HomeMy WebLinkAboutBLD2021-00106 - 05 STORMWATERMarc Horton – Washington Project Consultants Page 1 of 4 mueller stormwater report.docx
4/13/2021
212 S Rhododendron DR
Port Townsend, WA 98368
360-890-0752
STORMWATER REPORT & REVISED SITE INFORMATION
Bruce Mueller – Residence
Location: Spruce DR – Cape George Village
Parcel # 940500030
Design: Marc A Horton, PE
Date: 4/25/21
Attachments:
Attachment 1 – County Letter – Request for Information
Attachment 2 – Site Plan from BLD2021-00106
Attachment 3 –Revised Stormwater Site Plan
Attachment 4 - Soils Information from SEP09-00105 (now SEP2020-00160)
Attachment 5 – Soils Report (NRCS 2021)
Attachment 6 – Revised County Medium Project Stormwater Forms
Attachment 7 – Rain Garden Standard Design from WWSWMM (2019)
Attachment 8 – Rain Garden Profile (from “Rain Garden Handbook” -2013)
Proposed Development:
This report addresses stormwater management for construction of a residence on a 11,700-sf parcel on
Spruce Drive in the Cape George Village area. The home will have a roof-print of 2637 sf. The
driveway and concrete landings bring the total impervious surface to 3365 sf.
An application for a building permit has been submitted to the County (BLD21-00106). As part of the
review, a letter requesting additional information was issued by the County April 2, 2021 (Attachment
1). Specifically, the County requested clarification on handling of stormwater for this project.
A site plan and some stormwater information were submitted with the building permit application. With
regard to stormwater, this report and attachments revises and replaces previous information. The
building permit site plan is provided as Attachment 2. The Revised Stormwater Site Plan is provided
as Attachment 3.
4/25/21
BLD2021-00106
May 04 2021
May 24 2021
Page 2 of 4
Existing Parcel Conditions
This 0.27-acre parcel has had some development in the past. There are some remnants of a building
foundation on site, and septic design was completed in 2010 (SEP09-00105). The property slopes
from the east to west at about 8% for most of the parcel. Drainage from the southeast neighbor has
been intercepted in the past with construction of a curtain drain as shown on Attachments 1 and 2. The
details of this drain are unknown, but it appears to cut off any flow from the southwest and directs this
flow the west corner of the property where there is a natural drainage, and according to neighbors, a
buried conduit taking water to the Cape George storm system. Also, there is a depression at the east
corner of the property which contains flow (and may occasionally pond stormwater) from Spruce Dr.
and the neighbor’s parcel. This project will not affect drainage in this area because it already is
effectively blocked by existing topography and infiltrated. Existing topographic information is not at
detail to show this depression.
Parcels on the northwest side and the southwest side of the parcel are currently undeveloped while the
parcels on the remaining sides are developed with single family residences.
Soils
Soils on this site have been studied as part of septic system evaluations in 2009 (SEP09-00105) and
2020 (SEP2020-00160), including wet weather evaluation (Attachment 4). The results of this effort
show a mix of different soil, and evidence of fill. From a septic design perspective, these were
considered restrictive soils requiring use of an “alternative” septic system (e.g., Glendon System).
The NRCS (Natural Resources Conservation Service) soil survey for the parcel indicates that the soils
on the site include (Attachment 5):
1. CmC—Clallam gravelly sandy loam, and
2. HuD—Hoypus gravelly loamy sand.
The western portion of the site has Clallam soils indicated. These are generally well drained but have a
fairly shallow restrictive layer at about 2 feet. Often groundwater is indicated at this level. This makes
these soils difficult to use for septic and stormwater infiltration.
On the other hand, the eastern portion of the site show the Hoypus type soils which are very well
drained (Ksat of 6 to 20 inches per hour), and fairly deep (to 60 inches). This was not substantiated in
the septic soil analysis. Consequently, the approach here is a conservative one – soils limited
infiltration depth and infiltration in surface or constructed soils.
Regulatory Framework
The requirements for Stomwater management have been developed by Jefferson County and are
reflected in a series of worksheets and forms which are intended to meet the requirements of the
Western Washington Stormwater Management Manual. Jefferson County has cited the 2014 version,
although an update (2019) is currently being used throughout the Western Washington.
Page 3 of 4
A revised completed Jefferson County “medium” project stormwater packet is provided at Attachment
6. This attachment includes the following:
A Guide to Using Stormwater Forms.
Stormwater Calculation Worksheet: Impervious surfaces are shown on this worksheet.
