To: Members of the Castle Rock Water Commission
From: Mark Marlowe, P.E., Director of Castle Rock Water
David Van Dellen, Assistant Director
Title
An Ordinance Amending Title 15.36 of the Castle Rock Municipal Code Adopting by Reference as Primary Code the Storm Drainage Design and Technical Criteria Manual (First Reading) [Castle Rock Boundary]
Town Council Agenda Date: January 7, 2025
Body
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Executive Summary
The purpose of this memorandum is to seek Town Council approval of an Ordinance (Attachment A) amending Title 15.36 of the Castle Rock Municipal Code adopting by reference as a Primary Code the Town of Castle Rock, Colorado Storm Drainage Design and Technical Criteria (Drainage) Manual. The recommended changes to the Drainage Manual are primarily in response to issues that have arisen as a result of large storms more frequently impacting single family detached subdivisions over the last several years like the one which occurred on June 9, 2024. This strengthening of storms is a phenomenon being seen in many places, not just Castle Rock.
The Drainage Manual is a critical document governing the design and construction of new storm drainage infrastructure in the Town including storm sewer systems, street drainage, stormwater detention and stream channel improvements. The Manual is a key element in stormwater quality and floodplain regulations which the Town is responsible to implement based on state and federal regulations. Revisions to the Drainage Manual generally focus on 1) ensuring adequate street capacity in large storm events, 2) providing for additional inlets in residential subdivisions to address over icing of residential roads and reduce flow depths in smaller storm events and 3) ensuring designated emergency overflow paths for ponds and inlets (see Attachment B for a full list of proposed revisions).
Notification and Outreach Efforts
Castle Rock Water staff discussed proposed changes during the EDC Water Subcommittee on October 11, 2024 and November 15, 2024 and the Developer Roundtable Meeting held on November 20, 2024.
Town staff provided public notification on November 21, 2024 to the Town’s Economic Development partners outlining the proposed changes and a copy of the draft revisions were made available on the Town’s website on November 22, 2024.
Public notice was provided on this item in accordance with municipal code.
History of Past Town Council, Boards & Commissions, or Other Discussions
The Drainage Manual was originally adopted by Municipal Code in 2006 and updated in 2012 and 2019.
Discussion
The Drainage Manual was last updated in 2019. Since that time, the Town has experienced a rise in localized storm intensity overwhelming the 100-year major storm system especially impacting residential subdivisions. Additionally, the Mile High Flood District, formerly Urban Drainage and Flood Control District, who provides general guidance and standards for urban development along the Denver front range area, has adopted new standards with regard to runoff coefficients in response to broad sweeping changes in development densities and evolutions in landscaping practices and standards.
This revision will bring the Town’s Drainage Criteria more in alignment with the Mile High Flood District standards and provide for a more robust infrastructure to account for more frequent, higher intensity storms. Revisions will primarily influence local streets within new residential subdivisions to reduce flow depths in local streets and provide further protection of private residential dwellings. These revisions will ensure that in new development higher intensity storm flows will not escape the Right of Way prematurely or in undesignated locations. These revisions will ensure that for new development redundant systems are in place to handle clogging inlets due to influences such as hale and emergency overflow paths stay within designated open space tracts and not through private lots. These revisions will also serve to increase the number of inlets in new subdivisions to reduce the buildup of ice in the gutter during winter months.
Land disturbances on Town open space resulting from development have been an on-going concern given the challenges in establishing and maintaining native vegetation. This revision provides for the establishment of slope easements on Town open space that assign ground maintenance to a private entity such as a Homeowner Association or Metropolitan District. This allows for a higher level of service through these entities to keep up with revegetation and erosion control.
Budget Impact
There are no immediate direct cost impacts to the Town as a result of this action, however, these revisions will result in additional cost on residential development and increased public storm drainage infrastructure. Over the long term, the Town will have more infrastructure to maintain which will increase operational costs as a result of the proposed changes.
Staff Recommendation
Staff recommends approving an Ordinance to amend Title 15.36 of the Castle Rock Municipal Code adopting by reference as a Primary Code the Town of Castle Rock, Colorado Storm Drainage Design and Technical Criteria Manual.
Proposed Motion
“I move to recommend to Town Council approval of the Ordinance as presented”
Attachments
Attachment A: Ordinance
Exhibit 1: Redline version of Drainage Manual
Attachment B: Summary of Revisions