HomeMy WebLinkAboutBLD2001-00301 Stormwater/Erosion FD") ECEOWE ni
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JUL 3 2001
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JEFFERSON COUNTY
DEPT. OF COMMUNITY DEVELOPMENT
STORMWATER PLAN &
LARGE PARCEL EROSION & SEDIMENT CONTROL PLAN
KYLE RESIDENCE
Proponent:
KELLY SHIELDS, INC.
260 Kala Point Drive
Port Townsend, WA 98368
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VAL' 7/'2/c
July 2, 2001
Report By:
Tillman Engineering, Inc.
P.O. Box 1375
Port Hadlock, Washington 98339 APPROVED
(360) 379-9661 JUL 12 2001
JEFFERSO •U
DEPT.OF COM i NETY D P NT
SIGNATURE:
TABLE OF CONTENTS
I. PROJECT OVERVIEW 3
II. PLOT PLAN - (SEE ATTACHMENT II, SITE PLAN) 3
III. CONDITIONS SUMMARY 3
IV. OFF SITE ANALYSIS 4
V. ANALYSIS AND DESIGN OF STORMWATER FACILITIES 4
VI. SPECIAL REPORTS AND STUDIES 5
VII. BASIN AND COMMUNITY PLANNING AREAS 5
VIII. OTHER PERMITS 5
IX. EROSION AND SEDIMENT CONTROL PLAN 5
LARGE PARCEL EROSION AND SEDIMENT CONTROL PLAN 6
PERMANENT STORMWATER QUALITY CONTROL PLAN 9
X. BOND QUANTITIES WORKSHEET 13
XI. MAINTENANCE AND OPERATIONS 13
ATTACHMENT I SCS MAP OF THE PROJECT LOCATION
ATTACHMENT II SITE PLAN
ATTACHMENT III DOE DESIGN CRITERIA
ATTACHMENT IV HYDRAULIC CALCULATIONS
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I. PROJECT OVERVIEW
The applicant proposes to construct a single family residence on Lot 9, Port Ludlow
Division No. 6, located on Osprey Road at Sparrow Lane. The parcel is located in a
portion of the NE 'A of the NE '/4 of Section 17, Township 28 North, Range 1 East, W.M.
Construction at the site received approval for clearing from the property owners
association in December, 2000, and work was begun prior to January 1, 2001. The site
was cleared of vegetation, with portions of the timber cut in the area of the proposed
residence and stored on-site for future private use. Temporary erosion control currently
in-place consists of hay bales at the swales crossing the property and an over-all seeding of
the site with a pasture type grass mixture. Due to the lack of rain, the seeds have not
germinated to date.
The total area disturbed is less than 5.0 acres; however, this stormwater plan will comply
with the"Stormwater Management Manual for the Puget Sound Basin" as published by
the Washington State Department of Ecology, February, 1992. The report will address
the requirements for the Small Parcel Erosion Control Plan and a Permanent Stormwater
Quality Control Plan per the Manual.
Single family residences are located to north, south and east of the property which fronts
onto Osprey Ridge Road. The development is part of the overall Port Ludlow Resort
community which has not been totally developed such that there are many lots with no
residences.
The site receives off-site drainage from a swale running in a general north/south direction
which will remain unaffected by the proposed construction with the exception of the
clearing. As noted, hay bales have been set in the flow lines of the swale to protect
downstream properties. The current drainage pattern consists of general sheet flow with
concentrations in the swales noted..
With the overall seeding of the property, the owner anticipates utilizing the property
surrounding the residence as horse pasture with the major trees left in place as the site
currently exists.
II. PLOT PLAN - (See Attachment II, Site Plan)
III. CONDITIONS SUMMARY
The site is currently vacant. Water and sewer service to the site is provided by the Port
Ludlow community utility company, the Olympic Water& Sewer.
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Topography of the site is south sloping, rolling with small knolls, at a fairly uniform rate,
with a differential of approximately 30 feet of elevation.
