Wakes from Large Vessels and the Risk to the Shoreline Environment in Tampa Bay

The shoreline around Tampa Bay, Florida contains large areas of productive habitat, hosting some of the highest concentrations of wading birds in the US. The bay also contains one of the busiest seaports in the country. Thousands of transits by large vessels such as tankers, cargo ships, and cruise liners are recorded every year through the network of shipping channels within the bay. Their wakes potentially threaten critical shoreline habitat as well as increased erosion around infrastructure and property. Vessel wake heights for the bay were estimated from records of vessel length, draught, and speed extracted from the historical Automatic Identification System (AIS) data. Errors in these data such as unrealistic draught and speed were removed from the analysis. This initial study was limited to the year 2015 and vessel lengths greater than 30 m. Over 560 of these vessels, composed of 26 unique vessel types, passed through Tampa Bay for a total of similar to 6000 transits. These ships range in size from 30-300 m in length with maximum draughts around 15 m. Ship wake heights were computed using the empirical relation developed for displacement vessels by Kriebel and Seelig (2005) based on the Froude number. Almost all the estimated wake heights were < 0.6 m. About 15% of the AIS reports for ships underway were found to be subcritical, generating no significant wake. Estimated wake heights above 0.5 m were rare, found in similar to 0.02% of the vessel reports. Some of the high estimates may be due to se 'dinning vessels violating the assumptions of the method, yielding unrealistic results. Reducing the block coefficient of vessels moving above their hull speed largely eliminated estimated wake heights > 0.5 m.