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ARTICLE IN PRESS

248 C. Lo Iacono, J. Guille´n / Continental Shelf Research 28 (2008) 245–256

Fig. 3. Wave direction, height, and period recorded by Mazara del Vallo Buoy (see Fig. 2) in the period 1989–2005. 71,577 records were registered, of
which 13,757 were from the sector 270–2851N. The tables show the number of records with Hs44 m and Tp48 s for different wave directions.

(Apat Servizio Idrografico; www.idromare.com, Fig. 3). maximum diameter of 10 mm, based on a modification of
The buoy collected data every 6 h for a period of 17 years the critical Shields parameter (Shields, 1936):
(1989–2005). Wave characteristics displayed a strong
seasonal behavior, with lower significant wave heights ycr ¼ ð0:24=D50Þ þ 0:05½1 À expðÀ0:02 D50ފ, (1)
and peak periods recorded from May to September. The
most energetic winter storm events occurred from October where ycr is the critical Shields parameter and D50 the
to April. Maximum significant wave height (Hs) and peak median grain size.
period (Tp) values were 6.6 m and 12 s, respectively, and
storm duration was up to 4 days. The provenance of storm Based on Eq. (1), the critical bed shear stress tcr was
events coincides with the western, northwestern, and calculated for a grain size range of 2–10 mm, which is
southeastern sectors, corresponding to the longest fetch
areas from which the mistral (NW) and sirocco (SE) winds typical of the seabed sediment observed in the study area:
blow. However, waves coming from the north, north-
eastern, and eastern sectors are not much represented in the tcr ¼ ycrðrs À rwÞg D50, (2)
time series because the buoy is protected from the oriental
sectors by the Island of Sicily (Fig. 3). where rs is the density of sediment (2650 kg mÀ3), rw the
water density (1025 kg mÀ3), and g the acceleration due to
3.2. Hydrodynamic conditions for sediment motion gravity (9.81 m sÀ2). The corresponding critical friction

Threshold hydrodynamic conditions for the entrainment velocity u*cr is calculated as (3)
and transport of the sediments forming the dunes of the pffiffiffiffiffiffiffiffiffiffiffiffiffi
Marettimo inner shelf were estimated based on a theore-
tical approach. Firstly, the threshold parameter for uncr ¼ tcr=rw.
sediment motion was calculated as proposed by Soulsby
and Whitehouse (1997) for sediment grain sizes with a To estimate the current velocity at a distance z above the
bottom (U(z)), a logarithmic velocity profile in the near-
bottom layer is assumed and the Prandtl–Von Karman
equation is applied:

UðzÞ ¼ ðun=kÞ ln ðz=z0Þ, (4)

where k is the Von Karman constant ( ¼ 0.4), and z0 the
bed roughness length. In turbulent flow conditions,