HomeMy WebLinkAbout17 - Design Report - Westbrook - Laurel Glen Lift Station Analysis - Final Plat Response to Condition of Approval LAUREL GLEN LIFT STATION ANALYSIS
WESTBROOK FINAL PLAT RESPONSE TO
CONDITION OF APPROVAL #21
Prepared for:
Westbrook Development, LLC.
125 Central Avenue, Bozeman, MT 59718
Prepared by:
r
Engineering and Surveying Inc.
1091 Stoneridge Drive • Bozeman, MT 59718
Phone (406) 587-1 115 • Fax (406) 587-9768
www.chengineers.com • info@chengineers.com
All
�o ry
Project Number: 14693.9 , '
/07
March 2017 ��=' • ''�����'����
Condition #21 —Verification of Lift Station Capacity at Westbrook Subdivision
21. The proposed development had previously addressed sewer capacity with dependence on the
area lift station. Design calculations shall be provided to verify the new density of the
subdivision does not exceed what was previously accounted for.
1.0 SEWER SYSTEM CONFIGURATION
1.1 West Connection:
Phases 1, 2 and 3 of Westbrook will flow into an existing 8-inch main at the intersection
of Immanuel Way and Laurel Parkway and from there flow north in Laurel Parkway to
the Laurel Glen Lift Station.
1.2 East Connection:
Phase 4, Lot 1, Block 5 of the Westbrook Subdivision, will flow north to Annie Street
and from there flow down future Rosa Way and be routed west to the Laurel Glen Lift
Station. The mains located in Lot 1 will be installed with the future site plan for Lot 1,
Block 5 which is an R3 zoned multifamily lot. All sewer flows from the proposed
subdivision are routed through the Laurel Glen Lift Station installed in Phase 1 of the
Laurel Glen Subdivision and then upgraded in 2006 to accommodate the Rosa and
Flanders Subdivisions.
2.0 LIFT STATION CAPACITY ANALYSIS APPROACH
On the attached excerpt from the July, 2002 Laurel Glen Subdivision design report prepared by
Allied Engineering, titled Table 2, a design population for Westbrook Subdivision's west
connection is indicated to be 179 people (pipe segment S40) and the east connection is indicated
to be 75 people (pipe segment 65) with a catchment area of 6.2 acres which is approximately
equal in boundary to the current 5.41-acre lot. This equates to a total original design population
of 254 people in what is now Westbrook Subdivision. The sewer catchment areas from the 2002
Subdivision Design Report (shown on Table 2 and Figure 1) along with the original zoning map
from 2002 were used to break out the capacity allocation for the current boundary of Westbrook
Subdivision which consists of 62 single family lots and 1 R3 lot totaling 5.41 acres. These
supplements are provided at the end of the report for reference.
3.0 PREVIOUS AND CURRENT POPULATION ESTIMATES
The March, 2015 Westbrook Design Report, estimates 131 people for the west connection and
118 people for the east connection. The current projected population therefore equates to 249
people which is below the 2002 population estimate of 254 people. However, the flow rates used
in sizing the Laurel Glen lift station as indicated in Table 2 of the January 2003 Laurel Glen Lift
Station Design Report prepared by Allied Engineering, used a per capita wastewater generation
rate of 72 gallons per capita per day (gpcd). This rate is based on the 1998 Wastewater Facility
Plan. The City of Bozeman (the City) has updated the facility plan since this earlier version and
has increased the per capita wastewater generation rate to 89 gpcd as indicated in the 2007
Bozeman Wastewater Facility Plan prepared by Morrison Maierle. As such, the City
engineering department has required that all lift station analyses be converted to the current
wastewater generation rate of 89 gpcd which in effect reduces the original population estimates.
