HomeMy WebLinkAboutStormwaterReport_03-04-21
THE FLATS SITE PLAN
LOT 1, BLOCK 16 BAXTER MEADOWS SUB. PH. 2A
STORMWATER DESIGN REPORT
A. Introduction
This design report will give an overview of the proposed stormwater system for the proposed
0.42-acre site plan for ‘The Flats’ located in Lot 1, Block 16 of Baxter Meadows Sub. Ph. 2A,
Section 34, T01 S., R05 E., Bozeman, Montana.
The development proposes a live-work concept with general office units located on the first floor
and residential units located on the second floor. There are two proposed buildings, each with
four office units and four residential units.
The proposed storm water management system consists of overland sheet flow and roof drains
that will direct stormwater to on site ADS Stormtech SC-740 infiltration basins. There is also a
surface retention/infiltration basin located at the southwest corner of the site that will consist of
oversize cobbles to serve the driveways located in Basin C. The infiltration basin will be
excavated to native gravels to permit maximum infiltration.
The following references were used in the preparation of this report:
a. COB Design Standards and Specifications Policy, 2004. Addendum #7
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS)
The delineated basins, drainage plan, and grading plan are displayed on the sheets in Appendix A
of this report.
B. Runoff and Basin Estimates
10-year, 2-hour design rainfall frequencies were used for calculating the pertinent basins
utilizing the rational method in accordance with the COB design standards. A runoff coefficient
C of 0.90 was used for impervious areas and a C value of 0.20 was used for landscaped areas.
The tables below illustrate the calculated runoff coefficients for each basin.
Basin A
Area
(SF) C
Total Area 2,718 NA
Impervious 2,718 0.90
Weighted C NA 0.90
Basin B
Area
(SF) C
Total Area 5,595 NA
Impervious 5,595 0.90
Weighted C NA 0.90
Basin C
Area
(SF) C
Total Area 4,025 NA
Impervious 3,145 0.90
Landscaped 880 0.20
Weighted C 4,025 0.75
C. Conveyance Capacity
The proposed infrastructure improvements were designed to convey runoff per the City of
Bozeman standards. The conveyance structures include 8” ADS roof drains that direct runoff to
the infiltration basins located throughout the site. Runoff from Basins A & B enters 8” ADS roof
drains and is then piped to underground Stormtech SC-740 infiltration chambers. The storm
drain piping was sized to be 8” diameter based on the small area of the site and the relatively low
stormwater flows. Runoff from Basin C will sheet flow to the west into the infiltration basin
located at the southwest corner of the site.
E. Native Gravel Infiltration Rates
Soils on the subject property are typical for the area, including enbar loam and blackdog silt
loam. Both ADS Stormtech SC-740 infiltration systems and the infiltration basin consisting of
oversize cobbles should be excavated to native gravels to permit maximum infiltration of
stormwater. Backfill above the ADS chambers to consist of clean, crushed angular stone and
well-graded aggregate mixtures. Refer to Detail 1 on Sheet C1.3 for more information. NRCS
soil data is included in Appendix B.
F. Stormwater Detention/Retention Volumes and Infiltration System Calculations
All of the runoff is proposed to be conveyed to retention basins and ultimately infiltrate into
native gravels. Two types of underground systems are proposed: ADS infiltration chambers and
a gravel infiltration basin.
Groundwater monitoring from a nearby project (Four Points Subdivision, approximately 0.20
miles north of subject property) was used to determine the seasonal high groundwater levels.
Three test pits were used (TP4-6) that were closest to the subject property to determine the
average groundwater depth of 5.75 ft. Groundwater monitoring data can be found in Appendix
B. Table 1 below shows the finished grade elevations of each ADS infiltrator basin and the
depth to the bottom of gravel for each basin.
