HomeMy WebLinkAboutBLD2021-00276-05- Stormwater or Approved StormwaterMarc Horton – Washington Project Consultants Page 1 of 5 wright stormwater plan final 042121.docx
4/13/2021
212 S Rhododendron DR
Port Townsend, WA 98368
360-890-0752
STORMWATER MANAGEMENT PLAN
Brad Wright – Home and Detached Garage
Location: Jefferson County - Coyle
Parcel # 601103009
Design: Marc A Horton, PE
Date: 4/21/21
Attachments:
Attachment 1 – Site Plan Sheet 1
Attachment 2 – Site Plan Sheet 2
Attachment 3 –NRCS Soils Report
Attachment 4 - Soils Information from SEP99-00136
Attachment 5 – Geohazard Areas – Site Plan Sheet 3
Attachment 6 – Additional Critical Areas – Site Plan Sheet 4
Attachment 7 – A Guide to Using Stormwater Forms
Attachment 8 - Stormwater Calculation Worksheet
Attachment 9 - Flow Chart for Determining Minimum Requirements
Attachment 10 - Construction SWPPP
Attachment 11 – Dispersion Trench Design
Proposed Development:
This report addresses stormwater management for construction of a residence and a
detached garage on a 9.89-acre parcel on the Coyle Peninsula in Jefferson County. The
home will have a roof-print of 2370 sf. The detached garage will add an additional 1280 sf
of impervious surface bringing the total to 3650 sf. An existing road and circular drive will
provide access. The site plan is shown on Attachment 1 (Site Plan Sheet 1) and
Attachment 2 (Site Plan Sheet 2).
4/21/21
BLD2021-00276
Page 2 of 5
Existing Parcel Conditions
This 9.89-acre parcel has had some development over the past 20 years. The roadway
drive) has been developed along with installation of a septic system (SEP99-00136). The
property slopes from the east to west at about 17% for most of the parcel with steeper
slopes near Daybob Bay. This is shown with contours on Attachments 1 and 2. The road
approximately 1600’) enters the parcel on the southeast and traverses the southern
boundary to a location about 300’ from the “top of bank” on the property. This large parcel is
surrounded on three sides with other large parcels which are relatively undeveloped. The
geology of the site has been documented as a “Geology Assessment” prepared in 2007
Stratum Group).
Soils
The NRCS (Natural Resources Conservation Service) soil survey for the parcel
indicates that the soils for the home and garage construction sites include:
1. DaD—Dabob very gravelly sandy loam, 15 to 30 percent slopes, and
2. HuC—Hoypus gravelly loamy sand, 0 to 15 percent slopes
The NRCS Soils report is provided as Attachment 3. The boundaries for soil differentiation
in this case must be considered general since soils are shown to be variable across this
large site. Therefore, boundaries are considered general and not absolute.
Homesite Location: The existing septic system and the proposed home site are in the DaD
soils area. According to the NRCS, these soils are well drained to about 33 inches.
According to the NRCS Soils Report, the Ksat for these soils is in the range of 2 to 6 inches
per hour. A more restrictive layer is indicated below the 33-inch level. However, the soils
report as part of SEP99-00136 (Attachment 4) indicates the well-drained soils seem to
extend to 45 or 50 inches. This on-site deviation from the NRCS data is expected on a large
parcel such as this and given its geologic history.
Garage Location: According to the NRCS, the garage site is located in the HuC soil zone.
These soils are also well drained and deep (60 inches or more). The NRCS reports the Ksat
for these soils between 6 and 20 inches per hour.
BLD2021-00276
Page 3 of 5
Critical Areas
Critical Areas are shown on Attachments 5 & 6 (Site Plan Sheets 3 & 4).
Critical Areas include:
Geohazard Areas: Concerns here are landslides (slope stability) and erosion.
Stormwater flows should be dispersed / infiltrated in a manner to avoid concentrated
flow, and in a soil profile that can accommodate the expected quantity.