Flow Chart for Determining Minimum Requirements: This chart shows this project to be a
“medium” project under the County classification system. The following Minimum Requirements
(MR 1-5) are met are met as appropriate in the worksheets A1, B1, and C:
MR 1: Stormwater Site Plan (included in this report)
MR 2: Construction Stormwater Pollution Prevention Plan
MR 3: Source Control of Pollutants
MR 4: Preservation of Natural Drainages
MR 5: On-site Stormwater Management
Worksheets A1, B1, and C
Design
Bioretention (BMP T7.30) has been selected as the management approach for this project based on a
conservative approach to the soils’ capacity to handle infiltration and use of an “alternative” septic
system (plus downgradient protection zones) located on the western portion of the property.
Bioretention (Rain Gardens) have been sized according to the “Rain Garden Handbook for Western
Washington” (Department of Ecology, 2013)
For this site the more conservative infiltration rate of CmC soils (upper 2 feet) of 0.6 to 2 inches per
hour was used. Port Townsend is in Region 1 for design purposes, and lead to the following design
parameters:
Design Factor Value / Item Comment
(page # from the “Rain Garden
Handbook”)
Soil Drainage Rat 0.6 to 2 inches/hour NRCS / Septic Reports
Contributing Area 3237 sf Page 19 Calculations
Desired Ponding Depth 6 inches Option (6”)
Desired Performance Level Best Selected Option
Rainfall Region Region 1
Port Townsend
Page 22 Maps
Sizing Factor (the “Best”) 9% Page 21
Required Size (Top of Surface) 291 sf 2 ponds of 150 sf
Design Dimensions (Top of
Surface)
2 at 10x15’(300 sf)
Can vary
Overflow Containment Area 2 @ 12x 17’ – approx.
400sf
2:1 sloped side
Page 4 of 4
The southern rain garden will need to be constructed to accommodate the portion of the driveway sheet
flowing to the rain gardens. This can generally be accomplished with a gravel flow path off the drive to
the southern edge of the southern rain garden.
Both rain gardens will be set as close as possible to the property line to protect the 10 foot set back
from the home foundation. The northern rain garden will have an overflow to the north just off the
property line. The overflow area should be constructed as a 2x2x10’ trench running with the contour
and filled with gravel (similar to an infiltration trench). This area is a set back from the road provided by
the Cape George Spruce Dr. right of way. If this overflow is ever needed, the flows will infiltrate.
As standard procedure, care should be taken with septic tank / pump tank installation to assure a seal
around any risers. This will minimize any groundwater flow from entering the septic tank or pump tank
(septic system).
Rain garden construction should follow the design presented in the Stormwater Management Manual
for Western Washington (2014, 2019) as shown on Attachment 7. Another profile of a typical rain
garden is shown on Attachment 8 (taken from the “Rain Garden Handbook for Western Washington”
(Ecology, 2013).
Options for rain garden planting zones, plant selection, and maintenance also provided in the “Rain
Garden Handbook for Western Washington.” This document should be used as a reference and can
be found at: https://apps.ecology.wa.gov/publications/documents/1310027.pdf
Attachment 1
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Bruce Mueller Residence Parcel #940500030 Address: Spruce Drive, Cape George Village Lot Size: 11,700
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-Home: 2637 sf -Driveway: 600 sf -Concrete Landings: 128 sf Notes:To be printed at 11”x17”Base map information from -Creative Design Solutions: SEP2020-00160 -Quadra Engineering 2021 -County GIS 2021
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Attachment 3
Attachment 4
Soil Map—Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/24/2021
Page 1 of 3
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509310 509320 509330 509340 509350 509360 509370 509380 509390
509310 509320 509330 509340 509350 509360 509370 509380 509390
48° 5' 28'' N
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Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84
0 20 40 80 120
Feet
0 5 10 20 30
Meters
Map Scale: 1:423 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
Attachment 5
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:20,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Jefferson County Area, Washington
Survey Area Data: Version 19, Jun 4, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Apr 27, 2019—May
10, 2019
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Soil Map—Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/24/2021
Page 2 of 3
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
CmC Clallam gravelly sandy loam, 0
to 15 percent slopes
0.1 28.6%
HuD Hoypus gravelly loamy sand,
15 to 30 percent slopes
0.3 71.4%
Totals for Area of Interest 0.4 100.0%
Soil Map—Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/24/2021
Page 3 of 3
Physical Soil Properties
This table shows estimates of some physical characteristics and features that
affect soil behavior. These estimates are given for the layers of each soil in the
survey area. The estimates are based on field observations and on test data for
these and similar soils.
Depth to the upper and lower boundaries of each layer is indicated.
Particle size is the effective diameter of a soil particle as measured by
sedimentation, sieving, or micrometric methods. Particle sizes are expressed as
classes with specific effective diameter class limits. The broad classes are sand,
silt, and clay, ranging from the larger to the smaller.
