Infiltration Basin Design Notes Cont'd

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Copyright 2000, CWP

Design of

Infiltration

Practices

Center for
Watershed
Protection

Copyright 2000, CWP

Infiltration Systems


Primarily a water quality BMP


Helps meet groundwater recharge


Moderate to high pollutant removal


assumed, but seldom measured


not good for soluble pollutants (nitrate & chloride)


possible risks of groundwater contamination


Best used in conjunction with other treatment
practices


Longevity is less than 5 years without multiple
pretreatment practices


Cannot be used if contributing drainage is a hotspot



Copyright 2000, CWP

Infiltration Systems


Feasibility Criteria


Soil infiltration rate, f
c
,
>

0.5 inches/hour (requires
geotechnical tests)


Soil clay content < 20% and silt/clay content < 40%


Cannot be located on slopes > 15% or within fill soils


Runoff from hotspots must be pretreated before infiltration


May be prohibited within karst regions


Bottom should be
>

4 feet above high water table or
bedrock layer (requires soil borings)


100 feet horizontal setback from water supply well


Maximum contributing area < 5 acres


Horizontal Setback 25 feet down
-
gradient from structures


Copyright 2000, CWP

Infiltration Systems

Conveyance Criteria


Conveyance system to and from facility


Design to deliver and pass excess water at non
-
erosive velocities


Off
-
line design should be used if flow is delivered
by storm drain pipe or along main conveyance
system


Copyright 2000, CWP

Infiltration Systems

Pretreatment Criteria


>

25% of WQ
v

must be pretreated before entry (
>
50%
if soil infiltration rate is > 2 inches/hour) using the
following (or equivalent):


sedimentation basin


stilling basin


sump pit


Use redundant pretreatment methods


grass channel


grass filter strip


bottom sand layer


upper sand layer


washed bank run gravel as aggregate


Copyright 2000, CWP

Infiltration Systems

Pretreatment Criteria Cont’d


Camp
-
Hazen equation may also be used to calculate
pretreatment requirements



Pretreatment area based on WQ
V


Camp
-
Hazen equation: A
s

=
-
(Q
o
/W)*Ln(1
-
E),
where A
s

is surface area of sedimentation basin


A
s

= 0.066 (WQ
V
) ft
2

for impervious cover


75%


A
s

= 0.0081 (WQ
V
) ft
2

for impervious cover > 75%


Exit velocities from pretreatment shall be non
-
erosive
for 2
-
yr design storm

Copyright 2000, CWP

Infiltration Systems

T
reatment Criteria


Design to exfiltrate the WQ
v

less pretreatment
volume


Design storage reservoir to de
-
water WQ
v

within 48
hours after storm


Assume a porosity value (i.e., volume of voids/total
volume) of 0.4 used for stone reservoirs


Downstream detention often needed for Cp
v

and Q
p


Copyright 2000, CWP

Infiltration Systems

Landscaping Criteria


Dense vegetative cover should be established over
the contributing pervious drainage areas


Infiltration systems should not be constructed until
all of the contributing drainage area has been
completely stabilized


Infiltration basins should establish dense
vegetation on the basin side slopes and floor to
prevent erosion and sloughing

Copyright 2000, CWP

Infiltration Systems

Maintenance Criteria


Observation well should be installed in infiltration
trench


Extreme care should be taken during construction stage


no coverage with impermeable surface


avoid sediment entry


do not use as sediment control device


avoid compacting subsoils during construction


OSHA standards for trench excavation


Underdrain pipe system is recommended for de
-
watering where marginal soils exist


Direct access for maintenance and rehabilitation should
be provided

Copyright 2000, CWP

Infiltration Systems

Three Design Variations


Infiltration trench


Infiltration basin


Porous pavement

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Infiltration Trench

Design Notes


Field verification of soil permeability essential


f
c

> 0.5 in/hr


Require infiltration tests and test pit/soil boring at location of
proposed facility


1 test pit/soil boring per 50 ft of trench to depth of 4 ft
below bottom of proposed facility bottom


Geotextile fabric should interface between the trench
sidewalls and stone reservoir and top gravel filter


A 6
-
inch sand filter layer should be placed on the bottom of
the infiltration trench

Copyright 2000, CWP

Infiltration Trench

Design Notes Cont’d


Maximum trench depth is determined by:

d
max

= f T
s
/n

Where:

d
max
= maximum depth

f


= final infiltration rate (in/hr)

T
s

= maximum allowable storage time (hr)

n


= porosity

Copyright 2000, CWP

Infiltration Trench

Design Notes Cont’d


Surface area of trench is determined by:

A
t

= V
w

/ (nd
t

+ fT)

Where:

A
t


=

surface area of trench

V
w

=

design volume entering trench (e.g., WQ
v
)

n


=

porosity

d
t


=

trench depth based on the depth required above



seasonal groundwater table or a depth less than



d
max
, whichever is smaller

f


=

infiltration rate of trench

T


=

time to fill trench (generally assumed to be less



than 2 hours)

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Infiltration Basin

Design Notes


Failure rates of 25 to 100% recorded in the field


Pretreatment with settling basin is recommended


Partially excavated basins should not be used as
sedimentation basins during construction


Final excavation should be deferred until all
contributing areas have been stabilized or
protected


A 6 to 12 inch layer of filter material (e.g., coarse
sand) is recommended to prevent the builup of
impervious deposits on the soil surface. This layer
can be replaced when it becomes clogged.


Copyright 2000, CWP

Infiltration Basin

Design Notes Cont’d


Maximum basin depth is determined by:

d
max

= f T
p

Where:

d
max
= maximum depth

f


= final infiltration rate (in/hr)

T
p

= maximum allowable ponding time (hr)

Copyright 2000, CWP

Infiltration Basin

Design Notes Cont’d


Bottom surface area of basin is determined by:

A
b

= [2V
w

-

A
t

d
b
] / [d
b

-

2P + 2fT]

Where:

A
b


=

bottom surface area of basin

V
w

=

design volume entering basin (e.g., WQ
v
)

A
t


=

top surface area of basin

d
b


=

basin depth based on the depth required above



seasonal groundwater table or a depth less than



d
max
, whichever is smaller

P


=

design storm rainfall depth

f


=

infiltration rate of basin

T


=

time to fill basin (generally assumed to be less



than 2 hours)

Copyright 2000, CWP

Infiltration Basin

Design Notes Cont’d


Top length of basin (assuming a rectilinear shape) is
determined by:

L
t

= [V
w

+ Zd
b

(W
t



2 Zd
b
)] / [W
t

(d
b



P)


Zd
b
2
]

Where:

L
t


=

top length of basin

V
w

=

design volume entering basin (e.g., WQ
v
)

Z


=

side slope ratio of basin (h:v)

d
b


=

basin depth based on the depth required above


seasonal groundwater table or a depth less than


d
max
, whichever is smaller

W
t

=

top width of basin

P


=

design rainfall event


Note: The basin top length and width should be greater than 2Zd
b

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Porous Pavement

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Porous Pavement

Design Notes



Soils need to have a permeability between 0.5 and 3.0
inches per hour.


The bottom of the stone reservoir should be completely flat
so that infiltrated runoff will be able to infiltrate through
the entire surface.


Porous pavement should be sited at least 2 to 5 feet above
the seasonally high groundwater table, and at least 100 feet
away from drinking water wells.


Porous pavement should be sited on low traffic or
overflow parking areas, which are not sanded for snow
removal.

Copyright 2000, CWP

Porous Pavement

Design Notes

Cont’d


Asphalt, concrete or concrete
-
grid can be used


Vacuum sweeping needed


Construction stage sediment control is critical


Overflow inlets are recommended to convey
larger storms and as a relief for sealed
pavement surfaces


Needs an informed owner and long term
education commitment


Winter plowing/sanding can be a problem

Copyright 2000, CWP

Porous Pavement

Design Notes

Cont’d


Proper maintenance of porous pavement should
include the use of a carefully worded
maintenance agreement that provides specific
guidance, including how to conduct routine
maintenance, and how the surface should be
repaved. Signs should be posted on the site
identifying porous pavement areas.


Photo Copyright 1999, Center for Watershed Protection

Copyright 2000, CWP

Photo Copyright 1999, Center for Watershed Protection

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Copyright 2000, CWP

Infiltration Systems

Longevity


3 studies in Maryland indicate poor to moderate longevity
of first generation infiltration systems


Infiltration basins had poorest longevity (100% failure)


Infiltration trenches


about 20% failed initially


additional 30% failed within 5 years of construction


most trenches were never maintained


grass filter strips, alone, are inadequate pretreatment


sump pit pretreatment appeared useful

Copyright 2000, CWP

Infiltration Systems

Longevity (cont’d)


Porous pavement
-

significant evidence of failure


initial failure


clogging over time


resurfacing


Longevity can be improved with:


appropriate application (e.g., overflow parking only)


better soil testing


careful construction techniques


redundant pretreatment


regular maintenance