# The Influence of Hard Protection Structures on Shoreline Evolution in Riohacha, Colombia
## 4 Results and Discussion
### Zone 1: Effect of the Groins  
Table 2 shows the results of the analysis of shorelines with the DSAS program in Zone 1. This analysis has been divided into two time periods: 1987-2006 (before the construction of the groins) and 2009-2022 (after). For the 1987-2006 period, only three images are available, with the 1987 image serving as the baseline. Therefore, the regression data should be interpreted with caution. Nonetheless, it is evident that the coastal dynamics were predominantly erosive, with an average shoreline change of 5.3 \(\pm\) 1.2 m between 1987 and 2006. For the 2009-2022 period, the availability of more images allows for more reliable fitting results, better capturing the variability in shoreline dynamics. During this period, the construction of the groins led to a high mean accretion rate of 11.3 \(\pm\) 2.8 m/yr across the six analyzed transects. The sediment retention effect caused by the construction of the groins on littoral dynamics is clear (see shoreline evolution in Figure 4). From 2009 to 2013, significant beach growth was observed upstream of groins 1, 2, and 3 (Figure 5). Between 2014 and 2018, this growth was even more pronounced across all transects, leading to the almost complete silting up of groins 1 and 2. A similar trend is observed between 2018 (see Figure 2) and 2020, with notable beach advance between groins 3 and 6. Since 2020, the growth has slowed in nearly all groins, except for groins 5 and 6, which are the last to become silted (Figure 5). The further west, the later the gaps between groins are filled, all within a time interval of about 5 years. It is likely that, soon, sand will bypass these groins and advance again towards the areas located southwest, as has been observed in other coastal locations [65]. A one-way analysis of variance (ANOVA) was conducted to evaluate whether there were statistically significant differences in NSM between the two time intervals (see Table 3). The mean NSM for the 1987-2006 period was substantially lower (\(p\)-value \(<\) 0.001) compared to the 2009-2022 period. These findings suggest a statistically significant shift in shoreline dynamics over time, indicating a notable change in coastal behavior. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Period** & **Transect** & **NSM (m)** & **LRR (m/yr)** & **LCI (m/yr)** & **LR2** \\ \hline \multirow{8}{*}{1987–2006} & T1 & \(-2.1\) & \(-0.3\) & \(4.4\) & \(0.43\) \\  & T2 & \(-3.3\) & \(-0.4\) & \(3.3\) & \(0.72\) \\ \cline{1-1}  & T3 & \(-4.9\) & \(-0.5\) & \(0.7\) & \(0.99\) \\ \cline{1-1}  & T4 & \(-4.3\) & \(-0.5\) & \(2.7\) & \(0.85\) \\ \cline{1-1}  & T5 & \(-7.2\) & \(-0.8\) & \(3.8\) & \(0.89\) \\ \cline{1-1}  & T6 & \(-4.0\) & \(-0.5\) & \(5.7\) & \(0.58\) \\ \hline \multirow{8}{*}{2009–2022} & T1 & 81.1 & 6.5 & 3.7 & \(0.80\) \\ \cline{1-1}  & T2 & 118.6 & 9.3 & 3.9 & \(0.88\) \\ \cline{1-1}  & T3 & 146.6 & 13.2 & 3.6 & \(0.95\) \\ \cline{1-1}  & T4 & 138.0 & 13.1 & 2.5 & \(0.97\) \\ \cline{1-1}  & T5 & 134.9 & 13.6 & 4.9 & \(0.91\) \\ \cline{1-1}  & T6 & 129.5 & 12.3 & 4.4 & \(0.91\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Displacement rates of the coastline for the different transects created in Zone 1. It is not appropriate to apply prediction models based on the historical shoreline evolution of recent years in this case. This is because the coastal dynamics are likely to change once the shoreline reaches the end of the groins, altering the physical conditions. If the models were to follow the trend of the last decade, they would predict an unrealistic extension of beach accretion hundreds of meters beyond the groins. Additionally, it is not  
Figure 4: Shorelines and transects from 2009 to 2022 in Zone 1. Figure 5: New beach surface in Zone 1 since 2009. Quantities on the right represent the cumulative area, in m\({}^{2}\), gained in this period by each beach between groins.