HomeMy WebLinkAboutRS_TIR__V2_180402_v1STORM DRAINAGE
TECHNICAL INFORMATION
REPORT
FOR
FUPC ADDITION AND PARKING
3811 NE 21st St.
RENTON, WA 98056
on
PARCEL # 0423059237, 068 and 307
FOR:
FUPC
Prepared by: January 5, 2018
Anstey Engineering
CIVIL ENGINEERING/CONSULTING
8627 NE 180th Street
Bothell, Washington 98011
Phone: (206) 303-7639
Fax: (425) 658-9203
E-mail: benansey@ansteyengineering.com
Table of Contents
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Technical Information Report (TIR)
Cover 1
Table of Contents 2
Project Overview 3
Conditions and Requirements Summary 3
1. Discharge at the natural location 3
2.Offsite Analysis 3
3. Flow Control Design 5
4.Conveyance Design 8
5.Erosion Control D Design 8
6. Special Reports and Studies 9
7. Other Permits 9
8. Water Quality Design 9
9. Flow Control BMPs 10
Appendix
King County iMap of Sensitive Areas 11
Figure C.2.1.D Typical 50ft Dispersion Trench with Notched Board 12
Figure C.2.4.B Typical Gravel Filled Dispersion Trench 13
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PROJECT OVERVIEW
This storm drainage technical information report has been prepared to address the 2017 City of
Renton site development and storm drainage requirements for the proposed new single family
residence addition located at 3811 NE 21st St on Tax parcels #0423059237, 068 and 307 at East
side of Redmond Ave NE, South of NE 21st ST, between NE 21st St and NE 19st St in Renton, WA.
The Legal Description is: POR OF E 1/2 OF E 1/2 OF NW 1/4 OF NE 1/4 OF SE 1/4 LY N OF S
LN OF LOT 4 OF ROSE HAVEN PROD E in King County Washington.
The proposed project is located at the South of NE 21st ST, Renton, WA 98056. The
property is (3.38 Acre) 147,311 square feet in area. The property has existing
development, 13100sf buildings area and approximately 50,000sf impervious parking lot
and walk ways. The rest of the property is covered with grass and trees. The North and
East portion site is almost flat, South portion of the site has 3.5% +/- down slope toward
south.
The proposed project involves construction of a new 1,750 square foot building addition,
and 21,815 square foot pervious parking extension. Frontage improvements to include
new sidewalk will be done under a separate permit. As part of front improvement, 23’ at
North end, total 3755sf including 1295 impervious parking will be changed to ROW
dedicated. 574sf existing walk way will be demolished to construct addition and new 322
sf walkway will be added.
There is existing catch basin on North of site, which is connected to storm drainage
system via 12” drain under street 3811 NE 21st St, drains toward west . Major portion of
existing development on North and East section of site (parcels #0423059237 and 307)
drains to the existing North CB. There is also existing catch basin on South West of site
which is connected to drainage system on South side NE 19th St. Area from parcel #
0423059068 drains to South catch basin. The surface water from new impervious
construction will be added to south catch basin which will flow to existing natural
drainage system
Impervious Surfaces :
Existing Covered area
Existing Building 13,100 sq.ft.
Existing Asphalt Parking 40, 822 sq.ft.
Existing Walkway 10,788 sq.ft.
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Total existing Impervious Area 64, 710 sq.ft.
Proposed covered area
Proposed Building 1,750 sq. ft
Proposed walkway 322 sq. ft
Existing parking change to ROW (-) 1,294 sq. ft.
Demo existing walk way (-) 574 sq. ft
Total Impervious Area 64,914 sq. ft.
Target Impervious Surface = Residence Footprint = 1,750 sq. ft.
Pervious surface = Parking addition = 21815 sq. ft.
As more than 7000 sq ft land disturbing activity but new impervious addition is less than
2000 sq ft, the project needs Large Lot Drainage review per 1.3.2 of the 2017 Renton
SWDM. A site location and sensitive areas map can be seen in the Appendix. This map
shows that the site is not located near sensitive areas. The site is larger than 22,000 sf so
BMP’s will be used to the extent possible to contro l stormwater runoff.