Surface Soil Conditions
The site soils are mapped in the SCS Soil Survey of Jefferson County Area Washington,
September 1975. The project is mapped as Alderwood series soils (A1C) and (AID).
Alderwood series soils are considered moderately well drained, gravelly soils, have
moderately slow permeability above the cemented layer, slow to medium runoff, and the
hazard of erosion from runoff is slight to moderate.
Water table is described as varying from 20 inches to 40 inches to a cemented layer.
Hardpan, or glacial tills, occur with 24 inches of the surface for most of the site. Based on
the SCS information, we do not expect this soil type to be erodible in the upper horizons.
Storm Drainage
It is estimated that the majority of storm water that accumulates on-site and that run-off
originating off site, but routed through the site, crosses the parcel to the south as shallow
subsurface flow or surface runoff. Due to the gentle site grades which do not allow a
steep hydraulic gradient, it is estimated that the parcel holds some water for a portion of
the year at relatively shallow depths. The surface soil type would let water migrate slowly
to the south along the upper boundary of the till surface.
IV. OFF SITE ANALYSIS
The site is unaffected by any stream or natural flowing body of water. There is a Type 4
stream approximately '/4 mile to the west as shown on the"Stream/Water Classification
Eastern Jefferson County" per the County IDMS mapping.
There appears to be no significant flow onto the property from the north. The storm water
plan will incorporate an on-site pond to infiltrate the 100 yr storm increase of stormwater
from the proposed residential construction. Therefore, there is no expected impact to the
downstream stormwater swales.
V. ANALYSIS AND DESIGN OF STORMWATER FACILITIES
Stormwater runoff from the developed property will be collected through site grading,
surface swales, and roof drains that are to be directed to a detention pond as shown on the
site plan.
Stormwater is to be detained in the pond and infiltrated into the soil in accordance with
the design parameters of Washington State Department of Ecology Stormwater
Management Manual for the Puget Sound basin.
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Table I - Approximate Site Areas
Item Summary
SITE 5.0 Ac.
Residence 2702 sft. 0.062 Ac.
Garage 1109 sft. 0.025
Driveway 2000 sft 0.045
Table II - Summary of Design Flows
Item Pre- Post
SITE 5.0 Ac.
2yr storm 0.03 cfs 0.08 cfs
25yr storm 0.03 cfs 0.09 cfs
100yr storm 0.05 cfs 0.11 cfs
VI. SPECIAL REPORTS AND STUDIES
There are no other known special reports or studies relevant to this specific parcel.
VII. BASIN AND COMMUNITY PLANNING AREAS
There is not a basin plan for this locale. Community Plans are currently being reviewed as
a part of the new Storm Water Utility for Port Ludlow that was formulated in 2000.
VIII. OTHER PERMITS
A building permit together with associated electrical work permits will be required for the
construction of the new buildings. Road approach permits and fire number application are
required for the site.
IX. EROSION AND SEDIMENT CONTROL PLAN
The project creates an area of impervious greater than 5000 square feet, and will disturb 1
acre or more of land. Minimum Requirements #1 through#11 of Sections I-2.5 -I-2.15
of the SWMM are therefore applicable. The Large Parcel Erosion and Sediment Control
Plan (LPESC) included here addresses Minimum Requirement #2 through#11.
LARGE PARCEL EROSION AND SEDIMENT CONTROL PLAN
This LPESC plan addresses the 15 items required under Minimum Requirement#1,
Erosion and Sediment Control. The PSQC plan addresses Minimum Requirements#2
through#11.
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MINIMUM REQUIREMENT #1: EROSION AND SEDIMENT CONTROL
Because the project includes land disturbing activity of greater than 1 acre, Minimum
Requirements #1 through#15 of section I-2.5 apply:
1. Stabilization and Sediment Trapping
Criteria: All exposed and unworked soils shall be stabilized by suitable
application of Best Management Practices (BMPs). From October 1 to April 30,
no soils shall remain unstabilized for more than 2 days. From May 1 to
September 30, no soils shall remain unstabilized for more than 7 days. Prior to
leaving the site, stormwater runoff shall pass through a sediment pond or
sediment trap, or other appropriate BMPs.
