Montana

Montana Post-Construction Storm Water BMP Design Guidance Manual

StateMontanahighNo

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Other manuals in Montana: Montana

Document Metadata

Jurisdiction level

state

“In cooperation with: Montana Department of Environmental Quality”
Title Page

State

Montana

“Montana’s General Permit for Storm Water Discharges Associated with Small Municipal Separate Storm Sewer Systems (MS4s): Permit Number MTR040000”
Section 1.1, Page 1-1

Version / edition

First Edition, September 2017

“First Edition September 2017”
Title Page

Adoption / effective date

2017-09

“September 2017”
Title Page

Last revised date

Not found in document

No evidence cited

Relationship to state manual

independent

“This manual is a guidance document... With the exception of referenced permit language, this manual does not convey requirements for storm water management practices in the state of Montana. However, it is expected that municipalities or other governing entities may adopt this manual”
Preface, Foreword

Design Criteria

Design storm return periods (years)

2-year, 10-year, 25-year, 100-year

“Generally, a 10- or 25-year return design storm is used to size storm drainage infrastructure and a 100-year return design storm is used to address upstream and downstream flooding”
Section 3.4 Flood Control, Page 3-4

Water quality volume method

Post-Construction Performance Standard (MS4 General Permit Part II.A.5.b.iii): infiltrate, evapotranspire, and/or capture for reuse the runoff from the first 0.5 inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation (Runoff Reduction Volume, RRV = P×Rv×A/12 with P = 0.5 in and Rv = 0.05 + 0.9(I)). Remainder of that volume not reduced onsite (Runoff Treatment Volume) must be treated with BMPs expected to remove 80% TSS, or managed/treated offsite in the same subwatershed under MS4 offsite criteria.

“infiltrate, evapotranspire, and/or capture for reuse the post-construction runoff generated from the first 0.5 inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation... remainder... remove 80 percent total suspended solids (TSS)”
Section 1.3.1, Pages 1-2 to 1-3

Peak flow calculation method(s)

NRCS TR-55 Graphical Peak Discharge Method (adapted for Runoff Treatment Flow), NRCS Runoff Curve Number method (TR-55 Chapter 2)

“This procedure relies on the volume of runoff computed using Equation 3-1 and uses an adaptation of the NRCS TR-55 Graphical Peak Discharge Method”
Section 3.3, Page 3-2

Required hydrologic/hydraulic software

Not found in document

No evidence cited

Approved BMP categories

Infiltration Basin, Bioretention, Permeable Pavement Systems, Dispersion, Biofiltration Swale, Extended Detention Basin, Wet Detention Basin, Proprietary Treatment Devices

“Infiltration Basin ... Bioretention ... Permeable Pavement Systems ... Dispersion ... Biofiltration Swale ... Extended Detention Basin ... Wet Detention Basin ... Proprietary Treatment Devices”
Contents, Chapter 5