HomeMy WebLinkAbout20250807_PWD3985_Stormwater Report LAND TECHNOLOGIES, INC. OTECNNOLp6�Es
PLANNING • PERMITTING • ENGINEERING PRIM
MAKING A"WAY"OUT OF"NO WAY"
Harmony at Arlington LLC.
7417 204th St NE., Arlington, WA 98223
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LDA Submittal: July 2025
Stormwater Site Plan Report
for
Harmony at Arlington
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Prepared by:
Tyler Foster, P.E. �
53726
360-652-9727
SS/ON AU
Email:Tvler@landtechway.com
18820 Third Avenue NE, Arlington WA 98223 voice: 360.652-9727 fax:360.652.7553
Table of Contents
Section 1—Summary and Minimum Requirements............................................................. 1-1
1.1 Project Description............................................................................................................. 1-1
1.2 Project Data Summary: ...................................................................................................... 1-1
1.3 Construction Activities ....................................................................................................... 1-1
1.4 Minimum Requirements and Thresholds........................................................................... 1-3
Section 2- MR-1: PREPARE STORMWATER SITE PLAN..........................................................2-1
2.1 Existing Site Conditions ...................................................................................................... 2-1
Section 3— MR-2: Stormwater Pollution Prevention Plan....................................................3-1
3.1 Storm Water Pollution Prevention Plan............................................................................. 3-1
Section 4- MR-3: Source Control of Pollution......................................................................4-1
Section 5 - MR-4: Preserve Natural Drainage Systems and Outfalls......................................5-1
Section 6- MR-5: On-site Stormwater Management ...........................................................6-1
Section 7 - MR-6: Runoff Treatment....................................................................................7-1
Section8- MR-7: Flow Control............................................................................................8-1
8.1 Flow Control ....................................................................................................................... 8-1
Section 9 - MR-8: Wetlands Protection................................................................................9-2
Section 10 - MR-9: Operation and Maintenance................................................................ 10-1
Section11 - Support Data ................................................................................................. 11-1
11.1 Maps & Figures............................................................................................................... 11-1
Section12 Support Data................................................................................................... 12-6
12.1 Soils................................................................................................................................. 12-7
12.2 Reference Documents.................................................................................................... 12-8
12.3 Topographic Data ........................................................................................................... 12-8
12.4 Continuous Simulation Modeling................................................................................... 12-9
12.5 Modeling Methodology.................................................................................................. 12-9
12.6 Software Output........................................................................................................... 12-11
Drainage Report Harmony at Arlington 28-Jul-2025 1-i
Figures
Figure1 -Vicinity Map....................................................................................................................11-1
Figure 2 - Existing Conditions..........................................................................................................11-2
Figure 3 - NRCS Soil Map (Not to Scale)..........................................................................................11-3
Figure4 - Site Plan ..........................................................................................................................11-5
Tables
Table 1— Project Location Data........................................................................................................1-2
Table 2 - Project Theshhold Data Summary .....................................................................................1-2
Table3 - Soil Table..........................................................................................................................12-7
Drainage Report Harmony at Arlington 28-Jul-2025 1-ii
Section 1 — Summary and Minimum Requirements
1.1 Project Description
Harmony at Arlington, a proposed townhome community will reside on a 3.21-acre lot in the City of
Arlington. The development is located on Parcel Number 310511-003-040-00 and bears the
address 7417 204th St NE, Arlington, WA 98223. The development will contain 2.36-acres
impervious surface of which 1.19-acres will be pollution generating and 1.17-acres will be non-
pollution generating impervious surface.
The project will utilize bioretention swales for stormwater mitigation of driveway, aisle, and
parking. Swales will be located around the perimeter for stormwater infiltration. Stormwater will
sheet flow to the nearest swale. Stormwater generated from the rooftops will be directed to
downspouts and collection system. The roof drain collection system will be excavated to native
sand soils and backfilled with clean rock. The site will fully infiltrate 100% of the stormwater runoff
model.
The development of the parcel will conform to the requirements set forth in Arlington Municipal
Code (AMC).
1.2 Project Data Summary:
Existing and proposed project areas are presented in Table 2 for determination of stormwater
management requirements based on prescribed thresholds as outlined in the AMC 13.28.
1.3 Construction Activities
The site has been used for single-family residence but has changed uses and land coverage many
times through the years. The site has been vacant for at least a dozen years now.
The site is relatively flat and fill will be brought into the site to match the neighboring developments
grades. Minimal excavation will occur except for utility and stormwater management installations.
Gravel base and asphalt will be imported to create the parking, roadway, and driveways. Organic
strippings stockpiled on site will be used for soil amendments.
Project disturbance area is 3.35-acres. A separate document titled Construction Stormwater
Pollution Prevention Plan for Harmony at Arlington will detail construction methods for eliminating
pollution.
Drainage Report Harmony at Arlington 28-Jul-2025 1-1
Table 1 — Project Location Data
Project Data:
Applicant Harmony at Arlington LLC.
Project Name Harmony at Arlington
Project T.S.R. Location SW 1/4 of Section 11, Township 31 N, Range 5 W
Project Address 7417 204th St NE, Arlington, WA 98223
ParcellD(s) 310511-003-040-00
Watershed Mainstem Stillaguamish
WRIA number 5
Basin Stillaguamish
Sub-Basin Portage Creek
Table 2 - Project Theshhold Data Summary
Project Data:
............................... .................................................................................................................................................................................._................................................................................ ..............................................................................................................................................................
Project Name Harmony at Arlington
......................................................................................................................................................................................................................................................................................................................................................_...................................................................................................................................................................................
......................................................................................................................................................................................................................................................................................................................................................_...................................................................................................................................................................................
Existing Conditions:
Total Site Area 140,025 sf 3.21ac
( )
Proposed Activity:
p Y
Proposed Activity Townhomes
......................................................................................................................................................................................................................................................................................................................................................_...................................................................................................................................................................................
