HomeMy WebLinkAboutPAR2008-00039 Multi-parcel LPSSP - COM Engineering / Geo-Tech Reports - 10/21/2008 RECEIVED
OCT 21 2009
MASON COUNTY,
Mason County Stormwater Checklist
County Project# COM2008-00139, 00140 & 00141
Project Name McKay Commercial Property Stormwater Plan for Storage Buildings_
Twnsp,Sec,Range Twnsp 20, Sec 12, R 4 WWM Prcl No.s 42012-21-90003, 2 &1
Lots 1,2 &3 of SP#2413
Minimum Requirements of 1992 Stormwater Manual
The stormwater checklist identifies the minimum requirements of the 1992 Storm water
Manual. The checklist is intended to identify the locations within the plan that addresses
the minimum requirements. Mason County will not perform a technical evaluation of the
submittal. Rather, the checklist provides a guide to allow Mason County to review the
submittal and determine ifthe applicant has addressed the minimum features that make
up a stormwater plan.
It is incumbent upon the applicant and his/her engineer to fulfill all the applicable
requirements of the 1992 Storm water Plan as it relates to the proposed project. Review
by Mason County is intended to determine if the plan has addressed the minimum
requirements and should not be considered a review for technical accuracy of the plan.
During construction of the project,the storm water plan engineer of record or his/her
authorized representative shall inspect the site to ensure the storm water plan is being
implemented as designed. Upon completion of the project, the engineer or his authorized
representative shall be required to certify that the stormwater plan has been implemented
as designed.
Failure to meet the minimum requirements could result in delay or rejection of the
application until the deficiencies are corrected.
❑ #1: Erosion and Sediment Control.All new development and redevelopment that includes
land disturbing activities of one acre or greater shall comply with erosion and sediment control
requirements 1 through 14, below. Compliance with the Erosion and Sediment Control
Requirements shall be demonstrated through implementation of a large parcel erosion and
sediment control plan.
All new development and redevelopment that includes land disturbing activities of less than one
acre shall comply with the small parcel minimum requirements found in Section 14.48.130,
Compliance with the small parcel requirements shall be demonstrated through implementation of
a small parcel erosion and sediment control plan.
Erosion and Sediment Control Requirement#1 : Stabilization and Sediment Trapping.
See page/paragraph Page 2, Paragraph 3
Erosion and Sediment Control Requirement#2: Delineate Clearing and Easement Limits.
See page/paragraph Page 2, Paragraph 4
Erosion and Sediment Control Requirement#3: Protection of Adjacent Properties.
See page/paragraph_ Page 2, Paragraph 5
1
Erosion and Sediment Control Requirement#4: Timing and Stabilization of Sediment
Trapping Measures.
See page/paragraph Page 3, Paragraph 1
Erosion and Sediment Control Requirement#5: Cut and Fill Slopes.
See page/paragraph Page 3, Paragraph 2
Erosion and Sediment Control Requirement#6: Controlling Off-Site Erosion.
See page/paragraph Page 3, Paragraph 3
Erosion and Sediment Control Requirement#7: Stabilization of Temporary Conveyance
Channels and Outlets.
See page/paragraph Page 3, Paragraph 4
Erosion and Sediment Control Requirement#8: Storm Drain Inlet Protection.
See page/paragraph Page 3, Parag_raph 5
Erosion and Sediment Control Requirement#9: Underground Utility Construction.
See page/paragraph Page 4, Paragraph I
Erosion and Sediment Control Requirement#10: Construction Access Routes.
See page/paragraph Page 4, Paragraph 2
Erosion and Sediment Control Requirement#11: Removal of Temporary BMPs.
See page/paragraph Page 4, Paragraph 3
Erosion and Sediment Control Requirement#12: Dewatering Construction Sites.
See page/paragraph Page 4, Paragraph 4
Erosion and Sediment Control Requirement#13: Control of Pollutants Other than
Sediment on Construction Sites.
See page/paragraph Page 4, Paragraph 5
Erosion and Sediment Control Requirement#14: Maintenance.
See page/paragraph Page 5, Paragraph 1
Erosion and Sediment Control Requirement#15: Financial Liability.
See page/paragraph Page 5, Paragraph 1 The O&M Covenant fulfills this requirement
❑ #2: Preservation of Natural Drainage Systems. Natural drainage patterns shall be
maintained, and discharges from the site shall occur at the natural location to the
maximum extent practicable.
See page/paragraph Page 5, Paragraph 2
❑ #3: Source Control of Pollution. 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.
See page/paragraph Page 5, Paragraph 3
❑ #4: Runoff Treatment BMPs.All projects shall provide treatment of storm water.
Treatment BMPs shall be sized to capture and treat the water quality storm, defined as
the six-month, twenty-four hour storm. The first priority for treatment of stormwater shall
2
be to infiltrate as much as possible of the water quality design storm into the ground.
Pretreatment of storm water prior to infiltration into the ground may be required.
See page/paragraph Page 5, Paragraph 4
❑ #5: Streambank Erosion Control. 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
BMP's.
