بررسي ميزان آسيب‎پذيري تپه‎هاي ماسه‎اي ساحلي در شبه‎جزيره ميانكاله با مدل DVI

Vulnerability Assessment of Coastal Sand Dunes in Miankaleh Peninsula by DVI Model

اين پژوهش به بررسي ميزان آسيب‎پذيري تپه‎هاي ماسه‎اي ساحلي در منطقه حفاظت‎شده شبه‎جزيره ميانكاله، واقع در بخش شرقي استان مازندران پرداخته است. قسمت عمده اين منطقه را تپه‎هاي ماسه‎اي ساحلي تشكيل مي‎دهد. اين تپه‎ها در كل جهان، تحت تاثير فشارهاي گوناگون طبيعي و انساني سيستم‎هاي بسيار حساسي هستند كه حفظ آنها در نقش اكوسيستم‎هاي مهم، نياز به مديريت صحيح دارد. اين پژوهش بر آن است كه ميزان آسيب‎پذيري سيستم تپه‎هاي ماسه‎اي را كه زيستگاه گونه‎هاي گياهي و جانوريِ اين منطقه هستند، با استفاده از مدل DVI كه براي نخستين‎بار در ايران انجام مي‎شود، بررسي كند. براي انجام اين بررسي از داده‎هاي آماري، داده‎هاي مكاني و بازديدهاي ميداني استفاده شده است. ابتدا منطقه به چهار قطعه تقسيم شده و اين قطعات با پنج گروه از متغيرهاي تاثيرگذار در اين سيستم، مورد ارزيابي قرار گرفتند. مولفه‎هاي پنج‎گانه شامل شرايط ژيومورفولوژي تپه‎هاي ماسه‎اي، عوامل دريايي، عوامل جوي، شرايط پوشش گياهي و تاثير فعاليت‎هاي انساني، اندازه‎گيري و مورد ارزيابي قرار گرفته است. نتايج آسيب‎پذيري براي قطعه‎هاي اول، دوم، سوم و چهارم به‎ترتيب برابر 42، 37، 43 و 49 درصد به‎دست آمد. از آنجايي كه ضريب كمتر از 50 درصد آسيب‎پذيري پايين است، مي‎توان نتيجه گرفت كه اين منطقه براساس استاندارد‎هاي شاخص، دركل درصد آسيب‎پذيري پاييني دارد.

Introduction There are many theories about coastal sand dune management (Williams et al, 2001). Some try to stabilize these coastal dunes (Davies, 1995; Mitasova, 2005; Levin and Ben-Dor, 2004) and others accept a dynamic nature for them (Davies, 1995; Anthony, 2012). Therefore, for management of coastal dunes, it is essential to recognize the existing patterns and parameters on such areas. Many scientists believe that the coastal sand dunes have a vulnerable system (Davies et al, 1995). The vulnerability is a state of accelerated erosion, and ecosystem decay (Williams et al, 2001; Davies et al, 1995). An effective management is the best logical evaluation of the environment in order to discern the the main driving forces in these systems (Stoll-Kleenamm, 2010). Areas with well management policies have low vulnerability and in the areas with poor policies the vulnerability is high (Williams et al, 2001; Ghahroudi Tali, 2012). This is the first study in Iran to assess the vulnerability of coastal sand dunes using DVI index as a purpose. Study area Miankaleh peninsula is located in the southeast of Caspian Sea in Mazandaran, Iran. The wild life sanctuary with 13084 hectare is under the supervision of Iran Environmental Organization as a protected area. Methodology Coastal sand dunes are very complicated systems that response quickly to the driving forces (Geoarcia Mora M.R, 2001). The changes in the system are formed by an interaction between the natural and human factors (Davies P. et al, 1995). The vulnerability of the dunes is a blend of main pressures including geomorphology condition of dune (GCD), marine influence (MI), aeolian influence (AI), vegetation condition (VC) and human effect (HE). These pressures are considered as five group of variables each with corresponding parameters that can be modified in number to be tailored to a particular area (Geoarcia Mora M.R, 2001). The study area based on the physical and morphologic characteristics first is divided into some segments. Each parameter is assigned a value based on its state. The sum of values in column divided by the product of the numbers of parameters and number 4 as the maximum value they can get, the value for each variable can be calculated. The mean percentage of the five groups of variables gives the vulnerability index for that certain segment. 1. Geomorphologic Condition of Dune (GCD) The resilience of the dunes depends greatly on the typology and safety of the dunes. The satellite images and maps were used to measure the parameters in this variable. The parameters in this group includes (1) the length of active sand dune (Km), (2) the width of dynamic sand dune (Km) (3) average height of active sand dunes (m) (4) the slope of sand dune (degree) (5) size of particles (mm). 2. Marine Influence (MI) The erosion processes from sea and coastal waves can affect dunes. The parameters of this group are (1) the slope of beach (degree) (2) the width of internal zone of dune, in dispersion of waves (Km) (3) orientation waves relative to the coast (degree). 3. Aeolian Influence (AI) The wind plays a major role in developing the fronts of dunes and can form depositional and erosional morphologies in dunes. The Parameters of this group that mainly measured from images are (1) percentage of particles in surface (2) percentage of vegetation cover in seaward of dune (3) percentage of system without vegetation. 4. Vegetation Condition (VC) The vegetation plays a main role in development of sand dunes and can affect the strength of winds and some other factors. There can be three types of vegetation as Type I, mainly ephemerals, Type II, perennials, and Type III, resistant plants. The parameters in this group are (1) percentage of Type III in beach, (2) percentage of Type III in seaward front of dune (3) relative percentage of Type II in seaward front of dune (4) percentage of Type I in seaward front of dune (5) relative percentage of Type II and III in 100 meters of frontal dune inside (6) relative percentage resistant plants in seaward front of dune. 5. Human Activity (HA) Many human activities influence dune processes and shapes. Eight parameters of this group are (1) tourism pressure (2) accessibility difficulty (3) density percentage of paths in front of dune (4) percentage of agricultural cover on dune (5) the rate of grazing (6) number and density of digging animals (7) percentage of cultivated forests on 200 meter of dune front inside. Results and Discussion The first segment with 5 Km length is flat in morphology and DVI value is 43% and parameters are totally moderate in this segment. The second segment with 15 Km length in the west of first segment has a DVI value of 37% and least in vulnerability among the four segments. The third segment with 16 Km length has vulnerability value of 43% in dunes. The fourth segment is 16.5 Km in length has a DVI value of 49% and the greatest vulnerability value among the four segments and also in the area. All the segments have DVI values less than 50% and according to Garcia Morra all the coastal sand dunes in the area are not vulnerable. Conclusion The research with this method is the first in Iran and it can be seen that it is useful to evaluate vulnerability of coastal sand dunes in coastal regions like northern Iran. It is recommended to reduce the influence of human activities in the segments with more DVI values and also develop vegetation in these segments.