Soil CohserwtionHandbook

for Parks & Reserves in the Northern Territory

Kate HaddenCONSERVATION COMMISSION OF TI-IE NORTHERN TERRITORYTECHNICAL REPORT. NUMBER 54

March 1993DARWIN N.T.

3.3 FENCES AND FIREBREAKS

Soil Conserualion Hondbook for Porks and Reserues in the Nf 3.3 -l

MAIN POINTS IN THIS CHAPTER

o Locote fencelines ond firebreoks, where possible, olong ridgelinesor porollel to the contour. Avoid olignments which run diogonollyocross the slope.

o Existing fences not in ideol positions should be scheduled forrelocotion or upgroding in the long-term pork plon.

r Noturol borriers such os rivers or escorpments should be used tosupplement firebreoks ond fences.

. Creek crossings must be of right ongles to the flowline, ond neederosion control works obove eoch riverbonk.

o Plon oheod for the strotegic locotion of gotes ond occess roods,ond the need for erosion control bonks.

o In mony situotions, fencelines ond firebreoks con be constructedwith minimol disturbonce to the soil surfoce.

o Sloshed, rolled or ploughed breoks ore for more preferoble to flotbloded breoks, ond con be cheoper to construct.

o Mointenonce requirements ond the need for mochinery occessshould be decided qt the plonning stoge before constructionbegins.

3.3 -2 Soil Conservolion Handbook for Porks ond Reserves in the NT

t .

2,

3.3.I GENERAL STEPS TO PLANNING FENCES & FIREBREAKS

ldentify on mops or oeriol photos the oreo to be fenced or protected.

Drow existing fences, including the locotion of gotes ond occessroutes. lf the gotes ond occess ore not locoted in suitoble oreos,consider moving them where they con better fit in to the long termpork plon.

Drow o tentotive locqtion of the new fenceline/firebreok (does nothove to be in stroight line). Where possible use noturol feotures, mokeuse of ony ovoiloble informotion such qs lond unit mopping,topogrophic mopping or oeriol photogrophs, ond obtoin experlodvice. Keep in mind future occess requirements for mochinery orgenerol pork use.

Inspect oreos which moy couse problems (droinoge lines, sondy soils,etc.), relocote if possible.

Seek input from Soil Conservotion Officers if required.

Determine the type of fenceline or firebreok preferred (ploughed,rolled, sproyed etc.) ond work out construction ond droinogerequirements, with oppropriote odvice where required.

Monitor construction.

3.

4.

5.

6.

7.

Soil Conservation Hondbook for Parks and Reserves in the Nf 3.3 -3

FENCES & FIREBREAKS

Fences ond firebreoks, os withtrocks ond roods, require removolo f v e g e t o t i v e c o v e r f o rconstruction. This results inincreosed runoff, leoving the soilsurfoce vulneroble to occelerotederosion.

Troditionolly, firebreoks olongfencelines hove been constructedby bloding, leoving o bore soilsurfoce. Although this moy beeffective, it con olso be on erosionhozord.

Bloded breoks will olmost olwoysbe subject to rilling during wetperiods, especiolly on slopesgreoter thon 2olo. Annuol grodingwill smooth out rills, but only untilthe next roins. Annuol grodingolso creotes soil windrows whichdivert ond concentrote woter.Eventuolly this con leod to theformotion of o deep 'V' shopedchonnel olong the fenceline oredge of the breok, threoteningtheir integrity. Associoted gulliescon olso form. leoving thefirebreok ond fence in onunmonogeoble condition.

This situotion con eosily beovoided through odequoteplonning of both the locotion ondconstruction of fences ondfirebreoks. The locotion of gotesond occess roods should olso becorefully considered of theplonning stoge. Roods should notbe diverted lo go through gotes,os inoppropriote olignments moyresult. Goles should be plonned sof hot they ore locoted onoppropriolely oligned roods.

Existing structures con olso beutilised for fences ond firebreqks.F o r e x o m p l e , o p r o p e r l yconstructed ond well mointoinedrood locoted in o strotegic positioncon oct os on effective firebreok.Alternotively, noturol feotures suchos o cliff foce con oct os onoccess borrier, while creeks ondrivers con oct os effectivefirebreoks.

3.3.2 LOCATTON

Firebreoks ore locoted to controlfires, ond fences ore locoted tooid monqgement, Eose of occessond mointenonce requirementsore primory concerns in firebreokond fenceline locotion. Minimisingerosion should olso be o primoryconcern. Bodly eroded fencelinesond firebreoks moke occess moredi f f icul t , ond use voluobleresources in repoir work.

For erosion control, the locotion offencelines ond firebreoks shouldfollow the some principles os thoseopplied to roods ond trocks. Themost suitoble olignments ore thoseolong ridgelines or porollel to thecontour lines, ond in oreos of gooddroinoge.

