HomeMy WebLinkAboutBLD2021-00360-05- STORMWATER PLAN APPROVED
July 23, 2021
Shannen Cartmel
Assistant Planner
Jefferson County Department of Community Development
621 Sheridan Street
Port Townsend, WA 98368
Via e-mail: scartmel@co.jefferson.wa.us
Re: Stormwater Management Recommendations
Beckett Point Road (between 1210 & 1220)
Port Townsend, WA 98368
Jefferson County Parcel 932200702
0.04 Acres (1,742 ft2)
Building Permit BLD2021-00360
Hello Ms. Cartmel,
Evergreen Engineering Services (EES) is pleased to provide recommendations for stormwater
management at the referenced property where the Zagulas are planning to construct a new cabin.
With space constraints and limits on permissible excavation, attenuated surface dispersion of
stormwater onto highly pervious soils should maximize on-site infiltration while protecting nearby
surface waters.
PROPOSED IMPERVIOUS COVERAGE The proposed cabin would create 864 SF of
impervious surfaces on the 1,742 SF beachfront parcel, equating to 49.6% impervious land
coverage.
LIMITED OPTIONS for STORMWATER MANAGEMENT The 1,742 SF subject parcel
lacks land area and horizontal distances needed to comply with stormwater management options
outlined in the Department of water Management Manual for Western
Washington (Manual) Cultural resources identified in the Beckett Point area limit permissible
ground-disturbances making the installation of subsurface stormwater infiltration features
problematic. Fortunately, native soils on the subject site and surroundings consist primarily of
sands and gravels and readily allow stormwater permeation.
PROPOSED ROOF RUN-OFF DISPERSION We recommend routing roof downspout flows
onto coarse gravel dispersion features placed directly on the ground surface at the point of
downspout discharge. Each downspout should discharge onto a dispersion pad which can be
Evergreen Engineering Services
715 Grant Street Port Townsend, Washington 98368 360-821-9960 Aug 04 2021
Sep 27 2021
2
paired with a splash block. A typical dispersion pad could consist of a patch of drain rock or
crushed rock, washed of fine material, applied several inches deep and covering three to six (3-6)
square ft, extending several feet from the discharge point in the direction of downhill flow,
typically to the southwest.
This approach should effectively comply with the Manual's BMP T5.10B: Downspout Dispersion
Systems. From the 2019 Stormwater Manual (Vol V, Chapter 4, Page 713): Downspout
dispersion systems are splash blocks or gravel filled trenches, which serve to spread roof runoff
over vegetated pervious areas. Dispersion attenuates peak flows by slowing the runoff entering
the conveyance system, allowing some infiltration, and providing some water quality benefits.
Though BMP T5.10B stipulates a "flow path of at least 50 feet between the discharge point and
any property line...," the BMP intent is to maximize on-site infiltration and protect nearby surface
waters. The recommended approach should achieve these goals.
PROPOSED PARKING AREA SHEET FLOW DISPERSION Stormwater discharged as
sheet flow from two proposed parking spaces located between the proposed cabin and existing
gravel road will tend to flow onto permeable vegetated surfaces. This approach substantially
satisfies requirements of the Manual s BMP T5.12: Sheet Flow Dispersion (Vol V, Chapter 3,
Page 692). The practice applies to any impervious or pervious surface that is graded to avoid
concentrating flows and channeling. A transition zone that can consist of crushed rock applied
along the downhill perimeter of the parking area should prevent channeling. A narrow band of
adjacent vegetation attenuates and treats flow from the gently sloped surface. BMP guidance
recommends a 10-foot-wide vegetated buffer for up to 20 feet of width of impervious surface.
CONCLUSION In our opinion, the above-outlined stormwater management practices should
maximize on-site infiltration, minimize off-site impacts, and protect surface water quality. Please
let us know if you have any questions.
Sincerely,
Everett A. Sorensen, P.E.
Evergreen Engineering
360-821-9960
Copy to: Monica Mader, Architect, LEED Professional, studioSTL
Simon Little, studioSTL