HomeMy WebLinkAbout4_Stormwater Management Report_02-04-2020 STORMWATER MANAGEMENT
DESIGN REPORT
FOR:
COLLINS APARTMENTS
LOTS 2 & 3, BLOCK 1
CATTAIL CREEK SUBDIVISION, PH. 3
BOZEMAN, MT
Prepared By:
M
MADISON
ENGINEERING
Madison Engineering
895 Technology Drive, Suite 203
Bozeman, MT 59718
(406) 586-0262
FEBRUARY 2020
M
STORMWATER MANAGEMENT
DESIGN REPORT
FOR:
COLLINS APARTMENTS
LOTS 2 & 3, BLOCK I
CATTAIL CREEK SUBDIVISION, PH. 3
BOZEMAN, MT
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Madison Engineering
895 Technology Blvd Ste 203
Bozeman, MT 59718
(406) 586-0262
FEBRUARY 2020
COLLINS APARTMENTS SITE PLAN
LOT 2 & 3, BLOCK 1, CATTAIL CREEK SUBDIVISION, PHASE 3
STORMWATER DESIGN REPORT
A. Introduction
The proposed development for the Collins Apartments will be constructed on Lots 2 and 3,
Block 1, of Cattail Creek Subdivision Phase 3. The site will have four buildings, each with four
apartments, for a total of 16 dwelling units. The site will contain stormwater onsite and allow
infiltration/retention through underground infiltration chambers consisting of 36-inch diameter
perforated corrugated metal pipe(CMP) that capture runoff via stormwater inlets in the parking
lot and drive lanes.
The following references were used in the preparation of this report:
a. COB Design Standards and Specifications Policy, 2004. Addendum #6
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS)
B. Stormwater Management
The infrastructure for Lots 2 and 3 will be constructed in one phase although the construction of
the buildings may be phased. Stormwater from Lot 2 flows in a northerly direction from the
drive aisle and driveways into two storm inlets at the center of the lot. These flows are directed
into an underground infiltration facility on the west side of the site (straddling Lots 2 and 3).
The flows from Lot 3 flow in a northerly direction into an inlet at the northwest corner of the
drive aisle, which directs flows into an underground infiltration facility along the west side of the
site. All four buildings will tie their roof downspouts into an 8-inch ADS pipe that connects to
the storm system and ultimately drains into the infiltration chambers.
Storm water run-off is calculated based on the Rational Formula and infiltration/retention
facilities are sized per the City of Bozeman's design standards for a storm intensity of 10-year
frequency and a 2-hour duration. Table 1 below shows the required and proposed storage
volumes for the two (Lot 2: southern; Lot 3: northern) infiltration basins. The calculations that
produce these volumes can be found in Appendix A.
Table 1:Facilih'.Storage Eolunres
Facility ID Required Storage Volume cu ft Proposed Storage Volume cu ft)
Northern 409 452.2
Southern 400 409.8
Collins Apartments Stonnwater Design Report
Page 1 of 2
C. Curb inlet adequacy check
The three curb inlets that received surface flow from the site were checked for capacity. All
standard 24-inch by 36-inch curb inlets are adequate to handle the runoff from a 25-year storm
event. See Appendix B for more information.