See page/paragraph Page 6 Paragraph 1
❑ #6: Wetlands. Stormwater discharges to wetlands shall maintain the wetland's natural
h dro eriod and flows to the extent needed to reserve or enhance i
Y p p ace is existing functions
and values. Prior to proposing discharge of higher volumes of storm water to a wetland,
alternative discharge, detention, and infiltration practices located in areas outside the
wetland shall be evaluated and employed by the project engineer where feasible and
practicable.
See page/paragraph Page 6 Paragraph 2
❑ #7: Water Quality Sensitive Areas.Where the Mason County commissioners or their
designee 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. 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.
See page/paragraph Page 6 Paragraph 3
❑ #8: Off-Site Analysis and Mitigation. Downstream Analysis May Trigger Additional
Requirements. The project engineer shall provide a detailed qualitative analysis of the
flow path of the discharge from the project site to the receiving water. This requirement
shall apply to all projects where a drainage and erosion control plan is prepared,
including those proposing retention facilities. This analysis shall include flow routing, and
provide existing pipe and channel sizes and estimated capacities. In addition, the project
engineer shall discuss any known or expected downstream erosion, flooding, or water
quality problems, including those that may be caused by interflow from the proposed
retention facility. The director or designee shall have the discretion to specify the
distance and level of detail to be provided by the project engineer. In making this
determination, the director or designee shall consider such factors as the relative size of
the new development, availability of other hydrologic work for the drainage area, and the
extent to which storm water generated on the project site is to be infiltrated.
See page/paragraph Page 6 Paragraph 4
❑ #9: Basin Planning. Basin plan supersedes Manual
See page/paragraph Page 6 Paragraph 5
❑ #10: Operation and Maintenance. An operation and maintenance schedule shall be
provided for all proposed storm water facilities and BMPs, and the party (or parties)
responsible for maintenance and operation shall be identified. An operation and
maintenance (O&M) covenant will be required to cover all privately owned and
maintained storm water facilities approved by the director. O&M Covenant forms are
available at the Mason County public works office. A copy of the completed instrument
3
shall be recorded with the county auditors' office by the proponent and a copy of the
recorded instrument is required to be submitted to the public works department prior
to final approval of the completed permanent stormwater facilities.
See page/paragraph Page 7 Paragraphs 1. 2 & 3
❑ #11: Financial Liability. Performance bonding, or other appropriate instruments shall
be required for all projects to ensure compliance with these standards.
See page/paragraph Page 7 Paragraph 4
I certify that the stormwater plan submitted for this project fulfills the applicable provisions of the
1992 Stormwater Manual.
,
Engineer Date
Applicant Date
4
D ' s
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Mason County DCD
Planning Department
PO Box 279
Shelton, WA 98584
Dwight S. MaKay
590 E Island Lake Drive
Shelton, WA 98584-7821
(360)426-5975
Lame Parcel Stormwater Site Plan (LPSSP)
Mason Co. Case No.s PAR2008-00039
Mason Co. Prcl Nos. 42012-21-90003 9000
Stormwater Site Plan Narrative
Mr. McKay is proposing to develop two commercial properties he owns located off of the Shelton-
Springs Road north of Shelton. The two parcels are each over an acre in size and are proposed to
be developed with storage buildings and impervious surfaces exceeding 5000 square feet on each
of the two properties. Mason County's Stormwater Management Ordinance requires the
preparation of a Large Parcel Stormwater Site Plan(LPSSP)for development activities which
include in excess of an acre of clearing, or the creation of 5,000 square feet of new impervious
surfacing.
The Stormwater Site Plan is required to include a Large Parcel Erosion and Sediment Control
(ESC) Plan and a Permanent Stormwater Quality Control(PSQC)Plan which includes measures
needed to protect neighboring properties and State waters from adverse stormwater runoff impacts.
The McKay properties each drain towards the southwesterly corner of each site. Construction
sequencing should be initiated with the roughing out of the stormwater ponds designed for the
permanent stormwater quality control plan which will provide protection of neighboring properties
through the final development of each of the two sites. Directing construction area runoff into the
pond on each site, and maintaining the two ponds and removing any sediment build up resulting
from construction activities should satisfy the Large Parcel Erosion and Sediment Control Plan
requirements of the County Stormwater Management Ordinance.
This document is intended to provide the necessary documentation required for approval of a
Large Parcel Activity associated with clearing in excess of an acre of lightly vegetated land and
conversion of the cleared property to a predominantly impervious condition consisting of building
roofs and gravel driveways and parking areas. This conversion requires compliance with
Stormwater Management Ordinance requirements#1 through#11 in subsections (3)through(13)
of Mason County Chapter 14.48 Stormwater Management' Ordinance. The following pages are
intended to meet the requirements of the County's Stormwater Management Ordinance narrative
1 Mason County Stormwater Management version of Ecology's 1992 Stormwater Management Manual for the Puget
Sound Basin
File:C.1Projects1McKayWP 1 of 7 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
requirements. Plan sheets representing the Stormwater Site Plan and plan design features follow
this portion of the required documents. Included in the appendix of this report will also be found
supporting StormShed2G printouts which provide the basis for the retention pond sizing and
supporting documentation. Also included in the appendix material is a copy of the Operation and
Maintenance (O&M) Covenant required by the County to be completed, notarized, and recorded
with the County Auditor's Office as part of the LPSSP approval by the County.