All exposed and unworked soils shall be stabilized by suitable application of BMPs.
From October 1 to April 30, no soils shall remain unstabilized for more than 2
days. From May 1 to September 30, no soils shall remain unstabilized for more
than 7 days. Prior to leaving the site, stormwater runoff shall pass through a
sediment pond or sediment trap, or other appropriate BMPs.
Prior to on-site building construction and final grading of the site, a stabilized
construction access to Osprey Ridge Road shall be constructed to the
specifications of the SWMM, and Jefferson County Public Works Department
(JCPW).
The roadway surface will be stabilized by the application of gravel base and
crushed surfacing as soon as possible after the grading is accomplished. Areas that
are disturbed will be revegetated by seeding and/or landscaping concurrent with
the grading and paving operations. Additional measures such as straw bale
barriers or silt fences may be employed as needed.
With the absence of any substantial rain, the seeding has not germinated; however,
there have not been any runoff problems for the same reason.
2. Delineate Clearing and Easement Limits
Criteria: In the field, mark clearing limits and/or any easements, setbacks,
sensitive/critical areas and their buffers, trees and drainage courses.
The property lines have been marked in accordance with the plat monuments.
There are no wetlands on this project site.
3. Protection of Adjacent Properties
Criteria: Properties adjacent to the project site shall be protected from sediment
deposition.
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Hay bales have been installed at all low points along the perimeter of the property
that drains generally to the south. As noted above, all disturbed areas have been
re-vegetated by seeding. Future landscaping will be installed concurrent with the
final grading and paving operations adjacent to the residence.
4. Timing and Stabilization of Sediment Trapping Measures
Criteria: Sediment ponds and traps,perimeter dikes, sediment barriers, and
other BMPs intended to trap sediment on-site shall be constructed as a first step
in grading. These BMPs shall be functional before land disturbing activities take
place. Earthen structures such as dams, dikes, and diversions shall be seeded and
mulched according to the timing indicated in Erosion and Sediment Control
Requirement#1.
The infiltration pond will be installed upon approval of this stormwater plan. As
noted, the adjacent property has been protected by the installation of hay bales at
the low points. After each storm, the installations will be monitored and if
warranted, silt fences will be installed to trap any sediment.
5. Cut and Fill Slopes
Criteria: Cut and fill slopes shall be designed and constructed in a manner that
will minimize erosion. In addition, slopes shall be stabilized in accordance with
Erosion and Sediment Control Requirement#1.
The clearing of the site has not altered the natural contour of the land. No
substantial cut or fill slopes are anticipated.
6. Controlling Off-Site Erosion
Criteria: Properties and waterways downstream from development sites shall be
protected from erosion due to increases in the volume, velocity, and peak flow
rate of stormwater runoff from the project site.
The plan incorporates an infiltration pond and no increase in the peak flow rate will
occur.
7. Stabilization of Temporary Conveyance Channels and Outlets
Criteria: All temporary on-site conveyance channels shall be designed,
constructed and stabilized to prevent erosion from the expected velocity of flaw
from a 2-year, 24-hour frequency storm for the developed condition. Stabilization
adequate to prevent erosion of outlets, adjacent streambanks, slopes and
downstream reaches shall be provided at the outlets of all conveyance.systems.
With the seeding of the property and the installation of hay bales, the velocity of
flow should not be increased. The hay bale installations will be monitored at the
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end of each storm.
8. Storm Drain Inlet Protection
Criteria: All storm drain inlets made operable during construction shall be
protected so that stormwater runoff shall not enter the conveyance system without
first being filtered or otherwise treated to remove sediment.
The proposed system utilizes surface flow and a detention pond to capture the
stormwater without the aid of storm drain inlets.
9. Underground Utility Construction
Criteria: The construction of underground utility lines shall be subject to the
following criteria:
(;) (i) Where feasible, no more than 500 feet of trench shall be opened at one time.
