HomeMy WebLinkAboutBLD2016-00180 - 05 STORMWATER • •
SON f i vii Fr-' I 1
�w� e0 DEPARTMENT OF COMMUNITY DEVELOPME�FT
,,.,..,: ''il'''.7'' ',Nil 1 E,---. --;(,(1 ;-9,14,-,1 ' i
W , '� 62 i i I,.:Jt rc°et,Port t _'o t i i,y\.A ' 'i t
L� :„ a]
vVv t-.� )J ti _stet wa.0 ci_��l tuni < ael(}tyai ��J
W(.1):' l,-ntA I .l c.:;Ll(cr7 ,ltc t',i)I is.U.a:4s
�� ,
5�1 NG�O
,�F�rnr NT
STORMWATER CALCULATION WORKSHEET
L 1ibKE GARAGE /DWIGHT DOTY
MLA# PROJECT/APPLICANT NAME: TPN 983400112
DETERMINING STORMWATER MANAGEMENT REQUIREMENTS:This stormwater calculation worksheet should be completed first to
classify the proposal as "small," "medium," or "large." The size determines whether a Stormwater Site Plan is required in
conjunction with a stand-alone stormwater management permit application, building permit application, or other land use approval
application that involves stormwater review. The basic information will also be helpful for completing a Stormwater Site Plan, if
required.
PARCEL SIZE (I.E., SITE)
Size of parcel 0.15 acres An acre contains 43,560 square feet. Multiply the acreage by this figure.
Size of parcel in square feet 6381 sq/ft
Land-disturbing activity is any activity that results in movement of earth, or a change in the existing soil cover(both vegetative and
non-vegetative)and/or the existing soil topography. Land disturbing activities include, but are not limited to clearing,grading, filling,
excavation,and compaction associated with stabilization of structures and road construction.
Native vegetation is vegetation comprised on plant species, other than noxious weeds, that are indigenous to the coastal region of
the Pacific Northwest and which reasonably could have been expected to naturally occur on the site. Examples include species such
as Douglas fir, western hemlock, western red cedar, alder, big-leaf maple, and vine maple; shrubs such as willow, elderberry,
salmonberry, and salal; herbaceous plants such as sword fern,foam flower, and fireweed.
LAND DISTURBING ACTIVITY, CONVERSION OF NATIVE VEGETATION, AND VOLUME OF CUT/FILL
Calculate the total area to be cleared, graded,filled, Answer the following two questions related to
excavated, and/or compacted for proposed development conversion of native vegetation:
project. Include in this calculation the area to be cleared for:
Does the project convert 3/4 acres or more of
Construction site for structures 960 sq/ft native vegetation to lawn or landscaped areas?
Drainfield, septic tank, etc. 28 TRANSPORT LINE sq/ft Circle: Yes (No)
Well, utilities,etc. 0 sq/ft Does the project convert 21/2 acres or more of
native vegetation to pasture?
Driveway, parking, roads, etc. 476 sq/ft
100 Circle: Yes (No)
Lawn, landscaping, etc. sq/ft
Other compacted surface, etc. 0 sq/ft Indicate Total Volumes of Proposed:
Total Land Disturbance 2102 sq/ft Cut 45 Fill 0 (cu/yd)
Impervious surface is a hard surface that either prevents or retards the entry of water into the soil mantle as under natural
conditions prior to development. A hard surface area which causes water to run off the surface in greater quantities or at an
increased rate of flow from the flow present under natural conditions prior to development. Common impervious surfaces include,
but are not limited to roof tops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads,
packed earthen materials, and oiled, macadam or other surfaces which similarly impede the natural infiltration of stormwater.
stormwater calc worksheet-REV.10/20/2014
• •
STORMWATER CALULATIONS—IMPERVIOUS SURFACE
NEW EXISTING
Structures(all roof area) 1098 sq/ft Structures (all roof area) 0 sq/ft
Sidewalks 0 sq/ft Sidewalks 0 sq/ft
Patios 0 sq/ft Patios 0 sq/ft
0
Solid Decks 0
sq/ft Solid Decks sq/ft
(without infiltration below) (without infiltration below)
0
Driveway, parking, roads, etc 476 sq/ft Driveway, parking, roads, etc sq/ft
Other 0 sq/ft Other 0 sq/ft
Total New 1098 sq/ft Total Existing 0 sq/ft
TOTAL NEW+TOTAL EXISTING* 1574 sq/ft *This amount will be used to check total lot coverage.
The following questions will help determine whether the proposed project is considered development or redevelopment.
DEVELOPMENT v. REDEVELOPMENT
Divide the total existing impervious surface above by the size of the parcel and convert to a percentage: 24.6 %
Does the site have 35% or more of existing impervious surface? Circle: Yes No X
FURTHER INSTRUCTIONS: If the answer is yes, the proposal is considered redevelopment and the attached Figure 2 should be used
to determine the applicable Minimum Requirements. If the answer is no, the proposal is considered new development and the
attached Figure 1 should be used. At this juncture, the applicant should refer to the applicable Flow Chart to determine the
Minimum Requirements for stormwater management. DCD staff will help verify the classification of the project and the application
requirements.
For proponents of "small" projects who must comply only with Minimum Requirement #2—Construction Stormwater Pollution
Prevention—an additional submittal is not required. The proponent is responsible for employing the 12 Elements to control erosion
and prevent sediment and other pollutants from leaving the site during the construction phase of the project. Pick up the
Construction Stormwater Pollution Prevention (SWPP) Best Management Practices (BMPs) Packet. Proponents of "medium"
projects—those that must meet only Minimum Requirements#1 through #5—and for"large" projects—those that must meet all 10
Minimum Requirements—are required to submit a Stormwater Site Plan. DCD has prepared a submittal template of a Stormwater
Site Plan, principally for rural residential projects. Complete the template in the Stormwater Site Plan Instructions and Submittal
Template or prepare a Stormwater Site Plan using the step-by-step guidance in the Stormwater Management Manual.
