The Weather of Windblown Sediment: Aeolian Processes ... · Aeolian processes have shaped much of New Zealand's landscape through the transfer of terrestrial sediments from geomorphically

Weather and Climate (1996) 16 (1): 3-16 3

THE WEATHER OF WINDBLOWN SEDIMENT:AEOLIAN PROCESSES WITHIN THE NEW ZEALAND

LANDSCAPE.

Department of Geography, University of Canterbury, Private Bag 4800,Christchurch, New Zealand.

ABSTRACT

This paper examines meteorologicalphenomena such as the nor 'wester andSoutherly Change w i t h r e g a r d t o t h eentrainment - transportation - deposition offine grained sediment within the New Zealandlandscape. A t present, aeolian processes areconfined to geomorphically active areas suchas the dry braid channels of proglacial rivers,coastal sand dunes, exposed lake shorelines,and areas where the removal of vegetationhas left surface sediments unconsolidated andexposed t o t h e a i r s t r eam. W i n d b l o w nsediment that originates from such sourcescan present a considerable hazard to vehicletraffic, livestock and local residents, while alsothreatening the viability of local businesses.Two recent dust storm events are examinedby way o f case studies to h igh l i gh t thepotential hazard from such phenomena toresidents and property of areas affected bydust transporting winds. Contemporary ratesof dust deposition within an alpine lake basinare discussed wi th reference to favourablemeteorological conditions for wind erosion.These measurements are the f i r s t t o bedocumented for an area within the SouthernAlps and re f lec t d u s t deposi t ion ra tespreviously measured in desert environments.Finally, avenues fo r fu ture w ind erosionresearch are suggested.

INTRODUCTION

Scientific studies in to the nature andincidence o f aeolian sediment t ranspor ttypically have two pr incipal components,namely, that associated with the meteorology

Hamish A. McGowan

of wind erosion events and secondly, thegeomorphological processes responsible forfine particle formation and their presentationto t h e a i r s t r e a m . Consequent ly, a n ydiscussion on t he phys ica l na tu re andincidence o f aeolian sediment t ranspor tcannot take place without consideration ofboth the meteorological and geomorphologricalprocesses t h a t a re responsible f o r t h eentrainment - transportation - deposition ofwindblown sediment. The aim of this paperis therefore to review the contribution of windto aeolian processes observed within the SouthIsland of New Zealand and the downstreaminfluence on local residents. The physicalcharacteristics of areas prone to wind erosionand dust deposition are also discussed.

Aeolian processes have shaped much ofNew Zealand's landscape through the transferof terrestrial sediments from geomorphicallyactive areas to zones of deposition. During theQuaternary aeolian processes resulted in theformation of extensive loess deposits overlarge areas o f New Zealand, i nc l ud ingapproximately 1 million ha in the CanterburyProvince (Ives 1974). These deposits range indepth from less than 20 cm to more than 12mand are derived mostly from weathered parentgreywacke mater ia l . However, wh i le thestratigraphy of the loess deposits has receivedconsiderable scientific attention as reviewedby Smalley and Davin (1980), the principalprocess responsible f o r t he i r format ion,namely the entrainment- t ransportat ion-deposition o f terrestrial sediments by theairstream has largely been overlooked. Thisis unfortunate, as the South Island of NewZealand provides an ideal environment in

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environment i n which to conduct aeolianprocess studies.

Early scientif ic investigations in to thetransportation of fine grained sediments bythe w ind focused on aeolian processes i ndesert and pra i r ie environs, such as theSahara and the Dust Bowl region of the mid-western United States ofAmerica. These earlystudies by Free (1911), Bagrnold (1941), Chepil(1945a, 1945b ) a n d o t h e r s i d e n t i f i e dfundamental relationships governing thetransportation of fine grained sediments bythe wind which continue to form the basis formany contemporary aeolian process studies.With the development of high technology windtunnels as described by Nickling (1988) andWilliams et al. (1990) in conjunction wi thcomputer simulations of fine grain transport,advances i n t h e sc ience c o n t i n u e a sinvestigators examine such processes as thesplash function (Owen 1980, Werner 1990),and reptation (Ungar and Haff 1987).

Within the South Island of New Zealandthe entrainment of fine grained sediment bythe a i r s t r e a m a n d i t s s u b s e q u e n ttransportat ion and deposit ion was f i r s tdocument by early European explorers andrunholders as noted by Dick (1965), w i t harguably the first sound scientific account ofsuch processes by Hardcastle (1890). Kidson(1929, 1930) presented accounts o f thedeposition of Austral ian sourced dust oversouthern New Zealand from the 6-10 October1928. One resident from Winton stated that:"Towards 4 o'clock it suddenly grew dark andthere were portents of a violent thunderstorm.Then the sky became pinkish in colour, andshowers o f mud began to fall.... The mud-showers lasted for twenty minutes, then therain set i n and cleared the atmosphere."(Kidson 1929, p297). Zotov (1938) consideredwind erosion of the inner montane basins ofthe South Island to be no less widespread andextensive than soil erosion by running water.However, i t was almost 30 years later beforeButterf ie ld (1971) and Cox et al. (1973)presented their findings on the susceptibilityof high country soils to wind erosion and ratesof dust deposition downwind of the RakaiaRiver respectively.