4.0 CAPACITY ANALYSIS CONSIDERING AVERAGE DAILY FLOW
For this report, we considered the average flow rates given in Table 2 of the January 2003 Lift
Station Report and developed Table A and Table B using the 2002 zoning map and Table 2
supplements. Tables A, B and Table 2 from the 2003 Lift Station report have been included at
the end of the report for reference. Since Table 2 only lists an overall average flow rate for all
phases of Laurel Glen Subdivision, we developed Table A and Table B to reproduce the original
capacity allocations in average and peak flows for the area that is now known as Westbrook
Subdivision (originally Phase 3 of Laurel Glen). Based on Table A and Table B and converting
the average daily flows to the current wastewater generation rate of 89 GPCD the following
design populations result:
West Connection:
Population= (179 people x 72 GPCD)/89 GPCD= 145 people
East Connection.-
Population= (75 people x 72 GPCD)/ 89 GPCD=61 people
It appears that the Laurel Glen Lift Station was adequately designed to handle Westbrook Phases
1, 2 and 3 and the East connection (Phase 4) should have sufficient capacity for up to 61 people
by using the current COB design constraint for per capita wastewater generation of 89 gpcd.
Considering that Phases 1, 2 and 3 of Westbrook consists of 131 people on single family lots 14
additional people can be allocated to the 5.4-acre lot in Phase 4 which brings the population
allowed on that lot under the current lift station capacity to 75 people. It should also be discussed
that the current Bozeman Wastewater Facility Plan adopted in 2015 revises the 89 gpcd figure
down to 64.4 gpcd for residential uses based on current wastewater flow metering data from the
City wastewater treatment facility head works. This is discussed in Section 1.2 - Basis of
Planning of the 2015 Bozeman Wastewater Facility Plan prepared by Morrison Maierle. If the
design per capita wastewater rates were revised down in subsequent revisions to the COB Design
Standards and Specifications, the allowed dwelling units under the current lift station capacity on
the 5.4-acre lot would increase.
5.0 PEAK-HOUR FLOW DISCUSSION
As indicated in the Peaking Factor note on Table A and Table B, the peak-hour flow used for
sizing the submersible pumps was calculated based on the overall population served by the lift
station. In 2003 this population was estimated at 2,497 people. This equates to a peaking factor
of 3.52 using the Harmon Formula. In 2006 when the lift station was upgraded a peaking factor
of 3.4 was established with an overall population estimate of 3,329 (an additional 832 people
were added to the lift station design in 2006 with the pump and wet well upgrades). 3.4 is the
current established peaking factor and this number was used for this analysis and has also been
used to size the submersible pumps currently in operation at the lift station.
The following is a comparison of the Peak Hourly Flows from the original population projection
in the 2003 Lift Station Design Report to the current population projection as indicated in the
March 2015 Westbrook Subdivision Report:
5.1 -2003 Projections Peak Hour Analysis:
West Connection:
2003 Population Projection: 179 people
Infiltration= 3,225 GPD (150 gal/acre/day x 21.5 acre)
Average Flow= 179 people x 72 gpcd= 12,888 (not including infiltration)
Peak Factor= 3.4
Peak Hour Flow= 3.4 x 12,888 GPD + 3,225 GPD =47,044 GPD
East Connection:
2003 Population Projection: 75 people
Infiltration=930 GPD (150 gal/acre/day x 6.2 acre)
Average Flow= 75 people x 72 gpcd= 5,400 (not including infiltration)
Peak Factor= 3.4
Peak Hour Flow= 3.4 x 5,400 GPD + 930 GPD= 19,290 GPD
5.2 -2015 Projections Peak-hour analysis:
West Connection:
2015 Population Projection: 131 people
Infiltration=3,977 GPD (from 2015 Westbrook Design Report)
Average Flow= 131 people x 89 gpcd= 11,659 (not including infiltration)
Peak Factor=3.4
Peak Hour Flow= 3.4 x 11,659 GPD + 3,977 GPD =43,618 GPD
East Connection:
2015 Population Projection: 118 people
Infiltration= 1,325 GPD (from 2015 Westbrook Design Report)
Average Flow= 118 people x 89 gpcd= 10,502 (not including infiltration)
Peak Factor= 3.4
Peak Hour Flow= 3.4 x 10,502 GPD+ 1,325 GPD = 37,032 GPD
As shown above the currently projected peak-hour flow for the west connection is below the
peak hour flow from the 2003 population projection. The currently projected peak-hour flow
from the east connection comes out 92%higher than the original flow projection from 2003.