(ii) Where consistent with safety and space considerations, excavated material shall be
placed on the uphill side of trenches.
(;n) (iii) Trench dewatering devices shall discharge into a sediment trap or sediment pond.
Underground utility installation shall conform to the guidelines set forth in the
Washington State Department of Ecology Stormwater Management Manual for
the Puget Sound Basin, latest edition (SWMM).
10. Construction Access Routes
Criteria: Wherever construction vehicle access routes intersect paved roads,
provisions must be made to minimize the transport of sediment(mud) onto the
paved road If sediment is transported onto a road surface, the roads shall be
cleaned thoroughly at the end of each day. Sediment shall be removed from roads
by shoveling or sweeping and be transported to a controlled sediment disposal
area. Street washing shall be allowed only after sediment is removed in this
manner.
The contractor will be directed to maintain the approach to Osprey Ridge Road.
The stabilized access(s) will be kept free of mud and sediment.
11. Removal of Temporary BMPs
Criteria: All temporary erosion and sediment control BMPs shall be removed
within 30 days after final site stabilization is achieved or after the temporary
BMPs are no longer needed Trapped sediment shall be removed or stabilized on
site. Disturbed soil areas resulting from removal shall be permanently stabilized
All temporary erosion and sediment control measures shall be removed within
thirty days of final site stabilization or after the temporary measures are no longer
needed. Trapped sediment shall be removed or stabilized on site. Disturbed soil
areas resulting from removal of temporary BMPs shall be permanently stabilized.
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12. Dewatering Construction Sites
Criteria: Dewatering devices shall discharge into a sediment trap or sediment
Pond
No need for dewatering is anticipated.
13. Control of Pollutants Other Than Sediment on Construction Sites
Criteria: All pollutants other than sediment that occur on-site during
construction shall be handled and disposed of in a manner that does not cause
contamination of stormwater.
All pollutants that occur on-site during construction shall be handled and disposed
of in a manner that does not cause contamination of stormwater and that is
consistent with Washington State Department of Ecology Requirements.
14. Maintenance
Criteria: All temporary and permanent erosion and sediment control BMPs shall
be maintained and repaired as needed to assure continued performance of their
intended function. All maintenance and repair shall be conducted in accordance
with an approved manual.
All temporary and permanent erosion and sediment control measures employed
shall be maintained and repaired as needed to insure continued performance of
their intended function. All maintenance and repair shall be conducted in
accordance with the requirements of the SWMM.
15. Financial Liability
Criteria: Performance bonding, or other appropriate financial instruments, shall
be required for all projects to ensure compliance with the approved erosion and
sediment control plan.
Not applicable.
PERMANENT STORMWATER QUALITY CONTROL PLAN
The LPESC plan addresses the 15 items required under Minimum Requirement#1,
Erosion and Sediment Control. This PSQC plan addresses Minimum Requirements#2
through#11. This report describes the Minimum Requirement Criteria and proposed
mitigation. Please see site plan attached for details.
MINIMUM REQUIREMENT #2, PRESERVATION OF NATURAL DRAINAGE
SYSTEMS
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Criteria: Natural drainage patterns shall be maintained and discharges from the
site shall occur at the natural location, to the maximum extent practicable.
As described in the narrative, there are north/south traversing natural drainage
ways that cross the site. The natural flow will exit the parcel in sheet surface and
subsurface flows in the site soils to the southern side of the site. The natural
surface was not altered through the clearing and the re-vegetation by seeding of
native grasses will not alter the existing natural flow.
MINIMUM REQUIREMENT #3, SOURCE CONTROL OF POLLUTION
Criteria: Source control BMPs shall be applied to all projects to the maximum
extent practicable. Source control BMPs shall be selected, designed, and
maintained according to an approved manual.
An adopted and implemented basin plan (Minimum Requirement#9) may be used
to develop source control requirements that are tailored to a specific basin,
however, in all circumstances, source control BMPs shall be required for all sites.
