Preliminary Storm Drainage Report Preliminary Storm Drainage Report
For
Erika Court Subdivision
Tigard, Oregon
March 29, 2016
For
Pacific Evergreen Homes
Portland, Oregon
(WCI Project Number 2511-007)
Table of Contents
1.0 PROJECT OVERVIEW 2
2.0 WATER QUALITY ANALYSIS 3
3.0 DETENTION ANALYSIS SUMMARY 4
4.0 CONVEYANCE ANALYSIS SUMMARY 4
5.0 SUMMARY 4
APPENDICES:
Appendix A: Site Plan, Extended Dry Basin Details
Appendix B: Detention Analysis (HydroCAD)
Appendix C: Conveyance Analysis
Appendix D: NRCS Soil Reports
2511-007 Storm Report Page 1 of 5
PROJECT OVERVIEW
Introduction
This project is located at 14259 SW 100th Ave and has a total area of 3.31
acres. The site contains an existing residential home and various out buildings.
The remainder of the site consists of a gravel driveway, and a combination of
grass and tree areas.
The existing residence and out buildings will be retained on the site. This
development will provide 9 additional new single family residential homes with a
public street. Storm laterals will be provided to each lot and will be directed to a
new public storm main located in the proposed public street. The public storm
main will discharge into a new water quality and detention facility (Extended Dry
Basin) located at the northeast corner of the property. Additionally, storm runoff
from the adjacent property will be piped and bypassed with a separate storm
conveyance through the property and connected to the existing storm system in
SW 100th Avenue.
The water quality and detention facility will be designed according to the
requirements set forth in Clean Water Services "2007 Design and Construction
Standards for Sanitary Sewer and Surface Water Management" Chapter 4,
Section 4.06.3. The outfall from the stormwater facility will be into the existing
storm system in SW 100th Avenue.
The site topography slopes generally from the southwest to the northeast.
Analysis Purpose
The purpose of this Drainage Report is to analyze and size the following:
1. Water quality treatment facility (Extended Dry Basin)
2. Detention facility (Extended Dry Basin)
3. Pipe Conveyance
Discussion
It is assumed that there are downstream deficiencies; therefore a detention
facility will be constructed at the site. The detention facility will assure that the 2,
10, and 25-year post-development runoff rates will not exceed their respective 2,
10, and 25-year pre-development runoff rates per the Clean Water Services
"2007 Design and Construction Standards for Sanitary Sewer and Surface Water
Management" Chapter 4, Section 4.03.4.
The east portion of the existing pavement for SW 103rd Avenue, south of the
proposed public street into the site, will be not included in the stormwater
detention and water quality treatment calculations for the site, it will be piped into
the existing conveyance system on the west side of SW 103rd Avenue. In
addition, the west portion of the existing pavement for SW 100th Avenue, north of
the proposed public street into the site, will be not included in the stormwater
2511-007 Storm Report Page 2 of 5
detention and water quality treatment calculations for the site, it will be piped into
the existing conveyance system on the west side of SW 100th Avenue.
It is requested that the runoff from the existing development to the south be piped
through the site and bypassed, it will be piped into the existing conveyance
system on the west side of SW 100th Avenue.
See Site Plan in Appendix A.
WATER QUALITY ANALYSIS
For water quality, the system shall treat the total precipitation of 0.36 inches
falling in 4 hours with a storm return period of 96 hours. For water quality
treatment, the treatment will be completed by an Extended Dry Basin. The
Extended Dry Basin is designed per criteria included in Clean Water Services
"2007 Design and Construction Standards for Sanitary Sewer and Surface Water
Management" Chapter 4, Section 4.06.3.
Prior to runoff entering the water quality facility (Extended Dry Basin), a water
quality manhole will be constructed for pre-treatment per Clean Water Services
"2007 Design and Construction Standards for Sanitary Sewer and Surface Water
Management" Chapter 4, Section 4.06.1.
The total existing and future impervious areas for the site have been used to size
the water quality facility as shown below.
Water Quality Volume Calculations:
Total impervious area: = 53,800 sq. ft. = 1.2 acres
Water Quality Volume: WQV (cf) = 0.36 (in) x Impervious area (sf)
12 (in/ft)
= 0.36 (in) x 53,800 (sf)
12 (in/ft) = 1,614 cubic feet
Water Quality Flow: WQF (cfs) = WQV (cf) = 1,614 (cf)
14,400 seconds 14,400 seconds
= 0.11 cubic feet/second
Water Quality Surface Calculations:
The water quality volume of 1,614 cubic feet is required to be contained in the
extended dry basin at an elevation above the dead pool storage. The required
detention storage in the extended dry basin will be above the water surface
elevation required to store the water quality volume of 1,614 cubic feet.
