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Western Washington Division Eastern Washington Division
165 NE Juniper St., Ste 201, Issaquah, WA 98027 407 Swiftwater Blvd. , Cle Elum, WA 98922
Phone: (425) 392-0250 Fax: (425) 391-3055 Phone: (509) 674-7433 Fax: (509) 674-7419
www.EncompassES.net
TECHNICAL INFORMATION REPORT
For
Stewart Short Plat
2216 High Avenue NE
Renton, WA 98056
05/15/2017
Revised: 06/06/2018
Revised: 07/26/2018
07/26/18
Encompass Engineering Job No. 15671
Prepared
For
Hugh Stewart
3933 Lake Washington Blvd NE, Suite 100
Kirkland, WA 98033
Stewart Short Plat Technical Information Report
07/26/2018 Page i
Table of Contents
I. PROJECT OVERVIEW ................................................................................................................ 1
II. CONDITIONS AND REQUIREMENTS SUMMARY ...................................................................... 3
III. OFF-SITE ANALYSIS .................................................................................................................. 5
IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ........................... 14
V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ..................................................................... 17
VI. SPECIAL REPORTS AND STUDIES ............................................................................................ 18
VII. OTHER PERMITS ..................................................................................................................... 18
VIII. CSWPPP ANALYSIS AND DESIGN............................................................................................ 18
IX. BOND QUANTITIES and DECLARATION of COVENANT .......................................................... 20
X. OPERATION AND MAINTENANCE MANUAL .......................................................................... 20
Appendix A
Geotechnical Evaluation by Geospectrum Consultants, Inc. dated March 21, 2016
Appendix B
Bond Quantity Worksheet
Stewart Short Plat Technical Information Report
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I. PROJECT OVERVIEW
Site Address: 2216 High Ave NE, Renton WA, 98056
King County Tax Parcel: 334450-0189
Vicinity Map
This project involves the development of a 0.51-acre parcel into two single-family lots and one
tract containing a native growth protection area (open space). The site is bound by single-family
residences to the north, south and east and High Avenue NE to the west. Currently, the parcel
contains an existing house, rock retaining wall, driveway, concrete walk/patio, and about 24
significant trees. The site slopes to the east and to the west. Along the eastern edge of the
property, the site slopes eastward with slopes that are greater than 40%. The western portion
of the property slopes toward the west with slopes ranging from approximately 5% to 19%. Per
the SCS Soil maps (see soils map on next page), the site is underlain with Indianola loamy sand
with 5 to 15 percent slopes. This is confirmed in the Geotechnical Evaluation by Geospectrum
Consultants, Inc. dated March 21, 2016 (see Appendix A).
SITE
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Soils Map
Proposed Drainage Controls
The site storm drainage system has been designed according to the standards set forth by the
City of Renton 2017 Surface Water Manual. The project creates more than 5,000 sf of impervious
area, therefore full drainage review is required.
Flow control facilities will not be required since the project doesn’t generate at least 0.15-cfs
increase in the predeveloped 100-year peak flow. Runoff from the frontage improvements will
be controlled by a pair of bioretention cells that will be placed in the newly constructed planters.
The total required surface area for the bioretention cells is 5% of the frontage impervious area,
or 105 SF. The planter size allows for a total surface area of 289 SF. See Section IV for a detailed
discussion.
The project is exempt from the water quality requirement since it generates less than 5,000 SF
of new pollution generating impervious surface (PGIS).
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II. CONDITIONS AND REQUIREMENTS SUMMARY
CORE REQUIREMENTS
Core Requirement #1: Discharge at the natural Location
The site contains two natural discharge locations. The western portion of the site
generally sheet flows across the site to the west and collects in a channel along the edge
of the roadway and flows south toward an existing inlet. The eastern half of the site sheet
flows across the site toward the east and then across the property to the east until it
enters a roadway ditch along Jones Ave NE. Refer to the Level 1 Downstream Analysis in
Section 3 for a complete description of the existing drainage paths.
Core Requirement #2: Offsite Analysis
An offsite drainage analysis is provided in Section 3 of this TIR. A Level 1 Drainage Analysis
has been prepared and no problems have been identified.
Core Requirement #3: Flow Control
This project qualifies for Full Drainage Review. The proposed project site will produce
more than 5,000 square feet of new impervious surfaces. Flow control BMPs for full
infiltration will provide the required flow control for proposed improvements.
Core Requirement #4: Conveyance System
Conveyance system analysis and design is provided in Section 5.
Core Requirement #5: Erosion and Sediment Control
A temporary erosion and sediment control (TESC) plan provides BMPs to be
implemented during construction.
Core Requirement #6: Maintenance and Operations
See Section 10 for Maintenance and Operations.
Core Requirement #7: Financial Guarantees and Liability
The owner will arrange for any financial guarantees and liabilities required by the
permit.
