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Upper Goose Creek and Twomile Canyon Creek Floodplain Mapping Update

Upper Goose Creek and Twomile Canyon Creek Floodplain Mapping Update

The City of Boulder has updated the floodplain mapping for Twomile Canyon Creek and Upper Goose Creek. The updated mapping was approved by City Council on July 28, 2015. FEMA approved the mapping with an effective date of August 18, 2016.

To review the changes in floodplain designation and for more information about the study, please see links on the right side of this page.

Currently, these drainageways are undergoing a flood mitigation study and alternatives analysis. Please see the mitigation plan web page for more information. 


Project Background

The city contracted with ICON Engineering to develop updated floodplain maps for Upper Goose Creek and Twomile Canyon Creek. Anderson Consulting Engineers provided a peer review of the engineering study. The Urban Drainage and Flood Control District (UDFCD) is a co-sponsor of this project.

Floodplain mapping studies provide the basis for flood management by identifying the areas subject to flooding. This information is essential for determining areas where human safety is threatened and property damage is likely. Floodplain maps must be updated periodically to reflect changes in the floodplain resulting from land development, flood mitigation improvements, new study technologies and the impacts of major floods that may have occurred.

Upper Goose Creek and Twomile Canyon Creek were first studied by Greenhorne & O'Mara in 1987. The resulting Flood Hazard Area Delineation (FHAD) delineated the 100-year floodplain along these creeks. The Flood Insurance Study (FIS) and Flood Insurance Rate Map (FIRM) approved for these creeks were originally based on the 1987 FHAD and included a federally-regulated one foot rise floodway. In 1989, Love and Associates delineated the High Hazard Zone and City of Boulder Conveyance Zone (1/2 foot rise floodway). The delineations were based on the hydraulic models used in the 1987 FHAD.

In 1991, the city commissioned Love and Associates to restudy Twomile Canyon Creek. Based on that study, FEMA issued a Letter of Map Revision (LOMR) for Twomile Canyon Creek on April 5, 1994.The limit of the LOMR was between the confluence of Twomile Canyon Creek and Upper Goose Creek to just upstream of Linden Avenue. In 2004, McLaughlin Water Engineers, Ltd. updated flood mapping for Goose Creek to reflect channel improvements. The limits of the LOMR for this project extended from 30th Street to approximately 1,900 feet upstream of Folsom Street.

More Information

The Upper Goose and Twomile Canyon Creek Mapping Study was submitted to and was approved by FEMA as a Letter of Map Revision (LOMR). Traditionally, the city submits mapping updates as a Physical Map Revision (PMR). The main difference between a PMR and a LOMR is that a PMR changes the physical FIRM map and the LOMR is more like an attachment to the FIRM map. Find out if your property is located in a floodplain using the Interactive Floodplain Map. For property-specific information about the land development requirements for all floodplain zones, contact Planning and Development Services at [email protected] or 303-441-1880.

For information on Map Revision Process Options, please visit FEMA's webpage at:

If your flood insurance provider is requiring proof that your property has been removed from the floodplain, the LOMR Approved by FEMA pdf may be adequate documentation. However, the city can provide a floodplain information request stating that the property has been removed on city letterhead.  Please fill out the floodplain application form and follow instructions for submittal on the form. The process costs $28 and takes about two business days to process. Requests that are paid for by credit card must be dropped off in person or faxed to ensure that credit card information is secure.