Since the publication of Butterfield's andCox's research almost 25 years ago, scientificstudies into aeolian processes wi th in NewZealand have largely been confined to coastalsand transport including dune formation and

Weather of Windblown Sediment

stabilisation (Holland 1981, Lenihan 1984,Bradly 1993), while McGowan (1994) is theexception. This later study presented resultsfrom an investigation into the physical natureand incidence o f windblown dust a t LakeTekapo which captured local and nationalmedia headlines during the spring of 1989.Windblown dust during foehn windstorms inSeptember and October 1989 caused nuisanceand hazard to residents of the Lake Tekapoarea and was considered to seriously threatenthe local tour is t industry. The pr inc ipalentrainment zones identified by McGowan(1994) were broad dry fluvio-glacial meltwaterchannels, exposed l ake shorel ines a n ddegraded tussocklands. Namely, areas withinthe landscape considered to be geomorphicallyactive.

Sources of Wind Blown Sediment andTransportation Mechanisms

The entrainment of fine grained sedimentsby t h e w i n d m a y occur f r o m a n y d r y,unconsolidated and sparsely vegetatedsurface such as exposed fluvio-glacial deposits,braided riverbeds, a l luv ia l fans, zones o frecent mass movement, sand dunes, beaches,and areas where devegetat ion th roughcultivation and land clearance has exposedsurface sediments to the airstream. Withinthe con tex t o f N e w Zealand's aeo l iangeomorphology all of these environments arepresent within the contemporary landscapeand may become active during favourableconditions for particle entrainment by theairstream.

Particle movement by the wind may beinitiated by several mechanisms acting aloneor together to overcome the combined forcesof gravity and interparticle cohesion. Thesemechanisms include the fluid drag exerted onthe exposed particle(s) by the airstream,aerodynamic l i f t , impacts by ro l l i ng o rbouncing p a r t i c l e s a n d m e c h a n i c a ldisturbance by for example, vehicle traffic.

As the velocity of the near surface airstreamincreases, at some critical point the exposedgrains w i l l s tar t to move. This thresholdvelocity is referred to as the f lu id thresholdvelocity (u„,) and can be expressed by any oneof a number of equations, although Bagnold's(1941) equation is general ly accepted toprovide a good approximation of this value(Equation 1) where:

Weather of Windblown Sediment 5

(1) i i * t . A (cT pP)g..131/2

A i s a n emp i r i ca l coeff ic ient e q u a l t oapproximately 0.1 f o r part ic le Reynoldsnumbers >3.5, a and p are the grain and fluidmedium densi t ies ( w i t h respect t o a i r )respectively, g is acceleration due to gravity,and D is the part ic le diameter. Bagnolddemonstrated that u., attains a minimumvalue when D approaches 80 p,m as particlesfiner than this tend to give the surface anaerodynamically smooth appearance to theairstream and combine wi th the increasingdominance o f interparticle forces, such asmoisture f i lms and electrostatic chargesbetween grains to markedly increase the fluidthreshold velocity. For example, Belly (1964)showed that only 0.6% moisture by volumecould double the f lu id threshold velocitycompared to d ry beach sand, whi le bothNick l ing (1984) a n d P y e (1980) h a v ehighlighted the binding influence of solublesalts on sand grains. This process is mostimportant in marine coastal environmentswhere there is an abundance of salts as notedby Bradly (1993). Typical values for u rangefrom 4 ms-' to >16 ms-1 (Nickling 1984, Hal l1981).

As the near surface wind speed increasesand approaches the f luid threshold velocity

1.5m

TurbulentAirstream

Surface creep(>500pm)

3 3Modified saltation

( 7 0 - 1 0 0 p m )

. . . . . . - - 1 (Saltation

(70-500pm)

Long term suspension(<20pm)

Turbulenteddies

Short term suspension(20-70pm)

. • • • e • •)•.!••••••F•e•e••••_ _ . _ . ? . P. • • • P • • • • e • e • • • • • • • e • • • • • • • • • • • • • e • • • • e • e • o • o • e • e • e • e _ ! . . , • • •v....-1.......,....-*-:/..•••••••••••%.1....-.1.-..y..:41.•••••••*•••%•%•••••••%•%•%•%•%•%•%•%•%•%•%•%•%•%•%•-••-it

i.1%.71.-;•••:. ice" 1••••••:1•:.;.%••••••%%••••Ze...1.%; Zele. •••:%•••• 511 511 511 511 5•••1 51.. 5"; 51'11'. 5'1'. 5i :••••••••••:%:.%;

Figure 1: Modes of particle transport by wind (after Pye 1987,1;149).

the most exposed grains will begin to vibrateabout a stationary position before their fluidthreshold velocity is exceeded and they beginto move downwind. A t th is po in t grainstypically larger than 500 pm will begin to rollalong the surface in a process called surfacecreep (Figure 1), whi le smaller grains arelifted above the surface and become entrainedinto the airstream. Grains ranging in sizefrom approximately 70 to 500 pm will tend tobounce over the surface as they are carrieddownwind in saltation (Figure 1), while finergrained sediments w i l l become suspended(Figure 1) in the airstream. Grains <70 p,mwill typically remain in suspension until theyare scavenged or filtered from the airstream,or unt i l their settl ing velocity exceeds themean ve r t i ca l ve loc i t y o f the amb ien tairstream. For grains <10 p in this may beseveral weeks after init ial entrainment (Pye1987), t h e r e b y a l l o w i n g t h e m t o b etransported many thousands of kilometresfrom the entrainment zone.

