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Duy, D.V.; Tanaka, H.; Mitobe, Y.; Anh, N.Q.D., and Viet, N.T., 2018. Sand spit elongation and sediment balance at Cua Lo Inlet in Central Vietnam. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp. 32–39. Coconut Creek (Florida), ISSN 0749-0208. Erosion is significant on the downdrift beach of Cua Lo Inlet, which is located in Central Vietnam. The elongation of Cua Lo Inlet's updrift sand spit together with severe erosion of the downdrift bank is raising numerous social problems such as flood safety and land loss. In this study, a series of Landsat and Google Earth images were used to analyze and investigate the inlet recent morphological changes and evolution. Since 1970 the inlet has constantly migrated towards the sediment transport pathway, and its traveling distance has reached 1700 m. In order to quantitatively investigate such characteristics, the tip coordinates and the area of the sand spit updrift of the inlet were calculated. In particular, the area of the sand spit has increased remarkably, and the increasing rate is almost constant. Using this value, the rate of sand volume transported along the updrift sand spit by the waves was estimated and it is in good agreement with the longshore sediment transport rate of the surrounding beach. [ABSTRACT FROM AUTHOR]

Thanh, T.M.; Tanaka, H.; Mitobe, Y.; Viet, N.T., and Almar, R., 2018. Seasonal variation of morphology and sediment movement on Nha Trang Coast, Vietnam. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp. 22–31. Coconut Creek (Florida), ISSN 0749-0208. The Nha Trang beach, an embayed sandy beach located in the southeast of Vietnam, is a tropical touristic coastal environment that is being threat by persistent shoreline retreat. In order to investigate the processes underlying this erosive pattern, 1.5-years of video imagery, obtained from a local video system, was analyzed. Video products such as shoreline position and surf zone width (extracted from time-averaged images) were used to identify patterns of shoreline variability (using empirical orthogonal function – EOF) and investigate the link between the seasonal variations in the wave forcing and longshore sediment transport. Results indicate that shoreline retreat occurs essentially during the winter season, when energetic northeast monsoon waves dominate. The recovery phase (shoreline advance) was found to occur during the summer season, when milder south/southeast wind waves dominate. Shoreline variability was further investigated using EOF analysis and results show that the first mode, which explains 89% of the observed variance, corresponds to the seasonal longshore shoreline variations. This EOF mode was found to strongly related to the winter and summer monsoon seasons, thus reinforcing the role that seasonal changes in the wave climate play on the shoreline variations. Finally, the net of longshore sediment transport in Nha Trang beach was investigated using seasonal and monthly depth of closure (Dc) information, rather than a single fixed value of this parameter. The results highlight that the use of monthly or seasonal Dc values allow to capture the seasonal variations in the wave climate, thus providing more realistic patterns of sediment transport in this coastal area. [ABSTRACT FROM AUTHOR]

Thuy, M.T.T; Nagasawa, T.; Tanaka, H., and Viet, N.T., 2018. Sandy beach restoration using beach nourishment method: A case study of Nha Trang Beach, Vietnam. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp 57–66. Coconut Creek (Florida), ISSN 0749-0208. Nha Trang Beach, located in the Northern part of Cam Ranh Bay, is famous as a tourist destination for its beautiful sandy beach. Since 1975, the Southern part of the beach has been considered a tourist hotspot where accommodation and business infrastructure are concentrated. However, in the past decade, severe erosion and shoreline retreat has occurred in the area. Human intervention in terms of house construction and other tourism infrastructure building is supposed to be one of main causes for these erosion problems. On the Northern beach, tourism demand is low despite its considerable potential. Between 2004 and 2005, a road was built on the Northern area, resulting in the loss of the beach, consequently affecting local tourism economy. The restoration of the sandy beach in this area is therefore crucial for local economic development. In this study, a concept of beach nourishment design for the Northern beach was proposed. Beach nourishment was designed for the purpose of bringing the shoreline back to the state of equilibrium it once had been before the road construction during 2004 – 2005. This study considered original equilibrium beach profiles, sediment grain sizes and wave climate characteristics in the design method of beach nourishment. [ABSTRACT FROM AUTHOR]