Proposed Clearing Area (Total) 146,070 sf (3.35 ac)
Proposed Grading Area Total 146 070 sf 3.35 ac
p g (Total) ( )
Proposed New NPGIS (Total) 50,808 sf (1.17 ac)
Proposed New PGIS Total 51 764 sf 1.19 ac
p (Total) ( )
Proposed Replaced Impervious Area 0 sf (0.0000 ac)
Native Vegetation convert to Lawn 0 sf 0.0000 ac
g ( )
Native Vegetation convert to Pasture 0 sf (0.0000 ac)
......................................................................................................................................................................................................................................................................................................................................................_...................................................................................................................................................................................
Total New Impervious Area (Total) 102,572 sf (2.36 ac)
Proposed Disturbance Area 146,070 sf (3.35 ac)
......................................................................................................................................................................................................................................................................................................................................................_...................................................................................................................................................................................
Project Qualification:
Type Development T e New Development
p p
Minimum Requirements 1 thru 9
Drainage Report Harmony at Arlington 28-Jul-2025 1-2
1 .4 Minimum Requirements and Thresholds
The project qualifies as new development under Volume I, Chapter 2 of the 2024 DOE Stormwater
Management Manual for Western Washington (SMMWW). The SMMWW is specified as the
governing document for stormwater management per AMC 13.28.150. This specifies compliance
with Minimum Requirements 1-9 of the SMMWW.
Harmony at Arlington requires a full stormwater site plan and compliance with the minimum
requirements set forth in the SMMWW.
A full construction SWPPP is required. The SWPPP is provided as a separate stand-alone document
titled Construction Stormwater Pollution Prevention Plan for Harmony at Arlington
Minimum Requirements
1. Preparation of Stormwater Site Plans
2. Construction Stormwater Pollution Prevention
3. Source Control of Pollution
4. Preservation of Natural Drainage Systems and Outfalls
5. On-site Stormwater Management
6. Runoff Treatment
7. Flow Control
8. Wetlands Protection
9. Operation and Maintenance
Minimum Requirements are addressed sequentially in this document.
Drainage Report Harmony at Arlington 28-Jul-2025 1-3
Section 2 - MR-1 : PREPARE STORMWATER SITE PLAN
2.1 Existing Site Conditions
2.1 .1 Site Location
The project is located in the southwest quarter of Section 11 in Township 31 North, Range 5 West.
The site's address is 7417 204th St NE, Arlington, WA 98223.
The location of the site is shown in Figure 1 below. The parcel is located at the address of 7417 204th
St NE, Arlington, WA 98223 behind Starbucks and the Bartell Drugstore at the corner of 204th St and
Highway 9. The project is located in the Stillaguamish watershed (WRIA-5), in the Portage Creek Sub-
Basin.
2.1 .2 Site Description
The site has a total area of 140,025-square feet (3.21-acres). The property is zoned Commercial
Corridor and its current use is listed as 'Vacant.
2.1 .3 Vegetative Cover
The development area is predominately pasture or unvegetated. Some trees are located along the
north side of the parcel. An aerial photo can be seen in Section 11.1 as Figure 2.
2.1 .4 Topography
The pre-developed existing topography both onsite and offsite was evaluated using LiDAR generated
surfaces. The average slope as indicated by the LiDAR model is 0.8% through the parcel. A berm
appears to have been constructed along the north edge of the parcel adjacent to the stream corridor.
No structures will be placed along this berm but a open space path will be placed along the top edge
for residents. The site slopes to the west with a low point along the west boundary line towards the
north corner. The banks and edges of this stream corridor have been built up over the years by
neighboring parcels as well. The dike along the stream corridor is not consistent between parcels
and in some cases prevents surface flow from reaching the stream.
The site topography is generally flat within the development area.
2.1 .5 Critical Areas
Portage Creek is located to the north of the site within Parcel A of the Binding Site Plan. Parcel A will
be dedicated to the City of Arlington. There are no known wetlands or critical areas within the
development area.
An existing conditions map is presented in Figure 2.
Drainage Report Harmony at Arlington 28-Jul-2025 2-1
2.1 .6 Soils
The NRCS Soil mappings for the site consist of Norma loam. NRCS aerial photograph of the soil
mappings can be seen in Figure 3.
Norma loam and soils with a weathered layer represents "B/D" type hydrologic soils that range in
depth from about 0 to 60 inches. Below that is recessional outwash sand. Extensive grading appears
to have occurred in the past so surface soils may vary greatly from the NRCS mappings.
Analysis of the site indicates a silty sand and silty gravel layers underlain by fine to coarse sand with
silt. This is typical of geologic studies of the site area which define the site as the Marysville Sand
Member consisting of well-drained outwash sand.
A geotechnical report by ESNW indicates medium to coarse sand at a depth of about 1-2 feet to 7.5
feet below grade. (ESNW, LLC. , 2024) Groundwater elevation is deep, exceeds 9-feet BGS. A PIT
test was conducted in test pits 1 and 2. Soils were consistent throughout the site. Long-term
infiltration rate with correction is 12.0 inches/hour. For systems utilizing bioretention, the siltation
correction factor could be eliminated and long-term rates increased. However, the bioretention soils
are limited to 12-inches/hour and increasing the rate beyond this would not provide any measurable
effect. Long-term infiltration rate of 12-inches/hour will be used for a conservative stormwater
design across the site.
ESNW, LLC. provided an updated Geotech report specific to the project proposal with similar results
to the
2.1 .7 Existing Basin Analysis
The project falls within the Stillaguamish Basin that is a portion of the Mainstem Stillaguamish
Watershed. The project falls within the Portage Creek Sub-Basin.The development area lies in what
is colloquially called the Marysville Trough. The trough runs north to south and encompasses flat
terrain west of Highway 9 and East of the Lakewood/Tulalip areas. The site is situated in the
northwestern extent of this system.
