Sediment sorting at the Sand Motor at storm and annual time scales

Bed sediment composition, with a focus on the median grain size D50, was investigated at a large-scale nourishment (The ‘Sand Motor’) at the Dutch coast (∼21.5 million m3 sand). Considerable alongshore heterogeneity of the bed composition (D50) was observed as the Sand Motor evolved over time with (1) coarsening of the exposed part of the Sand Motor (+90 to +150 μm) and (2) a depositional area with relatively fine material (50 μm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the measured D50 values was most evident outside the surfzone (i.e. seaward of MSL −4 m). Coarsening of the bed after construction of the Sand Motor was attributed to hydrodynamic sorting processes, because the alongshore heterogeneity of the D50 showed a similar spatial pattern as the mean bed shear stresses. The observed alongshore heterogeneity of the D50 and correlation of D50 with modelled mean bed shear stresses suggest that preferential erosion of the finer sand fractions has taken place. The selective transport of finer sand fractions results in a coarser top layer of the bed at the Sand Motor. The preferential transport is most dominant during mild and moderate conditions when hydrodynamic forcing conditions are close to the critical bed shear stresses for transport. The measurements also show the impact of a storm, which consists of a ∼40 μm finer D50 of the offshore bed composition in front of the Sand Motor (i.e. where a considerably coarser bed was in place). Additionally, storms may generate a (temporary) zone with fine bed material at the toe of the deposition profile. This means that the coarsening of the bed is reduced by storms as a result of the mobilization of both coarse and fine sediment and mixing of the bed with the relatively finer substrate.


32
Spatial heterogeneity of bed sediment composition is observed at many coasts around the world 33 (Holland and Elmore, 2008), but seldom accounted for in morphological or environmental impact 34 studies of coastal interventions (e.g. modelling of sand nourishments; Capobianco et al., 2002). 35 Knowledge of the potential spatial variability of the bed sediment (i.e. grain size and grading) is 36 however considered essential for the understanding of the ecological impact of large-scale coastal 37 interventions. Firstly, bed composition changes affect the ecological habitats for benthic species 38 and fish (e.g. McLachlan, 1996;Knaapen et al., 2003). Small changes in the top-layer (i.e. cen-39 timeters) grain size can, for example, significantly affect the burrowing ability of juvenile plaice 40 (Gibson and Robb, 1992). Secondly, long-term morphological changes may be affected by bed 41 coarsening when finer sand fractions are predominantly eroded (Van Rijn, 2007). Furthermore,

224
A weighted average of the median grain size per cross-shore transect (referred to as D 50TR ) was 226 used to analyse the alongshore spatial heterogeneity of the bed. The D 50TR is defined as follows: The contribution of each sample (landward of the MSL-10m contour) is computed by multiplying          Alongshore heterogeneity of the bed composition was most prominent in deeper water seaward 403 of the sub-tidal bar (D 50TR,off of +90 to +150 µm with respect to T0 survey; Figure 11) as a 404 result of the relative coarse D 50 in deeper water at the Sand Motor (Table C. The gradual increase in the D 50TR at the Sand Motor peninsula in the first two years (from T1 416 to T4) exceeded the uncertainty as a result of the analysis methodology and short-term temporal 417 variability. Observed coarsening was therefore not considered due to initial construction of the 418 Sand Motor alone, but partly also the result of a gradual process in time.

420
The longer-term behaviour of the D 50TR from survey T3 onward was much more subtle (Fig-421 ure 12) and therefore makes it difficult to discern a trend. This may partly be due to a seasonal 422 influence on the D 50 of the measurement surveys, which was perceived to be present at transects 423 North of the Sand Motor (panel b in Figure 12). These transects show ∼30 µm coarser surveys 424 in summer (T4 and T6) than in winter (T3 and T5). In order to filter out the bias of the surveys 425 (e.g. due to seasonality) it is therefore proposed to use the difference in the D 50TR between the 426 coarsest and finest transect of each survey (respectively D 50TRmax and D 50TRmin ) with respect to 427 the average D 50TR of each survey (D 50TR ) as a proxy for the 'degree of alongshore heterogeneity' 428 of the D 50 (S alongshore ). The S alongshore is given by the following equation : Long-term development of S alongshore for transects B and D (i.e. finest and coarsest transect) 431 shows a considerably enhanced degree of alongshore heterogeneity (S alongshore ) compared to the 432 natural alongshore variability in the T0 survey ( Figure 13). This S alongshore decreased slowly Preferential transport of finer sand may take place during quiet and moderate wave con-497 ditions at the Sand Motor as a result of (tidal) flow contraction. This was shown from 498 the strong correlation between the time-averaged mean bed shear stresses (τ cw,mean ) and 499 alongshore spatial heterogeneity of the D 50 (Figure 15), which indicates that a mechanism 500 is present during moderate conditions (mainly due to the tide) which considerably affects 501 the development of the spatial heterogeneity of the D 50 . The added sediment at the Sand

502
Motor was similar to that of the surrounding coast, while the potential for mobilization 503 was increased due to the tidal flow contraction at the peninsula. Consequently, the critical 504 bed shear stresses for erosion of the fine fractions will be exceeded more frequently than 505 for the coarser fractions, which results in a larger entrainment of the finer fractions in the 506 28 water column (Komar, 1987) and enhanced alongshore transport rates (Steidtmann, 1982).
These derived properties can provide insight in the processes that were driving the bed com-