Journal of Coastal Research
300-3 15
West Palm Beach. Florida
Spring 2002
Measurement and Prediction of Aeolian Sediment Transport at Jandia Isthmus (Fuerteventura, Canary Islands) J. Alcantara-Carriof
and I. Alonso ]
t Dpto . de Geociencias
:j: Dpto . de Ffsica Un iver sidad de Las Palmas Marina s y 0 . T. de Gran Canaria Universidad de Vigo 36200 Vigo (Pontevedra), 35080 Las Palmas, Spain ignaci
[email protected] Spain
[email protected] ABSTRACT
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ALCANTARA-CARRI6, J . and ALONSO, 1., 2002. Measur ement and prediction of aeolian sediment tr ansport at J and fa Isthmus (Fuerteventura, Canary Islands). Journal of' Coastal Research, 18(2 ), 300-3 15. West Palm Beach (Florida), ISSN 0749-0208. Predictive models of aeolian sediment t ransport are calibrat ed and validate d with empirical measurements in the J andfa Isth mus (Canary islands ), which consist s of a wide diversity of aeolian environments, from dunes to sa nd sheets and serir areas . Empirical aeolian sediment tr ansport rates measur ed by verti cal sand tra ps simulta neously with wind velocity profiles permit validation of such models, as well as selection of the best performing equati on. The model of ZINGG (1953) for horizontal or nearly-horizontal surfaces and the model of HAI{I)ISTYand WHITEHOUSE (1988 ) applicable to dipping surfaces have shown the best agreement with measu rements. In th is paper, a new equat ion is defined and applied to predict the monthly and annual aeolian sa nd transport at the site. Sediment flux was found to be mainly to th e South or South -Southeast , caused by the dominant nort herly tr ade winds as well as the local topography. Wadis chann el the wind and associated tr ansport, but a high transport also occurs to the southwest along the windward coast. Therefore, supply of sediments to both coasta l sides has been quantified and the pattern of flux described. Sand blown from J andia Isthmus constit utes a significant source of materials for both the leeward and windward beaches. ADDITIONAL INDEX WORDS: Wind profiles, gra in-size, density, sand traps. calibration. CanQ/:Y Islands.
INTRODUCTION It is a well-known fact t hat t he relation ship between wind ener gy and se dim en t prop erties det ermines aeolian se dime nt transport mechanisms . Three ba sic mec hanisms are sus pe n sion, sa lt at ion a nd creep, a lt h oug h hybrid mechanisms h a ve a lso been described (TSOAR and PYE, 1987). Several mod els permit calcu lation of a eolian sand transport a s a function of wind pa ram eters (shear stress, threshold shear stress, a nd threshold velocity ), sedime nt properti es (size a n d den sity ) a nd surface roughn ess. The for mula e ofBAGNOLD (1941) and KAWAMURA (1951) a re the mo st accepted because t hey have a t h eoretical backgro u nd an d h a ve been veri fied by numerous experime nts, wh ile t he mod els of CHEPIL (194 5), ZINGG (1953), WILLIAMS (1964 ) a n d H su (197 3, 1977) ha ve been considered as em pir ica l modifications (HORIKAWA et al., 1986). However, all these above mo dels ha ve been developed for idea lised su rfaces : h or izont al , dry, unobstructed a n d unvegetated surfaces (SHERMAN and HOTTA. 1990 ), condit ions that a re se ldom found in true coas t a l aeolian environme nt s . Th ese environ me n ts a re ch a racterised by t he influ ence of va ri ous envir on m enta l factors , suc h as veget at ion (LANCASTER a nd 00 125 received 30 Augus t 2000; accepted in revision 20 August 200 1.