It is assumed and highly likely that the entirety of this area is drained through groundwater and the
groundwater gradient to the Portage Creek stream system. No discrete drainage systems drain the
site but surface flow is to the west — northwest. The site will drain to Portage Creek on the north
side of the site but may require a subsurface route to reach the creek. The creek channels have been
built up with what appears to be berms or dikes in an uncoordinated effort between landowners. It
is not known if this is the result of clearing activities on individual parcels or if there is flooding
concerns historic property owners were trying to alleviate. It is presumed that this is the result of
clearing activities only. Portage Creek drains to the Stillaguamish River before reaching Puget Sound.
There are no known drainage issues immediately downstream of the parcel.
There are no known flooding or erosion concerns adjacent to the site.
Drainage Report Harmony at Arlington 28-Jul-2025 2-2
2.1 .8 Upstream Analysis
There is no large expansive upstream area to the development site. The area is relatively flat. Small
portions of run-on may occur from the site perimeter and Highway 9 sheet flow. Upstream areas are
routed through Portage Creek in the north.
2.1 .9 Existing Drainage
The site topography trends westward. Due to the presence of outwash soils, surface runoff is not
expected to occur. There appears to be no existing discrete drainage systems on-site or offsite.
2.1 .10 Downstream Analysis
The site will largely infiltrate all stormwater except for extremely large storms following storm events
that overwhelm an already saturated groundwater system.The following is a theoretical assessment
of the drainage pathways assuming surface runoff occurs and recognizing the built-up berms
downstream from the site preventing stormwater to reach the creek.
The site drains to the sites west boundary along the northern portion of the west parcel line.The low
point along this west point is at roughly 119.60-feet MSL. Overflow from the sites bioretention cells
will be directed to this downstream property following the natural, pre-existing path. This route will
be armored with rip-rap in the event of emergency overflow.
Beyond this low point, stormwater will flow westward into the site to a confined low point that is
disconnected from any downstream path. Stormwater will need to pond before overtopping and
draining to Portage Creek to the northwest. Portage Creek flows west before reaching the
Stillaguamish River. See Figure 4.
There are no known drainage issues immediately downstream.
The project is not expected to alter hydrology to Portage Creek while discernibly posing no altered
hydrology to the Stillaguamish River/Puget Sound.
Drainage Report Harmony at Arlington 28-Jul-2025 2-3
Section 3 — MR-2: Stormwater Pollution Prevention Plan
3.1 Storm Water Pollution Prevention Plan
A Storm Water Pollution Prevention Plan (SWPPP) has been prepared for this project and
presented as a separate document titled "Stormwater Pollution Prevention Plan for Harmony at
Arlington", 28-Jul-2025
Drainage Report Harmony at Arlington 28-Jul-2025 3-1
Section 4 - MR-3: Source Control of Pollution
Per the 2024 SMWW Volumed, Ch-2.5.3, source control BMPs are required for all development
projects for the intended purpose to prevent stormwater from coming in contact with
pollutants. The activities and the associated source control BMPs are listed in Volume-IV of the
manual. These are primarily commercial industrial developments that involve significant pollutant
generation potential. The proposed stormwater facilities addressed in this report manage
stormwater generated from passenger vehicle parking and non-pollution generating rooftop from
residential development. No source control BMPs apply to these facilities controlling stormwater
from these surfaces.
No Volume-IV source control BMPs are specified.
For construction activities, source control BMPs prescribed in Volume-II are specified and described
in the construction SWPPP per MR#2.
Drainage Report Harmony at Arlington 28-Jul-2025 4-1
Section 5 - MR-4: Preserve Natural Drainage Systems and
Outfalls
All drainage courses are to be preserved to the maximum extent feasible. All outfalls will remain in
their current locations. Natural drainage patterns as they once existed shall be retained. Pre-
developed conditions experience a sheet flow drainage pattern to the sites western boundary and
infiltration. The developed site is designed to direct flows to maintain the hydrology at the same
boundary while fully infiltrating stormwater flows. Stormwater flows that exceed the infiltrative
capacity of soils will be under emergency conditions and will discharge to the low point along the
west side of the site.
Drainage Report Harmony at Arlington 28-Jul-2025 5-1
Section 6 - MR-5: On-site Stormwater Management
Minimum Requirement#5 specifies requirements for on-site stormwater BMPs. This requirement
mandates that on-site stormwater runoff be infiltrated, dispersed, and/or retained to the
maximum extent feasible without causing flooding or erosion impacts. Projects triggering
Minimum Requirements 1 through 5 must use On-site stormwater management BMPs from List#1
for all surfaces or demonstrate compliance with the LID Performance Standard. Projects triggering
Minimum Requirements 1 through 9 must meet the requirements of Table 2.5.1 in Vol. 1 of the
2024 SMMWW. Table 2.5.1 specifies the requirements for new or redevelopment depending on
UGA and parcel size to meet the requirements of the LID Performance Standard and/or List#2. List
#1 and List#2 specify stormwater BMPs in order of preference. The first BMP determined feasible
is required.
This project trigger MR's 1-9. This project is within the City's UGA. This project is required to
adhere to the LID Performance Standard or List#2 per Table 2.5.1.
List#1 and #2 contain appropriate BMPs to mitigate a particular developed surface. The surfaces
included in the list are Lawn and Landscaped Areas, Roofs, and other hard surfaces
(road/driveway/parking).
Lawn/Landscape is required to utilize BMP T5.13, Post-Construction Soil Quality and Depth.
Roofs are required to employ BMP T5.30 Full Dispersion or Downspout Infiltration, Rain Gardens or
Bioretention, BMP T5.10A Downspout Dispersion Systems, or perforated stub-out connections. The
first feasible BMP in this list must be used.
