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Technical report

Flourescein dye was used to trace water movements and to determine flow velocities and volume transports on two Guam fringing reef flats. Wave-driven water crossed most portions of the reef margin in a direction roughly perpendicular to the shoreline. As it moved across the reef flat it gradually changed direction until it was flowing as a longshore current in a deepened moat zone adjacent to the shoreline. After flowing in this longshore current for distances up to 1500 m, the water gradually moved seaward in a more dispersed pattern and exited the reef flat through major cuts in the reef margin. Smaller, more restricted portions of both reef flats had very sluggish water movements with less distinct patterns. Of the total water volume crossing transects which extended perpendicularly from the shoreline, 10-100% flowed in the moat zone within 80 m of the shore. Flow velocities ranged up to 0.6 m sec1. Mean volume transport across entire transects was up to 61.6 m3 sec1. and in the moat zone alone was up to 23.5 m3 sec-1. There was a significant correlation between surf and flow velocity, as well as between surf and volume transport in the moat, at all transects and tidal states tested in one bay; but the correlations were less conclusive in the other bay. Velocity was more strongly correlated with surf than was volume transport in the moat for most of the correlation analyses.