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REVISED DRAINAGE REPORT
FOR
REDWOOD LANE SHORT PLAT
27xx JONES ROAD N.E.
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SS10NAL S 43N303U
NOU 43H-40 JW:
EXPIRES 0
OCTOBER 2000
Contact: Engineer:
Rex Orkney Del Erickson, P.E
Orkney Homes & Development 15020 S.E. 46th St.
PMB No. 462 Bellevue, WA. 98006
218 Main Street Tel: 425-747-5825
Kirkland, WA. 98083
Tel: 425-766-3298
NARRATIVE
NARRATIVE
The subject site, located at 27xx Jones Ave. N.E., is presently
undeveloped. The site is covered with evergreen and deciduous trees, brush,
native undergrowth (See the existing conditions exhibit). The parcel is
rectangular in shape, 150.01' x 146.26', with a 24' x 150' handle to the west
that allows for access off of Jones Ave. N.E.. The proposed improvements
include the construction of a 20' wide asphalt access road, storm drainage,
sanitary sewer, water services and 4 single family residences (See the
developed conditions exhibit).
Contours fall across the parcel from the southwesterly corner to the
northeasterly corner on a slope of approximately 10%. No offsite flow enters
the parcel from the north or the east as they lie downslope. Minimal flow
enters from the south, as the property is presently developed with single
family structures and only their backyards will runoff onto the site. No flow
enters the site from the west as the street improvements from Jones Ave. N.E.
control drainage from the right of way and direct the flow to the north.
The site lies downslope from the improvements in Jones Ave. N.E.,
therefore storm drainage will be collected on site and directed to the existing
24" diameter storm drain system located about 60 feet to the east of the
parcel's easterly property line. Runoff from the access road will be collected
in 4 Type 1 and 1 Type 2 catch basins and conveyed easterly in 8 inch
diameter storm drain to an outfall into a bio Swale located along the the east
property line of Lot 1. This bio swale will convey the flow northerly to a
Type 1 catch basin located at the northeast corner of Lot 1. Outfall from this
catch basin is easterly to a connection with the City's existing 24 inch
diameter storm drain lying immediately to the east of the site. Each lot will
connect it's downspout and driveway drains to this outfall line through 4 inch
and 6 inch storm drain piping.
No detention is required for this site as the proposed project site post-
developed peak runoff rate for the 100 year, 24 hour duration design storm
event is calculated to be less than 0.5 cfs more than the peak runoff rate for
the existing site conditions. See the attached exhibits and calculations for
additional information.
Biofiltration is required for the site as there is more than 5000 square
feet of impervious area that is subject to vehicular use. Water Quality is not
required for the site as the proposed project contains less than 1 acre of new
impervious surface that will be subject to vehicular use. See calculations and
plans for bio swale sizing, design details, and location.
SIZE WATER QUALITY SYSTEM
M BE-DEEDED To
BY LOT LINE ADJUSTMENT TO RUN �
ENT WIT)4
10' S. RM ORAI FND. BAR
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******************** S.C.S. TYPE—IADISTRIBUTION ********************
********* 2—YEAR 24—HOUR STORM **** 2.00" TOTAL PRECIP. *********
----------------------------------------
ENTER: A(PERV), CN(PERV), A(RvIPERV), CN(IMPERV), TC FOR BASIN NO. 1
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DATA PRINT—OUT:
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A CN A CN
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PEAK—Q(CFS) T—PEAK(HRS) VOL(CV—FT)
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KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1.2.3 CORE REQUIREMENT #3: RUNOFF CONTROL
Proposed projects must provide runoff control through a combination of peak rate runoff control
and on-site biofiltration as described below.
Peak Rate Runoff Control
Proposed projects must provide peak rate runoff control to limit the developed conditions peak
rates of runoff from specific design storm events not to exceed the peak rates for the "existing
site conditions" described below. Three basic methods for peak rate runoff control are possible:
detention, retention, and infiltration. Detention is the collection and temporary storage of surface
water (typically over several hours) with the outflow rate restricted--usually to the pre-developed
outflow rate. Retention is the collecting and holding of surface and stormwater with effectively no
surface outflow (outflow occurs by evapotranspiration). Infiltration is the soaking of surface water
into the ground (typically for several hours or days).
Infiltration not only reduces or eliminates surface runoff, but also helps to maintain the hydrologic
balance of the surface water system. Infiltration can limit erosion and recharge groundwaters that
supply water to wetlands, streams, and wells. Preserving infiltration after development is by far
the most effective mechanism in preventing adverse impacts to the surface water system.
Because of these benefits, King County encourages the use of infiltration systems for runoff control
where the appropriate soil conditions exist.
Proposed project peak rate runoff control must be located on-site. An exemption from on-site peak
rate runoff control may be granted for the special conditions specified at the end of this core
requirement section.
Biofiltration
Proposed project runoff resulting from more than five thousand square feet of new impervious
surface' for each threshold discharge areas, and subject to vehicular use or storage of chemicals,
shall be treated prior to discharge from the project site by on-site biofiltration measures as
described in Section 4.6.4. Installation of a biofiltration facility (biofiltration swale or filter strip) to
meet Core Requirement #3 does not exempt the applicant from installing a special water quality
control facility (wetpond, wetvault or water quality swale) to meet Special Requirement #5.
. ,-_.
The biofiltration design flow rate shall be the peak rate of runoff for the 2-year 24-hour duration
design storm event. Note, biofiltration facilities installed following peak rate runoff control facilities
may be sized to treat the allowable release rate (existing site conditions) for the 2-year 24-hour
duration design storm event for the peak rate runoff control facility. Biofiltration facilities installed
prior to peak rate runoff control facilities shall be sized based on the developed conditions.
Proposed Project Site "Existing Site Conditions"
In performing the analysis for the design of runoff control, it is essential to first determine the
proposed project site "existing site conditions" from which the pre-development runoff rates can be
computed for specific design storms. Existing site conditions are not always synonymous with
those of the natural, totally undeveloped site. In some instances substantial modifications (such as
diversions, piping, clearing, and grading) have already increased and altered surface water runoff
leaving the site, but no permit, nor accompanying engineering plan, was ever approved. In other
instances, an approved drainage system exists and the existing system must be analyzed for its
performance. There are two definitions for proposed project site "existing site conditions"
depending on the site.
s See Definition Section
s A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple
natural discharge locations that combine within 1/4-mile downstream.
1.2.3-1 11/92
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1.3.5 SPECIAL REQUIREMENT #5: SPECIAL WATER QUALITY CONTROLS
Threshold Requirement
IF any threshold discharge area" of THEN the threshold discharge area"
the proposed project contains more shall have a wetpond meeting the
than 1 acre of new impervious standards described below employed
surface that will be subject to (in addition to peak rate control
vehicular use or storage of chemicals requirements) to treat a project's
and: runoff prior to discharge from the site.
A wetvault or water quality swale, as
(a) proposes direct discharge of described below, may be used when a
runoff to a regional facility, wetpond is not feasible because of
receiving water, lake, wetland, or physical site constraints or impacts to
closed depression without on-site sensitive areas.
peak rate runoff control; OR
(b) proposes discharge of runoff
through overland flow or on-site
infiltration into a Class 1 or 2
stream, or Class 1 wetland, within
one mile radius downstream from
the project site.
Wetponds or Wetvault
Wetponds and Wetvaults contain a permanent pool of water depending on the frequency and
quantity of inflow, and (for Wetponds) the rate of permeability of the underlying soil. The
Wetponds and Wetvaults fill with the initial onset of frequent storms such that the major portion of
the volume of runoff is treated. The principal mechanism of treatment is settlement due to
establishment of calm conditions. In Wetponds treatment is enhanced by*the mechanisms of
biofiltration and biologic activity.
The size (water surface area and volume) of the Wetpond or Wetvaults shall be determined as
follows:
(1) The design water surface area in the Wetpond or Wetvault shall be a minimum of
one percent of the impervious surface area in the drainage sub-basin contributing to
the facility.
(2) The design volume of the Wetpond or Wetvault shall be a minimum of the total
volume of runoff (see Chapter 3: Hydrologic Analysis) from the tributary sub-basin
proposed developed conditions using a water quality design storm event having a
total precipitation (Pt-wq), where Pt-wq is one-third of the two-year, 24-hour total
precipitation (PZ/3, P2 is obtained from Figure 3.5.1 C). This water quality design
storm event approximates the runoff from the mean annual storm event.
Other design criteria are found in Section 4.6.2.
A threshold discharge area is an on-site area which drains to a single natural discharge location
or multiple natural discharge locations that combine within 1/4-mile downstream.
1;3.5-1 11/92
SOILS INFORMATION
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
(2) CN values can be area weighted when they apply to pervious areas of similar CN's (within 20
�\ CN points). However, high CN areas should not be combined with low CN areas (unless the
low CN areas are less than 15% of the subbasin). In this case, separate hydrographs should be
generated and summed to form one hydrograph.
FIGURE 3.5.2A HYDROLOGIC SOIL GROUP OF THE SOILS IN KING COUNTY
HYDROLOGIC HYDROLOGIC
SOIL GROUP GROUP* SOIL GROUP GROUP*
Alderwood C Orcas Peat D
Arents, Alderwood Material C Oridia D
Arents, Everett Material B Oval) C
Beausite C Pilchuck C
Bellingham D Puget D
Briscot D Puyallup B
Buckley D Ragnar B
Coastal Beaches Variable Renton D
Earlmont Silt Loam D Riverwash Variable
Edgewick C Salal C
Everett A/B `Sammamish D
Indianola A . Seattle D
Kitsap lu Shacar D
Klaus C Si Silt C
Mixed Alluvial Land Variable Snohomish D
Neilton A Sultan C
Newberg B Tukwila D
Nooksack C Urban Variable
Normal Sandy Loam D Woodinville D
HYDROLOGIC SOIL GROUP CLASSIFICATIONS
A. (Low runoff potential). Soils having high infiltration rates, even when thoroughly wetted, and consisting
chiefly of deep, well-to-excessively drained sands or gravels. These soils have a high rate of water
transmission.
B. (Moderately low runoff potential). Soils having moderate infiltration rates when thoroughly wetted, and
consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of
water transmission.
C. (Moderately high runoff potentiai). Soils having slow infiltration rates when thoroughly wetted, and
consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately
fine to fine textures. These soils have a slow rate of water transmission.
D. (High runoff potential). Soils having very.slow infiltration rates when thoroughly wetted and consisting
chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a
hardpan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils
have a very slow rate of water transmission.
* From SCS, TR-55, Second Edition,June 19W, Exhibit A-1. Revisions made from SCS, Sol Interpretation
Record, Form #5, September 1988.
ii/92 3t5.2-2 ;
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gravelly coarse sand to very gravelly loamy sand. the presence of a consolidated substratum at a depth
Depth to the IIC horizon ranges from 18 to 36 of 7 to 20 feet. This substratum is the same made-
inches. rial' as that in the Alderwood soils.
Some areas are up to 5 percent included Alderwood Some areas are up to 5 percent included Norma,
soils, on the more rolling and undulating parts of Seattle, and Tukwila soils, all of which are poorly
the landscape; some are about 5 percent the deep, drained.
sandy Indianola soils; and some are up to 25 percent Runoff is slow to medium, and the erosion hazard
Neilton very gravelly loamy sands. Also included is slight to moderate.
in mapping are areas where consolidated glacial till, Most of the acreage is used for timber. Capabil-
which characteristically underlies Alderwood soils, ity unit VIs-1; woodland group 3f3.
is at a depth of 5 to 15 feet.