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Core Requirement #8: Water Quality
We have 2,578 SF of additional PGIS. This number includes the new road paving and the
driveway. Water Quality is not required since the project creates less than 5,000 sf of
PGIS.
Core Requirement #9: Flow Control BMPs
Full infiltration is proposed for the lots. Bioretention cells are proposed for the frontage
improvements.
SPECIAL REQUIREMENTS:
Special Requirement #1: Other Adopted Area-Specific Requirements
Critical Drainage Area – N/A
Master Drainage Plan – N/A
Basin Plan – N/A
Lake management Plan – N/A
Shared Facility Drainage Plan – N/A
Special Requirement #2: Floodplain/Floodway Delineation
The limits of this project do not lie in a 100-year floodplain.
Special Requirement #3: Flood Protection Facilities
This site does not contain any Class 1 or 2 streams; therefore, flood protection facilities
are not applicable.
Special Requirement #4: Source controls
This project is a 2-lot single-family residential project and this requirement is not
applicable.
Special Requirement #5: Oil Control
This project is not considered high-use in need of oil control.
Special Requirement #6: Aquifer Protection Area
This project is located in a Wellhead Protection Area Zone 2; no open channels or ponds
are proposed therefore no additional requirements apply to this project.
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III. OFF-SITE ANALYSIS
Existing Conditions West Drainage Basin
Runoff from the western portion of the site generally drains west from the subject site. The run
off drains toward a depression located along the edge of High Avenue NE, the overflow travels
south along the edge of the roadway into a CB located at point (A) and from there the flow travels
through an 8-inch pipe that runs underneath High Avenue NE and into the I-405 corridor. There
were no apparent downstream drainage problems.
Existing Conditions East Drainage Basin
Runoff from the eastern portion of the site generally drains east from the subject site. The run-
off sheet flows easterly across the site and then down a steeply sloped region along the eastern
edge of the property into the neighboring property along the east. From there, the flow empties
into a ditch located along the western side of Jones Ave NE. The flow travels through a 12”
concrete culvert located beneath the driveway at 2217 Jones Ave NE and into a ditch line toward
the north along Jones Ave NE. The flow is then conveyed through a 12” Corrugated Metal Pipe
located beneath the driveway at 2301 Jones Ave NE and empties into a channel that conveys the
storm flow into a wetland. From that point, the flow travels north through a culvert that runs
beneath NE 24th Street and into a channel located on the north side of NE 24th Street. The flow
continues to be conveyed north through the channel and empties into a drainage structure
located more than a quarter mile downstream of the subject property and then beneath I-405
until it eventually reaches a channel that carries the storm flow into Lake Washington. There
were no apparent downstream drainage problems.
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Developed Conditions
Runoff from the proposed impervious areas within each new lot will be directed to drywells for
full infiltration.
Western Drainage Basin
Photo 1: Depression area located on west side of the subject property
Photo 2: Downstream flowpath along High Ave NE on west side of subject property
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Photo 3: Catch Basin located on west side of High Ave NE
Photo 4: West side of High Ave SE where storm flow reaches I-405 corridor
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Eastern Drainage Basin
Photo 5: Neighboring property located just east of the subject property
Photo 6: Ditch along 2217 Jones Ave NE Photo 7: 12” concrete driveway culvert at
2217 Jones Ave NE
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Photo 8: Concrete culvert outflow location to channel at 2301 Jones Ave NE
Photo 9: Channel located at 2301 Jones Ave NE
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Photo 10: Upstream end of
culvert for driveway located
at 2301 Jones Ave NE
Photo 11: Downstream end of culvert for
driveway located at 2301 Jones Ave NE
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Photo 12: Channel located to the north of the
Property at 2301 Jones Ave NE
Photo 13: Channel located
in wetland sensitive area
just south of NE 24th St. &
East of High Ave
Photo 14: Culvert end on south
side of NE 24th Street
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Photo 15: Downstream channel located north of NE 24th St.
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IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN
Flow Control
The site is exempt from providing flow control as the difference between the 100-year existing
and developed peak flow is less than 0.15 CFS (see area calculations and MGS Flood analysis
below).
Area Calculations:
Area Breakdown:
Lot 1 – 7,890 SF – 4,340 SF impervious (55% per zoning)
Lot 2 – 7,777 SF – 4,277 SF impervious (55% per zoning)
Tract A – 6,715 – NGPA (not included in calculations)
Total Area = 22,382 SF or 0.51 Acres
Area Calculations for MGS Flood:
Per Section 5.2.2 of the City of Renton 2017 Surface Water Manual impervious areas that are
fully infiltrated can be subtracted from the total project area when determining eligibility from
Core Requirement #3 for flow control.
