HomeMy WebLinkAbout07 - Design Report - Laurel Glen Ph 2 Lot 1 Block 34 - Stormwater DESIGN REPORT FOR
STORM 'WATER MANAGEMENT
LOT 1 - BLOCK 34
LAUREL GLEN SUBDIVISION - PHASE 2
Prepared for:
HMP, LLC
126 Riverside Drive, Bozeman, MT 59718
Prepared by:
C & H Engineering and Surveying, Inc.
1091 Stoneridge Drive
Bozeman, MT 59718
(406) 587-1115
Project Number: 06070
January, 2007
INTRODUCTION
The proposed development is located in Lot 1 of Block 34 of Phase 2 of Laurel Glen
Subdivision. Laurel Glen Subdivision has been approved as a 272 lot residential subdivision,
located in the South Half of Section 4, Township 2 South, Range 5 East of P.M.M., Gallatin
County, Montana. The proposed development will consist of 12 condominiums with 3,4, and 5
unit buildings,providing a total of 43 units for the lot.
A design report, completed by Allied Engineering Services, Inc., has previously been approved
by the City of Bozeman. This report serves as a supplement to Allied's report, addressing items
for lot I not included in Allied's report.
Access to the lot will be provided in two locations; one at the intersection Saxon Way and Laurel
Parkway and the other at the south end of the lot on Glenellen Drive. An interior drive will be
constructed running from the north end of the property to the south end of the property,
approximately 825 feet, where it will intersect with Glenellen Drive. This street will be
constructed with curb and gutter to a distance of 26 feet from back of curb to back of curb,
except for a portion in the center of the lot, where the street will be 29 feet from back of curb to
back of curb. A 3 foot wide sidewalk will be constructed along the east side of the drive directly
adjacent to the back of curb. The sidewalk will connect to sidewalks on both Oak St. and
Glenellen Drive.
STORM WATER MANAGEMENT
Storm water run-off from Phase 2 of Laurel Glen Subdivision did not include calculations for lot
1 of block 34. Pond 42 of Laurel Glen Phase II design report, located at the northeast corner of
lot 1 was built to handle runoff from drainage area#2 from the Laurel Glen Subdivision Phase II
Design Report.
Runoff from both drainage area#2 of Laurel Glen Phase II and lot#1 are combined to determine
if pond #2 from Laurel Glen II is adequate to handle projected runoff . Also, an additional
section of Oak St. extending to the east was added to this drainage area, as it will be necessary
for future development of phase 4 of Laurgel Glen. These areas are collectively called drainage
area#1 in this report, as depicted on the included drainage map in appendix B.
For the existing development, drainage originates at the intersection of Glenellen Drive and
Laurel Parkway and flows via the street curb North to the intersection with Oak Street. From
here, it flows in the south curb to the east to the end of Oak Street, where it will enter a swale
conveying the water to the pond.
6
For the proposed development of lot 1, drainage will originate on the far south end of the lot and
travel north via the curb and gutter of the proposed drive. Drainage from areas 1-A and 1-C will
flow to the north in the east curb and gutter where it will be conveyed to existing pond #1 via
swales located in the turn-around and at the north termination of the drive. Drainage from area
1-13 will flow to the north in the west curb and gutter and will be conveyed to pond#2 via a curb
cut in the drive
COMPOSITE RUNOFF COEFFICIENT FOR DRAINAGE AREA 91
In order to proceed with sizing the storm retention pond, we must first determine the
composite runoff coefficients. An example of how these calculations were performed is
shown below. A comprehensive list of drainage area calculations can be found in the
appendix.
Drainage Area 91
The storm water runoff surface areas for combined subareas of drainage Area #1 were
calculated as follows:
CONTRIBUTING AREAS
Grass (C=.20) =69,255 ft2
Paved (C=0.90) = 86,296 ft2
Roof(C=.85) =25,313 ft2
Total = 180,864 ft2
=4.15 acres
COMPOSITE RUNOFF COEFFICIENT
Mean C= [(0.20x69,255)+(0.90x86,296)+(.85x25,313)]/180,864 =0.625
RETENTION POND #1 (EXISTING POND)
7
The combined drainage from sub areas of area #1 are combined to determine the runoff created
in a 10 year event - 2 hr. event. The volume of a retention pond sized to handle 10-yr 2-hr storm
event is calculated as follows:
Q=CiA Q=storm water runoff rate(cfs)
iio=0.64(t-0.65)=0.64(2 Ius)"0.6'=0.408 in/hr
d=(0.408 in/hr)x(1 ft/12 in)x(2 hr) C=runoff coefficient
d=0.068 ft d =depth of rainfall(ft)
V=CdA A=surface area(ft2)
V=total volume required
V(Area 41)=0.625 x 0.068 ft x 180,864 ft2=7,687 ft3 =volume to be retained
The existing pond is 1.5 feet deep and has a mid-depth area of 5,770 sq.ft., allowing a total
capacity of 8,655 ft3. Therefore, the existing pond is more than adequate to handle the projected
runoff from lot 1 as well as the runoff from the existing and projected runoff from Laurel
Parkway and Oak St.
The volumes for each drainage area were calculated as shown above. Calculations are included
at the end of this report.
