عنوان مقاله [English]
Coastal sand dunes are one of the important morphological landforms of coastal areas. These hills have been developed by interactions of sea waves, sea currents, winds and sediment materials on the coast. The sand dunes can protect the coast against waves and erosion. Thus, it is essential to protect these hills using management measures. The study area of this research is located between the Sirik port and the port of Ziarat, in the southeast part of Hormozgan province. Since the construction of new docks in recent years has led to changes in the coast of this region, so awareness of the vulnerability of the coastal hills can be effective in managing and protecting them. The purpose of this study is to assess vulnerability of the sand dunes by model DVI (Dune Vulnerability Index). A checklist is usually employed to evaluate the vulnerability index. In this research, effective factors in coastal sand dunes, geomorphologic conditions of sand dunes, marine factors, wind processes, vegetation, effects of human activities and management factors were evaluated using the checklist.
Materials and methods
In this study, the data from Google Earth, Landsat, and aerial photos, and questionnaires and field observations were used to complete the checklist. The steps to complete the checklist are as follows: in the first step, the value range for the quantitative variables was determined on the basis of the available structured checklists. Partial vulnerability indices are geomorphological condition (GC), marine influence (MI), aeolian influence (AI), vegetation condition (VC) and anthropogenic effects (AE). The indices were calculated as the ratio between the summations of given variable ranks within each variable class (PVi) and the total maximum possible rank within the class. A total DVI was calculated as the average of the five partial vulnerability indices (PV). The Protection Measure index (PM) was calculated separately from other factors. Lastly, the residual value as the difference between DVI and PM (DVI-PM) has been calculated to describe the stability of the dune system.
According to the studies by Davis (1995), to calculate the vulnerability index (VI), the calculation of the partial Vulnerability Index is not done and the vulnerability score from the division of the total variables obtained from all the group of variables, based on the maximum total sum of the concessions of the entire group of variables. This is obtained for each site.
Results and discussion
Partial Vulnerability (PV), for geomorphologic agent is above 0.75 in all sites. The degree of vulnerability is very severe because of characteristics of the sand dunes of the region with fine-to-good sorting on the slopes over 30 degrees.
The vulnerability to wind processes in all areas is above 0.5 and this shows high sensitivity of this factor. One of the reasons for vulnerability of the dunes to the wind processes is the small average diameter of coastal sediments in this area. The fine sands here are exposed to retrogressive waves and are more sensitive to erosion, so beaches with fine sand grains have less resistance to erosion.
The low vegetation of the dunes in this part of the sea, in the distance between the hills and relatively high percentage of uncovered areas are other factors of high sensitivity of dunes to wind processes.
The degree of vulnerability to vegetation condition is very severe in sites 4, 5 and 6, and is low in enclosures 1, 2 and 3. The vegetation condition was compared with 1966 aerial photos and satellite imagery from 2000 to 2016. This implies an increase in vegetation in 1966 and 2000, due to planting and plant care by government office like office of natural resource in Hormozgan.
The level of vulnerability to the human factors of sites 1 and 2 is moderate and in the other sites it is negligible. With the use of aerial photographs and satellite imagery, during the years mentioned above, the roads in this area have been increased and in some areas the roads cross the dunes. During these years, some parts of the dunes have been cleared for construction. In addition, the construction of three new wharfs in 3 Sirik (2007), Ziarat (2012) and Taheroi port (2013) has also been associated with shore changes.
According to the field observations and the results of the questionnaires (interviews with local people and experts), it can be asserted that no management action such as specifying buffer areas for access restrictions has been taken to protect the beaches.
The only protective measure is the planting of seedlings in sites 1, 2 and 3 through natural resources organization.
The overall values of DVI in all areas were moderate. Among the investigated factors, the morphological condition of sand dunes and the wind processes have the highest influence on the vulnerability of the dunes. The results for the DVI-PM index indicate that all sites need quick management.
Based on the obtained equilibrium index, there is no equilibrium between vulnerability and dune management in any of the sites. This indicated the lack of management or inadequacy. One of the reasons for lack of management or efficent management is that there is no a certain organization responsible for these kinds of protection measures.
Comparison of the two methods for calculation of vulnerability, DiPanjun (2014) and the Davis (1995) method), shows that the severity of the vulnerability of the sites is moderate in both methods. There is no difference between the qualitive results obtained in the two methods.