مجله علوم و مهندسی آبخیزداری ایران
پیاپی 18 (بهار و تابستان 1391)

The identification of prone landslide areas and its hazard zonation are considered as the first step in evaluating environmental hazards. There are a significant number of factors contributing to landslide occurrence so that their evaluation requires powerful analytical techniques. This study aims at selecting an appropriate model for the zonation of landslide hazard، and investigating the influence of effective factors in the process of landslides in the Akhlamad basin، Chenaran Town، Iran. Geological، geomorphological، climate and land-use specifications indicate that this area is a prone landslide area. In order to quantify the impact of the effective factors in landslide occurrence، and to select the best method in landslide zonation، LNRf and WLC conceptual models along with the GIS software were used. By reviewing air photos and looking at field surveys data، all effective factors were weighted using these models and finally weighted maps of the factors were generated. The results showed that by using the LNRf and WLC models، respectively، 50 % and 71 %of the study area are susceptible to high risk landslide. Both models indicated that the highest risk of landslide is seen in the north eastern part of the study area with slops exceeding 70%. In this study، the geological structures، geomorphologic features، fault and climate were introduced as natural landslide causes، and land-use changes and human activities as non-natural causes. The results also revealed that the fuzzy method not only increases the accuracy of landslide hazard zonation but also، due to selecting an appropriate membership function for each factor، it is of key importance. Hence، the fuzzy method can outperform the zonation of landslide hazards compared to the other available methods.

Bridge abutments are among vulnerable structures during flooding events. Local scour around abutments seems to be the main cause for structural damage. It is well known that this type of scour occurs due to the contraction of flow and subsequent vortices and eddies formation. The complexity of flow behavior in channels as well as the large number of involved parameters has limited the use of numerical models. In contrary، many laboratory investigations have been conducted and numerous techniques have been developed to reduce the size and depth of scour hole around the abutments. In the present research، breakwater was employed for reducing the scour. To do this، three lengths and five distances were tested and for each case the scour hole was measured. The results are applicable to finding the position and the length of the protecting breakwater.

Accelerated soil erosion and resultant sediment yield from catchments are key obstacles in reaching sustainable development. Hence، the evaluation of effective factors on erosion and sediment yield processes is considered as a prerequisite in proper watershed management. The erosive force of rain causes soil degradation، decreases surface permeability and then increases surface runoff، implying that rain plays the greatest role in water erosion. Different indices representing the rain erosivity factor have been proposed among which the EI30 index is considered as one the most important one with global application. However، researches have shown that I30 then is associated with efficient accuracy in all areas. Therefore، it is necessary that the most effective time basis for rain intensity is determined for any combination of physical and natural conditions in watersheds. In this regard، the present research is conducted in Chehelgazi Watershed of Gheshlagh Dam in Kurdistan Province for the purpose of determining the most effective time basis of rain intensity in storm sediment. The results of this research indicated that this index multiplying the kinetic energy in the rain intensity in the time basis of 20 min (EI20)، has the highest correlation (r=0. 810) with storm sediment yield.

Horizontal aprons are usually used at the downstream end of hydraulic structures. In most cases، because of the roughness difference at the apron and channel bed، scour occurs at the river bed downstream of the pron. As it deepens it damages the apron itself and ultimately leads to hydraulic structure failure. In order to reach the aim of this study، 24 experimental tests were carried out on four different aprons with (0، 2، 5، 10 and 14. 28 mm) heights، and two different bed material sizes (0. 8 and 1. 4 mm). The effects of different roughness heights on scour was investigated using the densimetric Froude numbers ranging from 2. 37- 3. 91. The results indicated that an increase in apron height and bed grain size significantly reduce the maximum scour depth. In this study، an experimental relationship was also developed for predicting the maximum scour depth at the downstream of an apron. The results were compared with those obtained from previous studies.

Mountain as an important factor controlling precipitation and has variable impacts depending on its direction to the wet air masses. Understanding the relationship between regional precipitation and the effect of topographic factors may improve the estimation of regional precipitation. This study aims to evaluate the role of topography on precipitation in the southeastern Khouzestan province. To do this، the regional stations with appropriate range of heights were firstly selected. Monthly and annual precipitation data were extracted at each station and homogeneity tests were performed. Thus، out-layers data were removed from data sets. Reconstructed data and topographic parameters including altitude، distance to the maximum elevation from where the wet air masses pass، latitude and longitude for each station. The single and multiple regressions of these measured factors versus annual and monthly precipitation were estimated. Error rate of each regression method was calculated using error measurement methods. To evaluate the role of altitude in wet، normal، and dry periods، an index of mean precipitation periods (dry، normal، and wet) to mean value of long periods (i. e. 20 yrs) was introduced. The results showed that the multiple regression presents less error compared with the linear regression of precipitation and elevation. However، the relationship between elevation and the annual precipitation was significant. The index of dry period and altitude is meaningful. Thus، the effects of dry and wet periods in higher altitudes are significantly less than lower altitudes

Exact recognition of effective factors in hydrological processes such as conversion of rainfall to runoff in the annual scale can improve the accuracy of estimations. Investigations on 13 sub-basins in the Ghezelozan river basin show that the effective parameters of mean annual potential yield (mean annual runoff accompanied by water utilities in basins) are adjusted precipitation، curve number، mean annual temperature، area and basin slope. Meanwhile، this paper focused on adjusted precipitation which is related to the time distribution of water utilities in the basins. Validation processes using runoff data in basins for minimizing the differences between estimated and observed discharge values have been conducted. Those processes recognized two equations according to basin area for validation. Investigations in validation of two basins show that runoff result tolerances are about 5 than that observed discharges

Soil is considered as the most important natural resources of each country. Soil erosion is a serious hazard to human life. Soil erosion not only causes destruction of soil، but also causes much detrimental effects with sediments deposited in waterways، reservoirs and dams which reduce their storage capacity. These sediments، due to their physical and chemical characteristics have the highest erodibility. In this study، the physicochemical properties of marl units on soil erosion have been determined on 31 samples in marl lithologic units based on erosion shapes and thru dominant slopes. Soil physical and chemical properties were determined by standard analytical methods. Analysis of variance and comparing of means was performed. Finally، the factor analysis was used to separate the affording factors on erosion shapes. Erosion increased with increasing SAR، EC، Na، Cl، SO4، silt and clay and decreased with increasing in HCO3، TNV and gravel. EC، SAR، Na، Cl، TNV، HCO3، SO4، gravel، silt and clay have statistically significant difference between erosion types and factor analysis showed that EC، SAR، Na، Cl، silt and clay booster and HCO3، TNV and gravel are halter factors of erosion.

Because of the increasing water demand، the study of the potential climate change and their impacts on droughts is unavoidable. In this research، the downscaling technique of LARS-WG is applied to predict the climate change، based on the General Circulation Models (GCM). By using this stochastic weather generator the climate of six synoptic weather stations in the East Azerbaijan province، northwestern of Iran is simulated. In this work، six GCM models and three emission scenarios of A1B، A2 and B1 in different horizons of 2020، 2055 and 2090 were used. Using the A2 scenario، the research outcomes show that the average annual temperature rises ~5°C and annual precipitation reduces 60 mm by the end of this century. The average results are also shown in suitable classification maps over the province. Based on the De Martonne aridity index، these changes shift the climate of the province from semi-arid to arid. The results of this study could advise the policy makers in water resources and managers to take suitable actions in securing water supply and drought management.