Table 1: Groundwater Depth vs. Proposed FG Elevations
Basin A Basin B
Proposed Finish Grade Ele (ft): 4706.75 4708.75
Bottom of Gravel Ele (ft): 4701.50 4703.5
Dist. From FG to Bottom of Gravel (ft): 5.25 5.25
Average Depth to Groundwater (ft): 5.75 5.75
There is a total of 5.25 ft. from the finish grade elevation to the bottom of the gravel for the ADS
infiltration systems. The average depth to groundwater is 5.75 ft., which means that the bottom
of the gravel for the ADS infiltrator systems will be above the high groundwater mark.
The retention storage volumes were sized based on the 10- year 2-hour design rainfall frequency
per the City Design Standards. The rational method was used to determine post-development
stormwater flows. The calculations for all stormwater storage facilities are included in Appendix
C. The proposed stormwater facilities reduce the post-development runoff rate to zero. All
impervious surface runoff is conveyed to the infiltration retention and detention systems and then
infiltrates into the ground.
G. Stormwater Facility Maintenance
The proposed storm drainage facilities will be privately operated and maintained by the property
owner(s) of the on-site development. Included in Appendix D is a proposed maintenance
program for the ADS Stormtech infiltration systems.
Appendices
A. Drawings and Details
B. NRCS Soil Data and Groundwater Monitoring Data
C. Retention System Calculations and ADS Stormtech SC-740 Chamber Design
D. Stormwater Maintenance Plan
Appendices
Appendix A – Drawings and Details
Appendix B – NRCS Soil Data and
Groundwater Monitoring Data
The Flats - Basin A (North)2/4/2021
Lot 1 Block 16 - Baxter Meadows Sub PH 2A
Stormwater Run-off Calculation
Calculation of Required Volume for Storm Water Retention & Infitration System
Design Rainfall Freq. 10 year - 2 Hour event
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Post-development Calculations:
C
Areas (ft2):Roof Area 2,282 0.90
Impervious 436 0.90 Sidewalk
Total: 2,718 0.90
Retention Basin Calculation:
Q = CIA
C = 0.90 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.06 acres
Qpost =0.02 cfs
Required retention storage (ft3) =166 ft3 (10-yr, 2-hr storm)
Retention Basin - A
User Inputs
Chamber Model: SC-740
Outlet Control Structure: No
Project Name: The Flats - Basin A
(North)
Engineer: Chris Budeski
Project Location: Montana
Measurement Type: Imperial
Required Storage Volume: 150 cubic ft.
Stone Porosity: 40%
Stone Foundation Depth: 6 in.
Stone Above Chambers: 6 in.
Average Cover Over Chambers: 18 in.
Design Constraint Dimensions:(8 ft. x 24 ft.)
Results
System Volume and Bed Size
Installed Storage Volume: 211.12 cubic ft.
Storage Volume Per Chamber: 45.90 cubic ft.
Number Of Chambers Required: 2
Number Of End Caps Required: 2
Chamber Rows: 1
Maximum Length:17.83 ft.
Maximum Width: 6.25 ft.
Approx. Bed Size Required: 111.46 square ft.
