silt fence_auckland.pdf
TRANSCRIPT
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 2
2.2 Silt Fence
Definition
A temporar y barr ier of woven geotextile fabric used to
intercept runoff, reduce its velocity and impound
sediment laden r unoff from small areas of disturbed soil.
Purpose
To detain flows from runoff so that deposition oftran sport ed sediment can occur thr ough settlement. Silt
fences can only be used to inter cept sheet flow. Do not
use them as velocity checks in chan nels or place them
where th ey will intercept concent rated flow.
Application
o On low gradient sites or for con fined ar eas where
the contribut ing catchmen t is small, such as short
steep batter fills and arou nd watercour ses.
o To delineate the l imit of dis turbance on an
earthworks site such as riparian areas or bush
reserves.
o To store runoff behind the Silt Fence without
dam aging the fence or the submerged area behind
the fence.
o Do not in stall Silt Fences across water cour ses or
in ar eas of concentrated flows.
Designo Ensure Silt Fence height is a min imu m o f 400m m
above groun d level.
o Place supp ortin g posts/warat ahs for Silt Fences no
mor e than 2m apart un less additional support is
provided by tensioned wire (2.5mm HT) along the
top of t he Silt Fence. Where a stron g woven fabr ic
is used in conjunction with a wire suppor t, the
distance between posts can be extended up to 4m .
Double the Silt Fence fabric over and fasten to t he
wire and posts with wire ties or cloth fastening clipsat 150mm spacings. Ensure supporting posts/
waratahs are emb edded a min imum of 400mm into
the ground .
Plate 2.2 Silt Fence
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 3
o Always install Silt Fences along th e contou r. Wher ethis is not possible or where there are long sections
of Silt Fence, install short Silt Fence returns
projecting up slope from the Silt Fence to min imise
concentration of flows. Silt Fence returns are a
minim um 2m in length, can incorporate a tie back
and are generally constr ucted by continuin g the Silt
Fence around the re turn and doubling back,
elimin ating joins.
o Join lengths of Silt Fence by doub ling over fabric
ends around a wooden post or batten or by staplingthe fabric ends to a batten and butting the two
batten s together as shown in Figure 2.2.
o Maximum slope lengths, spacing of return s and
angles for Silt Fences are sh own in Table 2.2.
o Install Silt Fence wings at either end of the Silt
Fence projecting upslope to a su fficient height to
prevent ou tflanking.
o Where imp ound ed flow may overtop th e Silt Fence,
crossing natu ral depression s or low points, ma ke
provision for a r iprap splash pad or other outletprotection d evice.
o Use of Silt Fences in catchmen ts of m ore th an 0.5ha
requires careful consideration of specific site
measures, and other control m easures maybe better,
such a s Super Silt Fence.
o Where water may pond behind the Silt Fence,
provide extra suppor t for th e Silt Fence with tie
backs from the Silt Fence to a centr al stable point
on the upward side. Extra support can also be
provided by stringing wire between support stakes
and connecting the filter fabric to this wire.
o T h e f a b r i c c lo th mu s t me e t t h e f o l l o win g
requ irem ents for Geotextile fabr ic.
Tension Strength: 0.345 p a (m in im um )
Tensile Modulus: 0.140 p a (m in im um )
Apparen t Open ing Size 100mm
Table 2.2 Silt Fence Design Criteria
Slope Steepness % Slope Leng th (o) (Maximum) Spacing of Returns (o) Silt Fence Leng th (o) (Max imum)
Flatter than 2 % Unlim ited N/ A Un lim it ed
2 1 0 % 4 0 6 0 3 0 0
1 0 2 0 % 3 0 5 0 2 3 0
2 0 3 3 % 2 0 4 0 1 5 0
3 3 5 0 % 1 5 3 0 7 5
> 5 0 % 6 2 0 4 0
Construction Specifications
o Use Silt Fence material app ropr iate to th e site
c o n d i t i o n s a n d i n a c c o r d a n c e w i t h t h e
manu factur ers specification s.
o Excavate a trench a minim um of 100mm wide and
200mm deep alon g the prop osed line of the Silt
Fence. Install the support posts on the down slopeedge of the trench and Silt Fence fabric on the
upslope side of the support p osts to the full depth
of the trench, then backfi l l the trench with
compacted soil.
o Use suppor t ing pos ts o f tana l i sed t imber a
minimu m of 50mm square, or steel waratahs at least
1.5m in length.
o Reinforce the top of the Silt Fence fabric with a
wire supp ort m ade of galvanised wire of a minim um
diameter of 2 .5mm. Tension the wire using
permanent wire s tra iners a t tached to angled
waratahs at th e end of the Silt Fence.
o Where ends of Silt Fence fabric come together,
ensur e they are overlapped , folded an d stapled to
prevent sediment bypass.
