River Jets Versus Wave-Driven Longshore Currents at River Mouths

At river mouths, fluvial jets and longshore currents (LSCs) generated by waves interact hydrodynamically. This idealized numerical modeling study simulates a large number of hydro-morphodynamic conditions (650) to explore the emergent hydrodynamics determined by different mouth bar volumes and geometries, river discharge, wave heights, and directions and their potential stress on river-mouth development. We find that in the absence of a river-mouth bar (RMB), interactions are driven by momentum balances, expressed either as the balance of wave momentum flux (Mw) and jet momentum flux (Mj), or the balance of river jet discharge (QJet) and longshore current discharge (QLSC). When a RMB is present, the topography modifies the structure of the jet by spreading it, and we quantify this mechanism through the lateral jet transfer rate (LJT). Secondly, topography generates complex longshore wave-driven circulation as a result of the protruding shoreface which serves as a platform on which counter LSCs develop. The balance in QJet/QLSC may be used as an indication of the type of circulation. High and oblique waves favor longshore circulation and RMB bypass, whereas low waves and normal-to-coast angles generate diverging LSCs on the mouth bar crest which interrupts the longshore circulation. A quantification of the dynamic diversion is proposed in the form of the non-dimensional Dynamic diversion index (DyD), which scales with the product of Mj and Mw, and can account for the absolute strength of hydrodynamic interactions occurring at river mouths. RMB morphology can affect DyD in multiple ways by strengthening or by weakening the interactions. The DyD effect seems to increase with increasing RMB size, indicating that the RMB scale regulates the interplay of the wave-driven circulation and the river jet which further controls the adjacent topography changes.

Investigasi Arus Sejajar Pantai (Longshore Current) di Daerah Abrasi Bengkulu Utara

The coastal area of Serangai Village is directly facing the open sea; as a result it is vulnerable to coastal abrasion. Longshore currents can accelerate the process of coastal abrasion. The study was aimed to determine the speed and direction of current as well as the existence of the longshore current at the Serangai. Field observation was conducted on 5-7 November 2018. The results showed that the frequency distribution of current at speed of 0-10 cm/s reached 69%, and at speed of 10-20 cm/s reached 25%. The highest speed of 20-30 cm/s has a frequency of 6 %. Moreover, the direction of the majority of ocean currents is to the southeast (120o-150o), showing that the current is more likely to be parallel to the shoreline called longshore current that can accelerate shoreline Serangai Village, Bengkulu. However, further research is needed to see variability of current associated with the season (monsoon). Wilayah pesisir Desa Serangai berhadapan langsung dengan laut lepas; sehingga rentan terhadap abrasi pantai. Arus sejajar pantai dapat mempercepat proses abrasi pantai. Penelitian bertujuan untuk mengetahui kecepatan dan arah arus serta keberadaan arus sejajar pantai di Serangai. Pengamatan lapangan dilakukan pada tanggal 5-7 November 2018. Hasil penelitian menunjukkan bahwa distribusi frekuensi arus pada kecepatan 0-10 cm/s mencapai 69%, dan pada kecepatan 10-20 cm/s mencapai 25%. Kecepatan tertinggi 20-30 cm/s memiliki frekuensi 6%. Selain itu, arah arus laut mayoritas ke arah tenggara (120o-150o), menunjukkan bahwa arus lebih cenderung sejajar dengan garis pantai yang disebut arus sejajar pantai yang dapat mempercepat garis pantai Desa Serangai, Bengkulu. Namun, penelitian lebih lanjut diperlukan untuk melihat variabilitas arus yang terkait dengan musim (monsun).

An Overview of the Expected Shoreline Impact of the Marine Energy Farms Operating in Different Coastal Environments

The aim of the present work is to provide an overview of the possible implications involving the influence of a generic marine energy farm on the nearshore processes. Several case studies covering various European coastal areas are considered for illustration purposes. These include different nearshore areas, such as the Portuguese coast, Sardinia Island or a coastal sector close to the Danube Delta in the Black Sea. For the case studies related to the Portuguese coast, it is noted that a marine energy farm may reduce the velocity of the longshore currents, with a complete attenuation of the current velocity for some case studies in the coastal area from Leixoes region being observed. For the area located close to the Danube Delta, it is estimated that in the proposed configuration, a marine energy farm would provide an efficient protection against the wave action, but it will have a relatively negligible impact on the longshore currents. Summarizing the results, we can conclude that a marine energy farm seems to be beneficial for coastal protection, even in the case of the enclosed areas, such as the Mediterranean or Black seas, where the erosion generated by the wave action represents a real problem.

Rip current simulation on some beaches in coastal Quang Nam province

This paper presents modelling results of rip currents on the main beaches along coastal Quang Nam province including Ha My, Binh Minh, Tam Thanh and Rang beaches during two typical wind seasons: Northeast monsoon (Northeast wind direction, wind levels: 4, 5, 6) and Southwest monsoon (Southeast wind direction, wind levels: 4, 5) using Mike 21 model. Calculation results show that during the Northeast monsoon, the rip current formed in all beaches. In the scenario of level 4 of wind speed, average rip speed was about 40–50 cm/s. In particular, at Tam Thanh beach area, the rip was a typical one with the components such as feeder current, rip neck and rip head. With the level 5 of wind field, the formation of the rip was clearer, the speed of the rip was stronger, average value was about 50–60 cm/s. Meanwhile, with the level 6 of wind field, the typical rip structure was broken, creating local eddies or longshore currents at some positions, but strengthened at other positions. During the Southwest monsoon, the rip current did not form at the beaches and the longshore currents were dominant.