Beach to Canyon Head Sedimentary Processes

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Publication . Ribeiro, M. A.; Taborda, Rui, 1966-; Rodrigues, Aurora, 1965-

Headland sediment bypassing (HSB) processes play an important role in the definition of the littoral sediment budget, having particular relevance on headland-bay coasts. However, HSB has received little attention compared to other topics of headland-bay beach research, such as the beach planform rotation and stability. Thus, the main goal of this thesis was to increase the knowledge on HSB and recognize its role on the sediment budget of headland-bay beaches. The study was carried out in a 100 km rocky coastal stretch located at the Portuguese western coast, between the Carvoeiro and Raso capes, where numerous headland-bay beaches develops in a wide range of different geomorphological settings. To achieve the proposed goals, the work was supported by several complementary approaches that gave insights on the processes that constrain headland-bay beaches evolution at a wide range of spatial and temporal scales. These approaches included: (1) detailed morphological and sedimentological analysis of the beaches and inner continental shelf; (2) the study of the longshore drift based on a fluorescent tracer experiment (short-term approach) and empirical modelling (long-term approach); (3) the development and application of a new shoreline evolution model (SEM-PLAT) that simulate shoreline changes on beaches develop over a rocky shore platform; and (4) the analysis of the sediment sources and sinks along the studied coastal stretch and study of long-term evolution trends. The results obtained allowed to classify dominant HSB processes in two main modes, which are in line with two of the existing conceptual models. The processes were classified according to the prevailing domain where sediment bypass the headlands as: (1) beach headland sediment bypassing (BSB) – a process that takes place at the subaerial section of the beach, and (2) inner shelf headland sediment bypassing (SSB) – a process that occurs on the submarine domain. BSB process is triggered by the persistence of incoming wave direction, inducing beach rotation that, especially in low-degree embayments, can result in a continuous inner sandbar along the headland coast, which migrates downdrift and welds to the downdrift beach. This process is dominated by the longshore drift, occurring mainly in the intertidal zone and, eventually, in very shallow depths of the submarine domain. SSB process, on the other hand, is triggered by high-energy wave conditions. Under these conditions, the strong offshore currents can transport sediment outside the embayments. The sediment that is pushed offshore is distributed on the inner shelf and eventually transported alongshore, bypassing the headlands. When wave energy start to reduce, the sediment is slowly transported onshore and start to weld to the downdrift beaches. This process depends on both cross- and longshore transport components and can occur in both low- and high-indented beaches. The results also showed that HSB is a selective process. Coarser particles such gravel and very coarse sand tend to be trapped inside the embayments, being their bypass very low or negligible. Coarse sand bypass occurs mainly in the subaerial beach, except during major storm events, when the wave energy triggers an extreme efficient selective process, transporting the lighter sediment particles offshore (including coarser sands), originating heavy mineral placer deposits. This selective process can generate large placer deposits (e.g. Grande beach) due to a long-term enrichment in heavy mineral particles. In the case of medium sand particles, headland bypass can occur either subaerial beach or inner shelf domain, depend on the wave energy; while fine sand bypass occur mainly at the inner shelf. This work also allowed to have a better understanding of the coastal dynamics of the target coastal stretch. The sediment transport patterns suggest that the studied coast is segmented in four sediment sub-cells: (1) Carvoeiro-Consolação; (2) Consolação-Lamporeira; (3) Lamporeira-Roca; and (4) Roca-Raso. The first sub-cell have particular characteristics due to the shelter promoted by the Peniche peninsula to the NW dominant waves and corresponds to a small closed system. The sub-cells (2) to (4) are macro scale embayments that are connected and have a similar sedimentary dynamics. The northern sectors of these embayments have a NW-SE or N- S general orientation and net longshore drift is southward for both winter and summer wave conditions. Thus, within these sectors, HSB occurs always southward, independently of the HSB process. The southern sectors, on the other hand, have a NE-SW general orientation and net longshore drift is southward, in the summer, and northward, in the winter. In these sectors, HSB occur mainly through the BSB, during summer, induced by the higher persistence of NW waves. While in winter, occur preferable through the SSB due to the increase of wave energy and considering that, BSB northward is unlikely due to the larger distances between the shoreline (beach berm) and the tip of the headlands and the lower persistence of W and SW waves. The balance between the longshore transport components within the southern sectors of the macro scale embayments, results in a sediment recirculation, which explains the sandier nature of inner shelf in these sectors compared to the northern ones. This also suggest that sediment can have a relatively long residence time inside these sub-cells. However, once sediment cross the sub-cell boundaries cannot return, because at the downdrift side the net longshore drift is permanently southward (gated boundaries). The frequency of the sediment linkage between these sub-cells depends on the seasonal and interannual wave climate variability; while the magnitude depends on the balance between the sediment sources and sinks of each sub-cell and was estimated in the order of 104 m3.yr-1. The sediment budget and beach long-term evolution study, indicate that the studied littoral is in equilibrium with present-day conditions. However, SEM-PLAT model results showed that seasonal (and interannual) changes in the incident wave climate can significantly affect the beach configuration or even its development, putting in evidence the delicate dynamic equilibrium of these beaches. This delicate equilibrium and the linkage of the beaches through the HSB processes, suggests that climate changes can have strong impacts on their evolution.

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Entidade financiadora

Fundação para a Ciência e a Tecnologia

Programa de financiamento

3599-PPCDT

Número da atribuição

PTDC/MAR/114674/2009