APPLICANT SIGNATURE
By signing the Stormwater Calculation Worksheet, I as the applicant/owner attest that the information provided herein is true and correct to the
best of my knowledge. I also certify that this application is being made with the full knowledge and consent of all owners of the affected
propert
3/24/16
(LANDOWNER OR AUTIORIZED REP ESENTATIVE SIGNATURE) (DATE)
FOR OFFICE USE ONLY
SMACL MEDIUM LARGE REDEVELOPMENT .Stormwatersite Plan:" Yes No
stormwater calc worksheet—REV.10/20/2014 2
Standard Conditions for Reside-ntial1velopment in Jefferson County •
1. LIGHTING: Lighting fixtures shall be designed and hooded to prevent the light source from being directly
visible from outside the boundaries of the property. The intensity or brightness of all lighting, during
construction and after project completion shall not adversely affect the use of surrounding properties or
adjoining rights-of-way.
Exterior lighting for residential uses shall not exceed twenty feet (20') in height from the finished grade,
excepting when such lighting is an integral part of a building or structure. Ground level lighting is
encouraged.
2. BUILDING HEIGHT: The building height is not to exceed 35 feet.
3. OUTDOOR STORAGE: Outdoor residential storage shall be maintained in an orderly manner and shall
create no fire, safety, health or sanitary hazard.
4. UNLICENSED VEHICLES: Not more than 2 unlicensed vehicles shall be stored on any lot unless totally
screened from view of neighboring dwellings and rights-of-way. Such screening shall meet all applicable
performance and development standards specific to the district in which the storage is kept, and shall be in
keeping with the character of the area. Screening shall meet the requirements of Chapter 18.30 JCC.
Outdoor storage of 3 or more junk motor vehicles is prohibited except in those districts where specified as
an automobile wrecking yard or junk(or salvage) yard and allowed as a permitted use in Table 3-1 or
Chapter 18.18 JCC, and such storage shall meet the requirements of JCC 18.211100,Automobile wrecking
yards and junk (or salvage) yards. In no case, shall any such junk motor vehicles be stored in a critical
area.
5. PARKING: A minimum of two (2) on-site parking spaces shall be provided for the single family residence.
6. This permit has been reviewed consistent with JCC 18.40.130 & 140. Notice: this permit does not excuse
the applicant from complying with other local, state and federal ordinances, regulations or statues,
applicable to the proposed development, but consistent with RCW 90.58. Future development beyond
that specified in this permit shall require further review.
7. BMPs: The project shall adhere to the Best Management Practices (BMPs)to control stormwater, erosion
and sediment during construction. BMPs shall address permanent measures to stabilize soil exposed
during construction, and in the design and operation of stormwater and drainage control systems.
8. NOTICE: This permit does not excuse the proponent from complying with other local, state, and federal
ordinances, regulations, or statutes applicable to the proposed development, but consistent with RCW
90.58.
Development pursuant to this permit shall be undertaken subject to the applicable policies and
performance standards of the Jefferson County Shoreline Management Master Program and the Jefferson
County Unified Development Code.
If during excavation or development of the site an area of potential archaeological significance is
uncovered, all activity in the immediate area shall be halted, and the Administrator shall be notified at once.
The Federal Endangered Species Act rules to protect threatened Chinook and Summer-run Chum salmon
became effective on January 8, 2001. Bull trout have been listed as threatened since early 2000. Under
the ESA, any person may bring lawsuit against any individual or agency that"takes" listed species (defined
as causing harm, harassing, or damaging habitat for the listed species). In addition, the National Marine
3/7/2013
2013-03-07 5FR StdConditions
1
Fisheries Service can levy pe s. Portions of Jefferson County, inclu marine waters; are included
as "critical habitat" for a listedies. Development of property along a arine shoreline, freshwater
shoreline,
or floodplains could harm habitat if protective measures are not taken. To minimize the potential to
damage habitat, all property owners developing adjacent to marine shoreline, freshwater shoreline, or
floodplains are advised to do the following:
All development activities should avoid unstable slopes, wetlands, and forested areas near surface
waters
Remove minimal vegetation for site development, especially large trees
Allow trees that have fallen into surface waters to remain there
Infiltrate stormwater from buildings and driveways onsite through drywells rather than discharging
directly into surface waters or roadside ditches
The Federal Bald and Golden Eagle Protection Act requires landowners within 660 feet (1/8th of a mile) of
an eagle nest to consult with the US Fish and Wildlife Service. This Eagle Act prohibits anyone from
"taking" bald eagles. This federal law defines the term "take" and describes the possible legal
consequences when a"take" occurs. Among other actions, "take" includes a disturbance of bald eagles or
their habitat_ Under federal law a permit may still be required for activities that impact bald eagles or their
habitat. Contact the US Fish and Wildlife Service (http://www.fws.gov/pacific/eagle/) to learn more about
how this law affects your project.
Any individual, group, or agency can bring suit for a listed species"taking", even if you are in compliance
with Jefferson County development codes. The risk of a lawsuit against you can be reduced by consulting
with a professional fisheries habitat biologist, and following the recommendations for site development
provided by the biologist. For more information, contact the National Marine Fisheries Service in Seattle,
or the U.S. Fish and Wildlife Service.
9/6/2013
2013-09-06_5rR_StdConditions
2
• •
3.1.1 Downspout Full Infiltration Systems (BMP T5.10A)
Downspout full infiltration systems are trench or drywell designs intended
only for use in infiltrating runoff from roof downspout drains. They are
not designed to directly infiltrate runoff from pollutant-generating
impervious surfaces.
