HomeMy WebLinkAbout20 - Design Report - Bozeman East Mixed Use - Drainage Bozeman East Mixed-Use
Storm Sewer Report
1 INTRODUCTION
The Bozeman East Mixed-Use development is the final phase of the Bridger Peaks Town
Center and consists of 4 apartment buildings, a clubhouse, commercial retail space and an
outdoor recreation space. The pre-development topography is gradual, about 1 to 2 percent
slope from south east to north west. Existing ground cover is undeveloped, formerly agricultural
fields, and is surrounded by previous retail and residential development phases. This report
provides hydraulic calculations for sizing of storm water conveyance and detention storage
facilities for the development of the Bozeman East Mixed-Use Site in accordance with the City
of Bozeman Design Standards.
2 APPROACH
The proposed development lies within the drainage basin of the following previously approved
drainage submittal, attached at the end of this report:
Revised Drainage Design Report for Bridger Peaks Town Center Bozeman, Montana
(October 16, 1998)
It will be shown that the proposed detention facilities will limit the post-development 10-year
peak runoff rate to the pre-development rate and provide effective treatment of storm runoff. All
storm drain pipes were sized to convey the 25-year peak runoff.
3 EXISTING CONDITIONS
As described in the Bridger Peaks Town Center Preliminary Plat/PUD application dated June
1998, the project will be constructed in phases. Phases I and II have already been constructed
with the remaining final phase being the Bozeman East development. In the original design of
Bridger Peaks, the subdivision was divided into 3 historic basins H-1, H-2 and H-3, see Figure 2
in the Revised Drainage Design Report. The post development strategy for the historic basins
was planned as follows:
H-1
H-1 is the combination of both H-1A and H-1B which had previously drained to the roadside
ditch along North 19th Aven ue/Sim mental Way. HI-A was calculated to have a post-
development release rate less than that for the existing conditions, therefore detention is not
necessary (p. 30). Detention and treatment for Basin H-1 B is provided along with Basin H-2.
H-2
Detention for Basins H-1 B and H-2 is provided by Pond 2 in the Northeast corner of the project
site (see Figure 2). Pond 1 in the Northwest corner of the project site is used primarily to
attenuate peak flows in order to reduce pipe sizes in the on-site storm drain system. Detention
storage from Pond 1 is neglected in the Pond 2 design; therefore no detention calculations are
provided for Pond 1. Pond 3 on the eastern side of the site exists for sediment retention during
construction of Phases I and II but is not intended as a permanent detention pond.
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00M� 11UNITY DEVL!_f:',__'___T
Pond 2 will limit the combined 10-year release rate of Basins H-1 B and H2 to a peak rate less
than or equal to the pre-development rate for basin H-2 alone. This requirement has been met
for Phase 1/II construction and will be the design standard for the fully developed conditions.
H-3
Basin H-3 covers a portion of the Oak Street right-of-way and entryway corridor. Pond 4 on the
east side of H-3 will be used to fully retain the 10-year, 2-hour storm runoff from this area.
Pond 2
As described above and in the Revised Drainage Design Report, most of the Bridger Peaks site
(historic basins H-1 B and H-2) are graded to drain through storm pipes and surface ditches to
Pond 2, a large detention basin at the northeast corner of the site. See the Revised Drainage
Design Report for the relevant sizing calculations for the existing on-site surface ditches and
storm drain pipe. As a part of Basin H-2, the Bozeman East development will also be designed
to drain to Pond 2. The current pond has been sized to detain runoff volume from Phases I & 11
and will need to be expanded per the Revised Drainage Design Report to store the runoff for the
fully developed conditions. Additionally, the existing orifice structure has a release rate equal to
the pre-development conditions of Phase 1/11. With the construction of the Bozeman East
development, the orifice will be modified control runoff under fully developed conditions. The
calculations are shown in section 5 of this report.
