SENSITIVITY OF BOTTOM STRESS AND BOTTOM ROUGHNESS ESTIMATES TO DENSITY STRATIFICATION, ECKERNFORDE BAY, SOUTHERN BALTIC SEA

Thermohaline density stratification may significantly alter the classi c near-bottom logarithmic velocity profile in many weak to moderately energetic, partially mixed estuaries. Results from Eckernforde Bay sug gest fits to log profiles which neglect thermohaline stratification ma y lead to overestimates of bottom stress and roughness of the order of 130 % and 600 %, respectively. Measurements of velocity obtained at f our heights within 1 m of the seabed are input to theoretical models f or velocity shear derived via dimensional arguments for the ''overlap' ' layer. Previous investigators applying dimensional arguments to ther mohaline stratification in estuaries have assumed buoyancy flux to be independent of height within the overlap layer. This may be a poor ass umption since there is no significant source or sink of thermohaline b uoyancy at the sediment-water interface. In this paper, dimensional ar guments which do not assume constant buoyancy flux are used to reduce estimates of the drag coefficient and bottom roughness to below the un reasonably high values predicted by simple log profiles. Formulations assuming very weak and relatively strong stratification are applied, a nd estimates of buoyancy frequency derived from fits to velocity profi les are compared with independent estimates of stratification. Estimat es of bottom stress and roughness derived from velocity profiles are a lso found to be sensitive to fluid acceleration, uncertainties in inst rument settling, and limitations in current meter accuracy, but these latter effects appear secondary in Eckernforde Bay to the impacts of t hermohaline stratification.