Application Projects subject to Minimum Requirement#5 (Section 2.5.5, Volume I)
must provide for individual downspout full infiltration systems or full
dispersion if feasible. Evaluate the feasibility, or applicability, of
downspout full infiltration unless full dispersion is proposed. Use the
evaluation procedure below to determine the feasibility of downspout full
infiltration.
Runoff Modeling If roof runoff is infiltrated according to the requirements of this section,
for Roof the roof area may be discounted from the project area used for sizing
Downspout Full stormwater facilities.
Infiltration
Procedure for 1. Have one of the following prepare a soils report to determine if soils
Evaluating suitable for infiltration are present on the site:
Feasibility
• A professional soil scientist certified by the Soil Science Society of
America(or an equivalent national program)
• A locally licensed on-site sewage designer
• A suitably trained person working under the supervision of a
professional engineer,geologist,hydrogeologist, or engineering
geologist registered in the State of Washington.
The report shall reference a sufficient number of soils logs to establish
the type and limits of soils on the project site. The report should at a
minimum identify the limits of any outwash type soils(i.e.,those
meeting USDA soil texture classes ranging from coarse sand and
cobbles to medium sand)versus other soil types and include an
inventory of topsoil depth.
2. If the lots or site does not have outwash or loam soils, and full
dispersion is not feasible, then consider a rain garden or bioretention
BMPs (the next lower priority on-site stormwater management
system).
3. Complete additional site-specific testing on lots or sites containing
outwash(coarse sand and cobbles to medium sand)and loam type soils.
Individual lot or site tests must consist of at least one soils log at the
location of the infiltration system,a minimum of 4 feet in depth from the
proposed grade and at least 1 foot below the expected bottom elevation
of the infiltration trench or dry well.
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-4
• •
Identify the NRCS series of the soil and the USDA textural class of the
soil horizon through the depth of the log, and note any evidence of high
ground water level,such as mottling.
4. Downspout infiltration is considered feasible on lots or sites that meet
all of the following:
• 3 feet or more of permeable soil from the proposed final grade to the
seasonal high ground water table.
• At least 1-foot of clearance from the expected bottom elevation of
the infiltration trench or dry well to theseasonal high ground water
table.
• The downspout full infiltration system can be designed to meet the
minimum design criteria specified below.
Design Criteria Figure 3.1.2 shows a typical downspout infiltration trench system, and
for Infiltration Figure 3.1.3 presents an alternative infiltration trench system for sites with
T:251.30 coarse sand and cobble soils. These systems are designed as specified
below.
General
1. The following minimum lengths(linear feet)per 1,000 square feet of
roof area based on soil type may be used for sizing downspout
infiltration trenches.
Coarse sands and cobbles 20 LF
Medium sand 30 LF
Fine sand, loamy sand 75 LF
Sandy loam 125 LF
Loam 190 LF
2. Maximum length of trench shall not exceed 100 feet from the inlet
sump.
3. Minimum spacing between trench centerlines shall be 6 feet.
4. Filter fabric shall be placed over the drain rock as shown on Figure
3.1.2 prior to backfilling.
5. Infiltration trenches may be placed in fill material if the fill is placed
and compacted under the direct supervision of a geotechnicalengineer
or professional civil engineer with geotechnical expertise, and if the
measured infiltration rate is at least 8 inches per hour.Trench length in
fill must be 60 linear feet per 1,000 square feet of roof area. Infiltration
rates can be tested using the methods described in Section 3.3.
6. Infiltration trenches should not be built on slopes steeper than 25%
(4:1). A geotechnical analysis and report may be required on slopes
over 15 percent or if located within 200 feet of the top of slope steeper
than 40%,or in a landslide hazard area.
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-5
• •
7. Trenches may be located under pavement if a small yard drain or catch
basin with grate cover is placed at the end of the trench pipe such that
overflow would occur out of the catch basin at an elevation at least one
foot below that of the pavement, and in a location which can
accommodate the overflow without creating a significant adverse
impact to downhill properties or drainage systems. This is intended to
prevent saturation of the pavement in the event of system failure.
Design Criteria Figure 3.1.4 shows a typical downspout infiltration drywell system.
for Infiltration These systems are designed as specified below.
Drywells General
1. Drywell bottoms must be a minimum of 1 foot above seasonal high
ground water level or impermeable soil layers.
2. When located in course sands and cobbles, drywells must contain a
volume of gravel equal to or greater than 60 cubic feet per 1000 square
feet of impervious surface served. When located in medium sands,
drywells must contain at least 90 cubic feet of gravel per 1,000 square
feet of impervious surface served.
3. Drywells must be at least 48 inches in diameter(minimum)and deep
enough to contain the gravel amounts specified above for the soil:-type
and impervious surface served.
4. Filter fabric(geotextile)must be placed on top of the drain rock and on
trench or drywell sides prior to backfilling.
5. Spacing between drywells must be a minimum of 10 feet.
6. Downspout infiltration drywells must not be built on slopesrtgreater
than 25% (4:1). Drywells may not be placed on or above a landslide
hazard area or on slopes greater than 15%without evaluation by a
professional engineer with geotechnical expertise or a licensed
geologist, hydrogeologist,or engineering geologist, and with
jurisdiction approval.
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-6
• !
roof
drain
PLAN VIEW
,— .4"rigid or 6"flexiile
NTS perforated pipe
CO
infiltration trench sump wlsolid lid
PLAN VIEW roof drain
NTS
overflow
4"rigid or 6"ilexbie splash block
perforated pipe _1111
(4-n,...;:v4vzef --!—H W
i 1 Q washed rock ‘;.=.a O"� ° a Zt!,_ 1'ren. 5.0'mirk 1
12" •°Q= 1 112"-3/4" ' Q' ° 04 .
l'min
fine mesh screen "--CB sump w/solid lid
I, L - 10'min
varies - _.]
