HomeMy WebLinkAboutApplication APPLICANT 3/9/2022380 Q Street, Ste 200 4Lftl�
Springfield, Oregon 97477
(541)302-9790
kylemorris@ao-engr.com A Et
Stormwater Report
project Thurston Estates
r-ocauon 7399 Thurston Road, Springfield, OR
Pmparea Br. Kyle Morris, PE _
Reviewed by: Scott Morris, PE Kyle Moms, PE°w�
Dato 111212022
Type dproiact: Residential Subdivision F f Iras
Project Overview
The applicant is proposing to construct a 10 -lot single family residential subdivision south of Thurston
Road on an approximate 2 -acre parcel. The subdivision will consist of publicly owned infrastructure
including roadways, utilities and stomnwater infrastructure. To accommodate stomiwater runoff it is
proposed to construct two stormwater ponds within publicly dedicated tracts to receive and process
runoff from the new rights-of-way and impervious surfaces on each lot. The primary outlet for these
ponds will be infiltration. The existing roadside ditch on the south side of Thurston Road will act as an
emergency overflow for both ponds should a flood level storm event occur.
Existing Conditions
The existing site currently has some structures but mostly consists of grass cover and some scattered
trees. The structures present are a single-family residence, a garage and some small sheds. The
existing home and shed present on the future lot 1 is proposed to remain, other structures currently on-
site are proposed to be removed. Web Soil Survey classifies on-site soils as Chapman -Urban Land
Complex with a Hydrologic Soil Group rating of B. Per LIDAR elevation data the site has a very slight
slope directed to the north towards Thurston Road. It is assumed existing runoff from the site sheet
flows to the north during large storm events.
Springfield requires post -construction peak flowrates be equal to or less than pre -construction peak
flowrates. To analyze the pre -construction peak flowrate from the site the following data will be utilized:
Land Type;Description: Grassland with HSG rating of B
Curve Number: 79
In addition to the on-site existing conditions flow, the adjacent property to the south slopes towards the
north. This creates the potential during large stone events for stormwaterflow off-site to flow across the
subject property following the historic drainage pattern. Using Google Earth the contributing area of
potential flow across the subject property was estimated to be 5.6 acres with a curve number of 79. To
be conservative in design this flow was assumed to be routed into the proposed stonmwater system for
the subject property. Full details for this offsite flow can be found in the HydroCAD report attached in
the appendix.
Infiltration
Infiltration testing was performed by Branch Engineering on July 15" 2021 and a geotechnical report
was prepared on July 16N 2021 which includes the details of the testing and the results. The rate
measured during the testing was 16.5 inflow. To meet the 2014 Eugene Stommater Management
Manual requirements a design infiltration rate of 6 inlhour for native soils will be utilized which provides
a safety factor of 2.75 when compared with the measured rate. Since this test was performed during
the summer, another infiltration test should be performed during the wet weather months (December,
January or February) to verify the infiltration rate and check the groundwater depth. Branch
Engineering's report is included with the appendix of this report.
To be consistent with the requirements outlined in the 2014 Eugene Stomnwater Management Manual,
a design infiltration rate of 2.5 in/hourwill be utilized for infiltration through imported soil growing mediums
such as is proposed in the pond.
Proposed Destination
The primary destination for stormwater runoff will be infiliration during more normal stone events. For
larger storm events the existing roadside ditch on the south side of Thurston Road will be utilized as an
escape route. An emergency overflow from Pond B is proposed to allow stonmwater to flow over the
new sidewalk in Thurston Road which then flows into the existing roadside ditch. Since Pond A and
Pond B are connected via piping this overflow will serve Pond A as well. This existing ditch generally
flows to the east to a natural drainage way approximately 400 feet away from the site. This drainage
way flows under Thurston Road by culvert and eventually drains into South Cedar Creek. The City of
Springfield basin map shows the entire site is to drain to the north as proposed.