Appendices
A. Stormwater calculations
B. Curb Inlet adequacy check
C. Stormwater Maintenance Plan
D. Drainage Exhibit
Collins Apartments Stormwater Design Report
Page 2 of 2
Appendices
Appendix A — Stormwater Calculations
Collins Apartments 1/9/2020
North Basin
Stormwater Run-off and Infiltration Calculation
Calculation of Required Volume for Storm Water Infitration System
Design Rainfall Freq ?c aQ-•,;,year
IDF coefficient a 064
IDF coefficient h
IDF coefficient n 065
Post-development Calculations
C
Areas(ft): Impervious 17.455 090
Landscapec 7.942 0.20
Total: 25,397
total area: 0 58 acres
composite C: 068
Overland tc
average slope: 075 percent
travel distance: >_•feet
tc: 10 minutes
intensity at t,(fig 23): 2.07 in/hr
post-devel peak runoff: 082 cfs
Infiltration Rate Calculation
Trench Width= 40
Trench Length= 64.0
Total Infiltration Surface Area= ?56 ft2
Infitration Rate= 5 inch/min
0.00069 f/s
Q= 0.18 cfs
Storrs Duration Intensity Future Runoff Runoff Infiltration Required
(minutes) (in/hr) Rate(cfs) Volume(cf) Volume(cf) Storage(cf)
10 2.07 0 82 486 105 381
12 1.84 0.73 518 126 392
14 1.66 0.66 547 148 400
16 152 0.60 574 169 405
18 141 0.56 598 190 408
20 131 0.52 621 212 409
22 123 0.49 642 233 409
24 117 0.46 662 254 408
26 111 0.44 681 276 406
28 105 0.42 699 297 402
30 101 0.40 716 318 398
32 0 97 0.38 733 340 393
34 0.93 0.37 749 361 388
36 0.89 0.36 764 382 382
38 086 0.34 779 404 375
40 0.84 0.33 793 425 368
42 0.81 0.32 806 446 360
REQ'D STORAGE(ft'): 409
Contech Corrugated Perforated CMP Pipe Storage Calculations
Pipe Area
A=314xR2
Diameter ft Radius(ft)
3 1.5 3.14 = 7.07 ft2
Infiltration Surface Area
Length = 64 ft
Pipe Diameter = 3 ft
Additional Trench Width = 1 ft
Infiltration Surface Width = 4 ft
Infiltration Surface Area = 256 ft2
Pipe Volume
A=3.14xR2
Diameter(ft) Radius(ft)
3 1.5 3.14 = 707 ft2
Length 64 ft
PROPOSED STORAGE: = 452.2 ft
Infiltration Calc-Corrugated CMP-North As
Collins Apartments 1/9/2020
South Basin
Stormwater Run-off and Infiltration Calculation
Calculation of Required Volume for Storm Water Infitration System
Design Rainfall Freq E'..i._ 14;:; ; year
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 065
Post-development Calculations
Areas(ft): Impervious 16.965 090
Landscape( 5,951 0.20
Total: 22,916
total area: 053 acres
composite C: 072
Overland t.
average slope: 075 percent
travel distance: 145 feet
tc: 9 minutes
intensity at tc(fig 23): 2.13 in/hr
post-devel peak runoff: 0 80 cfs
Infiltration Rate Calculation
Trench Width= 4,0
Trench Length= 58.0
Total Infiltration Surface Area= 232 ftZ
Infitration Rate= 0.50 Inch/min
0.00069 f/s
Q= 0.16 cis
Storm Duration Intensity Future Runoff Runoff Infiltration Required
(minutes) (in/hr) Rate(cfs) Volume(cf) Volume(cf) Storage(cf)
9 2.13 0.80 456 91 365
11 188 0.71 488 111 377
13 1,69 0.64 516 130 386
15 1.55 0.58 542 149 392
17 1.43 0.54 565 169 396
19 1.33 0.50 587 188 399
21 1.25 047 607 207 400
23 1.18 045 627 227 400
25 1.12 0.42 645 246 399
27 1.06 0-40 662 265 397
29 1.02 0.38 679 285 394
31 0.97 0.37 694 304 390
33 0.94 0.35 710 323 386
35 0.90 0.34 724 343 381
37 0.87 0.33 738 362 376
39 0,84 0.32 752 381 370
41 0.81 0.31 765 401 364
REQ'D STORAGE(ft):1 400
Contech Corrugated Perforated CMP Pipe Storage Calculations
Pipe Area
A=3.14xR2
