2020.0295.VA0002 2020-08-06 Preliminary Stormwater Drainage ReportPreliminary Stormwater Drainage Report Stevens St Development Yelm, WA Prepared for C & E Developments LLC PO Box 2983 Yelm, WA 98597-2983 (360) 400-0432Prepared by JMJ TEAM 905 Main St. Suite 200 Sumner, WA 98390 (206) 596-2020Justin Jones, PEAugust 5, 2020NOTE FROM OWNER: This report was prepared based on information given at initial pre-submission conference, which turned out to be incorrect, so the site plan had to be adjusted. This report shall be updated once the variance has been approved.
PROJECT ENGINEER’S CERTIFICATION I hereby certify that this Preliminary Stormwater Drainage Report for the Stevens St Development has been prepared by me or under my supervision and meets minimum standards of City of Yelm and normal standards of engineering practice. I hereby acknowledge and agree that the jurisdiction does not and will not assume liability for the sufficiency, suitability, or performance of drainage facilities designed by me. ____________________________________ Justin Jones, PE 08-05-20
TABLE OF CONTENTS Project Overview and Maps 1 Existing Conditions Summary 2 Proposed Conditions Summary 2 Summary of Minimum Requirements 3 Appendix A: Site Development Drawings
PROJECT OVERVIEW AND MAPS The Stevens St project is a new development project on a 15,246 sf lot (0.16 acres) along Stevens St NE in the City of Yelm. The project includes the construction of a new 3,663 SF 4-plex building, a new 3,320 SF asphalt driveway, and 326 SF of new concrete pavement. The project will add a total of 7,309 SF of new impervious surfaces, which will be managed through new drywells and bioretention cells on-site. The roof and concrete areas will drain to new drywells on site through new storm pipes and be infiltrated into native soils below. Stormwater runoff from the new asphalt driveway areas will surface flow to two new bioretention cells for treatment, and then infiltrated into native soils below. The bioretention cells have been designed to treat and infiltrate 100% of the runoff directed to the cells. Project Site
EXISTING CONDITIONS SUMMARY The existing project site is comprised of grass vegetation and is relatively flat. The existing site is 100% pervious. PROPOSED CONDITIONS SUMMARY The project proposes to add a new 4-plex building, a new driveway and new concrete walkways to the existing site, which will add 7,309 SF of roof area and hardscape. All proposed structures and hardscape will be directed to the new stormwater system comprised of new conveyance pipes, drywells, and bioretention cells. Stormwater management practices were evaluated for both the building roof and the pollution generating hardscape areas. Infiltration was selected as the most appropriate stormwater management practice for the building roof areas. A stormwater drywell infiltration system has been selected to be placed on the southeast edge of the building, and the northwest edge of the building, each receiving half of the building roof runoff. The proposed system will route stormwater from gutters through piped connections from the proposed building. Catch basins will be installed inline prior to the drywells to serve as sump to collect debris prior to stormwater infiltration. The drywells will allow for all roof stormwater to be infiltrated into native soils below. Bioretention was selected as the most appropriate stormwater management practice for the pollution generating hard surfaces on site. The proposed asphalt pavement shall be graded to direct stormwater surface runoff to two new bioretention cells on site. The bioretention cells shall contain a soil media treatment layer and plantings to remove pollutants from the stormwater prior to infiltration into native soils below.
DOE Stormwater Management Manual – Minimum Requirements City of Yelm utilizes the 2014 Department of Ecology Stormwater Management Manual. Volume 1 of this manual describes the minimum requirements for stormwater management for redevelopment sites. Using the flow chart below, minimum requirements 1-9 apply to the Stevens St Development site.