Creek crossings ond oreos of loosesondy soils should be ovoidedwhere possib le, crs shouldolignments diogonolly ocross theslope. Areos of erosion hozordshould be identified either throughfield inspection, lond unit moppingor odvice from Soil ConservotionOfficers. These oreos should beovoided, or hove odequote

3.3 -4 Soil Conservotion Hondbook for Porks ond Reserves in lhe IVI

erosion control works included inthe initiol design,

Where flowlines must be crossed, itshould be of right ongles. In somecoses, ossocioted occess trocksmoy need to be diverted owoyfrom the cleored line to oreoswhere crossings ore better suitedto trocks. By doing this, thepotentiol for erosion is minimised.

I-nc-orrecl

Corrq:

Fig. 3.3..| Locotion of fences on ridgelines.

Soil Conservolion Handbook lor Parks ond Reserves in the Nf 3.3 -5

3.3.3 DRAINAGE

The droinoge requirements offencelines ond firebreoks must bedecided before construction, ondespeciolly before fences oreerected. Once fences ore built, itis difficult for mochinery to workocross the cleored oreo. Efficientdroinoge of the surfoce is essentiolfor the prevention of erosion,

Treod crowning ond crossfolldroinoge

Treod crowning con be used onfencelines ond firebreqks todisperse runoff to both sides of thecleored line. lf lines ore regulorlygroded, core must be tqken tomointqin the crown, ond qvoidlowering the line to below thenoturol surfoce. lf the surfoce islowered th rough cont inuo lgroding, the line becomes ochonnel which will concentrotewoter ond couse scouring.

Crossfoll droinoge is suitoble forfencelines ond firebreoks whichore constructed porollel to thecontour line. This qllows woter toflow unimpeded ocross the breokto the downslope side, os long osthere ore no windrows to interferewith cross flow.

Fencelines with crossfoll droinogeshould not be groded, os o highpoint develops under the fencewhere the groder hos no occess.This high point intercepts flow ondcon leod to chonnelling onderosion (Fig. 3.2.2). Other methodsof construction (covered loter inthis chopter) should be used onfencelines with crossfoll droinoge.

Firebreoks without ossociotedfences moy be groded, howevercore must be token to ovoidlowering the surfoce ond creotinggroder windrows. Both theseqctions will impede cross flow ondcouse woter to chonnel down thecleored line.

lf required, side droinoge (mitredroins, toble droins, etc.) con beused in conjunction with treqdcrowning ond crossfoll droinoge tosofely dispose woter owqy fromthe cleored line.

Treod crowning ond crossfolldroinoge is covered more fully inChopter 3.2 (Roods).

3.3 4 Soil Conservation Handbook for Parks ond Reserues in the Nf

Na-t.rn2lSur€ace-undistdrbel

\

ltdz'

Nafonrl eqfrco-undisturbed Nafr.rral svr{aco

- tlndistc.rrbelFlow

Fire*&. crtV epaelFrtrled qt fldga

l% F" t l e rc t

Fig. 3.3.2 Treod crowning & crossfoll droinoge.

Soil Conseruation Hondbook lor Parks ond Reserves in the NT 3.3 -7

Nalrral 3,r€age, -undist<srbd

Nailsral gtrfaco-undis*rsrbed

Firabreak only orctrucfud on sjda-tslop€-

Ear+h wiMrclv

tfstrffwFlow

Fencejine,' 4 fi re,bresk eg9.d annwlly_

----F/ow_=_._r_<

---)

t%r , t r 7

Fig. 3.3.2 (cont,)

3.3 -8 SorT Conservation Hondbook for poks ond Reseryes in the NT

Cross bonks ond mitre droins

Cross bonks ond mitre droins conbe constructed to remove woterfrom the surfoce of the breok, ondto dispose of it owoy from thesides.

Areqs requiring erosion controlbonks should be identified beforeconstruction. This will primorilydepend on slope ond soil type.Ooble 3,2.1 in Roods givesindicotions of the erodibility ofsome soils). Further informotion isovoiloble through lond unit mops,topogrophic mops ond odvicefrom the Lond Conservotion Unit.

Cross bonks should be constructedocross the line to provide o lolo follto the outlet, lt is importont thotthe grode does not exceed lolo, qsthis moy couse scouring, Theoutlet must be stoble, ond locotedin undisturbed country.

Bonk design depends on slope,cotchment ond peok f lowestimotes. In generol, however,construction will require cut ond fill,forming o chqnnel on the insidewhich corries woter to the outlet(Fig. 3.3.3). The bonk should becompocted, ond ony topsoilrespreod over the whole oreo. lfon occess rood is required olongthe eosement, the bqnks moyneed romps built into them to oidtrofficobility (see Roods).