CONCLUSIONS
Based on the above discussion and calculations and Tables A and B developed for this report it
appears that Phases 1, 2 and 3 of Westbrook, what is known as the west connection in this report,
do have sufficient capacity for the proposed number of units as indicated in the 2015 Design
Report for Westbrook Subdivision. The east connection (Phase 4) would therefore be limited to
a population of 75 people considering the leftover capacity from Phases 1, 2 and 3 and also the
original capacity provided with the 2003 Lift Station Report when adjusted to the current
wastewater generation rate of 89 gpcd from the original wastewater generation rate of 72 gpcd.
Table A
2002 Acreage West People/ Equivalent
Segment Zoning Connection Acre Population
43 R2 2.04 7.62 15.54
42 R2 3.2 7.62 24.38
45 R2 2.4 7.62 18.29
44 R2 1.7 7.62 12.95
82 R2 0.83 7.62 6.32
81 R2 1.22 7.62 9.30
41 R2 2.62 7.62 19.96
46 R3A 0.92 12.19 11.21
48 R3A 0.66 12.19 8.05
50 R3A 0.75 12.19 9.14
83 R3A 0.93 12.19 11.34
39 R7 0.96 7.62 7.32
51 R8 0.93 7.62 7.09
49 R9 1.4 7.62 10.67
40 R10 0.98 7.62 7.47
Total 21.54 179.03
Table B
2002 Acreage East People/ Equivalent
Segment Zoning Connection Acre Population
67 R3 1.71 12.19 20.84
65 R3A 4.46 12.19 54.37
Tota I 6.17 75.21
Table 2
Sewer segment peak flow and sizing
Total Required
Sewer Downstream Tributary Upstream Total Tributary Upstream Upstream Peaking Peak Flow Pipe
-Segment# Segment# Flow( m) Flow( m) Flow( m) Population Population Population Factor ( m) Dia.(in)
0.0 78.6 78.6 0.0 92I.9 921.9 3.82
300 10
1.3 77.3 78.6 21.7 900.2 92 L9 3.82 300 10
3 2 L7 75.6 77.3 28.6 871.E 900.2 3.83 296 10
s 4 z 3 0.0 7.4 7.4 0.0 124.5 124.5 4.22 31 8
1.7 3.3 4.9 27.9 54.7 82.6 4.27 21 8
fi Y '4: 5 1.7 1.5 3.3 28.9 25.8 54.7 4.31 14 8
7 {t b LS 0.0 1.5 25.8 0.0 25.8 4.36 7 8
ti 8 N 4 1.2 1.3 2.5 19.9 22.0 41.9 4.33 11 8
9 =8 1.3 0.0 1.3 220 0.0 22.0 4.37 6 8
3.8 64.4 68.2 64.2 682.8 747.0 3.86 265 8
12 r 10 1.1 63.3 64.4 19.1 663.7 682.8 3.90 251 8
L x 11 16.1 35.2 51.3 7.2 451.6 458.9 3.99 205 8
0.3 34.8 35.2 5.5 446.2 451.E 4.00 141 8
14 @ 13 1.3 1.5 2.8 20.9 25.2 46.1 4.32 12 8
Ij 14 1.5 0.0 1.5 25.2 0.0 25.2 4.37 7 g
lti t' 11 0.8 0.6 1.4 13.2 10.5 23.6 4.37 6 8
17 16 0.6 0.0 0.6 10.5 0.0 10.5 4.41 3 8
IS , Pt 0.0 10.7 10.7 0.0 181.1 181.) 4.16 44 8
14 18 0.0 2.9 2.9 0.0 49.6 49.6 4.32 13 8
20 s19 2.9 0.0 2.9 49.6 0.0 49.6 4.32 13 8
Zl I8 " 1.3 6.5 7.8 21.5 110.0 131.5 4.21 33 8
22 :21 0.9 5.5 6.5 15.4 94.6 110.0 4.23 27 8
23 22 0.8 4.8 5.5 13.2 81.4 94.6 4.25 24 8
34 23 4.8 0.0 4.8 81.4 0.0 81.4 4.27 20 8
23 ?i3 0.0 32.0 32.0 0.0 400.1 400.1 4.02 129 6