Sand as a soil separate consists of mineral soil particles that are 0.05 millimeter
to 2 millimeters in diameter. In this table, the estimated sand content of each soil
layer is given as a percentage, by weight, of the soil material that is less than 2
millimeters in diameter.
Silt as a soil separate consists of mineral soil particles that are 0.002 to 0.05
millimeter in diameter. In this table, the estimated silt content of each soil layer is
given as a percentage, by weight, of the soil material that is less than 2
millimeters in diameter.
Clay as a soil separate consists of mineral soil particles that are less than 0.002
millimeter in diameter. In this table, the estimated clay content of each soil layer
is given as a percentage, by weight, of the soil material that is less than 2
millimeters in diameter.
The content of sand, silt, and clay affects the physical behavior of a soil. Particle
size is important for engineering and agronomic interpretations, for determination
of soil hydrologic qualities, and for soil classification.
The amount and kind of clay affect the fertility and physical condition of the soil
and the ability of the soil to adsorb cations and to retain moisture. They influence
shrink-swell potential, saturated hydraulic conductivity (Ksat), plasticity, the ease
of soil dispersion, and other soil properties. The amount and kind of clay in a soil
also affect tillage and earthmoving operations.
Moist bulk density is the weight of soil (ovendry) per unit volume. Volume is
measured when the soil is at field moisture capacity, that is, the moisture content
at 1/3- or 1/10-bar (33kPa or 10kPa) moisture tension. Weight is determined after
the soil is dried at 105 degrees C. In the table, the estimated moist bulk density
of each soil horizon is expressed in grams per cubic centimeter of soil material
that is less than 2 millimeters in diameter. Bulk density data are used to compute
linear extensibility, shrink-swell potential, available water capacity, total pore
space, and other soil properties. The moist bulk density of a soil indicates the
pore space available for water and roots. Depending on soil texture, a bulk
density of more than 1.4 can restrict water storage and root penetration. Moist
bulk density is influenced by texture, kind of clay, content of organic matter, and
soil structure.
Physical Soil Properties---Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2021
Page 1 of 5
Saturated hydraulic conductivity (Ksat) refers to the ease with which pores in a
saturated soil transmit water. The estimates in the table are expressed in terms
of micrometers per second. They are based on soil characteristics observed in
the field, particularly structure, porosity, and texture. Saturated hydraulic
conductivity (Ksat) is considered in the design of soil drainage systems and
septic tank absorption fields.
Available water capacity refers to the quantity of water that the soil is capable of
storing for use by plants. The capacity for water storage is given in inches of
water per inch of soil for each soil layer. The capacity varies, depending on soil
properties that affect retention of water. The most important properties are the
content of organic matter, soil texture, bulk density, and soil structure. Available
water capacity is an important factor in the choice of plants or crops to be grown
and in the design and management of irrigation systems. Available water
capacity is not an estimate of the quantity of water actually available to plants at
any given time.
Linear extensibility refers to the change in length of an unconfined clod as
moisture content is decreased from a moist to a dry state. It is an expression of
the volume change between the water content of the clod at 1/3- or 1/10-bar
tension (33kPa or 10kPa tension) and oven dryness. The volume change is
reported in the table as percent change for the whole soil. The amount and type
of clay minerals in the soil influence volume change.
Linear extensibility is used to determine the shrink-swell potential of soils. The
shrink-swell potential is low if the soil has a linear extensibility of less than 3
percent; moderate if 3 to 6 percent; high if 6 to 9 percent; and very high if more
than 9 percent. If the linear extensibility is more than 3, shrinking and swelling
can cause damage to buildings, roads, and other structures and to plant roots.
Special design commonly is needed.
Organic matter is the plant and animal residue in the soil at various stages of
decomposition. In this table, the estimated content of organic matter is expressed
as a percentage, by weight, of the soil material that is less than 2 millimeters in
diameter. The content of organic matter in a soil can be maintained by returning
crop residue to the soil.
Organic matter has a positive effect on available water capacity, water infiltration,
soil organism activity, and tilth. It is a source of nitrogen and other nutrients for
crops and soil organisms.
Erosion factors are shown in the table as the K factor (Kw and Kf) and the T
factor. Erosion factor K indicates the susceptibility of a soil to sheet and rill
erosion by water. Factor K is one of six factors used in the Universal Soil Loss
Equation (USLE) and the Revised Universal Soil Loss Equation (RUSLE) to
predict the average annual rate of soil loss by sheet and rill erosion in tons per
acre per year. The estimates are based primarily on percentage of silt, sand, and
organic matter and on soil structure and Ksat. Values of K range from 0.02 to
0.69. Other factors being equal, the higher the value, the more susceptible the
soil is to sheet and rill erosion by water.
Erosion factor Kw indicates the erodibility of the whole soil. The estimates are
modified by the presence of rock fragments.
