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About The Project, Left Hand Creek Flood Control Project

Left Hand Creek Channel Project InformationThe goal of this project is to reduce the floodplain so that most of the homes in Southmoor Park will not be damaged by the 100-year flood. Left Hand Creek is a multi-purpose project that will reduce the floodplain area, replace the South Pratt Parkway Bridge, add bridge structures at Main Street, enlarge the Left Hand Creek Channel between  Main Street and South Pratt Boulevard, improve the channel on the north side of Pike Avenue, and reconstruct the bike path to cross under South Pratt Parkway.

Left Hand Creek from Pike Avenue to Main Street is one of the areas of the City that is at risk of major flooding and property damage during a 100-year storm event.  This area has experienced about a 100-year flood in 1969 and two 20-year floods in 1995. This project was one of the four projects included as part of the 2007 Storm Drainage Bond election..

The original scope of the project  was to replace the South Pratt Parkway Bridge and regrade around the bridge.   The project was changed as the field  survey, floodplain mapping and watershed computer modeling showed the initial project would not remove any properties from the floodplain.  These changes will reduce the area of the 100-year floodplain

The additional projects components included adding box culverts under Main Street, re-grading the channel and lowering the bike path from Quebec Avenue to Main Street, raising the grade of Ridgeview Drive and re-grading the channel area downstream of Pike Road.

The new grading will need to be precise to provide the additional area needed to pass the stormwater.  This additional volume comes at the price of removing the existing trees and vegetation and planting new trees outside of the main channel flow area.

Federal government approvals are required for this project.  Approval will be required from the Federal Emergency Management Agency (FEMA), the U.S. Fish and Wildlife Service, and the U.S Army Corp of Engineers. 

FEMA will review the floodplain computer models to ensure their accuracy through the Conditional and Final Letter of Map Revision process.  This process will result in a new floodplain delineation for the project area which will be the official floodplain that is used to determine which houses will be required to have flood insurance.

U.S. Army Corps of Engineers must approve a 404 permit which includes environmental mitigation.

Image information Link to Larger Image

Draft Regulatory Floodplain

Current FEMA map copied from the latest Flood Insurance Rate Map

Lefthand Creek Draft Regulatory Floodplain

Draft Existing Floodplain

New floodplain model developed by the Mueller Engineering, City of Longmont Consultant

Lefthand Creek Existing Floodplain

Draft Proposed Floodplain


Reduced floodplain after the project is completed

Lefthand Creek Draft Proposed Floodplain
1995 Flood at Pike Road Lefthand Creek 1995 Flood at Pike Road
Site Plan of Proposed Channel Improvements Between S. Pratt Blvd. and Main St. Site Plan Near Main Street
Site Plan of Proposed Channel Improvements near Pike Road Site Plan Near Main Street
Typical Cross Section of Proposed Channel Improvements between S. Pratt Blvd. and Main St. Lefthand Creek Typical Section 1
Typical Cross Section of Proposed Channel Improvements near Pike Road Lefthand Creek Typical Section 2
Typical Cross Section of Proposed Bridge on S. Pratt Blvd. Lefthand Creek Typical Section 3


If you have more immediate questions or concerns, please contact Curtis Ansel, Project Manager, Public Works and Natural Resources Division, at (303) 651-8310 or by e-mail at