TITLE:
Legacy of Dam and Sediment Flushing Operation: Geomorphological Changes of Sefidrud Delta during 7 Decades, South of the Caspian Sea
AUTHORS:
Saeed Poorasadollah, Ziaoddin Shoaei, Mohsen Shariat-Jafari, Ali Sorbi
KEYWORDS:
Sefidrud Delta, Manjil Dam, Sediment Flushing Operation, Hydrological-Morphological Linkage, Sediment Load, Coastal Lagoons
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.13 No.3,
March
5,
2025
ABSTRACT: Human-induced changes profoundly affect deltaic systems. This paper examines the evolution of the Sefidrud Delta, impacted by the Manjil Dam and sediment flushing operation (SFO), through a hybrid approach that observes the hydrological and morphological linkages of the fluvial system and the delta over seven decades. All changes in the delta were detected by analyzing numerous aerial photographs and satellite images. The construction of the Manjil Dam altered the delta’s morphotype from a river-dominated bird’s foot to a wave-dominated arcuate shape. Coastal lagoons were formed by the progradation of mouth bars, influenced by wave actions. An increase of 343% in the mean annual sediment load, due to the SFO, significantly accelerated these changes and reinitiated delta development. The Boujagh point bar expanded by 1.7 km2 between 1981 and 1990, ultimately causing the Sefidrud channel to migrate by 45˚, along with a 2540-meter displacement of the river mouth. Sediment discharge increased by the flushing operation choked coastal lagoons. The sinusoidal trend in sediment load during the SFO transformed the delta into a fluvial and wave-dominated pointy cuspate shape. The sediment starvation induced by a sharp decrease in sediment load after 1998 resulted in 1.166 km2 of erosion. The effects of the dam construction and SFO on the Sefidrud deltaic system represent the Anthropocene in the southern Caspian Sea. We conclude that, the critical sediment demand to maintain delta is about 14 Mt/yr, while due to limiting the output of the Manjil Dam and construction of dam on the sefidrud main tributaries, only about 3.16 Mt/yr has been delivered since 1999. The continuation of current conditions will lead to severe erosion and the eventual disappearance of the delta. Repeating the SFO will result in further changes to the delta and cause environmental damage.