Erosion factor Kf indicates the erodibility of the fine-earth fraction, or the material
less than 2 millimeters in size.
Physical Soil Properties---Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2021
Page 2 of 5
Erosion factor T is an estimate of the maximum average annual rate of soil
erosion by wind and/or water that can occur without affecting crop productivity
over a sustained period. The rate is in tons per acre per year.
Wind erodibility groups are made up of soils that have similar properties affecting
their susceptibility to wind erosion in cultivated areas. The soils assigned to
group 1 are the most susceptible to wind erosion, and those assigned to group 8
are the least susceptible. The groups are described in the "National Soil Survey
Handbook."
Wind erodibility index is a numerical value indicating the susceptibility of soil to
wind erosion, or the tons per acre per year that can be expected to be lost to
wind erosion. There is a close correlation between wind erosion and the texture
of the surface layer, the size and durability of surface clods, rock fragments,
organic matter, and a calcareous reaction. Soil moisture and frozen soil layers
also influence wind erosion.
Reference:
United States Department of Agriculture, Natural Resources Conservation
Service. National soil survey handbook, title 430-VI. (http://soils.usda.gov)
Physical Soil Properties---Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2021
Page 3 of 5
Report—Physical Soil Properties
Three values are provided to identify the expected Low (L), Representative Value (R), and High (H).
Physical Soil Properties–Jefferson County Area, Washington
Map symbol
and soil name
Depth Sand Silt Clay Moist
bulk
density
Saturated
hydraulic
conductivity
Available
water
capacity
Linear
extensibility
Organic
matter
Erosion
factors
Wind
erodibility
group
Wind
erodibility
index
Kw Kf T
In Pct Pct Pct g/cc micro m/sec In/In Pct Pct
CmC—Clallam
gravelly
sandy loam,
0 to 15
percent
slopes
Clallam 0-3 -68--22-5-10- 15 0.85-0.98
-1.10
4.00-9.00-14.00 0.08-0.10-0.
11
0.0- 1.5- 2.9 3.0- 4.0-
5.0
.10 .15 3 5 56
3-23 -68--22-5-10- 15 0.90-1.03
-1.15
4.00-9.00-14.00 0.06-0.08-0.
10
0.0- 1.5- 2.9 1.0- 2.0-
3.0
.10 .28
23-60 -69--24-5- 8- 10 1.70-1.85
-2.00
0.01-0.20-0.42 0.00-0.00-0.
00
0.0- 1.5- 2.9 0.5- 0.5-
1.0
.20 .37
HuD—Hoypus
gravelly
loamy sand,
15 to 30
percent
slopes
Hoypus 0-2 -80--16-2- 4- 5 1.25-1.35
-1.45
42.00-92.00-14
1.00
0.03-0.04-0.
05
0.0- 1.5- 2.9 1.0- 2.0-
3.0
.05 .10 5 2 134
2-10 -84-- 9-5- 8- 10 1.35-1.45
-1.55
42.00-92.00-14
1.00
0.05-0.07-0.
08
0.0- 1.5- 2.9 0.5- 0.8-
1.0
.05 .15
10-26 -82--17-0- 2- 3 1.35-1.45
-1.55
42.00-92.00-14
1.00
0.01-0.02-0.
03
0.0- 1.5- 2.9 0.0- 0.3-
0.5
.10 .24
26-60 -82--17-0- 2- 3 1.40-1.48
-1.55
42.00-92.00-14
1.00
0.01-0.02-0.
03
0.0- 1.5- 2.9 0.0- 0.3-
0.5
.10 .24
Physical Soil Properties---Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2021
Page 4 of 5
Data Source Information
Soil Survey Area: Jefferson County Area, Washington
Survey Area Data: Version 19, Jun 4, 2020
Physical Soil Properties---Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/19/2021
Page 5 of 5
Attachment 6
Figure V-5.12: Typical Bioretention
2019 Stormwater Management Manual for Western Washington
Volume V -Chapter 5 -Page 776
Attachment 7
30
Buil
d
2
Rain Garden Cross Section
Rain Garden Soil Mix Depth
(12” to 24” Recommended)
1'
2'
oVERFLoW
iNFLoW—Water flowing off hard
surfaces (for example a roof or
driveway) can be delivered to
the rain garden through a swale
lined with decorative rock or
plants, through a pipe, or across
a landscaped area.
ExiSTiNG SoiL
GRADuAL SiDE SLoPES
(2:1 MAxiMuM)
MuLCH LAYERToP SuRFACE oF
PoNDiNG AREA
BoTToM oF ExCAVATioN
PoNDiNG DEPTH
(6” or 12” typical)
oVERFLoW
CoNTAiNMENT
RAiN GARDEN SoiL Mix
Attachment 8