Other Hard surfaces (Roads, Driveways, Parking Lots, Etc.) must utilize BMP T5.30 Full Dispersion,
BMP T5.15 Permeable Pavement, Bioretention, Sheet Flow Dispersion, or Concentrated Flow
Dispersion. The first feasible BMP in this list must be used.
All Lawn/Landscape areas are specified to utilize BMP T5.13, Post-Construction Soil Quality and
Depth. Geotechnical engineer will be require to confirm the final in-situ soil specification at project
completion.
Roofs will not be able to provide BMP T5.30 Full Dispersion. Full Dispersion requires 100 foot flow
paths within native areas. The project will not be able to maintain 65% open space while also
complying with the Growth Management Act and the City's comprehensive plan. There is not
adequate space for the Full Dispersion requirements while meeting other facets of code. Roofs will
be routed to Downspout Infiltration facilities which is the next feasible alternative (BMP T5.10A).
Sizing criteria of BMP T5.10A are provided in MR#7.
Driveways and parking will be routed to Bioretention areas. Bioretention used to meet MR#5 will
be designed in accordance with BMP T7.30 per Page 7-3 of Vol. V of the 2024 SMMWW.
Bioretention under BMP T7.30 is to be designed per the parameters of BMP T5.14 for infeasibility
and design criteria as well as hydrologic modeling credits. Stormwater is released to the
bioretention cells through sheet flow or a conveyance system.
A site plan showing the stormwater management and development can be seen in Figure 5.
Drainage Report Harmony at Arlington 28-Jul-2025 6-1
Section 7 - MR-6: Runoff Treatment
Minimum Requirement#6 specifies the requirements for providing runoff treatment. The
threshold for requiring a treatment BMP is 5,000 square feet of PGIS (Pollution Generating
Impervious Surface) or a total of more than % of an acre of PGPS (Pollution Generating Pervious
Surface).
This project is expected to generate 51,764 square feet(1.19 acres) of PGIS based on road and
driveway areas, therefore treatment facility BMPs are required for this project.
Runoff treatment facility selection is outlined in Vol. I, Ch. 4.2, Step V of the 2024 SMMWW. Step V
outlines the treatment facility selection flow chart based on the intended use of a project.
Treatment selection is based on if the site is a high-use site, if the downstream receiving waters are
phosphorous sensitive, and/or if the site is required to provide enhanced treatment. The
definitions of high-use, phosphorous control, and enhanced treatment can be found in Step V in
Section 4.2 of the 2024 SMMWW.
The project is not a high use site.
The project is not required to treat for phosphorous.
Enhanced treatment is not required for the project.
Basic treatment is required per Vol. I, Ch. 2 &4 of the 2024 SMMWW.
Enhanced and Basic treatment is provided through the use of a bioretention facility per Vol. III
Section 3.3.12 of the 2024 SMMWW. The bioretention specified will provide enhanced treatment.
The bio-cell treats stormwater through the infiltration of stormwater through soils and their ability
to absorb pollutants. Soils have a CEC of greater than 5 meg/100g to a depth of 12 inches. On-site
soils are suitable for use. See Vol III. Section 3.3.12 of the 2024 SMMWW for specific soil design
criteria.
Road, Driveway, and Parking Area
The project has PGIS areas exceeding the threshold for simple treatment BMPs. Therefore, the
development is required to employ basic treatment for its respective road, driveway, and parking
areas.
The project utilizes bioretention swales for treatment. The bioretention swales mitigate polluted
stormwater through physical, chemical, and biological treatment processes. The treatment process
reduces pollutant loads to downstream receiving waters. Pretreatment (flow entrance and
presettling) is controlled by 5/8" clean chip located adjacent to the edge of asphalt along its
entire length to prevent channelization of storm inflow. Stormwater then percolates through
compost amended soils and plantings to obtain treatment. Stormwater flows through this part of
the cell at a rate of 12.0 inches/hour. Infiltration is accounted for below the bio-cell at a long-term
adjusted rate of 12 inches/hour. This infiltration rate accounts for stormwater that will rehydrate
and restore pre-developed groundwater channels and accurately reflect the built condition. The
total percolated runoff through the bio-cell's amended soils is well over the 91%total runoff
volume treatment requirement, at 100%. See Section 12.6
Drainage Report Harmony at Arlington 28-Jul-2025 7-1
Section 8 - MR-7: Flow Control
8.1 Flow Control
Minimum Requirement#7 specifies the requirements for runoff flow control. The threshold for
requiring Minimum Requirement#7 is 5,000 square feet of impervious surface. Flow control shall
be provided if the project creates more than 10,000 square feet of effective impervious area in a
threshold discharge area, converts % of an acre or more of native vegetation to lawn, 2.5 acres or
more native vegetation is converted to pasture, or a combination of impervious and converted
pervious surfaces cause a 0.1 cfs increase in the 100-year flow frequency from a continuous
simulation runoff model.
The project exceeds this requirement and is required to provide flow control.
A total of 700 square feet of bioretention swale is proposed for stormwater facilities for the PGIS.
The bioretention swale varies in width at its base and is surrounded by 3:1 slopes that rise 1.0 feet.
The Swale consists of 0.5 feet of ponded water depth and 0.5 feet of freeboard. The swale is
constructed using 1.5 feet of amended soils. A rate of 12 inches/hour is measured below the
facilities and it shall be confirmed during construction that stormwater BMPs make a continuous
connection with these soils. Geotechnical confirmation shall be required.
The bioretention swale design and construction specification follows 2024 DOE SMMWW BMP
T7.30. The bioretention swale infiltrates 100% of stormwater design for the impervious areas
draining to them. Bioretention swales are located adjacent to the majority of PGIS inputs
throughout the site. The site is graded with the highest elevations in the east and the lowest in the
west following the predeveloped topography. The bioretention swales step through the site from
east to west. Overflow is directed to each bioretention swale on site through overflow risers to
equalize distribution throughout before releasing to the downstream emergency system. The
emergency system is not expected to be triggered unless complete failure of the system and
subsurface soils occur or back-to-back storms exceeding the 100-year design storm happen.
