Evolutionary trend of the Gorgan Bay (southeastern Caspian Sea) during and post the last Caspian Sea level rise

Abstract

The Gorgan Bay (GB) is a semi-confined sedimentary environment at the southeastern corner of the Caspian Sea (CS). High sensitivity to eustatic sea-level variations resulted in changes in the sedimentary facies and patterns of erosion/sedimentation for the GB in the last Caspian Sea-level cycle (1977–1995). Therefore, the evolutionary trend in coastal sedimentary environments was studied to determine the pattern of erosion and sedimentation related to sea-level fluctuation cycles. The sedimentology of the bay basin was studied in 500 km2 of the study area in 2001. In addition, coastal changes over the two past and post periods of sea-level rise were studied using satellite images (Landsat 7 with a resolution of 30 m) and ArcGIS 10.1 software. Waves, wave-induced currents and sediment transport were simulated using the MIKE-21 software package. Mixed sediments, particularly at the western and southern areas of the bay basin, were detected through reworking of coarser barrier island sediments into deeper areas. Well-sorted (0.35–0.5φ) sediments, with 98.31 ± 2.8% sand-size particles, were identified as the common texture along the wave-dominated coasts. Sea progradation, destruction of the beach profile and an increase in sea coverage of approximately 15,600 ha around the GB were other impacts of the CS-level rise. Additional impacts included severe erosion and submergence of a 9000 ha area of the flat coastal plains (slope~ 0.035%). The results demonstrate that water-level rise has strengthened the sediment accumulation in the bay inlet and basin at a mean rate of 0.430 MCM and 12.7 MCM, respectively. Both field measurements and simulation results revealed the notable shoaling of the bay entrance at a mean rate of 0.2 m. Simulations concluded that annual longshore transport toward the bay inlet has occurred at a net rate of 80,000 m3 during the last CS-level rise. Natural events after the eustatic cycle in the southeastern corner of the CS resulted in severe shoaling of the GB inlet and basin and restriction in the water exchange between the bay and open sea. Therefore, comprehensive management practices to restore the GB capacity and its natural habitats are critically needed.