5.1. Grain Size Trend Analysis Results
Establishing a characteristic distance (Dcr) is the first step in GSTA. The Dcr calculated for the Rossbeigh site was 1050 m, this was achieved iteratively. Firstly an arbitrary distance, the average distance between points was chosen, this was then varied incrementally to establish the characteristic distance that gave the most suitable vector plots to the site. There are 8 different cases for GSTA, these relate to the various permutations of the three parameters, mean grain size, sorting and skewness. All eight permutations were tested using the GiSedTrend tool. It was evident that after initial visual analysis of the 8 cases only 4 resembled coastal sediment transport behaviour, CB+, FP−, CP− and FB+.
The analysis was initially split over two datasets, the beach of Rossbeigh and the ebb tidal bar. However, no difference in correlation of trends discussed below on the beach were observed. The results below discuss the analysis of a combined data set of Ebb tidal bar and Rossbeigh beach.
The Coarser Poorer and more negatively skewed (CP−) trend case gave the most realistic plot of sediment transport trend when compared with sediment transport calculation, bathymetry surveys, hydrodynamic monitoring and morphological modelling.
The results of this case are shown in Figure 9
along with 3 other test cases, CB+, FP− and FB+. The CB+, Figure 10
and FB+, Figure 11
, both displayed some agreement with the sediment transport trends observed/calculated. The FP− case (Figure 12
) showed the least correlation with other methods with very small vector magnitudes and negligible variation in direction.
However, the strongest correlation in terms of trends and vector magnitude was the CP− case. The trend vectors of the CP− case showed strong onshore pathways on the ebb tidal delta. This was in agreement with the results of both tidal current monitoring and bathymetry surveys. The direction of the pathways was also significant, as it follows the pattern of the high tide wave direction. The majority of the vector arrows on the bar are in agreement with the drift aligned shore normal wave that occurs at high tide. As the bar is only covered and influenced by waves and tidal current at the upper stages of the tide, this reinforces both the validity of the trend analysis and also the influence of a dual directional wave climate at high tide in Rossbeigh drift aligned zone.
The results of the pathway analysis onshore at the drift aligned section of Rossbeigh were also pertinent. The trends show a strong offshore trend at both the island dune line and the distal edge. This was in agreement with the erosion trends shown on surveys [16
]. Further south along the shore in the drift aligned zone the trend vectors are running shore parallel or slightly angled to the shore. This conforms to the theory that the drift aligned zone sediment transport is dominated by shore parallel currents, documented by sediment formula comparison in O’Shea & Murphy [16
In the swash aligned zone the trends show vectors running perpendicular to the shoreline both offshore and onshore. This result gives confidence to the theory that the sediment transport in the swash aligned zone in predominantly cross shore.
There were also previously undocumented trends observed, for example, at the very edge of the island section a sediment pathway trend is running south in direction and is in contrast to the general trends. This trend may be caused by localised wave effects, by edge effects of the computation grid. It could also be a real trend as the bathymetry survey analysis shows this area to be morphological distinct. The neck of the channel does not erode like the entrance or middle section of the channel. There was very little change in bed level over the survey periods. There is a possibility that this trend vector identifies a sediment pathway previously not described.
As discussed earlier, the most common trends on beaches similiar to Rossbeigh are CB+ and FB−. Only several other cases have been validated including one case of FB+. However, until the present study there has been no CP− shown to be the dominant trend case. This was the first documented and validated case of CP− in a case study.
The coarser and poorer combination trend in the drift aligned zone it could be attributed to the intermitent dominance of wave and tidal forcings on sediment transport during the tidal cycle. However, for the trend case to be accurately sediment pathways on the swash aligned is unusual.
5.2. Comparison with Sediment Transport Simulations
As the GSTA sampling was undertaken during the modeling time period, direct comparisons of sediment transport results from both methods were possible. This presented a rare opportunity to assess the suitability of the GSTA method against a fully validated numerical morphodynamic model.
The modelled sediment transport regime was compared to the GSTA best case, CP−, Figure 9
. A corresponding plot of modelled accumulated sediment transport over the same time period as the GSTA is represented in Figure 13
Along the drift aligned dune section both GSTA and modelled sediment transport displayed alongshore and offshore wave dominated transport vectors. Although magnitude was notional in the GSTA analysis, the CP− case tended to display wave driven sediment transport as the dominant mode along the entire dune line of the drift aligned beach where as the modelled transport vectors displayed intermittent wave dominant sediment transport.
An interesting feature was the agreement at the Island terminus, where the sediment transport vectors appeared to be acting against the flood tidal current patterns and moving in an ebb tidal direction. Given the agreement of both numerical modelling derived sediment transport vectors and the GSTA trends, it was possible that this location may be ebb tidally dominated. This was a significant finding; given the nature of the bar migration described in O’Shea & Murphy [16
]. The large sediment transport vectors explain why the ebb tidal bar migration was fastest at this location.
Comparing the trend analysis plot, it was evident that there were discrepancies between the two in certain areas of the coastal cell. The direction of accumulated sediment transport on the ebb tidal bar differs in direction with the modelled. The modelled sediment transport vectors appeared to be driven in the same direction as peak tidal flood currents while the GSTA trend vectors follow a direct shore normal route. This suggests that the GSTA method showed a bias for wave dominated sediment transport in this area.
The other area of disagreement was at the breached inlet on the drift aligned beach. The model results show strong sediment transport going east wards through the breach but the GSTA trends were in the opposite direction showing sediment transport moving offshore in a westerly direction. This can be explained again by the mode of dominant sediment transport the GSTA method was biased towards. The GSTA adopted a wave dominant transport mode for this location, describing the sediment transport driven by wave erosive at the breach, while the model shows tidal current sediment transport through the breach was dominant.