Narrative - Drainage.pdfDevelopment and Drainage Narrative Report
for a 3 -Lot Short Plat for Gus and Morgan Kays
at 8705 Maplewood Lane in Edmonds, WA.
The existing parcel is a corner lot zoned RS -8 and consists of about 0.709 acres or 30,873
square feet.. An existing house, garage, long driveway and 2 unattached buildings are
located on the west half of the property. This area consists of about 7370 square feet of
impervious area. All the existing improvements were created prior to 1977. The east half
of the lot is undeveloped and is wooded with a few dozen large evergreen trees. The
parcel takes vehicular access from the north Right -Away of Maplewood Lane. The parcel
slopes downhill east to west at an average of about 8 %. The boundary corners of the
parcel have recently been survey staked and they all appear to create lines that agree with
the existing fence lines.
The proposal is to short plat the lot into 3 lots. This will create lot 1 on the west side
containing most of the existing developed area. The proposed Lot 2 and 3 is the
undeveloped wooded area to the east. Both Lot: 2 and 3 will take vehicle access from 86tH
Place West. No house construction is planned on lot 2 and 3 at this time. The impervious
surfacing on the parcel will not be significantly altered until these lots are built upon. All
the sanitary sewer, domestic water and power ,are presently in place in Maplewood i erne
and '861h Pl W. right -away for development of the east two lots. A 6 -inch sa ritary sewer
stubs exist on the southeast corner of proposed lot 2 as shown on the city's asbuilt sewer
plans.
The total site development, excluding the pre 1977 improvements, is proposed at more
than 2000 square feet but less than 5000 square feet of impervious area. Therefore is
classified by Edmonds as Category 1 Small Project. A stormwater mitigation is required
for the project to mitigate about 4800 square feet of proposed roof, driveway and
sidewalks/patio areas.
Since all the impervious area created will be on the undeveloped area lot 2 and 3 the
proposal is to select raingardens for storm water mitigation. The raingardens will be
proposed in the southwest corner of both lots 2 and 3. These areas are wooded with
relatively undisturbed soils conditions and at the lower portions of the lots. An 4 -inch
overflow will be designed to drain the raingardens to the open ditch in the Maplewood
Land right -away. Soils infiltration testing and a report will be prepared by a
Geotechnical Engineer to determine the design infiltration rates for the raingardens.
Preliminary sizing of the raingardens is enclosed for the preliminary short plat layout.
RMS Engineering 21101 - 0"' Ave W. # E Edmonds, WA. 98026
Requirement for Small Sites CategorV # ;1 Development
for: 3 lot short plat of a single family parcel at 8705 Maplewood Lane
in the City of Edmonds
tax account # 27041900202100
Small Site Minimum Requirement #1— Preparation of Stormwater Site Plan with two
design sheets are enclosed consisting of the civil engineering plans. The first is a Site
Plan and Notes, the second a Demolition and Erosion Control Plan.
Small Site Minimum Requirement #2 — Construction Stormwater Pollution Prevention
Plans: The seconds sheet shown the Silt Fencing and Construction Entrance details
which are both proposed to meet this requirement.
Small Site Minimum Requirement #3 — Source Control of Pollution: There is no
source control measures that are proposed for this residential short plat design.
Small Site Minimum Requirement #4 — Preservation of the Natural Drainage System
Outfall: All the drainage from the site will infiltrate into the natural topsoils. The site
drains due west and there are no surface water or cllan.nels that existing oil the site.
There is almost no upland drainage onto the property lironi the uphill paved 86th Place W.
The west side of the street pavement has thickened edge which directs the streets
pavement surface north. The drainage from the north side of Maplewood Lane slopes to
the open ditch along the south side of the proposal. This city ditch drains due west. The
only adjacent properties that are not lower than the property are to the north and they due
drain west not onto the parcel.
Small Site Minimum Requirement #5 — Onsite Stormwater Management: The
developed lot contains good permeable soils are acceptable for ground water infiltration.
The proposal is to install raingardens at the southwest corner of both lots 2 and 3. No
drainage will be altered on the existing impervious surfaces on lot 1. These surfaces were
created prior to 1977 and therefore exempt from mitigation requirements.
Small Site Minimum Requirement #6 — Runoff Treatment: Most of the rainfall water
runoff from the site will be from the roof tops which does not require treatment. The only
storm water considered for treatment is the two new driveways on lot 2 and 3 located east
of the houses. This area only total about 800 square feet of PGIS. This area much less
than the minimum area requiring treatment.
Small Site Minimum Requirement #7 — Flow Control: The site is classified as a
"Category 1 Small Site" project located is the Northstream drainage basin. Flow control
will via the infiltration rate of the subsoil of the proposed raingardens. The infiltration
rate of the soils will be test per city requirements prior to final design of the raingardens.
Small Site Minimum Requirement #8 — Wetland Protection: No wetlands are on or
close to the site and protection is not required.
RMS Engineering 21101 - 80th Ave. W #B Edmonds, WA. 98026
Small Site Minimum Requirement #9 — Operation and Maintenance: No maintenance
manuals for operation of the raingardens on lot 2 and 3.
Small Site Minimum Requirement #10 - Offsite Analysis and Mitigation: No offsite
problems exist and no mitigation is proposed or required.
Small Site Minimum Requirement #11— Financial Liability: No critical areas are
around the site and no downstream conveyance system capacity or flooding problems
have been located which require mitigation.
RMS Engineering 21101 - 80th Ave. W #B Edmonds, WA. 98026