4 PROPOSED CONDITIONS
In accordance with City of Bozeman design standard, the rational method was used to calculate
the peak runoff and detention requirements for the entire Bridger Peaks development. The post
development detention calculation (Appendix A) is an Excel spreadsheet with formulas that
calculate the required detention. The values used in each column of the table are as follows:
Area: Calculated in AutoCAD
C (Rational Method runoff coefficient): The previously approved Revised Drainage Design
Report for Bridger Peaks Town Center calculated the C for the full buildout using the measured
C of Phases 1/11 combined with a conservative assumption of 0.8 for the final phase. The runoff
coefficients for the final phase/Bozeman East development were more accurately calculated
using 0.2 for pervious area (pre-development and developed landscape areas), 0.65 for gravel
area, and 0.9 for impervious areas (pavement, concrete and rooftops). The actual C for the
Bozeman East development was determined to be 0.70 which is less than the conservative
estimate of 0.8 that was used in the previous calculations. The new runoff coefficient for the
proposed development was then combined with the coefficients for Phase 1/11 (p. 27) to obtain a
coefficient for the entire Bridger Peaks development. The following calculations show the
original Phase 1/11 Post Development and Fully Developed Calculations from the Revised
Drainage Design Report for Bridger Peaks Town Center. Lastly the C has been recalculated for
the Full Development replacing the assumed 0.8 coefficient with the actual coefficient from the
site plan of 0.7.
From p.27 Detention Calculations:
Phase 1/II Final Phase Undev.
Phase 1/11 Post Dev. C = 0.909.88) + 0.25(2.22) + 0.20(9.80) = 0.64
31.9 acres
From p. 28 Fully Developed Calculations:
Phase 1/11 Final Phase Dev.
Fully Developed Post Dev. C = 0.909.88) + 0.25(2.22) + 0.80 (11.4) = 0.82
33.5 acres
Revised Fully Developed Calculations:
Phase 1/II Final Phase Dev.
Fully Developed Post Dev. C = 0.909.88) + 0.25(2.22) + 0.70 (11.4) = 0.79
33.5 acres
T,_ (time of concentration): Determined based on the time required for runoff to travel from the
hydraulically most distant point in the watershed to the outlet. The Tc of 9.9 min from the
original drainage report was used as this number considers the most hydraulically distant point
from Pond 2 (p.27).
Storm Intensity: Determines the 10-year storm intensity at the time of concentration
Peak Runoff (Q,): Value calculated using the Ration Method formula (Q = C i A) and the
intensity formulas in the City's design standards (Rainfall Intensity— Duration Curve). The Peak
Runoff in this case is for the 10-year design storm.
Release Rate: From the original drainage design report, the 10-year allowable pre-development
release rate for the entire subdivision is 4.08cfs.
The resulting detention requirement for the full build out of the Bridger Peaks Town Center is
57,809 ft3, see Appendix A for spreadsheet calculations. Pond 2 will be sized to a volume of
59,432 ft3 based on the following contour information.
Stage Elevation Contour Incremental Total
Area Storage Storage
N (ft) (SO) (cam) (cuft)
0.00 4717.10 11895 0.000 0.000
0.90 4718.00 14835 12,029 12,029
1.90 4719.00 16948 15.842 27.870
2.90 4720.00 18825 17,837 45.707
3.60 47220.70 20391 13,726 59,432
5 DETENTION BASIN OUTLET STRUCTURE
The detention basin outlet structure for Pond 2 is located under the berm which impounds Pond
2, and consists of a 15-inch inlet and outlet pipes connected to a 24-inch riser within a concrete
manhole. The pipe and riser is made of corrugated outer wall, smooth interior HDPE pipe. The
control structures inlet and outlet pipe is sized to convey the maximum allowable site release
rate of 6.41 cfs as calculated in the Preliminary Plat/PUD application. The orifices on the riser
will be places such that the maximum releaser rate will not be exceeded during periods of low
tailwater.
With the final phase of construction, the Pond 2 orifice will be modified to control runoff under
fully developed conditions.
Orifice Calculations for Fully Developed Conditions:
Dewatering Orifice (2" dia.)
Q = Ca 2gh
Where: C= 0.6
a = orifice area, sq. ft
G = 32.2 ft/sec2
h = head to center of orifice
15" Outlet Pipe I.E. = 16.21
2
Q = 0.67r\12l 2(32.2)(20.8 — 16.21) = 0.23cfs
Primary Orifice (square):
Side dimension = Q 0.5
(CV-2gh)
Where: C= 0.6
a = orifice area, sq. ft
G = 32.2 ft/sec2
Inlet control on 15" outlet pipe = 17.49
Side dimension = (MV2(32.2)(20.7-17.49))
4.08-0.23 0.5 = 0.67' = 8.02"square
Therefore, the control structure will be as follows under fully developed conditions:
Top of Riser Elevation = 20.7
Dewatering Orifice = 2" diameter, center elevation 15.00
Primary Orifice = 8.0" square, center elevation 17.10
10-yr. peak release rate (post-development) = 4.08cfs
6 CONVEYANCE
The storm drain piping being proposed for the site will convey both existing and proposed runoff
to Pond 2 at the northeast corner of the site. The proposed site has been divided into 3 basins,
see Appendix B. Inlets throughout the site will collect the storm water. The piping, in accordance
with City Standards shall be designed to accommodate the 25-year storm event.
