Hydraulic Interpretation of Direct Velocity Measurements in the Bab al Mandab*

Abstract

Abstract Acoustic Doppler Current Profiler velocity measurements in the Bab al Mandab during the period June 1995–March 1996 are used to assess the hydraulic character of the exchange flow. The strait is 150 km long and contains two distinct geometrical choke points: the Hanish sill and Perim narrows. The authors use a three-layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal, long gravity waves. The first (second) mode is generally characterized by in-phase (out-of-phase) motions of the two interfaces. The calculations take cross-strait topographic variations into consideration by using a piecewise linear representation of the actual bottom. The resulting phase speeds are used to determine whether the flow is subcritical, supercritical, or critical with respect to the first and second modes. Subcritical (supercritical) means that the two wave pairs corresponding to a given mode propagate in opposite ...