Storm surge effects on a back-barrier tidal flat of the Danish Wadden Sea

The most severe storm passing Denmark in the 20th century occurred on 3-4 D ecember 1999. During this event, waves, currents and suspended sediment con centrations were measured on a muddy tidal flat vis-vis the salt marsh of S kallingen in the inner part of the Hobo Dyb tidal area on the Danish west c oast. Locally generated wave heights increased during the storm to H-3=0.38 m, with peak spectral wave periods of up to 3.6 s and wave-induced bed she ar stresses reaching a maximum of about 2 N m(-2). The highest current velo city measured over the tidal flat during the surge was 0.64 m s(-1) (simila r to1 N m(-2)). During the surge peak, the wind-induced water circulation c lose to the bed was about 0.2 m s(-1) which is equivalent to a bed shear st ress of about 0.2 N m The concentration of suspended matter on the tidal fl at increased from about 10 mg 1(-1) to about 200 mg 1(-1), and the mobile l ayer on the mud flat was removed. The estimated deposition on the salt mars h amounted to 133 g m(-2) (0.15 mm) which is only about 50% higher than tha t of a previously monitored gale event, and corresponds to less than 10% of the yearly deposition at the site. It is suggested that deposition on the salt-marsh surface caused by storms or gales depends on the prevailing meteorological and hydrographical condit ions as well as on the sequence of previous import and high-energy events. The results indicate that a single extreme storm can be of less importance for annual variations in saltmarsh deposition than more regularly occurring