Strongly aligned coastal boulders on Ko Larn island (Thailand): a proxy for past typhoon-driven high-energy wave events in the Bay of Bangkok

© 2019 Institute of Australian Geographers Recent studies of elevated carbonate boulder deposits on several rock islands near Bangkok have indicated that Thailand\u27s capital city may not be as protected from typhoon strikes as previously thought. Here, new evidence is presented for past high-energy wave (HEW) events in the form of statistically significant patterns of boulder alignment on exposed rocky shorelines of Ko Larn island. The long-axis orientations of 193 coastal sandstone boulders were analysed across four study sites. Several scenarios for the unimodal, bimodal, and polymodal patterns found can be envisaged. Either the most recent HEW event was the strongest—in which case most clasts were rearranged unimodally (one observation site), or the strongest HEW event was earlier and subsequent weaker ones realigned only smaller boulders to produce bimodal or polymodal patterns (three observation sites). Inferred northeastward or eastward onshore flow directions are consistent with palaeo-typhoons penetrating into the Bay of Bangkok on northwestward curving tracks. The calculated minimum flow velocities required to transport all sampled boulders are 5.5–7.8 m s−1, similar to other findings throughout the Asia-Pacific region. It was observed that the absence of a fitted boulder geomorphology lends credence to the earlier proposed time frame of 150–200 years between typhoon phases in the upper Gulf of Thailand. The current work has provided additional insights into the characteristics of past HEW events that have a possibility of reoccurring again at some time in the future. Our findings continue to raise awareness for a reassessment of the risks of coastal hazards for the Chao Phraya River delta and densely populated Bangkok, for which storm surge modelling should be an urgent priority, so as to give better perceptions on how typhoon-driven marine incursion would impact the city

Urban coral reefs: Degradation and resilience of hard coral assemblages in coastal cities of East and Southeast Asia

© 2018 The Author(s) Given predicted increases in urbanization in tropical and subtropical regions, understanding the processes shaping urban coral reefs may be essential for anticipating future conservation challenges. We used a case study approach to identify unifying patterns of urban coral reefs and clarify the effects of urbanization on hard coral assemblages. Data were compiled from 11 cities throughout East and Southeast Asia, with particular focus on Singapore, Jakarta, Hong Kong, and Naha (Okinawa). Our review highlights several key characteristics of urban coral reefs, including “reef compression” (a decline in bathymetric range with increasing turbidity and decreasing water clarity over time and relative to shore), dominance by domed coral growth forms and low reef complexity, variable city-specific inshore-offshore gradients, early declines in coral cover with recent fluctuating periods of acute impacts and rapid recovery, and colonization of urban infrastructure by hard corals. We present hypotheses for urban reef community dynamics and discuss potential of ecological engineering for corals in urban areas

Impacts of Typhoon Vae and Linda on wind waves in the Upper Gulf of Thailand and East Coast

The Upper Gulf of Thailand and east coast are the locations of the 3 important deepsea ports of Thailand, namely Bangkok port, Laem Chabang port and Map Ta Phut port which needed wind wave data for designs of port structures and navigation of cargo vessels and container ships to and from the ports. In the past, most wave data in the Gulf of Thailand were computed from wind data using the methodproposed by Silvester and Vongvisessomjai (1970). Recently, Pornpinatepong et al. (1999) used the WAM Model to predict wind waves in the Gulf of Thailand and Andaman Sea using waves initially from satellites ERS-2 and TOPEX and later from 9 oceanographic buoys in the Gulf of Thailand and 2 buoys in the Andaman Sea for model calibration. This study presents wind waves in 6 years (1997-2002) from the WAM Model at Petchaburi and Sichang buoys in the Upper Gulf of Thailand as well as Rayong and Ko Chang buoys on the east coast. Since the big waves in the study area are generated by strong winds of cyclones, emphasis is placed to the wind waves generated during the passage of cyclones Vae in 1952 formerly provided by Vongvisessomjai (1994b) and new data of Linda in 1997. Cyclonic waves are second in size to the tsunami waves in the Andaman Sea on December 26, 2004

Tropical cyclone disasters in the Gulf of Thailand

The origin of tropical cyclones in the South China Sea is over a vast deep sea, southeast of the Philippines. The severetropical cyclones in summer with northerly tracks attack the Philippines, China, Korea and Japan, while the moderate ones inthe rainy season with northwesterly tracks pass Vietnam, Laos and northern Thailand. In October, November and December, the tropical cyclones are weakened and tracks shift to a lower latitude passing the Gulf of Thailand. Tropical cyclone disasters in the Gulf of Thailand due to strong winds causing storm surges and big waves or heavy rainfall over high mountains in causing floods and land slides result in moderate damages and casualties. Analyses are made of six decades of data of tropical cyclones from 1951-2006 having averaged numbers of 3 and 13 in Thailand and the South China Sea respectively. Detailed calculation of surges and wave heights of the 5 disastrous tropical cyclones in the Gulf of Thailand reveal that the Upper Gulf of Thailand with a limited fetch length of about 100 km in north/south direction and about 100 km width in the east/west direction, resulted in a limited maximum wave height of 2.3-2.5 m and maximum storm surge height of 1.2 m generated by Typhoon Vae (1952), while the east coast, with longer fetch lengthbut still limited by the existence of its shoreline, resulted in an increased maximum wave height of 4 m and maximum storm surge height of 0.6 m in the Upper Gulf of Thailand generated by Typhoon Linda (1997). These are the Probable Maximum Cyclones here.The southern shoreline, with unlimited fetch length on the east by tropical cyclones approaching from the South China Sea, generated maximum wave height of 6-11 m by Typhoon Gay (1989), resulting in more casualties and damages. Note that storm surges on the southern shorelines with steep slopes are small due to the short distance of shallow shorelines in receiving wind stresses for piling up sea levels. These disasters can be alleviated from known characteristics of tropical cyclones and through proper warning before coming to the Gulf of Thailand