During construction, proper maintenance of construction equipment, to eliminate
leaks and spills, as well as adherence to the erosion and sediment control
requirements as set forth in the LPESC Plan, should help alleviate the potential for
pollution. At build out, proper maintenance of stormwater facilities, in compliance
with Section XI of this report, as well as the proper application of nutrients and
pesticides to landscape areas should help lessen the potential for pollution on a
long term basis.
MINIMUM REQUIREMENT#4, RUNOFF TREATMENT BMPs
Criteria: All projects shall provide treatment of stormwater. Treatment BMPs
shall be sized to capture and treat the water quality design storm, defined as the
6-month, 24-hour return period storm. The first priority for treatment shall be to
infiltrate as much as possible of the water quality design storm, only if site
conditions are appropriate and ground water quality will not be impaired Direct
discharge of untreated stormwater to ground water is prohibited All treatment
BMPs shall be selected, designed, and maintained according to an approved
manual.
Stormwater treatment BMPs shall not be built within a natural vegetated buffer,
except for necessary conveyance systems as approved by the local government.
An adopted an implemented basin plan (Minimum Requirement#9) may be used
to develop runoff treatment requirements that are tailored to a specific basin.
All runoff from impervious areas will be directed to the to the detention pond.
Surface swales will serve to assist in assimilation of hydrocarbons such as oil
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dripped from cars, and spill control oil water separators in the catch basins will
prevent accidental spills from reaching the Bound water.
MINIMUM REQUIREMENT #5, STREAMBANK EROSION CONTROL
Criteria: The requirement below applies only to situations where stormwater
runoff is discharged directly or indirectly to a stream, and must be met in
addition to meeting the requirements in Minimum Requirement#4, Runoff
Treatment BMPs:
Stormwater discharges to streams shall control streambank erosion by limiting
the peak rate of runoff from individual development sites to 50 percent of the
existing condition 2-year, 24-hour design storm while maintaining the existing
condition peak runoff rate for the 10-year, 24-hour and 100-year, 24-hour design
storms. As the first priority, streambank erosion control BMPs shall utilize
infiltration to the fullest extent practicable, only if site conditions are appropriate
and ground water quality is protected Streambank erosion control BMPs shall be
selected designed and maintained according to an approved manual.
Stormwater treatment BMPs shall not be built within a natural vegetated buffer,
except for necessary conveyance systems as approved by the local government.
An adopted and implemented basin plan (Minimum Requirement#9) may be used
to develop streambank erosion control requirements that are tailored to a specific
basin.
Not Applicable.
MINIMUM REQUIREMENT #6, WETLANDS
Criteria: The requirements below apply only to situations where stormwater
discharges directly or indirectly through a conveyance system into a wetland, and
must be met in addition to meeting the requirements in Minimum Standard#4,
Runoff Treatment BMPs.
(a) Stormwater discharges to wetlands must be controlled and treated to the extent necessary to
meet the State Water Quality Standards, Chapter. 173-201 WAC, or Ground Water Quality
Standards, Chapter. 173-200 WAC, as appropriate.
(b) Discharges to wetlands shall maintain the hydroperiod and flows of existing site conditions to
the extent necessary to protect the characteristic uses of the wetland Prior to discharging to a
wetland, alternative discharge locations shall be evaluated, and natural water storage and
infiltration opportunities outside the wetland shall be maximized.
(c) Created wetlands that are intended to mitigate for loss of wetland acreage,function and value
shall not be designed to also treat stormwater.
(d) In order for constructed wetlands to be considered treatment.systems, they must be constructed
on sites that are not wetlands and they must be managed for stormwater treatment. If these
systems are not managed and maintained in accordance with an approved manual for a period
exceeding three years these systems may no longer be considered constructed wetlands.
Discharges from constructed wetlands to waters of the state(including discharges to natural
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wetlands)are regulated under Chapter. 90.48 RCW, Chapter. 173-201 WAC, and Chapter. 173-
200 WAC.
(e) Stormwater treatment BMPs shall not be built within a natural vegetated buffer, except for
necessary conveyance systems as approved by the local government.
