Storm Water Drainage
Section I – References
Section II – Requirements
Section III – Sample Calculations
Section I:
References
A. City code of Helena, Montana – Title 6, Chapter 6 Storm water Utility
B.
Urban Storm Drainage Manual for the City of Helena – Stahley Engineering,
July 1978
C.
Drainage Master Plan for the City of Helena – Stahley Engineering, April 1980
D. Helena Storm Drain Master Plan update: Last Chance Gulch Basin – Stahley
Engineering, March 1989
E.
Helena Storm Drain Master Plan Update: Bull Run Basin – Stahley
Engineering, March 1989
F.
Helena Storm Drain Master Plan Update: West Area Basin – Stahley
Engineering, March 1989
G. Helena Storm Drain Master Plan Update: Davis Gulch Basin – Robert Peccia &
Associates, May 1985
H. Helena Comprehensive Plan 1981 (currently being updated)
I.
Storm water Retention by Stahre and Urbonas; Prentice Hall 1990
Section II:
Requirements
New subdivisions, changes in land use or developments that create 5000 square feet or
more of impervious area require a complete drainage plan design by a professional
engineer. Impervious area includes roofs, driveways, porches, patios, on-site sidewalks,
private roads, etc. Public roads and sidewalks are not included in the impervious area for
a property. Increased site drainage above the historical flow must be detained and
released at a lower, uniform rate or retained on-site.
Drainage plans shall at a minimum contain the following information:
A. Drainage Calculations:
1. Calculations for detention requirements
2. Calculations for detention pond (s)
3. Calculations for release from detention pond (s), if applicable
B.
Drainage Plan Showing:
1. Drainage flow on the site
2. Location and capacity of each detention pond
3. Drainage leaving the site
4. Downstream Impacts
Detail requirements are delineated on the Section I references.
Section III:
Sample Calculations
A. Facts
1. Use rational method for area less than 160 acres.
2. Use 1.25” for 1 hour for the 100 year storm
3. Use coefficient of runoff for unimproved area between .05 and .25
(engineer must justify use of values higher than .25).
4. Use coefficient of runoff for impervious area between .85 and .95
(engineer must justify use of values lower than .85).
5. Runoff in excess of the 100 year storm volume on the unimproved site
must be retaining to keep the release rate below the historical 100 year
storm rate.
B.
Sample Calculation for Retention
1. Assumptions
 Lot size 100,000 square fee
 Impervious improvements;
a. building – 32,000 square feet
b. parking lot – 26,000 square feet
c. walks and other impervious area 5,000 square feet
2. Calculation
 Storm water retention = improved site runoff – unimproved site
runoff
 Use rational method (ie…Less than 160 acres)
 V = T*C*I*A where:
1. V = runoff volume, in cubic feet
2. C = runoff coefficient
3. I = average storm intensity for storm duration
4. T = storm duration in inches/hour
5. A = area of the watershed, in acres
 Coefficient of runoff - .2 unimproved
 Coefficient of runoff = .9 impervious
 Unimproved runoff = .2*100,000/43,560 (acres)*1.25(*)*3600
(seconds) = 2067 cubic feet
 Improved site runoff = .2*(100,000-63,000)/43,560
(acres)*1.25*3600 seconds = 6622 cubic feet
 Detention required = 6622-2067 Cubic feet = 4555 Cubic feet