ABSTRACT Tanaka, H.; Duy, D.V.; Mitobe, Y., and Viet, N.T., 2018. Theory for Erosion Wave Propagation in Cua Dai Beach, Vietnam. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 621–625. Coconut Creek (Florida), ISSN 0749-0208. This paper deals with the propagation of erosion wave in Cua Dai Beach in Central Vietnam under the influence of longshore sediment movement using analytical results from the simplified equation of the one-line model. In addition, measured shoreline positions obtained from Landsat images are utilized to validate the calculated results. Effects of the waves on the shapes of the erosion/accretion zones along the Cua Dai Beach are observed. Using the analytical solutions, the propagation speed of the erosion wave is estimated as 37 m/y while this value is 54 m/y in reality. Coastal structures such as seawalls located along the Cua Dai Beach is causing more severe erosion along Cua Dai Beach. [ABSTRACT FROM AUTHOR]

Adityawan, M.B.; Tanaka, D., Mitobe, Y., and Widiyanto, W., 2016. A new computation method of bottom shear stress under tsunami waves, In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1247 - 1251. Coconut Creek (Florida), ISSN 0749-0208. The tsunami propagation in the river as well as sediment transport is highly affected by the bed stress beneath the wave. Unfortunately, most numerical models use an empirical method such as Manning's n, which was developed for steady flow. A new method in numerical modelling to assess bed stress and sediment transport induced by wave is developed based on the boundary layer approach. The boundary layer has a significant effect on the bed stress under unsteady flow. Bed stress under unsteady flow may exhibit phase shift and sign change to the free stream velocity. Recently, Simultaneous Coupling Method (SCM) was developed and verified by combining Shallow Water Equation (SWE) and k-ω model. The simplicity of SWE was maintained while k-ω model was used to assess the bed stress from the boundary layer. The velocity from SWE is assumed as the free stream velocity, which is used as the boundary condition for k-ω. The calculated bed stress from k-ω is used in SWE to obtain water depth and velocity. In this study, SCM is further developed by simulating a tsunami wave propagation around the Kitakami River mouth and verified using a laboratory experiment. The results show good comparison between the numerical and experimental. [ABSTRACT FROM AUTHOR]

Iida, T.; Kure, S.; Udo, K.; Mano, A., and Tanaka, H., 2016. Prediction of the 2011 Tohoku Tsunami scouring near structures. In: Vila-Concejo, A., Bruce, E., Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 872-876. Coconut Creek (Florida), ISSN 0749-0208. The 2011 Tohoku Tsunami left innumerable sites with large scale scouring near failed coastal structures. This was a major cause of failure, and the reconstruction of these structures in a robust way requires determination of the mechanisms and prediction of the scouring. Two mechanisms are known for tsunami scouring: erosion by the bottom shear stress and liquefaction by the pore pressure, however, these mechanisms have not been compared yet in their effects. This study aims to quantify these mechanisms and predict the scouring by the 2011 Tohoku tsunami. For this approach, we developed a numerical model by the quasi-3D finite volume method by meshing horizontally in water and vertically in ground. After validating our model, we have applied it to the breach point of a coastal levee hit by the tsunami. We can conclude that erosion by the bottom shear stress has the major role, however, the fluidisation still has innegligible effect in 28% to the maximum erosion depth. [ABSTRACT FROM AUTHOR]

Aoyama, Y.; Adityawan, M.B., Widiyanto, W., Mitobe, Y., Komori, D., and Tanaka, H., 2016. Numerical Study on Tsunami Propagation into a River. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1017 - 1021. Coconut Creek (Florida), ISSN 0749-0208. The Tohoku Earthquake and Tsunami in 2011 caused serious damage not only to coastal structures, but also to riverine infrastructure due to long distance of wave propagation into rivers located on the coast facing the Pacific Ocean. Although there have been numerous investigations on tsunami, limited number of studies have been made for tsunami propagation into a river channel. This study investigates tsunami propagation into a river numerically by comparing with laboratory experiment data. The present numerical simulation is based on shallow water equations, which are solved with the MacCormack scheme. Difference between calculated and experimental results are evaluated in terms of root mean square error. It is concluded that the present numerical simulation yields good agreement with experimental data in a wave flume. In addition it is observed that geographical characteristics in the river channel, such as sandbars and estuarine sand spits, highly affect tsunami propagation process in a river, causing lowering water level along with late arrival of tsunami peak. [ABSTRACT FROM AUTHOR]