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 4
Maintenanceo Insp ect Silt Fences at least o nce a week an d after
each rainfall. Make any necessary repairs when
bulges occur or when sediment accumulation
reaches 50% of the fabric height.
o An y a r e a s o f c o l l a p s e , d e c o mp o s i t i o n o r
ineffectiveness need to be imm ediately replaced.
o Remove sediment deposits as necessary to continueto allow for adequate sediment storage and reduce
pressure on the Silt Fence. Ensure that the sedim ent
is removed to a secure area.
o Do not rem ove Silt Fence mat erials and sedimen t
deposition until the catchment area has been
appr opr iately stabilised. Stabilise the area of th e
rem oved Silt Fence.
Figu re 2.2 Silt Fen ce
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 5
2.3 Super Silt Fence
Plate 2.3 Super Silt Fence
Definition
A tempor ar y bar rier of geotextile fabric over chain link
fence that is used to intercept flows, redu ce their velocity
and impound sediment- laden runoff from small
catchment areas.
Purpose
To redu ce run off velocity and allow the dep osition of
transpor ted sediment to occur.
A Super Silt Fence provides much m ore robust sediment
control than a stan dard Silt Fence and allows up to four
times the catchment area to b e treated by an equivalent
length of stan dard Silt Fence.
Application
o Provides a barrier that can collect and ho ld debris
and soil, preventing the mater ial from entering
critical areas, watercourses and streets.
o Can be u sed where the installation of an Earth or
Topsoil Bund would destroy sen sitive areas such as
bush and wetlands.
o Should be placed as c lose to the contour as
possible. No section of the fence shou ld exceed a
grade of 5% for a d istance of more th an 15m.
Design
When consider ing Super Silt Fence installation for larger
catchm ents (gr eater t han 0.5ha) as in Table 2.3, carefully
consider the specific site cond itions and oth er altern ative
contro l measu res available. Base the length of t he Super
Silt Fence is based on the lim its shown in Table 2.3.
Limits im posed by ultraviolet light affect the stab ility of
the fabric and will dictate the maximum per iod that the
Sup er Silt Fence may be u sed.
Where ends of the geotextile fabric come together,
overlap, fold and staple the fabric ends to prevent
sediment bypass.
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 6
Table 2.3 Super Silt Fence Design Criteria
Slope % Slope Length (o) Super Silt Fence
( m axim u m ) Leng th (o) ( max i mum)
0 1 0 Unlim ited Unlim ited
1 0 2 0 6 0 4 5 0
2 0 3 3 3 0 3 0 0
3 3 5 0 3 0 1 5 0
> 5 0 1 5 7 5
Construction Specifications
o Use a Silt Fence fabric that is appr opr iate to the
s i te cond i t ions and f it s the man ufac tu re r s
specifications.
o Excavate a trench 100mm wide by 200mm deep
along th e line of the Super Silt Fence.
o Position th e posts (No. 3 rou nd s, No. 2 half roun ds
or waratahs) at no greater than 3 ocentres on the
downslope side of the tr ench. While there is no needto set th e posts in concrete, ensure t he 1.8m long
p o s t s a r e d r i v e n t o a n a p p r o p r i a t e d e p t h
(1m m inimum).
o Insta ll tension ed galvanised wire (2.5 m m HT) at
400mm and again at 800mm above ground level
using permanent wire strainers.
o Secure chain link fence to the fence posts with wire
ties or stap les, ensur ing the chain link fence goes to
the base of the trench.
o Fasten t wo layers of geotextile fabr ic securely tothe Super Silt Fence with ties spaced every 60cm at
the top and mid section of the Super Silt Fence.
o Place the two layers of geotextile fabric to th e base
of the trench (a minimum of 200mm into the
ground ) and p lace compacted backfill back to the
original grou nd level.
o When t wo sections of geotextile fabric adjoin each
other, ensure th ey are doubled over a m inimum of
300mm, wrapped aroun d a batten an d stapled at
75mm spacings to p revent sediment bypass.
o The geotextile fabric must meet the following
requirements:
Tension Strength 0.345 pa (m in imum )
Tensile Modulus 0.140 pa (m in imum )
Apparent Opening Size 100 500mm
Maintenance
Inspect regularly and before and after storm events.
