Project Overview

The Twomile Canyon Creek drainage basin originates in the mountains west of Boulder near Pine Brook Hill Fire Station. The upper portion of the drainage basin is narrow, steep and rocky with a well-defined channel. The creek enters the City of Boulder and flows through a residential area near Spring Valley Road. Under normal flows, water is diverted into the Silver Lake ditch near the Linden Ave. and Wonderland Hill Ave. intersection. The South Juniper Lateral returns water into the creek near Foothills Elementary School. The rocky creek bed is approximately three to four feet wide, and ends on the west side of the intersection of Broadway and Iris. East of this intersection, water flows through a small ditch that runs along the north side of Iris and into a storm sewer. The storm sewer discharges into Elmer’s Two Mile Park, which flows into Goose Creek, just south of Valmont Road.

The Upper Goose Creek drainage basin is primarily composed of residential and commercial development, and is located just south of Twomile Canyon Creek. During minor storm events, runoff is collected in storm sewers and conveyed to a concrete lined channel at 19th St. and Tyler Rd. During major storm events, water naturally drains to the east side of North Boulder Park located on 9th St. From here to 19th St., floodwaters head east, along Balsam Ave., Alpine Ave. and through backyard areas, because there is no defined stream channel within this portion of the drainage basin. The concrete lined channel starts at 19th St. and continues for approximately 270 feet before becoming a more natural, vegetated channel. The stream channel runs south of and parallel to Edgewood Dr. which also carries flood waters. From approximately 600 feet upstream (west) of Folsom, Goose Creek has been improved to convey the 100-year storm event. The width of the channel is as large as 25 feet with 6 to 15-foot high channel walls.

Goose Creek Projects

Creek Assessments

City staff conducted physical assessments of Goose Creek and Twomile Canyon Creek in the fall of 2020 as part of the flood mitigation study. These assessments will allow staff to gain a detailed understanding of the current creek conditions and ascertain vital information.

If you received a right-of-entry letter request, please consider filling out the ROE Form and submitting the signed form by Nov. 13. In order to sign the form digitally, you will need to open it in Acrobat or you can print it out and take a photo of the completed form and email it to:

Goose Creek and Twomile Canyon Creek Mapping and Mitigation Information

Upper Goose and Twomile 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.

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 Mile High Flood District (MHFD) 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.

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

Goose Creek and Twomile Canyon Creek Flood Mitigation Studies

Current Status

The project is still in the mitigation planning phase. Staff is working with our consultant to develop a scoring matrix and benefit cost analysis. Creek assessments will be conducted in the spring of 2020.

Project Background

The City of Boulder is developing master plans to identify opportunities for flood mitigation improvements for Upper Goose Creek, and Twomile Canyon Creek. ICON Engineering has been hired as the project consultant team to assist city staff with developing and analyzing different mitigation options. Once completed, the flood mitigation plans will be a long-range planning tool to guide future flood mitigation projects in the area.

Several mitigation alternatives were developed and are being evaluated using the following criteria:

  • Life Safety (includes protecting vulnerable populations and providing emergency response capability)
  • Property Damage Mitigation
  • Water Quality Enhancement
  • Habitat Protection and Restoration
  • Operation/Maintenance Costs
  • Protection of Cultural Resources
  • Recreation Opportunities
  • Alternative Transportation Routes
  • City Construction Costs (including benefit to cost ratio)
  • Public Opinion/Community Acceptance
  • Resiliency/Adaptability to Climate Change and Uncertainty
  • Ability to Implement Quickly

Community Engagement

January 25, 2021 Water Resources Advisory Board Meeting

City staff presented the recommended plan to the WRAB which included proposed improvements based on the alternatives analysis and community engagement that was completed as part of the flood mitigation study.

March 18, 2019 Water Resources Advisory Board Meeting

City staff presented an update on the Upper Goose and Twomile Canyon Creek flood mitigation study to the Water Resources Advisory Board at their March 18, 2019 meeting. At this meeting, staff informed the WRAB how community feedback on the initial flood mitigation alternatives has helped shape the study process and discussed revisions made to the alternatives being evaluated.

View the original conceptual alternatives for Upper Goose and Twomile Canyon Creek, including maps and imagery.

March 19, 2018 Open House and Water Resources Advisory Board Meeting

City staff, engineering consultants and a representative from the Urban Drainage and Flood Control District were available to explain the flood mitigation alternatives under consideration and answer questions about the study process, creeks and ditches, and creek maintenance. Participants were asked to provide input using an online questionnaire. Below are materials related to the meetings:

July 20, 2017 Flood Mitigation Workshop:

Thank you to everyone who participated in the flood mitigation workshop on July 20, 2017! Below are links to the materials related to the workshops:

Next Steps

Moving forward, staff will be working on the following:

  • Continue to assess the refined alternatives
  • Geotechnical analysis/groundwater monitoring at North Boulder Park
  • Conduct creek assessments to understand current conditions
  • Continue to coordinate with the Alpine-Balsam team on the area plan development
  • Develop draft mitigation recommendations
  • Conduct additional public engagement to help refine the master plan recommendations which will be brought back to the WRAB at a future date for feedback