Report (36) I
c„,\
F4 IVF4 R I Q-: LA
S-,; . LvQ, 2,
RECEIVED kJ
...,....._0efi -n APR 8 2019
CITY Y OF TIGARD
CIVIL ENGINEERS & PLANNERS BUILDING DIVISION
TECHNICAL MEMORANDUM
Date: March 7, 2019
To: City of Tigard Engineering Staff
From: Eric Evans, PE - Emerio Design
Subject: Single-Family Residence Stormwater Analysis
This letter and its attachments are to serve as stormwater analysis for a new single residence. _
Proposed Project Description:
As seen in the attached site map, the proposed single-family home with attached garage will _
be constructed on an existing lot size of 9,323 SF, approximately 200 feet southeast of the
intersection of SW Taylors Ferry Road & SW 74th Avenue on tax lot 02500 of tax map 1S1W
25DB.
Proposed Treatment &Conveyance:
The proposed project will convey all impervious area from roof and driveway runoff toward a
standard LIDA flow through planter box sized at 6%. The planter box will receive driveway
runoff through curb cuts. Treated stormwater and flows greater than the design capacity will
be collected in an underdrain within the planter box and discharge to the north to an existing
ditch in the SW Taylors Ferry Road right-of-way (ROW). See attachment for LIDA flow-
through planter sizing.
Due to grading constraints for the driveway near the street frontage, 234 SF of impervious
driveway will flow into the existing ditch overland, untreated by the planter box. Using 1$ per
SF, it is proposed to have a fee-in-lieu assessed for a total of$234.00 for this area.
Engineering Conclusions:
The proposed design satisfies the conveyance and pollution reduction standards required by
the Clean Water Services 2017 Design Construction Standards manual. Pollution reduction is
accomplished by the treatment of on-site stormwater by a LIDA flow-through planter.
List of Attachments:
Site Map (Appendix A)
Clean Water Services LIDA Sizing Form (Appendix B) �tD PROO
$234C,Y4614431N 815
, . . "f,
,e,2
otel .
� Y11 Z*
t7,ANIEt.el
EXPIRES: 12/31 '
8285 SW Nimbus Avenue, Suite 180 Beaverton, Oregon 97008 TEL: 503.746.8812 Fax: 503.639.9592
411.1111111
APPENDIX B
Clean Wafer Services LIDA A Sizing Farm
Project Title: Taylors Ferry Residence
Project Location: 7304 SW Taylors Ferry Road
Contact Name/Title/Company:
Phone/e-mail:
STEP 1: Determine Impervious Area Requiring Treatment
Total Site Area (acres): 0.214 AC
Total Existing Impervious Area (sq.ft.): 0 SF
Proposed New Impervious Area (sq.ft.): 2 640 SF I
Impervious Area Requiring Treatment (sq.ft.) 2 640 SF
(Refer to Design&Construction Standards Chapter 4 for
instructions to calculate this area,which will be less than or
equal to the new plus existing site impervious area.)
STEP 2: Deduct Impervious Area LIDA Credits
Porous Pavement (sq. IL):
Green Roof(sq. ft.):
Other Credits as approved (sq. ft.):
Total Credits (sq. ft.):
Remaining Impervious area (sq. ft.)
(Total from Step 1 --Total Credits):
STEP 3: Size LIDA Facilities for Remaining Impervious Area
IA: Impervious SF, Sizing LIDA facility size
area treated Factor (sq.ft.) (IA x SF)
(sq.ft.)
Infiltration Planters/Rain
Garden 2,640 SF 0.06 160 SF
Flow-through Planter 0.06
LIDA Swale 0.06
Vegetated Filter Strip 0.06
Total Impervious (*Must equal total from Step 2 or additional LIDA2,640 SF facilities or Water Quality Facilities must be
Area treated (sq.ft.) added.)
ClearrWiter Services EOW impact Deve1ourrent f pproac nes Handbook 1 Q