Harbor waves yield secrets to analysis

New findings by ocean scientists may help port officials in Rotterdam, the Netherlands, predict potentially destructive waves in the city’s harbor.

Most of Rotterdam lies below sea level, and movable barriers near the mouth of the port’s main channel protect the region from storm surges originating in the North Sea. However, sloshing waves called seiches can arise within the harbor and cause water levels to vary as much as 1.8 meters in as little as 45 minutes. Those waves could damage the barriers if the gatelike devices were deployed at the wrong time, says Martijn P.C. de Jong, a civil engineer at Delft University of Technology in the Netherlands.

Several phenomena can cause seiches, including weather changes, earthquakes (SN: 7/7/01, p. 5: Available to subscribers at Is Nessie merely a bad case of the shakes?), and tsunamis (SN: 6/10/00, p. 378: Tsunami! At Lake Tahoe?). De Jong and his colleagues discovered that each of the 51 seiches in Rotterdam’s harbor from 1995 to 2001 that sloshed higher than 25 centimeters occurred as a cold front passed through the area.

Initially, the scientists found that their computer models could have predicted the five seiches associated with strong cold fronts but not the others, which occurred in conjunction with cold fronts marked by gradual changes in wind speed and direction. These findings will be reported in an upcoming Journal of Geophysical Research (Oceans).

De Jong’s team recently solved the mystery of the other seiches. When the air about 1.5 kilometers over the North Sea is more than 15C colder than the surface waters, lower-atmosphere instability produces waves in the North Sea with the same sloshing frequency that a seiche in Rotterdam harbor would have. When those waves sweep into the port, they resonate within the harbor and trigger the seiches.


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