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ARTICLE IN PRESS 249

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

Nikuradse (1933) found z0 ¼ (1/30)ks, where ks is the Threshold conditions for moving gravel were established
equivalent sand roughness that corresponds to 2.5D50 for by comparing the Shields parameter yw related to waves
a flat bottom and to 4Z when the bottom is covered by with the critical Shields parameter ycr related to different
bedforms (Z being the bedform height). The threshold grain sizes.
current intensity 1 m above the bottom for sediment
entrainment was estimated for different grain sizes and 4. Results and discussion
bedform heights using Eq. (4) (uà ¼ uÃcr for each grain size).
4.1. Sediment characteristics
Near-bottom wave orbital velocities uw were obtained for
a range of wave conditions using shallow water equations: The main sedimentary and morphological characteristics
of the study area can be interpreted in the SSS mosaic
uw ¼ pH=T sinhðkhÞ, (5) image (Fig. 4). Sediment distribution on the continental
shelf of Marettimo shows high spatial variability (Figs. 4
where H is the wave height, T the wave period, h the water and 5). High backscatter intensity (white tones) depicting
depth, and k the wave number (k ¼ 2p/L, where L is the rocky areas occurs near the coast and in sparse isolated
wavelength). Skin friction wave velocities uÃw were calculated outcrops. A lighter acoustic response represents coarse-
for different sediment grain sizes (Madsen and Grant, 1976). grained sediment fields (gravel–sandy gravel). Low back-
scatter intensity corresponds to darker shading and reflects
The wave Shields parameter (Shields, 1936) was calcu- finer sediments (coarse and medium sand).
lated as

yw ¼ u2nw=gðs À 1Þ D50, (6)
where s ¼ rs/rw.

Fig. 4. Sedimentological map of the Marettimo inner shelf based on side scan sonar data and sediment samples (a). The bathymetric contour interval is
5 m. Inserted boxes: (b) location of sediment samples and (c) SSS mosaic.