Rooftops on site are routed to an infiltration trench. The infiltration trench is designed to be 2 foot
thick, 2 feet wide and 1,525 feet long. The infiltration trench system is designed in accordance with
BMP T5.10A for medium sand. Medium Sand soils may accept rooftop at a rate of 1,OOOsf per 30-If
of trench draining to them. Therefore, 1,525sf of trench is required to fully infiltrate the rooftops.
The trench system will be dispersed throughout the site forming a broad infiltration field to further
mitigate stormwater by LID processes. The perimeter building drain and conveyance system which
would normally carry stormwater to a 'regional' facility will be composed of perforated storm pipe
and backfilled with clean rock. Infiltration will take place at the source from each connection.
Downspouts will incorporate screen filters and baffles prior to discharging to the gravel filled
trench. Hydrologic Analysis was conducted using the latest version of the WWHM software.
Software reports are in Section 12.6. Supplementary information for the continuous simulation
models is presented in Section 12.4. A site plan showing the location of bioretention swale can be
seen in Section 11.1.
Drainage Report Harmony at Arlington 28-Jul-2025 8-1
Section 9 - MR-8: Wetlands Protection
There is no detention or treatment in critical areas. There are no known wetlands on-site or
downstream from the site. Wetlands shall not be disturbed by the project. Protective and
mitigation measures are employed through the construction stormwater pollution prevention
plans.
Drainage Report Harmony at Arlington 28-Jul-2025 9-2
Section 10 - MR-9: Operation and Maintenance
Minimum Requirement#9 contains requirements for inspection, operation and maintenance of
stormwater facilities and BMPs. Specific maintenance standards and requirements are outlined in
Volume V of the 2024 SMMWW. The 2024 SMMWW requires the regular maintenance and
inspection of drainage facilities.
The Operations and Maintenance manual has been prepared for this project and presented as a
separate document titled "Operations and Maintenance manual for Harmony at Arlington", 28-
Jul-2025.
Drainage Report Harmony at Arlington 28-Jul-2025 10-1
Section 11 - Support Data
11.1 Maps & Figures
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Drainage Report Harmony at Arlington 28-Jul-2025 11-1
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Figure 2 - Existing Conditions
Drainage Report Harmony at Arlington 28-Jul-2025 11-2
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Drainage Report Harmony at Arlington 28-Jul-2025 11-3
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Figure 4 - Downstream Analysis
Drainage Report Harmony at Arlington 28-Jul-2025 11-4
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Figure 5 - Site Plan
Drainage Report Harmony at Arlington 28-Jul-2025 11-5
Section 12 Support Data
This appendix contains the following support data as applicable to this report:
• Soils Data
• Reference Documents
• Topographic Data
• Continuous Simulation Modeling
• Software Output
Drainage Report Harmony at Arlington 28-Jul-2025 12-6
12.1 Soils
The following data was extracted from Web Soil Survey, National Cooperative Soil Survey:
Table 3 - Soil Table
39—Norma loam
Map Unit Setting
National map unit symbol. 2hyx
Elevation: 0 to 1,000 feet
Mean annual precipitation: 35 to 60 inches
Mean annual air temperature:48 to 52 degrees F
Frost-free period: 150 to 200 days
Farmland classification:Prime farmland if drained
Map Unit Composition
Norma, undrained, and similar soils: 85 percent
Minor components: 15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Norma, Undrained
Setting
Landform: Drainageways, depressions
Parent material:Alluvium
Typical profile
H1 - 0 to 10 inches:ashy loam
H2- 10 to 28 inches:sandy loam
H3-28 to 60 inches:sandy loam
Properties and qualities
Slope: 0 to 3 percent
Depth to restrictive feature: More than 80 inches
Drainage class:Poorly drained
Capacity of the most limiting layer to transmit water(Ksat):Moderately high to high
(0.57 to 1.98 in/hr)
Depth to water table:About 0 inches
Frequency of flooding:None
Frequency of ponding:Frequent
Available water supply, 0 to 60 inches: Moderate (about 9.0 inches)
Interpretive groups
Land capability classification (irrigated):None specified
Land capability classification (nonirrigated): 5w
Hydrologic Soil Group: BID
Ecological site:F002XA007WA-Puget Lowlands Wet Forest
Forage suitability group: Wet Soils (G002XN102WA)
Other vegetative classification: Wet Soils (G002XN102WA)
Hydric soil rating: Yes
Minor Components
Norma, drained
Percent of map unit: 5 percent
Landform:Depressions
Other vegetative classification: Seasonally Wet Soils (G002XN202WA)
Hydric soil rating: Yes
Terric medisaprists, undrained
Percent of map unit: 5 percent
Drainage Report Harmony at Arlington 28-Jul-2025 12-7
Landform: Depressions
Other vegetative classification: Wet Soils (G002XN102WA)
Hydric soil rating: Yes
Bellingham, undrained
Percent of map unit: 5 percent
Landform: Depressions
Other vegetative classification: Wet Soils (G002XN102WA)
Hydric soil rating: Yes
Data Source Information
Natural Resources Conservation Service
Web Soil Survey, National Cooperative Soil Survey
7/23/2014
Soil Survey Area: Arlington, Washington (Snohomish County)
12.2 Reference Documents
Department of Ecology. (2005). Surface Water Management Manual for Western Washington. WA:
Department of Ecology.
ESNW, LLC. . (2024). Geotechnical Addendum - Harmony at Arlington. Redmond: ESNW.
Federal Emergency Management Agency. (1999, November 8). Flood Insurance Rate Map Number
392 of 1575. Snohomish County, Washington and Incorporated Areas, Panel 1061 of 1575.
Arlington, Washington.