Kure, S., Winarta, B., Takeda, Y., Udo, K., Umeda, M., Mano, A., Tanaka, H. 2014. Effects of Mud Flows from the LUSI Mud Volcano on the Porong River estuary, Indonesia. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 568-573, ISSN 0749-0208. On May 29, 2006, mud and gases began erupting unexpectedly from a hydrocarbon exploration well near Sidoarjo, East Java, Indonesia. The eruption, called the LUSI (Lumpur Sidoarjo [Lumpur means mud in Indonesian]) mud volcano, has continued since then at rates as high as 180,000 m3 per day. The mud inundated an area in excess of 6.5 km2, in spite of attempts to contain it via the construction of a series of levees. The mud is erupting and accumulating behind the levee system. It is also being pumped into the Porong River, which then carries the mud to the ocean approximately 20 km to the east. The main objective of this study is to assess the effect of mudflow from the LUSI volcano on the river and estuary environment such as sediment deposition and water quality using the observed river cross sections and bathymetry data after the eruption of the LUSI volcano. From the analysis it was found that a significant sediment deposition occurred at the river bed of Porong River due to increased sediment discharge from the volcano during the dry periods after which the sediment deposits on the river bed were gradually eroded and flushed away to the ocean by floods and high water flows during the wet periods. Environmental impacts due to increased sediment from the Porong River on the estuary and coastal region are discussed. [ABSTRACT FROM AUTHOR]

Iida, T., Kure, S., Udo, K., Mano, A., Tanaka, H., 2014. Breach process simulation of coastal levees broken by 2011 tsunami. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 302-307, ISSN 0749-0208. On March 11, 2011, a tsunami hit the Sendai Bay Coast and broke 80% of the coastal levees. The areas around the breached levees were scoured deeply, causing serious adverse effects on post-disaster reconstruction efforts. This motivated us to analyze the mechanism of the local scour in the areas around breached levees. We collected and investigated various data such as aerial photographs and tsunami videos. As the results of the investigation, we found the breach points progressing in two steps. Firstly, the brittle structure was destroyed by the first leading wave. Secondly, the concentration of the return flow eroded and breached levees with the broken points from the first step. In order to understand the details of the second step, a numerical simulation was conducted using a finite volume method. From the result of the simulation, the scouring mechanism around breached levees was analyzed and discussed. [ABSTRACT FROM AUTHOR]

Mano, A., IIda, T., Udo, K., and Tanaka. H., 2013. Breaking process and mechanism of coastal levees on Sendai Bay Coast hit by 2011 mega tsunami The 2011 mega tsunami induced by the East Japan Earthquake, Mw 9.0 hit the Sendai Bay Coast, overtopped the coastal levees and intruded deeply onto the Sendai and Ishinomaki plains by flushing people, houses, trees, etc. The levees with 6 to 7 m height had been constructed along 65 km-long sandy beach to protect the hinterland from the storm surges and wind waves. Eighty percent of the levees were broken by the tsunami in various degrees ranging from scatter of the surface blocks to the complete disappearance. The government decided to rebuild the levees with the target of the local major hazards as the storm surges but in a durable way even for the tsunami overtopping. Based on this background, this study aims to find the breaking process and mechanism of the coastal levees especially for the destructive cases. We conducted field survey, and collected aerial photos, tsunami records and videos, blue prints of the levees, etc. We integrated information of the tsunami records and videos, and matched the spatial information of the various data by GIS. These analyses enabled well understanding on the breaking process and mechanisms composed from two steps: (1) The impact of the leading bore with the maximum amplitude of 6 to 7m broke weaker parts like structural joints, parapets, landward soil slopes. (2) Return flow concentration to the broken parts in the first step or to the channels and swamps expanded erosion to make tsunami channels and bays and to lead levee breach. [ABSTRACT FROM AUTHOR]