Permeability is rapid. The effective rooting
depth is 60 inches or more. Available water capac- Indianola Series
ity is low. Runoff is slow, and the erosion hazard
is slight. The Indianola series is made up of somewhat
This soil is used for timber and pasture and for excessively drained soils that formed under conifers
urban development. Capability unit IVs-1; woodland in sandy, recessional, stratified glacial drift.
group 3f3. These undulating, rolling, and hummocky soils are on
terraces. Slopes are 0 to 30 percent. The annual
Everett gravelly sandy loam, 5 to 15 percent precipitation is 30 to 55 inches, and the mean
slopes (EvQ --This soil is rolling. Areas are annual air temperature is about 500 F. The frost-
irregular in shape, have a convex surface, and range free season is 150 to 210 days. Elevation ranges
from 25 acres to more than 200 acres in size. Run- from about sea level to 1,000 feet.
off is slow to medium, and the erosion hazard is In a representative profile, the upper 30 inches
slight to moderate. is brown, dark yellowish-brown, and light olive-
Soils included with this soil in mapping make up brown loamy fine sand. This is underlain by olive
no more than 25 percent of the total acreage. Some sand that extends to a depth; of 60 inches or more
areas are up to 5 percent Alderwood soils, which (pl. I, right) .
overlie consolidated glacial till; some are up to Indianola soils are used for timber and for urban
20 percent Neilton very gravelly loamy sand; and development.
some are about 15 percent included areas of Everett
soils where slopes are more gentle than 5 percent .;. Indianola loamy fine sand, 4 to 15 percent slopes
and where they are steeper than 15 percent. (InC):=-This undulating and rolling soil has convex
This Everett soil is used for timber and pasture slopes. It is near the edges of upland terraces.
and for urban development. Capability unit.VIs-1; Areas range from 5 to more than 100 acres in size.
woodland group 3f3. Representative profile of Indianola loamy fine
sand, 4 to 15 percent slopes, in forest, 1,000 feet
Everett gravelly sandy loam, 15 to 30 percent west and 900 feet south of the northeast corner of
slopes (EvD) .--This soil occurs as long, narrow sec. 32, T. 25 N., R. 6 E. :
areas, mostly along drainageways or on short slopes
between terrace benches. It is similar to�-Everett 01--3/4 inch to 0, leaf litter.
gravelly sandy loam, 0 to 5 percent slopes, but in B21ir--O to 6 inches, brown (IOYR 4/3) loamy fine
most places is stonier and more gravelly. •sand, brown (10YR 5/3) dry; massive; soft,
Soils included with this soil in mapping make up very friable, nonsticky, nonplastic; many
no more than 30 percent of the total acreage. Some roots; slightly acid; clear, smooth boundary.
areas are up to 10 percent Alderwood soils,-which 4 to 8 inches thick.
overlie consolidated glacial till; some are up to 5 B22ir--6 to 15 inches, dark yellowish-brown (10YR
percent the deep, sandy Indianola soils; some are 4/4) loamy fine sand, brown (10YR 5/3) dry;
up to 10 percent Neilton very gravelly loamy sand; massive; soft, very friable, nonsticky, non-
and some are about 15 percent included areas of plastic; common roots; slightly acid; clear,
Everett soils where slopes are less than 15 percent. smooth boundary. 6 to 15 inches thick.
Runoff is medium to rapid, and the erosion hazard C1--15 to 30 inches, light olive-brown (2.5Y 5/4)
is moderate to severe. loamy fine sand, yellowish brown (IOYR 6/4)
Most of the acreage is used for timber. Capa- dry; massive; soft, very friable, nonsticky,
bility unit VIe-1; woodland group 3f2. nonplastic; common roots; slightly acid;
gradual, smooth boundary. 12 to 17 inches
Everett-Alderwood gravelly sandy loams, 6 to 15 thick.
percent slopes (EwC) .--This mapping unit is about C2--30 to 60 inches, olive (5Y 5/4) sand, light
equal parts Everett and Alderwood soils. The soils brownish gray (2.5Y 6/2) dry; single grain;
are rolling. Slopes are dominantly 6 to 10 percent, loose, nonsticky, nonplastic; few roots;
but range from gentle to steep. Most areas are slightly acid. Many feet thick.
irregular in shape and range from 15 to 100 acres
or more in size. In areas classified as Everett There is a thin, very dark brown Al horizon at
soils, field examination and geologic maps indicate the surface in some places. The B horizon ranges
16
from very dark grayish brown to brown and dark and the mean annual air temperature is about 500 F.
yellowish brown. The C horizon ranges from dark The frost-free season is 150 to more than 200 days.
grayish brown to pale olive and from loamy fine sand Elevation ranges from about sea level to 500 feet.
to sand. Thin lenses of silty material are at a In a representative profile, the surface layer
depth of 4 to 7 feet in some places. and subsoil are very dark brown and dark yellowish-
Soils included with this soil in mapping make up brown silt loam that extends to a depth of about 24
no more than 25 percent of the total acreage. Some inches. The substratum is olive-gray silty clay
areas are up to 10 percent Alderwood soils, on the loam. It extends to a depth of 60 inches or more.
more rolling and undulating parts of the landscape; Kitsap soils are used for timber and pasture.
some are up to 8 percent the deep, gravelly Everett
and Neilton soils; some are up to 15 percent Kitsap Kitsap silt loam, 2 to 8 percent slopes (KpB) .--
soils, which have platy lake sediments in the sub- This undulating soil is on low terraces of the major
soil; and some are up to 15 percent Ragnar soils, valleys of the Area. Areas range from 5 acres to
which have a sandy substratum. more than 600 acres in size and are nearly circular
Permeability is rapid. The effective rooting to irregular in shape. Some areas are one-eighth to
depth is 60 inches or more. Available water capac- a half mile wide and up to 3 or 4 miles long.
ity is moderate. Runoff is slow to medium, and the Representative profile of Kitsap silt loam, 2 to
erosion hazard is slight to moderate. 8 percent slopes, in pasture, 820 feet west and 330
This soil is used for timber and for urban devel- feet south of east quarter corner of sec. 28, T. 25
opment. Capability unit IVs-2; woodland group 4s3. N., R. 7 E. :
Indianola loamy fine sand, 0 to 4 percent slopes Ap--O to 5 inches, very dark brown (10YR 2/2) silt
(InA) .--This soil occupies smooth terraces in long loam, dark grayish brown (10YR 4/2) dry; mod-
narrow tracts adjacent to streams. Areas range from erate, medium, granular structure; slightly
about 3 to 70 acres in size. hard, very friable, nonsticky, nonplastic;
Soils included with this soil in mapping make up many roots; medium acid; abrupt, smooth bound-'
no more than 20 percent of the total acreage. Some ary.
areas are up to 5 percent Alderwood soils, on the B2--5 to 24 inches, dark yellowish-brown (10YR 3/4)
more rolling and undulating parts of the landscape; silt loam, brown (10YR S/3) dry; 2 percent
some are about 10 percent the deep, gravelly Everett iron concretions; weak, coarse, prismatic
and Neilton soils; some are up to 10 percent Indian- structure; slightly hard, friable, slightly
ola loamy fine sand that has stronger slopes; and sticky, slightly plastic; many roots; slightly
some areas are up to 10 percent the poorly drained acid; abrupt, wavy boundary. 18 to 21 inches
Norma, Shalcar, Tukwila soils. thick.
Runoff is slow, and the erosion hazard is slight. IIC--24 to 60 inches, olive-gray (5Y 5/2) silty clay
This soil is used for timber. Capability unit loam, light gray (SY 7/2) dry; many, medium
IVs-2; woodland group 4s3. and coarse, prominent mottles of dark yellowish
brown and strong brown (10YR 4/4 and 7.SYR
Indianola loamy fine sand, 15 to 30 percent 5/8) ; moderate, thin and medium, platy struc-
slopes (InD) .--This soil is along entrenched-=-reams. ture; hard, firm, sticky, plastic; few roots
Soils included with this soil in mapping make up to a depth of 36 inches, none below; strongly
no more than 25 percent of the total acreage. Some acid.
areas are up to 10 percent Alderwood soils; some are
about S percent the deep, gravelly Everett and Neil- The A horizon ranges from very dark brown to dark
ton soils; some are up to 15 percent Kitsap .soi-Is, brown. The B horizon ranges from dark yellowish
which have platy, silty lake sediments in the sub- brown to dark brown and from silt loam to silty clay
soil; and some are up to 15 percent Indianola loamy loam. The platy IIC horizon ranges from grayish
fine sand that has milder slopes. brown to olive gray and from silt loam to silty clay
Runoff is medium, and the erosion hazard is moder- loam that has thin lenses of loamy fine sand in
ate to severe. places. Brownish mottles are common in the upper
This soil is used for timber. Capability unit part of the IIC horizon.
VIe-1; woodland group 4s2. Some areas are up to 10 percent included Alderwood
gravelly sandy loam; some are up to 5 percent the
very deep, sandy Indianola soils; and some are up to
Kitsap Series 5 percent the poorly drained Bellingham, Tukwila,
and Seattle soils.
The Kitsap series is made up of moderately well Water flows on top of the substratum in winter.
drained soils that formed in glacial lake deposits, Permeability is moderate above the substratum and
under a cover of conifers and shrubs. These soils very slow within it. The effective rooting depth is
are on terraces and strongly dissected terrace about 36 inches. Available water capacity is moder-
fronts. They are gently undulating and rolling and ate to moderately high. Runoff is slow to medium,
moderately steep. Slopes are 2 to 70 percent. and the erosion hazard is slight to moderate.
Platy, silty sediments are at a depth of 18 to 40 This soil is used for timber and pasture. Capabil-
inches. The annual precipitation is 35 to 60 inches, ity unit IIIe-1; woodland group 2d2.
17
EXHIBITS
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KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.2B SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS
SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS (Published by SCS in 1982)
Runoff curve numbers for selected agricultural, suburban and urban land use for Type 1A
rainfall distribution, 24-hour storm duration.
CURVE NUMBERS BY
HYD,P,Q.LOGIC SOIL GROUP
LAND USE DESCRIPTION A B C D
Cultivated land(1): winter condition 86 91 94 95
Mountain open areas: low growing brush and grasslands 74 82 89 92
Meadow or pasture: 65 78 85 89
Wood or forest land: undisturbed or older second growth 64 76 81
Wood or forest land: young second growth or brush 55 72 81 86
Orchard: with cover crop 81 88 92 94
Open spaces, lawns, parks, golf courses, cemeteries,
landscaping.
good condition: grass cover on 75%
or more of the area 80 86 90
fair condition: grass cover on 50%
to 75% of the area 77 85 90 92
Gravel roads and parking lots 76 85 89 91
Dirt roads and parking lots 72 82 87 89
Impervious surfaces, pavement, roofs, etc. 9 98 98 98
Open water bodies: lakes, wetlands, ponds, etc. 100 100 100 100
Single Family Residential (2)
Dwelling Unit/Gross Acre % Impervious (3)
1.0 DU/GA 15 Separate curve number
1.5 DU/GA 20 shall be selected
2.0 DU/GA 25 for pervious and
2.5 DU/GA 30 impervious portion
3.0 DU/GA 34 of the site or basin
3.5 DU/GA 38
4.0 DU/GA 42
4.5 DU/GA 46
5.0 DU/GA 48
5.5 DU/GA 50
6.0 DU/GA 52
6.5 DU/GA 54
7.0 DU/GA 56
Planned unit developments, % impervious
condominiums, apartments, must be computed
commercial business and
industrial areas.
(1) For a more detailed description of agricultural land use curve numbers refer to National Engineering
Handbook, Section 4, Hydrology, Chapter 9, August 1972.
(2) Assumes roof and driveway runoff is directed into street/storm system.
(3) The remaining pervious areas (lawn) are considered to be in good condition for these curve numbers.
3.52F3 11/92
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL.
where
Tc. = time of concentration (min), and
m = number of flow segments
Sheet Flow: Sheet flow is flow over plane surfaces. It usually occurs in the headwater of streams. With
sheet flow, the friction value (nj (a modified.Manning's effective roughness coefficient that includes the
effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and
erosion and transportation of sediment) is used. These n,values are for very shallow flow depths of about
0.1 foot and are only used for travel lengths up to 300 feet. Table 3.5.2.0 gives Manning's n,values for
sheet flow for various surface conditions.
For sheet flow up to 300 feet, use Manning's kinematic solution to directly compute T,:
Sheet flow: Tt = 0.42 (n,L)o.a
(P2) o.s (So) 0.4
where
T, = travel time (min),
n, = sheet flow Manning's,effective roughness coefficient (from Table 3.5.2C),
L = flow length (ft),
P2 = 2-year, 24-hour rainfall (in), (see Figure 3.5.1 C) and
S. = slope of hydraulic grade line (land slope, ft/ft)
Velocity Equation
A commonly used method of computing average velocity of flow, once it has measurable depth, is the
following equation: -
-
V = k /s—.—
where:
V = velocity (ft/s)
k = time of concentration velocity factor (ft/s)
s, = slope of flow path (ft/ft)
"ku is computed for various land covers and channel characteristics with assumptions made for hydraulic
radius using the following rearrangement of Manning's equation:
k = (1.49 (R) 0.667 )/n;
where
R = an assumed hydraulic radius
n = Manning's roughness coefficient for open channel flow (from Table 4.3.7E in Chapter 4)
J
3.5.2-6 1/90
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.2C "n" AND "k" VALUES USED IN TIME CALCULATIONS FOR HYDROGR"HS
'n,'Sheet Flow Equation Manning's Values(For the Initial 300 it of travel) n..