Minimum Requirement#1: Erosion
q os on and Sediment Control
The following E&S Control Requirements will be incorporated into the site development activities
to the greatest extent practical:
E&S Requirement #1: Stabilization and Sediment Trapping. All exposed and unworked soils shall be
stabilized by suitable application of BMPs. From October 1 to April 30, no exposed and unworked soils
shall remain unstabilized for more than two days. From May 1 to September 30, no exposed and
unworked soils shall remain unstabilized for more than seven days. Prior to leaving the site, stormwater
runoff shall pass through a sediment pond or sediment trap, or other appropriate BMPs.
As indicated in this document's introduction, the first step in the property's developments should
be the roughing out of the stormwater retention ponds designed for each site, and the rough
grading of the properties to direct all surface runoff to each pond. The ponds are designed to be
excavated below the surrounding ground levels and can be expected to intercept runoff from each
of the construction areas. The ponds are designed to infiltrate stormwater storm events up to and
including a 100 year 24 hour event.
E&S Requirement 42: Delineate Clearing and Easement Limits. In the field, mark clearing limits
and/or any easements, setbacks, sensitive%ritical areas and their buffers, trees and drainage courses.
The attached site plans indicate the general clearing limits and stormwater collection basins. Mr.
McKay is intending to reduce the County's required vegetated buffers to half the 20-foot
requirement by doubling the required plantings in the buffer strips surrounding each property.
These buffer areas should be planted as early as practical during the construction phase of the site
development, and should include a grass or clover groundcover vegetation or beauty bark
application to protect the surface soils from erosion. The two retention ponds will encompass most
or all of the southerly vegetated buffer on each property, but the slopes above the two year storm
event water elevation should be planted with grass in addition to the County's planting
requirements to provide stabilization of the pond slopes. Only rarely (less the 98% of the time) will
the slopes above the 2 year 24 hour event will be submerged, and the ponds are designed to drain
themselves in roughly 24 hours.
E&S Requirement#3. Protection of Adjacent Properties. Properties adjacent to the project site shall be
protected from damage by sediment deposition.
The two retention ponds were sized by modeling 24-hour storm events from the 6-month storm
size to the 100-year event storm size. All surface runoff from the properties will be eliminated,
providing the required protection of adjacent properties.
E&S Requirement#4: Timing and Stabilization of Sediment Trapping Measures. Sediment ponds and
traps, perimeter dikes, sediment barriers, and other BMPs intended to trap sediment on-site shall be
File:C:TrojectsWcKay1SSP 2 of 7 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
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 two stormwater retention ponds developed for this stormwater management plan should be
roughed out as soon as possible, and the areas surrounding each pond should be graded to deliver
stormwater runoff to them to provide runoff treatment as soon as possible.
E&S Requirement#5: Cut and Fill Slopes. 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 current site development plans do not create any major cut or fill slopes. The two ponds
included in this plan are designed with 2:1 (H:V) sideslopes which should be stable when
submerged, and should be shallow enough to sustain vegetation.
E&S Requirement#6: Controlling Off-Site Erosion. Properties and waterways downstream from
development sites shall be protected from damage by erosion due to increases in the volume, velocity,
or peak flow rate of stormwater runoff from the project site.
The retention ponds designed for the built-out sites eliminates post development surface
discharges. The pre-developed site may have had a discharge of surface stormwater runoff but no
point discharge was or is easily discernable, so no developed site point discharges are being
promoted in this stormwater control plan.
E&S Requirement#7: Stabilization of Temporary Conveyance Channels and Outlets. All temporary on-
site conveyance channels shall be designed, constructed and stabilized to prevent erosion from the
expected velocity of the peak flow from a two-year, twenty-four 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.
Stormwater runoff delivery from driving& parking areas to the retention ponds is intended to be in
a sheet flow regime. Roof runoff will either be delivered directly to the retention ponds, or to
dispersion pads installed in the perimeter vegetated buffer of each parcel. Parking and driveway
area runoff will be sheeted to the retention ponds by grading the site to deliver un-concentrated
flows of stormwater runoff to each retention pond. Without a surface discharge from the site for
storm events through the 100 year 24 hour event,there should be no surface runoff delivered off
the site, or to neighboring properties.
E&S Requirement 48: Storm Drain Inlet Protection. All storm drain inlets made operable during
construction shall be protected so that stormwater runoff shall not be discharged offsite or be
introduced into any fill area without first being filtered or otherwise treated to remove sediment.
The McKay development plans require no catch basins, but relies on shallow surface flow and site
grading to deliver stormwater runoff to the retention/treatment ponds.
E&S Requirement 99: Underground Utility Construction. The construction of underground utility lines
shall be subject to the following criteria:
(i) No more than five hundred feet of trench shall be opened at one time, unless provisions are
made to protect against adverse stormwater impacts.
File: C TrojectsWcKaySSP 3 of 7 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
(ii) Where consistent with safety and space considerations, excavated material shall be placed on
the uphill side of trenches.
These requirements should be made conditions for project approval and permitting.
E&S Requirement#10: Construction Access Routes.
(i) Construction vehicle access shall be, whenever possible, limited to one route. Access points
shall be stabilized to minimize the tracking of sediment onto public roads.