Undertake maintenance as needed and remove silt
buildup s when bulges develop in the Super Silt Fence or
when sed iment deposition reaches 50% of the Super SiltFence height.
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 7
Figu r e 2.3 Su p er Silt Fen ce
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 8
2.4 Hay Bale Barrier
Definition
Tempor ary barr iers of hay bales used to intercept and
direct sur face ru noff from sma ll areas.
Purpose
To intercept or direct sediment laden r unoff from sm all
areas to a sediment r etention facility so that d eposition
of tran sport ed sedim ent can o ccur. Hay Bale Bar riers d o
not filter sediment.
Application
o Hay Bale Barriers are n ot prim ary sediment contr ol
measures. They easily deteriorate and require
frequent maintenance.
o Only use Hay Bale Barr iers to meet short t erm needs
of less than one m onth duration.
o Only use Hay Bale Bar rier s to int ercept sh eet flow.
Do not use th em as velocity checks in channels or
place them where they will intercept concentrated
flow. They do not act as filters and are easily
overtopped or scoured out.
o Do not use with a catchm ent area of more th an
0.2ha per 100o
length of ha ybales.o Do not use Hay Bale Barriers on slopes exceeding
20%.
Design
Not Applicable.
Plate 2.4 Hay Bale Barr ier
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 1 9
Figu re 2.4 H ay Ba le Ba rr ier
Construction Specificationso Place Hay Bale Barriers along the contour with
bales in a row with the ends tightly abutting
adjacent bales.
o Dig each bale into the groun d 100mm and place so
the bale bindings are horizontal.
o Do not place bales more th an on e bale high.
o Secure bales in place by two stakes dr iven thro ugh
the bale 300 to 400m m int o the grou nd . Drive the
first stake toward the previously laid bale at an angle
to force the b ales togeth er. Drive stakes flush with
the top of th e bale.
MaintenanceInspect Hay Bale Barr iers frequent ly and after each r ain
event. Undert ake mainten ance as necessary.
Remove all bales when the site has b een fully stabilised.
Stabilise the tr ench where the bales were located an d
grade flush.
F l o w
S t r i n g B i n d e r
U n d i s t u r b e d G r o u n d
1 0 0 m m V e r t i c a l F a c e
Bedding Detail
A n g l e f i r s t s t a k e t o w a r d
t h e p r e v i o u s l y p l a c e d b a l e
E n t r e n c h b a l e s a m i n i m u m o f
1 0 0 m m i n t o t h e g r o u n d
T r e n c h c u t i n t o g r o u n d
2 R e - b a r s o r 5 0 m m x 5 0 m m s t a k e s
d r i v e n 3 0 0 m m m i n i m u m i n t o t h e
g r o u n d . S t a k e s a r e t o b e d r i v e n
f l u s h w i t h t h e t o p o f t h e b a l e s
B o u n d b a l e s
p l a c e d a l o n g
c o n t o u r
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 2 0
2.5 Stormwater Inlet Protection
P lat e 2.5 Sto rm water In let Pr otect ion
Application
o Do not use Stormwater Inlet Protection as a
primary m ethod of treatment instead of other
sediment retention facilities.
o Use only in sma ll catchm ents of less than 0.5 ha.
o Use only where the catchment area to an inlet is
disturbed and it is not possible to temporar ily divert
the storm drain out fall into a sediment retention
facility.
Stormwater Inlet Protection only offers limited t reatment
of sedimen t-laden water, because of the concentr ated
flows arriving at them. Stormwater systems are, by
design , very efficient at con duct ing flows away from
inlets, and therefore, once any sediment reaches the
storm water system, it will be discharged directly to the
receiving environment .
