1. 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

2. 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.

3. Overall scheme of modelling

4. 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

5. 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

6. Examples of 1D model behaviour
Momental cross-shore distributions

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

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

9. Meteorological (forcing) data
Meteorological datasets
wind measurement data for stations Arkona and Barth - observations of wind velocity and direction for the time period 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.

10. 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

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

12. 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.

13. 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.