(ii) 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 creating a hazard, the roads shall be cleaned immediately, and thoroughly cleaned 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 site already includes an existing access located off of the Shelton Springs Road which includes
an asphalt paved surface. The access driveway is constructed from the relatively clean natural
gravels underlying the site and is stable in its present condition. Mr. McKay has expressed his
intention of applying a crushed rock or rock chip driving surface on the parking and driving areas
of the finished site. The surfacing materials are expected to provide additional runoff treatment to
stormwater runoff developed on the site in addition to the treatment/retention of the 6 month 24
hour storm event.
E&S Requirement #H: Removal of Temporary BMPs. All temporary erosion and sediment
control BMPs shall be removed within thirty 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.
Completion of the sites' developments will include excavating the roughed out stormwater
retention ponds to their design dimensions and planting of the vegetations recommended in this
report, and required by the County.
E&S Requirement 412: Dewatering Construction Sites. Trench dewatering devices shall be
discharged in a manner that will not adversely affect flowing streams, drainage systems or off-site
property. Water discharged from dewatering pumps shall be routed through a sediment pond or
trap unless it is clear.
The site enjoys highly permeable soils and no deep excavations will be required for site
development grading.
E&S Requirement#13: Control of Pollutants Other than Sediment on Construction Sites. 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.
This requirement should be made a condition of permit approval. Construction machinery used in
the sites' developments should be maintained to a degree that neither lubricants or fuels are
released or spilled within the construction area or elsewhere.
File:C:TrojectsWcKayISSP 4 of 7 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
E&S Requirement 414: Maintenance. All temporary and permanent erosion and sediment control
BMPs shall be maintained and repaired as needed and in a timely manner to assure continued
performance of their intended function. All maintenance and repair shall be conducted in
accordance with an approved manual.
An Operation and Maintenance (O&M) Covenant form has been prepared for each of the two
commercial properties. The main maintenance the stormwater management features are expected
to require is the periodic inspection of the ponds to verify that they are functioning in compliance
with their design target rates (infiltration rates and storm event water elevations). Water elevations
exceeding design parameters should be considered a signal that silt removal and disposal off-site is
needed. Silt and sediment removed from the retention ponds should be disposed of at an approved
(County permitted) disposal location. Silts and sediments removed from the retention pond dead
storage areas of the ponds is not expected to have very high levels of contamination, but should be
checked for possible contamination from ruptured tanks or other automotive related contaminants
and disposed of accordingly.
Minimum Requirement#2:Preservation of Natural Drainage Systems. Natural drainage
patterns shall be maintained, and discharges from the site shall occur at the natural location to the
maximum extent practicable.
The existing site does not exhibit any heavy flow discharge locations to off-site receiving areas.
The proposed retention of stormwater precipitation is felt to be a reasonable preservation of pre-
developed conditions and drainage patterns. Site soils are found to be a Soil Conservation Service
Grove Gravelly Sandy Loam termed excessively drained. Stormwater management of the site
includes the development of two stormwater retention ponds which preserve the pre-developed
natural drainage system.
Minimum Requirement#3: Source Control of Pollution. 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.
This is integral to the stormwater management plan design.
Minimum Requirement#4: Runoff Treatment BMPs. All projects shall provide treatment of
stormwater. Treatment BMPs shall be sized to capture and treat the water quality storm, defined as
the six-month, twenty-four hour storm. The first priority for treatment of stormwater shall be to
infiltrate as much as possible of the water quality design storm into the ground.
The retention pond designs provide for complete retention of the water quality design storm, and
storms sized through the 100 year 24 hour event storm. The infiltration rate used in the retention
pond modeling is considered to represent half the natural infiltration rate available.
Minimum Requirement#S: Streambank Erosion Control. 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 fifty percent of the existing condition two-year, twenty-
four hour design storm while maintaining the existing condition peak runoff rate for the ten-year,
twenty-four-hour and one hundred-year, twenty-four-hour design storms. As the first priority,
streambank erosion control BMPs shall utilize infiltration to the fullest extent practicable, only if site
File:C:TrojectsWcKaySSP 5 of 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
conditions are appropriate and ground water quality is protected. Streambank erosion control BMPs
shall be selected, designed, and maintained according to an approved manual.
The proposed retention pond designs reduces each sites' discharge to zero, providing erosion and
silt protection to downstream properties and compliance with downstream erosion control
discharge rates.
Minimum Requirement#6: Wetlands. Stormwater discharges to wetlands shall maintain the
wetland's natural hydroperiod and flows to the extent needed to preserve or enhance its existing
functions and values. Prior to proposing discharge of higher volumes of stormwater to a wetland,
alternative discharge, detention, and infiltration practices located in areas outside the wetland shall
be evaluated and employed by the project engineer where feasible and practicable.
No wetlands have been identified on or near the McKay property.
Minimum Requirement V. Water Quality Sensitive Areas. Where the Mason County
commissioners or their designee 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. 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.
The McKay property is not located
ed in an area requiring a higher degree of protection than the
proposed plan provides, nor is it located within a known or adopted basin plan.
