silt fence_Auckland.pdf

8/14/2019 silt fence_Auckland.pdf

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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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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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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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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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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


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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Figu r e 2.3 Su p er Silt Fen ce


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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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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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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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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.


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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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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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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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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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

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