هيدروديناميك دريا و تاثير آن در تشكيل توده‌هاي ماسه‌اي ساحلي مطالعه موردي: سواحل غربي مكران

Affect of Sea Hydrodynamic on Coastal Sand Masses Formation Case study: Western Makran Coastal Palin

ساحل محل تلاقي پهنه‌هاي آبي و خشكي و ديناميك هوا بوده و از ديدگاه ژيومورفولوژي به عنوان يك منطقه بسيار مورفوديناميك قلمداد مي‌گردد. توده‌هاي ماسه‌اي بر اثر تاثير عوامل مختلف در مناطق ساحلي ايجاد مي‌شوند و بررسي چگونگي تشكيل و تكوين آن‌ها در برنامه‌ريزي هاو مديريت محيطي منطقه ساحلي داراي اهميت فراوان است. هدف اصلي اين پژوهش شناسايي عملكرد ديناميك دريا بر مكان‌گزيني توده‌هاي ماسه‌اي ساحلي در بخش غربي جلگه مكران است. توزيع فضايي لندفرم‌هاي ساحلي، سرعت و جهت باد، طول بادگير سواحل منطقه، شاخص‌هاي گرانولومتري رسوب و اطلاعات موج و جريان‌هاي دريايي داده‌هاي اين تحقيق مي‌باشند. نقشه‌هاي توپوگرافي، نقشه‌هاي زمين‌شناسي، عكس‌هاي هوايي، تصاوير ماهواره‌اي، GPS، بينوكلر و نرم‌افزارهاي رايانه‌اي از جمله Gradistat , WRPlot View و Arc GIS، ابزار تحقيق بوده‌اند. پس از تهيه نقشه محدوده پراكنش توده‌هاي ماسه‌اي ساحلي، با استفاده از معادله موليتور و نرم‌افزار WRPlot View، گل‌بادها و گل‌موج‌هاي ماهانه و سالانه ساحلي منطقه ترسيم شد. تغييرات ماهانه دبي آب و رسوب رودخانه‌هاي اصلي منطقه نيز به كمك گردآوري نتايج ساير مطالعات و آمار ايستگاه‌هاي هيدرومتري بررسي شد. براي مطالعات رسوب‌سنجي، چهار نمونه رسوب از محدوده‌‌هاي جزر و مدّي، به موازات هر يك از توده‌هاي ماسه‌اي ساحلي برداشت و پس از تفكيك با الك‌هاي استاندارد ASTM، به كمك نرم‌افزار Gradistat تحليل شد. بر اساس يافته‌هاي تحقيق نسيم دريا ماسه‌هاي برداشت‌شده از پيش‌كرانه جزر و مدّي را به فاصله كمي از ساحل و به موازات خط ساحلي بر روي پس‌كرانه خشكي برجاي مي‌گذارد. همچنين با توجه به انحناهاي خط ساحلي و هيدروديناميك رودخانه‌ها، امواج با رانش رسوب در جهات مختلف، بر مكان‌گزيني توده‌هاي ماسه‌اي ساحلي به شكل فعلي تاثير داشته‌اند. در ساحل جنوبي، امواج ساحلي رانش شرقي رسوب را سبب مي‌شوند. در دماغه كوه‌مبارك، رسوبات در فصل زمستان رانش جنوبي و در ساير فصل‌ها رانش غربي دارند. در سواحل سيريك و كوهستك(ساحل تنگه هرمز) جهت رانش رسوبات شمالي است.

Introduction Coastal zone is a dynamic part of the Earth where land surface influenced by both marine and atmospheric processes, from geomorphologic point of view. The coastal ocean, where land, air and sea meet, is a region of very high physical energy and biological diversity that is heavily exploited by man (Murthy et al., 2002). Dunes are an integral part of our coastal environment. They are the basis of important ecosystems, supporting valuable communities of plants and animals (Kidd, 2001). Coastal dunes develop where there is a supply of sand from the beach and where there is a frequent strong onshore wind. The form of dune systems will be dictated by a number of factors, including the shape of the coastline, shape of the beach, currents and swell of the ocean, prevailing wind, frequency of storm events, and particle size of the sand (Hesp, 2000).Coastal dunes generally form where sand on the shore has dried out and been blown to the back of the beach, to accumulate above high tide level; They differ from inland (desert) dunes in that they are subject to a wider variety of processes, including wave action and vegetation (Bird, 2008). Waves and tides deliver sediment to the intertidal zone, so that wind is able to transport it landwards from the intertidal to the supratidal area (French, 2001). Wind-generated waves are the most important energy input into the littoral zone and together with wave-generated currents, are responsible for coastal erosion and sediment transport (Davidson-Arnott, 2010). The study area is located at 25o31’-27o09’N, 56o54’-59o19’E, in western of coastal plain of Makran, south of Iran at the north of Oman Sea and east of Strait of Hormoz. In general the region under research could be assumed a dry land with very low rain and its remarkable nature is windy, sand storm, torrent shower, thunder-storm, higher humidity and hazy down(Akbarian et al., 2006). From geological aspect, this area affected by Makran region general construction and mainly composed of shale, marl and sandstone layers. The aim of this study is to identify performance of sea dynamics on coastal sand masses locations and their forming processes. Methodology Research data included spatial distributions of coastal dunes, velocities and directions of wind, fetch length, granolometric indicators of sediments, waves and currents data. Maps of geology, topography, satellite images, aerial photos, GPS, binocular microscope, and computerʹs software including ArcGIS, WRPlot View and Gradistat were used as tools also. Coastal sand masses spatial distribution map was plotted. After that by using wind data, WRPlot View software and Molitor equations, wind and wave characteristics were studied, the wind and wave roses were plotted also. Main riversʹ monthly changes in water and sediment flow were investigated with the help of hydrometric stations data and gathering the results of other studies. For sediment measurement studies, 4 samples were taken from the tidal zone parallel to the sand masses. Samples were analyzed using ASTM standard sieves and Gradistat software. Results and discussion According to the result: 1. Apart from interior dunes, sand Masses were formed only on spatial part of backshores. They are stuck to the tidal ranges parallel to the beach. In north of Sirik County, the height of sand masses is greater than the height of neighbor palm groves. 2. Wind roses show that sea breeze is approximately 19.48 percent of winds in South and 13.29 percent of winds in West Coast. 3. In this region the main rivers are seasonal. Most floods are happening at late autumn to early spring. The riversʹ beds are dry in summer and flooding is minimal. 4. On south coast, the southwest waves are predominant in October until the end of May. In June, the percentage of southeastern waves is increased. They are predominant waves in July until the end of September. 5. On Cape of Kouhmobark (between south and north coast), prevailing waves are from west (SWW to NWW) in October until the end of June. In July to the end of September they decline rapidly so southeast waves (SE to SSE) will be dominant. 6. On west coast, almost in whole year the southwest waves are predominant, just in February and May, northwest waves are dominant. 7. According to the grain size distribution curves and mod of particles diameter, wind removal threshold for tidal foreshores sediments is about 7.21 ms-1. This means sea breeze velocity wont able to transport sediment far from tidal zone. Conclusion The present study shows that sands are removed from tidal foreshores and deposited on backshores by sea breezes. Also from forming processes aspect and according to the coastline curves and riversʹ hydrodynamics, coastal sand masses were located by coastal waves; On the south coast, sediments are drifted to the east, on Cape of Kohmobark, to the north in winter and to the west in other seasons, finally on west Strait of Hormoz coastal zone (Sirik and Kohestak), they are drifted to the north by coastal waves.