Minimum Requirement#8: Off-Site Analysis and Mitigation. Downstream Analysis May Trigger
Additional Requirements. The project engineer shall provide a detailed qualitative analysis of the
flow path of the discharge from the project site to the receiving water. This requirement shall apply
to all projects where a drainage and erosion control plan is prepared, including those proposing
retention facilities. This analysis shall include flow routing, and provide existing pipe and channel
sizes and estimated capacities. In addition, the project engineer shall discuss any known or
expected downstream erosion, flooding, or water quality problems, including those that may be
caused by interflow from the proposed retention facility.
The McKay property receives little or no runoff from adjacent properties. Retention of stormwater
runoff and infiltration into the ground is expected to relieve the project from downstream analysis
or mitigation requirements. Interflow is expected to roughly match pre-development interflow
rates.
Minimum Requirement#9: Basin Planning. Basin plan supersedes Manual.
If a proposed project is located in a basin or sub basin for which the county has an adopted basin
plan, stormwater requirements specifically identified in the basin plan shall take precedence over
those provided in this chapter. However, all other elements detailed in this chapter shall continue to
apply to such projects. Basin plans are required to be developed according to an approved manual.
The McKay property is not located within a known adopted basin plan area.
Minimum Requirement#10: Operation and Maintenance. An operation and maintenance
schedule shall be provided for all proposed stormwater facilities and BMPs, and the party(or
parties) responsible for maintenance and operation shall be identified. An operation and
maintenance (O&M) covenant will be required to cover all privately owned and maintained
stormwater facilities approved by the director.
File:C:TrojectsWcKaySSP 6 of 7 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Mr. McKay, as owner of the completed commercial properties, and subsequent owners will be
responsible for operation and maintenance of the stormwater management features.
The re
tention ponds, gutters and downspouts, downspout piping& splash pads and graveled areas
should be inspected at a minimum twice each year(in the Fall before the wet season and in the
Spring after the wet season), and after any major storm event(more than an inch of precipitation in
a 24 hour period).
An Operation and Maintenance Covenant will be implemented as required by the County's
adopted requirements.
Minimum Requirement#11: Financial Liability. Performance bonding, or other appropriate
instruments shall be required for all projects to ensure compliance with these standards.
The proposed McKay development is a relatively modest development proposal and the O&M
Covenant is expected to satisfy this requirement.
Variances from these Standards. Variances from these standards may be allowed at the
discretion of the director or designee provided that the proponent will substantially meet flow control
and water quality goals established by or implicit in these standards.
No variances are deemed necessary from the current County Stormwater Management Ordinance
requirements.
This Stormwater Site Plan and supporting documents have been prepared by myself or under my
direct supervision as required under the Stormwater Management Ordinance requirements. I hope
this document and supporting elements meet the standards and needs of Mason County and the
McKays in permitting and developing their property as they currently intend.
Sincerely,
Alan A. Tahja, P.E. y�Of WAsk
Attached: Stormwater Site Plan Sheets
O&M Covenant
StormShed2G Modeling Printouts
30784
�sSI�NAL E��l
File: C:TrojectsWcKayISSP 7 of 7 10/13/2008
Pr perty Line —220'
_ —Ve
— '8 elated Bufi'er (�
r ----- — b-
�� 4 012-21-00003 1.27 cres N
Buffer Planting Schedule:
rnzo lo<w
Quantities per 100 feet N � I � '-•
(Doubled to reduce buffer to 50%) r I vi
6 Canopy Trees C 1U25 I c l w E in 14
12 Understory Plants 18 Shrubs , / 30'Gate
y IE S N b
^�0— z.w acw 72,20 I y
I„
CZ /
w
I I �
I b
1 Prope y Line 2 6.88' I �
Po Floor 120'x 8' _ Po Floor
—J
P nd Top 132'x 20' — Pond T 72'x 2V
1 10n20 10 1 2:1(H:V)sideslopes, b
I 3'Deep(Intl.1'Fn,ob.rd) I C)W
C
t MI I0r25 10, I 14 k
.� N1 14W 15
I �o S w Jn
o I.x, I t I h7
of wAsxl a` I E S rin wood Lane °p
/ izxw w.w �
zrw I —
L 42012 21-00004 1.31 Acres I I
A Property Line —310'
30784 9 [yam Pond Top Dimensions 16'x 282' (2:1 SS) > I ^ y
�CISTv p 4'x 270'x 3'Deep Retention Pond •� w �b b 11
o� cu o O
�SSIONAL
Conjectural Storage Buildings:
U e a y
Q' Bldg A 8,500 Sqr Ft 14
� x
aC Bldg B 9,240 Sqr Ft W
/ I
\ CO
Bldg C 8,550 Sqr Ft N fD h
\ � I
Bldg D 9,120 Sqr Ft I1,'
She ft Total— 36,150 Sqr Ft
SCALE: I"= 100' pringwood t
0 25 50 100
w
PAR2008-00039
Disperse downspout discharge into vegetated buffer
Property Line —220' by way of buried 6"PVC transport pipe&dispersion pad.
