Criteria Manual Updates

USDCM Volumes 1 and 2 Update – What’s Changed?

Over the last several years UDFCD has been working on a major update of the Urban Storm Drainage Criteria Manual, Volumes 1 and 2. UDFCD developed a group of stakeholders consisting of communities within the region, consultants, and other interested parties. Many provided input and review for the update. Below is a summary of the biggest changes.

Drainage Law: This chapter was updated with new and more recent case law. New law related to detention of stormwater as it related to water rights was also added.

Planning: The Planning chapter was rewritten as guidance for communities and consultants who seek information on developing a stormwater master plan.

Flood Risk Management: This is a new chapter that takes the place of the Floodproofing chapter. It includes guidance on floodproofing but also covers the fundamentals of floodplain management, mapping changes, flood insurance; and UDFCD, local, and State programs. The chapter also includes information for property owners regarding potential sources of financial assistance and selection of a floodproofing method.

Rainfall: UDFCD developed new Depth Reduction Factors (DRFs) based on data from a study completed in 2010 by the City of Colorado Springs. Data from the Fountain Creek Rainfall Characterization Study were used to develop new DRFs for minor (2- through 10-year) events. Since areal-average rainfall over a large watershed is generally lower than the point rainfall, a DRF is applied to reduce point precipitation values to areal-average precipitation values.

Although the new NOAA Atlas 14 Precipitation-Frequency Atlas of the United States, Volume 8-Midwestern States was published with new precipitation values in 2013, UDFCD determined that the difference between the new values and those from the previous NOAA Atlas 2 was not statistically significant, and therefore did not adopt the new values. For more information on this decision see the UDFCD position paper regarding this issue dated September 11, 2013.

Runoff: The Runoff chapter contains new runoff coefficients and a new time of concentration equation for urban catchments. The basis for these changes is documented in a paper titled Derivation and Calibration of Volume-based Runoff Coefficients for Denver Area, Colorado (Guo, 2013). These changes also resulted in a need to modify a number of Excel-based workbooks. Changes to UD-Inlet were made to update peak runoff calculations as well as allow for design of multiple inlets within one workbook. The change will also impact the UD-Rational workbook. However, further updates are planned for this workbook and it will be re-released in the summer of 2016.

Streets, Inlets, and Storm Drains: From 2006 to 2011, hundreds of street and area Inlet physical model tests were conducted at the CSU Hydraulics Laboratory. This facilitated refinement of the HEC-22 methodology for inlets common to Colorado. This study informed revisions to the Streets, Inlets, and Storm Drains chapter as well as the UD-Inlet workbook.

Open Channels: This chapter is a rewrite of the Major Drainage chapter. It includes discussion of the functions and benefits of natural streams and lists principles of stream restoration. It includes guidance for design of naturalized channels as well as vegetated and rock-lined swales. It also provides guidance relevant to both preparing and reviewing a hydraulic model using HEC-RAS.

Hydraulic Structures: The Hydraulic Structures chapter now includes guidance for sculpted concrete drop structures and allows use of the simplified method for design only with grouted (stepped or sloping) boulder and sculpted concrete drop structures. Additionally, all details were updated and several details were added. Discussion on bridges was removed from this chapter and is now located in the Culverts and Bridges chapter.

Stream Access and Recreational Channels: This is a new chapter that includes design of shared use paths adjacent to streams and guidance for design of recreational channels. This chapter is largely related to safety issues and also contains a summary list of all USDCM criteria pertaining to public safety as well as considerations for reviewing safety of a proposed project.

Culverts and Bridges: Guidance on bridges, including hydraulic modeling recommendations using HEC-RAS, was added to this chapter. Also, information regarding safety grates was expanded including straightforward guidance as to when a safety grate is recommended.

Storage: Previously, the Storage chapter included a number of different approaches for designing detention basins. This was simplified with the single recommendation of full spectrum detention in the new chapter. The concept of full spectrum detention has been expanded with detailed guidance on how to implement full spectrum detention while incorporating many of the stormwater quality treatment measures described in Volume 3 of the USDCM. This resulted in significant changes to the UD-Detention workbook which was released at the same time as the update.

Revegetation: The Revegetation chapter was completely rewritten and includes guidance for upland, riparian, and wetland vegetation. The chapter includes site preparation, material selection and installation, mulching, maintenance, and post-construction monitoring.