تحلیل پیامدهای اکوژئومورفولوژیک احداث سد بر پایاب رودخانه‌های ساحلی مطالعه‌ی موردی: جلگه‌های سدیچ، گابریک و جگین

تغییر دینامیک محیط در نتیجه احداث سد بر روی رودخانه‌های ساحلی، تاثیر زیادی بر فرایندهای ژئومورفولوژیک جلگه‌ها به‌جا می‌گذارد. هدف این پژوهش پیش‌بینی تغییرات ژئومورفولوژیکی و پیامدهای اکولوژیکی احداث سد بر روی رودخانه‌های سدیچ، گابریک و جگین در جلگه ی غربی مکران و شرق شهرستان جاسک با مطالعه ی چگونگی عملکرد این رودخانه‌ها بر ویژگی‌های ژئومورفولوژک و اکولوژیک کنونی این جلگه است. داده‌های تحقیق شامل توزیع فضائی لندفرم‌های ژئومورفولوژی، داده‌های هیدرودینامیک رودخانه‌ها شامل دبی آب و رسوب سالانه آن‌ها، داده‌های مورفومتری سطحی و توپوگرافیک شامل شیب و نوع و تراکم پوشش گیاهی است. نقشه‌های توپوگرافی، نقشه‌های زمین‌شناسی، عکس‌های هوائی، تصاویر ماهواره‌ای، سیستم موققیت‌یاب جهانی و نرم‌افزارهای رایانه‌ای نظیر فری‌هند و آرک‌جی‌آی‌اِس ابزار تحقیق هستند. با استفاده از آمار ایستگاه‌های هیدرومتری، مقایسه زمانی عکس‌های هوائی در پایه‌های زمانی مختلف و با رجوع به مطالعات موجود، تغییرات مورفودینامیکی رودخانه‌های اصلی مطالعه شد. با مطالعه ی شرایط توپوگرافی جلگه، وضعیت زمین‌شناسی و ژئومورفولوژی و پوشش گیاهی، نقشه‌های ژئومورفولوژی و پوشش گیاهی منطقه تهیه شد. سپس بر مبنای تحلیل‌های ژئومورفولوژیک و اطلاعات کسب شده از محیط و بازدیدهای میدانی، عملکرد هیدرودینامیک رودخانه‌ها بر اکولوژی و ژئومورفولوژی جلگه و تغییرات اکوژئومورفولوژیکی احتمالی در صورت احداث سد بر رودخانه‌های جگین، گابریک و سدیچ، مورد مطالعه قرار گرفت. نتایج نشان ‌دهنده ی عملکرد ویژه ی رودخانه‌ها بر شکل‌دهی لندفرم‌ها، ایجاد پوشش‌های جنگلی تنک و نیز منابع تامین رسوب توده‌های ماسه‌ای سطح جلگه و توده‌های ماسه‌ای ساحلی است. به نظر می‌رسد احداث سد بر روی این رودخانه‌ها، تاثیرات اکوژئومورفولوژیکی فراوانی بر پوشش گیاهی جنگلی سطح جلگه، پوشش جنگل‌های حرا در خورها و تالاب‌های ساحلی و لندفرم‌های بادی سطح جلگه داشته باشد.

Consequences of Dam Construction on the Ecogeomorphology of Coastal PlainsCase Study: Coastal Plains of Sedijch, Gabric and Jagin