/ t0 4k
OF WA$�f / � za 00003 ————� 10'Vegetated Buffer
20' 1 Lines(Dashed)
/ iuczu un-w iA y
Pond Notes: j, b
j 1,30
¢ I The pond designed for Prcl.No. o It's'Apt 1p�, 7g4 E9 ��4 / N / 5 2 "'"' 1 42012-21-90003(Floor Area=1440 SF) N
Ly, ISTEg 1� 1 will accomodate a total building area
s`�I�NAL up to 19,200 SF
N p � 1_� I w
/ 30'Gate �
t 1 1 1 The pond igned for Parcl.No. tit+
> 1s 1 10 42012-21-900 (Floor Area= 1080 SF) b
will accomodate a al building area $
Conjectural Storage Buil ngs: ' C I N
up to 20,000 SF
Bldg A 8,500 S r Ft ' "^"'
Bldg B 9,240 qr Ft j o Y
1 I 4 A
Bldg C 8,550 qr Ft 1 2rN „�c.15° ^°^p W P
'V
Bldg D 9,12 Sqr F / 25' I Security Fenced
Pro erty Line 246.881 zr 1 Property Boundaries �1
Total— 36,15 Sqr t
J 10' 00
I 10' I
1 00004 Connecting Driveway wrw 1
1u,Y) with 12"Dia.x 40' I n
I 1 long corrugated I
1 Plastic or Metal Pipe I SCALE: I"=60' CA
[2.k1 I 1 W �eby
N h j5 70' I o 20 ao bo j ►� p O
W E v c I 'CS p rA'rd)
04/ C 00Q o 1
10
x IS' IIO' tAb
00
O 'a1' tA CO
/ 10'Vegetated Buffer > o 00 o
Lines(Dashed) ,�, > ,,,w a
O~/ / 12,N1 > •� > N
'fl C 12x30 I
/ I�' C7 C7 v � a° I Direct Gutter Downspouts
——— into 6"Diameter solid PVC
�/ ---------------------- pipes discharging to Rock I K)
y° ------- Armored Dispersion Pads
/ (F thick)located at discharge I w
roperty me � �
end of pipe near retention
pond floor bottom.
x
0 0
SCALE: 1"=14 R
0 1 2 3 4 feet own
20'Top Pond Dimension w
1—
Surrounding w 3'Pond Depth([ncl. 1'Freeboard) w Surrounding y
Ground Ground
N
100 Yr tJ
`aes�o4
-- — —_ 2 Yr
24 Hour Storm Event L.`l�•
Peak Water Elevations 8' Wide Pond Floorb
Pond Notes: Pond Notes:
The pond designed for Prcl.No. a o Pond slopes and pond floor below
42012-21-90003 will accomodate i the 2 Year 24 Hour Storm Event w 'b
a total building area up to 19,200 p elevation(s)should be planted with c O
Sqr Ft. o water tolerant vegetaion. o 19 a
0 o V k M
The pond designed for Parcl.No. c Silt shall be removed from pond b
42012-21-90004 will accomodate m a floors if and when the ponds do not t^b
a total building are up to 20,000 infiltrate/function as designed. N y txt
Sqr Ft. Parcel No.42012-21-90003 8' x 180'x 3' Deep Retention Pond ,o
Ia
00
SCALE: 1"=4' Chain Link
Security Fence f7
0 1 2 3 4 feet
ON
C
16'Top Pond Dimension 2 e
tJ" in
Surrounding Surrounding b y Q Z
Ground Ground 114
--- ---- ---------------------- --------- ---------
00
_ 100vr cSISSI 14 a•
Buffer Planting Schedule: 10Yr _
YF- _ vls'acs`o4 v�of WA
Quantities per 100 feet — __ _ _ _ lY1• R �, (�
(Doubled to reduce buffer to 50%)
6 Canopy Trees 4' Wide Pond Floor y
12 Understory Plants 3'Pond Depth O/
18 Shrubs (Incl. 1'Freeboard) o
o��SSIpIVAL
Parcel No.42012-21-90004 4'x 290'x 3' Deep Retention Pond
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Dwight S. McKay
590 E Island Lake Drive
Shelton, WA 98584
(360) 426-5975
StormShed Modeling Parcel No. 42012-21-90003.