System Components
Amount Of Stone Required: 11.05 cubic yards
Volume Of Excavation (Not Including
Fill):
14.45 cubic yards
Project:
Chamber Model - SC-740
Units -Imperial
Number of chambers - 3
Voids in the stone (porosity) - 40 %
Base of Stone Elevation - 4702.00 ft
Amount of Stone Above Chambers - 6 in
Amount of Stone Below Chambers - 6 in
9
Area of system - 146 sf Min. Area -
Height of
System
Incremental Single
Chamber
Incremental
Total Chamber
Incremental
Stone
Incremental
Ch & St
Cumulative
Chamber Elevation
(inches) (cubic feet) (cubic feet) (cubic feet) (cubic feet) (cubic feet) (feet)
42 0.00 0.00 4.87 4.87 287.11 4705.50
41 0.00 0.00 4.87 4.87 282.24 4705.42
40 0.00 0.00 4.87 4.87 277.38 4705.33
39 0.00 0.00 4.87 4.87 272.51 4705.25
38 0.00 0.00 4.87 4.87 267.64 4705.17
37 0.00 0.00 4.87 4.87 262.78 4705.08
36 0.05 0.16 4.80 4.97 257.91 4705.00
35 0.16 0.49 4.67 5.16 252.95 4704.92
34 0.28 0.85 4.53 5.37 247.79 4704.83
33 0.60 1.81 4.14 5.95 242.41 4704.75
32 0.80 2.41 3.90 6.31 236.46 4704.67
31 0.95 2.85 3.73 6.58 230.15 4704.58
30 1.07 3.22 3.58 6.80 223.57 4704.50
29 1.18 3.54 3.45 6.99 216.77 4704.42
28 1.27 3.80 3.35 7.14 209.78 4704.33
27 1.36 4.07 3.24 7.31 202.63 4704.25
26 1.45 4.36 3.12 7.48 195.33 4704.17
25 1.52 4.57 3.04 7.61 187.84 4704.08
24 1.58 4.75 2.97 7.71 180.23 4704.00
23 1.64 4.93 2.90 7.82 172.52 4703.92
22 1.70 5.10 2.83 7.93 164.69 4703.83
21 1.75 5.26 2.76 8.02 156.77 4703.75
20 1.80 5.41 2.70 8.11 148.75 4703.67
19 1.85 5.56 2.64 8.21 140.64 4703.58
18 1.89 5.68 2.59 8.27 132.43 4703.50
17 1.93 5.80 2.55 8.35 124.16 4703.42
16 1.97 5.92 2.50 8.42 115.81 4703.33
15 2.01 6.03 2.45 8.48 107.39 4703.25
14 2.04 6.13 2.41 8.55 98.90 4703.17
13 2.07 6.22 2.38 8.60 90.35 4703.08
12 2.10 6.31 2.34 8.66 81.75 4703.00
11 2.13 6.40 2.31 8.70 73.10 4702.92
10 2.15 6.46 2.28 8.74 64.39 4702.83
9 2.18 6.53 2.25 8.79 55.65 4702.75
8 2.20 6.60 2.23 8.82 46.86 4702.67
7 2.21 6.62 2.22 8.84 38.04 4702.58
6 0.00 0.00 4.87 4.87 29.20 4702.50
5 0.00 0.00 4.87 4.87 24.33 4702.42
4 0.00 0.00 4.87 4.87 19.47 4702.33
3 0.00 0.00 4.87 4.87 14.60 4702.25
2 0.00 0.00 4.87 4.87 9.73 4702.17
1 0.00 0.00 4.87 4.87 4.87 4702.08
StormTech SC-740 Cumulative Storage Volumes
The Flats - Basin A
101 sf min. area
Include Perimeter Stone in Calculations
Click Here for Metric
The Flats - Basin B (South)2/4/2021
Lot 1 Block 16 - Baxter Meadows Sub PH 2A
Stormwater Run-off Calculation
Calculation of Required Volume for Storm Water Retention & Infitration System
Design Rainfall Freq. 10 year - 2 Hour event
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Post-development Calculations:
C
Areas (ft2):Roof Area 2,282 0.90
Roof Area 2,202 0.90 Garage
Impervious 1,111 0.90 Sidewalk
Total: 5,595 0.90
Retention Basin Calculation:
Q = CIA
C = 0.90 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.13 acres
Qpost =0.05 cfs
Required retention storage (ft3) =341 ft3 (10-yr, 2-hr storm)
Retention Basin - B
User Inputs
Chamber Model: SC-740
Outlet Control Structure: No
Project Name: The Flats - Basin B
(South)
Engineer: Chris Budeski
Project Location: Montana
Measurement Type: Imperial
Required Storage Volume: 341 cubic ft.
Stone Porosity: 40%
Stone Foundation Depth: 6 in.