Existing Conveyance
At the northwest corner of the site, an existing 30" RCP pipe daylights to the north of Building
4/D, these existing flows then swale to the east and into Pond 2. Based on Figure 1 Phase 1/II
Developed Drainage Map in the approved Revised Drainage Design Report this pipe, P1, has a
contributing area of 14.5 acres.
Existing flows at the south of the site will be conveyed by proposed storm drains connecting to
two existing southern storm drain stubs. Based on Figure 1 Phase 1/II Developed Drainage Map
in the approved Revised Drainage Design Report that the stubs are P16 and P18 with
contributing areas of 1.6 and 5.8 acres respectively
The original detention calculations for Phase 1/11 Post development used a C value of 0.64 and a
Tc of 9.9 for the existing flows. These will be used when calculating the contribution of the
existing flows in the development.
Proposed Conveyance
With the construction of Bozeman East, this existing pipe P1 will be extended to Pond 2 while
also collecting contributing flows from proposed Basin 1 & Basin 4. The flows from P16 and
P18 will be conveyed to Pond 2 while also collecting flows from Basin 5. The pipe sizing is
shown in Appendix C while the following table summarizes the existing and proposed
conveyance calculations.
Existing Conveyance Area C T, Q25 Pipe Dia. Slope Capacity Full
P 16-Ex 18"Stub 1.60 0.64 9.9 2.53 18" 3.13 23.41cfs
P 18-Ex 24"Stub* 5.80 0.64 j 9.9 9.17 24"
P 1 -Ex 30"Stub 14.50 0.64 9.9 22.93 30" 0.16 20.67cfs
*The slope for the 24"Stub is unknown
Proposed Conveyance Area C T, Q21 Pipe Dia. Slope Capacity Full
Basin 1 4.80 0.79 5.0 14.51 j 24" 0.30 15.61 cfs
Basin 2 1,10 I 0.79 5.0 3.32 12" j 0.75 3.89 cfs
Basin 3 0.52 0.79 5.0 1.57 8" 1.20 1.69 cfs
-- .
Basin 4 w/P 1** 15.27 0.65 9.7 24.83 30" 0.45 34.66cfs
Basin 5 w/P 16& P 18** r 9.58 ' 0.67 8.8 1 17.23 j 21" 1.10 21.86 cfs
**The combined flows were calculated using the total areas weighting the C and Tc values
Appendix A
Bozeman East Mixed Use
Total Detention Requirement (10 year storm)
MODIFIED RATIONAL METHOD
Qp=CiA
POST-DEVELOPMENT
BASIN AREA POST= 33.50 AC
POST-DEV C= 0.79
POST-DEVTc= 9.9 MIN
Release rate = 4.08 cfs
STORM INTENSITY= 2.06 IN/HR
TIME STEP
POST-DEV Qp= 54.50 CFS DURATION= 5.0 min.
Max.Volume, Max.Volume, Required Detention
Triangle Rel.(cu.ft.)Constant Rel.(cu.ft.) Volume(cu.ft.)