Conditions and Requirements Summary
The 9 core requirements will be addressed in this TIR:
CORE REQUIREMENT #1 DISCHARGE AT THE NATURAL LOCATION
The flows on this site for building addition are kept dispersed using the basic dispersion
and are allowed to continue to flow to the natural downhill flow path. The flows will drain to
the southwest as they are now. Parking addition will be pervious asphalt pavement, the flows
are allowed to continue infiltrate and disperse natural as existing.
CORE REQUIREMENT #2 OFFSITE ANALYSIS
This is an urban area and the properties around are small and fully developed. There are
no significant on or off site flows on this property. No downstream drainage problems were
observed.
Offsite Analysis – Level One Downstream Analysis
Task 1. Study Area Definition and Maps
The properties to the north of this site are small single family residences, and do not
discharge any significant amount of runoff onto this site. 21st st has curbs and drainage system
and Frontage of site will be developed to construct curbs and walk way which does not drain any
significant run off to site. South side NE 19th St and properties are lower than site, do not
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discharge any significant amount of runoff onto this site. East and West side properties small
single family residences and fully developed, do not discharge any significant amount of runoff
onto this site.
Task 2: Resource Review
Using the King County iMap interactive tool, it was found that there are no immediate
critical areas upstream or downstream of the property.
Task 3: Field Inspection
There are no reported problems to be investigated. The amount of space and soil types
present on the property clearly support the flow control BMPs being implemented. There will
not be any destruction of aquatic habitat on-site or downstream. The Site was visited on
October 11, 2017 and a 1/4 mile downstream investigation was made. As there are no signs of
surface flow no problems were identified.
Task 4: Drainage System Description and Problem Descriptions
For this site Basic Dispersion using roof downspout to gravel filled dispersion
trench and permeable pavement will be used.
No more than 700 square feet of the roof impervious surface (or 5,000 square feet
of non-native pervious surface) may be drained to a simple 10-foot dispersion trench. The
dispersion trench is calculated as follows: 1750 sf/700sf = 2.5 or 25 ft. trench. Therefore
we will use 25ft x 2ft. trench.
Task 5: Mitigation of Existing or Potential Problems
There are no known existing or potential drainage problems and / or water quality
problems.
CORE REQUIREMENT #3 FLOW CONTROL
Proposed Site and Drainage
Part A: Existing Site Hydrology
The lot is developed, the storm water runoff flows from existing roof and parking lot to
existing catch basin and drainage system on North of site. South side of site has grass and trees;
it is gently sloped with slope of 3.5%+/- down to south. The storm water runoff from the South
side currently disperses across the site toward South and flows to existing catch basin and
drainage system under NE 19th St of South side. There is no evidence of any other on-site flows
going off-site. The site does not receive any significant off-site flows from the adjacent
properties
Part B: Developed Site Hydrology
When the addition building is constructed it will use Basic Dispersion using roof
downspout dispersion trenches per Figure C.2.4.B.
For parking addition permeable asphalt paving will used as per appendix C section C 2.7 of
RSWDM 2017.
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Part C: Performance Standards
Flow control BMPs per C.1.3.2 are used on this Large Lot to control storm water from
roof by using Basic Roof Downspout Dispersion using dispersion trench per C.2.4.B, and
permeable asphalt paving for parking area.
Part D: Flow Control System
The Erosion and Sediment Control Plans along with the Site Plans include the locations
of the flow control systems. In addition, cross-sections of the flow control BMPs can be found in
the Appendix.
BMPs were analyzed for their practicality and basic dispersion using a roof downspout
dispersion trench in the back yard and permeable asphalt paving for the parking area were
selected the only practical BMPs.
Part E: Water Quality System
The site will have less than 5,000sf of pollution new generating surface. Therefore no
water quality treatment is required. This is not a high use site. No additional facilities will be
necessary.