Minimum Requirement 1 – Preparation of Stormwater Site Plans Stormwater Site Plan drawings are submitted with this Permit. Minimum Requirement 2 – Construction Stormwater Pollution Prevention As part of the Civil Permit application, the applicant will ensure a Stormwater Pollution Prevention Plan meets Department of Ecology code. Minimum Requirement 3 – Source Control of Pollution This minimum requirement focuses on post-construction, such as the formation of a pollution prevention team, good housekeeping on the site, preventative maintenance, and spill prevention and cleanup. This will be outlined as part of the stormwater report at time of the Civil Permit application. Minimum Requirement 4 – Preservation of Natural Drainage Systems and Outfalls Stormwater will be captured, treated, and infiltrated within the site by a 60 SF of bioretention cell area and two 110 CF drywells. No stormwater will leave the site, preserving the natural drainage of stormwater infiltration into native soils on-site. Minimum Requirement 5 – On-Site Stormwater Management Minimum Requirement #5 states projects shall utilize either On-Site Stormwater Management BMP’s from List #1 or demonstrate compliance with the LID Performance Standard. The Stevens St Development Project is selecting Stormwater BMP’s from List #1 to manage site stormwater. List #1 requires the evaluation of BMP’s in the order listed to determine the most appropriate stormwater management system for landscaped areas, roofs, and hard surfaces, see Appendix B for details. Below are evaluations of each stormwater BMP for the Morse Redevelopment Project: · Lawn and Landscaped Areas o Post Construction Soil Quality and Depth In Accordance with BMP T5.13 shall be utilized for grass and landscaped areas of the Project. · Roofs o Full Dispersion (BMP T5.30) is determined to be infeasible due to insufficient vegetated flow path within project site. Project does not maintain 65% of the site in a “native condition”. o Downspout Infiltration (BMP T5.10A) is determined to be feasible for roof runoff, and shall be utilized for all roof areas of the project. The project is selecting drywells to be used on-site for infiltration of roof runoff. Drywells have been designed based upon the following criteria, as stated in the Ecology manual: Bottom of Drywell must be 1 foot above seasonal high groundwater table In Course Sands, Volume of Drain Rock = 60 CF per 1000 SF of roof area. Min. 48” diameter Geotextile on Top of drain rock The project proposes 3,663 SF of roof area. The required drain rock to serve this area is as follows: (3,663 SF Roof Area / 1000 SF) x 60 CF = 220 CF required.
Due to the slope of the proposed building roof, the roof runoff shall be directed to two separate drywells, on opposing sides of the house. Each drywell shall be designed to have a minimum drain rock volume of 110 CF, resulting in a minimum drywell size of 48-inches in diameter, and 9 feet in depth. Stormwater will be conveyed to these drywells through roof downspouts, new storm lines and a catch basin sump for debris collection and maintenance. Stormwater will then enter the drywell system and be infiltrated into native soils on site. · Hard Surfaces o Full Dispersion (BMP T5.30) is determined to be infeasible due to insufficient vegetated flow path within project site. Project does not maintain 65% of the site in a “native condition” o Permeable Pavement (BMP T5.15) will not be used on-site due to maintenance and longevity concerns. o Bioretention (BMP 7.30) was selected to manage stormwater runoff from the pollution generating hard surfaces on-site. The bioretention cells were designed based upon the following criteria: 1-foot ponding depth 6-inches of freeboard 3-inches of course compost 18-inches of Bioretention Soil Media 3:1 Side Slopes The bioretention cell bottom areas were determined through WWHM modelling, and an assumed infiltration rate of 3 inches per hour based on site soil characteristics. Stormwater runoff from the proposed pollution generating asphalt pavement will be directed to two new bioretention cells on site. The bioretention cells will remove pollutants from the stormwater prior to infiltrating into native soils below. The cells will have the capacity to treat 100% of the pollution generating runoff, and infiltrate 100% of the stormwater directed to them. All stormwater on-site will be managed through these BMPs, preserving the natural drainage and resulting in no increase in runoff or pollutants generated from the development of the site.