Where flowlines qre crossed, bonksshould be built obove the tops ofeoch creek or river bonk (Fig.3.3.4). Agoin the outlets fromthese bonks must be in stoble, wellvegetoted oreos.

The following toble offers o generolguide to bonk spocing in the TopEnd. This con olso be used os orough guide for centrol Austrolio,olthough on some soils the bonksmoy need to be closer together.Further odvice is ovoiloble from SoilConservotion Officers.

Toble 3.3..| Intervols betweenerosion control bonks on fencelinesond firebreoks.

Low to moderote soil erodibility

Slope (o/o)

2-45-9t0- t 5> . |5

Bonk spocing (m)

20010030t 5

High soil erodibility

Slope (o/o) Bonk spocing (m)

I2345

&r01 t - 1 5> ] 5

r30907565604030t 5

Mitre droins (olso colled offshoot,offlet or spur droins) toke woterfrom the surfoce of the cleoredline for disposol onto stoble,undisturbed oreos Gig. 3.3.5). Theconstruction of these droins iscovered in Chopter 3.2, Roods.

Soil Conserualion Hondbook for Porks and Reserves in the NT 3.3 -9

=60- 4&rnm Reepread +@J

Tcps,oilI'q..

Cro>s srction

Y, ffi,'1,

% / trTAYlWH{az// ,.arQE?",,.*,-

Fig. 3.3.3 Bonks.

Soil Consewalion Handbook for Porks ond Reserves in lhe NT3.3 -t0

1 Flow

,'\' J j\o)c.\

/9zz\- ,/)<r.'- ,/a,3r---" o,'6-----\ry) ol<\ ,'qx ' qr4rh

\ \

'- '-tt" I i\,*.*".o4,, { '

o

I outlef rn---. undist.rrbed- YeAdalioi

. / l o/ l l

. / l tt ---- | le.ncO

- t l

Oe ribo - PIqD

Mifra droin

Firobr.ak

- - Ocrtlef invndietorbedv?gda+\Dn

On cxo*-slo?o-?lan

N,i..\I '1.\| |

'J.r

Fenco I

Fig. 3.3.5 Mitre droins,

3.3 -t2 Soil Conservation Handbook lor Parks ond Reserues in the Nf

3.3.4 CONSTRUCTTON

When bui ld ing fences ondfirebreoks, occess for constructiontroffic will be required. Whenproviding occess, the someprinciples should be opplied osthose covered in Chopter 3,2(Roods). Temporory qccessthrough dry flowlines mqy beprovided by cleon fill, however thiswill need to be removed beforethe onset of ony wet weother,

l_f working in weed infested oreos,it is importont thot topsoil or plontmoteriol is not tronsported out of

Fig. 3.3.6 Fence construction of creeks.

the oreo. lt moy be necessory toprovide wosh down oreos, oropply herbicide before leoving thesite.

When building q fence ocross ocreek or gully, weighted tin,pigwire, or lengths of plostic pipeshould be suspended from oreinforced strond of the top of thefence (Fig. 3.3.6). This ollows debristo poss under the fence, ovoidinginterference with chonnelflow ondsubsequent flooding ond fencedomoge.

6ronq a??"r cable(?9' ?oWe.r a\e,,)€orAOaQZTd|M iron oWs wirh'breaYa\t/ay fies

WeakitwaYIie.&wasf?;-uQM{ %k1on'3oft ircn g\eete

3heets oF iron

Sfrai\ne( Wt ForgecD(elv Aruhoriwonu znd of-*t*o€ iron

Soil Conservotion Handbook lor Porks ond Reserves in the NT 3.3 - t3

Rolling or drogging

Another olternotive to blodedbreoks is the construction of rolledbreoks. These ore formed by usingo light roller or gross leveller.Rollers or drogs con be built byplocing old tyres or o weightedmesh frome on on oxle, bydrogging o sleeper behind crvehicle or similor method.

ln the Top End, the vegetotion isrolled eorly in the dry seoson(Morch to April), left to dry, ondthen burnt. Loter in the dryseoson, the oreo outside this conbe burnt bock into the rolled oreq,providing the finol firebreok.

In the Southern Region, timing willdepend on loco l c l imot icconditions ond the omount ofbuild-up of vegetotive moteriol.Drogging should, however, becorried out once the vegetotionhos hod sufficient time to dry ofterroins, but before long periods ofdry weother.

Rolling or drogging is o cheopermethod thon ploughing or bloding.

Poisoning

Herbicides con olso be used onfirebreoks ond fencelines. The firstopplicotion is corried out eorly inthe growing seoson, ond furtheropplicotions corried out osrequired. The costs in the first fewyeors will be relotively high, but willreduce substontiolly thereoffer.