A
filter fabric
} compacted backfill
V'
c:_drr'r iffy
6• , `.:' >�
4"rigid or 6"flexible
f 9 9a° ,'°p, , perforated pipe
24' ..Q.. . °o<fp .
°v''°
°O CS°Q°p nCi' washed rock
12' I:° 4° ° ° z 1 1/2"-3/4'
N
i � DA°OQa9Ootiro'
'
i\�"iN'�i� I,',, �
k-r--24=-
.
SECTION A
•
NTS •
Figure 3.1.2-Typical Downspout Infiltration Trench
Source: King County
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-7
•
from roof
i
•j
ograss
ove ow 6"min
ao
1112"-3/s' O p °V°.V°
washed rock.. /'413'.z a a o° q a° 18"min° NOTE:Same length dimensions
,vo and site limitations as typical system
�Llau Vo°.
o�oV° a'
2'min , . 15'mat
X-SECT1ON
NTS -
•
Figure 3.1.3 -Alternative Downspout Infiltration Trench System for Coarse Sand and Gravel
Source: King County
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-8
• •
Roof Downspout
House f,/
/ Catch Basin
/ (Yard Drain)
//////3
tow � _-
Rolf
DaanspoU
0
DRY WELL 48Inch Dia meter
Hole Filled with
PLAN VIEW 11i`-3"Washed
Drain Rock
NTS
f R cof Downspout
Mark Center of Hole
with 1"'Capped PV•
Nausa d V Overflow or Other means
M Splash Block Topsal Flushwith Sixface
1 '
l'min.
111111111111111111111111111111111111111r~At
—� ill
_ 4.T:..-r.tr s--., . t -
v.a�r e�,��,._{r Gts�rtsf
MeshtiOw
l'IMOINt-1451,
ili
Screen '� I 1-V 4"d a `,4.4
Fine1
1-VC Pipe - .. v.s.-
Catch Basin °r" Q
(1rd Drain)
' 48 Inch Diameter • ' ''snn,.
Sides of Hole ? Hole Filled with -
Lined with kg' 11/2-3'Washed -.
Filter Fabric 6. Drain Rods •f
' Li
,{ 15'min. ',�r�r�wf �a+sskali
tat ....,..,aa- .
Min. 1'a5ove SAasorial
DRY WELL H igti Grouidwatar Table
SECTION f
NTS
Figure 3.1.4-Typical Downspout Infiltration Drywell
Source: King County
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-9
• •
Setbacks
Local governments may require specific setbacks in sites with slopes over
40%,land slide areas, open water features, springs,wells,and septic tank
drain fields.Adequate room for maintenance access and equipment should
also be considered. Examples of setbacks commonly used include the
following:
1. All infiltration systems should be at least 10 feet from any structure,
property line,or sensitive area(except slopes over 40%).
2. All infiltration systems must be at least 50 feet from the top of any slope
over 40%. This setback may be reduced to 15 feet based on a
geotechnical evaluation,but in no instances may it be less than the
buffer width.
3. For sites with septic systems,infiltration systems must be downgradient
of the drainfield unless the site topography clearly prohibits subsurface
flows from intersecting the drainfield.
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-10
0 t .
3.1.2 Downspout Dispersion Systems (BMP T5.10B)
Downspout dispersion systems are splash blocks or gravel-filled trenches,
which serve to spread roof runoff over vegetated pervious areas.
Dispersion attenuates peak flows by slowing the runoff entering into the
conveyance system, allowing some infiltration, and providing some water
quality benefits.
Applications& Downspout dispersion may be used in all subdivision lots where
Limitations downspout full infiltration, full dispersion,and bioretention/rain gardens
are not feasible. /
{
Runoff Modeling In WWHM,roof areas may be modeled as grass d surfaces (landscape) if
for Roof roof runoff is dispersed according to the requi ments of this section on
Downspout ` ,�lots greater than 22,000 square feet, and the egetated flowpath is 50 feet
Dispersion ,or_larger through undisturbed native landsc pe or lawn/landscape area that
mets BMP T5.13. If the available veget ed flowpath is 25 to 50 feet,
use of-q dispersion trench allows model' g the roof as 50%
impervi s/50%landscape. This is do e in WWHM on the Mitigated
Scenario sc en by entering the roof rea into one of the entry options for
dispersal of i�`m`pervious area runoff or WWHM2012, see Appendix III-C
in this Volume.
Design Criteria 1. Use downspout -nches des' ed as shown in Figures 3.1.5 and 3.1.6
for all downspout •"a s ersio' applications except where splash blocks
are allowed below.
2. Splash blocks shown ini pure 3.1.7 may be used for downspouts
discharging to a veget ed fl• lath at least 50 feet in length as
measured from the dwnspout t•,the downstream property line,
structure,slope over/15%,stream, , etland, or other impervious surface.
Sensitive area buff'rs may count tow..d flowpath lengths.
3. Cover the vegetated flowpath with well-e .blished lawn or pasture,
landscaping with well-established groundco -r, or native vegetation
with natural groundcover. The groundcover sha be dense enough to
help disperse and infiltrate flows and to prevent e z sion.
4. If the vegetated flowpath(measured as defined above 's less than 25
feet, a perforated stub-out connection per Section 3.1.3 ,ay be used in
lieu of downspout dispersion. A perforated stub-out may a o be used
where implementation of downspout dispersion might cause -rosion or
flooding/problems,either on site or on adjacent lots. For exam. e,this
provision might be appropriate for lots constructed on steep hills here
downspout discharge could culminate and might pose a potential .zard
for lower lying lots, or where dispersed flows could create problems for
adjacent off-site lots. This provision does not apply to situations where
lots are flat and on-site downspout dispersal would result in saturated
yards. Perforated stub-outs are not appropriate when seasonal water
table is<1 foot below trench bottom.