Stone Above Chambers: 6 in.
Average Cover Over Chambers: 18 in.
Design Constraint Dimensions:(8 ft. x 40 ft.)
Results
System Volume and Bed Size
Installed Storage Volume: 390.74 cubic ft.
Storage Volume Per Chamber: 45.90 cubic ft.
Number Of Chambers Required: 4
Number Of End Caps Required: 2
Chamber Rows: 1
Maximum Length:32.07 ft.
Maximum Width: 6.25 ft.
Approx. Bed Size Required: 200.42 square ft.
System Components
Amount Of Stone Required: 19.18 cubic yards
Volume Of Excavation (Not Including
Fill):
25.98 cubic yards
Project:
Chamber Model - SC-740
Units -Imperial
Number of chambers -3
Voids in the stone (porosity) - 40 %
Base of Stone Elevation -4704.00 ft
Amount of Stone Above Chambers - 6 in
Amount of Stone Below Chambers - 18 in
9
Area of system - 146 sf Min. Area -
Height of
System
Incremental Single
Chamber
Incremental
Total Chamber
Incremental
Stone
Incremental
Ch & St
Cumulative
Chamber Elevation
(inches)(cubic feet)(cubic feet)(cubic feet)(cubic feet)(cubic feet)(feet)
54 0.00 0.00 4.87 4.87 345.51 4708.50
53 0.00 0.00 4.87 4.87 340.64 4708.42
52 0.00 0.00 4.87 4.87 335.78 4708.33
51 0.00 0.00 4.87 4.87 330.91 4708.25
50 0.00 0.00 4.87 4.87 326.04 4708.17
49 0.00 0.00 4.87 4.87 321.18 4708.08
48 0.05 0.16 4.80 4.97 316.31 4708.00
47 0.16 0.49 4.67 5.16 311.35 4707.92
46 0.28 0.85 4.53 5.37 306.19 4707.83
45 0.60 1.81 4.14 5.95 300.81 4707.75
44 0.80 2.41 3.90 6.31 294.86 4707.67
43 0.95 2.85 3.73 6.58 288.55 4707.58
42 1.07 3.22 3.58 6.80 281.97 4707.50
41 1.18 3.54 3.45 6.99 275.17 4707.42
40 1.27 3.80 3.35 7.14 268.18 4707.33
39 1.36 4.07 3.24 7.31 261.03 4707.25
38 1.45 4.36 3.12 7.48 253.73 4707.17
37 1.52 4.57 3.04 7.61 246.24 4707.08
36 1.58 4.75 2.97 7.71 238.63 4707.00
35 1.64 4.93 2.90 7.82 230.92 4706.92
34 1.70 5.10 2.83 7.93 223.09 4706.83
33 1.75 5.26 2.76 8.02 215.17 4706.75
32 1.80 5.41 2.70 8.11 207.15 4706.67
31 1.85 5.56 2.64 8.21 199.04 4706.58
30 1.89 5.68 2.59 8.27 190.83 4706.50
29 1.93 5.80 2.55 8.35 182.56 4706.42
28 1.97 5.92 2.50 8.42 174.21 4706.33
27 2.01 6.03 2.45 8.48 165.79 4706.25
26 2.04 6.13 2.41 8.55 157.30 4706.17
25 2.07 6.22 2.38 8.60 148.75 4706.08
24 2.10 6.31 2.34 8.66 140.15 4706.00
23 2.13 6.40 2.31 8.70 131.50 4705.92
22 2.15 6.46 2.28 8.74 122.79 4705.83
21 2.18 6.53 2.25 8.79 114.05 4705.75
20 2.20 6.60 2.23 8.82 105.26 4705.67
19 2.21 6.62 2.22 8.84 96.44 4705.58
18 0.00 0.00 4.87 4.87 87.60 4705.50
17 0.00 0.00 4.87 4.87 82.73 4705.42
16 0.00 0.00 4.87 4.87 77.87 4705.33
15 0.00 0.00 4.87 4.87 73.00 4705.25
14 0.00 0.00 4.87 4.87 68.13 4705.17
13 0.00 0.00 4.87 4.87 63.27 4705.08
12 0.00 0.00 4.87 4.87 58.40 4705.00
11 0.00 0.00 4.87 4.87 53.53 4704.92
10 0.00 0.00 4.87 4.87 48.67 4704.83