Smooth surfaces(concrete,asphalt,gravel,or bare hard packed soli) 0.011
Fallow fields or loose sod surface(no residue) 0.05
Cultivated soil with residue cover(s <-0.20 tt/ft) 0.06
Cultivated sop with residue cover(S>0.20 tt/ft) 0.17
Short prairie grass and lawns 0.15
0.24
Dense grasses
0.41
Bermuda grass
Range(natural)
0.13
Woods or forest with light underbrush 0.40~-
Woods or forest with dense underbrush 0.80
*Manning values for sheet flow only,from Overton and Meadows 1976(See TR-55,1986)
'k'Values Used In Travel Time/Time of Concentration Calculations
Shallow Concentrated Flow (After the ini ial.300 ft.of sheet now,R -0.1) k,
1. Forest with heavy ground litter and meadows(n=0.10) 3
2. Brushy ground with some trees(n-0.060) 5
3. Fallow or minimum tillage cultivation(n -0.040) 8
4. High grass(n = 0.035) 9
5. Short grass,pasture and lawns(n-0.030) 11
e Nearly bare ground(n-0.025) 13
7. Paved and gravel areas(n-0.012)_--}_: 27
Channel Flow(Intermittent)(At the beginning of visible channels:R-0.2) k,
1. Forested Swale with heavy ground litter(n -0.10) 5
2. Forested drainage course/ravine with defined channel bed(n-0.050) 10
3. Rock-lined waterway(n=0.035) 15
4. Grassed waterway(n=0.030) 17
5. Earth-lined waterway(n=0.025) 20
6. CMP pipe(n=0.024) 21
7. Concrete pipe(0.012) 42
8. Other waterways and pipes 0.508/n
Channel Flow(Continuous stream,R -0.4) k,
9. Meandering stream with some pods(n -0.040) 20
10. Rock-lined stream(n-0.035) 23
11. Grass-lined stream(n-0.030) 27
12. Other streams,man-made channels and pipe 0.807/n••
--See Chapter S.Table 5.3.6C for addit nal Mannings'n'values for open channels
i
3.5.2-7 1/90
i
CALCULATE Qexisting
TO BE`DEEDED TO BY LOT UNE ADJU T TO RUN
CONCURRENT WITH ORT FND. BAR •
10' S. RM DRAI & CA LS#827 ZX
30.00 S 1'07"E 330A7 EASE ENT `10-1 99
1 _— 150.00
00 � L 180•
v$' 4�C�I �S
N ACCESS
PRIVAOAD 9'01'07'E
I-
70.0 J T
FND. REBAR
S89-43.59-E 150.00 I I o
.I 20.
& CAP LS#8271 & CAPE 0- cq n T 1 /_ I
w 10-7-99 0 f 049
\ � 1
\ `� I 10' STORM OR
(� �c.r EASEMENT
I \ S8 1'07' 110.01
LOT 1
o LOT 4 I Sa �
Q� p
�Q) FND. REBAR N �sb S89'01 Q 40.0 I �k
O� & CAP LS#8271
V� `L 10-7-99 I J
w
in
62 SF L
N G $ �0 55 T S W 7544 S
N z
in toLOT 2 I
tO p
c0 p
I�
N FND. REBAR
a & CAP LS#8271 --- - -- - - - -- - - - - —
10-7-99
50.00 .00 01
N89'43'59" .01
ND. REBAR FND. REBA 10' SANI Y
-=ALF. & CAP LS#30427 & CAP LS#3 -7 EASEMEN
10-7-99 10-7-99 & CAP LS#8271 v'
10-7-99
Q
PARCEL LOT 1 LOT 2 L
LOT 4
a LLA-010-86
�+ w WHISPERING PINE LANE
N KC REC. N0. 86092290. LUA-99-066-FP
KC REC NO. 19990702001247
z z
r
�ALC- CD
Z
1 o%,c 145.S9 = Z.Co ZCo
Z9r1y _S
O ca 8 PwC �---
Ct.I = 55
CL
wow = 18�'
40
O.lo8y
i
L-
j -
l
II�
i
I
I
--------------------------------------------------- -----------------
******************** S.C.S. TYPE-IA DISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP.
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(I PERV), CN(HAPERV), TC FOR BASIN NO. 1
0.68,55,0,0,22.7
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MIrtUTES)
A CN A CN
.7 .7 55.0 .0 .0 22.7
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.02 23.83 1208
CALCULATE Qdeveloped
TO BE-DEEDED TO
BY LOT LINE ADJI}S T TO RUM Z��b
FIND. BAR
1 30.00 S l'07'E 330.01 EASE ENT DR 271
10-7 99
$ N 346a r—
FPA'TBROAD
FND. REBAR S89 43'59'E 150.00 D. REB —f t _ .f n I C
10 CAP & CAP \ \ ° n 049 r t
I
\ `N I 10' STORM OR,
EASEMENT
• \ S8n1'07'E 10.01
LOT 1
I I $ a \ LOT 41
` r0
i' Q� FND. REBAR N `'o- S89'01 Q 40.0
�` O�� & CAP LS#8271
w . . . . . . . . .i��/ ��� . 10-7-99
^ ^ Q. _ d �G
I �J �Q T 4 • :`' n g ��
' 62 SF? Io T 3 7544 S
55N
�G
W I
N04
LOT 2
co
N FND. REBAR
I & CAP LS#8271 --- -- - - -- - - -
10-7-99 � _
. . . . . 50.00 .00 .01
CAP N89'43'59"W 01
do OEBS#30427 FNO. REBA 10' SANI Y
& CAP LS#3 .7 EASEMEN
10-7-99 10-7-99 & CAP LS#8271
10-7-99
PARCEL LOT 1 LOT 2
J LOT 4
LLA-010-86 J WHISPERING PINE LANE
W
r N KC REC. N0. 86092290 LUA-99-066—FP
N
o KC REC NO. 19990702001247
_ z
\ I
I
C�� r
ar.
QLr
Rc ovL'-5 &Am
Zti
l�s 1 - 4
t
y -iZt.S- S C 84 53.
14 a-a.
-
n . COS - o .-tZ
!hi fl i .w�
----------------------------------------------------------------------
******************** S.C.S. TYPE-IA DISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP. *********
---------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IIVIPERV), CN(IMPERV), TC FOR BASIN NO. 1
0.36,68,0.32,98,6.3
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TCNINUTES)
A CN A CN
.7 .4 68.0 .3 98.0 6.3
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.38 7.83 5750
LASS O.S �'fS
tJo 'i�'i-�,t�T1 onl
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
Infiltration facilities must be designed based on infiltration testing and a soils report prepared by a
professional civil engineer with expertise in soil engineering. To maintain outflow rates of the
infiltration tanks and ponds, all inflow must be pretreated for sediment removal (see Section 5.4).
An emergency overflow path must be identified for infiltration facilities and noted on the engineering
plan. This overflow path must be analyzed to meet the requirements of Core Requirements #1 (see
Section 1.2.1) and #2 (see Section 1.2.2) for the 100-year, 24-hour duration design storm, except
Downspout Infiltration Systems (see Section 4.5.1).
Infiltration facilities may be especially useful in the following circumstances, provided the proper soil
conditions are present and all requirements can be met.
(1) The proposed project discharges to a closed depression.
(2) The proposed project discharges to a severely undersized conveyance system that
restricts the runoff volume that can be accommodated.
(3) The proposed project is in a Critical Drainage Area requiring runoff volume control.
----3111P Exemptions From On-Site Peak Rate Runoff Control
On-site peak rate runoff control will not be required for a proposed project in the following
situations.
Neolioible Peak Runoff Rate Increase':
(1) The proposed project site post-developed peak runoff rate for the 100-year, 24-hour
` . duration design storm event is calculated for each discharge location' to be less
than 0.5 cfs more than the peak runoff rate for the existing site conditions; OR,
(2) The project proposes to construct 5,000 square feet, or less, of new impervious
surface.
Direct Discharae: The proposed project will discharge surface and stormwater runoff without on-
site peak rate runoff control directly-;to:
A Regional Facility. Direct discharge of surface and stormwater runoff to a regional facility
will be allowed if:
o the facility has been demonstrated to adequately control the proposed project's
increased peak rate of runoff by an adopted King County basin plan or by a detailed
drainage analysis approved by the SWM Division; AND
o the facility will be available by the time of construction of the project; AND
o the conveyance system to the regional facility can accommodate, with no significant
adverse impact to the drainage systems, the design peak runoff for the proposed
project site and the equivalent area' developed to the full zoning potential.
' Proposed projects in adopted critical drainage areas, basin plans and community plans requiring peak runoff rate
or runoff volume controls more strict than standard controls shall not qualify for this exemption.
' A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple
natural discharge locations that combine within 1 i4-mile downstream.
' The equivalent area is the area tributary to the receiving water body equal to or less than the shortest, straight
line distance from the discharge point from the receiving water body (or regional facility) to the furthermost
point of the proposed development.
1.2.3-5' 11194
y
REVISED DRAINAGE REPORT
FOR
REDWOOD LANE SHORT PLAT
27xx JONES ROAD N.E.
CITY of REKTON
RECEIVED
h'I OCT 2 3 2000
CUS t ON,,a,
z
9800 w
S
0NAL�
EXPIRES ,Z-I 1 - Q�
OCTOBER 2000
Contact: Engineer:
Rex Orkney Del Erickson, P.E
Orkney Homes & Development 15020 S.E. 46th St.
PMB No. 462 Bellevue, WA. 98006
218 Main Street Tel: 425-747-5825
Kirkland, WA. 98083
Tel: 425-766-3298
t
NARRATIVE
NARRATIVE
The subject site, located at 27xx Jones Ave. N.E., is presently
undeveloped. The site is covered with evergreen and deciduous trees, brush,
native undergrowth (See the existing conditions exhibit). The parcel is
rectangular in shape, 150.01' x 146.26', with a 24' x 150' handle to the west
that allows for access off of Jones Ave. N.E.. The proposed improvements
include the construction of a 20' wide asphalt access road, storm drainage,
sanitary sewer, water services and 4 single family residences (See the
developed conditions exhibit).
Contours fall across the parcel from the southwesterly corner to the
northeasterly comer on a slope of approximately 10%. No offsite flow enters
the parcel from the north or the east as they lie downslope. Minimal flow
enters from the south, as the property is presently developed with single
family structures and only their backyards will runoff onto the site. No flow
enters the site from the west as the street improvements from Jones Ave. N.E.
control drainage from the right of way and direct the flow to the north.
The site lies downslope from the improvements in Jones Ave. N.E.,
therefore storm drainage will be collected on site and directed to the existing
24" diameter storm drain system located about 60 feet to the east of the
parcel's easterly property line. Runoff from the access road will be collected
in 4 Type 1 and 1 Type 2 catch basins and conveyed easterly in 8 inch
diameter storm drain to an outfall into a bio swale located along the the east
property line of Lot 1. This bio swale will convey the flow northerly to a
Type 1 catch basin located at the northeast corner of Lot 1. Outfall from this
catch basin is easterly to a connection with the City's existing 24 inch
diameter storm drain lying immediately to the east of the site. Each lot will
connect it's downspout and driveway drains to this outfall line through 4 inch
and 6 inch storm drain piping.
No detention is required for this site as the proposed project site post-
developed peak runoff rate for the 100 year, 24 hour duration design storm
event is calculated to be less than 0.5 cfs more than the peak runoff rate for
the existing site conditions. See the attached exhibits and calculations for
additional information.
Biofiltration is required for the site as there is more than 5000 square
feet of impervious area that is subject to vehicular use. Water Quality is not
required for the site as the proposed project contains less than 1 acre of new
impervious surface that will be subject to vehicular use. See calculations and
plans for bio swale sizing, design details, and location.
SIZE WATER QUALITY SYSTEM
TO BE-DEEDED TO
BY LOT LINE-ADJtJ TO RtJM Z?b:=; .
/ FND. BAR I
-11
30.00 1'0TE 330.Ot EASE AI
ENT OR 271 r-
;7 � w� s'7n1i:�+..`: ,tc�`�+,�-an-..L,. J,� .:C 'r_. �.',�'' _� �^� .'��Nv+�r.�.—t. ��•_—/� �''-•. 1. s.� ..
99
p N�^ x, g eEis1J- .JC a+l. "s 4. `S F Lt 4ci.r f:1►li . . __.— _. — —
FND. REBAR S89 43'59'E 150.00 D. RE8
I & CAP LS#8271 t �'
10-7-99 do CAP 10- g � ao f c
W � I 049 I L"
Z ;
99
N I 10' STORM OR
EASEMENT j
+ 1 S89Q71'07"E 10. cm
LOT 1 — Q LOT a
tn
XX
lbI5�O� FND. REBAR N Tb S89ro1 p 401:6 .0
. 00 1 CAP lS#8271�
i� . . . . . . . .. . .
0-7-99 c �
p• °�
T 4 co
� $ G
G _ S r 62 SF i4 I T 3 55 544 S I
n tn LOT 2
N N In
-
Cf
N FND. REBAR
1 do CAP LS#8271 --- -- - -- -- - _
10-7-99
. . 50.00 .00 .01
N89'43'59'W _ Ot
FND. REBAR FND. REBA 10' SANI Y _
& CAP LS#30427 do CAP LS#3 7 F EASEMEN
I 10-7-99 10-7-99 do CAP LS#8271 ".
- 10-7-99
PARCEL LOT 1 LOT -2 L LOT 4
LLA-010-86
KC REC. N0. 86092290 WHISPERING PINE LANE
N _ LUA-99-066-FP
M I KC REC NO. 19990702001247
n
o '
o
z
1-�a•�lt�t-�� Zoe- �S = -1 oc�.o
Z Co Z�S xZSZ J - ZCoS 'a
Z.43• ZD,c.�tS
o k CIO ac.