As saltating grains bounce over the surfacethey usual ly eject other grains f rom thesurface through ballistic impact processes.This usually results in the expulsion of manygrains from the surface with lower energiesthan the incident grain, which often reboundsitself with increased vertical velocity throughthe conversion of horizontal momentum to

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vert ical momentum. Consequently, t h i sprocess results in grains being ejected into theairstream at velocities lower than the f luidthreshold velocity. This new value is referredto as the impact threshold velocity (zit) andwas def ined b y Bagnold (1941) th roughEquation 2, where D is the particle diameter.

(2) = 6804T3.1og ( 3D0 )

Saltating grains are highly effective i ndislodging f iner gra ined sediments f romaerodynamically smooth surfaces, such as siltand clay deposits that have developed surfacecrusts in response to rain compaction, sunbaking or micro-organisms. Consequently,saltating grains are often instrumental in theent ra inment process o f f i n e r g r a i n e dsediments tha t become suspended i n theairstream to produce dust plumes.

The transportation of particles by the windwill typically continue as long as there is asupply of free sediment to the airstream andsurface/meteorological conditions promoteaeolian grain transport. Suspended sedimentconcentrations wil l decrease downwind fromthe entrainment zone wi th the dust plumedisplaying g r a n u m e t r i c s t r a t i f i ca t i on .Namely, smaller and more rounded particlestend to be carried higher into the airstreamand subsequently fur ther downwind thanlarger less rounded grains, un t i l they aredeposited (Goossens 1985, McGowan 1994).

Deposition of aeolian transported sedimentmay occur in several ways. Larger particlestravelling via surface creep or saltation wil lbe depos i ted w h e n t h e n e a r su r faceairstream's velocity fal ls below ut, wh i levegetation wi l l effectively f i l ter windblownsediment from the airstream and shelter i tfrom re-entrainment. Changes in surfacetype, par t icu lar ly surface roughness andmoisture content will also cause deposition ofwindblown particles and lead to the formationof sand tails and adhesion ripples respectively.Creeping and saltating grains in dune systemsare usually deposited in the lee of dunes asthey travel over the dune crest and into a zoneof reduced w ind speed. Th is mechanismlargely accounts for the general geometry ofsand dunes that may be used in identifyingthe predominant wind regime of a dune fieldor sand sea. Sand grains may also becomeburied as they impact on loose unconsolidated


surfaces, although thei r impact can causeother grains to move i n a net downwinddirection by the process of reptation.

The deposition of finer grained dust (<100Udden 1894) also occurs by any of the

processes a l r e a d y ou t l i ned , i n c l u d i n gscavenging f r o m t h e a i r s t r e a m b yprecipitation. However, recent f ie ld andlaboratory experiments by Goossens (1988)and Goossens and Offer (1990) have identifiedsilt and clay (<63 ilm) deposition by the windto be highest on the concave windward slopesof topographic obstacles, rather than on theirlee slope as observed during sand transportin dunes. Goossons suggests that fine particlestend to follow the winds streamlines so thatover a concave windward slope where thestreamlines a r e c o m p r e s s e d , d u s tconcentrations w i l l be h ighest therebyenhancing the deposition process. Near thecrest o f the topographic obstacle wherestreamline separation occurs, suspended dustgrains wil l be carried downwind beyond theturbulent wake region situated in the lee ofthe obstacle. I f the ambient airstream'svelocity exceeds the deflation velocity over thewindward slope, high rates of deposition onthe windward slopes will not be observed. Thiscritical wind speed is dependent on surfaceroughness elements such as vegetation andthe geomet ry o f t h e w i n d w a r d s lope.Currently this value is derived empiricallythrough f i e l d a n d l a b o r a t o r y s t ud ies(Goossens 1988, Goossens and Offer 1990).

Sediment Transporting WindsAlthough many different weather systems

initiate aeolian sediment transport, only thosethought to be most important w i th in thecontext of New Zealand's aeolian processes arediscussed i n th i s section. The reader istherefore referred to Goudie (1983) and Pye(1987) f o r a more extens ive rev iew o fl i terature on dust and sand transport ingweather systems.