MGS Flood Analysis
Soil Type: Existing –Outwash (Forest)
Developed – Outwash (Grass)
Precipitation Station: Puget East 40
Total Area: -0.51 AC
NGPA: -6,715 SF / 0.15 AC
Fully Infiltrated Roof Area: -8,617 SF / 0.20AC
Area Analyzed: -7,050SF / 0.16 ac
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The difference in peaks is 0.030 CFS-0.000 CFS = 0.03 < 0.15 CFS therefore flow control is not
required.
Flow Control BMPs
1. This project is a residential two lot short plat. The total impervious area for this project
exceeds 5,000 square feet, and therefore this project qualifies for the Full Drainage Review
process.
· Full Drainage Review is required.
· 55% impervious per zoning
Lot 1 7,890 SF, 4,340 SF impervious
Lot 2 7,777 SF, 4,277 SF impervious
2. Considering the size of the existing parcel, the topography of the project site, and the
configuration of the proposed improvements, the most feasible flow control approach for
the project site is Full Infiltration. SCS soil maps define this area as consisting of Indianola
loamy sand and a Geotechnical Investigation was performed to confirm that the soils would
support the Full Infiltration design approach (see Appendix A).
· Implement Dry Wells for Full Infiltration. The design is based on drywells containing at
least 90 cubic feet per each 1,000 square feet of impervious surfaces served.
Lot 1 – 4,340 SF = 4,340/1,000 * 90 = 390.6 CF
Lot 2 – 4,277 SF = 4,277/1,000 * 90 = 384.9 CF
· A total of 8 dry wells, containing 98 cu. ft. each are proposed to service the new lots.
3. Off-site frontage improvements will cause a total increase in impervious area of 2,100 SF.
This is made up of 1,280 SF of additional roadway construction, 635 SF of sidewalk
construction and 185 SF of off-site driveway construction. This area runoff will be mitigated
using one two bioretention cells. This cell will be placed in the planter and is sized as
follows.
· Minimum bioretention footprint as measured at the overflow elevation shall be 5%
of the impervious surface
· 5% of 2,100 SF = 105 SF
· Total available bioretention surface area available = 289 SF
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BMP Type Mitigation
Area
Volume of
Facility
Lot 1 Drywells (4) 1085 SF per
Drywell
98 CF per
Drywell
Lot 2 Drywells (4) 1069 SF per
Drywell
97 CF per
Drywell
Offsite Area Bioretention
Cell N
1,050 SF 59 CF
Bioretention
Cell S
1,050 SF 47 CF
Water Quality BMPs
Water quality design is not required, since the overall pollution generating impervious area of
1,280 SF (New Roadway) + 1,298 (Driveway) = 2,578 SF is less than 5,000 square feet.
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V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
The proposed conveyance system from the frontage improvements will tightline flows to the
natural discharge point. The conveyance calculations were performed using Manning’s Equation.
The conveyance system was checked to ensure that during the 100-year storm event, the system
would function adequately. The 100-year peak flow from the developed site using KCRTS with
15 minute time steps was compared to the maximum capacity of the pipe. Using the Manning’s
Equation, the maximum capacity of an 8” and 12” pipe was calculated:
Pipe Diameter
(inches)
Pipe Slope Capacity (CFS)
8 0.50% 1.02
12 0.50% 2.73
As shown in the table the capacity of all of the pipe diameters is much greater than the actual
peak discharge rate of 0.317CFS (see KCRTS below).
Flow Frequency Analysis
Time Series File:stewartconv.tsf
Project Location:Sea-Tac
---Annual Peak Flow Rates--- -----Flow Frequency Analysis-------
Flow Rate Rank Time of Peak - - Peaks - - Rank Return Prob
(CFS) (CFS) Period
0.095 6 8/27/01 18:00 0.317 1 100.00 0.990
0.070 8 1/05/02 15:00 0.215 2 25.00 0.960
0.215 2 12/08/02 17:15 0.134 3 10.00 0.900
0.077 7 8/23/04 14:30 0.132 4 5.00 0.800
0.132 4 11/17/04 5:00 0.117 5 3.00 0.667
0.117 5 10/27/05 10:45 0.095 6 2.00 0.500
0.134 3 10/25/06 22:45 0.077 7 1.30 0.231
0.317 1 1/09/08 6:30 0.070 8 1.10 0.091
Computed Peaks 0.283 50.00 0.980
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VI. SPECIAL REPORTS AND STUDIES
Geotechnical Engineering Report
Arborist Report
VII. OTHER PERMITS
Building permits will be required
VIII. CSWPPP ANALYSIS AND DESIGN
The potential for erosion within the site will be mitigated by use of erosion control measures
during clearing, grading, and site development activities. Filter fences will be installed along the
downhill perimeter of the site to protect adjacent properties from sediment-laden water.
Stockpiles and exposed disturbed areas will be covered to protect from erosion and sediment
runoff.
