HomeMy WebLinkAbout3731 Park Ave. N TIRv8
Prepared by: /December 14, 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
Technical Information Report (TIR)
Cover 1
Table of Contents 2
Project Overview 3
Conditions and Requirements Summary 4
1. Discharge at the natural location 4
2. Offsite Analysis 4
3. Flow Control Design 6
4. Conveyance Design 7
5. Erosion Control D Design 7
6.Special Reports and Studies 8
7.Other Permits 8
8. Water Quality Design 8
9. Flow Control BMPs 9
SPECIAL REQUIREMENTS 9
Appendix
King County iMap of Sensitive Areas 10
Soil Data 11-23
PROJECT OVERVIEW
This storm drainage technical information report has been prepared to address the City of Renton site development and storm drainage requirements for the proposed new single family residence
located at 3731 Park Ave. North on Tax parcel # 3342700440 located at the west side along the alley along N 37th St. of Park Ave. N between N 38th St. and N 36th St. in Renton,
WA. The Legal Description is: TRACT 91. C.D.HILLMAN’S LAKE WASHINGTON GARDEN OF EDEN ADDITION TO THE CITY OF SEATTLE, DIVISION NUMBER 2, ACCORDING TO THE PLAT THEREOF RECORDED IN
VOLUME 11OF PLATS, PAGE 64, RECORDS OF KING COUNTY, WASHINGTON;
The proposed project is located at 3731 Park Ave. North, Renton, WA 98056. The property is 15,064 square feet in area. The property is grass with an old house and shed built in 1932,
and an asphalt drive serving 3737 Park Ave N to the west. Total existing impervious area = 5,040sf+/- or 33.45% with two trees on the left rear and one at the right side of the old
house. The site is moderately sloped down to the northwest at 9%+/-. The proposed project involves constructing a new 4977 square foot single-family residence (5,382 sf roof), with
patio 442 sf , with a 791 square foot concrete driveway and walk, with a 791 sf access road and fire department turnaround. A water main and gas line will be installed under the existing
access road. The total new/replaced impervious area will amount to 9,257 square foot.
There is no storm drainage in the area. There are single family residences to the north, east, and west of the parcel. Currently, the stormwater runoff sheetflows towards the northwest
onto the adjacent yards and then to N 38th Street and west to Lake Washington.
Impervious Surfaces 15,064SF LOT
Existing AC Drive 2,098sf
Proposed New Residence W OH 5,382 sq.ft.
Patio 442 sf
Driveway and Walkway 791 sq.ft.
Widened Access Road and Fire Turn Around 544 sq ft.
Total new Impervious Area 7,159 sq.ft. including off-site driveway widening
Total Impervious Area = 9,257 sq.ft = 61,45% on site
Since this lot is smaller than 22,000 square feet, it is subject to the Small Lot BMP Requirements in Appendix C of the KC Surface Water Design Manual (SWDM) which specify the target
impervious surface is equivalent to the roof area and driveway.
Target Impervious Surface = Residence Footprint plus driveway = 9,257 sq.ft.
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 and displays that the runoff will travel north
and west to North 38th Street and then west to lake Washington. The site is smaller 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 and 5 Special requirements will be addressed in this TIR:
CORE REQUIREMENT #1 DISCHARGE AT THE NATURAL LOCATION
The flows on this site are kept dispersed and are allowed to continue to flow to the natural downhill flow path. The flows will drain to the northwest or be infiltrated as they are
now.
CORE REQUIREMENT #2 OFFSITE ANALYSIS
This is an urban area and the properties are small and fully developed. There are no significant on or off site flows on this property. No downstream drainage problems were observed.
See Level One Downstream Drainage Study below:
Offsite Analysis – Level One Downstream Analysis
Task 1. Study Area Definition and Maps
The properties to the north, east and west of this site are small single family residences, and do not discharge any significant amount of runoff onto this site. The access drive to
the south does not drain any significant amount of water onto the site. No other significant flows enter the site.
No concentrated surface water runoff will be discharged off-site. There are no downstream effects anticipated from the proposed development. The downstream flowpath of stormwater runoff
during very large storms is to flow northwest to the adjacent properties. A site map and topographical map with downstream flow path can be found in the Appendix on page 13 per
KC Imap.
