Longmont, Colorado

Drainage project completed just in time for a major flood event

In 2012, the Left Hand Creek channel in Longmont was expanded, removing 110 properties from the Special Flood Hazard Area (SFHA) and reducing the risk of flood damage to a key highway and the downtown area. Nine months after the expansion was completed in December, a 1-percent-annual-chance flood hit the town—the worst flash flood in the city’s history. Residents were evacuated as creeks overtopped, but 87% of the 204 homes in the SFHA experienced less or no damages.

Photo by Patsy Lynch - FEMA

Community facts

Population
90,719
Homes
35,802
Businesses
9,675
Source: US Census Bureau.

Mitigation actions

Longmont had not seen a 1-percent-chance-annual flood since the late 1960s, but city leaders knew it could happen again. In 2010, the city proposed the Left Hand Creek flood project as a pre-disaster mitigation project. Some community members were initially resistant to the project, owing to concerns about the environment and damage to recreational paths.

The project was jointly funded by the city’s monthly residential and commercial stormwater fees and by a FEMA grant. In total, the project cost $5.7 million and took nearly a year to complete. A year after completion, in 2013, the city experienced a 1-percent-chance-annual flood event. The total losses avoided as a result of the mitigation project were tallied at $22.5 million, the majority of which were savings in building, content, and displacement costs. The figure also includes losses from mental anxiety and loss of productivity, which were assessed to be over $1 million.

Benefits of mitigation

Economic benefits
$22.5 million
Total monetized benefits
Return on investment
291.0%
Estimated return on investment
Benefit-cost ratio
3.91
Benefit-cost ratio
Source: FEMA (2015). Reducing Losses through Higher Regulatory Standards: 2013 Colorado Floods Case Study.

Project challenges

When the project costs became larger than expected, federal grants helped cover the cost

The city had originally planned to build $800,000 worth of new bridges along the South Pratt Parkway to divert the damaging effects of the floodplain. However, once FEMA released new flood maps for the area, Longmont determined that the mitigation project would have to be far bigger and more costly to have the same effect. The city did not have the funds to cover such a costly project, but it successfully sought funding from FEMA and moved forward with the project. In the end, the project encompassed channel improvements that increased the flow capacity of the channel and of two culverts at road crossings.

“It was an extremely successful project. Very rarely do you have the opportunity to see the benefits of your project so quickly”
David Hollingsworth, former floodplain manager, City of Longmont, Colorado

Future considerations

A bridge over a road in Longmont, Colorado, shows extensive damage due to flooding.
Photo by Patsy Lynch - FEMA
The community continues to invest in flood mitigation

Most of the community had never experienced a major flood event, according to Hollingsworth. By providing data at community meetings about floodplain risks and the impact a 1-percent-annual-chance flood would have on the area, city leaders were able to make the case that this mitigation effort was important. Longmont is currently pursuing a program of flood mitigation projects along St. Vrain Creek to reduce the size of the floodplain. Projects include resizing the creek channel and restoring the St. Vrain greenway. The timeline for the Resilient St. Vrain program is 7-10 years, and the projects are estimated to cost up to $140 million. The city continues to seek grants and other financial partners to help fund the projects.

Flood mitigation resources

Below is a non-exhaustive list of other flood mitigation information resources.