Dust Devils (Whirlwinds)These features typically occur over very

warm surfaces or along micro-fronts and arecharacterised by vertical columns of rotating,.air which are made visible by tracers, such asdust and vegetation. Dust devils tend to formin s t rongly convective condit ions whenintense s u r f a c e h e a t i n g p roduces asuperadiabatic lapse rate in the near surface


atmosphere. Such conditions are most oftenobserved in New Zealand dur ing summerunder clear anticyclonic conditions when localwinds are l ight . A s a resul t , convectivethermals develop which acquire angularmomentum from the ambient airstream aloft.This is subsequently transferred down thevortex towards the surface and may give dustdevils tangential velocities of 22 ms-' or more,as observed by Schweisow and Cupp (1976).Strong vertical motion around the dust devilof up to 3.5 ms-' or more (Kaimal and Businger1970) is balanced by downward motion in thecore of the vortex which is often clearly visible.Dust devils mostly form over relatively leveland fairly uniform terrain such as riverbeds,road sides, beaches and sparsely vegetatedbadlands. They may range in size from 0.5 to3 m i n diameter and f rom 20 to severalhundred metres in height. While most dustdevils are often on ly observed for a fewminutes, some may prevail for as long as 20minutes ( S i n c l a i r 1969 ) , w h i l e t h e i rtrajectories may e i ther be migra to ry o rstationary.

Where convergence of opposing airflowsoccurs, dust devils can also be observed, suchas along lake and sea breeze fronts, or aroundtopographic obstacles. I n this situation thedust devils obtain their angular momentumfrom wind shear and are often observed tomigrate along the convergence of the opposingairflows. These vort ices d isp lay s im i la rphysical properties as those which developdue to convective processes.

Within New Zealand, dust devils do notappear to play a significant role in the overalltransportation of sediment by the airstream,although their site specific importance inrespect to local dust budgets is thought to beconsiderable. No studies have been conductedwithin New Zealand on dust devils, or theirrole a s g e o m o r p h i c a g e n t s i n t h etransportation o f sediment which may beconsiderable i n areas l ike the MacKenzieCountry and Central Otago. For example,observations from Arizona indicate that theoccurrence of dust devils on hot summer dayscan raise as much as 250 kg of dust per km2(Sinclair unpublished, cited i n Hal l 1981).Presently, the occurrence of dust devils in NewZealand is seen as one of curiosity or nuisancevalue r a t h e r t h a n o f a n i n t e r e s t i n gmeteorological phenomenon that is potentially

7

an i m p o r t a n t geomorphic agent i n t hetransfer of terrestrial sediments.

Cold FrontsThe passage o f f ronta l depressions i s

probably the most widespread cause of duststorms on a global basis (Pye 1987). In NewZealand i t is the passage o f the f ron ta ldisturbance known as the Southerly Change(Sturman e t a l . 1 9 9 0 ) t h a t producesfavourable atmospheric conditions for theentrainment of fine grained sediment by thewind in the absence of precipitation. Southerlychanges a re shor twave t roughs i n t h eprevailing westerlies that advance northwardover s o u t h e r n N e w Z e a l a n d be tweenanticyclones. Over eastern South Is landdistricts, the southerly change is usual lypreceded by warm and dry foehn conditions,while moist northerlies generally prevail inareas west of the Southern Alps.

Nowhere else is the southerly change morepronounced than over the expanse o f theCanterbury Plains (Figure 2). In this regionthe prefrontal foehn nor'wester results in lowatmospheric humidities and warm ambientair temperatures that promote the drying ofexposed surface sediments, particularly large

170E1

1

MiddlerrlarchDunedin

Scale100krn

Christchurch

Land over 1000 metres

45S

Figure 2: Location map of South Island sites mentionedin the text.

8

cultivation. As moisture is a principle grainbinding agent, any reduction in soil moisturecontent significantly increases the potentialfor wind erosion as the near surface windspeed approaches the fluid threshold velocityof the exposed surface sediments.

Southerly Changes were noted by Sturmanet al. (1990) to advance northwards over theCanterbury Plains w i t h average f ron ta lvelocit ies o f 30 -50 k m h-1-. H o w e v e rinstantaneous f r o n t a l speeds m a y b esignificantly higher as the strong temperatureand pressure gradients observed across manycold fronts generate severe turbulence, assuggested by Revell et al. (1987). Turbulencein the head of the front effectively transportsentrained dust particles vertically into theatmosphere, where increasing dynamic andthermal stability aloft dampens this processso t h a t t he m a j o r i t y o f the suspendedparticulates remain in the first few hundredmetres above the surface. A s a resu l t ,southerly changes that are not accompaniedby precipitation often produce blowing dustevents over the Canterbury Plains, in whichcase suspended pa r t i cu l a tes ( t race rs )effectively mark the front and associated windchange as shown in Figure 3. Such events aremost common dur ing spring (September,

Figure 3: Blowing dust effectively marking the leading edgeof a Southerly Change on the Canterbury Plains (courtesyof Bob Crowder).


October, November) when large areas of theCanterbury Plains are subject to cultivationand in late summer (February) when soilmoisture content is lowest.