Downstream Flow Path
Length
A. to northwest to N 38th Street
150 feet
N 38th Street to Lake WA
900 feet+/-
Task 2: Resource Review
Using the King County iMap interactive tool, and COR Maps it was found that there are no immediate critical areas upstream or downstream of the property. In addition, the site is
not on or near any wetlands. The maps also confirmed that there are no mitigating rivers, floodplain issues, there are no adopted basin plans, and the water quality is listed as “Basic
Water Quality Treatment.” Per Soil Map 5 of the SCS Soil Survey of King County the soils are Indianola loamy fine sand, InC. which is an outwash soil. A soil investigation by Cobalt
Geosciences found that the soil was adequately permeable for roof downspout infiltration and pervious pavement. See attached Cobalt Soil Report
Task 3: Field Inspection
There are no reported problems to be investigated. The amount of space and soil types present on the property limit the flow control BMPs being implemented. There will not be any destruction
of aquatic habitat on-site or downstream. The Site was visited on July 9, 2017 and a 1/4 mile downstream investigation was made. As there are no signs of surface flow no problems
were identified.
See Cobalt GeoSciences Soil Evaluation in the Appendix
Task 4: Drainage System Description and Problem Descriptions
For this small site limited infiltration and basic dispersion BMPs will be used to the extent practical. No problems were observed in the downstream drainage system.
Task 5: Mitigation of Existing or Potential Problems
There are no 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 site is currently occupied by an abandoned house and shed and asphalt driveway, with two trees remaining at the left rear end. The site is gently sloped with slopes of 9% +/- down
to the northwest. The storm water runoff from the site currently disperses across the site and drains onto the neighboring properties to the northwest. There does not appear to be
any significant surface flow. 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 asphalt driveway to the south does not have any drainage but does not appear to discharge a significant amount of runoff onto the site. The existing driveway currently sheet flows
onto the adjacent grass yards and will continue to do so.
Part B: Developed Site Hydrology
When the new house is constructed it will use limited infiltration to control storm water runoff. 20% of the site is required to be mitigated using BMPs per C.1.3 20% of the site
is 3,012.8sf.
The existing AC driveway will used sheet flow per C.2.4.D and a 10 foot Basic Dispersion sheet flow path which will treat the access road as it currently does. The Fire department turn
around, driveway and walk will be made of pervious pavement
Two roof downspout infiltration trenches will totally infiltrate the roof area of 5,382sf C.2.2.A
Therefore the total area treated will be = 9,257 sf of the site which is greater than the 3,012.8sf required. 100 % of the impervious area will be treated.
Part C: Performance Standards
Flow control BMPs are used on this site to control stormwater using Limited Infiltration using roof downspout infiltration trenches per Fig C.2.2.A and pervious concrete pavement.
This is a single family residence on a small lot and as such does not pose any significant threat for pollution. No pollution generating activities are planned. This is not a ‘High
Use Site’. It is a single family residence.
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 limited infiltration using roof downspout infiltration Trenches per Fig. C.2.2.A and pervious concrete pavement will be used as the most
practical BMPs.
Part E: Water Quality System
The site will have less than 5,000sf of pollution generating surface. Therefore no water quality treatment is required. This is not a high use site. No additional facilities will be
necessary.
New Impervious Surfaces
Proposed Residence 5,382 sq.ft.
New Driveway and Walkway 791 sq.ft
New Fire Departmetn turnaround and widended drive = 544sf.
Total New Impervious Area 7,159 sq.ft
Existing AC Drive 2,098sf
Total site Impervious. = 9,257 sf = 61.45%
Since this lot is smaller than 22,000 square feet, it is subject to the Small Lot BMP Requirements in Appendix C of the KC Surface Water Design Manual (SWDM) which specify the target
impervious surface is equivalent to the roof area and driveway.
Target Impervious Surface = Residence Footprint plus driveway = 9,257 sq.ft.
Flow control BMPs are used on this site to control stormwater. See discussion below:
Full Dispersion – Unable to achieve the minimum 100 feet of native vegetated flowpath. Therefore, full dispersion is not feasible.
Full Infiltration – According to the attached soils information from the SCS King County Soil Survey, the soils on the site are listed as Indianola Loamy Fine Sand, InC. Full infiltration
is only allowed with medium sands, coarse sands, or cobbles. Therefore, full infiltration is not applicable.
Since full dispersion and full infiltration of roof runoff is not feasible, one or more of the alternate BMPs (listed in C.1.3.1 A.3) must be applied to (or used to mitigate for) an
impervious area equal to at least 20% of the lot. [Paraphrased from C.1.3.1 A.3].
Limited Infiltration – The soils on the site are specified as InC Indianola Loamy Fine Sand . The Goetechnical investigation by Cobalt GeoSciences found the soil to be loamy sand which
allow 21 feet of gravel trench per 1,000 square feet of impervious area for roof downspout infiltration trenche per Fig C.2.2.A per C.2.3.3. Anstey Engineering has decided to use a
more conservative 52lf of trench per 1000sf of roof. There is sufficient space for the minimum amount of gravel trench needed, as 5,382sf roof/1000= 5.382 x 52 = 280lf of 2 foot trench
or 560 sf of trench. We propose to use two trenches with a width of 4.5 feet and combined length of 126 feet for 126x4.5= 567sf of trench bottom. Pervious Concrete pavement will be
used for the new driveway and walks.