Therefore, the n eed to u se Stor mwater Inlet Protection
can indicate poor erosion and sedim ent control and/or
inadeq uate stabilisation on th e site.
Design
There are various design options for reducing sediment
inputs to the storm water cesspits.
Silt Fence Design
A Silt Fence can be er ected arou nd the inlet (see Part B
Section 2.2). This meth od is app ropr iate where cesspits
have been connected to a stor mwater system an d are
collecting ru noff from distur bed soil surfaces.
Filter Media Design
Two common methods use geotextile and scoria or gravel
to tr eat sedimen t laden flows. All points where r un off
can enter th e cesspit must be pr otected with suitable
geotextile fabric.
o Wrap geotextile fabric arou nd the cesspit grate as
a bar rier to flow directly from the roadside gutter.
Pay special attention to th e inlet above the grate
back of the cesspit wh ere a geotextile fabric sockfilled with gravel must be placed to intercept ru noff.
o Lay coarse geotextile fabric over the cessp it an d
up on to the kerb with a layer of aggregate material
to act as a pr imary filter and to hold th e fabric in
Definition
A barrier across or ar ound a cesspit (stormwater inlet).
Purpose
To intercept an d filter sedim ent-laden r un off before it
enters a reticulated stor mwater system via a cesspit,
thereby preventing sediment -laden flows from enter ing
receiving environments. The protection m ay take various
forms dependin g upon the type of inlet to be protected.
Storm water Protection is a secondar y sediment control
device. It must on ly be used in con junct ion with oth er
erosion and sediment control measures.
If good erosion and sedim ent control measures are inplace on the site, then Stor mwater Inlet Protect ion will
not be required.
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A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 2 1
place.
Check Dams
Place a series of low sandba g check dams u p th e gutter
from cesspits to act as a series of sediment tr aps. The
checkdams requ ire a spillway lower than the kerb to
ensure that runoff does not encroach onto the ber m area
and cause scourin g. Const ruct checkdam s out of up to
six sandbags laid end to end with no gap s in an arc away
from the kerb and up the road to create a series of
impound ment areas.
Excavated Inlet Pr otection
Excavating aroun d a stor mwater inlet creates storage
capacity where suspended m aterial can settle out. Ensure
that seep age holes allow for filtered dewatering, and th at
the capacity provided arou nd t he inlet for storage is a
minimu m of 1% of the catchm ent (1m3of capacity per
100 m 2of contr ibuting catchm ent).
Maintenance
Maintenance requirements for cesspit protectionmeasures are high because they clog easily. When clogging
occurs, remove accumulated sediment and clean or
replace the geotextile fabric an d aggregate.
Inspect all Stormwater Inlet Protection measures
following any rainfall event and m aintain as n ecessary
to en sure they oper ate effectively.
Stor mwater Inlet Protection pr ovides at best limited
sediment retent ion. Do not use it as a prim ary meth od
of sediment contr ol. Use add itiona l measures up -slope,
such as topsoil bunds an d cut-off drains, to m inimisethe volume of sediment reaching any stormwater
inlets. Cesspits mu st at all times re main able to convey
flow from the site to pr event large concent rated h ighly
erosive flows from bu ilding up and causing washouts
in secondary overland paths.
Construction Specifications
Constr uct Silt Fences for Storm water In let Protection a s out lined in Part B Section 2.2 of these Guidelines.
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 2 2
Figure 2.5 Stormwater Inlet Protection Filter Media Design
2.6 Earth Bund
Plate 2.6 Ear th Bund
Definition
A te mp o r a r y b e r m o r r i d g e o f c o mp a c t e d s o i l
(including topsoil) constru cted to create imp oun dm ent
areas where ponding of runoff can occur and suspended
mat erial can settle before r unoff is discharged.
Purpose
Used to intercept sediment-laden r unoff and redu ce the
amount of sediment leaving the site by detaining
sediment-laden runoff.
Application
Earth Bunds can be constructed across disturbed areas
and ar ound constr uction sites and subd ivisions. Keep
them in place until the disturbed areas are perm anently
stabilised or adeq uately replaced by other m eans. Earth
Bun ds can assist the settling of sedime nt-laden r un off.