Water Quality Volume Draw Down:
The orifice sizing for the Extended Dry Basin, which requires water quality
drawdown time of 48 hours, will be calculated at final engineering.
2511-007 Storm Report Page 3 of 5
DETENTION ANALYSIS SUMMARY
Detention calculations will be performed using the HydroCAD Version 10.00-16
design and analysis software. Calculations are based on the Santa Barbara
Urban Hydrograph runoff method (SBUH) using the Type 1A, 24-hour storm
events as required in the Clean Water Services "2007 Design and Construction
Standards for Sanitary Sewer and Surface Water Management".
In conformance with Clean Water Services "2007 Design and Construction
Standards for Sanitary Sewer and Surface Water Management" Chapter 4,
Section 4.05.5, impervious areas for each lot of 2,640 square feet will used in the
design and analysis of the detention facility for the site. Pre and Post curve
numbers used in the detention analysis will be based on the existing and
proposed soil conditions at the site based on soil Hydrologic Soil Group C. See
Appendix D for existing soil information at the site.
Rainfall depths used in the detention analysis for the 2, 10 and 25 year Type 1A
24-hour storm events are as follows:
• 2-Year 2.50 inches
• 10-Year 3.45 inches
• 25-Year 3.90 inches
• 100-Year 4.50 inches
The detention facility will be sized to assure that the 2, 10, and 25-year post-
development runoff rates will not exceed their respective 2, 10, and 25-year pre-
development runoff rates.
CONVEYANCE ANALYSIS SUMMARY
Stormwater runoff from this site will discharge into an existing storm sewer pipe
located in SW 100th Avenue. The storm sewer system for the site will be sized to
convey the 100-year storm event.
Emergency overflow from the stormwater facility (Extended Dry Basin) will
overflow through the ditch inlets in the stormwater facility and discharge into the
existing storm drain pipe in SW 100th Avenue.
SUMMARY
This analysis demonstrates that the stormwater design for the Erika Court
Subdivision will comply with the requirements set forth by Clean Water Services
and the City of Tigard upon final engineering.
2511-007 Storm Report Page 4 of 5
Appendix A:
Site Plan, Extended Dry Basin Details
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WALL SECTION AND
INTERIOR DIMENSIONS
IN ACCORDANCE WITH MAXIMUM WATER SURFACE ELEVATION YR)
•
STD. DWG 390 ° 3" MINIMUM WHEN USED WITH
'•d 1• . ///\ EXTENED DRY POND
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GUIDE (SEE iN. ? ANCHOR TO WALL WITH
DRAWING NO. 730 �e, 4• + ,o' STAINLESS STEEL RISER
e , — A CLAMP OR 2" MIN. STAINLESS
v — STEEL BAND AND STAINLESS
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CLAMP DETAIL
r SELF TAPPING CONCRETE
ANCHORS, PHILLIPS 5-12 OR
APPROVED EQUAL.
r X 1-4" STAINLESS STEEL
BOLT.
NOTES:
1. CONNECTING PIPE AND TEE SHALL BE 4", 6", OR 8" AWWA C-900 OR ASTM 3034 PVC,AND
ONE SIZE LARGER THAN THE ORIFICE OPENING.
2. MAXIMUM ORIFICE OPENING SHALL BE 6" DIAMETER.
3. STRUCTURES SHALL CONFORM TO STANDARD DRAWING NO. 390 DITCH INLET.
4. FRAME AND GRATE SHALL CONFORM TO STANDARD DRAWING NO. 400, DITCH INLET FRAME
AND GRATE.
5. PLATE AND GUIDE SHALL BE SECURED FLUSH AGAINST WALL OF STRUCTURE AS APPROVED.
6. MAINTAINANCE ACCESS REQUIRED TO WITHIN 10' OF CENTER OF BOTH STRUCTURES.
7. FOR APPROVAL OF ALTERNATE STRUCTURES SEE SECTION 1.17.
OUTFLOW CONTROL
STRUCTURE C1eanWate\ Services
DRAWING NO. 720 REVISED 11-06 Our commitment is clear.
3/s" STAINLESS STEEL CHAIN OR CABLE
ATTACHED TO ORIFICE PLATE AND
STRUCTURE AS APPROVED. CHAIN OR CABLE
.11
SLOT SHALL BE !1 SHALL BE SMALL ENOUGH TO ALLOW ORIFICE
I PLATE TO BE REMOVED FROM GUIDE. ORIFICE
"X5" CENTERED 1i1rr PLATE AND GUIDE TO BE MANUFACTURED
FROM Y2" HDPE OR Y4" STAINLESS STEEL.