Post construction
It is proposed to construct two publicly owned and maintained stonnwater ponds within tracts as shown
on the plan. The ponds would collect stornwater runoff from the new impervious rights-of-way on the
site and the impervious surfaces on lots including roofs. The impervious surface of each lot was
assumed to be 2,640 SF per the City of Springfield EDSP section 4.11. In the table below physical
characteristics of the proposed pond are summarized:
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Table 1: Pond Physical Characteristics
The ponds vAll have 12" thick growing mediums and 12" thick rock chambers. The growing medium is
proposed under the entire facility. Each pond also is proposed to have a landscape block retaining wall
around the upper 2.5 feet providing more storage volume. The total storage area the proposed pond
provides is summarized in the table below in units of cubic feet:
Table 2: Total Pond Storage
Open
Bottom
Depth
Side
Top
Facility
Area [SF]
[FT]
Slopes
Area
(35% Voids)
Storage
Tract A Pond 4,086
[H:V]
[SF]
Water Quality
Tract B Pond 5,978
77
191
6,246
Detention Pond Tract
307
4
3:1
1,097
A
Water Quality
Detention Pond Tract
545
4
3:1
1,568
B
The ponds vAll have 12" thick growing mediums and 12" thick rock chambers. The growing medium is
proposed under the entire facility. Each pond also is proposed to have a landscape block retaining wall
around the upper 2.5 feet providing more storage volume. The total storage area the proposed pond
provides is summarized in the table below in units of cubic feet:
Table 2: Total Pond Storage
Open
Growing
Area:
1.99 acres
Facility Storage
Medium (10%
Rock Chamber
Total
(100% Voids)
Voids)
(35% Voids)
Storage
Tract A Pond 4,086
47
232
4,365
Tract B Pond 5,978
77
191
6,246
Hydraulic Calculations
HydroCAD software was utilized to perform all hydraulic calculations. Below are parameters utilized in
calculating the existing conditions peak flowrates leaving the site from the development area:
Runoff Method:
Santa Barbara Urban Hydrograph (SBUH)
Area:
1.99 acres
Curve Number.
98 Unconnected roofs
98 Impervious ROW
Time of Concentration:
61.9 minutes (Details for this included with HydroCAD node report)
Storm Type:
Type IA 24 -hr
Storm Events:
Water Quality -0.83 inches
2 -Year Storm — 3.30 inches
25 -Year Storm — 4.80 inches
To show detention requirements are met, both pre -construction and post -construction peak gowrates
leaving the site were calculated without the Offsite Flow included. Those values are summarized in
the table below
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Table 3: Peak Fbwrates Summery
Storm Existing Conditions Post Construction
Event Peak Flowrate [cis] Peak Flowrate [cis]
2-Year0.29 0.00
T5Year
0.63 0.00
As shown above, detention requirements are met as the majority of stormwater runoff is proposed to
infiltrate within the ponds.
Along with peak flowrates, hydraulic grade lines were analyzed to ensure no flooding occurs within the
subdivision. Below is a table summarizing the pond elevations and outflow structure elevations.
Table 4. Pond Elevations fall numbers in units of Feet)
Facility Pond FL. Overflow Pond Top
Pond TractA 522.40 526.50 526.90
Pond Trac[ B 522.40 526.50 526.90
The overflow from Pond Tract A will be routed into Pond Tract B for further storage to allow for
infiltration. The overflow from Pond Tract B is a small overflow weir with a rock channel directed
towards the new sidewalk in Thurston Road as discussed in the Proposed Destination section of the
report above. Below is a table summarizing the calculated HGLs within the pond in comparison to the
above pond elevations:
Table 5: Calcurated Pond HGLs
Facility Water 2 -Year 25 -Year Pond
Quality Top
Pond Tract 522.55 526.61 526.86 526.90
Pond Tract B 520.40 526.60 526.77 526.90
As shown in the above table, all stormwater is contained within the ponds during the design 2 -year
storm event. During the 25 -year flood event stormwater is allowed to surcharge into the roadway by
approximately 3 inches for a short time -period and thus meets City of Springfield standards. The
above HGL's assume the full calculated inflow from the adjacent property to the south flows into the
ponds during each modeled storm event.
Pollution Control
As discussed, infiltration is proposed as the primary outflow during normal storm events. For the
impervious roadways and lot surfaces stormwater runoff wilt be routed through the proposed water
quality detention ponds. This pond will be vegetated to meet City of Springfield requirements. As shown
in the tables above all stormwater is infiltrated during the Water Quality storm event as the peak HGL is
below the overflow elevations.
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Conclusion
The proposed stonnwater system will meet City of Springfield requirements. Detention requirements
will be met and pollution control also knit be provided with the proposed infiltration and vegetation within
the pond. During large stone events stormwater will be allowed to flowthrough the emergency overflow
towards Thurston Road and not cause damage to habitable structures. The proposed system as
modeled will not pose any public safety or health risks.
Appendix
• HydroCAD Report
• Branch Engineering Site infiltration Test Results
• Stomrwater Scoping Sheet
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