9 0.00 0.00 4.87 4.87 43.80 4704.75
8 0.00 0.00 4.87 4.87 38.93 4704.67
7 0.00 0.00 4.87 4.87 34.07 4704.58
6 0.00 0.00 4.87 4.87 29.20 4704.50
5 0.00 0.00 4.87 4.87 24.33 4704.42
4 0.00 0.00 4.87 4.87 19.47 4704.33
3 0.00 0.00 4.87 4.87 14.60 4704.25
2 0.00 0.00 4.87 4.87 9.73 4704.17
1 0.00 0.00 4.87 4.87 4.87 4704.08
StormTech SC-740 Cumulative Storage Volumes
The Flats - Basin B
101 sf min. area
Include Perimeter Stone in Calculations
Click Here for Metric
The Flats - Basin C 2/4/2021
Lot 1 Block 16 - Baxter Meadows Sub PH 2A
Stormwater Run-off and Basin Capcity Calculation
Calculation of Required Volume for Storm Water Retention & Infitration System
Design Rainfall Freq. 10 year - 2 Hour event
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Post-development Calculations:
C
Areas (ft2):Impervious 3,145 0.90
Landscaped 880 0.20
Total: 4,025 0.75
Retention Basin Calculation:
Q = CIA
C = 0.75 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.09 acres
Qpost =0.03 cfs
Required retention storage (ft3) =204 ft3 (10-yr, 2-hr storm)
Volume held between contours:
Cumulative
Contour Area (ft2) Delta V (ft
3) Volume (ft
3)
4706.00 40
4706.50 96 34 34
4707.00 220 79 113
4707.35 364 102 215
Design storage at 1.35' depth (ft3) =215 ft3
Retention Basin - C
Piezometer Original Top of Casing*Allied
Well Ground Elevation Elevation 02.15.14 02.15.14 Elevation 04.08.14*04.26.14*05.09.14*05.21.14*06.24.14*07.30.14*09.06.14*10.17.14*12.12.14*01.14.15*
#4 4698.12 4699.03 6.00 4692.12 5.30 5.70 5.76 5.58 5.65 6.77 6.00 6.77 6.16 6.14
#5 4699.24 4700.43 5.50 4693.74 5.31 5.70 5.63 5.61 5.74 6.40 5.90 6.67 5.85 5.87
#6 4698.7 4700.22 5.50 4693.20 5.80 5.80 5.90 5.90 5.85 6.19 5.95 6.86 5.79 5.70
Average 5.47 5.73 5.76 5.70 5.75 6.45 5.95 6.77 5.93 5.90
Dewatering taking place. TP 10-12 had mud instead of groundwater. Elevation marked indicates level of mud. These TPs were near the dewatering.
Four Points Major Subdivision - Depth to Groundwater
Appendix C – Retention System Calculations
And ADS Storm Chamber Design
Appendix D – Stormwater Maintenance Plan
STORMWATER MAINTENANCE PLAN
The Flats
The recommended stormwater facility maintenance is displayed below. It is the Property
Owner’s Associations responsibility for routine inspection and maintenance of the following
items:
1. Keep roof drains and storm drain manholes free of leaves, litter, and other debris.
2. Keep the inlets of the facilities free of leaves, rocks, and other debris.
3. Quarterly inspect and clean roof drains and infiltration systems.
4. Follow the O&M procedures for the Isolator Row Plus and ADS Stormtech SC-740
infiltration system.
_______________________________
Property Owners Association