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 by suspended particles) were observed in a near-bottom mooring. Using moored current meters together with aircraft remote imaging overflights and profiling anchor stations, we established that the spikes were associated with the trailing edges of internal solitary waves of depression of tidal frequency. Each trailing edge had a bore-like face with strong convergent bottom currents followed by a rank-ordered packet of solitons. The near bottom optical attenuation spikes were correlated with these bottom convergent currents at the trailing edge and were not correlated with high bottom stress. Higher optical attenuation was also found near surface at the leading edge of the internal solitary wave of depression. We suggest that in a similar manner to the accumulation and shoreward transport of particles in a surface convergence area (e.g., slick lines), the bottom convergence area at the trailing edge also accumulates and transports particles shoreward. Bottom convergence must be associated with vertical motions which can help in lifting the nepheloid layer off the bottom. This scenario has implications for near bottom optical searches, for transport of non-buoyant zooplankton and for sediment redistribution.