Investigating Process of Geomorphological Variation of Tombolo in Tang Seaport during 1972-2015

Coasts are among the most important and strategic places with special importance for countries. Having 673 km of water border in the north and 2098 km in south, Iran has a great potential for commercial, touristic, and military purposes. The beaches, as one of the most endangered places under the influence of sea waves, are constantly undergoing geomorphological changes. Tombolo is one of the effects of coastal sand compression that is affected by waves and the resulting hazards always threaten the inhabitants of the area, so it is necessary to study this effect. The purpose of this study was to monitor the time series of geomorphological changes in Tombolo of Bandar Tang on the coasts of Sistan and Baluchestan province using remote sensing and field surveys. The Tang Seaport tombolo is located on the southeastern coasts of Iran between longitude of °59 ′53 ″10 to °59 ′54 ″10 E and latitude of °25 ′20 ″29 to °25 ′21 ″01 N. This geomorphological feature with the area of 0.3 km2 is located at the south of Sistan and Baluchistan Province, Konarak County, 97 km distance from the west of Konarak at the south of Tang Seaport. therefore, it is essential to study the geomorphological shape of coasts in order to manage the reduction of such hazards.

The present study was conducted using statistical analysis, field survey, and remote sensing. In order to investigate the geomorphological changes in the tombolo of Tang Seaport, different information was used. In this regard, the Landsat satellite imagery from 1972 to 2016 was used. To analyze the regime of the atmospheric and sea currents in this region, the statistics of the stations close to the region was used, which included the daily statistics of Chabahar and Jask synoptic stations, and statistics of the PMO buoy station as well as marine buoy station at city of Chabahar. In order to analyze the geomorphological features of the tombolo in Tang Seaport, Gis & Rs was used. Satellite imagery was provided during the statistical period and underwent band composition using ENVI software. Then, it was digitized in GIS software and the morphometric changes of the tombolo and sandspit were calculated. Afterwards, using WRPlot software, the wind rose and wave rose of the region were depicted and their association with geomorphological changes was investigated.

The obtained results showed that during the studied statistical period, the tombolo’s connection to the mainland was interrupted in four cases in 1972, 1989, 2000, and 2016, the main reason of which was the surges along the wind direction that have eroded and destroyed the tombolo sand spit. The main cause of these currents was the summer monsoons of the southeastern Asia, which cause the anomalies in the Sea of Oman currents every year. Since the tombolo sand spit is located at the north of island, the changes and power of the waves in the Sea of Oman from west and east have eroded and removed the sand spit so that the western direction of the waves in 1988 and 2015 and their eastern direction in 1999 have resulted in the disconnection of this geomorphological feature. The existing evidence showed that the eastern currents in the Sea of Oman were the cause of the disconnection of the tombolo sand spit in 1972, which had acceptable correlation with the sand spit separation in 1999. According to the results of the above table, physiography of the tombolo and its sand spit as well as position of Tang Seaport and its sand spit had a close relationship with the relatively calm years in the Sea of Oman, and whenever the seasonal currents of Southeastern Asia had less energy, the erosion processes were weaker and sedimentation was increased, so that the specific geomorphological changes occurred proportionate to the direction of the waves in that year.

Studies have shown significant geomorphological changes in this phenomenon. The results of field visits, time-series analyzes (45 years) of Landsat satellite data and statistics of synoptic and buoy stations showed that in some years severe sea currents and waves caused extensive geomorphological changes, such as discontinuation of sandbags and elimination of tombolo. The harbor has been narrowed, so that during the statistical period, waves in 1972, 1989, 2000, and 2016 interrupted the island's connection with the land through sandy tongue erosion. Marked with time intervals of 17, 11 and 16 years, respectively. Therefore, the active dynamics of the waves in Tombolo, the port of Tang, have prevented the hazards of the waves from creating human settlements in the area. Due to the existing conditions and for the coastal management of Tombolo Port Tang and the use of existing potentials, the construction of artificial barriers for sedimentation of coastal sands seems necessary. It should be noted that this geomorphologic complication has never been eliminated during the period under study and has only undergone changes that have.