Wind erosion associated with the advanceof Southerly Changes over the South Island(including the Canterbury Plains) has notbeen invest igated, a l t h o u g h sou the r l ychanges were the subject of an intensive fieldinvestigation programme during January andFebruary 1 9 8 8 k n o w n a s S O U C H E X(Southerly Change Experiment, Steiner et al.1987). S o u t h e r l y Changes m a y h a v esignificant impacts on farm costs by exposingnewly sown crops through the removal of topsoil, particularly small seeds such as cloverand grasses that are usually sown near to thesurface. Windblown soil particles originatingfrom arable farmland may contain chemicalresidues from herbicides and pesticides. Suchresidues may be transported downwind fordeposition on livestock, homes, vehicles andonto neighbouring properties. Fertilizer mayalso be removed from the top soil dur ingblowing dust events, while "sandblasting" ofcrops by entrained sediments can causesignificant damage to plants resulting in theneed to reseed and replace fertil izers andherbicides. As a result, the on-farm costs ofwind erosion during southerly changes maybe considerably more than previously thought.

Foehn winds - the nor'westerThe foehn nor'wester undoubtedly produces

the most favourable conditions for aeolianprocesses in New Zealand, namely warm, dryand gusty winds t h a t may exceed 30 to40 m s-1 (McGowan and Sturman 1996a). Aspreviously outlined, such conditions promotethe drying of surface sediments and thei rsubsequent entrainment by the airstream.Often the nor'wester is confined to the inlandregions o f the South Island and the r ivergorges of the eastern foothills. However, whenthe gradient wind is sufficiently strong andboth thermal and dynamic mixing promotethe grounding of the foehn airstream, thenor'wester may be experienced across mosteastern districts.

Both topographic channel l ing and thegrounding of atmospheric wave phenomenain the lee of the Southern Alps wil l enhancenear surface wind speeds during foehn windevents so tha t the airstream can entra insignificantly large particles. For example,


significantly large particles. For example,McGowan (1994) observed grains 2 to 3 mmin diameter sa l ta t ing over f luvio-glacialoutwash deposits to heights exceeding 2 mabove the surface. Such high energy saltatinggrains were considered to be the principleagents responsible for the entrainment of finergrained sediments f rom aerodynamicallysmooth surfaces through bal l ist ic impactprocesses. Saltating grains of this size werealso observed to effectively erode silt and claytype deposits resulting in dust storms at LakeTekapo (Figure 2) as described by Kirk (1989)and McGowan (1994).

Foehn northwesterlies were undoubtedlyresponsible for the majority of loess depositsthat blanket the eastern South Island, asdiscussed by Raeside (1956) and Ives (1974).During interstadial (and interglacial) periodswhen rivers were rapidly aggrading, vastamounts of silt and clay size material wouldhave been available to the airstream fortransportation. Any removal of vegetation byfor example, fire, would have exposed thesurface to deflation by the airstream. Winderosion throughout much of the Pleistocenewould have been enhanced by needle iceactivity associated wi th the colder climaticconditions. The a b i l i t y o f needle ice t oeffect ively f r a g m e n t sur face c rus t s i spresently thought to contribute significantlyto the severity of wind erosion observed inthe degraded tussock grasslands of the innermontane basins of the South Island.

The entrainment of fine grained sedimentby the airstream following flood events in thelarge braided rivers that cross the CanterburyPlains during foehn northwesterlies, typicallyproduces dus t p lumes t h a t t r ave l i n asoutheasterly direction over the plains. Suchdust plumes can be seen for 30 or 40 km andbecome less concentrated as they t raveldownwind d u e t o t h e d i spe rs ion a n ddeposition o f sediment. Cox et al. (1973)presented results f rom a network of dustdeposition traps located on the south bank ofthe Rakaia River near Barrhill. He concluded,that the entrainment of silt/clay size materialfrom the dry channels in the Rakaia River bythe nor'wester and its subsequent depositiondownwind accounted for the greater depth offine material over the Burnham Formationoutwash gravels (Suggate 1963) on the southbank, than on the north. Contemporary ratesof dust deposition on the south bank of the

Rakaia are thought to be significantly morethan those monitored by Cox et al. (1973) thataveraged 1540 kg ha-' yr-1 at a site located only20 m from the edge of the river terrace, to490 kg ha-' y r ' at a site 1750 m south of theterrace edge. T h i s i s because passivedeposition traps similar to those employed byCox and his colleagues have recently beenshown to significantly under-catch settlingdust grains by as much as 80% (Hall et al.1994).

Other Dust Transporting WindsWhile dust devils, cold fronts and the foehn

nor'wester may be considered the mostimportant and frequently observed sedimenttransporting winds over the South Island andmost of New Zealand in general, they are notthe only winds capable o f entraining andtransport ing f ine gra ined sediments. A spreviously discussed, any weather system ableto generate near surface wind velocities thatexceed the f lu id threshold velocity o f theexposed surface sediments w i l l i n i t i a teincidents of blowing sand and/or dust in theabsence of precipitation. For example, gustfronts associated with large convective stormsor the onset of strong sea breeze events whichplay a significant role in the transportationof sand in the coastal zone as noted by Bradly(1993), w i l l bo th i n i t i a t e aeol ian g ra intransport. The grounding of synoptic airflow,such as gradient northeasterlies along theeast coast of the South Island or westerliesalong the West Coast are often observed toinitiate sediment transport wi th in coastalsand dune systems.