Basic Dispersion – There is the minimum 50 feet required space to implement splash blocks for one downspout. Gravel filled trenches and sheet flow are not feasible due to the inability
to achieve 25 feet of vegetated flowpath for downspouts and trenches. A 10 foot vegetated flow path will be used for the existing driveway per C.2.4.D.
Rain Garden – The use of a raingarden for stormwater mitigation is infeasible due to constraints of the site, slope, lack of outlet and as open-space on the lot is limited.
Permeable Pavement – Permeable Concrete Pavement can be used as the slope of the new driveway is under 6% which is less than the 10% maximum.
Rainwater Harvesting – Rainwater harvesting methods would be fairly large and therefore difficult to site on the lot due to space constraints. The landscaped area that would receive
irrigation is also fairly small. Considering the costs and benefits rainwater harvesting is not practical for this site.
Vegetated Roof – A Vegetated roof is not proposed for this residence.
Reduced Impervious Surface Credit – This site does not meet the requirements for the reduced Impervious surface Credit
Since full dispersion, and full infiltration, have been ruled-out, we will use limited infiltration using a roof downspout infiltration trenches per Fig C.2.2.A and pervious pavement
for the new drive and walk. Sheet flow dispersion is used for the existing asphalt drive for drainage control.
CORE REQUIREMENT #4 CONVEYANCE SYSTEM
As the flows on this small site are kept dispersed and are minimal there are 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 driveway and house, 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.
Topsoil will be stockpiled in a designated area to be reapplied after the completion of construction
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 site vegetation.
CORE REQUIREMENT #6 MAINTENANCE AND OPERATION
The pervious pavement and roof downspout infiltration trenches will require little maintenance. See attached Maintenance requirements
CORE REQUIREMENT #7 FINANCIAL GUARANTEES AND LIABILITY
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 than 5,000sf of new pollution generating surface will be created no special water quality treatment is necessary.
Core Requirement #9 FLOW CONTROL BMPS
BMPs will be used to the extent feasible to control Stormwater on this site. See
Discussion of BMPs in Requirement #3 Flow Control. The BMPs most practical for this small lot are Roof Downspout Infiltration Trenches per Fig C.2.2.A, and pervious concrete pavement
per C.2.3.4 and Basic Dispersion per C.2.4.D.
Full Dispersion Cannot be used as there is not a 100 foot flow path.
Full Infiltration Cannot be used as the soils are not medium sand.
Limited Infiltration can be used as there is adequate space for the required trench. Limited infiltration requires 52lf of trench per 1000sf of impervious area. 52lf of trench per 1000sf
of roof. There is sufficient space for the minimum amount of gravel trench needed, as 5,382sf roof/1000= 5.382 x 52 = 280lf of 2 foot trench or 560sf of trench. We propose to use two
trenches with a width of 4.5 feet and combined length of 126 feet for 126x4.5= 567sf of trench bottom.
Basic Dispersion will be used for the existing pavements using sheet flow per C.2.4.D
Rain Garden cannot be used as there is not adequate space and no overflow
Pervious Concrete Pavement will be used as the new driveway pavement will be sloped at 9% which has been determined to be feasible for pervious concrete, per Fig C.2.7.A
Rainwater Harvesting cannot be used as Rainwater harvesting methods would be fairly large and therefore difficult to site on the lot due to space constraints. The landscaped area that
would receive irrigation is also fairly small. Considering the costs and benefits rainwater harvesting is not practical for this site
Vegetated Roof cannot be used as it will not fit this house.
Reduced Impervious Surface Credit cannot be used as the imperious area is too large
Native Growth Protection Credit cannot be used as there is no Native Growth on the lot and there is not adequate space remaining to establish a NGPE
A Perforated Pipe Connection cannot be used as there is no drainage system to connect to.
SPECIAL REQUIREMENTS:
Special Requirement #1: Other Adopted Requirements – Do not apply to the site
Special Requirement # 2: Flood Hazard Delineation – Not required, not in flood area
Special Requirement #3: Flood Protection Facilities – Not required, not in flood area
Special Requirement #4: Source Control – Not require this is a Single Family residential development
Special Requirement #5: Oil Control – Not required this is not a high use site.
Special Requirement #6: Aquifer Protection Area – Not required, not in Aquifer Protection area
Appendix: Figure 1:I Imap
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