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ORIFICE SIZE 0
ORIFICE ELEVATION
ALIGN INVERT OF ORIFICE TO SPACER REQUIRED FOR
INVERT OF PIPE. MULTIPLE ORIFICES
3
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1 1/2" MIN. —NI—
TOP OF ODE
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ORIFICE PLATE GUIDE SHALL FIT STOP
GATE AND INCLUDE BOTTOM CHANNEL
ORIFICE PLATE GUIDE.
NOTE:
Y2" SELF TAPPING CONCRETE aa FOR MULTIPLE ORIFICE APPLICATION
ANCHORS, PHILLIPS 5-12 OR d o A 3" MIN. SPACER IS REQUIRED AS
APPROVED EQUAL. SHOWN. SPACER TO MATCH PLATE GUIDE
Y2" X 1—Y2" STAINLESS STEEL a
BOLT. DIMENSIONS, WIDTH, MATERIAL
WITH A WATER TIGHT SEAL.
O�IEICE PLATE
A\ D GUIDE CleanWate Services
DRAWING NO. 730 REVISED 12-06 Our commitment is clear.
Appendix B:
Detention Analysis
(To Be Completed at Final Engineering]
Appendix C:
Conveyance Analysis
(To Be Completed at Final Engineering]
Appendix D:
NRCS Soils Report
1� Hydrologic Soil Group—Washington County,Oregon
516560 516980 517000 517020 517040 517060 517080 5171C0 517120 517140 517160 517180 517200
45°25'3"N _ 45°25 3"N
7 � 1 • .
s ." 9913 I 0
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r no G _ -r20oe be valid at'.hic' -I Is'
45°24'57"N — - - +. 45°24'57"N
516960 516980 517000 517020 517040 517060 517080 517100 517120 517140 517160 517180 517200
3 3
Map Scale:1:1,130 if printed on A landscape(11"x 8.5")sheet
N N Meters
0 15 30 60 90 ..
Feet
0 50 100 200 300
Map projection:Web Mercator Corner coordinates:WGS84 Edge tics:UrM Zone 10N WGS84
USIM Natural Resources Web Soil Survey 3/29/2017
Conservation Service National Cooperative Soil Survey Page 1 of 4
Hydrologic Soil Group—Washington County,Oregon
MAP LEGEND MAP INFORMATION
Area of Interest(AOI) ® C The soil surveys that comprise your AOI were mapped at
Area of Interest(AOI) CID 1:20,000.
Soils D Warning:Soil Map may not be valid at this scale.
Soil Rating Polygons
I A p Not rated or not available Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
I A/D Water Features line placement.The maps do not show the small areas of
Streams and Canals contrasting soils that could have been shown at a more detailed
0 B scale.
Transportation
B/D
EFF Rails
Please rely on the bar scale on each map sheet for map
CInterstate Highways measurements.
I C/DUS Routes Source of Map: Natural Resources Conservation Service
whoD Web Soil Survey URL:
Major Roads Coordinate System: Web Mercator(EPSG:3857)
Not rated or not available Local Roads
Maps from the Web Soil Survey are based on the Web Mercator
Soil Rating Lines Background projection,which preserves direction and shape but distorts
• , A _ distance and area.A projection that preserves area,such as the
Aerial Photography Albers equal-area conic projection,should be used if more
• r A/D accurate calculations of distance or area are required.
B This product is generated from the USDA-NRCS certified data as
1..-w+ B/D of the version date(s)listed below.
C Soil Survey Area: Washington County,Oregon
Survey Area Data: Version 14,Sep 16,2016
rV C/D
Soil map units are labeled(as space allows)for map scales
r�r D 1:50,000 or larger.
• • Not rated or not available Date(s)aerial images were photographed: Aug 3,2014—Aug
Soil Rating Points 23,2014
p A The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
p A/D imagery displayed on these maps.As a result,some minor
O B shifting of map unit boundaries may be evident.
O B/D
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Conservation Service National Cooperative Soil Survey Page 2 of 4
Hydrologic Soil Group—Washington County,Oregon
Hydrologic Soil Group
Hydrologic Soil Group—Summary by Map Unit—Washington County,Oregon(OR067)
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
11B Cornelius and Kinton silt C 2.1 37.2%
barns,2 to 7 percent
slopes
11C Cornelius and Kinton silt C 3.5 62.8%
loarns,7 to 12 percent
slopes
Totals for Area of Interest 5.6 100.0%
Description
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D)and
three dual classes (ND, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential)when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential)when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (ND, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
USDA Natural Resources Web Soil Survey 3/29/2017
Conservation Service National Cooperative Soil Survey Page 3 of 4
Hydrologic Soil Group—Washington County,Oregon
Rating Options
Aggregation Method: Dominant Condition
Component Percent Cutoff.None Specified
Tie-break Rule: Higher
USDA Natural Resources Web Soil Survey 3/29/2017
Conservation Service National Cooperative Soil Survey Page 4 of 4