Minimum Requirement 6 – Stormwater Runoff Treatment Stormwater run-off treatment will be achieved through the construction of two new bioretention cells on site. The asphalt pavement is considered a pollution generating surface as it will have moderate vehicle traffic throughout its lifespan. and will drain to the bioretention cell on site. The bioretention cell is sized to treat the asphalt pavement, as well as receive a portion of the surrounding landscape areas. The bioretention cell has been sized using the Western Washington Hydrology Model (WWHM) based on the following criteria: 1-foot ponding depth 6-inches of freeboard 3-inches of course compost 18-inches of Bioretention Soil Media 3:1 Side Slopes Infiltration rate of 3 inches per hour The WWHM modelling of the required bioretention cell area assumes a minimum of 93% of stormwater treatment, and 100% of stormwater infiltration as shown below: WWHM modelling shows that a minimum bottom area of 50 SF is required to achieve full treatment and infiltration of the 3,320 SF of pollution generating asphalt pavement. Due to existing grades on site, it was determined that the most appropriate method for achieving this was to split the bioretention cell area into two distinct cells, with one cell to the north of the asphalt, and one to the southeast. Each cell was designed to maintain a minimum of 30 SF bottom area. The proposed asphalt pavement shall be graded to direct equal parts of the surface runoff to each cell through grade breaks and curb cuts. Each bioretention cell will contain a minimum 18-inch treatment layer, along with plantings to allow for the removal of pollutants prior to stormwater infiltration into native soils below the cells.
Minimum Requirement 7 – Flow Control Based on the DOE’s minimum requirement flow chart for redevelopment, flow control is required for new and replaced hardscape, and converted vegetation areas. This results in allowable release rates equal to half the 2-year and the full 50-year flows from the site based on the forested condition. The existing roof will use a 157 CF downspout full infiltration drywell sized per BMP T5.10A to infiltrate 100% of roof runoff. The asphalt pavement and landscape areas will drain to a bioretention cell sized to infiltrate 100% of runoff from these areas. Sizing is based on the 3” per hour infiltration rate of the DOE bioretention soil mix governing the design due high infiltration capabilities in the native soil. Drywell and bioretention sizing results in 100% infiltration of site runoff and no stormwater discharging from the site. Minimum Requirement 8 – Wetlands Protection There are no wetlands located within the development site. Minimum Requirement 9 – Operations and Maintenance The project will include the development of an O&M manual in accordance with the DOE Stormwater Manual. This manual will be developed at the time of permitting and include a log of maintenance activities to be kept on-site for city inspection.
APPENDIX A
\ °
\ a \ —
\\ ° \\ Downspout
\
(Typ-)
110 Ft' 48" Dia. by 9' Deep
\ - \ Drywell per DOE BMP T5.10A \
i
\ \\
Existing Concrete Type 40Catch
\ Sidewalk (Typ.) \ 8asln (Typ-) m
i
\ \ Downspout
\ ° a. \ ITYP•} Building Powe
Connection (Typ.
\ Existing Landscape
\
Planter (Typ.) y'
\\ Existing Curbline �\
\ (TYP.)
\t
0
0
— — — — Property Line ID
\ Bioretention Cell
\ P Bottom Area=30 SF
� 0
)
CS P
Building Sewer \
Connection (Typ.)
Proposed Building
Area=3,663 SF
6" PVC Storm Line
S=0.5% Min,
\Water Meters (Typ.) \
Building Water
Connect to Existing ` s� Connection (Typ.)
Proposed Concrete
Pavement (Typ.)
110 Ft' 48" Dia. by 9' Deep Drywell _ Type 40 Catch
per DOE BMP T5.10A Basin (Typ.)
i
i
i
i ® Asphalt
STEP Tanks (Typ.) - Pavement (Typ.)
Downspout
(Typ
s 1
\/SS
\ \ r
\ Property Line 4
\ \ - Curb Cut for Storm -
\ ° \ Runoff to Bioretention
Cell (Typ.)
\ Stormwater
\ Connect to Existing \ - Flowpath (Typ.)
\ Sewer Stub
\ 4° \
II
6" Extruded Curb (Typ.)
I
0
I
o
ID
Grade Break LineT
( YP.)