LPOOLCOMPUTE [Retention Pond] SUMMARY using Puls
Start of live storage: 270.0000 ft
Event Match Q (cfs) Peak Q (cfs) Peak Stg (ft) Vol (cf) Vol (acft) Time to Empty
6 month 0.0039 0.3401 270.0964 142.45 0.0033 24.67
2 year 0.0309 j 0.4284 270.4834 785.12 0.0180 24.83
10 year 0.0665 0.5321 271.0000 1821.33 0.0418 24.83
100 year 0.2725 0.7569 272.0840 4682.63 0.1075 24.83
Summary Report of all Detention Pond Data
Event Precip (in)
6 month 2.9000
2 year 4.5000
10 year 6.0000
100 year 9.0000
BASLIST2
[Un-03] Using [TYPEIA] As [6 month]
[Dev-03] Using [TYPE 1 A] As [6 month]
[Un-03] Using [TYPEIA] As [2 year]
[Dev-03] Using [TYPEIA] As [2 year]
[Un-03] Using [TYPEIA] As [10 year]
[Dev-03] Using [TYPEIA] As [10 year]
[Un-03] Using [TYPEIA] As [100 year]
[Dev-03] Using [TYPEIA] As [100 year]
LSTEND
Peak Q Peak T Peak Vol Area
BasinID Event (cfs) (hrs) (ac-cf) (ac) Method/Loss Raintype
E � I
�
Un-03- 0.0039 24.00 0.0009 1.28 SBUH/SCS TYPEIA
month
4
Dev-03 month 0.3596 T8.00 01380 1.28 SBUH/SCS TYPEIA
jUn-03 2 year 0.0309 �24.00 0.0271 1.28 SBUH/SCS TYPEIA
File:C:TrojectsWcKay StrmShed Prc13 Basin Printout 1 of 3 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Dev-03 2 year 0.8181 7.83 0.2803 T 1.28 SBUH/SCS TYPEIA
Un-03 10 year 0.0665 F 17.00 0.0775 F1.28 SBUH/SCS TYPEIA
Dev-03 10 year 1.3013 [- 7.83 j� 0.4242 F 1.28 SBUH/SCS TYPEIA
Un-03 y�° �0.2725 8.17 �0.2294 1.28 SBUH/SCS TYPEIA
100
Dev-03 year 2.3065 7.83 0.7256 I.28 SBUH/SCS 'TYPEIA
BASLIST [TYPEIA] AS [6 month] DETAILED [Un-03] [Dev-03] LSTEND
Record Id: Un-03
,,Design Method SBUH Rainfall type -F-"TYPEIA
jHyd Intv 10.00 min Peaking Factor 484.00
- �-��-�Abstraction Coeff 1 0.20
lPervious Area (AMC 1) 1.28 ac DCIA 0.00 ac
CPervious CN 43.82 DC CN - 0.00
Tervious TC F15.88 min DC TC �0.00 min
Pervious CN Cale
Description SubArea Sub cn
Meadow or Pasture 1.28 ac 65.00
C---- Pervious Composited CN(AMC 2) 65.00
Pervious TC Cale
Type Description Length Slope Coeff Misc F �TT
Sheet Short prairie grass and lawns.: 0.15 320.00 ft 4.00% 0.1500 4.50 in 15.88 min
Pervious TC 15.88 mini'.
Record Id: Dev-03
jDesign Method SBUH Rainfall type TYPEIA
Hyd Intv �10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area (AMC 1) 1.28 ac DCIA 0.00 ac
Pervious CN �81.89 DC CN F0.00
Pervious TC 1.96 min DC TC 0.00 min'
Pervious CN Cale
File:C:TrojectsWcKay StrmShed Prc13 Basin Printout 2 of 3 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
j Description SubArea Sub en
Gravel Roads & Parking Lots 0.64 ac r 76.00
Impervious surfaces (pavements, roofs, etc) j 0.49 ac F 98.00
Open spaces, lawns,parks (50-75%grass) 0.15 ac 77.00
Pervious Composited CN(AMC 2) 84.54
Pervious TC Cale
Type Description Length Slope Coeff J Misc TT
Sheet Smooth Surfaces.: 0.011 320.00 ft [4.00% 0.0110 4.50 in 1.96 min
Pervious TC 1.96 min
HYDLIST SUMMARY [6 month out] [2 year out] [10 year out] [100 year out]
LSTEND
HydID Peak Q (cfs) Peak T (hrs) Peak Vol (ac-ft) Cont Area (ac)
6 month out I 0.3401 ( 8.00 0.1380 1.2800
2 year out F 0.4284 8.33 0.2802 1.2800
_ - 0.4242 I 1.2800-.
10 year out 0.5321 � 8.67
100 year out 0.7569 9.00 0.7256 �1.2800
STORLIST [Pond 1] LSTEND
Record Id: Pond 1
A.
Descrip: Retention Pond Increment 0.10 ft v�0 WA
Start El. 270.0000 ft Max El. 273.0000 ft
Length 180.0000 ft Width 8.0000 ft
Length ss 1 2.00h:1 v ILength ss2 2.0000h:1 v
Width ssl 2.00h:ly ��EN -%S w�
_Width ss2 2.000Oh:Iv �SSI01 ST 7, `S'
DISCHLIST [Infiltration] LSTEND
Record Id: Infiltration
Descrip: Prototype Structure Increment 0.10 ft
Start El. 270.0000 ft Max El. 105.0000 ft
Infiltration rate 10.0000 in/hr WP Multiplier 1.00
Licensed to: TAHJA ENGINEERING. Inc.
