BROCHURE # 53
RESIIDENTIA
AL
INFIL
LTRATIO
ON PIT
T/TRENC
CH
Wha
at is an inffiltration pit?
An in
nfiltration pit/trench is a stormwater
s
facility
f
much
h
like a gravity sep
ptic system, that mitigate
es stormwater
flowss from roofto
op runoff by providing a reservoir tha
at
allow
ws for water storage and
d infiltration into the nativ
ve
soilss located ben
neath the pitt.



Wha
at size infilltration pit do I need?
?
DCD
D employees
s will size the
e infiltration pit required
for yyour residenttial building based
b
on infformation
provvided to them
m by the app
plicant. Sizing
g of your I-p
pit
is de
etermined by
y 3 factors:
 T
The amount of imperviou
us area bein
ng directed to
o
the proposed
d infiltration pit.
The recorded
d rainfall tota
als for the arrea your
 T
p
project will be built in.
 T
The infiltratio
on rate of the
e soils where
e the
infiltration pitt will be plac
ced.
How
w is the soiils infiltration rate de
etermined?
?
An a
analysis of th
he soil where
e the infiltrattion pit is
prop
posed is needed in orderr to properly size the
infiltrration pit. A soil sample must be obttained from
the immediate area in which
h the infiltration pit will be
e
locatted, taken att a depth between 3.5 and 4 feet
below
w the ground surface.
The soil sample must be tak
ken to a soil testing lab
and analyzed to determine the
t percent of
o soil
retained by a #200 sieve. Th
his test identtifies the soil
grad
dation, the am
mount of cou
urse (gravel and sand),
and fine (clay an
nd silt) materrial in the so
oil. The
perccent of fines is used to de
etermine the
e soils
infiltrration rate.
Whe
ere can an infiltration
n pit be loc
cated?
Therre are severa
al things tha
at affect plac
cement of an
n
infiltrration pit, as
s follows:
 1
10’ setback from
f
propertty lines, foun
ndations, or
ssensitive are
eas.
 3
30’ setback ifi located uphill of an on--site septic
ssystem.
Kitsap
p County Departm
ment of Commun
nity Developmentt
614 Division Street, MS-36
Orchard, WA 98366-4682
Port O
www.kkitsapgov.com/dc
cd/
Revisiion Date: 11/01/2
2013

0’setback if lo
ocated to the side, or do
ownhill of an
n
10
on
n-site septic system.
20
00’ setback* from top of a 30% or grreater slope.
*T
This setback may be reduced if supp
ported by a
ge
eotechnical a
analysis and
d report.
Inffiltration pitss may not be
e located on slopes
ste
eeper than 2
20%. A geottechnical ana
alysis and
re port is required if the inffiltration pit iss to be built
on
n a slope bettween 15% a
and 20%.
Th
here must be
e a minimum
m of 12” of se
eparation
be
etween the b
bottom of the
e infiltration p
pit/trench
an
nd both hard
dpan and the
e seasonal h
high water
tab
ble
Can iit be locate
ed under a driveway?
?
Yes, iff the drivewa
ay is paved with either cconcrete or
aspha
alt.
Whatt type of Ca
atch Basin
n (CB) can I use?


If tthe CB will n
not be in a lo
ocation subje
ect to
ve
ehicular load
ding one of th
he following may be
ussed as a CB::
o A black ma
anufactured ADS CB. *A
ADS pipe
with a pou
ured concrete
e bottom will not be
allowed.
o A fiberglasss septic tan
nk riser with a welded
bottom, minimum 24” diameter.
o A concrete
e CB (Type 3
30, Type 1, or Type 2).
If tthe CB will b
be in a location subject tto vehicular
adjacent to a driveway),, you have
loa
ading (in or a
on
ne option:
o A concrete
e CB (Type 3
30, Type 1, or Type 2).
How deep is the
e infiltratio
on pit?
Infiltra
ation pits size
ed by DCD h
have a stand
dard depth
of 18” inches of ro
ock. Other depths may b
be used
when designed byy an enginee
er.
Whatt shape mu
ust the tren
nch be?
Infiltra
ation pits size
ed by DCD m
must meet the
calcul ated square
e footage. Th
hey may be a
any shape
meets the squ
uare footage
e requirement, as long
that m
the folllowing criteria are met:
Phone
e: (360) 337-5777
Faxx: (360) 337-4925
Form
m Number: 5353B
B
Email: Kitsap1@
@co.kitsap.wa.us
Page 1 of 2



Infiltration tre
enches mustt be oriented
d parallel to
topographic contours, with a level bo
ottom.
T
The maximum length of the trench must
m
not
e
exceed 100’.
If multiple tre
enches will be
b used the minimum
sspacing betw
ween centerllines is 6’.
Wha
at type of pipe
p
needs
s to be use
ed in the
infiltration pit?
?
Perfo
orated Polyv
vinyl Chloride (PVC) pipe, a minimum
of 4”” in diameterr must be us
sed.
At w
what depth
h within the
e drain roc
ck should
the drain pipe be placed
d?
The perforated PVC
P
pipe should be loca
ated a
minim
mum of 6” above the bo
ottom of the trench
t
and a
minim
mum of 6” below
b
the top
p of the rock.
Kitsap
p County Departm
ment of Commun
nity Developmentt
614 Division Street, MS-36
Orchard, WA 98366-4682
Port O
www.kkitsapgov.com/dc
cd/
Revisiion Date: 11/01/2
2013
Whatt type of ro
ock needs to be used
d in the
infiltrration pit/trrench?
The ro
ock required
d to be used in the infiltra
ation
pit/tren
nch is round
d washed rocck, ¾”- 1 ½”” in diameterr.
Is an inspection
n port requ
uired?
An insspection porrt is not required, but is
recom
mmended as it can functiion as an em
mergency
overlo
ow in case th
he infiltration
n pit is unable to mitigate
e
all flow
ws during a heavy or pro
olonged rain
n event rain
event..
Whatt does an infiltration pit/trench look like?
See th
he detail below which sh
hows the layyout of the
constrructed infiltra
ation pit/tren
nch.
Phone
e: (360) 337-5777
Faxx: (360) 337-4925
Form
m Number: 5353B
B
Email: Kitsap1@
@co.kitsap.wa.us
Page 2 of 2