T�z -Z . o `
7L = L.-5
--------------------------------------------
****** ************* S.C.S. TYPE-IA DISTRIBUTION ********************
********* 2-YEAR 24-HOUR STORM **** 2 00" TOTAL PRECIP. *********
------------------------------------- --
ENTER A(PERV), CN(PERV), A(IMPERV), CNM"ERV), TC FOR BASIN NO. 1
0,0,0.16,98,6.3
DATA PRINT-OUT: -
AREA(ACRES) PERVIOUS IMPERVIOUS TCNINUTES)
A CN A CN
.2 .0 .0 .2 98.0 6.3
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.08 7.67 1030
'i
�1"
,
= C> o,2) G-
mA
Q - t.4�A
+
A b
0. o
D. alb Z.1
= (C .-:5 410 4-0.3S '
�Ili
�II
- u Z
+ �
i
o.Z + Z ti o 25 4 .4
- �o
1 i`
I►
i -A 70 =
S.A. =
b = 71zs
H E bo--m M Q trrn4 = -t.ZS'
. t�
CY de%4 cm
i
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1.2.3 CORE REQUIREMENT #3: RUNOFF CONTROL
Proposed projects must provide runoff control through a combination of peak rate runoff control
and on-site biofiltration as described below.
Peak Rate Runoff Control
Proposed projects must provide peak rate runoff control to limit the developed conditions peak
rates of runoff from specific design storm events not to exceed the peak rates for the "existing
site conditions" described below. Three basic methods for peak rate runoff control are possible:
detention, retention, and infiltration. Detention is the collection and temporary storage of surface
water (typically over several hours) with the outflow rate restricted--usually to the pre-developed
outflow rate. Retention is the collecting and holding of surface and stormwater with effectively no
surface outflow (outflow occurs by evapotranspiration). Infiltration is the soaking of surface water
into the ground (typically for several hours or days).
Infiltration not only reduces or eliminates surface runoff, but also helps to maintain the hydrologic
balance of the surface water system. Infiltration can limit erosion and recharge groundwaters that
supply water to wetlands, streams, and wells. Preserving infiltration after development is by far
the most effective mechanism in preventing adverse impacts to the surface water system.
Because of these benefits, King County encourages the use of infiltration systems for runoff control
where the appropriate soil conditions exist.
Proposed project peak rate runoff control must be located on-site. An exemption from on-site peak
rate runoff control may be granted for the special conditions specified at the end of this core
requirement section.
Biofiltration
Proposed project runoff resulting from more than five thousand square feet of new impervious
surface' for each threshold discharge areas, and subject to vehicular use or storage of chemicals,
shall be treated prior to discharge from the project site by on-site biofiltration measures as
described in Section 4.6.4. Installation of a biofiltration facility (biofiltration swale or filter strip) to
meet Core Requirement #3 does not exempt the applicant from installing a special water quality
control facility (wetpond, wetvault or water quality swale) to meet Special Requirement #5.
The biofiltration design flow rate shall be the peak rate of runoff for the 2-year 24-hour duration
design storm event. Note, biofiltration facilities installed following peak rate runoff control facilities
may be sized to treat the allowable release rate (existing site conditions) for the 2-year 24-hour
duration design storm event for the peak rate runoff control facility. Biofiltration facilities installed
prior to peak rate runoff control facilities shall be sized based on the developed conditions.
Proposed Project Site "Existing Site Conditions"
In performing the analysis for the design of runoff control, it is essential to first determine the
proposed project site "existing site conditions" from which the pre-development runoff rates can be
computed for specific design storms. Existing site conditions are not always synonymous with
those of the natural, totally undeveloped site. In some instances substantial modifications (such as
diversions, piping, clearing, and grading) have already increased and altered surface water runoff
leaving the site, but no permit, nor accompanying engineering plan, was ever approved. In other
instances, an approved drainage system exists and the existing system must be analyzed for its
performance. There are two definitions for proposed project site "existing site conditions"
depending on the site.
5 See Definition Section
s A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple
natural discharge locations that combine within 1/4-mile downstream.
1.2.3-1 11/92
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1.3.5 SPECIAL REQUIREMENT #5: SPECIAL WATER QUALITY CONTROLS
Threshold Requirement
IF any threshold discharge area" of THEN the threshold discharge area"
the proposed project contains more shall have a wetpond meeting the
than 1 acre of new impervious standards described below employed
surface that will be subject to (in addition to peak rate control
vehicular use or storage of chemicals requirements) to treat a project's
and: runoff prior to discharge from the site.
A wetvault or water quality swale, as
(a) proposes direct discharge of described below, may be used when a
runoff to a regional facility, wetpond is not feasible because of
receiving water, lake, wetland, or physical site constraints or impacts to
closed depression without on-site sensitive areas.
peak rate runoff control; OR
(b) proposes discharge of runoff
through overland flow or on-site
infiltration into a Class 1 or 2
stream, or Class 1 wetland, within
one mile radius downstream from
the project site.
Wetponds or Wetvault
Wetponds and Wetvaults contain a permanent pool of water depending on the frequency and
quantity of inflow, and (for Wetponds) the rate of permeability of the underlying soil. The
Wetponds and Wetvaults fill with the initial onset of frequent storms such that the major portion of
the volume of runoff is treated. The principal mechanism of treatment is settlement due to
establishment of calm conditions. In Wetponds treatment is enhanced by the mechanisms of
biofiltration and biologic activity.
The size (water surface area and volume) of the Wetpond or Wetvaults shall be determined as
follows:
(1) The design water surface area in the Wetpond or Wetvault shall be a minimum of
one percent of the impervious surface area in the drainage sub-basin contributing to
the facility.
(2) The design volume of-the Wetpond or Wetvault shall be a minimum of the total
volume of runoff (see Chapter 3: Hydrologic Analysis) from the tributary sub-basin
proposed developed conditions using a water quality design storm event having a
total precipitation (Pt-wq), where Pt-wq is one-third of the two-year, 24-hour total
precipitation (PZ/3, P2 is obtained from Figure 3.5.1 C). This water quality design
storm event approximates the runoff from the mean annual storm event.
Other design criteria are found in Section 4.6.2.
" A threshold discharge area is an on-site area which drains to a single natural discharge location
or multiple natural discharge locations that combine within 1/4-mile downstream.
i 1.3.5-1 11/92
SOILS INFORMATION
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
(2) CN values can be area weighted when they apply to pervious areas of similar CN's (within 20
�\ CN points). However, high CN areas should not be combined with low CN areas (unless the
low CN areas are less than 15% of the subbasin). In this case, separate hydrographs should be
generated and summed to form one hydrograph.
FIGURE 3.5.2A HYDROLOGIC SOIL GROUP OF THE SOILS IN KING COUNTY
HYDROLOGIC HYDROLOGIC
SOIL GROUP GROUP* SOIL GROUP GROUP*
Alderwood C Orcas Peat D
Arents, Alderwood Material C Oridia D
Arents, Everett Material B Ovall C
Beausite C Pilchuck C
Bellingham D Puget D
Briscot D Puyallup B
Buckley D Ragnar B
Coastal Beaches Variable Renton D
Earlmont Silt Loam D Riverwash Variable
Edgewick C Salal C
Everett A/B �,Sammamish D
Indianola A . Seattle D
Kitsap Shacar D
Klaus C Si Silt C
Mixed Alluvial Land Variable Snohomish D
Neilton A Sultan C
Newberg B Tukwila D
Nooksack C Urban Variable
Normal Sandy Loam D Woodinville D
HYDROLOGIC SOIL GROUP CLASSIFICATIONS
A. (Low runoff potential). Soils having high infiltration rates, even when thoroughly wetted, and consisting
chiefly of deep, well-to-excessively drained sands or gravels. These soils have a high rate of water
transmission. ,
B. (Moderately low runoff potential). Soils having moderate infiltration rates when thoroughly wetted, and
consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of
water transmission.
C. (Moderately high runoff potentiai). Soils having slow infiltration rates when thoroughly wetted, and
consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately
fine to fine textures. These soils have a slow rate of water transmission.
D. (High runoff potential). Soils having very.slow infiltration rates when thoroughly wetted and consisting
chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a
hardpan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils
have a very slow rate of water transmission.
* From SCS, TR-55, Second Edition,June 1986, Exhibit A-1. Revisions made from SCS, Soil Interpretation
Record, Form #5, September 1988.
f 315.2-2 11/92 ,
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'' A I m Ur
II Ev!B v) t
•KPDyr BeC
$ 2 AgD
BM 0
PD ♦ r1 1 I: I E9 BeD •eC
qgC 8
• �• Bh •.b' .. � Bh �••ti
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BM May a KpC Ya �. •EvC I .
N.
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----- OvD Q g N
\\ —. -- ------ — ---
• AgD -- 90�ABM
•• AgC AgC 692
No ` BMA o I
♦ ii• •�: I 605
Sm
j c�, '1 �• •i n BeC
A C �1 _ ••. i AgC
qgD•' 1 AgD 34
gennydale w Agc �. ;i, is
y •InC AkF I•"•� i�l AkF
IAkF AgC EvB i
EvC
Coleman Poin Q• Py GRAVEL
P/T
AkF w 306
InA' J j' zl AgC
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BM• 0g AgD
Evc/ Z=LL gC
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C AMeV
Plant
- :RdE goo•ate'
tENTON /.9 M/. 12'30" (Joins sheet 7 1) RdC 10'
RENTON 1.7 Ml.
N
Scale 1:24 000
gravelly coarse sand to very gravelly loamy sand. the presence of a consolidated substratum at a depth
Depth to the IIC horizon ranges from 18 to 36 of 7 to 20 feet. This substratum is the same mate-
inches. rial� as that in the Alderwood soils.
Some areas are up to 5 percent included Alderwood Some areas are up to 5 percent included Norma,
soils, on the more rolling and undulating parts of Seattle, and Tukwila soils, all of which are poorly
the landscape; some are about 5 percent the deep, drained.
sandy Indianola soils; and some are up to 25 percent Runoff is slow to medium, and the erosion hazard
Neilton very gravelly loamy sands. Also included is slight to moderate.
in mapping are areas where consolidated glacial till, Most of the acreage is used for timber. Capabil-
which characteristically underlies Alderwood soils, ity unit VIs-1; woodland group 3f3.
is at a depth of 5 to 15 feet.
Permeability is rapid. The effective rooting
depth is 60 inches or more. Available water capac- Indianola Series
ity is low. Runoff is slow, and the erosion hazard
is slight. The Indianola series is made up of somewhat
This soil is used for timber and pasture and for excessively drained soils that formed under conifers
urban development. Capability unit IVs-1; woodland in sandy, recessional, stratified glacial drift.
group 3f3. These undulating, rolling, and hummocky soils are on
terraces. Slopes are 0 to 30 percent. The annual
Everett gravelly sandy loam, 5 to 15 percent precipitation is 30 to 55 inches, and the mean
slopes (EvQ --This soil is rolling. Areas are annual air temperature is about 500 F. The frost-
irregular in shape, have a convex surface, and range free season is 150 to 210 days. Elevation ranges
from 25 acres to more than 200 acres in size. Run- from about sea level to 1,000 feet.
off is slow to medium, and the erosion hazard is In a representative profile, the upper 30 inches
slight to moderate. is brown, dark yellowish-brown, and light olive-
Soils included with this soil in mapping make up brown loamy fine sand. This is underlain by olive
no more than 25 percent of the total acreage. Some sand that extends to a depth of 60 inches or more
areas are up to 5 percent Alderwood soils, which (pl. I, right).
overlie consolidated glacial till; some are up to Indianola soils are used for timber and for urban
20 percent Neilton very gravelly loamy sand; and development.
some are about 15 percent included areas of Everett
soils where slopes are more gentle than 5 percent —low Indianola loamy fine sand, 4 to lSpercent slopes
and where they are steeper than 15 percent. (InC)-.=-This undulating and rolling soil has convex
This Everett soil is used for timber and pasture slopes. It is near the edges of upland terraces.
and for urban development. Capability unit.VIs-1; Areas range from 5 to more than 100 acres in size.
woodland group 3f3. Representative profile of Indianola loamy fine
sand, 4 to 15 percent slopes, in forest, 1,000 feet
Everett gravelly sandy loam, 15 to 30 percent west and 900 feet south of the northeast corner of
slopes (hvD) .--This soil occurs as long, narrow sec. 32, T. 25 N., R. 6 E.
areas, mostly along drainageways or on short slopes
between terrace benches. It is similar to;�-Everett 01--3/4 inch to 0, leaf litter.
gravelly sandy loam, 0 to 5 percent slopes, but in B21ir--0 to 6 inches, brown (10YR 4/3) loamy fine
most places is stonier and more gravelly. -sand, brown (IOYR 5/3) dry; massive; soft,
Soils included with this soil in mapping make up very friable, nonsticky, nonplastic; many
no more than 30 percent of the total acreage. Some roots; slightly acid; clear, smooth boundary.
areas are up to 10 percent Alderwood soils,-which 4 to 8 inches thick.
overlie consolidated glacial till; some are up to 5 B22ir--6 to 15 inches, dark yellowish-brown (10YR
percent the deep, sandy Indianola soils; some are 4/4) loamy fine sand, brown (10YR 5/3) dry;
up to 10 percent Neilton very gravelly loamy sand; massive; soft, very friable, nonsticky, non-
and some are about 15 percent included areas of plastic; common roots; slightly acid; clear,
Everett soils where slopes are less than 15 percent. smooth boundary. 6 to 15 inches thick.
