Process-mechanism information contained in typhoon deposits, Hainan Island, South China

Shu Gao

State Key Laboratory for Estuarine and Coastal Research, East China Normal University,

3663 North Zhongshan Road, Shanghai 200062

E-mail：sgao@sklec.ecnu.edu.cn

Good afternoon, ladies and gentlemen. The theme of my presentation is about coastal sedimentary records. Information on material composition and age of sedimentary deposits can be obtained by analyzing cores and surficial sediment samples, which reveals the evolutionary history of the sedimentary system. Furthermore, the sedimentary record can be also analyzed to understand the processes and mechanisms that are responsible for the formation of the record itself. To illustrate this point, I’ll use some examples of typhoon deposits from Hainan Island, southern China.

        Deposits by storm events have been studied for a long period of time, particularly those associated with the western Atlantic hurricanes and western Pacific typhoons. They are analyzed against the background facies so that the layers produced during storms can be identified. Furthermore, sedimentary record has been used to extend the storm intensity-frequency curve beyond the instrument observation period, for coastal defense purposes.

Typhoons have enormous influences on the southern China coasts. In Hainan Island, there are 6-8 typhoon vents every year, on average. These storms are either from the western Pacific Ocean, or within the South China Sea. Hainan is a pear-shaped, large island, with an area of around 33 thousand km². The typhoon action is highly different on the eastern and western sides, in terms of wind speed, storm surge level and wave height. Geomorphologically, the coastlines of eastern Hainan Island are characterized by sandy beaches, coastal aeolian dunes, coastal lagoons, and coral reefs. As such, the deposits are often modified by the typhoon events.

        There are three major types of typhoon deposits, i.e., storm layers in dune deposits, coarse grained layers in coastal lagoon muds, and storm boulders. Let’s first of all have a look at the dune deposits at LA and JL sites on the southeastern Hainan coast.

The coastal dunes, up to 30 m in height, are formed by wind-induced transport and accumulation of beach sediments. At both LA and JL sites, transverse sections perpendicular to the shoreline reveal storm layers, which are characterized by coarser grained sediments. The elevation of the storm layers reaches more than 12 m above sea level, within the sediment column. Grain size data and OSL dating enable us to establish a chronology of the storm events, which are compared well with the written historical matewrials.

Using the energy transfer principle, the information on the storm wave conditions can be obtained. For instance, for a storm layer located at an elevation of 12 m, the initial up-wash speed due to storm wave breaking will be 15 m/s in minimum. Calculations of the up-wash and deposition patterns using typhoon data sets are consistent with the observed storm layers. Thus, the correlation between the storm layer and the typhoon record can be determined.

        In the micro-tidal coastal lagoons of this region, the seabed sediment is dominated with fine-grained materials under normal weather conditions, according to ROMS predictions. In conjunction with modeling work, we carried out in situ observation of tide range, tidal current and suspended sediment concentration, and collected more than 10 short cores and 3 long cores covering the entire Holocene period. In the two lagoons we studied, muds are the basic facies. This feature is important, because any storm deposits can be easily identified against the background facies.

        We carried out grain size and geochemical analyses in laboratory, in order to identify the modern storm events in the sedimentary record. Sandy layers with shell debris with varied thicknesses are found in the cores, which have resulted from storm modifications to the original mud deposits. Pb-210 dating indicates that these events match well with the regional meteorological records of typhoons. In the areas of water depths 5-10 m, strong typhoons may generate storm layers of 1 cm in thickness.

        These observations agree well with the results of sediment dynamic analysis based on in situ measurements and numerical calculations of current velocities, suspended sediment concentrations, and transport/accumulation rates. Under calm weather conditions, the tidal current rarely exceed 0.1 m/s, with weak resuspension. However, typhoon events interrupted the environment: the enhanced water level and current velocity, together with relatively big waves during the storm period, have caused seabed erosion and re-deposition. Generally, a coarse-grained layer will be formed, followed by a thin layer of fine-grained layer after the storm event.

        At the moment we are carrying out further modelling investigations, in an attempt to predict the thickness of storm layers in different water depths, which can be compared with the sedimentary record. The final goal is to establish a regional history of typhoon action of 10³ yr timescale on the basis of core analysis and numerical modelling. It should be noted that the thickness of storm layers is a function of wind speed, storm surge water level and water depth. Hence, the storm intensity can be simulated, with the model being calibrated against the storm layers. In this way, long-term (i.e. Holocene) variations in the storm activity can be identified using long sediment cores.

        For the long sediment cores, the information on the process and mechanism has not only a potential of deriving a storm intensity – return period relationship with a time scale of 10³ years, but it may also reveal the storm patterns influenced regime shifts in the climate system during the Holocene times.

        Storm boulders are widely distributed on the Hainan coasts. Here, we report the coral boulders at three locations in the Xiaodonghai and Dadonghai bays. At the Xiaodonghai site, more than 1200 boulders with a mass of exceeding 0.3 t are present, near the coral reef edge. The spatial distribution patterns of the storm boulders are recorded by aerial imagery and in situ measurements. The largest of these boulders has a weight of more than 35 t. However, over the inner part of the reef flat, only smaller boulders (< 0.3 t) are found. Similar distribution patterns occur at the west and east Dadonghai sites.

Strong currents are required to cause the upward movement of the boulders. For instance, to lift a boulder of 3 m in diameter, the current speed is at least 11 m/s. Using empirical equations the threshold for each of the boulders have been calculated, which indicates that only during storms can the upward transport occur on the reef front.

For an explanation of the timing of boulder accumulation and spatial distribution pattern, computer simulation methods are useful. The timing of the large boulder deposits can be estimated if the reef evolution history and the position of the boulder on the reef (in terms of the distances to the reef apex and edge) are known. Likewise, based on an analysis of dissolution and bio-erosion processes after the boulder was exposed in air, its size as a function of time can be determined. This may explain why there are only relatively small boulders on the inner reef flat.

To summarize, firstly, there are three types of typhoon deposits on Hainan Island coasts. Secondly, each of the types contains information on typhoon processes, together mechanisms that are responsible for the formation of the sedimentary records. Thirdly, typhoon intensity-frequency relationships, in relation to time-scale and climate change, can be established.

        I wish to thank the Natural Science foundation of China for financial support to this study, and the team members of the research group for their effort made in field measurements and sampling, laboratory analyses and numbering simulations. Thank you for your attention.