Pass Cavallo, Texas: Case Study of Tidal-Prism Capture

Pass Cavallo was until 1963 the only permanent inlet for the Matagorda Bay System, and has not been stabilized or protected in any way. In 1963, a deepdraft ship channel was constructed to provide access from the Gulf of Mexico, for which the Entrance Channel was dredged across Matagorda Penninsula 18,000 ft (5.5 km) up the coast from the Pass. This landcut proved to be scour-unstable, enlarging spontaneously to depths on the order of 50-60 ft (15-18 m) and widths on the order of 1,000 ft (300 m). Current measurements in Pass Cavallo prior to the creation of the landcut and approximately ten years after are analyzed to determine hydraulic characteristics These indicate that the Entrance Channel has captured approximately half of the tidal prism of Pass Cavallo. From both critical current and critical stress criteria, the reduction in current speed in Pass Cavallo associated with this loss of tidal prism is sufficient to render the Pass shoal-unstable, hence more susceptible to deposition.