Level 1 Off-Site DRNG Analysis-06-08-2020-V1
LEVEL 1 OFF-SITE DRAINAGE ANALYSIS
7-Eleven Convenience Store and Fueling Station
35620 Pacific Hwy South
Federal Way, WA 98003
Prepared for:
Visconsi Companies, Ltd.
30050 Chagrin Boulevard, Suite 360
Pepper Pike, OH 44124
May 29, 2020
Our Job No. 20948
20948.004-Level 1 Off-Site DRNG Analysis.doc
TABLE OF CONTENTS
TASK 1 STUDY AREA DEFINITION AND MAPS
TASK 2 RESOURCE REVIEW
Exhibit A - Reconnaissance Summary Report - Hylebos Creek Basin
TASK 3 FIELD INSPECTION
3.1 Conveyance System Nuisance Problems (Type 1)
3.2 Severe Erosion Problems (Type 2)
3.3 Severe Flooding Problems (Type 3)
TASK 4 DRAINAGE SYSTEM DESCRIPTION AND PROBLEM DESCRIPTIONS
Exhibit B - Downstream Drainage Basin Map
TASK 5 MITIGATION OF EXISTING OR POTENTIAL PROBLEMS
20948.004-Level 1 Off-Site DRNG Analysis.doc
TASK 1 STUDY AREA DEFINITION AND MAPS
The proposed 7-Eleven fueling facility and convenience store is located within Section 29,
Township 21 North, Range 4 East, Willamette Meridian, in King County, Washington. More
specifically, the site is located at 35620 Pacific Highway South, Federal Way. The vicinity
map located seen as Figure 2 of the Technical Information Report and provides a graphic
depiction of the site location.
The site consists of 3 parcels of land containing an area of approximately 2.16 acres. The site
contains a 165-foot buffer zone from the Hylebos Wetland located on the east and south
borders of the site. Approximately 38 percent of the property is within the wetland buffer
zone. The existing topography of the site exhibits a central high point and generally slopes
down towards the adjacent streets and lots. Existing grades on the proposed area for the
development range from 2 to 12 percent, and existing grades on the existing we tland buffer
range from 10 to 30 percent. The majority of the site’s ground cover consists pervious
pavement comprising 79 percent of the property and 21 percent existing impervious surfaces.
UPSTREAM DRAINAGE ANALYSIS
The site receives little to no flow from any of the adjacent sites as topography exhibits the si te
higher than all of the surrounding public roadways and areas. The north and east frontage
which include existing driveways are sloped away such that run-off drains off-site. Areas to
the east and south are within a wetland buffer and/or the Hylebos Wetland, and run-off from
this area is contained and does not flow to the proposed development.
20948.004-Level 1 Off-Site DRNG Analysis.doc
TASK 2 RESOURCE REVIEW
Adopted Basin Plans: The site is located within the Hylebos Creek Basin.
Finalized Drainage Studies: This is not applicable.
Basin Reconnaissance Summary Report: This site is located in the Hylebos Creek
drainage basin. A Basin Reconnaissance Summary Report is included as Exhibit A in this
report.
Critical Drainage Area Maps: This project development will not discharge to any critical
areas or wetlands as it will be discharging run-off to an existing storm network in Pacific
Highway South.
Floodplain and Floodway FEMA Maps: This project is not located within a 100-year
floodplain. See Figure 4 of the Technical Information Report for the FEMA Map.
Other Off-Site Analysis Reports: There are no other off-site reports.
Sensitive Areas Folios: Based on the King County Sensitive Areas Map Folios, the site
does not contain any on-site wetlands, erosion areas, sensitive areas nor land slide
areas. See Figure 6 of the Technical Information Report for the Sensitive Areas Map.
Road Drainage Problems: No road drainage problems were encountered during off -site
drainage analysis.
United States Department of Agriculture King County Soils Survey: A soils map found as
Figure 5 has been included in the Technical Information Report. The soils map identifies
the site underlain with Everett-Alderwood gravelly sandy loam and Bellingham silt loam.
Wetland Inventory Map: As mentioned, the property is adjacent to the Hylebos Wetland.
There is an area buffer 165 feet on the east portion of the site from the wetland.
Migrating River Studies: This is not applicable.
20948.004-Level 1 Off-Site DRNG Analysis.doc
TASK 3 FIELD INSPECTION
3.1 Conveyance System Nuisance Problems (Type 1)
Conveyance system nuisance problems, in general, are defined as any existing or
predicted flooding or erosion that does not constitute a severe flooding or erosion
problem. Conveyance system nuisance problems are defined as flo oding or erosion that
results in the overflow of the constructed conveyance system for runoff events less than
or equal to a 10-year event. Examples include inundation of a shoulder or lane of a
roadway. Examples include overflows collecting in yards or pastures, shallow flows
across driveways, minor flooding in crawlspaces or unheated garages/outbuildings , and
minor erosion.
There were no known conveyance system nuisance problems reported and observed.
3.2 Severe Erosion Problems (Type 2)
Severe erosion problems are defined as downstream channels, ravines, or slopes with
evidence of or potential for erosion/incision, sufficient to pose a sedimentation hazard to
downstream conveyance systems or propose a landslide hazard by undercutting
adjacent slopes. Severe erosion problems do not include roadway or minor ditch erosion.
There were no known severe erosion problems within the downstream drainage course,
nor have any such problems been observed.
3.3 Severe Flooding Problems (Type 3)
Severe flooding problems can be caused by conveyance system overflows or the
elevated water surfaces of ponds, lakes, wetlands, or closed depressions. Severe
flooding problems are defined as follows:
Flooding of the finished area of a habitable building for runoff events less than or
equal to the 100-year event. Examples include flooding of finished floors of
homes and commercial or industrial buildings. Flooding in electrical/heating
systems and components in the crawlspace or garage of a home. Such problems
are referred to as "severe building flooding problems."
Flooding over all lanes of a roadway or severely impacting a sole access
driveway for runoff events less than or equal to the 100-year event. Such
problems are referred to as "severe roadway flooding problems."
Based on a review of the FEMA Map (Figure 4 of the TIR), the proposed site is outside of
the 500-year floodplain, and there is no known evidence of severe flooding problems.
20948.004-Level 1 Off-Site DRNG Analysis.doc
TASK 4 DRAINAGE SYSTEM DESCRIPTION AND PROBLEM DESCRIPTIONS
The existing conditions of the site exhibit runoff to sheet flow along the perimeter of the site
and is drained to an existing curb and gutter and finally to catch basins located along Pacific
Highway South and S.W . 356th Street. Part of the existing site's run-off (east of the proposed
development area) also sheet flows towards the east and south towards the existing pervious
areas. A Downstream Basin Map is shown in Exhibit B.
In the developed condition, stormwater run-off will be collected by proposed catch basins and
will be treated through an oil/water separator and a water quality unit following conveyance
through an on-site detention facility (StormTech Facility).The point of discharge is an existing
catch basin located along Pacific Highway South.
No known drainage problems were encountered during the off-site analysis.
Storm
20948.004-Level 1 Off-Site DRNG Analysis.doc
TASK 5 MITIGATION OF EXISTING OR POTENTIAL PROBLEMS
No existing or potential problems were during the off-site analysis
The project proposes to provide a detention facility that will allow on-site flows to meet the
Level 2 flow control standard. Additionally, the enhanced basic water quality treatment will be
provided by a Modular Wetland water quality unit. This project intends to control flow rates to
the extent required by the 2016 KCSWDM and is not anticipated to negatively impact the
downstream conveyance systems or properties.