Wi th in comp lex a l p i n e topography,topographic jetting of airflow through narrowriver valleys and gorges may result in theentrainment of exposed surface sediments aswind speeds increase due to streaml inecompression. B o t h the rma l l y generatedcirculations and gradient a i r f low can beaffected in this manner resulting in localisedinstances of windblown sediment.

DUST STORM CASE STUDIES

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Lake TekapoAs previously mentioned in this paper, the

degraded inner montane basins and broadfluvio-glacial r i ve r valleys o f the easternSouthern Alps are ideal environments for theentrainment of fine grained sediment by the

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airstream. A t Lake Tekapo (F igu re 2) ,instances o f blowing dust have long beenreported by residents and visitors to the region(Bateson 1964, Dick 1965, K i rk 1989, Owen1993). Bateson (1964) reported dust plumesregular ly b e i n g s i g h t e d d u r i n g f oehnnorthwesterly conditions in the Tasman andGodley River Valleys, and over the associatedbraid deltas along the northern shores o fLakes Pukaki and Tekapo respectively. K i rk(1989) reported observing six distinct duststorms while visiting the Lake Tekapo areaon 11 October 1989 during a foehn windstorm,whi le O w e n ( 1 9 9 3 ) d i s c u s s e d t h eestabl ishment o f the Lake Tekapo So i lConservation Reserve in the 1950's to controlsevere wind erosion on 165 ha of pastoralleasehold land adjacent to State Highway 8,1 km east of Tekapo Village.

Undoubtedly, the most recent and widelypublicized period of windblown dust at LakeTekapo occurred during the spring of 1989when extremely low lake levels exposed aconsiderable area of lake shoreline to aeolianprocesses. Dust plumes were observed to carrydust down the lake basin during moderate tostrong foehn wind events to affect Tekapo


Village. Residents of the Lake Tekapo areaconsidered such incidents of blowing dust toseriously t h rea ten t h e regions t o u r i s tindustry, while some claimed that their healthwas adversely affected by the inhalation ofhigh concentrat ions o f suspended d u s tparticles. I t was even reported that the duststorms resulted in dirtier than usual sheep'swool leading to below average prices for thewool at sale (Timaru Herald Newspaper, 7/10/89).

In October 1991, a three year investigationinto the physical nature and incidence o fwindblown dust in the Lake Tekapo basin wasstarted b y t he au tho r. A s p a r t o f t h i sinvestigation, rates of dust deposition withinthe lake bas in were measured betweenAugust 1992 and February 1993 by a networkof passive dust deposition traps. The trapsconsisted of 60 cm lengths of PVC (polyvinylchloride) pipe sealed at one end and filled witha solution of distilled water and antifreeze.They were attached to stakes 1 m above thesurface so as to avoid contamination by locallysourced sediment ejected into the airstreamby rain splash, or large saltating sand grains.Throughout the dust deposition monitoring

Figure 4: Schematic display of monthly dust deposition rates recorded in the Lake Tekapo studyarea from August 1992 to February 1993 in kg ha-1 mth-1.


programme w i n d speed a n d d i rec t ion ,temperature, relative humidity, precipitationand solar radiat ion were monitored by anetwork o f anemographs and automaticweather stations deployed wi th in the lakebasin as part of the field study programme(McGowan 1994). Meteorological observationsfrom this monitoring network were used toidentify favourable conditions for blowing dustevents and dust plume paths.

Figure 4 presents dust deposition ratesrecorded around Lake Tekapo by the dustdeposition traps in kilograms per hectare permonth. The highest rates of dust depositionwere recorded near the northern (D1; 214 kg

Figure 5: Potential dust roses for Site 1 and Site 2 withinthe Lake Tekapo Basin.

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ha-1 mth-1 and D8; 449 kg ha-1 mth-1) andsouthern (D6; 512 kg ha-' mth-' and D7; 340kg ha m t h ') lake shores respectively. Thesedust deposition rates were considerably largerthan those documented by Cox et al. (1973)and more closely resemble those monitoredby Goossens and Offer (1990) in the Avdatregion of the Negev Desert, Israel. Potentialdust roses (McGowan et al. 1995) presentedin Figure 5 indicate the direction from whichwindblown sediment caught by the depositiontraps was most likely to have originated fromdur ing the d u s t deposi t ion mon i to r i ngprogramme. These dust roses were calculatedfrom the total time during the dust depositionmonitoring programme tha t mean hourlywind speed at Sites 1 and 2 was 7.5 m s-1 orgreater, relative humidity was less than 55%and no precipitation was recorded at eithersite. These criteria were derived empiricallyfrom observations made throughout this studyand from observations made by residents ofthe Lake Tekapo area at times correspondingto the onset of blowing dust events.