Puget Sound Action Team. (2005,January). Low Impact Development Technical Guidance Manual for
Puget Sound. Publication No. PSAT 05-03. Washington: Washington State University - Pierce
County Extension.
Puget Sound LIDAR Consortium. (2003, April). LIDAR Bare Earth DEM File. g47121h24be.e00.
Snohomish County, Washington. Retrieved May 2013, from
http://pugetsoundlidar.ess.washington.edu/index.htm
12.3 Topographic Data
• Snohomish County 2005 LiDAR survey was used to augment the existing site topography and
the downstream and surrounding areas. Citation: LiDAR Bare Earth DEM Files:
be_48122b22.zip. Available: Puget Sound LiDAR Consortium, Seattle, WA
• Modeled coordinate system: Lateral - Washington State Plan Plane - North, FIPS 4601;
Vertical— NAVD 88
Drainage Report Harmony at Arlington 28-Jul-2025 12-8
12.4 Continuous Simulation Modeling
12.4.1 Continuous Simulation Background
HSPF based continuous simulation modeling was used to evaluate the hydrologic performances of
the pre-developed and developed sub-basins in order to accurately assess flow rates.
The currently adopted continuous simulation models use the HSPF (Hydraulic Simulation Program
in FORTRAN) software engine. The HSPF model uses a robust and detail accounting of the 'water
budget', including evaporation, evapotranspiration, interception, interflow, and groundwater. The
modeling accounts for and assesses land segment areas that include vegetation or impervious
cover, soil types, and slopes. The modeling also utilizes over 50 years of continuous rainfall data
(precipitation) and evaporation data for the area. The HSPF continuous modeling is considered the
best available science for hydrologic analysis.
12.5 Modeling Methodology
HSPF modeling was managed via the Western Washington Hydrology Model (WWHM) interface
program. The current professional version of WWHM by Clearcreek Solutions, Inc., WWHM-2012
was used. The current data precipitation and evaporation set provided by DOE with the WWHM-
2012 software interface was used that includes quantized data in 15-minute time steps from
October 1948 to October 2009.
The WWHM program comes packaged with generic, well considered HSPF modeling parameters.
These settings allow for the modeling of the majority of the topological conditions found in the
Puget Sound area. Where conditions fall reasonably outside the range of the default HSPF
parameters, adjustments should be made to more accurately reflect those conditions. These is
generally limited to the pervious land segments (IMPLNDS) and are mostly limited to slopes
(SLSUR), infiltration rates (INFILT), and length of flow path (LSUR). For this project, HSPF parameters
were not adjusted to reflect site conditions.
12.5.1 Existing Conditions
The surface vegetative cover is assumed forested with a flat slope. The NRCS soil maps indicate
Lynnwood soils throughout the project area. For hydrologic modeling, Lynnwood soils are A type
soils. Note: the predeveloped scenario for the site is included but not required since the project is
complying with 100% infiltration. The comparison of predeveloped flows is for discharges to
downstream conveyances. The existing site is pasture. Outside of the disturbance area, there is no
land conversion.
12.5.2 Developed Conditions
The site is flat and grading will consist of stripping existing soils. Roof and driveway area will be
added to the developed site. These areas are accounted for and used in the WHMM program. The
Drainage Report Harmony at Arlington 28-Jul-2025 12-9
remaining area of the site will be used for stormwater management. The stormwater bioretention
cells will be excavated down below the sealed surface layer to allow stormwater to readily infiltrate.
The bio-swale areas are accounted for in the WWHM bio-retention element and are therefore
discounted from the basin area. The preceding section contains input and output of parameters
relevant to the existing and developed site to achieve 100% infiltration meeting current DOE
standards. CAS areas are modeled as pasture as allowed and recommended by DOE.
Drainage Report Harmony at Arlington 28-Jul-2025 12-10
12.6 Software Output
WWHM2012
PROJECT REPORT
Project Name: LDA
Site Name: Harmony Village
Site Address:
City : Arlington
Report Date: 4/29/2025
Gage : Everett
Data Start : 1948/10/01
Data End : 2009/09/30
Precip Scale: 1.20
Version Date: 2023/03/31
Version : 4.2.19
Low Flow Threshold for POC 1 50 Percent of the 2 Year
High Flow Threshold for POC 1: 50 year
PREDEVELOPED LAND USE
Name : Basin 1
Bypass: No
GroundWater: No
Pervious Land Use acre
A B, Forest, Flat 3.03
Pervious Total 3.03
Impervious Land Use acre
Impervious Total 0
Basin Total 3.03
Element Flows To:
Surface Interflow Groundwater
MITIGATED LAND USE
Name : Basin 1
Bypass: No
GroundWater: No
Pervious Land Use acre
C, Pasture, Flat .71
Pervious Total 0.71
Impervious Land Use acre
DRIVEWAYS FLAT 0.97
SIDEWALKS FLAT 0.22
Impervious Total 1.19
Drainage Report Harmony at Arlington 28-Jul-2025 12-11
Basin Total 1.9
Element Flows To:
Surface Interflow Groundwater
Surface retention 1 Surface retention 1
Name Bioretention 1
Bottom Length: 750.00 ft.
Bottom Width: 1.00 ft.
Material thickness of first layer: 1.5
Material type for first layer: SMMWW 12 in/hr
Material thickness of second layer: 0.5
Material type for second layer: Sand
Material thickness of third layer: 0.5
Material type for third layer: GRAVEL
Infiltration On
Infiltration rate: 12
Infiltration safety factor: 1
Wetted surface area On
Total Volume Infiltrated (ac-ft.) : 285.275
Total Volume Through Riser (ac-ft.) : 0.017
Total Volume Through Facility (ac-ft.) : 285.292
Percent Infiltrated: 99.99
Total Precip Applied to Facility: 2.39
Total Evap From Facility: 1.181
Underdrain not used
Discharge Structure
Riser Height: 0.5 ft.