68907.02 46710.03 57,809
POND VOLUME CALCULATIONS:
Triangle Release Constant Release
DURATION INTENSITY Qp POND VOLUME POND VOLUME
(MIN) (IN/HR) (CFS) (CF) (CF)
9.41 2.13 56.35 29,433 27,246
19.31 1.34 35.31 37,322 34,027
29.21 1.02 26.98 42,486 38,066
39.11 0.85 22.32 46,360 40,805 ORIFICE CALCULATIONS
49.01 0.73 19.27 49,451 42,754 MATCHING Qp= 4.08 cis
58.91 0.65 17.10 52,008 44,165 Max.Depth= 3.21 ft
68.81 0.59 15.46 54,173 45,180
78.71 0.54 14.16 56,036 45,888
88.61 0.50 13.11 57,656 46,353 SOUARE ORIFICE(as-built):
98.51 0.46 12.24 59,077 46,616 DIAMETER= 8.000 inches
108.41 0.44 11.50 60,331 46,710 Area= 0.44 ftZ
118.31 0.41 10,87 61,443 46,660 ORIFICE FLOW= 3.83 cfs
128.21 0.39 10.31 62,431 46,486
138.11 0.37 9.83 63,312 46,203
148.01 0.36 9.39 64,098 45,823
157.91 0.34 9.01 64,798 45,356
167.81 0.33 8.66 65,422 44,813
177.71 0.32 8.34 65,977 44,199
187.61 0.31 8.05 66,469 43,522
197.51 0.30 7.79 66,903 42,787
207.41 0.29 7.54 67,285 41,997
217.31 0.28 7.32 67,618 41,159
227.21 0.27 7.11 67,906 40,275
237.11 0.26 6.92 68,152 39,348
247.01 0.26 6.73 68,359 38,382
256.91 0.25 6.56 68,529 37,379
266.81 0.24 6.41 68,665 36,341
276.71 0.24 6.26 68,770 35,270
286.61 0.23 6.11 68,843 34,169
296.51 023 5.98 68,889 33,038
DURATION INTENSITY Qp POND VOLUME POND VOLUME
(MIN) (IN/HR) (CFS) (CF) (CF)
306.41 0.22 5.85 68,907 31,880
316.31 0.22 5.73 68,900 30,697
326.21 0.21 5.62 68,868 29,488
336.11 0.21 5,51 68,814 28,256
346.01 0.20 5.41 68,737 27,002
355.91 0.20 5.31 68,640 25,727
365.81 0.20 5.22 68,522 24,431
375.71 0.19 5.13 68,386 23,116
385.61 0.19 5.04 68,231 21,783
395.51 0.19 4.96 68,059 20,431
405.41 0.18 4.88 67,870 19,063
415.31 0.18 4.80 67,665 17,678
425.21 0.18 4.73 67,444 16,277
435.11 0.18 4.66 67,208 14,861
445.01 0.17 4.59 66,959 13,431
454.91 0.17 4.53 66,695 11,987
464.81 0.17 4.47 66,418 10,529
474.71 0.17 4.40 66,128 9,057
484.61 0.16 4.35 65,826 7,574
494.51 0.16 4.29 65,512 6,078
504.41 0.16 4.23 65,186 4,570
514.30 0.16 4.18 64,849 3,051
524.20 0.16 4.13 64,501 1,521
534.10 0.15 4.08 64,142 -20
544.00 0.15 4.03 63,773 -1,572
553.90 0.15 3.98 63,395 -3,133
563.80 0.15 3.94 63,007 -4,704
573.70 0.15 3.89 62,609 -6,285
583.60 0.15 3.85 62,202 -7,875
593.50 0.14 3.81 61,787 -9,474
603.40 0.14 3.77 61,363 -11,082
613.30 0.14 3.73 60,930 -12,698
623.20 0.14 3.69 60,490 -14,323
633.10 0.14 3.65 60,041 -15,956
643.00 0.14 3.62 59,585 -17,596
652.90 0.14 3.58 59,121 -19,244
662.80 0.13 3.55 58,650 -20,900
672.70 0.13 3.51 58,172 -22,563
682.60 0.13 3.48 57,687 .24,233
692.50 0.13 3.45 57,194 -25,910
702.40 0.13 3.41 56,696 -27,594
712.30 0.13 3.38 56,191 -29,285
722.20 0.13 3.35 55,679 -30,982
732.10 0.13 3.32 55,161 -32,685
742.00 0.12 3.29 54,637 -34,394
751.90 0.12 3.27 54,108 -36,110
761.80 0.12 3.24 53,572 -37,831
771.70 0.12 3.21 53,031 -39,559
781.60 0.12 3.18 52,484 41,292
791.50 0.12 3.16 51,932 43,030
801.40 0.12 3.13 51,374 -44,774
811.30 0.12 3.11 50,811 -46,524
821.20 0.12 3.08 50,243 48,278
831.10 0.12 3.06 49,670 -50,038
841.00 0.12 3.04 49,092 -51,803
