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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.