6 ?5 8.3 12.5 20.8 10.4 210.E 221.1 4.13 86 8
2] „26 0.2 12.3 12.5 3.7 206.9 210.E 4.14 52 8
28 27 0.1 11.4 11.5 1.9 191.6 193.5 4.15 48 6
?9 z 28 0.0 3.9 3.9 0.0 65.1 65.1 4.29 17 8
0.7 0.0 0.7 11.3 0.0 11.3 4.41 3 B
1.7 1.5 3.2 27.8 26.t 53.9 4.31 14 8
32 31 1.5 0.0 1.5 26.1 0.0 26.1 436 7 8
33 28 1.2 0.0 1.2 20.3 0.0 20.3 4.38 5 $
34 28 I.7 4.6 6.2 28.1 78.0 106.2 4.24 26 8
35 34 0.5 4.1 4.6 8.5 69.5 78.0 4,27 20 8
36 35• 1.6 2.4 4.1 27.7 41.8 69.5 4.28 17 8
3T 36 _ L5 ).0 2.4 25.2 16.6 41.8 4.33 11 8
38 37 1.0 0.0 1.0 16.6 0.0 16.6 4.39 4 8
39 27 0.8 0.0 0.8 13.4 0.0 13.4 4.40 3 8
40 25 0.0 11.2 11.2 0.0 179.0 179.0 4.17 47 8
41`' 40 1.0 0.0 1.0 15.5 0.0 15.5 4.39 4 8
42; 40 0.0 10.2 10.2 0.0 163.5 163.5 4.18 43 8
1.2 9.0 10.2 183 145.2 163.5 4.18 43 8
0.8 8.2 9.0 13.0 132 145.2 4.20 38 B
95 44 1.6 0.0 1.6 24.4 0.0 24.4 4.37 7 8
4ti 44 0.4 6.3 6.7 6.3 101.5 107.9 4.23 28 8
3
Table 2 (continued)
Sewer segment peak flow and sizing
"i4� 46 0.0 6.3 6.3 0.0 101.5 101.5 4.24 27 8
0.0 3.3 3.3 0.0 53.0 53.0 4.31 14 8
0.5 0.5 0.9 7.5 7.3 14.8 4.40 4 8
50 `0. ..;; 0.5 0.0 0.5 7.3 0.0 7.3 4.43 2 8
1.3 1.0 2.3 22.0 16.2 38.2 434 10 8
LO 0.0 1.0 16.2 0.0 16.2 4.39 4 8
1.3 0.0 1.3 20.0 0.0 20.0 4.38 6 8
0.5 1.2 1.7 8.0 20.5 28.6 4.36 7 8
r55 s' S r54 f a 12 0,0 1.2 20.5 0.0 20.5 4.38 5 8
56 76 15 21.3 24.8 59.2 363.7 423.0 4.01 99 8
5 5T56 0.0 21.3 21.3 0.0 363.7 363.7 4.04 86 8
58 57 0.0 21.3 21.3 0.0 363.7 363.7 4.04 86 8
5�4` 58 1.7 19.6 21.3 29.3 334.5 363.7 4.04 86 8
.................60 = 59 1.5 15.8 17.3 26.3 270.0 296.3 4.08 71 8
61 t �Q 1.7 11.9 13.6 29.9 203.2 233.1 4.12 56 8
62n p' 61 0.7 6.9 7.7 12.2 118.E 130.8 4.21 32 8
r 63 " t,621 0.4 5.6 6.0 7.2 96.1 103.2 4.24 26 8
64 63 1.2 4.4 5.6 20.8 75.2 46.1 4.25 24 8
65,z j 64 3.2 1.2 4.4 54.4 20.8 75.2 4.28 19 8
66 65 0.0 1.2 1.2 0.0 20.8 20.8 4.38 5 8
67 66 i 1.2 0.0 1.2 20.8 0.0 20.8 4.38 5 8
z68 61 0.0 2.4 2.4 0.0 40.2 40.2 4.33 10 8
69 68 2.0 0.4 2.4 34.0 6.2 40.2 4.33 10 8
70 69 0.4 0.0 0.4 6.2 0.0 6.2 4.43 2 8
`7E 0.9 0.0 0.9 15.4 0.0 15.4 4.39 4 8
72„ 61 1.9 0.0 1.9 32.2 0.0 32.2 4.35 8 8
73 60 0.9 1.2 2.2 16.2 20.7 36.9 4.34 9 8
74 73 L2 0.0 1.2 20.7 0.0 20.7 4.38 5 8
>75 1 59 2.2 0.0 2.2 38.2 0.0 38.2 4.34 10 8
76 0.0 103.4 103.4 0.0
TOTALS 103A 1344.9 1344.9 1344.9 3.71
PVC pipe meeting COB specifications will be used for all the sewer mains. Since all sewer main
segments will be 10-inch diameter and smaller, a maximum manhole spacing of 400 feet was
used and a manhole barrel size of 48-inches is required, except for inside drop manholes which
will be 60-inches in diameter. Manholes will be constructed according to COB specifications.