An adopted and implemented basin plan (Minimum Requirement#9) may be used
to develop requirements for wetlands that are tailored to a specific basin.
Not Applicable.
MINIMUM REQUIREMENT #7, WATER QUALITY SENSITIVE AREAS
Criteria: Where local governments determine that the Minimum Requirements do
not provide adequate protection of water quality sensitive areas, either on-site or
within the basin, more stringent controls shall be required to protect water
quality.
Stormwater treatment BMPs shall not be built within a natural vegetated buffer,
except for necessary conveyance systems as approved by the local government.
An adopted and implemented basin plan (Minimum Requirement#9) may be used
to develop requirements for water quality sensitive areas that are tailored to a
specific basin.
Not Applicable.
MINIMUM REQUIREMENT#8, OFF-SITE ANALYSIS AND MITIGATION
Criteria: All development projects shall conduct an analysis of off-site water
quality impacts resulting from the project and shall mitigate these impacts. The
analysis shall extend a minimum of one-fourth of a mile downstream from the
project. The existing or potential impacts to be evaluated and mitigated shall
include, at a minimum, but not be limited to:
0) excessive sedimentation
streambank erosion
(;n) discharges to ground water contributing or[sic]recharge zones
(iv) violations of water quality standards
(v) spills and discharges of priority pollutants
(i) Excessive sedimentation
See Small Parcel Erosion and Sediment Control Plan above.
(ii) Streambank Erosion
Not Applicable.
(iii) Discharges to groundwater contributing or [sic] recharge zones
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Not Applicable.
(iv) Violations of water quality standards
Runoff entering the stormwater system is not expected to contain significant
contaminants. Treatment of this stormwater, as described under Runoff Treatment
BMPs, will minimize the potential for violation of water quality standards.
(v) Spills and discharges of priority pollutants
Not Applicable.
MINIMUM REQUIREMENT #9, BASIN PLANNING
Criteria: Adopted and implemented watershed-based basin plans may be used to
modify any or all of the Minimum Requirements,provided that the level of
protection for surface or ground water achieved by the basin plan will equal or
exceed that which would be achieved by the Minimum Requirements in the
absence of a basin plan. Basin plans shall evaluate and include, as necessary,
retrofitting of BMPs for existing development ardor redevelopment in order to
achieve watershed-wide pollutant reduction goals. Standards developed from
basin plans shall not modify any of the above requirements until the basin plan is
formally adopted and fully implemented by local government. Basin plans shall
be developed according to an approved manual.
Not Applicable.
MINIMUM REQUIREMENT#10, MAINTENANCE AND OPERATIONS
Criteria: An operation and maintenance schedule shall be provided for all
proposed stormwater facilities and BMPs, and the party (or parties) responsible
for maintenance an operation shall be identified
See Section XI, Maintenance and Operations, in the Stormwater Report.
MINIMUM REQUIREMENT#11, FINANCIAL LIABILITY
Criteria: Performance bonding or other appropriate financial instruments shall
be required for all projects to ensure compliance with these standards.
As required by Jefferson County.
X. BOND QUANTITIES WORKSHEET
Not Applicable.
XI. MAINTENANCE AND OPERATIONS
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Silt fences and hay bale dams as well as all other BMPs employed shall be inspected and
cleaned after each large storm event, when necessary. The construction access shall be
monitored and either top dressed or cleaned when necessary. Silt and other material
removed shall be disposed of in accordance with the guidelines established in the SWMM.
All other temporary and permanent measures employed shall be maintained in accordance
with the SWMM.
The measures represented in this report are considered to be the minimum required to
stabilize the site and protect adjacent properties. The project area should be closely
monitored and other BMPs employed as needed.
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ATTACHMENT III
STORM/ATER MANAGIMENT MANUAL FOR THI PUGET SOUND BASIN
Table 1II-1.3 SCS Western Washington Runoff Curve Numbers
(Published by SCS in 1982) Runoff curve numbers for selected agricultural.
suburban and urban
land use for Type lA rainfall distribution, 24-hour storm duration.