Volume III—Hydrologic Analysis and Flow Control BMPs—December 2014
3-11
•
2.5 Minimum Requirements
This section describes the minimum requirements for stormwater
management at development and redevelopment sites. Section 2.4 should
be consulted to determine which requirements apply to any given project.
Figures 2.4.1 and 2.4.2 should be consulted to determine whether the
minimum requirements apply to new surfaces,replaced surfaces, or new
and replaced surfaces. Volumes II through V of this manual present Best
Management Practices (BMPs) for use in meeting the Minimum
Requirements.
Throughout this chapter, requirements are written in bold and
supplemental guidelines that serve as advice and other materials are
not in bold.
2.5.1 Minimum Requirement#1: Preparation of Stormwater
Site Plans
All projects meeting the thresholds in Section 2.4 shall prepare a
Stormwater Site Plan for local government review. Stormwater Site
Plans shall use site-appropriate development principles, as required
and encouraged by local development codes,to retain native
vegetation and minimize impervious surfaces to the extent feasible.
Stormwater Site Plans shall be prepared in accordance with Chapter
3 of this volume.
Objective
The 2,000 square feet threshold for hard surfaces and 7,000 square foot
threshold for land disturbance are chosen to capture most single family
home construction and their equivalent.Note that the scope of the
stormwater site plan only covers compliance with Minimum Requirements
#2 through#5 if the thresholds of 5,000 square feet of hard surface or
conversion of 3/4 acre of vegetation to lawn or landscape, or conversion of
2.5 acres of vegetation to pasture are not exceeded.
Supplemental guidelines
Projects proposed by departments and agencies within the local
government with jurisdiction must comply with this requirement. The
local government shall determine the process for ensuring proper project
review, inspection, and compliance by its own departments and agencies.
Volume I—Minimum Technical Requirements—August 2012
2-16
411 III
thresholds. Local governments should be prepared to demonstrate that by
comparing the number and types of historical projects that would have
been regulated using the Ecology thresholds versus the local government's
thresholds.
Local governments are allowed to institute a stop-loss provision on the
application of stormwater requirements to replacedhard surfaces. A stop-
loss provision is an upper limit on the extent to which a requirement is
applied.For instance,there could be a maximum percentage of the
estimated total project costs that are dedicated to meeting stormwater
requirements. A project would not have to incur additional stormwater
costs above that maximum though the standard redevelopment
requirements will not be fully achieved. The allowance for a stop-loss
provision pertains to the extent that treatment, flow control and wetlands
protection requirements are imposed on replaced hard surfaces.It does not
ply to meeting stormwater requirements for new hard surfaces.
Lo al governments can also establish criteria for allowing redevelopment
proje is to pay a fee in lieu of constructing water quality or flow control
faciliti on a redeveloped site. At a minimunX the fee should be the
equivalen of an engineering estimate of the/cost of meetingall applicable
stormwater; quixernents,for the:project.. 'helocal government should use
such funds for eimplementation of stdrmwater control projects that
would have simi abenefits to the sar}e.receiving water as if the project
had constructed its required improvements. Expenditure of such funds is
subject to other state s tutory.reirements.
Ecology cautions local go rrrients about the potential long-term
�
consequences`of allowing a,fi3.e-in-lieu of stormwater facilities. Sites that
are allowed to pay a fee c ntinwithout stormwater controls. If it is
determined,throughure basin phanning for:instance,that controls on
such sites are necess to achieve water quality goals or legal
requirements,the public may bear the osts for providing those controls.
Underground tn. ty projects that replac the ground surface with in-kind
material or m erials with similar runoff c aracteristics should not be
subject to re evelopment requirements exc t construction site erosion
control.
Local gOivernments are also encouraged to revieall road projects for
chan es in elevations or drainage flowpath that co Id;cause flooding,
upl nd or stream erosion, or changes to discharges t6wetlands.For
e ample, adding curbs will result in redirecting flows a d possibly causing
ew downstream impacts.The local government should s t project-
specific requirements to avoid or mitigate those impacts.
/
Volume I—Minimum Technical Requirements—August 2012
2-15
• •
2.5.2 Minimum Requirement#2: Construction Stormwater
Pollution Prevention (SWPP)
Thresholds
All new development and redevelopment projects are responsible for
preventing erosion and discharge of sediment and other pollutants
into receiving waters.
Projects which result in 2,000 square feet or more of new plus
replaced hard surface area, or which disturb 7,000 square feet or
more of land must prepare a Construction SWPP Plan (SWPPP) as
part of the Stormwater Site Plan (see Section 2.5.1).
Projects that result in less than 2,000 square feet of new plus replaced
hard surface area, or disturb less than 7,000 square feet of land are
not required to prepare a Construction SWPPP, but must consider all
of the 13 Elements of Construction Stormwater Pollution Prevention
and develop controls for all elements that pertain to the project site.
General Requirements
The SWPPP shall include a narrative and drawings. All BMPs shall
be clearly referenced in the narrative and marked on the drawings.
The SWPPP narrative shall include documentation to explain and
justify the pollution prevention decisions made for the project. Each
of the 13 elements must be considered and included in the
Construction SWPPP unless site conditions render the element
unnecessary and the exemption from that element is clearly justified
in the narrative of the SWPPP.
Clearing and grading activities for developments shall be permitted
only if conducted pursuant to an approved site development plan (e.g.,
subdivision approval) that establishes permitted areas of clearing,
grading, cutting, and filling. These permitted clearing and grading
areas and any other areas required to preserve critical or sensitive
areas,buffers, native growth protection easements, or tree retention
areas shall be delineated on the site plans and the development site.