Runoff is medium to rapid, and the erosion hazard C1--15 to 30 inches, light olive-brown (2.5Y 5/4)
is moderate to severe. loamy fine sand, yellowish brown (10YR 6/4)
Most of the acreage is used for timber. Capa- dry; massive; soft, very friable, nonsticky,
bility unit VIe-1; woodland group 3f2. nonplastic; common roots; slightly acid;
gradual, smooth boundary. 12 to 17 inches
Everett-Alderwood gravelly sandy loamy, 6 to 15 thick.
percent slopes (EwC) —This mapping unit is about C2--30 to 60 inches, olive (5Y 5/4) sand, light
equal parts Everett and Alderwood soils. The soils brownish gray (2.SY 6/2) dry; single grain;
are rolling. Slopes are dominantly 6 to 10 percent, loose, nonsticky, nonplastic; few roots;
but range from gentle to steep. Most areas are slightly acid. Many feet thick.
irregular in shape and range from 15 to 100 acres
or more in size. In areas classified as Everett There is a thin, very dark brown Al horizon at
soils, field examination and geologic maps indicate the surface in some places. The B horizon ranges
16
from very dark grayish brown to brown and dark and the mean annual air temperature is about SO F.
• yellowish brown. The C horizon ranges from dark The frost-free season is 150 to more than 200 days.
grayish brown to pale olive and from loamy fine sand Elevation ranges from about sea level to 500 feet.
to sand. Thin lenses of silty material are at a In a representative profile, the surface layer
depth of 4 to 7 feet in some places. and subsoil are very dark brown and dark yellowish-
Soils included with this soil in mapping make up brown silt loam that extends to a depth of about 24
no more than 25 percent of the total acreage. Some inches. The substratum is olive-gray silty clay
areas are up to 10 percent Alderwood soils, on the loam. It extends to a depth of 60 inches or more.
more rolling and undulating parts of the landscape; Kitsap soils are used for timber and pasture.
some are up to 8 percent the deep, gravelly Everett
and Neilton soils; some are up to 1S percent Kitsap Kitsap silt loam, 2 to 8 percent slopes (KpB) .--
soils, which have platy lake sediments in the sub- This undulating soil is on low terraces of the major
soil; and some are up to 15 percent Ragnar soils, valleys of the Area. Areas range from 5 acres to
which have a sandy substratum. more than 600 acres in size and are nearly circular
Permeability is rapid. The effective rooting to irregular in shape. Some areas are one-eighth to
depth is 60 inches or more. Available water capac- a half mile wide and up to 3 or 4 miles long.
ity is moderate. Runoff is slow to medium, and the Representative profile of Kitsap silt loam, 2 to
erosion hazard is slight to moderate. 8 percent slopes, in pasture, 820 feet west and 330
This soil is used for timber and for urban devel- feet south of east quarter corner of sec. 28, T. 25
opment. Capability unit IVs-2; woodland group 4s3. N. , R. 7 E. :
Indianola loamy fine sand, 0 to 4 percent slopes Ap--O to 5 inches, very dark brown (10YR 2/2) silt
(InA) .--This soil occupies smooth terraces in long loam, dark grayish brown (10YR 4/2) dry; mod-
narrow tracts adjacent to streams. Areas range from erate, medium, granular structure; slightly
about 3 to 70 acres in size. hard, very friable, nonsticky, nonplastic;
Soils included with this soil in mapping make up many roots; medium acid; abrupt, smooth bound-'
no more than 20 percent of the total acreage. Some ary.
areas are up to 5 percent Alderwood soils, on the B2--5 to 24 inches, dark yellowish-brown (10YR 3/4)
more rolling and undulating parts of the landscape; silt loam, brown (10YR 5/3) dry; 2 percent
some are about 10 percent the deep, gravelly Everett iron concretions; weak, coarse, prismatic
and Neilton soils; some are up to 10 percent Indian- structure; slightly hard, friable, slightly
ola loamy fine sand that has stronger slopes; and sticky, slightly plastic; many roots; slightly
some areas are up to 10 percent the poorly drained acid; abrupt, wavy boundary. 18 to 21 inches
Norma, Shalcar, Tukwila soils. thick.
Runoff is slow, and the erosion hazard is slight. IIC--24 to 60 inches, olive-gray (5Y 5/2) silty clay
This soil is used for timber. Capability unit loam, light gray (5Y 7/2) dry; many, medium
IVs-2; woodland group 4s3. and coarse, prominent mottles of dark yellowish
brown and strong brown (10YR 4/4 and 7.5YR
Indianola loamy fine sand, 15 to 30 percent 5/8); moderate, thin and medium, platy struc-
slopes (InD) .--This soil is along entrenched-st-reams. ture; hard, firm, sticky, plastic; few roots
Soils included with this soil in mapping make up to a depth of 36 inches, none below; strongly
no more than 25 percent of the total acreage. Some acid.
areas are up to 10 percent Alderwood soils; some are
about 5 percent the deep, gravelly Everett and Neil- The A horizon ranges from very dark brown to dark
ton soils; some are up to 15 percent Kitsap soils, brown. The B horizon ranges from dark yellowish
which have platy, silty lake sediments in the sub- brown to dark brown and from silt loam to silty clay
soil; and some are up to 15 percent Indianola loamy loam. The platy IIC horizon ranges from grayish
fine sand that has milder slopes. brown to olive gray and from silt loam to silty clay
Runoff is medium, and the erosion hazard is moder- loam that has thin lenses of loamy fine sand in
ate to severe. places. Brownish mottles are common in the upper
This soil is used for timber. Capability unit part of the IIC horizon.
VIe-1; woodland group 4s2. Some areas are up to 10 percent included Alderwood
gravelly sandy loam; some are up to S percent the
very deep, sandy Indianola soils; and some are up to
Kitsap Series 5 percent the poorly drained Bellingham, Tukwila,
and Seattle soils.
The Kitsap series is made up of moderately well Water flows on top of the substratum in winter.
drained soils that formed in glacial lake deposits, Permeability is moderate above the substratum and
under a cover of conifers and shrubs. These soils very slow within it. The effective rooting depth is
are on terraces and strongly dissected terrace about 36 inches. Available water capacity is moder-
fronts. They are gently undulating and rolling and ate to moderately high. Runoff is slow to medium,
moderately steep. Slopes are 2 to 70 percent. and the erosion hazard is slight to moderate.
Platy, silty sediments are at a depth of 18 to 40 This soil is used for timber and pasture. Capabil-
inches. The annual precipitation is 35 to 60 inches, ity unit IIIe-1; woodland group 2d2.
17
EXHIBITS
Op AD i
awe;®�
poll
a low ��M� � �j 11� ��•
I sm
���� ' ' �� °yak ��L�,.,'!�•-� .,
w` � •��1 lit ' -►�__- � •
��IIMIi���i A Ir.�•.
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.2B SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS
SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS (Published by SCS in 1982)
Runoff curve numbers for selected agricultural, suburban and urban land use for Type 1A
rainfall distribution, 24-hour storm duration.
CURVE NUMBERS BY
HYD OGIC SOIL GROUP
LAND USE DESCRIPTION 25 B C D
Cultivated land(1): winter condition 86 91 94 95
Mountain open areas: low growing brush and grasslands 74 82 89 92
Meadow or pasture: 65 78 85 89
Wood or forest land: undisturbed or older second growth 64 76 81
Wood or forest land: young second growth or brush 55 72 81 86
Orchard: with cover crop 81 88 92 94
Open spaces, lawns, parks, golf courses, cemeteries,
landscaping.
good condition: grass cover on 75%
or more of the area 80 86 90
fair condition: grass cover on 50%
to 75% of the area 77 85 90 92
Gravel roads and parking lots 76 85 89 91
Dirt roads and parking lots 72 82 87 89
Impervious surfaces, pavement, roofs, etc. 9 98 98 98
Open water bodies: lakes, wetlands, ponds, etc. 100 100 100 100
Single Family Residential (2)
Dwelling Unit/Gross Acre % Impervious (3)
1.0 DU/GA 15 Separate curve number
1.5 DU/GA 20 shall be selected
2.0 DU/GA 25 for pervious and
2.5 DU/GA 30 impervious portion
3.0 DU/GA 34 of the site or basin
3.5 DU/GA 38
4.0 DU/GA 42
4.5 DU/GA 46
5.0 DU/GA 48
5.5 DUJGA 50
6.0 DU/GA 52
6.5 DU/GA 54
7.0 DU/GA 56
Planned unit developments, % impervious
condominiums, apartments, must be computed
commercial business and
industrial areas.
(1) For a more detailed description of agricultural land use curve numbers refer to National Engineering
Handbook, Section 4, Hydrology, Chapter 9, August 1972.
(2) Assumes roof and driveway runoff is directed into street/storm system.
(3) The remaining pervious areas (lawn) are considered to be in good condition for these curve numbers.
3.5.2(3 11/92
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
j where
T� = time of concentration (min), and
m = number of flow segments
Sheet Flow: Sheet flow is flow over plane surfaces. It usually occurs in the headwater of streams. With
sheet flow, the friction value (nj (a modified.Manning's effective roughness coefficient that includes the
effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and
erosion and transportation of sediment) is used. These n,values are for very shallow flow depths of about
0.1 foot and are only used for travel lengths up to 300 feet. Table 3.5.2.0 gives Manning's n,values for
sheet flow for various surface conditions.
For sheet flow up to 300 feet, use Manning's kinematic solution to directly compute T,:
Sheet flow: Tt = 0.42 (n,L)o.a
T2) 0.5 (So) 0.4
where
T, = travel time (min),
n, = sheet flow Manning's.effective roughness coefficient (from Table 3.5.2C),
L = flow length (ft),
P2 = 2-year, 24-hour rainfall (in), (see Figure 3.5.1 C) and
S. = slope of hydraulic grade line (land slope, ft/ft)
}
Velocity Equation
A commonly used method of comp-uting average velocity of flow, once it has measurable depth, is the
following equation: -
V = k �
where:
V = velocity (ft/s)
k = time of concentration velocity factor (ft/s)
so = slope of flow path (ft/ft)
"k" is computed for various land covers and channel characteristics with assumptions made for hydraulic
radius using the following rearrangement of Manning's equation:
k = (1.49 (R) 0.667 )/n;
where
R = an assumed hydraulic radius
n = Manning's roughness coefficient for open channel flow (from Table 4.3.76 in Chapter 4)
,i
I 3.5.2-6 1/90
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.2C "n" AND 'rk" VALUES USED IN TEk4E CALCULATIONS FOR HYDROGRAPHS
'n,'Sheet Flow Equation Manning's Values(For the Initial 300 It of travel) n..
Smooth surfaces(concrete,asphalt,gravel,or bare hard packed sod) 0.011
Fallow fields or loose sod surface(no residue) 0'05
Cultivated sod with residue cover(s <- 0.20 ft/ff) 0.06
Cultivated sod with residue ewer(S>0.20 ft/ft) 0.17
Short prairie grass and lawns 0.15
0.24
Dense grasses
0.41
Bermuda grass
Range(natural)
0.13
Woods or forest with light underbrush 0-40-d*---
Woods or forest with dense underbrush 0.80
*Manning values for sheet flow only,from Overton and Meadows 1976(See TR-55.1986)
Y Values Used In Travel Time/Time of Concentration Calculations
Shallow Concentrated Flow (After the inhial-500 R of sheet flow,R -0.1) k,
1. Forest with heavy ground litter and meadows(n-0.10) 3
2. Brushy ground with some trees(n-0.060) 5
3. Fallow or minimum tillage cultivation(n -0.040) 8
4. High grass(n - 0.035) 9
S. Short grass,pasture and lawns(n-0.030) 11
6, Neady bare ground(n-0.025) 13
7. Paved and gravel areas(n-0.012)- -i_ 27
Channel Flow(intermittent)(At the beginning of visible channels:R-0.2) k�
1. Forested Swale with heavy ground liner(n =0.10) 5
2. Forested drainage course/ravine with defined channel bed(n=0.050) 10
3. Rock-lined waterway(n=0.035) 15
4. Grassed waterway(n-0.030) 17
5. Earth-lined waterway(n=0.025) 20
6. CMP pipe(n=0.024) 21
7. Concrete pipe(0.012) 42
8. Other waterways and pipes 0.508/n
Channel Flow(Continuous stream,R -0.4) kit
9. Meandering stream with some pods(n-0.040) 20
10. Rock-lined stream(n-0.035) 23
it. Grass-lined stream(n-0.030) 27
12. Other streams.man-made channels and pipe 0.807/n••
**See Chapter 5.Table 5.3.6C for additional Mannings'n'values for open channels
3.5.2-7 1/190
CALCULATE Qexisting
TO BE`DEEDED TO
BY LOT UNE AD,JU T TO RUN
CONCURRENT WITH ORT
FND. BAR
/ - EA S. M DRAT r& CA LS#827116(
30.00 — —— 150.00 S 1 0TE 330.d1 EASE ENT �! 10-7 99
L1���,9*01'0-rE
t
PRIVATBROAD 70.0 J I
S89143'591E 150.00 —� 20. 0 I ( c FND. REBAR 0. REB � aT 1
.I & CAP LS#8271 & CAP 10- 9 ^ n 049
w 10-7-99
N
'z 10' STORM OR
w L_ --1 EASEMENT
` 58n1'07' 10.01
co
— �
LOT 1 $ a �� LOT 4
2
�o
i4 OOP FND. REBAR N �sb S89'01 Q 40.0 ( 3
4blo �c & CAP LS#8271�
. . . . . . . . . . . . . o��r� � . . . . . . . . . 10-7-99 . .