Extended AbstractIntroductionEach part of a shore should be thouroughly considered with its hydrodynamic, lithodynamic, geological, geomorphological, and other peculiarities. In modern conditions, anthropogenic factors should also be included. Only such a multilateral approach can produce results which reflect the actual situation as closely as possible. Arid lands and coastal zones are the most favourable areas for wind processes. The analysis of the forms and coastal conditions provides a valuable key for coastal management. A beach can be composed of a wide variety of materials of many sizes and shapes. The river load is one of the hydrological elements that affects the beach. Particle size windows (PSWs) are interpreted as reflecting different modes of sediment transport and deposition. In a related research on western Makoran coastal plain, Shayan et.al (2014) found that the distribution of the internal sand masses on the plain was affected by the extent of the old beds and floodplain that were exposed to the prevailing wind rather than wind function. The aim of this study was to identify the effect of river’s flows on sedimentological characteristics of the shoreline in the western part of Mokran coastal plain, Iran. The major process that formed coastal plains were fluvial processes. In semi arid areas, the geomorphological impacts of dams are very major. The purpose here was to identify the consequences of dam construction on vegetation and aeolian geomorphology with an emphasis on the ecogeomorphology of the western part of  Makran coastal plain.  The study area is located at 25o31’25o50’N, 57o52’59o02’E, in the western part of the coastal plain of Makoran, the southern part of Iran, the northern part of the Oman Sea, and the eastern part of Strait of Hormoz. In general, the region under research could be assumed a dry land with very low rain, windy, sand storm, torrent shower, thunderstorm, higher humidity, and hazy down (Akbarian et al., 2006). Geologically, it is affected by Mokran (Makoran) region's general construction and it is mainly composed of shaile, marl, and sandstone layers.MethodologyResearch data included spatial distributions of coastal lanforms, rivers' hydrodinamic data, data of surface morphometric specialy slope, and type of vegetation. Maps of geology and topography, satellite imagery, aerial photos, GPS and computer softwares such as Freehand and ArcGIS were used. Morphodynamic changes of the rivers were investigated by using hydrometric station's data, comparing the satelite images and areal photos, and gathering the results of other studies. The Coastal plain's topographical and geological charecteristics, the distribution map of landfoms, the vegetatin cover, and coastal dunes were also studied. Then, based on the geomorphological analysis and information from field works, the possible ecogeomorphological consequences of dam construction were studied.  DiscussionThe exploitation of coastal rivers have had an enormously negative impact on coastal zone and a deleterious effect on the natural dynamics of river loads. The basins leading to the Coastal Plains of Sedijch, Gabric, and Jagin have high sedimentation discharge. According to the results, the hydrodynamic effects of  these  rivers have a special effect on the formation of plain landforms. The sources of the sedimentation of sand masses and the establishment of forest cover on adjacent coastal plain are affected by river’s hydrodynamic. Once dams constructions on these rivers are complete, sediment load virtually comes to an end. In the near future, major changes can be expected at the mouth of the rivers and along the coastal zones of this region. However, dams could cause serious environmental damage in the downstream of these rivers. They may increase the severity of wind erosion, the performance of waves in sediment drift along the coastline. They will also have negative effect on the amount of forest cover on upland of these coastal plains and especially mangrove wetlands. These results are consistent with the following researchers. According to Nanson and Croke (1992), as a result of environmental modifications on rivers, flood plains affected by major rivers evolve over time. In a study of hydrology, geomorphology and vegetation in the southeastern coastal rivers of the United States, Hop (2000) concluded that the vegetation diversity of the coastal plain has adapted to long flood periods and anaerobic conditions. Based on Berkun (2014), the natural course of fluvial alluvium transport is almost completely altered because of planned and constructed dams and coastal protection measures. Decreasing volume of sediment carried to the sea cause intensification of the shore erosion (Berkun, 2012). Sulimanirad et al. (2011) stated that the habitat of the protected area of Gabrij (the whole range of Gabriel coastal wetlands, Jengin, and Sourgelm) is threatened and possibly destroyed due to the construction of a dam on the Jegin River, camel grazing and the destruction of mangroves.ConclusionThe present study showed that water and sediment discharge of rivers and their hydrodynamic has a special role in the formation of coastal landforms, specially on aeolian landforms, and the stablishment of mangrove on lagoon and Persian GulfOman(SaharaSendi) forest habitat on upland deltas. Lake of fluvial processes in the long period will probably have destructive consequences on the ecology and geomorphology of this coastal region.