HomeMy WebLinkAboutRS_TIR__V7_181116_v1Prepared by: October 30, 2018
Anstey Engineering
CIVIL ENGINEERING/CONSULTING
8627 NE 180th Street
Bothell, Washington 98011
Phone: (206) 303-7639
Fax: (425) 658-9203
E-mail: benanstey@ansteyengineering.com
Table of Contents
Technical Information Report (TIR)
Cover 1
Table of Contents 2
⦁ Project Overview – Purpose
⦁ Existing Condition
⦁ Developed Condition
⦁ TIR Worksheet
⦁ Basin Map
⦁ Soil Map
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⦁ Site Map
2. Conditions and Requirements
Core Requirements
#1 Discharge at Natural Location 5
#2 Off-Site Analysis 5
#3 Flow Control 5
#4 Conveyance System 7
#5 Erosion and Sedimentation Control 7
#6 Maintenance and Operation 7
#7 Financial Guarantee and Liability 7
#8 Water Quality 8
#9 Flow Control BMPs
Special Requirements 1-6
⦁ Other Adopted Area Specific Requirements
⦁ Flood Hazard Area Delineation
⦁ Flood Protection Facilities
⦁ Source Controls
⦁ Oil Control
⦁ Aquifer Protection Area
3.Offsite Analysis
Task 1 – Study Area Definition and Maps
Task 2- Resource Review
Task 3 – Drainage System description
4. Flow Control
4.1 Existing Site Hydrology
Developed Site Hydrology
Performance Standards
Flow Control System
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4.2 Water Quality
5. Conveyance System
6. Special Reports – See Geotech Reports by Cobalt GeoSciences
7. Other Permits
8. CSWPPP – See attached
9. Bond Quantity , Facility Summary & Declaration of Covenant
10. Operations and Maintenance
11. Conclusions
Appendix
King County iMap of Sensitive Areas 11
Figure AquaFilter Filtration System 12
Figure C.2.7.1.A Schematic Representation of Permeable Pavement 13
Figure City of Renton Std Plan 235.00 Typical Detention Tank
1 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 Church
Building addition located at 3811 NE 21st St on Tax parcels #0423059237, 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 (2.49 Acre) 108,480 square feet in area. The property has existing
development, 13,100sf Church building 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 north portion of the site, existing church building and paved
parking drains to the north to the NE 21st Street Drainage System. The existing gravel
parking on the east is a pothole that infiltrates int o the ground.
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The proposed project involves construction of a new 1,750 square foot building addition,
and 21,815 square foot pervious parking extension and Frontage Improvements will be
made to NE 21st Street to include new sidewalk curb and gutter. As part of frontage
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 sheet flow across the site
to the existing drainage system in NE 19th Street.
Impervious Surfaces :
Existing Covered area
Existing Church Building 11,176 sq.ft.
Existing Asphalt Parking 39,762 sq.ft.
Existing Walkway 10,788 sq.ft.
Ex other pavement 733 sq. ft
Total existing on site Impervious Area 62,919 sq.ft
+ Area in ROW 2,938.5 = 65,857.5sq ft
Proposed new replaced impervious area
Proposed Building 1,750 sq. ft
Proposed walkway 322 sq. ft
Existing Pvmt to landscape change to ROW (-) 1,294 sq. ft
New Sidewalk in ROW = 665sf
New Curb and gutter in ROW = 187lf = 280.5sq ft
= increase in impervious area = 1,723.5 sq ft
. New pervious asphalt 21,815sq. ft.
Demo existing walk way (-) 574 sq. ft
Total Impervious Area 64,914 sq. ft.
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Target Impervious Surface = New Church Addition Footprint = 1,750 sq. ft.
Pervious surface = Parking addition pervious asphalt = 21,815 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 control 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 the building addition enter a detention tank, from which it
continues to flow to the natural downhill flow path. The flows will drain to the NE 21st Street
and NE 19th Street drainage systems or be kept on site they are now. Parking addition will be
pervious asphalt pavement, the flows will also enter the detention tank. To divert flow from the
gravel parking area to the 19th Street Drainage System should require a Drainage Adjustment as
it will be a diversion.
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.
The existing paved surfaces on this site drain to the piped drainage systems in NE 21st Street.
The gravel parking area ponds on site.
See Level One Downstream Analysis in section 3
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 and into the NE 21st Street Drainage
system. 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. The soil is glacial till. The existing gravel
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parking on the east of the site infiltrates on site.
Part B: Developed Site Hydrology
For the parking addition a 305LF 72" detention tank, outletting to NE 19th St, will used per
section 5.1.2 of RSWDM 2017. The existing area to be paved is currently mostly gravel so there
will be no significant change of condition.
When the 1,750 sf addition to the building is constructed it will also use the detention tank.
Part C: Performance Standards
Flow control BMPs are used on this Large Lot to control storm water from roof by using a
detention tank per 5.1.2 for the parking area and building addition. All BMPs were found to be
infeasible due to site constraints. Pervious pavement will be used for the new 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 pervious pavement and a detention tank for the
parking area and building addition was selected as the only practical BMP. See Section 9 BMP
Analysis
Part E: Water Quality System
The site will have more than 5,000sf of pollution new pollution generating surface,which
will be treated at the end of the detention tank by an AquaFilter system. 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 Renton Surface Water Design Manual
(SWDM) which specify the target impervious surface is equivalent to the roof area of
addition.