Diameter(ft Radius(ft)
3 15 314 = 7.07 ftZ
Infiltration Surface Area
Length = 58 ft
Pipe Diameter = 3 ft
Additional Trench Width = 1 it
Infiltration Surface Width = 4 it
Infiltration Surface Area = 232 ftz
Pipe Volume
A=3.14xR2
Diameter(ft) Radius(it)
3 15 3.14 = 7.07 ftZ
Ln_ngih = 58 ft
PROPOSED STORAGE: = 409.8 ft
Infiltration Calc-Corrugated CMP-South As
Appendix B — Curb Inlet adequacy check
I
Collins Apartments
Peak flow calculations
Curb Inlet Drainage Basins-Post Development
Peak Flow Summary(cfs)
Area Drainage Area No. Area(Ac.) Weighted C 100 Yr 25 Yi i o Yr
S.drive aisle DA 1 0.420 0.72 1.62 1.22 1.06
E.drive aisle DA 2 0.110 0.72 0.81 0.61 0.53
N.drive aisle DA 3 0.580 0.68 2.00 1.59 1.41
OF Equations from Bozeman Stormwater Master Plan for City of Bozeman,March 1982
General Equation i=a/(b+D)"where D is duration in hours,i=intensity in inches/hour
Storm Information
Design Rainfall Freq. 100 25 10
OF coefficient a 1.01 0.78 0.64
OF coefficient b 0 0 0
OF coefficient n 0.67 0.64 0.65
Adjustment Factor Cf: 1.25 1.1 1
Peak Q Values
Drainage Area 1
Area 0.420 acres
Weighted C 0.72
Average slope 0.75 percent
Travel Distance 145 feet
Design Rainfall Freq. 100 25 10 years
C`Cf 0.90 0.79 0.72 (Shall not exceed 1.00)
Total tc: 4.96 7.63 9.42 minutes
intensity at t. 5.37 4.02 3.49 in/hr
peak runoff: 1.62 1.22 1.06 cfs
Drainage Area 2
Area 0.110 acres
Weighted C 0.72
Average slope 2.72 percent
Travel Distance 50 feet
Design Rainfall Freq. 100 25 10 years
C'Cf 0.90 0.79 0.72 (Shall not exceed 1.00)
Total t�: 1.89 2.92 3.60 minutes
intensity at tc 10.23 7.66 6.65 in/hr
peak runoff: 0.81 0.61 0.53 cfs
Drainage Area 3
Area 0.580 acres
Weighted C 0.68
Average slope 0.75 percent
Travel Distance 130 feet
Design Rainfall Freq. 100 25 10 years
C'Cf 0.85 0.75 0.68 (Shall not exceed 1.00)
Total tc: 5.87 8.26 9.86 minutes
intensity at tc 4.80 3.81 3.39 in/hr
peak runoff: 2.00 1.59 1.41 cfs
Page 1 of 1
Collins Apartments
Gutter Capacity Calculations
Allowable Pavement Encroachment
Given: T= 9 feet(max per city)
T W= 1.5 feet
-W--Ts - Ts= 7.5 feet
Sw= 0.0625 ft/ft
Sx= 0.03 ft/ft
a a= 0.59 inches
a S• d= 3.24 inches
° n= 0.015
S.
Sw/Sx=
TM=
Capacity for Gutter
equations:
Q=QW +Qs Where
Q W =EoQ Qs.= Discharge within the Roadway
above the depressed section(cfs)
QS Qw= Discharge within the depressed
Q= I—E (gutter)section(cfs)
° Cf= 0.56 for English units
QS =Cr SX 5TS8S�,1 Sx= Pavement cross slope(ft/ft)
n Ts= Width of flow in the roadway above
depressed section
So= Gutter longitudinal slope(ft/ft)
S /S Sw= Gutter depression cross slope(ft/ft)
E = 1+ W "Sx T= Spread (ft)
( �T/W
1+ S W/)—] —1 W= Width of gutter depression (ft)
Capacity solution
Gutter Capacity-Drainage Area 1 Gutter Capacity-Drainage Area 2
So= 0.0075 So= 0.0272
QS= 2.02 cfs Qs= 3.84 cfs
Eo= 0.41 cfs Eo= 0.42 cfs
Q= 3.50 cfs Q= 6.67 cfs
Gutter Capacity-Drainage Area 3
So= 0.0075
QS= cfs
Eo= cfs
Q= cfs
DA AREA 25 Yr Design Flow(cfs)
1 S.drive aisle 1.22
2 E.drive aisle 0.61
3 N.drive aisle 1.59
Summary
The gutter capacity is adequate for all three proposed curb inlets
Page 1 of 1
Collins Apartments
Inlet Capacity Calculations
Gutter Section
Given T= 9.0 feet
T W= 1.50 feet
Ts= 7.50 feet
W Ts SW= 0.06 ft/ft
Sx= 0.03 fUft
_ a = 0.59 inches
d = 3.24 inches
n = 0.015
d "� Where: So= 0.016
s.