Riser Diameter: 12 in.
Element Flows To:
Outlet 1 Outlet 2
Bioretention 1 Hydraulic Table
Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) In£ilt(cfs)
0.0000 0.0172 0.0000 0.0000 0.0000
0.0275 0.0172 0.0002 0.0000 0.0000
0.0549 0.0172 0.0004 0.0000 0.0000
0.0824 0.0172 0.0006 0.0000 0.0000
0.1099 0.0172 0.0009 0.0000 0.0000
0.1374 0.0172 0.0011 0.0000 0.0000
0.1648 0.0172 0.0013 0.0000 0.0009
0.1923 0.0172 0.0015 0.0000 0.0014
0.2198 0.0172 0.0017 0.0000 0.0019
0.2473 0.0172 0.0019 0.0000 0.0025
0.2747 0.0172 0.0022 0.0000 0.0032
0.3022 0.0172 0.0024 0.0000 0.0040
0.3297 0.0172 0.0026 0.0000 0.0050
0.3571 0.0172 0.0028 0.0000 0.0061
0.3846 0.0172 0.0030 0.0000 0.0073
0.4121 0.0172 0.0032 0.0000 0.0086
0.4396 0.0172 0.0035 0.0000 0.0100
0.4670 0.0172 0.0037 0.0000 0.0116
0.4945 0.0172 0.0039 0.0000 0.0134
0.5220 0.0172 0.0041 0.0000 0.0153
0.5495 0.0172 0.0043 0.0000 0.0173
Drainage Report Harmony at Arlington 28-Jul-2025 12-12
0.5769 0.0172 0.0045 0.0000 0.0195
0.6044 0.0172 0.0048 0.0000 0.0219
0.6319 0.0172 0.0050 0.0000 0.0244
0.6593 0.0172 0.0052 0.0000 0.0271
0.6868 0.0172 0.0054 0.0000 0.0300
0.7143 0.0172 0.0056 0.0000 0.0330
0.7418 0.0172 0.0058 0.0000 0.0363
0.7692 0.0172 0.0061 0.0000 0.0397
0.7967 0.0172 0.0063 0.0000 0.0432
0.8242 0.0172 0.0065 0.0000 0.0470
0.8516 0.0172 0.0067 0.0000 0.0510
0.8791 0.0172 0.0069 0.0000 0.0551
0.9066 0.0172 0.0071 0.0000 0.0595
0.9341 0.0172 0.0074 0.0000 0.0640
0.9615 0.0172 0.0076 0.0000 0.0688
0.9890 0.0172 0.0078 0.0000 0.0738
1.0165 0.0172 0.0080 0.0000 0.0789
1.0440 0.0172 0.0082 0.0000 0.0843
1.0714 0.0172 0.0084 0.0000 0.0899
1.0989 0.0172 0.0087 0.0000 0.0957
1.1264 0.0172 0.0089 0.0000 0.1018
1.1538 0.0172 0.0091 0.0000 0.1080
1.1813 0.0172 0.0093 0.0000 0.1145
1.2088 0.0172 0.0095 0.0000 0.1212
1.2363 0.0172 0.0097 0.0000 0.1281
1.2637 0.0172 0.0100 0.0000 0.1353
1.2912 0.0172 0.0102 0.0000 0.1426
1.3187 0.0172 0.0104 0.0000 0.1503
1.3462 0.0172 0.0106 0.0000 0.1581
1.3736 0.0172 0.0108 0.0000 0.1662
1.4011 0.0172 0.0110 0.0000 0.1745
1.4286 0.0172 0.0112 0.0000 0.1830
1.4560 0.0172 0.0115 0.0000 0.1918
1.4835 0.0172 0.0117 0.0000 0.2006
1.5000 0.0172 0.0118 0.0000 0.2083
Surface retention 1 Hydraulic Table
Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) To Amended(cfs) Wetted Surface
1.5000 0.0172 0.0118 0.0000 0.2083 0.0344
1.5275 0.0201 0.0123 0.0000 0.2083 0.0688
1.5549 0.0229 0.0129 0.0000 0.2160 0.1032
1.5824 0.0257 0.0136 0.0000 0.2198 0.1377
1.6099 0.0286 0.0143 0.0000 0.2236 0.1721
1.6374 0.0314 0.0152 0.0000 0.2274 0.2066
1.6648 0.0343 0.0161 0.0000 0.2312 0.2411
1.6923 0.0371 0.0170 0.0000 0.2350 0.2756
1.7198 0.0400 0.0181 0.0000 0.2389 0.3101
1.7473 0.0428 0.0192 0.0000 0.2427 0.3446
1.7747 0.0457 0.0204 0.0000 0.2465 0.3792
1.8022 0.0486 0.0217 0.0000 0.2503 0.4137
1.8297 0.0514 0.0231 0.0000 0.2541 0.4483
1.8571 0.0543 0.0246 0.0000 0.2579 0.4829
1.8846 0.0571 0.0261 0.0000 0.2618 0.5175
1.9121 0.0600 0.0277 0.0000 0.2656 0.5521
1.9396 0.0628 0.0294 0.0000 0.2694 0.5868
1.9670 0.0657 0.0312 0.0000 0.2732 0.6214
1.9945 0.0686 0.0330 0.0000 0.2770 0.6561
2.0220 0.0714 0.0349 0.0346 0.2778 0.6907
Drainage Report Harmony at Arlington 28-Jul-2025 12-13
2.0495 0.0743 0.0369 0.1165 0.2778 0.7254
2.0769 0.0772 0.0390 0.2257 0.2778 0.7602
2.1044 0.0800 0.0412 0.3555 0.2778 0.7949
2.1319 0.0829 0.0434 0.5015 0.2778 0.8296
2.1593 0.0858 0.0457 0.6597 0.2778 0.8644
