design Thunderstorm

Lower Basin Storm

The study group estimated the discharge within the sub-basins (shown in black outline on the maps above) and their conclusions were rather shocking. Of the 27 sub-basins within the watershed the 3 northern ones bear the brunt of the flood impacts:

Lower Basins

The table below is a summary of the discharge estimates for these lower sub-basins.

Sub-Basin	Design Storm (CFS)	Lower Basin Storm (CFS)	Difference
C1	25	651	2600%
C2	38	1,658	4300%
C3	20	461	2300%

The report describes these discharge estimates:

However, particularly for the lower urban storm center, the peak inflow from local basins may be 20 to 40 times higher than the design storm. These flows must also make their way to the mainstem but are very likely to create problems along the way.

As an example, the peak flow in basin C2 alone is over 1600 cfs, a discharge of over 40% of the mainstem flows and 40 times the tributary inflow from the adopted design storm. The systems in the tributary are not likely to handle these high flows and are likely to cause localized flooding, some of which may be severe.

In describing the flood impacts the report states:

In fact, in some cases, the highest hazard to properties located in the floodplain not immediately adjacent to the mainstem may be the result of a storm center at a location other than the one that was used to define the mainstem peak flow.

It goes on:

Flood hazard exists as a result of flows other than those emanating from the mainstem. In fact, as the discharge estimates in Table 2 point out, in some cases, the flood hazard from localized storms may far exceed the hazard associated with mainstem floods. While the evaluation of this tributary hazard is outside the scope of the current project, these impacts may be of greater concern to many of the residents within the lower portion of the watershed.

In the conclusion the report recommends:

It is important to remember that the flood hazard associated with localized storms falling on other parts of the watershed should be defined and factored into any floodplain management and flood mitigation strategies.

We wholeheartedly agree with this recommendation. The city should acknowledge this flood hazard and act on it. In particular the City will be faced with two major decisions in the coming year regarding mitigation and flood plain management.

Flood Mitigation Expenditures

There is 3 million dollars in the 2008 budget slated for South Boulder Creek flood mitigation. We agree that if there are flood risks to the residents of South Boulder then they should be mitigated if possible. However, lets make sure that we spend this mitigation money wisely and mitigate the flood impacts that bring the highest hazard to South Boulder. As the lower basin report describes the highest hazard may be the result of the lower basin storm:

While these conditions represent the mainstem flooding design condition, it may not represent the greatest risk to property off the mainstem.

We understand that the flood study has been driven by the need to redefine the FEMA Flood maps. However, mitigation expenditures should be driven by the need to reduce risk to life and property. Before the City spends millions of dollars it should make sure that they get they mitigate the flood that presents the highest hazard to South Boulder.

Land Use Decisions

The flood impacts described in the report are especially alarming in the context of the Hogan/Pancost development. As described in our discussion on general flood issues the Hogan/Pancost development is already impacted by and may exacerbate for the surrounding neighborhoods and schools the mainstem flood.

This development may be especially impacted by the lower basin storm. The following image shows the Hogan/Pancost property in blue and the Dry Creek Ditch Drainage corridor in green. As can be seen the Hogan/Pancost property is in the center of these two sub-basins and is adjacent to one of the major drainageways of those basins.

Basins C1 and C2 and Hogan/Pancost Property

With this largely unknown and undefined hazard that we face in South Boulder, any development or annexation proposals in the area must be put on hold until these risks are fully defined and understood.

City Responses

When brought to the attention of the Citys flood coordinator the response has been rather dismissive of our concerns. One of the responses was:

Oh, you see this is more of an urban drainage problem.

Well, in our estimation if a storm hits and put our families and neighbors lives at risk and damages our properties we don't care what its called. Flood hazard is flood hazard. After all, one could have characterized the Ft. Collins Spring Creek flood as an "urban drainage problem" and 5 people lost their lives.

One of the other responses has:

There will always be risks. Micro-bursts can happen anywhere.

This seems to simply characterize this storm as some fluky random event. We disagree. The City has spent more than 1 million dollars and years of effort in studying the mainstem flood impacts of the design storm events. Now, the impacts described in this report are simply from that same storm shifted just a few miles north. If one can describe and dismiss the lower basin storm as simply a "micro-burst" then it follows that one could also dismiss the main-stem storm as simply a "micro-burst". I don't think that the City should be so dismissive of these risks.