The SWPPP shall be implemented beginning with initial land
disturbance and until final stabilization. Sediment and Erosion
control BMPs shall be consistent with the BMPs contained in chapters
3 and 4 of Volume II.
Seasonal Work Limitations -From October 1 through April 30,
clearing, grading, and other soil disturbing activities shall only be
permitted if shown to the satisfaction of the local permitting authority
that silt-laden runoff will be prevented from leaving the site through a
combination of the following:
1. Site conditions including existing vegetative coverage, slope, soil
type and proximity to receiving waters.
Volume I—Minimum Technical Requirements—August 2012
2-17
• •
2. Limitations on activities and the extent of disturbed areas.
3. Proposed erosion and sediment control measures.
The following activities are exempt from the seasonal clearing and
grading limitations:
1. Routine maintenance and necessary repair of erosion and
sediment control BMPs,.
2. Routine maintenance of public facilities or existing utility
structures that do not expose the soil or result in the removal of
the vegetative cover to soil.
3. Activities where there is one hundred percent infiltration of
surface water runoff within the site in approved and installed
erosion and sediment control facilities.
Project Requirements - Construction SWPPP Elements
Element 1: Preserve Vegetation/Mark Clearing Limits
• Before beginning land disturbing activities, including clearing and
grading, clearly mark all clearing limits, sensitive areas and their
buffers, and trees that are to be preserved within the construction
area.
• Retain the duff layer, native top soil, and natural vegetation in an
undisturbed state to the maximum degree practicable.
Element 2: Establish Construction Access
• Limit construction vehicle access and exit to one route, if possible.
• Stabilize access points with a pad of quarry spalls, crushed rock,
or other equivalent BMPs, to minimize tracking of sediment onto
public roads.
• Locate wheel wash or tire baths on site, if the stabilized
construction entrance is not effective in preventing tracking
sediment onto roads.
• If sediment is tracked off site, clean the affected roadway
thoroughly at the end of each day, or more frequently as necessary
(for example, during wet weather). Remove sediment from roads
by shoveling, sweeping, or pick up and transport the sediment to a
controlled sediment disposal area.
• Conduct street washing, only after sediment is removed in
accordance with the above bullet.
Volume I—Minimum Technical Requirements—August 2012
2-18
•
• Control street wash wastewater by pumping back on-site, or
otherwise prevent it from discharging into systems tributary to
waters of the State.
Element 3: Control Flow Rates
• Protect properties and waterways downstream of development
sites from erosion and the associated discharge of turbid waters
due to increases in the velocity and peak volumetric flow rate of
stormwater runoff from the project site.
• Where necessary to comply with the bullet above, construct
stormwater retention or detention facilities as one of the first steps
in grading. Assure that detention facilities function properly
before constructing site improvements (e.g. impervious surfaces).
• If permanent infiltration ponds are used for flow control during
construction, protect these facilities from siltation during the
construction phase.
Element 4: Install Sediment Controls
• Design, install, and maintain effective erosion controls and
sediment controls to minimize the discharge of pollutants.
• Construct sediment control BMPs (sediment ponds,traps, filters,
etc.) as one of the first steps in grading. These BMPs shall be
functional before other land disturbing activities take place.
• Minimize sediment discharges from the site. The design,
installation and maintenance of erosion and sediment controls
must address factors such as the amount, frequency, intensity and
duration of precipitation,the nature of resulting stormwater
runoff, and soil characteristics,including the range of soil particle
sizes expected to be present on the site.
• Direct stormwater runoff from disturbed areas through a
sediment pond or other appropriate sediment removal BMP,
before the runoff leaves a construction site or before discharge to
an infiltration facility. Runoff from fully stabilized areas may be
discharged without a sediment removal BMP, but must meet the
flow control performance standard in Element#3, bullet#1.
• Locate BMPs intended to trap sediment on-site in a manner to
avoid interference with the movement of juvenile salmonids
attempting to enter off-channel areas or drainages.
• Where feasible, design outlet structures that withdraw impounded
stormwater from the surface to avoid discharging sediment that is
still suspended lower in the water column.
Volume I—Minimum Technical Requirements—August 2012
2-19
• •
Element 5: Stabilize Soils
• Stabilize exposed and unworked soils by application of effective
BMPs that prevent erosion. Applicable BMPs include, but are not -
limited to: temporary and permanent seeding,sodding,mulching,
plastic covering, erosion control fabrics and matting, soil
application of polyacrylamide (PAM), the early application of
gravel base early on areas to be paved, and dust control.
• Control stormwater volume and velocity within the site to
minimize soil erosion.
• Control stormwater discharges, including both peak flow rates
and total stormwater volume,to minimize erosion at outlets and to
minimize downstream channel and stream bank erosion.
• Soils must not remain exposed and unworked for more than the
time periods set forth below to prevent erosion:
• During the dry season (May 1 - Sept. 30): 7 days
• During the wet season (October 1 -April 30): 2 days
• Stabilize soils at the end of the shift before a holiday or weekend if
needed based on the weather forecast.
• Stabilize soil stockpiles from erosion, protected with sediment
trapping measures, and where possible, be located away from
storm drain inlets,waterways and drainage channels.
• Minimize the amount of soil exposed during construction activity.
• Minimize the disturbance of steep slopes.
• Minimize soil compaction and, unless infeasible, preserve topsoil.
Element 6: Protect Slopes
• Design and construct cut-and-fill slopes in a manner to minimize
erosion. Applicable practices include, but are not limited to,
reducing continuous length of slope with terracing and diversions,
reducing slope steepness,and roughening slope surfaces (for
example,track walking).
• Divert off-site stormwater(run-on) or ground water away from
slopes and disturbed areas with interceptor dikes, pipes and/or
swales. Off-site stormwater should be managed separately from
stormwater generated on the site.