All sewer services will be 4-inch or 6-inch PVC installed at 2 to 4% grade. Sewer services will
connect into the main with in-line gasketed wyes. A 4-inch sewer service with a grade of 2%
can convey approximately 60 gpm at 50% capacity. Our flow calculations show that no
individual lot generates over 60 gpm of peak flow.
A plan view of the proposed sewer infrastructure is shown in Figure 3. Sewer mains were placed
along the centerline of the street on tangential alignments. On curvilinear street alignments,
sewer mains were kept a minimum of five feet from the edge of the concrete gutter for collector
and arterial streets and 3.5 feet for local streets. On curvilinear street alignments we attempted to
keep manholes in the center of the street. However, to keep the required separation distance
between water mains and the edge of the concrete gutter in addition to maintaining an 11-foot
separation water and sewer mains, some sewer manholes do not fall on the street centerline.
Laurel Glen subdivision will utilize a lift station for pumping wastewater effluent into the City of
Bozeman collection system. The downstream manhole in Laurel Glen is approximately 15 feet
lower than the tie in manhole for the City of Bozeman, thus preventing gravity flow into the
collection system. The City of Bozeman engineering department has noted the need for a lift
station in the pre-application plan review.
4
S56 557 S58
LEGEND
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N0. REVISIONS DRAM 8Y DATE tso NO 4p
LAUREL GLEN SUBDIVISION •.:.:a C1vilEngineeringPRaECT it oD-1B5 a
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i Land Surveying BOZEN1AN,aIT597I8 Cora, E.hiblt-2d,•.g 1®1
��� i ALLIED Geutechnical Engineering PHONE(a06)582.0221 LAUREL GLEN
ENGINEERING Structural Engineering FAX(406)582.3770
J CiRYIGE`J.INC. MASTER PLAN
MDEQ
00-185 Laurel Glen Subdivision
January 13,2003 Pr.
Business Park zoning designation. The largest generator of wastewater per acre in the facility plan
was the B2 zoning designation at 3,495 gallons per acre,which was used in lieu of a flow for B-1.
Table 2. Population Served by Lift Station
Population Number of
Zone Classification (per acre) Developed Acres Total Population Flow(gpd)
Laurel Glen
R-2 7.62 34.74 265 19,060
R-3 12.19 32.56 397 28,578
R-3A 12:19 38.06 464 33,405
R-4 12.19 21.84 267 19,169
B-1 N/A 8.72 30,477
Valley West
R-3 12.19 40 487.6 35,108
B-1 N/A _ 4 13,980
Totals 1.79.92 179,777
The average daily flow rate, including infiltration, is then calculated as follows; ..
179,777 gal/day/ 1440 min/day= 124.85 gpm
(150 gal/acre/day x 228 total acres)/ 1440 min/day=.23.75gpm
124.85 gpm+23.75 gpm= 148.6 in
An equivalent population was estimated in order to calculate a peaking.factor. According to the
1998 Wastewater Facility Plan for the City of Bozeman, people generate 72 gpcd. The total flow
of 179,777 gpd has an equivalent population of 2,497 people.
Using the following equation from DEQ2, the peaking factor for the peak hourly flow rate was
estimated as follows:
F=(18 +po.5)/(4+po.$)
Where:
F=Peaking Factor
P=Population(in thousands)
F= (18 +2.40.$)/(4+2.40")
F=3.52
The peak hourly flow rate is then estimated to be 124.85 gal/min * 3.52 =439.47 gal/min+ 23.75
gal/min (infiltration)= 463.22 gal/min. A peak hourly flow rate of 465gpm will be used for all
calculations.
S.-Wrojects12000100-185 Laurel Glen SublDesien ReporALi t Stationwesi n Report 1-7-03 doc
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