LAND USE DESCRIPTION CURVE NUMBERS BY
HYDROLOGIC SOIL GROUP
A B C D
Cultivated land(1) : winter condition 86 91 94 95
Mountain open areas: low growing brush & grasslands 74 82 89 92
w
Meadow or pasture: 65 78 85 89
I Wood or forest land: undisturbed 42 64 76 81 !
P
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 L75% of the 68 80 86 90
area
Fair condition: grass cover on 50-75% of 77 85 90 92
the area
Gravel roads & parking lots: 76 85 89 91
Dirt roads & parking lots: 72 82 87 89 Impervious surfaces, pavement, roofs etc. 98 98 98 98 I (
Open water bodies: lakes, wetlands, ponds etc. 100 100 100 100
Single family residential(2) :
Dwelling Unit/Gross Acre %Impervious(3) Separate curve number
1.0 DU/GA 15 shall be selected for
1.5 DU/GA 20 pervious & impervious f
2.0 DU/GA 25 portions of the site
2.5 DU/GA 30 or basin
3.0 DU/GA 34
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
5.5 DU/GA 54
7.0 DU/GA 56
PUD's, condos, apartments, %impervious
commercial businesses & must be
industrial areas computed
(1) For a more detailed description of agricultural land use curve numbers refer
to National Engineering Handbook, Sec. 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.
IIZ-1-12 FESRUARY, 1992
•
STORMWATER MANAGEMENT MANUAL FOR THE PUGET SOUND BASIN
Table __I-1.4 'n" AND "k" Values Used in Time Calculations for Hydrographs
Sheet Flow Equation Manning' s Values (for the initial 300 ft. of travel) n,
Smooth surfaces (concrete, asphalt, gravel, or bare hand packed
soil)
0.011
Fallow fields or loose soil surface (no residue) 0.05
Cultivated soil with residue cover (s 0.20 ft/ft) 0.06
Cultivated soil with residue cover (s> 0.20 ft/ft) 0.17
Short prairie grass and lawns 0.15
Dense grasses 0.24 U5 E
Bermuda grass 0.41 0.2
Range (natural) 0.13
Woods or forest witn light underbrush 0.40
Woods or forest with dense underbrush 0.80 Sii£E'-
*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 initial 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
5. Short grass, pasture and lawns (n = 0.030) 11
6. Nearly bare ground (n = 0.25) 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 ckomnivil 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 pipe 0.508/n
Channel Flow (Continuous stream, R = 0.4) k,
9. Meandering stream with some pools (n = 0.040) 20
Rock-lined stream (n = 0.035) 23
-. Grass-lined stream (n = 0.030) 27
12. Other streams, man-made channels and pipe 0.807/n**
I1I-1-16 FEBRUARY, 1992
STORMWATER MANAGEMENT MANUAL FOR THE PUG_" .- _
-24
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III-1-44 FEBRUARY, 1992
S':ORMWATER MANAGEMENT MANUAL FOR THE ?': :: _ ;. __.
49
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=:I-1-45 7EEFUARY. :992
•
S•:CRMWATER MANAGEMENT MANUAL FOR HE ?UZI-_ _:N.: ___-.
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ISOPL(JVIALS OF 100-YR 24-HR PRECIPITATION 55 NationalPnececea andAtmosp of Commerce
IN o++oaoe«+c AtfmrrosetY r
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•
TENTHS OF{AN INCH h. n„ Service.w..tf„rService.o+tKa,il
Pr,preo for US.Department or ayr+cutars.
L
Sod Conservation Servs:a.Entwtssrn uo t Drvn
124 122 :2:
III-1-46 =EBRUARY, 1992
ATTACHMENT IV
• JOB Y- -Re / kie
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TILLMAN
SHEET NO.CALCUUTED BY DATE .6/39/0,
ENGINEERING, INC. CHECKED BY DATE
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141 OAK BAY ROAD/P.O. BOX 1375 PORT HADLOCK, WA 98339 360-379-9661 360-379-0150 FAX
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ENGINEERING, INC. CHECKED BY DATE
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