• At the top of slopes, collect drainage in pipe slope drains or
protected channels to prevent erosion.
• Temporary pipe slope drains must handle the peak 10-minute
velocity of flow from a Type 1A, 10-year,24-hour frequency
Volume I—Minimum Technical Requirements—August 2012
2-20
• •
storm for the developed condition. Alternatively,the 10-year
and 1-hour flow rate predicted by an approved continuous
runoff model, increased by a factor of 1.6, may be used. The
hydrologic analysis must use the existing land cover condition
for predicting flow rates from tributary areas outside the
project limits. For tributary areas on the project site, the
analysis must use the temporary or permanent project land
cover condition,whichever will produce the highest flow rates.
If using the Western Washington Hydrology Model (WWHM)
to predict flows, bare soil areas should be modeled as
"landscaped" area.
• Place excavated material on the uphill side of trenches, consistent
with safety and space considerations.
• Place check dams at regular intervals within constructed channels
that are cut down a slope.
Element 7: Protect Drain Inlets
• Protect all storm drain inlets made operable during construction
so that stormwater runoff shall not enter the conveyance system
without first being filtered or treated to remove sediment.
• Clean or remove and replace inlet protection devices when
sediment has filled one-third of the available storage (unless a
different standard is specified by the product manufacturer).
Element 8: Stabilize Channels and Outlets
• Design, construct, and stabilize all on-site conveyance channels to
prevent erosion from the following expected peak flows:
• Channels must handle the peak 10-minute velocity of flow
from a Type 1A, 10-year,24-hour frequency storm for the
developed condition. Alternatively,the 10-year, 1-hour flow
rate indicated by an approved continuous runoff model,
increased by a factor of 1.6,may be used. The hydrologic
analysis must use the existing land cover condition for
predicting flow rates from tributary areas outside the project
limits. For tributary areas on the project site, the analysis must
use the temporary or permanent project land cover condition,
whichever will produce the highest flow rates. If using the
Western Washington Hydrology Model (WWHM) to predict
flows, bare soil areas should be modeled as "landscaped area.
• Provide stabilization,including armoring material, adequate to
prevent erosion of outlets, adjacent stream banks,slopes and
downstream reaches at the outlets of all conveyance systems.
Volume I—Minimum Technical Requirements—August 2012
2-21
• •
Element 9: Control Pollutants
• Design, install, implement and maintain effective pollution
prevention measures to minimize the discharge of pollutants.
• Handle and dispose of all pollutants, including waste materials
and demolition debris that occur on-site in a manner that does not
cause contamination of stormwater.
• Provide cover, containment, and protection from vandalism for all
chemicals, liquid products, petroleum products, and other
materials that have the potential to pose a threat to human health
or the environment. On-site fueling tanks must include secondary
containment. Secondary containment means placing tanks or
containers within an impervious structure capable of containing
110% of the volume contained in the largest take within the
containment structure. Double-walled tanks do not require
additional secondary containment.
• Conduct maintenance,fueling, and repair of heavy equipment and
vehicles using spill prevention and control measures. Clean
contaminated surfaces immediately following any spill incident.
• Discharge wheel wash or tire bath wastewater to a separate on-site
treatment system that prevents discharge to surface water, such as
closed-loop recirculation or upland application, or to the sanitary
sewer,with local sewer district approval.
• Apply fertilizers and pesticides in a manner and at application
rates that will not result in loss of chemical to stormwater runoff.
Follow manufacturers' label requirements for application rates
and procedures.
• Use BMPs to prevent contamination of stormwater runoff by pH
modifying sources. The sources for this contamination include, but
are not limited to: bulk cement, cement kiln dust,fly ash,new
concrete washing and curing waters,waste streams generated
from concrete grinding and sawing, exposed aggregate processes,
dewatering concrete vaults, concrete pumping and mixer washout
waters.
• Adjust the pH of stormwater if necessary to prevent violations of
water quality standards.
• Assure that washout of concrete trucks is performed off-site or in
designated concrete washout areas only. Do not washout concrete
trucks onto the ground, or into storm drains, open ditches, streets,
or streams. Do not dump excess concrete on-site,except in
designated concrete washout areas. Concrete spillage or concrete
discharge to surface waters of the State is prohibited.
Volume I—Minimum Technical Requirements—August 2012
2-22
• •
• Obtain written approval from Ecology before using chemical
treatment other than CO2 or dry ice to adjust pH.
Element 10: Control De-Watering
• Discharge foundation,vault, and trench de-watering water,which
has similar characteristics to stormwater runoff at the site, into a
controlled conveyance system before discharge to a sediment trap
or sediment pond.
• Discharge clean, non-turbid de-watering water, such as well-point
ground water,to systems tributary to, or directly into surface
waters of the State, as specified in Element#8, provided the de-
watering flow does not cause erosion or flooding of receiving
waters. Do not route clean dewatering water through stormwater
sediment ponds.Note that "surface waters of the State" may exist
on a construction site as well as off site; for example, a creek
running through a site.
• Handle highly turbid or otherwise contaminated dewatering water
separately from stormwater.
• Other treatment or disposal options may include:
1. Infiltration.
2. Transport off-site in a vehicle, such as a vacuum flush truck,
for legal disposal in a manner that does not pollute state
waters.
3. Ecology-approved on-site chemical treatment or other suitable
treatment technologies.
4. Sanitary or combined sewer discharge with local sewer district
approval, if there is no other option.
5. Use of a sedimentation bag with outfall to a ditch or swale for
small volumes of localized dewatering.
Element 11: Maintain BMPs
• Maintain and repair all temporary and permanent erosion and
sediment control BMPs as needed to assure continued
performance of their intended function in accordance with BMP
specifications.