Li Q CG
W JP� � - / ccrn ��
.. I M n
-, �, o' T 4 L �, o
w o I �Q G� M 62 SF 1 T
N ' 'to cl ��� Q-F' $ ( 55 W 7544 S
to LOT 2 l M
tn
Oi
N .i & CAPELS#8271 --- - -- - - - \--
- - - - —
10-7-99 50.00 .00 .01
N89'43'59" .01
ND. REBAR FND. REBA 10' SANI Y
& CAP LS#30427 & CAP LS#3 7 EASEMEN l
10-7-99 10-7-99 do CAP LS#8271 \
-�-r-� 10-7-99
PARCEL LOT 1 LOT 2 L T L T 4 }
J
°W"- w LLA-010-86 WHISPERING PINE LANE
C KC REC. NO. 86092290,
LUA-99-066—FP
( KC REC NO. 19990702001247
z z I
` I
\ I
CP�LC. CR)
z
t 5.a9 = ZCo ZCc t .-1
M% %A 14
Z9"1Zt_,_S
A. �---
o.
t o1b n = o .40
w �ZsZ .CD
S u e
Oy
GJ$ _ ._.4�C'
;I
" 27 1 rn1nu�lt �
�i
1 -
i
I
I�
----------------------------------------------------------------------
******************** S.C.S. TYPE-IA DISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP.
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(RAPERV), CN(EMPERV), TC FOR BASIN NO. 1
0.68,55,0,0,22.7
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(NIINUTES)
A CN A CN
.7 .7 55.0 .0 .0 22.7
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.02 23.83 1208
CALCULATE Qdeveloped
TO BE-DEEDED TO
BY LOT LINE .ADJUS TO RUM
FND. BAR
30.00 S 1'07'E 330.01 EASE ENT DR 1`0AI
271 A
AI & CA LS#8
TR14C - 80.
3460-:SF C — — —
PRIV�'TEROAIJ -�
. ACCF.S EMENT
99
� T �
FND. REBAR S89-43-59-E 150.00 D. REB ( C o
.I p C7 P99S#8271 & CAP 10- g \ 0. aq
^ OT 1 f o
W \ CO I 049 r
Z !
0*4 ( 10' STORM DR,-
EASEMENT
` • \ S89V1'3TE 10.01
om
LOT 1 — pi °' LOT 4
O I Cj N d
. �- Z
i� Q) FND. REBAR N '66 S89'01 Q 40.0 �c
& CAP LS8271
. . . . . . . . . . . . .�4v� 10'. . 10-7-99. I uj
�G
P "
►.-, ���Q��G en62 SF I o T 3 544 S
C14N _ 55 _ W
n LOT 2 SC.PrLE�
N �� ,
N FND. REBAR
& CAP LS#8271 — —— — — — - - -- - —
10-7-99
. . . . . . . . . . . . . . . . . . . . . . . . . . 50.00 .00 .01
-41
N89'43'59"W .01
FND. REBAR FND. REBA 10' SANI Y
& CAP LS#30427 & CAP LS#3 7 EASEMEN
10-7-99 10-7-99 & CAP LS8271
10-7-99
PARCEL LOT 1 LOT 2
L07 4
LLA-010-86
' '�' WHISPERING PANE LANE
N KC REC. NO. 86092290 LUA-99-066-FP
PO
KC REC NO. 19990702001247
c
0
z
CND
To-�-p► S o.Co 8 Aq-
CQ =98
NCC5 ,9S-,m ZQ �� AC--0a.-Q---
L 1 - 4 Cxg*-M�
C� 3ZAve_
o . Cos - o .3Z = o.-!-.G Ate.
----------------------------------------------------------------------
******************** S.C.S. TYPE-IA DISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP. *********
---------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(IlVIPERV), CN(RvIPERV), TC FOR BASIN NO. 1
0.36,68,0.32,98,6.3
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
.7 .4 68.0 .3 98.0 6.3
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.38 7.83 5750
LASS O. S G'�S
Knob.
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
Infiltration facilities must be designed based on infiltration testing and a soils report prepared by a
professional civil engineer with expertise in soil engineering. To maintain outflow rates of the
infiltration tanks and ponds, all inflow must be pretreated for sediment removal (see Section 5.4).
An emergency overflow path must be identified for infiltration facilities and noted on the engineering
plan. This overflow path must be analyzed to meet the requirements of Core Requirements #1 (see
Section 1.2.1) and #2 (see Section 1.2.2) for the 100-year, 24-hour duration design storm, except
Downspout Infiltration Systems (see Section 4.5.1).
Infiltration facilities may be especially useful in the following circumstances, provided the proper soil
conditions are present and all requirements can be met.
(1) The proposed project discharges to a closed depression.
(2) The proposed project discharges to a severely undersized conveyance system that
restricts the runoff volume that can be accommodated.
(3) The proposed project is in a Critical Drainage Area requiring runoff volume control.
Exemptions From On-Site Peak Rate Runoff Control
On-site peak rate runoff control will not be required for a proposed project in the following
situations.
Nealioible Peak Runoff Rate Increase':
(1) The proposed project site post-developed peak runoff rate for the 100-year, 24-hour
`— duration design storm event is calculated for each discharge location' to be less
than 0.5 cfs more than the peak runoff rate for the existing site conditions; OR,
(2) The project proposes to construct 5,000 square feet, or less, of new impervious
surface.
Direct Discharge: The proposed project will discharge surface and stormwater runoff without on-
site peak rate runoff control directfy,.;t
A Regional Facility. Direct discharge of surface and stormwater runoff to a regional facility
will be allowed if:
o the facility has been demonstrated to adequately control the proposed project's
increased peak rate of runoff by an adopted King County basin plan or by a detailed
drainage analysis approved by the SWM Division; AND
o the facility will be available by the time of construction of the project; AND
o the conveyance system to the regional facility can accommodate, with no significant
adverse impact to the drainage systems, the design peak runoff for the proposed
project site and the equivalent area' developed to the full zoning potential.
' Proposed projects in adopted critical drainage areas, basin plans and community plans requiring peak runoff rate
or runoff volume controls more strict than standard controls shall not qualify for this exemption.
' A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple
natural discharge locations that combine within 1/4-mile downstream.
' The equivalent area is the area tributary to the receiving water body equal to or less than the shortest, straight
line distance from the discharge point from the receiving water body (or regional facility) to the furthermost
point of the proposed development.
AM 1.2.3-5 I1/94
,wr°-7- - Z8'g 2
REVISED DRAINAGE REPORT
FOR
REDWOOD LANE SHORT PLAT
27xx JONES ROAD N.E.
OF �j Ony OF REM(W
RECE WD
OCT 3
�Q 9800 w� c,�. .
IS
9fON ti
EXPRES
OCTOBER 2000
Contact: Engineer:
Rex Orkney Del Erickson, P.E
Orkney Homes & Development 15020 S.E. 46th St.
PMB No. 462 Bellevue, WA. 98006
218 Main Street Tel: 425-747-5825
Kirkland, WA. 98083
Tel: 425-766-3298
NARRATIVE
NARRATIVE
The subject site, located at 27xx Jones Ave. N.E., is presently
undeveloped. The site is covered with evergreen and deciduous trees, brush,
native undergrowth (See the existing conditions exhibit). The parcel is
rectangular in shape, 150.01' x 146.26', with a 24' x 150' handle to the west
that allows for access off of Jones Ave. N.E.. The proposed improvements
include the construction of a 20' wide asphalt access road, storm drainage,
sanitary sewer, water services and 4 single family residences (See the
developed conditions exhibit).
Contours fall across the parcel from the southwesterly corner to the
northeasterly corner on a slope of approximately 10%. No offsite flow enters
the parcel from the north or the east as they lie downslope. Minimal flow
enters from the south, as the property is presently developed with single
family structures and only their backyards will runoff onto the site. No flow
enters the site from the west as the street improvements from Jones Ave. N.E.
control drainage from the right of way and direct the flow to the north.
The site lies downslope from the improvements in Jones Ave. N.E.,
therefore storm drainage will be collected on site and directed to the existing
24" diameter storm drain system located about 60 feet to the east of the
parcel's easterly property line. Runoff from the access road will be collected
in 4 Type 1 and 1 Type 2 catch basins and conveyed easterly in 8 inch
diameter storm drain to an outfall into a bio swale located along the the east
property line of Lot 1. This bio swale will convey the flow northerly to a
Type 1 catch basin located at the northeast corner of Lot 1. Outfall from this
catch basin is easterly to a connection with the City's existing 24 inch
diameter storm drain lying immediately to the east of the site. Each lot will
connect it's downspout and driveway drains to this outfall line through 4 inch
and 6 inch storm drain piping.
No detention is required for this site as the proposed project site post-
developed peak runoff rate for the 100 year, 24 hour duration design storm
event is calculated to be less than 0.5 cfs more than the peak runoff rate for
the existing site conditions. See the attached exhibits and calculations for
additional information.
Biofiltration is required for the site as there is more than 5000 square
feet of impervious area that is subject to vehicular use. Water Quality is not
required for the site as the proposed project contains less than 1 acre of new
impervious surface that will be subject to vehicular use. See calculations and
plans for bio swale sizing, design details, and location.
SIZE WATER QUALITY SYSTEM
TO BE-DEEDED TO
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' KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1.2.3 CORE REQUIREMENT #3: RUNOFF CONTROL
Proposed projects must provide runoff control through a combination of peak rate runoff control
and on-site biofiitration as described below.
Peak Rate Runoff Control
Proposed projects must provide peak rate runoff control to limit the developed conditions peak
rates of runoff from specific design storm events not to exceed the peak rates for the "existing
site conditions" described below. Three basic methods for peak rate runoff control are possible:
detention, retention, and infiltration. Detention is the collection and temporary storage of surface
water (typically over several hours) with the outflow rate restricted--usually to the pre-developed
outflow rate. Retention is the collecting and holding of surface and stormwater with effectively no
surface outflow (outflow occurs by evapotranspiration). Infiltration is the soaking of surface water
into the ground (typically for several hours or days).
Infiltration not only reduces or eliminates surface runoff, but also helps to maintain the hydrologic
balance of the surface water system. Infiltration can limit erosion and recharge groundwaters that
supply water to wetlands, streams, and wells. Preserving infiltration after development is by far
the most effective mechanism in preventing adverse impacts to the surface water system.
Because of these benefits, King County encourages the use of infiltration systems for runoff control
where the appropriate soil conditions exist.
Proposed project peak rate runoff control must be located on-site. An exemption from on-site peak
rate runoff control may be granted for the special conditions specified at the end of this core
requirement section.
Biofiltration
Proposed project runoff resulting from more than five thousand square feet of new impervious
surfaces for each threshold discharge areas, and subject to vehicular use or storage of chemicals,
shall be treated prior to discharge from the project site by on-site biofiltration measures as
described in Section 4.6.4. Installation of a biofiltration facility (biofiltration swale or filter strip) to
meet Core Requirement #3 does not exempt the applicant from installing a special water quality
control facility (wetpond, wetvault or water quality swale) to meet Special Requirement #5.
- r-
The biofiltration design flow rate shall be the peak rate of runoff for the 2-year 24-hour duration
design storm event. Note, biofiltration facilities installed following peak rate runoff control facilities
may be sized to treat the allowable release rate (existing site conditions) for the 2-year 24-hour
duration design storm event for the peak rate runoff control facility. Biofiltration facilities installed
prior to peak rate runoff control facilities shall be sized based on the developed conditions.
Proposed Project Site "Existing Site Conditions"
In performing the analysis for the design of runoff control, it is essential to first determine the
proposed project site "existing site conditions" from which the pre-development runoff rates can be
computed for specific design storms. Existing site conditions are not always synonymous with
those of the natural, totally undeveloped site. In some instances substantial modifications (such as
diversions, piping, clearing, and grading) have already increased and altered surface water runoff
leaving the site, but no permit, nor accompanying engineering plan, was ever approved. In other
instances, an approved drainage system exists and the existing system must be analyzed for its
performance. There are two definitions for proposed project site "existing site conditions"
depending on the site.
5 See Definition Section
s A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple
natural discharge locations that combine within 1/4-mile downstream.