Analytical model of interaction of tide and river flow

Hydrodynamic characteristics of a river resulting from interaction of tide and river flow are important since problems regarding flood, salinity intrusion, water quality and sedimentation are ubiquitous. The lower reach of the river strongly influenced by tides from the sea, when interacting with river flows, results in a complicated pattern which is simplified to its interaction with four main constituents of tides obtained from harmonic analysis. An analytical model is developed in this study for simulating the hydrodynamic processes in estuarine waters, with the emphasis being given to the interaction between tides and river flows. The perturbation method is used to derive the analytical solution, in which the estuarine flow is separated into steady and unsteady components. Thus the analytical solutions derived consist of two distinct parts; one represents the influence of river flows and the other represents the influence of tides. The application of the model to a case study, the Chao Phraya river, which requires a time series of discharges and loadings at the river mouth to model water quality in the Gulf of Thailand, shows that the model can beautifully and completely simulate the hydrodynamic features of tide and river flow interaction especially in the rainy season when the river discharge is high. Data of tidal discharges are scarce because of high cost of measurement especially in the lower reach of the river strongly influenced by tides from the sea. From this study of relation between tidal discharges and tides, the analytical model can compute tidal discharges from tides correctly. The results of tides and tidal flow can subsequently be used to calculate eddy viscosity and dispersion coefficient for describing salinity and water quality profiles

2004 Sumatra Tsunami

A catastrophic tsunami on December 26, 2004 caused devastation in the coastal region of six southern provinces of Thailand on the Andaman Sea coast. This paper summaries the characteristics of tsunami with the aim of informing and warning the public and reducing future casualties and damage.The first part is a review of the records of past catastrophic tsunamis, namely those in Chile in 1960, Alaska in 1964, and Flores, Java, Indonesia, in 1992, and the lessons drawn from these tsunamis. An analysis and the impact of the 2004 Sumatra tsunami is then presented and remedial measures recommended.Results of this study are as follows:Firstly, the 2004 Sumatra tsunami ranked fourth in terms of earthquake magnitude (9.0 M) after those in 1960 in Chile (9.5 M), 1899 in Alaska (9.2 M) and 1964 in Alaska (9.1 M) and ranked first in terms of damage and casualties. It was most destructive when breaking in shallow water nearshore.Secondly, the best alleviation measures are 1) to set up a reliable system for providing warning at the time of an earthquake in order to save lives and reduce damage and 2) to establish a hazard map and implement land-use zoning in the devastated areas, according to the following principles:- Large hotels located at an elevation of not less than 10 m above mean sea level (MSL)- Medium hotels located at an elevation of not less than 6 m above MSL- Small hotel located at elevation below 6 m MSL, but with the first floor elevated on poles to allow passage of a tsunami wave- Set-back distances from shoreline established for various developments- Provision of shelters and evacuation directionsFinally, public education is an essential part of preparedness

The interaction between tide and salinity barriers

Presently, there is a number of salinity barrier utilization and this kind of structure becomes more common in estuarine areas. However, the construction of barrier at the river mouth or inside the river results in amplification of tide due to creation of standing tide at the barrier. This standing tide creates two major problems, namely, the overspill of salinewater during high water and bank erosion during low water along the tidal reach downstream of the barrier. In this study, the analytical model is developed to determine the river hydraulic behaviors which affects by tide, river flow and barrier structure of the Bang Pakong River, Thailand. The analytical model of tide and river flow of the Chao Phraya River is adopted and adjusted to determine the tide characteristics modified by river flow. Moreover, the analytical model of tide and salinity barrier would then be developed by cooperating of the analytical model of tide and river flow interaction together with tidal flow cooscillating tide theory. It is found from this study that the analytical model of the Chao Phraya River which is suitable for high freshwater discharge underestimates damping modulus and friction slope which requires adjustment for low freshwater discharge of the Bang Pakong River. The analytical model of tide and salinity barrier can be finally used to predict the water level downstream of the barrier. The model overestimated the water level fluctuation during the unsteady flow from upstream which may be because of the assumption of steady flow condition in the model development due to limited data available after the construction

Effects on the upstream flood inundation caused from the operation of Chao Phraya Dam

During the flooding events, the operation of Chao Phraya Dam to control downstream water discharge is one of the causes of the inundation occuring over the upstream area. The purposes of this research are to study the effects of the operation of Chao Phraya Dam upon the upstream flood inundation and to find out the new measures of the flood mitigation in the upstream areas of Chao Phraya Dam by using a hydrodynamic model. The results show that Manning's n in the Chao Phraya River and its tributaries is 0.030-0.035 in the main channels and 0.050-0.070 in the flood plain areas. The backwater due to the operation of the Chao Praya dam affects as far as 110 kilometers upstream. New methods of water diversion can mitigate the flood inundation without the effect on the floating rice fields. The construction of reservoirs in the Upper Sakaekang River Basin and the Upper Yom River Basin will mitigate the flood not only in their own basins but also in the Lower Chao Phraya River Basin. The coordinated operation of the Chao Phraya Dam, the regulators and the upper basin reservoirs will efficiently mitigate the flood inundation