A p p r o p r i o t e c h e m i c o l s ,opplicotion rotes ond costs oreovoiloble in Deportment of PrimoryIndustry ond Fisheries Agnotes

81/02,226 ond 230, or on odvicefrom DPI&F Officers.

Eorly burning moy olso be corriedout in conjunction with chemicolcontrol.

Ploughing

Ploughed breoks should leove thesoil surfoce level in the sense thqtthere ore no bonks or furrowswhich cqn divert or concentrqtewoter. For this reoson, discploughs ore not suitoble; tynedimplements or offset disc horrowsore preferoble.

To ovoid long slopes, loops con beincorporoted into the breok so thoto n y w o t e r w h i c h d o e sconcentrote runs off into thesurrounding country. Whereprocticol, loops should be spocedsimilorly to bonks.

The position of o ploughed breqkshould be chonged every 3 to 5yeors to ovoid the destruction ofsoil structure, which itself leods toincreosed runoff ond potentiol forerosion. The plough directionshould olso be reversed eoch timeto prevent the formotion ofwindrows. Ploughed breoks shouldnot be used in oreos where woterconcentrotes (for exomple indroinoge lines or gullies).

Soil Conservalion Handbook for Parks and Reserves in lhe NT 3.3 - t5

3.3.5 MANAGEMENT

Fences ond firebreoks will requireong o ing m onogem ent ondperiodic mointenqnce. ln thecose of firebreoks, qnnuol sloshing,rolling or ploughing moy berequired. Fences will need regulorinspection ond periodic repoir,porticulorly of droinoge lines. lfexisting fences ore poorly locotedond require extensive upkeep,they should be morked for removqlor relocotion.

Erosion control bonks ond mitredroins should be regulorly checkedfor bonk foilure ond blockoges,ond the sites of finol disposolchecked for erosion domoge,especiolly ofter heovy roin.

Monogement requirements shouldbe considered qt the plonningstoge, ond occess routes forsloshers, groders, rollers, herbicidetroilers ond other equipment mustbe provided. lf permonent roodsore required for oll yeor occess,then they need to be constructedto minimise the risk of erosion (seeRoods). Temporory trocks shouldbe ripped ofter use, ond respreodwith topsoil (see Chqpter 4.3,Disused Sites).

Common problems occurring onfencelines ond firebreoks

The most common problemoccuning on fencelines ondfirebreoks is erosion ocross thesurfoce. This con eosif beprevented by using other methodsof construction, ond by providingodequote droinoge in the form ofcross bonks ond mitre droins.

Groded lines moy provide on eosysuffoce to work from, but orehighly susceptible to erosion ondprone to heow mointenoncerequirements. They will olmostolwoys prove to be moreexpensive in the long run.

3.3 - t6 Soil Conseruation Handbook for Porks and Peserves in lhe NT

REFERENCES

Environment Protection Unit (undoted) Guidelines for EnvironmentqlAssessment ond Control of Fencinq Operotions on NT Porks (droft),Conservotion Commission of the NT.

Houghton, D. (1984) Protectino Firebreoks Aqoinst Erosion, Qld Dept ofPrimory Industries, Agdex 0651570, FT9lApr 84,

Letts, M.A. (undoted) Tipperorv Stotion Erosion Report, ConservotionCommission of the NT, lnternol Report (unpublished).

Mocnish, S. (undoted) Sofe Desions for Roods, Fences ond Firebreoks in theNorthern Tenitorv, Animol Industry & Agriculture Bronch, Dept of the NorthernTerritory.

Soil Conservqtion Service of New South Woles (undoted) Propertv PlonMonuol - o Resource Monooement Plon for Sustoinoble Lond Use, SoilConservotion Service of New South Woles,

Sofl Conservation Hondbook for Porks and Reserves in the NT 3.3 - t7

At the completion of extroctionoperotions, cleored oreos must beproperly rehobilitoted. This willinclude:

' r e m o v o l o f d i s c o r d e dequipment,

. bottering bock plt wolls (3horizontolto I verticol for pits upto I m deep, ond 6 horizontol toI verticol for pits greoter thonI m deep),

deep ripping of the pit floor,with 3m spocings betweenrip lines,

construction of diversionbonks ond other erosioncontrol structures, on odvicefrom Soil ConservotionOfficers,

respreoding topsoil, ond

revegetotion (see Chopter4..|, Revegetotion).

Plote E.l Borrow pit not rehqbilitoted. Note rilling on pit bqtter.

E - 5 Soil Conselation Hondbook for Parks and Reserves tn the NT

Plotes E.2 & E.3 Adequote rehobilitotion.

Soil Consewolion Hondbook for Parks ond Reserves in the Nf E-6