Impervious Surfaces
Impervious Surfaces
Proposed Addition 1,750 sq.ft.
Total Impervious Area 1,750 sq.ft.
Since this lot is larger than 22,000 square feet, it is subject to the Large Lot BMP
Requirements in Appendix C - C.1.3.2of the 2017 Rent on Surface Water Design Manual
(SWDM) which specify the target impervious surface is equivalent to the roof area of
addition.
Target Impervious Surface = Roof area addition = 1,750 sq.ft.
CORE REQUIREMENT #4 CONVEYANCE SYSTEM Not Applicable
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As the flows on this site are totally dispersed and are minimal there is no conveyance
systems needed.
Core Requirement #5 EROSION AND SEDIMENT CONTROL
Temporary ESC measures will be required as there will be disturbance of the soil to
construct the additional parking and building addition, and clearing of the site. All of the flows
from the driveway and house will flow onto the lot so no adjacent properties will be affected. A
silt fence around the construction area and construction entrance should be provided at a
minimum. No special requirements are needed
In order to prevent erosion and trap sediment within the project site, the following BMPs
will be used approximately as shown on the ESC plan:
⦁ Clearing limits will be marked by fencing or other means on the ground but in this
case are the limits of the property.
⦁ Extra excavated soil will be removed from the site or spread on site.
⦁ A rocked construction entrance will be placed at the location of the driveway
throughout construction.
⦁ Runoff will not be allowed to concentrate and no water will be allowed to point
discharge.
⦁ Silt fencing will be placed along slope contours at the downslope limit of clearing.
⦁ Mulch will be spread over all cleared areas of the site when they are not being
worked. Mulch will consist of air-dried straw and chipped vegetation.
CORE REQUIREMENT #6 MAINTENANCE AND OPERATION
The dispersal trenches and Pervious Pavement will require little maintenance. See
attached Maintenance requirements
CORE REQUIREMENT #7 FINANCIAL GUARANTEES AND LIABILITY – Not Applicable
The property owner will be responsible to insure that the project is constructed to the
approved plans and that the drainage does not cause any problems.
CORE REQUIREMENT #8 WATER QUALITY – Not Applicable
As less than 5,000sf of new pollution generating impervious area will be created no
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special water quality treatment is necessary.
Core Requirement #9 FLOW CONTROL BMPS PER C.1.3.2 for Large Lot
BMPs will be used to the extent feasible to control Stormwater on this site per C.1.3.2. .
The only BMPs practical for this large lot are roof downspout dispersion for building addition
and pervious asphalt pavement for addition of parking area. The roof downspouts will be
routed to the dispersion trench through catch basins per Fig C.2.1.D and C.2.4.B. Pervious
asphalt pavement will be used as per section 2.7.
Flow control BMPs are used on this site to control stormwater. See discussion below:
C.2.1 Full Dispersion – Roof area is more than 700 sf. A 100 foot vegetated flow
path is not available, therefore, full dispersion is not feasible.
C.2.2 Full Infiltration – Not feasible as the soils are till per the Cobalt GeoSciences
report dated January 19, 2018
C.2.3 Limited Infiltration – Not feasible due to high winter ground water table and
till soil per the Cobalt GeoSciences report dated January 19, 2018
C.2.6 Bioretention – Not feasible as there is not a practical overflow connection to
the City Storm Drain
C.2.7 Permeable Pavement – Permeable Asphalt Pavement is proposed for
proposed addition of parking area. This is light duty overflow parking for the
church so pervious asphalt pavement should work adequately. The area is gravel
parking now.
C.2.4 Basic Dispersion – There is the minimum 50 feet required space to
implement splash blocks and rock pads. In addition, gravel filled trenches and sheet
flow are feasible due to the ability to achieve 25 feet of vegetated flow path. As
more than 700 sf (1,750sf new addition) of new impervious surface is created a
Roof Downspout Dispersion Trench will be used per C.2.4.B. 1750/700= 2.5 x 10
= 25 feet of trench required.
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Appendix: Imap
SITE
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