HomeMy WebLinkAboutApplication APPLICANT 1/8/2022380 Q Street, Ste 200
Springfield, Oregon 97477
(541)302-9790
dylanmciver@ao-engr.com
A a: O Engineering LLC
Stormwater Report
RUI-b Woodland Ridge Subdivision Phases 3 & 4
tacatimc Jasper Rd & Mt Vemon Rd, Springfield, OR
Prepared syr Dylan McIver, EIT
$0
03%
Reviewed Scott Morris, PE
b7r
Dater 12/20/2021
Type of Praiecc Residential Subdivision
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Project Overview
The Woodland Ridge subdivision on the south side of the ridge is proposed to be constructed in 6
separate phases. This report includes a detailed analysis of phase 3 & 4 and a general design overview
for phases 5 through 8. Woodland Ridge Subdivision (south) is a proposed single family residential
subdivision with 265 lots on a 72.8 -acre parcel. The property is located north of Jasper Rd and east of
Holly St. There is an existing 18" culvert passing beneath Jasper Rd right of way flowing to the south
and west through a series of ditches and culverts before eventually discharging into the Willamette River
seveml miles downstream. Stormwater runoff from the subdivision is proposed to discharge treated
stormwater upstream of the existing 18" culvert beneath Jasper Rd at pre -construction peak flow rates.
To meet City of Springfield detention and treatment requirements for stormwater runoff in the subdivision
it is proposed to construct three separate stormwater quality ponds at different locations within the
development. Stormwater runoff will be routed to these ponds via underground piping within the
subdivision. Runoff within the right of way will be collected via curb inlets, with houses proposed to
discharge roof drainage to streets via weepholes in the curb or storm lateral connections depending on
the lot location within the proposed subdivision.
Phases 3 & 4 includes 108 lots, located in the northwest quadrant of the development, and includes the
south 51s Place connection to Woodland Ridge phase 1.
Existing Conditions
The property currently is an open grass field with a private gravel driveway running through that
previously provided access to a home that has since burned down. A gravel maintenance path exists
along the southern site boundary that services a series of sanitary manholes. Web Soil Survey lists the
following soil types as present on the site:
#73
Linslaw loam
HSG
D
#121 B
Salkum silty clay loam, 2 to 8 percent slopes
HSG
B
#121C
Salkum silty clay loam, 8 to 16 percent slopes
HSG
B
The total parcel area is 72.8 acres; however, the new development area is approximately 49.68 acres,
and this will be used for existing conditions analysis. The remaining acreage flows down the ridge or
south along the historic drainage path detailed next: The existing site within the development area slopes
towards the Southwest, but a ditch along the southern site boundary carries all runoff east into a series
of culverts beneath the existing railroad track where runoff would leave the site. After the discharge
passes the site boundary, it flows west along an existing ditch north of Jasper Rd where an existing 18"
culvert conveys stormwater runoff south under the road. Within the property south of Jasper Rd is a
series of check dams conveying the stormwater runoff south and west towards the Willamette River.
Below is a table summarizing the physical characteristics of the existing site:
Table f: Existing site physlcatcharectenstics.
Land Type & Description
Total
Curve
TP -3
Acreage
Number
Meadow, non -grazed,
10
0.1
TP -16
49.698
71
HSG C
Infiltration
Infiltration testing was performed by Geopacific, Inc. In summary, three infiltration tests were performed
throughout the site and the following recommendations were made:
Table 2: Summary ofgeotechnioal sal tests.
Test Number
Test Depth
(Ft)
Measured Infiltration
Rate (In./hr.)
TP -3
7
0.2
TP -11
10
0.1
TP -16
6
0
Tests 3, 11, and 16 were conducted on the southwest, south, and southeast portions of the site
respectively. To provide a factor of safety, an infiltration rate of zero inthr was used for analysis in all
ponds.
The test pits were dug between 8/31/2021 and 91112021. No seasonal adjustments for the design were
used. All test pits except for TPA 7 found no groundwater seepage between 7.5 to 16 feet below ground
surface. TP -17 observed groundwater seepage at 7 feet below ground surface which is located at an
elevation of 508 feet.
Proposed Destination
Treated and detained on -she stormwater runoff is proposed to discharge from the pond outlet through
a flow spreader into an existing ditch south of the existing maintenance path. The stormwater runoff the
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flows west along the maintenance path until it reaches 4 parallel 12" culverts crossing beneath the
railroad tracks. Stormwater runoff then flows west along an existing ditch between Jasper road and the
railroad tracks until it reaches an existing 16" stonnwater culvert beneath Jasper Road. Stormwater
runoff then flows south through this culvert before eventually reaching the Willamette River. Post -
construction peak flowrate will be controlled by detention facilities to be at or below pre -development
peak flow rates.
To work with the proposed layout and the existing site contours it is proposed to split the stormwater
drainage from the site into three separate systems: the north and western third will be routed through
the detention pond #1 in the southwest section of the site. The central third of the site will be routed
through detention pond #2 along the central southern development boundary. The eastern third of the
site will be muted through detention pond #3 in the southeast section of the site.