This presence of a Critical Area prompted development of a Geology Assessment in
2007 (Stratum Group 2007). This report is currently being updated. However, the
previous report concluded: “Roof drains and any other drainage should be discharged
into a level dispersion/infiltration trench excavated perpendicular to the slope.”
Critical Aquifer Recharge Area: This area includes infiltration locations important to
support aquifer(s). From a stormwater perspective, flows should be infiltrated or
dispersed as much as possible in a predevelopment pattern if possible. The HuC
soils on this parcel are designated as high infiltration soils.
Seawater Intrusion: This zone shows susceptibility of the aquifer to groundwater
withdrawals, and not generally an issue for stormwater except to infiltrate or disperse
stormwater as much as possible in a predevelopment pattern if possible.
Riparian Cover: This area appears to include a buffer of 300’ from Ordinary High
Water. All structures for this project are outside this zone.
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.
The following County forms have been completed and are part of this report.
A Guide to Using Stormwater Forms (Attachment 7): Consistent with instructions on this
form, this project will use and engineered approach. This report provides the information
requested as part of County Forms (A1 and B1).
Stormwater Calculation Worksheet (Attachment 8): Impervious surfaces are shown on this
worksheet.
Flow Chart for Determining Minimum Requirements (Attachment 9): This chart shows this
project to be a “medium” project under the County classification system. The following
BLD2021-00276
Page 4 of 5
Minimum Requirements (MR 1-5) are met are met as appropriate and described in this
report:
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
A Construction Stormwater Pollution Prevention Plan (CSWPPP - Attachment 10):
Completion of this form along with notations on the SW Site Plan will serve to meet the
requirement for a CSWSPPP (MR 2).
Design
Full Infiltration (BMP T5.10A) has been selected as the management approach for this
project based on the soils’ capacity to handle infiltration. Generally, drywells are also an
option, but given the downgradient bluff, indications of an impervious layer at depth, it
seems prudent to keep the water near the surface and avoid concentrated sources. The
infiltration trench design will help achieve that (Attachment 11).
Infiltration trenches will need to be constructed with the level running with the land contour,
and be protected with a catch basin.
The infiltration trench length is governed by the soil texture. The WWSWMM calls for 75
feet of trench for each 1000 sf of roof, and for fine sand soil (e.g., fine sandy loam), and 30
feet per 1000 sf for medium sand (e.g., loamy sand). Of course, there is a range of soil
texture and infiltration rates between these values.
For this site, a trench length of 40 feet per 1000 has been selected for both structures. This
is based on the soils report filed with SEP99-00136 which indicates a medium sand soil, and
the NRCS data (and County Infiltration Soils mapping) showing high infiltration potential in
this area.
Six 25-foot infiltration trenches need to be constructed downslope (downgradient) from the
structures and 10 feet from any foundation. Four will be needed for the home, and two for
the garage. These are shown on Attachment 2. These are constructed in pairs with a
minimum centerline spacing of the trenches of 3 feet.
Facility Maintenance
Clean catch basins annually prior to the onset of the wet season.
Use gutter protection to help prevent materials from entering the roof drain system.
BLD2021-00276
Page 5 of 5
Construction Stormwater Management
Construction during the time between October and May should be considered the most
vulnerable to erosion and sediment loss from the site. Sediment laden water needs to be
contained. To mitigate for this vulnerability, (references are to the 2019 WWSWMM and are
referenced in Attachment 10)
All earth work and foundation construction should only occur from May to October.
Natural vegetation will be preserved as much as possible (BMP C101: Preserving
Natural Vegetation p.271).
Straw mulch (BMP C121: Mulching p.289) will be used on exposed soil after the
foundation is constructed to prevent erosion
If necessary, before permanent stormwater systems are installed, silt fence (BMP
C233: Silt Fence p.370) will be installed down-gradient from the site to prevent
movement of soil from the site.
