The effective bed roughness in estuaries can be determined either from known bed form dimensions (including the shape) or from bulk parameters. As regards the smaller bed forms (mini-ripples), the shape with relatively steep lee-side slopes is almost invariant and does not need to be included as a separate parameter. The bed roughness of the smaller mini-ripples is almost fully determined by flow separation effects. The available predictors for the dimensions and roughness of these smaller bed forms based on bulk parameters are sufficiently accurate (Van Rijn, 1993; Van Rijn and Walstra, 2003). As regards the larger bed forms (mega-ripples and dunes) in estuaries, the bed form shape is of crucial importance for the effective bed roughness because it determines the generation of flow separation effects or not. This effect can be included by using a shape correction factor. At present stage of research, the available bed form predictors for mega-ripples and dunes in estuaries are not sufficiently accurate. These predictors, generally, produce estimates of the bed form height and length, but do not give information of the bed form shape. The shape correction factor of mega-ripples in estuaries will probably be in the range of 0.3 to 0.7 depending on the precise shape of the mega-ripples in the crest region (degree of roundness). Since a generally-accepted method for the accurate prediction of bed form characteristics (Δ,λ and shape) in estuaries is not yet available, it is concluded that the prediction of bed roughness from predicted bed form dimensions will not lead to very accurate results at present stage of research. More accurate bed roughness values can be obtained if the bed form dimensions are known from detailed field measurements (including the bed form shape) in combination with detailed flow velocity measurements. This requires the detailed mapping of the bed forms in various geomorphological units of each specific system (river, estuary or coastal sea). Given the fact that bed form data often is inaccurate or totally missing, it may be more attractive to relate the bed roughness (ks) directly to bulk hydrodynamic and sedimentdynamic parameters (ks/d50=f(ψ) with ψ=parameter characterizing the hydrodynamic regime). Such a method was recently developed by Van Rijn (see Van Rijn and Walstra, 2003) for combined current and wave conditions. However, the proposed method needs extensive verification as regards the predicted roughness values for mega-ripples and dunes in estuaries and seas.
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