The full buildout ofthis subdivision will include 265 single family lots. A basin map summarizing drainage
basin details for the finished subdivision is included in the appendix of this report. The table below
summarizes the physical characteristics of the ponds:
Table 3 Open storage area.
In addition to the open pond area provided above, the ponds are proposed to have 12" of growing
media. The following table summarizes storage capacities for the ponds:
Table 4: Storage capacities foreach pond. (Soil media 8 rock chamber)
Pond:
Open
Storage
Oa1adtyfdl
Bottom
Top of
Top of Slope
Side
3,050
Bottomof
Of(ft
Pond 2
90,673
2,817
93,490
Pond:
Stupe EL
Square
Slope EL
Square Footage
Slopes
Footage(sf)
(ft)
(so
(H:V)
Pond 1
510.00
21,065
514.00
30,495
3:1
Pond 2
513.00
17,377
517A0
28,168
3:1
Pond 3
518.00
13,069
522.00
1 19,392
3:1
In addition to the open pond area provided above, the ponds are proposed to have 12" of growing
media. The following table summarizes storage capacities for the ponds:
Table 4: Storage capacities foreach pond. (Soil media 8 rock chamber)
Pond:
Open
Storage
Oa1adtyfdl
Growing Media Storage
Upadtyl0%void space
kf)
Total
Storage
(cf)
Pond 1
105,523
3,050
108,572
Pond 2
90,673
2,817
93,490
Pond 3
64,538
1,939
66,477
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Hydraulic Calculations
HydroCAD software was utilized to calculate existing condition peak flows leaving the site from the
development area. Below are parameters utilized in calculating the existing condition peak flowrates
leaving the site from the development area:
Runoff Method:
Santa Barbara Urban Hydrograph (SBUH)
Area:
49.698 acres
Curve Number:
90: 1/8 acre lots, 65% imp, HSG C
100 -Year
98: Paved roads wicurbs & sewers, HSG C
Land Description:
71: Meadow, non -grazed, HSG C
Time of Concentration:
Existing Phase 3 & 4: 212 minutes
2 -Year
Existing Macro Basin is 27.0 minutes
Existing Macro Basin 2:152 minutes
Stomr Type:
Type IA 24 -hr
Stone Events:
Water Quality -0.83 inches
2 -Year Storm - 3.30 inches
5 -Year Stone - 3.60 inches
Peak
10 -Year Storm - 4.30 inches
Peak Elev.
25 Year Storm - 4.80 inches
To ensure stormwater structures will not cause flooding, the following hydraulic elevations were
calculated for a post -construction site:
Ta Me 5: Hydraulic elevations whin ponds dunng each storm event.
Curve numbers used in the model include 98 for the impervious ROW and 90 for the 65% impervious
lots. The city of Springfield requires a type IA 24 -hr design storm for each of the water quality, 2, 5, 10,
25, and 10D -year rainfall events. The city of Springfield also requires an emergency spillway system that
safely passes the 100 -year, 24-hour design stone over the pond embankment without overtopping any
part of the pond in the event of a control structure failure. As shown, the ponds completely contain
stormwater runoff from the site dud ng each storm event with the required 1 -foot of freeboard per city of
Springfield stormwater design standards and thus are adequate to protect the subdivision and
surrounding properties from damage.
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Water
5 -Year
30 -Year
25 -Year
100 -Year
Top -of-
Top -of -
2 -Year
Overflow
Quall[y
Peak
Peak
Peak
Peak Elev.
Bank
Berm
Pond:
Peak Elev.
Peak Elm.
ElmElm.
Elm. (ft)
(ft)
Elev. (ft)
Eley. (ft)
ElWeir
(ft)
(ft)
(ft)
Pond 1
510.19
512.77
513.20
513.53
513.69
513.76
513.70
514.00
514.69
Pond 2
513.43
515.87
516.17
516.38
516.62
516.75
516.65
517.00
517.62
Pond 3
518.37
520.40
520.69
520.95
521.23
521.33
521.25
522.00
522.23
Curve numbers used in the model include 98 for the impervious ROW and 90 for the 65% impervious
lots. The city of Springfield requires a type IA 24 -hr design storm for each of the water quality, 2, 5, 10,
25, and 10D -year rainfall events. The city of Springfield also requires an emergency spillway system that
safely passes the 100 -year, 24-hour design stone over the pond embankment without overtopping any
part of the pond in the event of a control structure failure. As shown, the ponds completely contain
stormwater runoff from the site dud ng each storm event with the required 1 -foot of freeboard per city of
Springfield stormwater design standards and thus are adequate to protect the subdivision and
surrounding properties from damage.
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The following table lists the peak flow rates for run-off from both the existing site and the proposed site:
Table 6, Pre-mnstrucflon peak flowrdtes
As shown in the above tables 6, the proposed peak flow rates are lower than the peak flows for existing
conditions. All calculation details can be found in the attached HydroCAD report. This is an aggregate
analysis that covers all eight phases of the proposed subdivision. The design for future phases will be
refined at the time of the tentative subdivision and PIP process for the respective phases.