I I1/92
1.2.3-1
i
I
' KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1.3.5 SPECIAL REQUIREMENT #5: SPECIAL WATER QUALITY CONTROLS
Threshold Requirement
IF any threshold discharge area" of THEN the threshold discharge area"
the proposed project contains more shall have a wetpond meeting the
than 1 acre of new impervious standards described below employed
surface that will be subject to (in addition to peak rate control
vehicular use or storage of chemicals requirements) to treat a project's
and: runoff prior to discharge from the site.
A wetvault or water quality swale, as
(a) proposes direct discharge of described below, may be used when a
runoff to a regional facility, wetpond is not feasible because of
receiving water, lake, wetland, or physical site constraints or impacts to
closed depression without on-site sensitive areas.
peak rate runoff control; OR
(b) proposes discharge of runoff
through overland flow or on-site
infiltration into a Class 1 or 2
stream, or Class 1 wetland, within
one mile radius downstream from
the project site.
Wetponds or Wetvault
Wetponds and Wetvaults contain a permanent pool of water depending on the frequency and
quantity of inflow, and (for Wetponds) the rate of permeability of the underlying soil. The
Wetponds and Wetvaults fill with the initial onset of frequent storms such that the major portion of
the volume of runoff is treated. The principal mechanism of treatment is settlement due to
establishment of calm conditions. In Wetponds treatment is enhanced by the mechanisms of
biofiltration and biologic activity.
The size (water surface area and volume) of the Wetpond or Wetvaults shall be determined as
follows:
(1) The design water surface area in the Wetpond or Wetvault shall be a minimum of
one percent of the impervious surface area in the drainage sub-basin contributing to
the facility.
(2) The design volume of"the Wetpond or Wetvault shall be a minimum of the total
volume of runoff (see Chapter 3: Hydrologic Analysis) from the tributary sub-basin
proposed developed conditions using a water quality design storm event having a
total precipitation (Pt-wq), where Pt-wq is one-third of the two-year, 24-hour total
precipitation (P213, Pz is obtained from Figure 3.5.1 C). This water quality design
storm event approximates the runoff from the mean annual storm event.
Other design criteria are found in Section 4.6.2.
" A threshold discharge area is an on-site area which drains to a single natural discharge location
or multiple natural discharge locations that combine within 1/4-mile downstream.
1.3.5-1 11/92
SOILS INFORMATION
KING COUNTY, ' WASHINGTON, SURFACE WATER DESIGN MANUAL
(2) CN values can be area weighted when they apply to pervious areas of similar CN's (within 20
CN points). However, high CN areas should not be combined with low CN areas (unless the
low CN areas are less than 15% of the subbasin). In this case, separate hydrographs should be
generated and summed to form one hydrograph.
FIGURE 3.5.2A HYDROLOGIC SOIL GROUP OF THE SOILS IN KING COUNTY
HYDROLOGIC HYDROLOGIC
SOIL GROUP GROUP* SOIL GROUP GROUP*
Alderwood C Orcas Peat D
Arents, Alderwood Material C Oridia D
Arents, Everett Material B Ovall C
Beausite C Pilchuck C
Bellingham D Puget D
Briscot D Puyallup B
Buckley D Ragnar B
Coastal Beaches Variable Renton D
Earlmont Silt Loam D Riverwash Variable
Edgewick C Salal C
Everett A/B °Sammamish D
Indianola A Seattle D
Kitsap Shacar D
baus C Si Silt C
Mixed Alluvial Land Variable Snohomish D
Neilton A Sultan C
Newberg B Tukwila D
Nooksack C Urban Variable
Normal Sandy Loam D Woodinville D
HYDROLOGJC SOIL GROUP CLASSIFICATIONS
A. (Low runoff potential). Soils having high infiltration rates, even when thoroughly wetted, and consisting
chiefly of deep, well-to-excessively drained sands or gravels. These soils have a high rate of water
transmission.
B. (Moderately low runoff potential). Soils having moderate infiltration rates when thoroughly wetted, and
consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of
water transmission.
C. (Moderately high runoff potentiai). Soils having slow infiltration rates when thoroughly wetted, and
consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately
fine to fine textures. These soils have a slow rate of water transmission.
D. (High runoff potential). Soils having very.slow infiltration rates when thoroughly wetted and consisting
chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a
hardpan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils
have a very slow rate of water transmission.
* From SCS, TR-55, Second Edition,June 1986, Exhibit A-1. Revisions made from SCS, Soil Interpretation
Record, Form #5, September 1988.
i 3J5.2-2 11/92 ;
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eENTON I'D mi. 1290" (Joins sheet 7 1) RdC 10'
RENTON 7.7 Ml.
Scale 1:24 000 N
gravelly coarse sand to very gravelly loamy sand. the presence of a consolidated substratum at a depth
Depth to the IIC horizon ranges from 18 to 36 of 7, to 20 feet. This substratum is the same mate-
inches'. rial as that in the Alderwood soils.
Some areas are up to 5 percent included Alderwood Some areas are up to 5 percent included Norma,
soils, on the more rolling and undulating parts of Seattle, and Tukwila soils, all of which are poorly
the landscape; some are about 5 percent the deep, drained.
sandy Indianola soils; and some are up to 25 percent Runoff is slow to medium, and the erosion hazard
Neilton very gravelly loamy sands. Also included is slight to moderate.
in mapping are areas where consolidated glacial till, Most of the acreage is used for timber. Capabil-
which characteristically underlies Alderwood soils, ity unit VIs-l; woodland group 3f3.
is at a depth of 5 to 15 feet.
Permeability is rapid. The effective rooting
depth is 60 inches or more. Available water capac- Indianola Series
ity is low. Runoff is slow, and the erosion hazard
is slight. The Indianola series is made up of somewhat
This soil is used for timber and pasture and for excessively drained soils that formed under conifers
urban development. Capability unit IVs-1; woodland in sandy, recessional, stratified glacial drift.
group 3f3. These undulating, rolling, and hummocky soils are on
terraces. Slopes are 0 to 30 percent. The annual
Everett gravelly sandy loam, 5 to 15 percent precipitation is 30 to 55 inches, and the mean
slopes (EvC) .--This soil is rolling. Areas are annual air temperature is about 50' F. The frost-
irregular in shape, have a convex surface, and range free season is 150 to 210 days. Elevation ranges
from 25 acres to more than 200 acres in size. Run- from about sea level to 1,000 feet.
off is slow to medium, and the erosion hazard is In a representative profile, the upper 30 inches
slight to moderate. is brown, dark yellowish-brown, and light olive-
Soils included with this soil in mapping make up brown loamy fine sand. This is underlain by olive
no more than 25 percent of the total acreage. Some sand that extends to a depth; of 60 inches or more
areas are up to 5 percent Alderwood soils, which (pl. I, right).
overlie consolidated glacial till; some are up to Indianola soils are used for timber and for urban
20 percent Neilton very gravelly loamy sand; and development.
some are about 15 percent included areas of Everett
soils where slopes are more gentle than 5 percent -top Indianola loamy fine sand, 4 to 15 percent slopes
and where they are steeper than 15 percent. (InC)-.=-This undulating and rolling soil has convex
This Everett soil is used for timber and pasture slopes. It is near the edges of upland terraces.
and for urban development. Capability unit.VIs-1; Areas range from 5 to more than 100 acres in size.
woodland group 3f3. Representative profile of Indianola loamy fine
sand, 4 to 15 percent slopes, in forest, 1,000 feet
Everett gravelly sandy loam, 15 to 30 percent west and 900 feet south of the northeast corner of
slopes (EVD) .--This soil occurs as long, narrow sec. 32, T. 25 N., R. 6 E.:
areas, mostly along drainageways or on short slopes
between terrace benches. It is similar io�Everett 01--3/4 inch to 0, leaf litter.
gravelly sandy loam, 0 to 5 percent slopes, but in B21ir--0 to 6 inches, brown (10YR 4/3) loamy fine
most places is stonier and more gravelly. -sand, brown (10YR 5/3) dry; massive; soft,
Soils included with this soil in mapping make up very friable, nonsticky, nonplastic; many
no more than 30 percent of the total acreage. Some roots; slightly acid; clear, smooth boundary.
areas are up to 10 percent Alderwood soils, which 4 to 8 inches thick.
overlie consolidated glacial till; some are up to 5 B22ir--6 to 15 inches, dark yellowish-brown (10YR
percent the deep, sandy Indianola soils; some are 4/4) loamy fine sand, brown (10YR 5/3) dry;
up to 10 percent Neilton very gravelly loamy sand; massive; soft, very friable, nonsticky, non-
and some are about 15 percent included areas of plastic; common roots; slightly acid; clear,
Everett soils where slopes are less than 15 percent. smooth boundary. 6 to 15 inches thick.
Runoff is medium to rapid, and the erosion hazard C1--15 to 30 inches, light olive-brown (2.5Y 5/4)
is moderate to severe. loamy fine sand, yellowish brown (10YR 6/4)
Most of the acreage is used for timber. Capa- dry; massive; soft, very friable, nonsticky,
bility unit VIe-1; woodland group 3f2. nonplastic; common roots; slightly acid;
gradual, smooth boundary. 12 to 17 inches
Everett-Alderwood gravelly sandy loamy, 6 to 15 thick.
percent slopes (EwC) .--This mapping unit is about C2--30 to 60 inches, olive (5Y 5/4) sand, light
equal parts Everett and Alderwood soils. The soils brownish gray (2.5Y 6/2) dry; single grain;
are rolling. Slopes are dominantly 6 to 10 percent, loose, nonsticky, nonplastic; few roots;
but range from gentle to steep. Most areas are slightly acid. Many feet thick.
irregular in shape and range from 15 to 100 acres
or more in size. In areas classified as Everett There is a thin, very dark brown Al horizon at
soils, field examination and geologic maps indicate the surface in some places. The B horizon ranges
16
from very dark grayish brown to brown and dark and the mean annual air temperature is about 500 F.
yellowish brown. The C horizon ranges from dark The frost-free season is 150 to more than 200 days.
grayish brown to pale olive and from loamy fine sand Elevation ranges from about sea level to 500 feet.
to sand. Thin lenses of silty material are at a In a representative profile, the surface layer
depth of 4 to 7 feet in some places. and subsoil are very dark brown and dark yellowish-
Soils included with this soil in mapping make up brown silt loam that extends to a depth of about 24
no more than 25 percent of the total acreage. Some inches. The substratum is olive-gray silty clay
areas are up to 10 percent Alderwood soils, on the loam. It extends to a depth of 60 inches or more.
more rolling and undulating parts of the landscape; Kitsap soils are used for timber and pasture.
some are up to 8 percent the deep, gravelly Everett
and Neilton soils; some are up to 15 percent Kitsap Kitsap silt loam, 2 to 8 percent slopes (KpB).--
soils, which have platy lake sediments in the sub- This undulating soil is on low terraces of the major
soil; and some are up to 15 percent Ragnar soils, valleys of the Area. Areas range from 5 acres to
which have a sandy substratum. more than 600 acres in size and are nearly circular
Permeability is rapid. The effective rooting to irregular in shape. Some areas are one-eighth to
depth is 60 inches or more. Available water capac- a half mile wide and up to 3 or 4 miles long.
ity is moderate. Runoff is slow to medium, and the Representative profile of Kitsap silt loam, 2 to
erosion hazard is slight to moderate. 8 percent slopes, in pasture, 820 feet west and 330
This soil is used for timber and for urban devel- feet south of east quarter corner of sec. 28, T. 25
opment. Capability unit IVs-2; woodland group 4s3. N., R. 7 E. :
Indianola loamy fine sand, 0 to 4 percent slopes Ap--O to 5 inches, very dark brown (10YR 2/2) silt
(InA) .--This soil occupies smooth terraces in long loam, dark grayish brown (10YR 4/2) dry; mod-
narrow -tracts adjacent to streams. Areas range from erate, medium, granular structure; slightly
about 3 to 70 acres in size. hard, very friable, nonsticky, nonplastic;
Soils included with this soil in mapping make up many roots; medium acid; abrupt, smooth bound-'
no more than 20 percent of the total acreage. Some ary.
areas are up to 5 percent Alderwood soils, on the B2--5 to 24 inches, dark yellowish-brown (10YR 3/4)
more rolling and undulating parts of the landscape; silt loam, brown (10YR 5/3) dry; 2 percent
some are about 10 percent the deep, gravelly Everett iron concretions; weak, coarse, prismatic
and Neilton soils; some are up to 10 percent Indian- structure; slightly hard, friable, slightly
ola loamy fine sand that has stronger slopes; and sticky, slightly plastic; many roots; slightly
some areas are up to 10 percent the poorly drained acid; abrupt, wavy boundary. 18 to 21 inches
Norma, Shalcar, Tukwila soils. thick.