During construction pollution can be better controlled with the following BMPs:
BMP C151: Concrete Handling (p.315)
BMP C152: Sawcutting and Surfacing Pollution Prevention (p.317)
BMP C153: Material Delivery, Storage and Containment (p.318)
BLD2021-00276
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Wright Residence Site Plan - Sheet 2 Parcel #610003109 Structures Home 2040 sf Decks & Steps 520 sf Garage 1008 sf Notes -
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from Jefferson County GIS (4/2021)General Slope of Parcel 15-20%Average Road Slope - 17%Scale 1 inch = 40 feet Drawing Developed by Marc Horton, PEImpervious Surfaces Home 2370 sf Garage 1280 sf
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Wright Residence Site Plan - Sheet 2 Parcel #610003109 Structures Home 2040 sf (not rectangular - see design drawings)(building envelope 41’x75’)Decks & Steps 520 sf Garage 1008 sf (28’x36’)Notes -
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from Jefferson County GIS (4/2021)General Slope of Parcel 15-20%Average Road Slope - 17%Scale 1 inch = 40 feet Drawing Developed by Marc Horton, PEImpervious Surfaces Home 2370 sf Garage 1280 sf 6/24/21
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Soil Map—Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/18/2021
Page 1 of 3
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515100 515110 515120 515130 515140 515150 515160 515170 515180 515190 515200 515210 515220 515230 515240 515250 515260
515100 515110 515120 515130 515140 515150 515160 515170 515180 515190 515200 515210 515220 515230 515240 515250 515260
47° 45' 34'' N
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47° 45' 34'' N
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47° 45' 30'' N
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Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84
0 35 70 140 210
Feet
0 10 20 40 60
Meters
Map Scale: 1:775 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
BLD2021-00276
Attachment 3
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
4/18/2021
Page 2 of 3
BLD2021-00276
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
DaD Dabob very gravelly sandy
loam, 15 to 30 percent
slopes
1.3 41.0%
HuC Hoypus gravelly loamy sand, 0
to 15 percent slopes
1.8 59.0%
Totals for Area of Interest 3.1 100.0%
Soil Map—Jefferson County Area, Washington
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
4/18/2021
Page 3 of 3
BLD2021-00276
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
4/18/2021
Page 1 of 5
BLD2021-00276
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
4/18/2021
Page 2 of 5
BLD2021-00276
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
4/18/2021
Page 3 of 5
BLD2021-00276
http://soils.usda.gov
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
DaD—Dabob
very gravelly
sandy loam,
15 to 30
percent
slopes
Dabob 0-3 -68--22-5-10- 15 0.85-0.98
1.10
14.00-28.00-42.
00
0.05-0.06-0.
07
0.0- 1.5- 2.9 2.0- 2.5-
3.0
10 .20 3 6 48
3-20 -69--24-5- 8- 10 1.35-1.43
1.50
14.00-28.00-42.
00
0.04-0.06-0.
07
0.0- 1.5- 2.9 1.0- 1.5-
2.0
10 .28
20-33 -69--24-5- 8- 10 1.35-1.43
1.50
14.00-28.00-42.
00
0.04-0.06-0.
07
0.0- 1.5- 2.9 0.5- 0.8-
1.0
15 .37
33-60 -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
HuC—Hoypus
gravelly
loamy sand,
0 to 15
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
4/18/2021
Page 4 of 5
BLD2021-00276
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
4/18/2021
Page 5 of 5
BLD2021-00276
BLD2021-00276 Attachment 4
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Jun 30 2021
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BLD2021-00276
Attachment 6
BLD2021-00276 Attachment 7
BLD2021-00276
Attachment 8
6/24/21
Jun 24 2021
BLD2021-00276
Attachment 9
BLD2021-00276 Attachment 10
BLD2021-00276
BLD2021-00276
BLD2021-00276
BLD2021-00276
Jun 24 2021
BLD2021-00276
Figure V-4.1: Typical Downspout Infiltration Trench
2019 Stormwater Management Manual for Western Washington
Volume V -Chapter 4 -Page 709
BLD2021-00276 Attachment 11