City of Helena
Montana Circular DEQ-8 — Subdivision Storm Water Drainage Standards (capital)
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DEQ-8 is subdivision stormwater drainage standards — partial design coverage for capital MS4.
Document Metadata
Jurisdiction level
City
No evidence cited
State
Montana
No evidence cited
Version / edition
2024 Edition
No evidence cited
Adoption / effective date
2024
No evidence cited
Last revised date
2024
No evidence cited
Relationship to state manual
adopts state manual directly
No evidence cited
Design Criteria
Design storm return periods (years)
2-year, 10-year, 100-year
No evidence cited
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). BMPs per Montana Post-Construction Storm Water BMP Design Manual may satisfy if they treat/remove 80% TSS.
“first 0.5 inches of rainfall on impervious areas”
DEQ-8
Peak flow calculation method(s)
Rational Method, Modified Rational Method, TR-55 / SCS Curve Number Method, Storage-Indication Routing
No evidence cited
Hydrologic/hydraulic software
Not found in document
No evidence cited
Approved BMP categories
Initial Storm Water Facility, Retention / Detention / Infiltration Facilities, Bioretention, Pre-treatment Facilities, Conveyance Structures
No evidence cited