Pollution Control
The stormwater from the pollution control event will be 100% treated prior to discharging into the public
system. Pond #1 contains an interior grassy swale for stormwater treatment for each of two inlet
locations. Ponds #2 and #3 are connected to exterior grassy swales upstream of the pond inlets for
stormwater treatment. To meet water quality standards, see the table below for grassy swale
dimensions, travel times, and flow depth.
Table 7: Grdseyswafe dmensions and tmafinentcapability dwnga water quat tystorm.
Swale
Proposed
Swale
Existing Peak
Swale
Storm Event
Average
Peak Flowrate
Length
Flowrate (cfs)
Side
Bottom
Bottom
(cfs)
2 -Year
5.48
5.42
5 -Year
8.55
7.21
10 -Year
11.99
9.98
25 -Year
15.68
13.59
As shown in the above tables 6, the proposed peak flow rates are lower than the peak flows for existing
conditions. All calculation details can be found in the attached HydroCAD report. This is an aggregate
analysis that covers all eight phases of the proposed subdivision. The design for future phases will be
refined at the time of the tentative subdivision and PIP process for the respective phases.
Pollution Control
The stormwater from the pollution control event will be 100% treated prior to discharging into the public
system. Pond #1 contains an interior grassy swale for stormwater treatment for each of two inlet
locations. Ponds #2 and #3 are connected to exterior grassy swales upstream of the pond inlets for
stormwater treatment. To meet water quality standards, see the table below for grassy swale
dimensions, travel times, and flow depth.
Table 7: Grdseyswafe dmensions and tmafinentcapability dwnga water quat tystorm.
Each swale will be vegetated to assist in treatment of stormwater runoff. Each pond also has 1 -foot of
growing medium to hold vegetation. Stormwater runoff filters through this vegetation prior to discharge
off-site.
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Swale
Swale
Swale
Swale
Swale
Average
Pond:
Length
Depth
Side
Bottom
Bottom
FMInImum Travel
Depth at
(ft)
(ft)
Slope
Slope
Widthme(mIn)
Peak
(ft/ft)
(111W
(ft)
Storage (ft)
Pond #1
251
1.0
4.0
0.0025
14
0.36
West Grassy
29.6
Swale
Pond#1 East
129
1.0
4.0
0.0025
21
0.39
Grassy Swale
14.2
Pond #2
120
3.0
4.0
0.003
23
0.40
Grassy Swale
11.8
Pond 93
131
3.0
4.0
0.0025
23
0.34
Grassv Swale
15 7
Each swale will be vegetated to assist in treatment of stormwater runoff. Each pond also has 1 -foot of
growing medium to hold vegetation. Stormwater runoff filters through this vegetation prior to discharge
off-site.
• Page 5
As shown in table 7, the minimum travel times within the swales are above 9 minutes and the flow depths
meet the maximum 0.4 feet. Thus, stormwater from the site will be adequately treated. Proposed
treatment methods adequately provide treatment for water quality storms while using the 0.83 -inch water
quality stone from the City of Springfield Code.
As the proposed ponds do not include rock chambers and the existing soil conditions at each pond
location shows poor infiltration capacity, the only means of treatment during the water quality storm is
by discharging through a grassy swale. For treated stormwater discharge to drain from the pond
completely between water quality storm events, the lowest orifice must be constructed at the flowline of
the pond.
Table 9Gmssyswale drawdown hares, during the waterqueW slam event.
Conclusion
The proposed stormwater system will adequately serve all phases of the proposed development. The
system is designed to keep post construction peak flows lower than the pre -development peak flows.
Therefore, the storm management system meets flood control requirements.
Peak water flow elevations during water quality storm events within the grassy swales do not rise above
the 0.4' maximum. This means the swales are adequately sized to allow the runoff to be completely
treated by flowing through vegetation and thus the stone management system meets pollution control
requirements.
Emergency overflow weirs are designed to be located on the south side of each proposed pond. This
will allow water to flow away from structures in an unusual event to protect public safety and prevent
property damage.
Lastly, the system will contain runoff thereby protecting public safety and removing the potential for
properly damage.
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Drawdown
Storm Event
time (hours)
Pond 1 West
34.00
Grassy Swale
Pond 1 East
29.00
Grassy Swale
Pond 2 Grassy
2
28.00
Swale
Pond 3 Grassy
28,00
Swale
Conclusion
The proposed stormwater system will adequately serve all phases of the proposed development. The
system is designed to keep post construction peak flows lower than the pre -development peak flows.
Therefore, the storm management system meets flood control requirements.
Peak water flow elevations during water quality storm events within the grassy swales do not rise above
the 0.4' maximum. This means the swales are adequately sized to allow the runoff to be completely
treated by flowing through vegetation and thus the stone management system meets pollution control
requirements.
Emergency overflow weirs are designed to be located on the south side of each proposed pond. This
will allow water to flow away from structures in an unusual event to protect public safety and prevent
property damage.
Lastly, the system will contain runoff thereby protecting public safety and removing the potential for
properly damage.
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