a Qs= Discharge within the Roadway
above the depressed section (cfs)
Qw= Discharge within the depressed
(gutter) section (cfs)
Capacity for Inlets on Grade Cf= 0.56 for English units
(Standard 24x36 Curb inlet) Sx= Pavement cross slope (ft/ft)
Ts= Width of flow in the roadway above
Drainage Area 1 depressed section
So= Gutter longitudinal slope(ft/ft)
Qw= 1.48 cfs Sw= Gutter depression cross slope (ft/ft)
Qs= 2.02 cfs T= Spread (ft)
Cross-sectional area of flow W= Width of gutter depression (ft)
A= 1.22 ft2
Gutter Velocity
V= 2.88 ft/sec
Fraction of side flow intercepted
Rs= 0.27
Total flow capacity intercepted b the inlet
Qint= 2.07 cfs
Qbypass= 1 4' cfs
Design Q for inlet#1
Q25= 1.22 cfs Single Inlet Sufficient
Drainage Area 2
Qw= 2.83 cfs
Qs= 3.84 cfs
Cross-sectional area of flow
A= 1.22 ft2
Gutter Velocity
V= 5.49 ft/sec
Fraction of side flow intercepted
Rs= 0.10
Total flow capacity intercepted by the inlet
Page 1 of 2
Qint= 3.32 cfs
Qbypass= 3.34 cfs
Design Q for inlet#2
Q25= 0.61 cfs Single Inlet Sufficient
Drainage Area 3
Qw= 1.48 cfs
Qs= 2.02 cfs
Cross-sectional area of flow
A= 1.22 ftz
Gutter Velocity
V= 2.88 ft/sec
Fraction of side flow intercepted
Rs= 0.27
Total flow capacity intercepted by the inlet
Qint= 2.07 1 cfs
Qbypass= 1.43 cfs
Design Q for inlet#3
Q25= 1.59 cfs Single Inlet Sufficient
Page 2 of 2
Appendix C — Stormwater Maintenance
Plan
a
STORMWATER MAINTENANCE PLAN
Collins Apartments
Owner's responsibility for routine inspection and maintenance
1. Keep the inlets of the facilities free of leaves, rocks, and other debris.
2. Re-sod damaged or maintained areas immediately, or use grass plugs from the
adjacent up-slope area.
3. See that litter and other debris are removed from inlets, swales, and vegetated and
paved areas.
4. Maintenance of the underground systems are as follows:
• Minimum required maintenance includes a quarterly inspection during the
first year of operation and a yearly inspection thereafter. Utilize inspection
ports for inspections.
• The inspection ports can be used to pump water into the system and re-
suspend accumulated sediment so that is may be pumped out. Flush and
pump as inspections deem necessary.
• Utilize the adjacent upstream storm sewer manhole or cleanout for removal
(vacuuming) of debris from the end of the storm drain at the connection
point with the underground chamber.
5. Owner to maintain and fund Operation and Maintenance of stormwater facilities.
Shane Collins, Owner's Representative
GAMADISON ENGINEERING\PROJECTS\2019\19-116 Springer-Collins\Stonn'.STORMWATER MAINTENANCE PLAN.doe
Appendix D — Drainage Exhibit
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