The high rates of dust deposition recordedover the northern margins of the lake are mostlikely associated with northerly airflow thattypically displays classic foehn characteristicsand w h i c h h a s b e e n topog raph i ca l l ychannelled down the large braided r i vervalleys that enter the lake basin north of Site1. A t Site 2, adjacent to Tekapo Village, theabsence of dominant topographic forcing onairflow results in this region of the lake basinbeing affected by dust transporting windsfrom other directions. I n particular, Site 2 isaffected most frequently by northwesterlydust transporting winds that are most likelyto entrain dust from the degraded tussockgrasslands situated west-northwest of thissite. Persistent over grazing of the tussockgrasslands by domestic livestock and rabbitshas seriously depleted the vegetation coverin large areas of the lake basin and adjacentlandscape, thereby exposing the topsoil towind erosion. During early winter and spring,needle ice ac t iv i ty effectively fragmentssurface crusts and relic deflation surfaces. Asa result, a fr iable f ine grained surface ispresented to the airstream that is h ighlysusceptible to wind erosion.

Dust plumes originating from the degradedtussock grasslands are thought to display avery fine mean grain size estimated to beapproximately 15-25 p . m i n d i a m e t e r

12

compared to windblown sediment entrainedfrom the dry braided riverbeds that consist ofmuch larger grains as outlined by McGowanand Sturman (1996b). The sedimentologicalcharacteristics o f the windblown sedimenthave a significant affect on the transportationof dust grains, w i th the smaller and morerounded grains being transported fur therfrom the entrainment zone than larger andless rounded grains. As a result, high dustdeposition rates recorded near the northernmargins of Lake Tekapo give way to muchlower deposition rates only a few kilometresdown valley at for example, traps D2, D3, andD9 (Figure 4). However, significantly greatervolumes of dust entrained from the degradedtussocklands are able to remain in suspensionfor transportation downwind, with dust grains<10 p m i n d iamete r able t o r ema in i nsuspension for weeks or months. During May1991, suspended dust particles originatingfrom a dust storm in the MacKenzie Basinwere observed to reduce horizontal visibilityat Cannington in South Canterbury (Figure2) to less than 4 km. This was approximately35 km downwind of the entrainment zone.Such dust storms typically produce dust hazeover much of coastal South Canterbury wherewind speeds during synoptic northwesterlyconditions may be considerably less than inthe MacKenzie Basin due to the shelteringeffect o f coastal foothills, as discussed byMcGowan a n d S t u r m a n (1993) a n d amaritime inversion. In the MacKenzie Basin,such dust storms cause nuisance to visitorsand res idents o f the reg ion ,wh i le a lsopresenting a serious hazard to vehicle traffic

Figure 6: A dust storm across Haldon Road i n thenortheastern MacKenzie Basin, May 1991.


using roads throughout the basin as can beappreciated from Figure 6.

Middlemarch, 7 January 1995The most recent blowing dust event to

capture national media headlines i n NewZealand occurred near Hyde, approximately19 km northeast of Middlemarch (Figure 2)in Central Otago at 10am on the 7 January1995. The 0600NZST (New Zealand StandardTime) synoptic analysis for the 7 January 1995presented i n F i g u r e 7 shows a s t rongnorthwesterly a i rs t ream over the SouthIsland that resulted in northwesterly galesover most central and eastern South Islanddistricts. A t approximately 1000NZDT (NewZealand Day l igh t Time) winds began toentrain top soil f rom a freshly cult ivatedpaddock adjacent to State Highway 87 nearHyde, while only a few kilometres down valleya light breeze was experienced by residents.Cars using the highway were engulfed by thedust storm and bombarded wi th materialranging in size from silt size grains (<63 pm)to med ium s ize pebbles (15-25 m m i ndiameter, B. Johnston pers. corm). Severalcars caught in the storm had windows blown-out, while others were badly frosted. Paintwork on vehicles caught in the storm wasseverely sandblasted causing many thousandsof dollars o f damage. One caravan wascompletely destroyed. Some motorists wereforced from their vehicles after they filled withdust and pebbles only to be knocked down bythe wind, receiving cuts and bruises from

Figure 7: Synoptic analysis at 0600NZST (New ZealandStandard Time) 7 January 1995.


windblown sediment (Otago Daily Times, 9/01/95). The storm lasted for nearly 20 minuteswith the dust plume being clearly visible for30-40 km as i t travelled in a southeasterlydirection towards the coast.

Although no measurements of wind speedare ava i lab le f o r t h i s e v e n t f r o m t h eentrainment zone, i t is possible to estimatethe velocity required to transport particles 15-25 mm in diameter by employing Equation 1.Assuming that near neutra l atmosphericconditions prevailed at the time of the duststorm and that a logarithmic wind profileexisted over the entrainment zone, a meanwind speed of between 25-30 m s-1 at 2 m abovethe surface i s calculated. However, asEquation 1 does not consider the influence ofbinding forces such as moisture or organiccompounds on surface particles, actual windspeeds may have been significantly higherthan those theoretically predicted, perhaps ashigh as 40 m s-1. Such extreme wind speedsmay result from several processes acting aloneor together when strong gradient air f lowinteracts with complex mountain topography,such as the Rock and Pillar mountain rangesituated adjacent to the entrainment zone.

