تحليل عوامل مؤثر بر هيدروديناميك خط ساحلي طي بازه زماني 1955-2016 (مطالعۀ موردي: قاعده دلتاي اروندرود)

Factors Affecting Hydrodynamics of Arvandrud Delta Coastline during 1955-2016

پهنه مورد مطالعه در اين پژوهش بخش ايراني قاعده دلتاي اروندرود واقع در شمال غربي خليج فارس به طول تقريبي Km 23 است. هدف از اين مطالعه بررسي عوامل مؤثر بر هيدروديناميك ساحلي طي شصت سال گذشته به روش DSAS است. از اين رو، داده هاي دبي و رسوب رودخانه اروند، باد، موج، جريان دريايي، و نوسانات جزرومد همراه عكس هوايي، نقشه توپوگرافي، و تصوير ماهواره اي چندزمانه ابزار و داده هاي اصلي اين پژوهش را تشكيل داده اند. با استفاده از تكنيك سيستم تحليل ديجيتالي نوار ساحلي (DSAS)، ميزان جابه جايي هاي خط ساحلي (1955-2016) در نرم افزار Arc GIS استخراج شده است. نتايج نشان مي دهد بيشترين ميزان جابه جايي به صورت پيشروي خط ساحلي به ويژه در دهانه رودخانه اروندرود بين سال هاي 1955 - 1973 رخ داده است. بررسي روند تغييرات دبي آب و رسوب ايستگاه هاي هيدرومتري حاكي از ارتباط مستقيم ميزان پيشروي و پسروي هاي صورت گرفته با افزايش و كاهش نرخ تراكم رسوب وارده به مصب اروند بين سال هاي 1968 - 2011 بوده است. همچنين، مشخص شد كه عامل اصلي تغيير شكل تحدب مورفولوژي خط ساحلي به سمت غرب در مقياس محلي نقل و انتقال جريان رسوب توسط جريان هاي دريايي و در مقياس منطقه اي از جريان هاي متأثر از نيروي كوريوليس تأثير پذيرفته اند.

A shoreline is defined as the line of contact between land and a body of water. The most effective processes on the changes of coastal morphology are wind, wave, marine current, tides and human activity. Prodelta of Arvandrud delta, as the largest delta of the Persian Gulf, has very low slope in foreshore. Historical data of the last 60 years show that the shoreline experienced transgression and degradation several times. The aim of this study is to assess shoreline displacement rate and the factors affecting that. Materials and methods In order to gain an understanding of the effective factors on shoreline changes, we have analyzed flow rate and sedimentary changes of Ahvaz (Karun) hydrometric stations statistics, area tidal data, wave rose, wind rose and current rose statistics. To determine the spatial and temporal changes in the shoreline, we have also used Cartosat (2011) and Landsat (1973, 1982, 1994, 2002, and 2016) satellite imagery and aerial photographs (2016). To calculate the rate of the shoreline changes, DSAS (Digital Shoreline Analysis System) method was used. In this study, upper limit of tidal zone (Diurnal) was selected as shoreline for all time periods. Results and discussion Shoreline position in periodic changes in the years 1955-2016 show that shoreline has had progradation in most areas. Increases in 3.98 km during the last 60 years indicate this sedimentation rate in shoreline. Shoreline progradation, has been notable at the mouth of the Arvandrud. Recent progress is related to the period 1955 - 1973. Relatively stable parts are mostly in the east part of the area. These areas have been the old mouths of Karun River, which has been now converted into estuary. The speed of dominant wind is 3.5 m/s and frequency is 62%. Since the direction of dominant winds of the region are northwest, wind factor can’t play an effective role on the shoreline morphology changes of this region. The directions of dominant waves also are from the NW. About 72% of the wave’s heights are at 0.02- 0.99 meters. Only affective waves in this area are those with southwest and south east (diagonal) directions. These waves, according to the frequency and lower height, have the longest distance from the shore. The directions of marine currents are from NW-SE in this area. This current carries water and sediment of Arvandrud and Bahmanshir rivers from east to west. More than 51% of the marine flows have the velocity about 0 - 0.05 cm/s that can affect shoreline morphology. The results can also show the level of the tide in this area of the Persian Gulf (except Khore Musa) at its maximum. The maximum and minimum heights of sea water are 322 cm and 32 cm progress in land (backshore), respectively. To understand the reasons of the shoreline progress, during dam construction upstream Karun River in Ahvaz, especially from 1975 until now, trend of water and sediment discharges into the mouth of Arvandrud has been gradually reduced. In the years of 1973-1955, Karun River has experienced one of the highest peaks of water and sediment flow to the Arvandrud. This has been coincident with the most shoreline progradation. An increase in the flow rate and deposition in this period correspond to the time with less number of built dams on the Karun River in comparison with the next periods. Conclusion Dynamics of water and sediment flow transport of Arvandrud to the shoreline has been introduced as the main factor of propagation in this part of Persian Gulf shoreline. Delta-making process had been the largest in the area between the years 1955 to 1973. This progradation has been associated with one of the highest peaks of water and sediment flow to Arvandrud. Marine currents (local scale) with direction of east to west and Coriolis factor with regional scale have played a major role in shaping shoreline convexity and development of spits to the west. The results of this study have been conformed more or less to the findings of Yamani et al (2013) for the period 1977-2005 and Ranjbar and Iranmanesh (2011) for the period 1955-1992.