Earth Bun ds are par ticularly useful for cont rolling runoff
after topsoiling and grassing before vegetation becomes
established. Where works are occurrin g within t he berm
area, compact the topsoil over the berm area as a bun d
adjacent and parallel to the ber m. This will act as an
impoun dm ent area while also keeping overland flow
away from the constru ction area.
Design
o Earth Bund s need a constructed outlet structure
and spillway as designed for Sedim ent Retention
Pond s (Par t B, Section 2.1 of these Guidelines).
Altern atively, constr uct an outlet of perforated pipe
connected to a non -perforated pipe that passes
through t he Earth Bund and either d ischarges to
the gutter or d irectly to a stor mwater inlet. Ensure
that the section of pipe within the impoundm ent
area is suppor ted by mean s of a rigid post, allowingfiltrat ion to occur.
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 2 3
o Ensur e the top opening of the perforated pipe is100 mm lower than the st abilised spillway.
o Ensure the section of pipe leading thr ough the Earth
Bun d and cont inuing down slope below the Earth
Bund is non-perforated.
which may cause overtop ping. Check any discharge points for signs of scour ing and install furt her arm our ing or other
o Constru ct the Earth Bund such that th e maximumcontributin g catchm ent do es not exceed 0.3 ha.
Maintenance
Inspect and m aintain Earth Bund s regularly and after
each rainfall event to check for accumulated sedim ent
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 2 4
stabilisation if scouring is evident .
Figu re 2.6 Ear th Bu nd
2.7 Sump / Sediment Pit
Plate 2.7 Su mp / Sedim en t Pit
Definition
A temp orar y pit which is constr ucted to tr ap and filter
water before it is pu mped to a suitable discharge area.
Purpose
To tr eat sediment-laden water that h as been r emoved
from areas of excavation, or areas where ponded
sediment laden-water can not drain by other m eans.
Application
o When water collects dur ing the excavation ph ase
of constru ction.
o Particularly useful in urban areas dur ing excavation
for building foun dation s.
o
May also be used to de-water sediment retentionmeasures.
Design
The design is based on a per forated vertical standpipe
placed in the centre of a pit which is then backfilled
with aggregate.
o Determine th e num ber of Sum p/Sediment Pits and
their locations on site in accordance with the
required dewatering facilities and procedures
outlined below.
o Pump water from the centre of the pipe to a suitable
discharge area.
o Direct the discharge to an appr opriate out let.
o If the water is pumped directly to a receiving
environm ent, then wr ap a geotextile fabr ic aroun d
the s tandpipe to help achieve a c lean water
discharge. When a geotextile fabric is used, the
surface area of the standpipe will need to be
increased and the pumping rate decreased to
prevent th e geotextile becoming ra pidly blocked.
o Sum p/Sediment Pit dimensions ar e variable, but
require a minimum depth of 1m an d a m inimum
volum e of 2m3.
o Construct the s tandpipe from 300 600mm
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erosion&sedimentc o n t r o lG u i d e l i n e s f o r L a n d D i s t u r b i n g A c t i v i t i e s i n t h e A u c k l a n d R e g i o n
A u c k l a n d R e g i o n a l C o u n c i l S e d i m e n t C o n t r o l : 2 2 5
diameter pipe with a grid of 10 mm diameterperforations at 60mm spacings along the stan dpipe.
o Place a base of 50mm aggregate in the Sump/
Sedim ent Pit to a depth of 300mm .
o After placing the sta ndp ipe in position , backfill the
area with 50mm aggregate.
o Extend t he stan dpip e 300m m abo ve the lip of the
Sum p / Sediment Pit with th e aggregate extended
100 mm above the anticipated standing water
elevation.
Maintenance
Undertake ongoing checks throu ghout the use of theSum p/Sedimen t Pit to ensur e effective oper ation .
For isolated areas where dewatering must occur to
facilitate progress, other method s may be appr opriate.
These alternat ives include th e following.
o Pump ing accum ulated sediment-laden water to a
sediment retention pond.
o Constru cting a Silt Fence and pum ping water to
behind th e Silt Fence to be retained for tr eatment.
Do not let water to b e treated enter the Silt Fence
as a concentr ated flow or out flan k the Silt Fence.
o Discharge accum ulated sediment laden water to
land where soakage may occur. Ensure that th is untreated sedim ent-laden r un off cannot ente r to a storm water
t t