File: C:TrojectsWcKay StrmShed Prcl 3 Basin Printout 3 of 3 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Dwight S. McKay
590 E Island Lake Drive
Shelton, WA 98584
(360) 426-5975
StormShed Modeling Parcel No. 42012-21-90004
LPOOLCOMPUTE [Retention Pond] SUMMARY using Puls
Start of live storage: 260.0000 ft
Event Match Q (cfs) Peak Q (cfs) Peak Stg(ft) Vol (cf) Vol (acft) Time to Empty
6 month 0.0040 0.2889 260.1369 159.45 0.0037 24.67
2 year 0.0316 0.4232 j 260.6053 856.03 0.0197 24.67
10 year 0.0680 0.5673 j 261.1010 1860.64 0.0427 24.83
100 year 0.2789 0.8463-11-262.0415 4535.40 0.1041 24.83
Summary Report of all Detention Pond Data
Event Precip (in)
6 month 2.9000
2 year 4.5000
10 year 6.0000
100 year 9.0000
BASLIST2
[Un-04] Using [TYPE 1 A] As [6 month]
[D-04] Using [TYPE 1 A] As [6 month]
[Un-04] Using [TYPE 1 A] As [2 year]
[D-04] Using [TYPEIA] As [2 year]
[Un-04] Using [TYPE 1 A] As [10 year]
[D-04] Using [TYPE 1 A] As [10 year]
[Un-04] Using [TYPE 1 A] As [100 year]
[D-04] Using [TYPE 1 A] As [100 year]
LSTEND
Peak Q Peak T Peak Vol Area
BasinID Event (cfs) (hrs) (ac-cf) (ac) Method/Loss Raintype
Un-04 month 0.0040 24.00 0.0009 1.31 SBUH/SCS TYPEIA
_
D-04 month 0.3547 8.00 0.1377 1.31 SBUH/SCS TYPEIA
Un-04 2 year 0.0316 24.00 0.0277 1.31 SBUH/SCS TYPEIA
D-04 2 year 0.8164 7.83 0.2818 r 1.31 SBUH/SCS TYPEIA
File: C:TrojectsWcKay StrmShed Prcl 4 Basin Printout 1 of 3 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Un-04 10 year 0.0680 -I 17.00 0.0793 1.31- SBUH/SCS TYPEIA
D-04 10 year 1.3078 r 7.83 0.4283 1.31 SBUH/SCS TYPEIA
00
Un-04 y a 0.2789 T 8.17 0.2348 1.31 SBUH/SCS TYPEIA
i
----------._ _.
D-04 100 2.3343 7.83 0.7357 I 1.31 SBUH/SCS TYPEIA
year
BASLIST [TYPEIA] AS [6 month] DETAILED [Un-04] [D-04] LSTEND
Record Id: Un-04
Design Method 7 SBUH Rainfall type TYPEIA
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area (AMC 1) 1.31 ac DCIA 0.00 ac
Pervious CN 43.82 DC CN 0.00
Pervious TC 15.88 min DC TC 0.00 min
� Pervious CN Cale _.--
Description SubArea Sub cn
Meadow or Pasture 1.31 ac 65.00
Pervious Composited CN (AMC 2) 65.00
J Pervious TC Cale
Type Description Length Slope Coeff Misc TT
Sheet Short prairie grass and lawns.: 0.15 320.00 ft 4.00% 0.1500 4.50 in 15.88 min
Pervious TC 15.88 min
Record Id: D-04
Design Method SBUH Rainfall type j TYPEIA
Hyd Intv 10.00 min Peaking Factor 484.00
Abstraction Coeff 0.20
Pervious Area (AMC 1) 1.31 ac DCIA 0.00 ac
Pervious CN 81.37 DC CN 0.00
Pervious TC 1.96 min DC TC i 0.00 min
Pervious CN Cale
Description ( SubArea Sub cn
File: C:TrojectsWcKay StrmShed Prcl 4 Basin Printout 2 of 3 10/13/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Gravel Roads & Parking Lots 0.64 ac 76.00
Impervious surfaces (pavements, roofs, etc) 0.51 ac ; 98.00
j- Open spaces, lawns,parks (<50% grass) 0.16 ac 68.00
Pervious Composited CN (AMC 2) 83.59
Pervious TC Calc
Type Description Length Slope Coeff Misc TT
r-Sheet Smooth Surfaces.: 0.011 320.00 ft 4.00% 0.0110 4.50 in 1.96 min
_
Pervious TC 1.96 min
HYDLIST SUMMARY [6 month out] [2 year out] [10 year out] [100 year out]
LSTEND
HydID Peak Q (cfs) Peak T (hrs) Peak Vol (ac-ft) Cont Area (ac)
6 month out 0.2889 I 8.17 0.1377 1.3100
12 year out 0.4232 8.33 0.2818 1.3100
1110 year out 0.5673 �� 8.67 r 0.4283 1.3100
100 year out 0.8463 T 8.67 0.7357 F-1.3100
STORLIST [Pond 1] LSTEND
Record Id: Pond 1
Descrip: Retention Pond Increment 0.10 ft I A. T
;Start El. 260.0000 ft Max El. 263.0000 ft °F 'rAr
(Length 270.0000 ft Width 4.0000 ft
jLength ss 1 2.00h:1 v Length ss2 2.0000h:1 v w v �0
'Width ssl 2.00h:Iv Width ss2 2.0000h:Iv ��' �7g4 w
_ ��',���'CISTEg�9
SSYlo L SIA
DISCHLIST [Infiltration] LSTEND
Record Id: Infiltration
'Descrip: Prototype Structure Increment 0.10 ft
Start El. 260.0000 ft Max El. 265.0000 ft
[Infiltration rate 10.0000 in/hr WP Multiplier 1.00
Licensed to: TAHJA ENGINEERING. Inc.
File: C:TrojectsWcKayStrmShed Prcl4 Basin Printout 3 of 3 10/13/2008