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Larimer County Stormwater Design Standards

Contact:   Eric Tracy     (970) 498-5729    Send E-mail

The Larimer County Stormwater Design Standards , have been updated (June, 2005) as an Addendum to the Urban Storm Drainage Criteria Manuals - Volumes 1, 2 and 3.

The coefficient in Equation RA-3, page 24, of the Larimer County Stormwater Design Standards, has been corrected as of 12/17/10

Another area affected by adoption of this adendum is the Urban Area Street Standards which controls street design standards in Larimer County within the cities' urban growth areas.

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Overview

Previously, design of facilities to accommodate stormwater for new development in Larimer County was based on the Larimer County Stormwater Management Manual, dated 1979. Since that time, there have been advances in the technological science of stormwater management as well as changes in practices used to address stormwater and stormwater quality. The need for adequate engineering standards to accommodate stormwater runoff has become more apparent with the intense growth of the County population and urban areas in recent years. The revised design standards will provide consistent and accurate engineering design of facilities for stormwater management in all developments within the County.

For these reasons, the County has adopted stormwater design standards based on the Denver Urban Drainage and Flood Control District's (UDFCD) Urban Storm Drainage Criteria Manual, dated 2001, with appropriate revisions to address local conditions. The use of common design standards for design of stormwater facilities along the Front Range will provide consistency for the engineering design community

The Larimer County Stormwater Design Standards are based on the Denver Urban Storm Drainage Criteria Manuals-Volumes 1, 2 and 3. This Addendum to the Urban Storm Drainage Criteria Manual, dated June, 2005 addresses the specific needs of the County. Each section of the Addendum corresponds to a respective section of the Denver Urban Storm Drainage Criteria Manual. Except where otherwise noted, the term "Denver Region" or "District" in the Urban Storm Drainage Criteria Manual can be considered to mean "Larimer County" in the Addendum. The Urban Storm Drainage Criteria Manual along with this Addendum have been adopted as a Technical Supplement to the Larimer County Land Use Code, and referred to as Larimer County Stormwater Design Standards.

One significant change from the Stormwater Management Manual previously used by Larimer County is that the rainfall frequency - intensity relationships are based on those used by the respective major city in each river watershed. The Cache La Poudre River Watershed uses a slightly higher intensity of rainfall. The 100-year rainfall total is 3.67 inches in 2 hours, based on the rainfall frequencies adopted by the City of Fort Collins. Correspondingly, the Big Thompson and Little Thompson river watersheds use the relationship adopted by the City of Loveland, for which the 100-year rainfall is 3.02 inches in a two hour period. This change is necessary, since the design standards must be consistent with the respective drainage basin master plans. Stormwater master plans prepared by the City of Fort Collins are also used in the County, as is the City of Loveland Drainage Master Plan.

Another significant change from the Stormwater Management Manual previously used is the adoption of Volume 3, a more comprehensive guide to Best Management Practices. Volume 3 also provides a detailed approach to handling stormwater quality issues as required by Phase II of National Pollutant Discharge Elimination System (NPDES).

The Larimer County Stormwater Design Standards utilizes up to date technology and procedures. Due to the dynamic nature of urban storm drainage, amendments and revision are expected as experience is gained in the use of The Urban Drainage Criteria Manual and Larimer County Addendum. Amendments and revisions will be posted with the Larimer County Engineering Department link of the Larimer County Web Page, www.larimer.org/engineering.

Questions concerning the material in the Manual or suggestions for improvements should be directed to:

Larimer County Engineering Department
Stormwater Engineer
200 West Oak Street, Suite 3000
P.O. Box 1190
Fort Collins, CO 80522-1190
(970) 498-5700