20-104505_Stormwater Narrative V2 11-3-2021
P & M Development, LLC – 35001 NE 147TH AVE, YACOLT, WA 98675 – (360) 263-4873
September 15, 2021 (REVISED October 29, 2021)
TECHNICAL MEMORANDUM – STORMWATER REQUIREMENTS
To: Ms. Chaney Skadsen
City of Federal Way
From: Michelle Henry, PE
RE: Stormwater Narrative
Henry Reasonable Use, Process III
Applications 20-105405-UP, 21-100016-UP, 21-100017-UP
This is an executive summary of the stormwater Technical Information Report (TIR) and stormwater plan
provided as basis for design with the proposed project seeking Reasonable Use for each single family lot
through the above referenced Process III applications. This is to show the stormwater requirements
have been met for the proposed intrusion into the stream buffer and into the wetland buffer.
In general, the site is comprised of three residential lots zoned RS15.0 with a wetland to the north and a
creek through the middle that divides the drainage area into two as shown in Figure 1.3 on page 11 of
the stormwater TIR. The site’s topology is sloped such that all stormwater that falls on the site flows to
the creek and then to the wetland or directly to the wetland with a natural discharge location on the
north end shown in figure 1.28 on page 9 of the stormwater TIR.
The stormwater TIR and plan meet the 2016 King County Surface Water Design Manual (KCSWDM) for
runoff control and water quality treatment. A Level 1 downstream analysis was included in the
stormwater report to show the proposed development would not adversely impact areas offsite
downstream of the development.
The project protects critical areas from stormwater runoff by enhancing areas within the buffers to to
provide flow control, ensure water quality, and improve the buffers ecological function. The enhanced
buffer areas provided in the stormwater report are for full dispersion per the KCSWDM best
management practices (BMP) for flow control and water quality using naturally vegetated flow paths
(NVFP) which are allowed within the buffers of critical areas per KCSWDM, Appendix C.2.1 p.31. These
enhanced areas improve the buffer function to protect the stream and wetland, and support habitat.
The following summary table from page 16 of the TIR identifies how each of the 9 core elements and
special requirements in drainage design for meeting the KCSWDM stormwater requirements were
addressed if applicable. Pages 16-19 in the stormwater TIR summarize these in more detail and
subsequent chapters in the stormwater TIR go into further detail.
P & M Development, LLC – 35001 NE 147TH AVE, YACOLT, WA 98675 – (360) 263-4873
REQUIREMENTS: How being met if applicable:
Core Reqmt #1: Discharge at the Natural
Location
No change to the natural discharge
location.
Core Reqmt #2: Offsite Analysis Level 1 offsite analysis completed (Section
3)
Core Reqmt #3: Flow Control Full Dispersion (Section 4)
Core Reqmt #4: Conveyance System Gravel channels and oversized drainpipes
for roof downspouts for capacity exceeding
the peak flow during a 100-year rainfall
event. (Sizing analysis in Section 5)
Core Reqmt #5: Erosion and Sediment Control ESC measures during construction and in
final design.
Core Reqmt #6: Maintenance and Operations Property owners will be responsible to
maintain natural vegetated areas.
Core Reqmt #7: Financial Guarantees and
Liability
Private Facilities/NVFP Area Covenant
Core Reqmt #8: Water Quality Full Dispersion (Section 4)
Core Reqmt #9: Flow Control BMPs NVFP -Full Dispersion (onsite), Sheet Flow
Dispersion (offsite), Simple Detention
Special Reqmt #1: Other Adopted Area-Specific
Requirements
Critical Areas: wetlands and stream to be
protected. The project area lies within the
South Lower Puget Sound Subbasin.
Special Reqmt #2: Flood Hazard Area
Delineation
N/A – Not within the 100-yr flood plain.
Special Reqmt #3: Flood Protection Facilities N/A – Not within an area of flooding.
Special Reqmt #4: Source Control N/A - Single Family
Special Reqmt #5: Oil Control N/A – Sindle Family
The stormwater plan shows the project will retain approximately 64,062 SF of natural vegetation within
the combined three lots of 76,826 SF, making up 83% of the overall project site. The remaining 17% will
consist of the existing roadway, roofs, driveways, and lawn areas (TIR p.35).
Below is a table from p.10 of the stormwater TIR that provides a breakdown of the stormwater surfaces
shown within the project and shown on the stormwater plan which will require stormwater
management. The stormwater design provides treatment of the existing impervious surfaces (SW 333RD
Place) as well as new impervious surfaces (building and driveways), along with pollution generating
pervious surfaces (grass lawns). The project plans provide construction details for each stormwater
management BMP per the KCSWDM on plan sheet 7 of the project plan set.
DESCRIPTION TYPE AREA (SF) METHOD/BMP
Pavement (onsite, max
20-ft wide)
PGIS 4939 FULL DISPERSION/BMP FC-NVFP
KCSWDM C2.1
P & M Development, LLC – 35001 NE 147TH AVE, YACOLT, WA 98675 – (360) 263-4873
Pavement (offsite, 10-ft
wide strip)
PGIS 1900 STRIP (SHEET FLOW)
DISPERSION
/BMP KCSDM C2.2
LOT -9043 - SFR Non-PGIS 1486 FULL DISPERSION/BMP FC-NVFP
KCSWDM C2.1
LOT -9087 - SFR Non-PGIS 1300 FULL DISPERSION/BMP FC-NVFP
KCSWDM C2.1
LOT -9069 - SFR Non-PGIS 1563 FULL DISPERSION/BMP FC-NVFP
KCSWDM C2.1
LOT -9043 -Lawn PGPS 1225 SIMPLE DETENTION
POND/KCSWDM Chapter 5.1.7
LOT -9087- Lawn PGPS 1200 SIMPLE DETENTION
POND/KCSWDM Chapter 5.1.7
LOT -9069 - Lawn PGPS 1300 SIMPLE DETENTION
POND/KCSWDM Chapter 5.1.7
Note, because the buffer intrusion areas are exempt from including the existing roadway of 333rd Place
per the FWRC 19. 145.440 (4) and FWRC 19.145.270 (4) Permanently altered buffers, the areas for
impervious and pervious surfaces listed above are different than the areas of intrusion as defined by the
clearing and grading limits for each lot.
The stormwater plan provides details showing the direction and flow of surface discharges for control
and treatment in the enhanced buffer regions recognized as an approved BMP for flow control and
water quality. These areas are identified as NVFP on the site improvement plans.
The applicant recognizes that all naturally vegetated flow paths proposed with the project will need to
be constructed on all lots with the construction of the first lot to meet public works requirement for
stormwater treatment of the improved existing roadway. Though the new paved surface replaces an
existing impervious surface, replacement impervious is not exempt from stormwater treatment.
Attached: Stormwater TIR