The influence of bed forms on reference concentration and suspension under waves and currents

Hydrodynamic, suspension and bed-form measurements were made 2 km off the Dutch coast near Noordwijk aan Zee in ∼14 m water depth for a period of 32 days in 2003. Tidal currents were just able to suspend sand at the bed at peak spring tide but most suspension and transport occurred as a result of the combination of waves and currents. Burst-average (17 min) sand concentration profiles (C¯-profiles) from an acoustic backscatter instrument were used to define the (varying) location of the sea-bed, following the method used by Green et al. [Green, M.O., Dolphin, T.J., Swales, A., Vincent, C.E., 1999. Transport of mixed-size sediments in a tidal channel. Coastal Sediments ‘99, edited by N.C. Kraus, and W.G. McDougal, ASCE, Long Island, New York, pp. 644–658]. Reference concentrations at the sea-bed (C₀) and at 1 cm (C₁) were examined in relation to both the hydrodynamic conditions and the type of bed forms present. The C₀ predictive equations of Green and Black [Green, M.O., Black, K.P., Suspended sediment reference concentration under waves: field measurements and critical analysis of two predictive models, Coastal Engineering, 38, 115–141, 1999](short-wave ripples) and Nielsen [Nielsen, P., Suspended sediment concentrations under waves, Coastal Engineering, 10, 23–31, 1986](all bed forms; includes ripple steepness), both of which require knowledge of the bed-form type, were not as successful in explaining the variance in our C₀ data as a regression of C₀ against the skin-friction Shields parameter θ′_cw that ignored bed-form type (73% of variance explained). The values of the reference concentration C₁ were compared with the Lee et al. [Lee, G.-H., Dade, W.B., Friedrichs, C.T., Vincent, C.E., Examination of Reference Concentration Under Waves and Currents on the Inner Shelf., Journal of Geophysical Research, 109, 1–10, 2004] equation which predicts C₁ from the product of the Shields parameter and the inverse Rouse parameter; 51% of the variance in C₁ was explained.     

Benthic status of near-shore fishing grounds in the central Philippines and associated seahorse densities

Benthic status of 28 near-shore, artisanal, coral reef fishing grounds in the central Philippines was assessed (2000-2002) together with surveys of the seahorse, Hippocampus comes. Our measures of benthic quality and seahorse densities reveal some of the most degraded coral reefs in the world. Abiotic structure dominated the fishing grounds: 69% of the benthos comprised rubble (32%), sand/silt (28%) and dead coral (9%). Predominant biotic structure included live coral (12%) and Sargassum (11%). Rubble cover increased with increasing distance from municipal enforcement centers and coincided with substantial blast fishing in this region of the Philippines. Over 2 years, we measured a significant decrease in benthic 'heterogeneity' and a 16% increase in rubble cover. Poor benthic quality was concomitant with extremely low seahorse densities (524 fish per km(2)). Spatial management, such as marine reserves, may help to minimize habitat damage and to rebuild depleted populations of seahorses and other reef fauna.     

Oblique and near collision subduction, Sagami and Suruga Troughs —preliminary results of the French-Japanese 1984 Kaiko cruise, Leg 2

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the Suruga and the Sagami Troughs, which lie on both sides of the northwestward moving and colliding Izu-Bonin Ridge, the northernmost part of the Philippine Sea plate. The transition from the Nankai Trough to the Suruga Trough is characterized by northward decrease in width of the accretionary prism, in good agreement with the increasing obliquity between the through axis and the direction of the convergence, as the strike of the convergent boundary changes from ENE-NNE to south-north. South of the area, the southern margin of the Zenisu Ridge shows contractional deformations. This supports the interpretation made by the team of Leg 1 who studied the western extension of the area we studied, that it is an intra-oceanic thrusting of the ridge over the Shikoku Basin. In the Sagami Trough, where the relative motion is highly oblique to the plate boundary, active subduction is mostly confined in the east-west trending portions of the trough located south of the Boso Peninsula and along the lower Boso Canyon, near the TTT triple junction. In between, the present motion is mainly right-lateral along the northwest trending Boso escarpment. However, an inactive but recent (Pliocene to lower Pleistocene) accretionary prism exists south of the Boso escarpment, which suggests that the relative motion was more northerly than at present before about 1 Ma ago.