Element 1: Mark Clearing Limits.
All clearing, grading, sensitive areas, and buffers will be clearly marked in the field prior to
construction in accordance to the plans and specifications. Prior to beginning land disturbing
activities, including clearing and grading, clearly mark all clearing limits, sensitive areas and their
buffers, and trees that are to be preserved within the construction area. These shall be clearly
marked, both in the field and on the plans, to prevent damage and offsite impacts. Plastic, metal,
or stake wire fence may be used to mark the clearing limits.
Element 2: Establish Construction Access.
Construction access will be provided for the site. Driveway re-alignment should be completed
after all other construction is complete. Stabilize the construction access with rock per the storm
water plans if the driveway is disturbed. Access points shall be stabilized with a pad of quarry
spalls, crushed rock, or equivalent BMP prior to traffic leaving the construction site to minimize
the tracking of sediment onto all roads and accesses.
Element 3: Control Flow Rates.
Flow rates from the construction site are not expected to negatively impact the downstream
corridor. At all times, flow rates shall be controlled for this project. Natural drainage patterns
shall be protected as much as possible during construction, and concentrated flow should not be
permitted. Properties and waterways downstream from development sites shall be protected
from erosion due to increases in the volume, velocity, and peak flow rate of stormwater runoff
from the project site.
Element 4: Install Sediment Controls.
Silt fence should be used to protect all sensitive area slopes. Soils should be covered if not worked
for 7 days during the dry season or 2 days during the wet season. The street should be swept
each night or as required. If the minimum BMPs fail to retain sediment to the sight, additional
BMPs will be used.
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Element 5: Stabilize Soils.
Soils shall be covered if not worked for 7 days during the dry season or 2 days during the wet
season. Soil stockpiles will be covered unless worked. Soil stockpiles shall be located away from
drain inlets and surface water discharge locations. Soil stockpiles shall be stabilized and covered
as needed or removed to an approved disposal site. Soils shall be stabilized at the end of the shift
before holidays or weekends if needed based on weather forecast.
Element 6: Protect Slopes.
The site does not contain any steep slope areas; however, it borders a steep slope to the west.
(Highway 405 embankment) The proposal includes routing drainage to the east therefore slope
protection is not anticipated to be needed.
Element 7: Protect Drain Inlets.
There are existing catch basins located downstream of the project. Filter fabric protection shall
be used for sediment control.
Element 8: Stabilize Channels and Outlets.
Existing or proposed channels or drainage outlets are not components of this project, thus
stabilization of these elements is not required.
Element 9: Control Pollutants.
Pollution generated from construction must be controlled at all times. Control of pollutants other
than sediments include the following:
All pollutants other than sediments shall be handled and disposed of in a manner that
does not cause contamination of stormwater.
Cover, containment and protection from vandalism shall be provided for all chemicals,
liquid products, petroleum products, and non-inert wastes present on the project site.
Maintenance and repair of heavy equipment and vehicles involving oil changes, hydraulic
system drain down, solvent and de-greasing cleaning operations, fuel tank drain down
and removal, and other activities which may result in discharge or spillage of pollutants
to the ground or into stormwater runoff must be conducted using spill prevention
measures, such as drip pans. Contaminated surfaces shall be cleaned immediately
following any discharge or spill incident. Emergency repairs may be performed on-site
using temporary plastic placed beneath and, if raining, over the vehicle.
Element 10: Control De-Watering.
De-watering is not anticipated for the site. In the event that dewatering is necessary, storm shall
be treated such that sediment remains on site. This shall be done by routing the storm water
through a straw filter, silt fence, and/or sediment trap.
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Element 11: Maintain BMPs.
BMPs shall be inspected monthly and after every significant storm event, sediment shall be
removed from the BMPs as necessary for them to continue operating at the required
performance level. In the event that a BMPs has been damaged, it shall be replaced immediately.
Element 12: Manage the Project.
Construction activities shall be phased such that the impact to the area will be kept at a minimum.
Coordination will occur with all utility agencies that are affected by this project. BMPs shall be
inspected regularly and after each significant storm event. The Contractor will provide a Certified
Erosion and Sediment Control Specialist. If for any reason a BMPs is not sufficient for the project,
additional BMPs will be installed.
IX. BOND QUANTITIES and DECLARATION of COVENANT
Bond Quantities
The bond quantity worksheet is included in Appendix B.
Facility Summaries
Not applicable.
Declaration of Covenant
Declaration of Covenant for maintenance will have to be approved and recorded.
X. OPERATION AND MAINTENANCE MANUAL
Please refer to Appendix A in the 2016 King County Surface Water Design Manual with regards to the
Operation and Maintenance.
Stewart Short Plat Technical Information Report
Appendix A
Geotechnical Report
Stewart Short Plat Technical Information Report
Appendix B
Bond Quantity Worksheet