• Remove all temporary erosion and sediment control BMPs within
30 days after achieving final site stabilization or after the
temporary BMPs are no longer needed.
Volume I—Minimum Technical Requirements—August 2012
2-23
• •
Element 12: Manage The Project
• Phase development projects to the maximum degree practicable
and take into account seasonal work limitations.
• Inspection and monitoring—Inspect, maintain and repair all
BMPs as needed to assure continued performance of their
intended function. Projects regulated under the Construction
Stormwater General Permit must conduct site inspections and
monitoring in accordance with Special Condition S4 of the
Construction Stormwater General Permit.
• Maintaining an updated construction SWPPP —Maintain, update,
and implement the SWPPP.
• Projects that disturb one or more acres must have site inspections
conducted by a Certified Erosion and Sediment Control Lead
(CESCL). Project sites disturbing less than one acre may have a
CESCL or a person without CESCL certification conduct
inspections. By the initiation of construction, the SWPPP must
identify the CESCL or inspector,who must be present on-site or
on-call at all times.
• The CESCL or inspector(project sites less than one acre) must have
the skills to assess the:
• Site conditions and construction activities that could impact the
quality of stormwater.
• Effectiveness of erosion and sediment control measures used to
control the quality of stormwater discharges.
• The CESCL or inspector must examine stormwater visually for the
presence of suspended sediment, turbidity, discoloration, and oil
sheen. They must evaluate the effectiveness of BMPs and determine if
it is necessary to install,maintain, or repair BMPs to improve the
quality of stormwater discharges.
Based on the results of the inspection, construction site operators must
correct the problems identified by:
• Reviewing the SWPPP for compliance with the 13 construction
SWPPP elements and making appropriate revisions within 7 days
of the inspection.
• Immediately beginning the process of fully implementing and
maintaining appropriate source control and/or treatment BMPs as
soon as possible, addressing the problems not later than within 10
days of the inspection. If installation of necessary treatment BMPs
is not feasible within 10 days, the construction site operator may
request an extension within the initial 10-day response period.
Volume I—Minimum Technical Requirements—August 2012
2-24
• S
• Documenting BMP implementation and maintenance in the site log
book(sites larger than 1 acre).
• The CESCL or inspector must inspect all areas disturbed by
construction activities, all BMPs, and all stonnwater discharge points
at least once every calendar week and within 24 hours of any discharge
from the site. (For purposes of this condition, individual discharge
events that last more than one day do not require daily inspections. For
example, if a stormwater pond discharges continuously over the course
of a week, only one inspection is required that week.) The CESCL or
inspector may reduce the inspection frequency for temporary
stabilized, inactive sites to once every calendar month.
Element 13: Protect Low Impact Development BMPs
• Protect all Bioretention and Rain Garden BMPs from
sedimentation through installation and maintenance of erosion
and sediment control BMPs on portions of the site that drain into
the Bioretention and/or Rain Garden BMPs. Restore the BMPs to
their fully functioning condition if they accumulate sediment
during construction.Restoring the BMP must include removal of
sediment and any sediment-laden Bioretention/rain garden soils,
and replacing the removed soils with soils meeting the design
specification.
• Prevent compacting Bioretention and rain garden BMPs by
excluding construction equipment and foot traffic. Protect
completed lawn and landscaped areas from compaction due to
construction equipment.
• Control erosion and avoid introducing sediment from surrounding
land uses onto permeable pavements. Do not allow muddy
construction equipment on the base material or pavement. Do not
allow sediment-laden runoff onto permeable pavements or base
materials.
• Pavement fouled with sediments or no longer passing an initial
infiltration test must be cleaned using procedures in accordance
with this manual or the manufacturer's procedures.
• Keep all heavy equipment off existing soils under LID facilities
that have been excavated to final grade to retain the infiltration
rate of the soils.
Volume I—Minimum Technical Requirements—August 2012
2-25
•
Objective
To control erosion and prevent sediment and other pollutants from leaving
the site during the construction phase of a project. To have fully functional
stormwater facilities and BMP's for the developed site upon completion of
construction.
Supplemental Guidelines
If a Construction SWPPP is found to be inadequate (with respect to
erosion and sediment control requirements),then the Plan Approval
Authority' within the Local Government should require that other BMPs
be implemented, as appropriate.
The Plan Approval Authority may allow development of generic
Construction SWPPP's that apply to commonly conducted public road
activities, such as road surface replacement,that trigger this minimum
requirement. They may also develop an abbreviated SWPPP format for
project sites that will disturb less than 1 acre.
Based on the information provided and/or local weather conditions,the
local permitting authority may expand or restrict the seasonal limitation on
site disturbance. The local permitting authority shall take enforcement
action- such as a notice of violation, administrative order, penalty,or
stop-work order under the following circumstances:
• If, during the course of any construction activity or soil disturbance
during the seasonal limitation period, sediment leaves the construction
site causing a violation of the surface water quality standard;or
• If clearing and grading limits or erosion and sediment control
measures shown in the approved plan are not maintained.
Coordination with Utilities and Other Contractors-The primary project
proponent shall evaluate, with input from utilities and other contractors,
the stormwater management requirements for the entire project, including
the utilities,when preparing the Construction SWPPP.
Element#13,Protect Low Impact Development BMPs, is not yet included
as a permit condition in the NPDES Construction Stormwater General
Permit. That permit is not scheduled for reissuance until December,2015.
Until that permit is reissued with element#13 added as a permit condition,
the element may be enforceable only through the requirements of local
stormwater codes that may have been updated to include it.Municipal
Stormwater Permittees must incorporate this element into local
requirements per the timelines in their Municipal Stormwater Permit.
The Plan Approval Authority is defined as that department within a local government that has been delegated
authority to approve stormwater site plans.
Volume I—Minimum Technical Requirements—August 2012
2-26