Eyewitness accounts confirm that the foehnnorthwesterly on 7 January 1995 was blowing

perpendicular t o t h e Rock a n d P i l l a rmountain range and across the Taieri RiverValley as shown in Figure 8. It therefore seemsthat the grounding of atmospheric lee wavesor the occurrence of shooting flow (Figure 8),may have caused such extreme surface levelwinds as described by Barry (1992). Shootingflow due to the compression of streamlinesover the crest of an obstacle, such as the Rockand Pillar Range, by an overlying inversionhas been observed to produce very strongleeslope winds, as discussed by L i l l y andKlemp (1979), Ne iman et al . (1988) andDurran (1990). I n th is process, potentialenergy gained by the a i rst ream whi le i tascends an obstacle is converted into kineticenergy over the leeslope as the descendingairstream accelerates down slope. A t somedistance downw ind t h e a i r s t r eam w i l ldecelerate and ad jus t t o local ambien tconditions, possibly through some form ofhydraulic jump mechanism.

Without meteorological observations suchas temperature, humidi ty and wind speedprofiles the exact process responsible for thesevere winds and associated dust stormexperienced near Middlemarch on 7 Januarycannot be confirmed. However, this event hashighlighted the potential hazard from dust

Rock and PillarRange (970m)

4

(INVERSION)

ShootingFlow

4km

Gradient Northwesterly

• k

Dust Plume

1 Okm

Taieri Ridge(730m)

13

Figure 8: Schematic representation of hypothesised shooting flow during the Middlemarch dust storm on 7 January 1995.

14

storms when the interaction of topographyand the airstream combine to produce verystrong n e a r sur face w i n d s ove r areassusceptible to wind erosion.

SUMMARY

Although the ro le o f wind i n aeol ianprocesses is often overlooked by contemporarygeomorphologists within New Zealand, thispaper has i l l us t ra ted i t s importance i nlandscape evolution through the transfer ofterrestrial sediments from geomorphicallyactive areas to zones of deposition. Recently,windblown sediment has been shown to posea significant hazard to vehicle traff ic andpersons caught w i t h i n dus t storms, ashighlighted by the Middlemarch case study.At Lake Tekapo, dust storms during foehnevents in the spring of 1989 were consideredto seriously threaten the local tourist industryas visitors bypassed the village in search ofless dusty locations. In coastal regions sanddune blow-outs have resulted in the advanceof dune systems into urban areas and ontofarmland. Over the Canterbury Plains andother farming areas where the potential forwind erosion is high due to the removal ofvegetation by cultivation and grazing, aeoliansediment transport may result in significanton-farm costs. These include crop damage, lossof soil fertility, exposure of newly sown cropsto aerial processes and the redistribution ofchemical residues.

The translocation of chemical residues fromfarmland by windblown dust has not beenstudied w i t h i n New Zealand. However,investigations conducted by Cohen andPinker ton (1966) a n d Spencer (1975)ident i f ied s u c h processes a s b e i n g o fconsiderable importance. Recently, studies byLarney (pers. coin) i n A lbe r ta , Canadaidentified high concentrations of Mecoprop(5500 ppb), 2-4-D (6000 ppb) and Bromoxynil(1000 ppb) in windblown dust collected overfarmland. These chemicals are used on NewZealand farms and along with organo-chlorineresidues from chemicals such as DDT whichwas used widely for grass-grub control, posea serious unseen hazard. I t is thereforesuggested that a dust monitoring programshould be established in regions such as theCanterbury Plains. Furthermore, fine dustparticles (PM10 ) are becoming increasingly thefocus of air quality studies in both urban and


rural centres as a i r emission inventoriesattempt to quantify emissions to air from bothanthropogenic and natural sources.

While the entra inment o f fine grainedsediment by the airstream from such dynamicenvironments as braided rivers and exposedshorelines may not be controllable, w inderosion o f farmland can be through theimplementation of appropriate managementstrategies. Such strategies should focus onreducing the exposure of surface sedimentsto h i g h w i n d s p e e d s t h r o u g h t h eestablishment of shelterbelts, revegetationand the implementation of minimum tillagepractices o n a r a b l e f a r m s . S c i e n t i f i cinvest igat ions i n t o s u c h managementstrategies s h o u l d b e encouraged, i nassociation with process studies that examinethe dynamics of fine grain transport by theairst ream w i t h i n t h e N e w Z e a l a n denvironment. A t present, these are not wellknown.

ACKNOWLEDGEMENTS

The author acknowledges the continuedf inancia l a n d techn ica l suppor t o f theGeography Depa r tmen t , U n i v e r s i t y o fCanterbury and the financial support of theElectricity Corporation of New Zealand inmaking this project possible as part of themuch larger Lake Tekapo dust storm genesisinvestigation. The author is also grateful tothe residents o f the Lake Tekapo area whoallowed the installation of equipment on theirland and for comments on a draf t o f thismanuscript by Dr Andrew P. Sturman.

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The Weather of Windblown Sediment: Aeolian Processes ... · Aeolian processes have shaped much of New Zealand's landscape through the transfer of terrestrial sediments from geomorphically