850.90 0.11 3.01 48,509 -53,572
860.80 0.11 2.99 47,922 55,347
870.70 0.11 2.97 47,330 -57,126
880.60 0.11 2.95 46,733 -58,910
890.50 0.11 2.93 46,132 -60,699
900.40 0.11 2.91 45,526 -62,492
910.30 0.11 2.88 44,916 -64,289
920.20 0.11 2.86 44,302 -66,091
Appendix B
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Appendix C
Channel Report
Hydraflow Express Extension for Autodesk0 AutoCADO Civil 3D0 by Autodesk, Inc. Tuesday,Feb 18 2020
24in PVC @ 0.3%
Circular Highlighted
Diameter (ft) = 2.00 Depth (ft) = 1.80
Q (Cfs) = 15.61
Area (sqft) = 2.98
Invert Elev (ft) = 100.00 Velocity (ft/s) = 5.24
Slope (%) = 0.30 Wetted Perim (ft) = 5.00
N-Value = 0.011 Crit Depth, Yc (ft) = 1.43
Top Width (ft) = 1.20
Calculations EGL (ft) = 2.23
Compute by: Q vs Depth
No. Increments = 10
Elev ft p (
) Section Depth (ft)
103.00 3.00
102.50 2.50
102.00 2.00
101.50 _ 1.50
101.00 1.00
100.50 - 0.50
100.00 0.00
99.50 -0.50
0 1 2 3 4
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk0 AutoCADO Civil 31DO by Autodesk, Inc. Tuesday, Feb 18 2020
12in PVC @ 0.75%
Circular Highlighted
Diameter (ft) = 1.00 Depth (ft) = 0.90
Q (cfs) = 3.886
Area (sqft) = 0.74
Invert Elev (ft) = 100.00 Velocity (ft/s) = 5.22
Slope (%) = 0.75 Wetted Perim (ft) = 2.50
N-Value = 0.011 Crit Depth, Yc (ft) = 0.84
Top Width (ft) = 0.60
Calculations EGL (ft) = 1.32
Compute by: Q vs Depth
No. Increments = 10
Elev ft p (
( ) Section Depth (ft)
102.00 2.00
101.50 1.50
101.00 1.00
loop-
100.50 0.50
100.00 0.00
99.50 -0.50
0 1 2 3
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Tuesday, Feb 18 2020
8in PVC @ 1.2%
Circular Highlighted
Diameter (ft) = 0.67 Depth (ft) = 0.60
Q (cfs) = 1.689
Area (sqft) = 0.33
Invert Elev (ft) = 100.00 Velocity (ft/s) = 5.05
Slope (%) = 1.20 Wetted Perim (ft) = 1.68
N-Value = 0.011 Crit Depth, Yc (ft) = 0.60
Top Width (ft) = 0.40
Calculations EGL (ft) = 1.00
Compute by: Q vs Depth
No. Increments = 10
Elev (ft)
Section
101.00
100.75
v
100.50
100.25
100.00
99.75
0 1
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk0 AutoCADO Civil 3D0 by Autodesk, Inc, Tuesday,Feb 18 2020
3Oin RCP @ 0.45%
Circular Highlighted
Diameter (ft) = 2.50 Depth (ft) = 2.25
Q (cfs) = 34.66
Area (sqft) = 4.66
Invert Elev (ft) = 100.00 Velocity (ft/s) = 7.45
Slope (%) = 0.45 Wetted Perim (ft) = 6.25
N-Value = 0.011 Crit Depth, Yc (ft) = 2.00
Top Width (ft) = 1.50
Calculations EGL (ft) = 3.11
Compute by: Q vs Depth
No. Increments = 10
Elev (ft)
Section
103.00
102.50
102.00
101.50
101.00
100.50or
100.00
99.50 - _-
0 1 2 3 4
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk0 AutoCADO Civil 3D0 by Autodesk,Inc. Tuesday,Feb 18 2020
21in @ 1.1%
Circular Highlighted
Diameter (ft) = 1.75 Depth (ft) = 1.58
Q (cfs) = 20.93
Area (sqft) = 2.28
Invert Elev (ft) = 100.00 Velocity (ft/s) = 9.18
Slope (%) = 1.10 Wetted Perim (ft) = 4.38
N-Value = 0.011 Crit Depth, Yc (ft) = 1.62
Top Width (ft) = 1.05
Calculations EGL (ft) = 2.88
Compute by: Q vs Depth
No. Increments = 10
Elev (ft)
Section
102.00
101.50
101.00
100.50
100.00
99.50
0 1 2 3
Reach (ft) I