Runoff is slow, and the erosion hazard is slight. IIC--24 to 60 inches, olive-gray (5Y 5/2) silty clay
This soil is used for timber. Capability unit loam, light gray (5Y 7/2) dry; many, medium
IVs-2; woodland group 4s3. and coarse, prominent mottles of dark yellowish
brown and strong brown (10YR 4/4 and 7.5YR
Indianola loamy fine sand, 15 to 30 percent 5/8) ; moderate, thin and medium, platy struc-
slopes (InD) .--This soil is along entrenched'str.eams. ture; hard, firm, sticky, plastic; few roots
Soils included with this soil in mapping make up to a depth of 36 inches, none below; strongly
no more than 25 percent of the total acreage. Some acid.
areas are up to 10 percent Alderwood soils; some are
about 5 percent the deep, gravelly Everett and Neil- The A horizon ranges from very dark brown to dark
ton soils; some are up to 15 percent Kitsap .s.oijs, brown. The B horizon ranges from dark yellowish
which have platy, silty lake sediments in the sub- brown to dark brown and from silt loam to silty clay
soil; and some are up to 15 percent Indianola loamy loam. The platy IIC horizon ranges from grayish
fine sand that has milder slopes. brown to olive gray and from silt loam to silty clay
Runoff is medium, and the erosion hazard is moder- loam that has thin lenses of loamy fine sand in
ate to severe. places. Brownish mottles are common in the upper
This soil is used for timber. Capability unit part of the IIC horizon.
VIe-1; woodland group 4s2. Some areas are up to 10 percent included Alderwood
gravelly sandy loam; some are up to 5 percent the
very deep, sandy Indianola soils; and some are up to
Kitsap Series 5 percent the poorly drained Bellingham, Tukwila,
and Seattle soils.
The Kitsap series is made up of moderately well Water flows on top of the substratum in winter.
drained soils that formed in glacial lake deposits, Permeability is moderate above the substratum and
under a cover of conifers and shrubs. These soils very slow within it. The effective rooting depth is
are on terraces and strongly dissected terrace about 36 inches. Available water capacity is moder-
fronts. They are gently undulating and rolling and ate to moderately high. Runoff is slow to medium,
moderately steep. Slopes are 2 to 70 percent. and the erosion hazard is slight to moderate.
Platy, silty sediments are at a depth of 18 to 40 This soil is used for timber and pasture. Capabil-
inches. The annual precipitation is 35 to 60 inches, ity unit IIIe-1; woodland group 2d2.
17
EXHIBITS
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KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5.213 SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS
SCS WESTERN WASHINGTON RUNOFF CURVE NUMBERS (Published by SCS in 1982)
Runoff curve numbers for selected agricultural, suburban and urban land use for Type 1A
rainfall distribution, 24-hour storm duration.
CURVE NUMBERS BY
HYDaQ,LOGIC SOIL GROUP
LAND USE DESCRIPTION .�) B C D
Cultivated land(1): winter condition 86 91 94 95
Mountain open areas: low growing brush and grasslands 74 82 89 92
Meadow or pasture: 65 78 85 89
Wood or forest land: undisturbed or older second growth 64 76 81
Wood or forest land: young second growth or brush 55 72 81 86
Orchard: with cover crop 81 88 92 94
Open spaces, lawns, parks, golf courses, cemeteries,
landscaping.
good condition: grass cover on 75%
or more of the area O 80 86 90
fair condition: grass cover on 50%
to 75% of the area 77 85 90 92
Gravel roads and parking lots 76 85 89 91
Dirt roads and parking lots 72 82 87 89
Impervious surfaces, pavement, roofs, etc. 98 98 98 98
Open water bodies: lakes, wetlands, ponds, etc. 100 100 100 100
Single Family Residential (2)
Dwelling Unit/Gross Acre % Impervious (3)
1.0 DU/GA 15 Separate curve number
1.5 DU/GA 20 shall be selected
2.0 DU/GA 25 for pervious and
2.5 DU/GA 30 impervious portion
3.0 DU/GA 34 of the site or basin
3.5 DU/GA 38
4.0 DU/GA 42
4.5 DU/GA 46
5.0 DU/GA 48
5.5 DU/GA 50
6.0 DU/GA 52
6.5 DU/GA 54
7.0 DU/GA 56
Planned unit developments, % impervious
condominiums, apartments, must be computed
commercial business and
industrial areas.
(1) For a more detailed description of agricultural land use curve numbers refer to National Engineering
Handbook, Section 4, Hydrology, Chapter 9, August 1972.
(2) Assumes roof and driveway runoff is directed into street/storm system.
(3) The remaining pervious areas (lawn) are considered to be in good condition for these curve numbers.
i 3.5.2F3 j 11/92
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL.
` where
T, = time of concentration (min), and
m = number of flow segments
Sheet Flow: Sheet flow is flow over plane surfaces. It usually occurs in the headwater of streams. With
sheet flow, the friction value (nj (a modified Manning's effective roughness coefficient that Includes the
effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and
erosion and transportation of sediment) is used. These n,values are for very shallow flow depths of about
0.1 foot and are only used for travel lengths up to 300 feet. Table 3.5.2.0 gives Manning's n,values for
sheet flow for various surface conditions.
For sheet flow up to 300 feet, use Manning's kinematic solution to directly compute T,:
Sheet flow: Tt = 0.42 (n,L)°'8
(P2) 0.5 (So) 0.4
where
T, = travel time (min),
n, = sheet flow Manning's.effective roughness coefficient (from Table 3.5.2%
L = flow length (ft),
PZ = 2-year, 24-hour rainfall (in), (see Figure 3.5.1 C) and
S. = slope of hydraulic grade line (land slope, ft/ft)
t
I .
Velocity Equation
A commonly used method of computing average velocity of flow, once it has measurable depth, is the
following equation:
V = k /s—.
where:
V = velocity (ft/s)
k = time of concentration velocity factor (ft/s)
s, = slope of flow path (ft/ft)
"k" is computed for various land covers and channel characteristics with assumptions made for hydraulic
radius using the following rearrangement of Manning's equation:
k = (1.49 (R) 0.11e7 )/n;
where
R = an assumed hydraulic radius
n = Manning's roughness coefficient for open channel flow (from Table 4.3.76 in Chapter 4)
3.5.2-6 1/90
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TABLE 3.5ZC "n" AND "k" VALUES USED IN T WE CALCULATIONS FOR HYDROGRAPHS
'n,'Sheet Flow Equation Mar"ng's Values(For the Initial 300 it of travel) n..
Smooth surfaces(concrete,asphalt,gravel,or bare hard packed sod) 0.011
Fallow fields or loose sod surface(no residue) 0.05
Cultivated sod with residue cover(s<-0.20 ft/ft) 0.06
Cultivated sod with residue cover(S>0.20 ft/ft) 0.17
Short prairie grass and lawns 0.15
0.24
Dense grasses
0.41
Bermuda grass
Range(natural)
0.13
Woods or forest with light underbrush 0.40-N&- �'
Woods or forest with dense underbrush 0.80
*Manning values for sheet flow only,from Overton and Meadows 1976(See TR-55,1986)
'k'Values Used In Travel Time/Time of Concentration Calculations
Shallow Concentrated Flow (After the inrtlal.300 ft.of sheet flow,R -0.1) k,
1. Forest with heavy ground litter and meadows(n-0.10) 3
2. Brushy ground with some trees(n-0.060) 5
3. Fallow or minimum tillage cultivation(n -0.040) 8
4. High grass(n -0.035) 9
S. ShoR grass.pasture and lawns(n-0.030) 11
e Nearly bare ground(n-0.025) 13
7. Paved and gravel areas(n-0.012)_�_: 27
Channel Flow(Intermittent)(At the beginning of visible channels:R-0.2) k�
1. Forested Swale with heavy ground litter(n =0.10) 5
2. Forested drainage course/ravine with defined channel bed(n-0.050) 10
3. Rock-lined waterway(n=0.035) 15
4. Grassed waterway(n-0.030) 17
5. Earth4ined waterway(n=0.025) 20
6. CMP pipe(n=0.024) 21
7. Concrete pipe(0.012) 42
8. Other waterways and pipes 0.508/n
Channel Flow(Continuous stream,R -0.4) k•
9. Meandering stream with some pods(n -0.040) 20
10. Rock-lined stream(n-0.035) 23
11. Grass-lined stream(n-0.030) 27
12. Other streams.man-made channels and pipe 0.807/n-
••See Chapter S.Table 5.3.6C for additional Manning$'n'values for open channels
3.5.2-7 V90
CALCULATE Qexisting
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---------------------------------------------------- -----------------
******************** S.C.S. TYPE-1ADISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP. **
----------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(RVIPERV), CN(RvIPERV), TC FOR BASIN NO. 1
0.68,55,0,0,22.7
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(NIINUTES)
A CN A CN
.7 .7 55.0 .0 .0 22.7
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.02 23.83 1208
CALCULATE Qdeveloped
TO BE- DEEDED TO
BY LOT LINE.ADJUS TO RUM Z���
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1 - ..
1 330.0t EASE ENT
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----------------------------------------------------------------------
******************** S.C.S. TYPE-IADISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP. *********
---------------------------------------------------------------------
ENTER: A(PERV), CN(PERV), A(HYIPERV), CN(Ev DERV), TC FOR BASIN NO. 1
0.36,68,0.32,98,6.3
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TCWJNUTES)
A CN A CN
.7 .4 68.0 .3 98.0 6.3
PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT)
.38 7.83 5750
ti
' KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
Infiltration facilities must be designed based on infiltration testing and a soils report prepared by a
professional civil engineer with expertise in soil engineering. To maintain outflow rates of the
infiltration tanks and ponds, all inflow must be pretreated for sediment removal (see Section 5.4).
An emergency overflow path must be identified for infiltration facilities and noted on the engineering
plan. This overflow path must be analyzed to meet the requirements of Core Requirements #1 (see
Section 1.2.1) and #2 (see Section 1.2.2) for the 100-year, 24-hour duration design storm, except
Downspout Infiltration Systems (see Section 4.5.1).
Infiltration facilities may be especially useful in the following circumstances, provided the proper soil
conditions are present and all requirements can be met.
(1) The proposed project discharges to a closed depression.
(2) The proposed project discharges to a severely undersized conveyance system that
restricts the runoff volume that can be accommodated.
(3) The proposed project is in a Critical Drainage Area requiring runoff volume control.
Exemptions From On-Site Peak Rate Runoff Control
On-site peak rate runoff control will not be required for a proposed project in the following
situations.
Nealioible Peak Runoff Rate Increase':
(1) The proposed project site post-developed peak runoff rate for the 100-year, 24-hour
duration design storm event is calculated for each discharge location' to be less
than 0.5 cfs more than the peak runoff rate for the existing site conditions; OR,
(2) The project proposes to construct 5,000 square feet, or less, of new impervious
surface.
Direct Discharne: The proposed project will discharge surface and stormwater runoff without on-
site peak rate runoff control direct+*-,W:
A Regional Facility. Direct discharge of surface and stormwater runoff to a regional facility
will be allowed if:
o the facility has been demonstrated to adequately control the proposed project's
increased peak rate of runoff by an adopted King County basin plan or by a detailed
drainage analysis approved by the SWM Division; AND
o the facility will be available by the time of construction of the project; AND
o the conveyance system to the regional facility can accommodate, with no significant
adverse impact to the drainage systems, the design peak runoff for the proposed
project site and the equivalent area' developed to the full zoning potential.
' Proposed projects in adopted critical drainage areas, basin plans and community plans requiring peak runoff rate
or runoff volume controls more strict than standard controls shall not qualify for this exemption.
' A threshold discharge area is an on-site area which drains to a single natural discharge location or multiple
natural discharge locations that combine within 1/4-mile downstream.
' The equivalent area is the area tributary to the receiving water body equal to or less than the shortest, straight
line distance from the discharge point from the receiving water body (or regional facility) to the furthermost
point of the proposed development.
1.2.34 11/94
o�
oo -
Short Plat'(SHPL#'
REQUEST FOR PROJECT#f Prelim Plat (PP#
CAG 9
060 4P Y
To: .Technical Services Date 1 2 - p (� WOE CP 7 rccn#
From: Plan Review/Project Manager
Project Name )K2 La S e a r Pj e4
(H)charact=mu)
Description of Project:
Circle Size of Waterline: 10" 12" Circle One: New or Extension
Circle Size of Sewerline: 8" 10" 12" Circle One: New or Extension
Circle Size of Stom-dine: 12" 15" 18" 24" Circle One: New or Extension l
• - � 2 -� �-� � � �� rat E Address or Street Na-me(s)
Dvlpr/Contractor/Owner/Cnslt::
(0 characters mu)
Check each discipline involved in Project Ltr Drwg #of sheets per discipline
Trans-Storm
(Roadwaybrainagc) (Off site improvcmentsKndudc basin name) (include TESC sheds)
❑ Transportation (sigsulizadon.Channclizadon•Lighting) ❑ ❑
4QII" Wastewater O. —
Sanitary Scwu Main(include basin name) /
la�Water (Mains,Valves,Hydrants) ®� t�
(Include composite G:Horizontal C
❑ Su face Water O ❑
[mprovements (CLP ONLY)(indudc basin name)
TS Use Only
w-rl2- - ;Lj - 2$1 W
st�v -a - 2��j� - -
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