BAAS, 1998), humidity content (LOWE. 1982 , SEELIGEH et al., 2000 ), slope (HARDISTY a nd WH I TEIl O US~: , 1988 ) and h uman pressure (NORDSTROM , 1994 ). Th erefor e, t he a pplica t ion of t he aforementioned a eolian transp ort mod els in coastal a re a s mu st be ca libra t ed with measurem ents of env iron me nta l fa ctors a nd ae olia n sa nd transport ra tes. Uncerta inty in long-term pr edi ctions is obvious ly hi gher t h an in sh ort -te r m predictions, but long-t erm predictions a re necessary to a n a lyse evolution of se dime n ta ry envir onme nts. Mea surement of wind- sp eed profi les over rever sing dun es show t h e feedb ack rel ation ship between flow and form a s morphology te nds to a n equilibr iu m sh a pe with respect to th e prevailin g wind in ea ch se as on (BUHKINsHAW et al ., 1993). Thi s r esult s ug ges t s that, a lt houg h focal points vary (Mr.» EWAN and WILLETTS, 1993 ), a n a verage value during sa lta tion can be obtained for ea ch season a nd su r face, which is u sefu l to predict t he ann ual a eolia n sa nd transp ort ra te. Aeolian se di men t t ransport st u dies are hen ce necessary to understand t he sedime ntary dynamics of man y coas t a l a re as . Ero sio n or a ccret ion on beac h es is gen erally reflect ed in t he associat ed ae olia n env iron me nts , whic h in t u r n act as a source or sin k of se dime n ts. Severa l marine and aeolian pro cesses a r e invo lved in th is bal a nce of m at eria ls, ea ch on e wit h th eir own direct ion, intensit y and t emporal va riability (SHORT
Measurement an d Predictio n of Aeolian Sedimen t Tr an sport
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28" 30 ' N
28" IS' N
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Figur e 1. Loca tion of th e st udy a rea . A) Fu er tevent ur a Island. B) Aeri al photogr aphy of 1992. C) Topogr aphic map an d positi on of th e st ations in Februar y and August of 1998.
and R ESP, 1982). Orientation of coastline is another factor that mu st be consider ed (BAUER et al., 1996). It is genera lly accepte d th at this in ter action work s in th e following way: sma ll waves pr oduce a net onshore tran sport to th e foreshore, and strong wind s remove this materi al norm all y to th e back-
shore to form ae olian dunes. Th ese dunes supply sedime nts in both directions, either to adjacent lagoons and coastal plains, or to th e beach, depending on wind direction. By contrast , th e architecture of contin ental arid environments generally consists of bare rock sur faces, pediments,
Tabl e 1. Main equations used to evaluate the aeolian sand tra nsport. Author
Equat ion
BAGNOLD (1941)
KAWAMURA (1951)
q = K (p'/g)·(Vo + V o,)2.(U. - V .,)
ZINGG (1953) WILLIAM S (1964)
q = Z·(p/ gJ.(dID)3I4·V.'
q = a '·( p/g)·V.··
Hs u (1973, 1977)
q = H.Fr3 = H.V , ' .(g.d) ""2
LETTAU and LETTAU (1978)
q = L (dID)w·(p/ g)·V.2·(U . - V ,,) 'I V. > V .,
'I V. > V .,
J ourn al of Coasta l Resea rch, Vol. 18, No. 2, 2002
Footnotes q: flux of sediments (kg m - I s 1) C: empirica l coefficien t related to sorting and me an gra in size. C = 1.5; 1.8 or 2.8 d: mean grai n size of sa mple in mm 0 : sta nda rd mean gra in size of 0.25 mm K: empirical coefficient. 1 :5 K :5 2.78 V .,: Thresh old shear st res s Z: empirica l coefficient (Z = 0.83) a' a nd b': empirical coefficien ts a ' = 0.17; b' = 3.42 for sa nd H: empirical va riable H = 1O- 4·exp (- 0.47 + 4.97·d) Fr: Froude num ber (Fr = V.·(g·d)- 1I2) L: empirica l coefficient (L = 4.2)
Alcantara-Carrie and Alonso
302
1998
Ranges of velocity
o a 3 ms'
Annual scale
3 a 6 ms'
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6 a 9 rns'
1000 2000 3000 hour
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Monthly scale
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JANUARY
MAY
~"lei SEPTEMBER
250
500 hour
FEBRUARY
MARCH
JUNE
JULY
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