Interactions between beachrock formations and shoreline evolution. Case study

Abstract

Beachrocks are lithified coastal sedimentary formations which form inside the beach body on short time scales, possibly within a year. Beachrocks can play an important role in the evolution of a shoreline as they fix the normally loose sediments. On the other hand, the evolution of a shoreline could play an important role in the genesis of beachrock and their preservation. A better understanding of these interactions will aid their use in science, to understand their genesis and to reconstruct paleo-climates. The knowledge derived is also useful for coastal engineering practice, to evaluate the response of a coastline to beachrock exposure. This study evaluated the interactions between beachrocks and shoreline evolution both from a theoretical point of view as well as in a practical case study in Togo, West Africa. In the theoretical review, the variability of shore and shoreline has been related to the diagenetic environment of a beachrock. Furthermore, existing literature was reviewed in order to assess the existing knowledge on shoreline response to the exposure of beachrock. A 2-month field campaign was carried out in Togo to gather data on beachrock characteristics and shoreline development. Finally, a synthesis was made of the theoretical findings and the results of the field campaign. The process of beachrock genesis and the controls on this process are not fully clear. Multiple mechanisms have been proposed but cannot be confirmed for all reported occurrences of beachrock. It is thus likely that no single process is responsible for the formation of beachrock. The availability of carbonate and the temperature of the beach water are likely to be among the controls on beachrock formation. It has been proposed that the stability of a beach is another control, as agitation of beach particles would not allow cementation to develop. This proposition does however not properly consider the variability of shore and shoreline. The results show that short term variations of shore and shoreline (hours to decades) are likely to be a control on beachrock genesis. The variability of the diagenetic environment related to these variations determines the cementation characteristics of beachrock. Shoreline variations thus determine the diagenetic history of a beachrock, which in turn influences the rock mass properties of the beachrock. Longer term trends in shoreline evolution are probably not linked to the genesis of beachrock, thus limiting its paleo-environmental significance. This theoretically derived conclusion needs to be confirmed by further more detailed investigation in the field. The analysis of shoreline variability related to beachrock genesis can be used to further evaluate the different mechanisms of formation that have been proposed. Analysis of weathering patterns can be the next step in increasing the paleo-environmental significance of beachrock formations. In the case study of Togo, the impact of a very large outcrop of beachrocks on shoreline evolution was investigated, by defining an approach that could be applied in similar cases. The beachrock in Togo can be divided into two formations: a main formation which shows high continuity and strength, and effectively blocks cross-shore sediment transport along major parts of the coast of Togo. Another less profound formation is found adjacent to the main formation. The beachrock in Togo fulfills a role similar to hard engineered structures which are often used in coastal engineering to protect sandy coastlines. The placement of these engineered structures in the nearshore zone triggers a certain response that is adequately understood and can be predicted using modeling software or engineering guidelines. The retreat of a sandy coastline with buried discontinuous beachrock formations poses difficult questions that have not yet been accounted for in science. The complex coupling between sediment transport and hydrodynamic conditions in the nearshore zone is not well understood, and the activation of non-erodible elements inside or as a continuous part of beaches has received only minor attention. Furthermore, beachrocks differ fundamentally from engineered hard structures as they exhibit a large natural variation in cementation characteristics and thus rock mass properties. Whereas for engineered structures the mechanical properties are known and can be used to calculate the life time of the structure, for beachrock a more thorough understanding of the genesis needs to be developed. The dominant mode of weathering of the main formation appears to be undercutting by scouring around the seaward base of the beachrock. The response of a retreating sandy coast to buried beachrock formations can be qualitatively analyzed using the Phased Retreat Model presented in this study. This model uses equilibrium shoreline profiles to determine different phases in the process of retreat. Longshore processes play a dominant role in the retreat of a coast behind a beachrock barrier. Starting from locations where the beachrock barrier has been breached, erosion landward of the beachrock travels in the opposite direction of the longshore current seaward of the beachrock. The Phased Retreat Model can be used to further analyze and predict shoreline development along the coast of Togo. Using a more data-driven analysis and by investing in structural solutions. the Togolese government can upgrade their shoreline protection policy to the philosophy of Integrated Coastal Zone Management. Furthermore, the presented model could be used in similar cases elsewhere. Lastly, the Togo case can be used for further generic research on morphodynamic response to barriers in the near-shore zone.