Internal tidal bores and bottom nepheloid layers

During a littoral optical environment project field study in shallow water off Oceanside, California, large spikes in optical attenuation (dominated b y suspended particles) were observed in a near-bottom mooring. Using moored current meters together with aircraft remote imaging overflights and profi ling anchor stations, we established that the spikes were associated with t he trailing edges of internal solitary waves of depression of tidal frequen cy. Each trailing edge had a bore-like face with strong convergent bottom c urrents followed by a rank-ordered packet of solitons. The near bottom opti cal attenuation spikes were correlated with these bottom convergent current s at the trailing edge and were not correlated with high bottom stress. Hig her optical attenuation was also found near surface at the leading edge of the internal solitary wave of depression. We suggest that in a similar mann er to the accumulation and shoreward transport of particles in a surface co nvergence area (e.g., slick lines), the bottom convergence area at the trai ling edge also accumulates and transports particles shoreward. Bottom conve rgence must be associated with vertical motions which can help in lifting t he nepheloid layer off the bottom. This scenario has implications for near bottom optical searches. for transport of non-buoyant zooplankton and for s ediment redistribution. (C) 2001 Published by Elsevier Science Ltd.