Semi Diurnal Tides : Sea level rises & falls twice a day(24 hours). The duration of tide is about 6 hours. Period of Tide (From one High to another High or from one low to another low) is 12 hours. These tides occur in most parts of the world, specially at European & North American Ports.
You may also know :
Diurnal Tides : One single High or Low Water occurs each tidal day. These tides occur along the northern shores of Gulf of Mexico & in the Java Sea.
Mix Tides : The diurnal & semi-diurnal oscillations are both important factors & the tide is characterized by a large inequality in the high water heights, low water heights, or in both. There are usually two high & two low waters each day, but occasionally tide may become diurnal. Such tides are prevalent along the coast of USA.