Montana

Circular DEQ-8: Montana Standards for Subdivision Storm Water Drainage

StateMontanahighNo

Fields not found in document: design_criteria.required_hydrologic_hydraulic_software

Other manuals in Montana: Montana

Document Metadata

Jurisdiction level

state

“These standards, based on demonstrated technology, set forth the requirements for the design and preparation of plans and specifications of storm water drainage systems in subdivisions in the State of Montana.”
Foreword, page 2

State

Montana

“Montana Department of Environmental Quality (Department)”
Section 1.1

Version / edition

2024 Edition

“2024 Edition”
Title Page

Adoption / effective date

2024

“These standards replace Department of Environmental Quality (DEQ) Circular DEQ-8, 2017 Edition.”
Foreword, page 2

Last revised date

2024

“2024 Edition”
Title Page

Relationship to state manual

independent

“The standards were developed by the Montana Department of Environmental Quality (Department) in compliance with Section 76-4-104, MCA, of the Sanitation in Subdivisions Act, and ARM 17.36.310.”
Section 1.1

Design Criteria

Design storm return periods (years)

2-year, 10-year, 100-year

“Calculations showing peak flow, at the time of concentration from all offsite basins impacting the site for the following storm events: (1) a 2-year storm event; (2) a 10-year storm event; and (3) a 100-year storm event.”
Section 2.2.2.G, page 11–12

Water quality volume method

Initial Storm Water Facility must infiltrate, evapotranspire, and/or capture for reuse post-development runoff from the first 0.5 inches of rainfall on impervious areas (V = 0.5 × Aimp / 12). For Standard Plans, BMPs designed per the Montana Post-Construction Storm Water BMP Design Manual may satisfy this volume if they treat/remove 80% TSS. Simplified Plan retention may use 250 ft³ per 1,000 ft² impervious (min 750 ft³) or Appendix B for the 100-year storm.

“Storm drainage designs must include an Initial Storm Water Facility sized to infiltrate, evapotranspire, and/or capture for reuse the post-development runoff generated from the first 0.5 inches of rainfall on impervious areas.”
Section 3.4, page 15

Peak flow calculation method(s)

Rational Method, Modified Rational Method, TR-55 / SCS Curve Number Method, Storage-Indication Routing

“The Rational Method may be used for calculating peak flow rate and volume of storm water runoff for areas less than 200 acres.”
Section 3.7.1, page 17

Required hydrologic/hydraulic software

Not found in document

No evidence cited

Approved BMP categories

Initial Storm Water Facility, Retention Facilities, Detention Facilities, Infiltration Facilities, Pre-treatment Facilities, Conveyance Structures (open channels, storm sewers, culverts), Infiltration basins, Bioretention, Biofiltration swales, Extended detention basins, Wet detention basins, Proprietary treatment devices

“The BMPs may include infiltration basins, bioretention, biofiltration swales, extended detention basins, wet detention basins, and proprietary treatment devices.”
Section 3.4, page 16