HomeMy WebLinkAboutDRAINAGE REPORT - PLN General - 1/14/2020 Verizon WA4 Speedways
REDI
DrainageReport
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PROJECT LOCATION : Street
100 W PUBLIC WORKS DRIVE
Engineer: Duncanson Company
145 SW 1551h Street, # 102
Seattle, WA 98166
Contact: Harold Duncanson
P: 206-244-4141
e: haroldd@duncansonco.com
Date: 1/14/2020
DCI Project: 99544.1786 pk1S
4, rt r
29647
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DUNCANSON
Company, Inc.
Section A—Introduction
This drainage report evaluates the Project in relation to the applicable Minimum Requirements of the
2005 Stormwater Management Manual for Western Washington (SWMMWW).
The Project involves a 40-foot square wireless telecommunication equipment compound and associated
access road and utility improvements. The Site is located on a small knoll to the west of the Mason County
Public Works yard at 100 W Public Works Drive on tax parcel 420022190010. The proposed access route
follows an existing road bed extending from the Public Works yard below. The equipment compound is
located on a previously cleared and partially graded/benched area. New and/or replaced impervious
surface is approximately 6,700 SF (0.15 acres).
Section B—Minimum Requirement Evaluation
Minimum Requirement 1—Preparation of Stormwater Site Plan
This report and associated plans constitute the Stormwater Site Plan
Minimum Requirement 2—Construction Stormwater Pollution Prevention
Element 1—Mark Clearing Limits
Clearing Limits can be designated in the field with flagging. Most work is proposed in areas that are
already cleared and partially revegetated with scotch broom and other small brush.
Element 2—Establish Construction Access
A stabilized construction entrance is shown on the TESC/SWPPP plan.
Element 3—Control Flow Rates
This project is below the thresholds for requiring flow control. Flow rates will be mitigated by dispersal
into the surrounding forested vegetation.
Element 4—Install Sediment Controls
Vegetated buffers consisting of the forested and brushy areas downslope from the project are proposed
for sediment control. There are no natural or constructed surface water bodies, collection or conveyance
systems within at least several hundred feet of the project area.
Element 5—Stabilize Soils
Soil stabilization for stockpiles and disturbed areas is identified on the TESC/SWPPP plans.
Element 6—Protect Slopes
Project grading will involve only minor slopes of 2 to 4 feet. Typical cover practices discussed under soil
stabilization are anticipated to be sufficient. The larger natural slope to the east will not be disturbed and
preserving existing vegetation will provide protection.
Element 7—Protect Drain Inlets
No existing catch basins or drain inlets are near the Project and none are proposed.
Element 8—Stabilize Channels and Outlets
No channels or outlets are proposed.
Element 9—Control Pollutants
Notes for controlling pollutants are included on the TESC/SWPPP plans.
Element 10—Control Dewatering
A note for managing dewatering discharge is included on the plans.
Element 11—Maintain BMPs
Implementation and maintenance of the TESC/SWPPP BMPs will be the responsibility of the Contractor.
Element 12—Manage the Project
The Project is below the threshold for requiring a CESCL; however, the Contractor shall designate an
Erosion and Sediment Control Supervisor at the pre-construction meeting to act as a point of contact and
who is knowledgeable and experienced with erosion and sedimentation control practices.
Minimum Requirement 3—Source Control of Pollution
Several typical source control practices are included as SWPPP notes on the plans. A lined concrete
washout area is also specified on the plans.
Minimum Requirement 4--Preservation of Natural Drainage Systems and Outfalls
The natural drainage pattern is sheet flow through vegetation to the southeast of the Project area. This
pattern will be maintained.
Minimum Requirement 5—On-Site Stormwate Management
On-Site Stormwater Management BMPs include Sheet Flow Dispersal for the access drive and Permeable
Pavement for the equipment compound.
The access drive is outsloped so that existing and proposed runoff from the road sheet flows into the
forested slope to the southeast. The 12-foot wide road requires at least a 10-foot vegetated flow path.
The available vegetated flow path is between 30 and nearly 200 feet.
The equipment compound/lease area is proposed to be surfaced with open graded crushed rock over a
permeable ballast base material. This effectively acts as a permeable pavement surface.
Minimum Requirement 6—Runoff Treatment
This Project does not propose any pollution generating impervious surface (PGIS). The access drive is an
"infrequently used maintenance access road", which by definition is non-PGIS surface. The Project is
below the threshold for requiring treatment facilities. Runoff treatment is limited to the On-Site
Stormwater BMPs discussed above.
Minimum Requirement 7—Flow Control
The Project is below the 10,000 SF impervious surface threshold for requiring flow control facilities. Flow
control is limited to the On-Site Stormwater BMPs discussed above.
Minimum Requirement 8—Wetlands Protection
Project runoff does not contribute to a wetland either directly or indirectly through a conveyance system.
This requirement does not apply to this Project.
Minimum Requirement 9—Basin/Watershed Planning
This Project does not meet the threshold for requiring Basin/Watershed Planning.
Minimum Requirement 10—Operation and Maintenance
Operation and maintenance of the On-Site Stormwater BMPs will be the owner/manager of the wireless
facility.
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