2.1868 0.0887 0.0481 0.8261 0.2778 0.8991
2.2143 0.0915 0.0506 0.9966 0.2778 0.9339
2.2418 0.0944 0.0532 1.1671 0.2778 0.9687
2.2692 0.0973 0.0558 1.3333 0.2778 1.0036
2.2967 0.1002 0.0585 1.4914 0.2778 1.0384
2.3242 0.1030 0.0613 1.6378 0.2778 1.0732
2.3516 0.1059 0.0642 1.7695 0.2778 1.1081
2.3791 0.1088 0.0671 1.8845 0.2778 1.1430
2.4066 0.1117 0.0701 1.9818 0.2778 1.1779
2.4341 0.1146 0.0732 2.0620 0.2778 1.2128
2.4615 0.1174 0.0764 2.1274 0.2778 1.2477
2.4890 0.1203 0.0797 2.1826 0.2778 1.2617
2.5000 0.1215 0.0810 2.2635 0.2778 1.1783
Name Surface retention 1
Element Flows To:
Outlet 1 Outlet 2
Bioretention 1
ANALYSIS RESULTS
Stream Protection Duration
Predeveloped Landuse Totals for POC #1
Total Pervious Area:3.03
Total Impervious Area:O
Mitigated Landuse Totals for POC #1
Total Pervious Area:0.71
Total Impervious Area:1.19
Flow Frequency Return Periods for Predeveloped. POC #1
Return Period Flow(cfs)
2 year 0.003478
5 year 0.007543
10 year 0.0121
25 year 0.021132
50 year 0.031227
100 year 0.045296
Flow Frequency Return Periods for Mitigated. POC #1
Return Period Flow(cfs)
2 year 0
5 year 0
10 year 0
25 year 0
50 year 0
100 year 0
Drainage Report Harmony at Arlington 28-Jul-2025 12-14
Stream Protection Duration
POC #1
The Facility PASSED
The Facility PASSED.
Flow(cfs) Predev Mit Percentage Pass/Fail
0.0017 2355 5 0 Pass
0.0020 1326 5 0 Pass
0.0023 434 5 1 Pass
0.0026 112 5 4 Pass
0.0029 102 5 4 Pass
0.0032 89 5 5 Pass
0.0035 77 5 6 Pass
0.0038 66 5 7 Pass
0.0041 61 5 8 Pass
0.0044 58 5 8 Pass
0.0047 54 5 9 Pass
0.0050 50 4 8 Pass
0.0053 49 4 8 Pass
0.0056 47 4 8 Pass
0.0059 43 4 9 Pass
0.0062 40 4 10 Pass
0.0065 36 4 11 Pass
0.0068 36 4 11 Pass
0.0071 32 4 12 Pass
0.0074 31 4 12 Pass
0.0077 31 4 12 Pass
0.0080 29 4 13 Pass
0.0083 29 4 13 Pass
0.0086 27 4 14 Pass
0.0089 26 4 15 Pass
0.0092 26 4 15 Pass
0.0095 26 4 15 Pass
0.0098 25 4 16 Pass
0.0101 23 4 17 Pass
0.0104 23 4 17 Pass
0.0107 23 4 17 Pass
0.0110 23 4 17 Pass
0.0113 23 4 17 Pass
0.0116 21 4 19 Pass
0.0119 19 4 21 Pass
0.0122 18 4 22 Pass
0.0125 17 4 23 Pass
0.0128 16 4 25 Pass
0.0131 16 4 25 Pass
0.0134 15 4 26 Pass
0.0137 15 4 26 Pass
0.0140 14 4 28 Pass
0.0142 13 4 30 Pass
0.0145 13 4 30 Pass
0.0148 13 4 30 Pass
0.0151 13 4 30 Pass
0.0154 13 4 30 Pass
0.0157 13 4 30 Pass
0.0160 13 4 30 Pass
0.0163 13 4 30 Pass
0.0166 13 4 30 Pass
Drainage Report Harmony at Arlington 28-Jul-2025 12-15
0.0169 11 4 36 Pass
0.0172 11 4 36 Pass
0.0175 11 4 36 Pass
0.0178 11 4 36 Pass
0.0181 11 4 36 Pass
0.0184 11 4 36 Pass
0.0187 11 4 36 Pass
0.0190 11 4 36 Pass
0.0193 11 4 36 Pass
0.0196 11 4 36 Pass
0.0199 11 4 36 Pass
0.0202 11 4 36 Pass
0.0205 11 4 36 Pass
0.0208 10 4 40 Pass
0.0211 10 4 40 Pass
0.0214 9 4 44 Pass
0.0217 9 4 44 Pass
0.0220 8 4 50 Pass
0.0223 8 4 50 Pass
0.0226 8 4 50 Pass
0.0229 8 3 37 Pass
0.0232 8 3 37 Pass
0.0235 8 3 37 Pass
0.0238 8 3 37 Pass
0.0241 8 3 37 Pass
0.0244 8 3 37 Pass
0.0247 8 3 37 Pass
0.0250 8 3 37 Pass
0.0253 8 3 37 Pass
0.0256 8 3 37 Pass
0.0259 8 3 37 Pass
0.0262 8 3 37 Pass
0.0265 7 3 42 Pass
0.0268 7 3 42 Pass
0.0271 7 3 42 Pass
0.0274 7 3 42 Pass
0.0277 6 3 50 Pass
0.0280 6 3 50 Pass
0.0282 6 3 50 Pass
0.0285 6 3 50 Pass
0.0288 6 3 50 Pass
0.0291 6 3 50 Pass
0.0294 6 3 50 Pass
0.0297 6 3 50 Pass
0.0300 6 3 50 Pass
0.0303 6 3 50 Pass
0.0306 6 3 50 Pass
0.0309 5 3 60 Pass
0.0312 5 3 60 Pass
Drainage Report Harmony at Arlington 28-Jul-2025 12-16