1D Long-term Modelling of Longshore Sediment Transport U.Bethers (1), J.Sennikovs (1), K.-P. Holz (2) (1) Laboratory for Mathematical Modeling of Environmental and Technological Processes, University of Latvia (2) Institute of Information Technology in Civil and Hydraulic Engineering, Brandenburg University of Technology

Area of study The Gellen Bay is a shallow water area in the south-western part of the Baltic Sea. The eastern coast of the Bay is island Hiddensee, the southern coast is formed by islands Darss, Zingst and Bock. The system of inland water basins and interconnecting channels (Bodden area) is separated from the sea by these narrow and prolonged islands. The system of channels also forms a waterway from the Gellen Bay to the Bay of Greifswald.

Overall scheme of modelling

Classification of flow patterns Morphodynamical scenario Classification of flow patterns yielded from the steady state calculations by a 2D model. Multi-level modelling include 2D shallow water wave-current-sediment transport models describe characteristic processes’ patterns (steady-state) or short-term processes’ development (transient) in open water body Integral (0D) models describe mass balance of enclosed inner water bodies 3D HD model would be needed for channel entrance zone 1D littoral drift model is suitable for long-term assessment of sediment transport rates

One-dimensional model Wave field: ky(x)=const, k(x) calculated from dispersion equation yields kx(x) and wave group celerity. Wave energy calculated from ECgx=const and breaking criteria Bottom roughness: calculated from waves Hydrodynamics: momentum conservation yields equations for elevation and longshore velocity 1D wave vector and longshore current Load transport: littoral drift calculated assuming saturation concentration c* in suspension Bed level: assumed constant

Examples of 1D model behaviour Momental cross-shore distributions

Examples of 1D model behaviour Time-averaged (1993 to 1997) cross-shore distributions

Examples of 1D model behaviour Cumulative (1993 to 1997) integral long-shore transport

Meteorological (forcing) data Meteorological datasets wind measurement data for stations Arkona and Barth - observations of wind velocity and direction for the time period 1973-1997 with time resolution 1 hour. wave height and period are predicted from wind speed, duration and fetch. Windrose for Arkona Significant wave height for a particular point in the area of interest for wind speed 20 m/s Fetch for a particular point in the area of interest.

Description of models Selected profiles for 1D longshore load transport calculations. Local rapid changes of the coastline orientation have not be accounted for. The profiles for calculation are taken at an average distance of 3 km between them Rate of deposition/erosion between the two profiles are calculated as a divergence of the load flux

Calculations by a 1D longshore model Calculations were performed for the time period 01.01.1973 to 01.07.1997 Cumulative load transport (million m3) along Hiddensee Calculated annual mean load transport, thousands m3.

Calculations by a 2D depth averaged model Steady state calculations were performed for a variety of storm conditions. Calculated annual mean load transport, thousands m3 Distribution of sediment concentration. Load transport rate, thousands m3 per year. NW-wind 10 m/s Hs=2.0m Annual load transport values were obtained from calculated steady state values taking into account storm event probability.

Conclusions Selection of appropriate meteorological forcing is of the most importance as any model is extremely sensitive to it. Ranking of time periods according to their morphodynamical impact should be performed using calculated transport rates instead of meteorological data. Mean load transport rates calculated by 1D longshore and 2D depth averaged models are close to each other. Transport rates calculated by a present model and HR Wallingford software COSMOS are in reasonable agreement 2D calculations for a typical meteorological situations allow classification of flow and load transport patterns. 2D model should be used for short-term (e.g. duration of storm) simulations involving rapid morphodynamical changes both in time and in space. 1D model is better suitable for long-term simulations, hind- and forecasts due to its simplicity. Models of every level have their own areas of application and they should not be treated as alternatives. In the case of absence of good data for the verification plausibility of separate models increases if the results obtained by models of different levels agree.