On-Site Detention Requirements When Regional Detention Is Provided
The planning and conceptual design of regional detention is frequently provided as part of a master planning process. This document is intended to clarify criteria regarding on-site detention requirements when regional detention is constructed downstream of a development or redevelopment project.
The 2014 City of Colorado Springs Drainage Criteria Manual Provides the following guidance:
- Volume 1, Chapter 6, Section 4 states, ‘Detention facilities shall be provided for all new development sites larger than 1 acre unless an approved basin plan includes the site being developed. In cases where project-specific conditions cause detention to be infeasible or ineffective, a variance may be requested.”
- Volume 1, Chapter 6, Section 5 states, “Full spectrum detention is a relatively new approach to detention that is expected to effectively limit peak flow rates to near predevelopment levels. In addition to reducing runoff rates, full spectrum detention can also provide some mitigation of increased runoff volume and water quality benefits. Unless an alternative detention concept is approved through a master planning process, the full spectrum detention approach, as defined in Chapter 13 of this Manual, shall be implemented as the standard detention approach. Alternative detention approaches will be evaluated based on their ability of achieve results similar to full spectrum detention and not only based on potential cost reduction.”
Policy Statement and Clarification
When a separate existing regional detention facility is provided downstream of the site as part of a master planning process and detention requirements have been triggered per the DCM, detention requirements will be satisfied according to the following:
- If the existing downstream regional detention facility provides detention for the minor storm event (5 or 10 year) in addition to providing detention for the major storm event (100 year) and the facility was part of a previously approved master study, then only water quality capture volume (WQCV) treatment is required on-site.
- If the existing downstream regional detention facility does not provide detention for the minor storm event, but does provide detention for the major storm event, then excess urban runoff volume (EURV) treatment is required on-site.
- If the existing downstream regional detention facility does not provide detention for the major storm event, then full spectrum detention is required on-site.
For more information, please contact the City of Colorado Springs Engineering Division.