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Target Impervious Surface = Roof area addition = 1,750 sq. ft .
Pervious Pavement = 21,815 sq. ft.
See section 9 for BMP analysis
CORE REQUIREMENT #4 CONVEYANCE SYSTEM Not Applicable
As the flows on this site are totally dispersed and are minimal there is no conveyance
systems needed. Flows from the pervious parking area and building addition will be conveyed
through the detention tank, no additional conveyance will be necessary.
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
construction entrance should be provided at a minimum. No special requirements are needed
In order to prevent erosion and trap sediment within t he 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.
⦁ 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.
⦁ Mulch will be spread over all cleared areas of the sit e when they are not being
worked. Mulch will consist of air-dried straw and chipped vegetation.
CORE REQUIREMENT #6 MAINTENANCE AND OPERATION
The Pervious Pavement will require little maintenance. See attached Maintenance
requirements. This is a private system that will be maintained by the property owner.
CORE REQUIREMENT #7 FINANCIAL GUARANTEES AND LIABILITY – Not Applicable
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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 –
As less more 5,000sf of new pollution generating impervious area will be created water
quality treatment is necessary. The Pervious pavement is self treating by infiltration and an
AquaFilter will be provided near the outlet of the detention system to treat any runoff.
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 BMP practical for this large lot is a detention tank per 5.1.2 for the additional previous
parking area. The roof downspouts will be routed to the detention tank. Pervious asphalt
pavement will be used as per section 2.7 and will also use the detention tank.
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 suitable area due to surfacing,
slopes and setbacks.
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. Underneath the parking area will be a 305 LF 72" detention tank.
See WWHM printout. See Cobalt Geotechnical Report on Soils dated May 15,
2018 with letter of June 26, 2018 did 3 holes and found goundwater between 3.5
and 8 feet below the ground. As the proposed pervious pavement will be placed
on top of the existing ground at the lower end of the parking lot, the 1 foot
minimum separation required for pervious pavement will be maintained. Per
C.1.3.2 Pervious Pavement is required to be used if possible and we have
adequately permeable soil and an adequate amount of permeable soil above the
ground water table which are the only two applicable criteria.
A control manhole and detention tank are provided for flow control.
C.2.4 Basic Dispersion – There is not the minimum 50 feet required space to
implement splash blocks and rock pads. In addition, gravel filled trenches and sheet
flow are not feasible due to the inability to achieve 25 feet of vegetated flow path.
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However, since a detention tank is already needed for the parking area, the
building addition will utilize the detention tank.
Special Requirements:
⦁ Other Adopted Area Specific Requirements – none known
⦁ Flood Hazard Are Delineation – site is not in a flood plane
⦁ Flood Protection Facilities – No Flood Protection Facilities are
required or proposed
⦁ Source Control-The site is a commercial site but we are not adding
any impervious pollution generating surface so no source control is
required.
⦁ Oil Control- This is not a high use site so oil control is not required
⦁ Aquifer Protection Area- This site is not in an Aquifer protection
area.
⦁ Off-Site Analysis
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 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
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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. The piped drainage systems flow west.
Task 4: Drainage System Description and Problem Descriptions
For this site, permeable pavement with a detention t ank will be used.
The building addition will also use the detent ion tank. Detention of the
stormwater from the new addition using the tank will prevent any flooding
problems on-site or downstream.
NE 21st Street and NE 19th Street both have piped drainage systems that
appear to be functioning properly.
⦁ There are no other significant on or off-site flows. The existing church and
parking drain to on-site catch basins which drain to the NE 21st Street Drainage
System.
Task 5: Mitigation of Existing or Potential Problems
There are no known existing or potential drainage problems and / or water
quality problems.
4 Flow Control
⦁ Existing Hydrology –The site is currently developed with the church building and
asphalt and gravel parking. The Church building and asphalt parking drain to an
existing catch basin in the parking lot near NE 21st Street which drains to the NE
21st Street piped drainage system. The gravel parking area infiltrates into the
ground.
Developed Site Hydrology-The existing church building and asphalt parking will
continue to drain as they do now. The new 1,750sf Church addition and new paved
parking will drain to a detention tank which will discharge to the NE 19th Street
storm system.
⦁ Conveyance
No special conveyance design is required as a 6” pvc pipe is
adequate to take the roof water to the detention tank
⦁ Special Reports – See Geotech Report by Cobalt GeoSciences
⦁ Other Permits – A building Permit will be required. A ROW permit
will be required for frontage improvement work in the NE 21st Street
ROW.
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⦁ CSWPPP – See attached
⦁ Bond Quantity, Facility Summary and Declaration of Covenant – See
attached
⦁ Operations and Maintenance- Operations and Maintenance
instructions are attached
⦁ Conclusions – The Church Addition, pervious parking and frontage
improvements on NE 21st Street can be installed per City standards
and without changing the character of the site.
Appendix: Imap
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