RETENTION POND#2
Drainage area #2 will drain to retention pond 42 located just south of the entrance on Laurel
Parkway. The volume of retention pond #2 sized to handle 10-yr 2-hr storm event is calculated
as follows:
Q=CiA Q=storm water runoff rate (cfs)
iio=0.64(t-0.65)=0.64(2 hrs)-o.61= 0.408 in/hr
d=(0.408 in/hr)x(1 ft/12 in)x(2 hr) C=runoff coefficient
d=0.068 ft d =depth of rainfall (ft)
V =CdA A=surface area(ft)
V=total volume required
8
V(Area 42) =0.543 x 0.068 ft x 32,183 ft2= 1,189 ft3=volume to be retained
The proposed pond #2 is 1.5 feet deep and has a mid-depth area of 980 sq.ft., allowing a total
capacity of 1,469 ft3. Therefore, pond #2 is more than adequate to handle the projected runoff
from drainage area 42.
DRAINAGE AREA#1C CULVERT SIZING
As indicated on the included drainage map,the runoff from drainage area#1 C will flow to the
north where it will run under the proposed sidewalk via a culvert,and onto pond#1. In order to
size the pipe,the time required for a drop of water to travel from the furthest point in drainage
area#1 C to the culvert is calculated as follows:
Time of Concentration
Overland flow(22 ft @ I%, C=0.20) =7.89 min. (see Appendix B)
Gutter flow(208 ft @ 1.54%avg.slope)
V=(1.486/n)R2`Su2 (n=0.013, A=1.24 ft,P=9.23, R=0.134, S=0.0154)
V=3.72 ft/s
T=208 ft/3.72 ft/s/60s/min =.93 min
Total Time of Concentration(overland+gutter)= 8.82 minutes(0.147 hours)
For a 25-year storm event Its =0.78X-,"=0.78(0.147)".54=2.66 in/hr
Q25=CIA=0.825(2.66 in/hr)(.513 acres)= 1.12 cfs
Autocad's Manning calculator was used to determine the capacity of a 12" pvc pipe at a slope of
1%. A 12" pipe will flow at 39% full during the 25 year event, and is therefore more than
adequate to handle these flows. This calculation is included in the appendix.
A Landscape swale was used in lieu of this culvert.
9
Watershed Area Calculations
Lot 1,13lock 34-Laurel Glenn Phase If
Job#06070
Land Use Watershed#1A Watershed#1B Watershed#1CWatershed#1C Watershed#1 Watershed#2 Watershed#3'
Category Combined
Grass 5532.00 50328.00 1979.00 11416.00 69255.00 16132.00 14719.00
Total Paved 19594.00 52164.00 14460.00 78.00 86296.00 11974.00 0.00
Roof 9513.00 7208.00 5889.00 2703.00 25313.00 4077.00 9460.50
Total Area 34639.00 109700.00 22328.00 14197.00 180864.00 3218300 24179.50
Watershed#1A
Grass C= 0.20 Paved C= 0.90 Roof C= 0.85
Area(Ft2)= 5532.00 Area(Ft2)= 19594.00 Area(Ft2)= 9513.00
Area(ac)= 0.13 Area(ac)= 0.45 Area(ac)= 0.22
Weighted C= 0.774
Watershed#113
Grass C= 0.20 Paved C= 0.85 Roof C= 0.85
Area(Ft2)= 50328.00 Area(Ft2)= 52164.00 Area(Fe)= 7208.00
Area(ac)= 1.16 Area(ac)= 1.20 Area(ac)= 0.17
Weighted C= 0.552
Watershed#1 C
Grass C= 0.20 Paved C= 0.90 Roof C= 0.85
Area(Ft2)= 1979.00 Area(Ft2)= 14460.00 Area(Ft2)= 5889.00
Area(ac)= 0.05 Area(ac)= 0.33 Area(ac)= 0.14
Weighted C= 0.825
Watershed#1 D
Grass C= 0.20 Paved C= 0.85 Roof C= 0.85
Area(Ft2)= 11416,00 Area(Ft2)= 78.00 Area(Ft2)= 2703.00
Area(ac)= 0.26 Area(ac)= 0.00 Area(ac)= 0.06
Weighted C= 0.327
Watershed#1 Combined
Grass C= 0.20 Paved C= 0.90 Roof C= 0.85
Area(Ft2)= 69255.00 Area(Ft2)= 86296.00 Area(Ft2)= 25313.00
Area(ac) 1.59 Area(ac)= 1.98 Area(ac)= 0.58
Weighted C= 0.625
Watershed#2
Grass C= 0.20 Paved C= 0.90 Roof C= 0.85
Area(Ft2)= 16132.00 Area(Ft2)= 11974.00 Area(Ft2)= 4077.00
Area(ac)= 0.37 Area(ac)= 0.27 Area(ac)= 0.09
Weighted C= 0.543
Watershed#3
Grass C= 0.20 Paved C= 0.90 Roof C= 0.85
Area(Ft2)= 14719.00 Area(Ft2)= 0.00 Area(Ft2)= 9460.50
Area(ac)= 0.34 Area(ac)= 0.00 Area(ac)= 0.22
Weighted C= 0.454
`11
DRAINAGE AREAS
T A
PIT B x1t= CREEK CONDOS
LOCATED IN LOT 1, BLOCK 34 0 LAUREL GLEN SUBD. PHASE li
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DRAINAGE AREA BOUNDARY
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PROPERTY UNE
ADJOINING PROPERTY UNE
—' — — — — EASEMENT UNE
Date Pre pored. 1/16/07 PROPOSED CURB AND GUTTER
— -— •—.— PROPOSED SWALE/RETENTION POND CONTOUR
s to s r.x WER AND MITIGATION BOUNDARY
PROPOSED INLET/CATCH BASIN
Engineering and Surveying Inc.
x van SPOT ELEVATIONS(EXISTING do PROPOSED)
1091 S-1w or •8-,mr 59716 nit.- DRAINAGE DIRECTION
Phao F4061587.1115•Fu 14061587-97
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