Classification of coastal landforms and analysis of temporal-spatial changes of the coastline of Qeshm Island with the approach of integration coastal zone construction management

Qeshm Island is located on the southern coast of Iran in the Persian Gulf and the Strait of Hormuz, whose shores, like other coastal environments, are affected by processes and morphological changes due to sea hydrodynamics and geohydrology of the island's land environment. Mangrove forests on the north coast of Qeshm Island are a gentle environment for sedimentation of Qeshm Island runoff and sedimentation of rivers that enter this environment from the mainland. Also, along the coastline of the Strait of Hormuz and the Persian Gulf, there is the highest tidal range. Under these conditions, monitoring and digitization of temporal-spatial changes of Qeshm Island coastline can be important in sustainable coastal development and coastal strip integrity management, and by detecting and predicting it, a comprehensive plan for changes can be designed depicting developed morphodynamics, and sea retreat and forward patterns. Therefore, the present study, focusing on the coastal environment development approach, tries to classify coastal landforms and monitor the spatial changes of the coastline of Qeshm Island, which is analyzed through the Shepard classification method and the coastline system analysis tool (DSAS).

In this study, in order to classify landforms and analyze the shoreline changes of Qeshm Island based on integrated shoreline management, first by using library resources, analytical tools and field visits and by applying Shepard method, the coastal areas of Qeshm Island were classified into different zones. The data and tools used included topographic maps, geology and land use of the island, digital data and elevation model, and LANDSAT satellite imagery of the ETM + and OLI sensor series. For spatial analysis and drawing maps, ArcGIS software was used. After increasing the recovery and contrast power of water and land in ENVI software, the satellite images were transferred to Arc map software and from the images, Qeshm Island coastline polyline related to each image were drawn and the shoreline was prepared as the required layer. In this way, shorelines were prepared to monitor spatio-temporal changes with the DSAS tool. In order to statistically analyze and quantify the trend of backward and forward (moving) shorelines, statistical methods including endpoint (EPR), linear regression (LRR), weighted linear regression (WLR) and final confidence interval (NSM) embedded in DSAS tools were used in this study.

By studying the geological and topographic maps (Landsat 8 (OLI)) of the study area which was used to classify the coastline of Qeshm Island, five different morphological units including slopes, marine sediments, residential areas (man-made), terraces and the high hills in the area were identified and classified. The linear regression index (LRR) was obtained by fitting the least squares of the regression line to all points at a 95% confidence level in a particular transect, in which a positive value indicates coastal sedimentation and a negative value indicates coastal erosion. In the morphological units of the primary coasts of Qeshm Island, the highest advancement of the coastline has been in the high hills, which often ends in the hills of Basaeido port. It has flowed towards the shore and has caused sedimentation in the coastline, and its average has increased by about 11.3 meters per year during 31 years. Of course, in the coastline of Namakdan Cave Mountain, due to the lack of runoff and also the direction of the prevailing south winds, the waves have caused the coast to regress and erode; this area is located in the morphological unit of high hills, which is at least -6.3 meters. Therefore, among the morphological units of the high coastal hills, in Laseido, the LRR index is positive and indicates the advancement of the coast and sedimentation, but in the salt cave, the LRR index is negative and indicates the regression and erosion of the coast. The lowest LRR index among the primary beaches is in the morphological unit of coastal terraces (5.4 m per year) with a maximum sedimentation rate of 34 m/year in Goran in the west of the mangrove forests and a minimum of 27 meters per year in southern parts of the the island as well as the port of Laft. On Qeshm Island, the shores of mangrove forests, Dolab port, Dastko, Direstan and eastern man-made shores are considered as parts of the primary beaches. Among these morphological units, the highest rate of shoreline movement per year is LRR and EPR occurring on the shoreline of the mangrove forests, which are positive indicators and show that due to the massive sedimentation in this forest, the coast is progressing and on average annually between 21.7 up to 23.1 meters on the beach is added. The maximum of this positive displacement is about 76.6 meters per year and the minimum is 3.3 meters per year.

The surface runoff of Qeshm Island, which originates from the hills and terraces and reaches the shore, loses its initial energy by decreasing the slope and, in contact with the seawater, causes the deposition of its sedimentary load. Accumulation of these sediments has caused the shore (land) to advance towards the sea. In the mangrove forests of the north of Qeshm Island, the northeastern runoff of the island as well as marine sediments from the rivers of the mainland of Iran, especially the sediments of the Mehran River, have caused the shoreline to advance. Also, in Dolab, Basaido and Direstan ports, sediments from runoffs have caused the coastline of Qeshm Island to advance towards the sea. But on the southern coast of Qeshm Island, due to waves and winds from the west and southwest, we see regressive erosion of the coastline and the coastline recedes to the mainland by at least 27 meters annually. The results of the present study can be used in Qeshm Island coastal strip integrity management plans and experts must be notified that strong waves cause erosion of the south coast of Qeshm Island, but sedimentation of the resulting runoffs can advance the coastline in the north of Qeshm Island.