0
0
O
Bioretention Cell
Bottom Area=30 SF
o
10
TC@/- d1ATC@
1. Contractor to install temporary erosion and sediment control
measures as necessary to ensure stormwater leaving the site is
free of settleable solids.
2. Roads shall be cleaned thoroughly as needed to protect
stormwater infrastructure and downstream water resources.
Sediment shall be removed from roads by shoveling or pickup
sweeping and be transported to a controlled sediment disposal
area.
3. Install storm drain inlet protection in all existing catch basins
within the project vicinity per WSDOT Std Plan 1-40.20-00.
4. Install straw bale barriers, wattles, and other TESL measures as
necessary.
5. Exposed soils shall be watered as necessary to prevent dust
from leaving the site.
6. Contractor to mark clearing limits with lath and flagging.
7. Concrete handling and equipment washing shall be in
accordance with DOE BMP C151.
S. Intall Silft Fence per DOE BMP 0233 as necessary. Install High
Visibility Construction Fence where Silt Fence is not Required
Per DOE BMP C103.
—� GENERAL NOTES
1. Storm Pipes to Maintain a Minimum Cover of 1.5' from Finished
Grade Surface
2. Storm Pipes to be SDR 35 PVC Piping.
3. Bioretention Cells to be Constructed Per DOE Figure 7.4.1a.
4. Stormwater Drywells to be Constructed per DOE Figure 3-t a.
CONSTRUCTION NOTES
1. Bioretention Cell Sizing Based on WWHM Modeling:
• Pollution Generating Hardscape Area = 3,320 SF
•
Infiltration Rate =3.0"per hr
• Soil Media Depth = 18"
• Ponding Depth= 12"
• Freeboard = 6"
• Sidewall Slope 3:1
• Minimum Bottom Area =50 SF
2. Drywell Sizing Based on DOE Specifications:
• Roof Area Sent to Drywells = 3,663 SF
•
Infiltration Rate =3.0"per hr
• Required Gravel Volume= 60 Ft' per 1,000 SF of
impervious surface area
• (3,663 SF Roof Area/ 1,000 SF) x 60 Ft3=220 Ft; of
Gravel Required.
1 C/'�CL11"1
Asphalt Pavement
° - I Concrete Pavement
W ° Water Line
SS Sewer Line
P Power Line
SD Storm Line
C & E Developments LLC
PO Box 2983
Yelm, WA 98597-2983
(360)400-0432
JV J V JTEAM
Justin Jones, PE
905 Main St. Suite 200
Sumner, WA9B390
(206]595-2020
Project:
Stevens St
Development
ONE IAFCH AT FULL SCALE.
W NOT, SCALE ACCCIR DI NGLY
, cyrti .
08/05/20
BRAWN BY: DESIGN BY: 11
PRO. NO:
\
DAT. August 05, 2020
DWG.
\
\
\
NG
"-18
C-01
Curb Cut for Storm
\
Runoff to Bioretention
\
Cell (Typ.)
Ppm
\
P
\
\
\
6" PVC Storm Line
\ -
S=0.5% Min. -
\
\ -
- Downspout. - -
(TYP•)
21'
110 Ft' 48" Dia. by 9' Deep Drywell _ Type 40 Catch
per DOE BMP T5.10A Basin (Typ.)
i
i
i
i ® Asphalt
STEP Tanks (Typ.) - Pavement (Typ.)
Downspout
(Typ
s 1
\/SS
\ \ r
\ Property Line 4
\ \ - Curb Cut for Storm -
\ ° \ Runoff to Bioretention
Cell (Typ.)
\ Stormwater
\ Connect to Existing \ - Flowpath (Typ.)
\ Sewer Stub
\ 4° \
II
6" Extruded Curb (Typ.)
I
0
I
o
ID
Grade Break LineT
( YP.)
0
0
O
Bioretention Cell
Bottom Area=30 SF
o
10
TC@/- d1ATC@
1. Contractor to install temporary erosion and sediment control
measures as necessary to ensure stormwater leaving the site is
free of settleable solids.
2. Roads shall be cleaned thoroughly as needed to protect
stormwater infrastructure and downstream water resources.
