HomeMy WebLinkAboutDrainage Report - PLN General - 2/2/2021 Engineered Abbreviated
Drainage Report PLANMNG
Weldon Garage RECEIVED
286 E. Cronquist Rd. s APR 19 2021
Allyn, WA 98524 15 W. A/der Street
TPN 12232-40-01140
Mason County Project No.
Olympic Engineering No. 20058
February 2, 2021
Prepared by:
OLYMPIC
ENGINEERING
PO Box 12690
Olympia WA 98508
360.705.2474
www.olyeng.com
Project Overview
The proposal is to construct a detached garage on a developed f0.54-acre parcel off E. Cronquist
Rd. on Puget Sound. All existing improvements (e.g. house, sheds, garage, septic, well) will
remain. Access will be taken from an existing driveway off Cronquist Rd. The parcel and
surrounding area are zoned RR5 (Rural Residential 5 Acres).
The parcel abuts Puget Sound on the east side. There are no other known critical areas
(wetlands, steep slopes etc.) on-site or within the immediate vicinity of the site and all
improvements are proposed outside of the 100' fish & wildlife habitat conservation area.
Current.site vegetation consists mostly of maintained lawn/landscape.
The Natural Resource Conservation Service (MRCS) classifies the soils within the project area as
Alderwood Gravelly Sandy Loam(HSG B) (see enclosed NRCS soils report).
The parcel is bordered by Puget Sound to the east and by developed residential parcels on all
other sides. The project has a single-threshold discharge area(TDA).
The majority of stormwater runoff from the subject parcel is collected in catch basins and
tightlined to Puget Sound. Stormwater runoff from the proposed roof and approximately one-
half of the driveway area will also be collected and tightlined to Puget Sound. Stormwater
runoff from the other half of the driveway area will be sheet flow dispersed. See Minimum
Requirement 45 below for additional information.
Project Areas
Parcel Area: f0.54 ac (f23,522 sf)
Existing Hard Surface Area:
Roof: ±2,360 sf
Driveway: ±1,520 sf
Concrete/Misc.: ±1,085 sf
Total: t4,965 sf
New/Replaced Hard Surface Area:
Roof: 1,300 sf
Driveway: 2,400 sf
Total: 3,700 sf
Total Hard Surface Area after Project Completion:
Roof: 3,660 sf
Driveway: 3,920 sf
Concrete/Misc.: 920 sf
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Total: 8,500 sf
Hard Surface Coverage: 36%
Total Area to be Cleared/Graded: ±5,100 sf
Minimum Requirements
The total "new and/or replaced" hard surface area is greater than 2,000 sf but less than 5,000 sf;
therefore, this project is required to address Minimum Requirements (MR) #1-5 per Volume I,
Section 1-2.4 of Volume I of the WSDOE Stormwater Management Manual for Western
Washington, 2014 edition (Stormwater Manual). These Minimum Requirements have been
addressed as follows:
Minimum Requirement#1 —Preparation of Stormwater Site Plans
An Engineered Abbreviated Drainage Plan has been prepared (see enclosed).
Minimum Requirement#2—Construction Stormwater Pollution Prevention (SWPP)
A Short Form SWPP plan has been prepared (see enclosed).
Minimum Requirement #3—Source Control of Pollution
A Residential Pollution Source Control Program will be prepared, if required.
Minimum Requirement 44—Preservation of Natural Drainage Systems and Outfalls
No new discharge points, changes to existing drainage patterns, or revisions to any
existing discharge locations are being proposed; therefore, this Minimum Requirement is
not applicable.
Minimum Requirement#5—On-Site Stormwater Management
Per Section 1-2.5.5 and I-2.5.7 in Volume I of the Stormwater Manual, this project is
exempt from flow control; therefore, it is not required to achieve the LID Performance
Standard or consider bioretention,rain gardens, permeable pavement, and full dispersion.
The project will provide Post-Construction Soil Quality and Depth (BMP T5.13) and
Sheet Flow Dispersion(BMP T5.12)to the maximum extent possible.
The proposed BMP's are as follows:
Lawn and Landscape Areas:
• All disturbed and/or new lawn and landscape areas will contain soils meeting the
Post-Construction Soil Quality and Depth (BMP T5.13)requirements.
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Roof Areas:
• Downspouts from the roof gutters will be tightlined to Puget Sound.
Other Hard Surface Areas:
• Stormwater runoff from the western portion of the driveway will be sheet flow
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dispersed (BMP T5.12) onto adjacent lawn/landscape areas containing soils
meeting the Post-Construction Soil Quality and Depth (BMP T5.13)
requirements.
• Stormwater runoff from the eastern portion of the driveway will be collected and
tightlined to Puget Sound.
Additional Reauirements
Financial Liability
This is a single-family residential project; therefore, a Financial Guarantee is not
required.
Off-Site Analysis and Mitigation
It appears some off-site drainage enters the western portion of the parcel and a portion of
this is collected in an existing catch basin and routed to Puget Sound. This is proposed to
remain as-is. There does not appear to be any other noticeable stormwater run-on from
adjacent parcels or noticeable runoff from the subject parcel.
All stormwater runoff generated by the new improvements will be dispersed on-site
and/or routed directly to Puget Sound. Therefore, a quantitative off-site analysis or
mitigation is not warranted.
2/2/2021
"I hereby state that this Engineered Abbreviated Drainage E`ER M.
Report for the Weldon Garage project has been prepared by
me or under my supervision and meets the requirements of �
Mason County and normal standards of engineering practice.
I hereby agree that Mason County does not and will not
assume liability for the sufficiency,suitability, orperformance 7401 w4
of drainage facilities designed by me.
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