Sediment shall be removed from roads by shoveling or pickup
sweeping and be transported to a controlled sediment disposal
area.
3. Install storm drain inlet protection in all existing catch basins
within the project vicinity per WSDOT Std Plan 1-40.20-00.
4. Install straw bale barriers, wattles, and other TESL measures as
necessary.
5. Exposed soils shall be watered as necessary to prevent dust
from leaving the site.
6. Contractor to mark clearing limits with lath and flagging.
7. Concrete handling and equipment washing shall be in
accordance with DOE BMP C151.
S. Intall Silft Fence per DOE BMP 0233 as necessary. Install High
Visibility Construction Fence where Silt Fence is not Required
Per DOE BMP C103.
—� GENERAL NOTES
1. Storm Pipes to Maintain a Minimum Cover of 1.5' from Finished
Grade Surface
2. Storm Pipes to be SDR 35 PVC Piping.
3. Bioretention Cells to be Constructed Per DOE Figure 7.4.1a.
4. Stormwater Drywells to be Constructed per DOE Figure 3-t a.
CONSTRUCTION NOTES
1. Bioretention Cell Sizing Based on WWHM Modeling:
• Pollution Generating Hardscape Area = 3,320 SF
•
Infiltration Rate =3.0"per hr
• Soil Media Depth = 18"
• Ponding Depth= 12"
• Freeboard = 6"
• Sidewall Slope 3:1
• Minimum Bottom Area =50 SF
2. Drywell Sizing Based on DOE Specifications:
• Roof Area Sent to Drywells = 3,663 SF
•
Infiltration Rate =3.0"per hr
• Required Gravel Volume= 60 Ft' per 1,000 SF of
impervious surface area
• (3,663 SF Roof Area/ 1,000 SF) x 60 Ft3=220 Ft; of
Gravel Required.
1 C/'�CL11"1
Asphalt Pavement
° - I Concrete Pavement
W ° Water Line
SS Sewer Line
P Power Line
SD Storm Line
C & E Developments LLC
PO Box 2983
Yelm, WA 98597-2983
(360)400-0432
JV J V JTEAM
Justin Jones, PE
905 Main St. Suite 200
Sumner, WA9B390
(206]595-2020
Project:
Stevens St
Development
ONE IAFCH AT FULL SCALE.
W NOT, SCALE ACCCIR DI NGLY
, cyrti .
08/05/20
BRAWN BY: DESIGN BY: 11
PRO. NO:
DAT. August 05, 2020
DWG.
Storm Plan
o zo
NG
"-18
C-01
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DRY WELL
PLAN VIEW
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stow el Wnhlnplen ft".dMBNy,sdd..wk-
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74
C & E Developments LLC
PO Box 2983
Yoh, WA 98597.2983
J-; 0j 400-0432
E10-
0
10—
JOJTF–AM
lvstln Junes, PE
905 Main SL Suite 200
Sumner, WA 98390
{206j 596.2020
Pr,
Stevens St
Development
ONE INCH AT FULL SCALE.
IF NOT, SCALE ACCORDINGLY
a.".
DRAWN BY. DESIGNBY, J1
PU.N0.
DAT. August 05, 2020
x
Details
CLS
C-02
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Typical 8iotention
DEPRRTMEHT DF P.W.M..r2Qsi
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stow el Wnhlnplen ft".dMBNy,sdd..wk-
V fame V Run pTrearment BMP§ Oerember 2014
74
C & E Developments LLC
PO Box 2983
Yoh, WA 98597.2983
J-; 0j 400-0432
E10-
0
10—
JOJTF–AM
lvstln Junes, PE
905 Main SL Suite 200
Sumner, WA 98390
{206j 596.2020
Pr,
Stevens St
Development
ONE INCH AT FULL SCALE.
IF NOT, SCALE ACCORDINGLY
a.".
DRAWN BY. DESIGNBY, J1
PU.N0.
DAT. August 05, 2020
x
Details
CLS
C-02