مجله مهندسی و مدیریت آبخیز
سال سوم شماره 3 (پاییز 1390)

Gully erosion is one of water erosion types that happen due to improper management of agricultural lands. The main objectives of this research are zonation and characterisation of gully erosion. For this purpose، affected areas by gully erosion were noticed in Arial photographs and satellite images. Areas with more than 500 ha and their dominant climates were specified with climate map and for each climate two areas in which one representative gully and two repetition gullies were chosen. Morphology characteristics، soil sampling and other related studies were performed in each gully. Consequently، according to the main characteristics، such as climate and view plan، these gullies were classified by cluster analysis and their digitized zonation maps were provided. According to the results، six regions with a total area of 7437. 3 ha were affected by gully erosion with three different types of climates، from which، three regions had moderately arid climate (Qeshlagh Alborz and Naizar) and warm semi-arid (Rahjerd) with more than 500 ha. These regions that are classified in two classes based on cluster analysis، have lateral gullies which are developed beside the natural drainages and are classified as un continuous gullies. The texture of soil surface is usually sandy-loam and in some cases loam and silty-loam and sandy-loam and silty-loam in the subsurface layer. Highly sensitive lands، flood occurrence، past heavy rainfalls and high amount of salts in some areas، are from the most morpho-climatic factors causing gully erosion in dry regions and deserts of Qom province.

Spur dikes are used widely to prevent outer bank river bends and path-modification river projects. Local scouring in spur dile nose is due to contraction and strong eddy is among the important problems occurring in designing Super dikes. Experimental study has been done on effect of length and alignment of minor spur dike to reduction of main spur dike scouring. In order to do this study a shorter spur dike was used in two stages، one perpendicular and one angled to flow channel direction. By non dimensional parameters، contraction ratio L/B=0. 3، 0. 35، 0. 4، 0. 45، (L=length of spur dike، B=width of channel)، Length ratio L’/L=0. 5، 0. 6، 0. 7، (L’= length of minor spur dike) and distance ratio x/L=0. 5، 1، 1. 5، 2، 2. 5 have been studied. Scour depths at nose of minor and main spur dike have been measured. Data collection were fitted by regression analysis and a relation for reducing scour depth at nose of first spur dike is provided، with and without minor spur dike.

Landslide is one of the natural hazards that makes numerous financial and life damages each year. By landslide hazard zonation، we able to detect susceptible areas to landslide and with applying developed methods and suitable management the abundance of land sliding and the amount of damages will be reduced. In this study potential landslide hazard evaluated using multivariate regression model at a part of Alamout watershed in general level (1:50000 scale). So first، landslide distribution map of area prepared using study of air photos and field surveying. After reviewing available resources along with reviewing the benefit of experts، all factors that can affect landslide were extracted and among them eight parameters including (lithology، slope percent، aspect، height، distance to fault، land use، rainfall and earthquake acceleration) were selected as landslide effective factors. AHP and pair comparing technique were used for numerical weighting to qualitative categories of land use، aspect، and lithology parameters. Homogeneous units map prepared using overlaying 8 maps of landslide key factors، and by crossing of homogeneous map and landslide distribution map. Categories of each parameter were detected in each landslide، and with weighting average of them، the effect of each parameter was determined in each landslide. Therefore 84 observations prepared for statistical analysis of landslides. Results showed that 5 parameters including lithology، slope، height، distance to fault and land use have meaningful relation with landslides that determining coefficient between these parameters as independent variables and logarithm of landslides area as dependent variable was 60. 7%. Landslide hazard zonation map and landslide distribution map were crossed and efficiency of model was evaluated. The Chi square test was used for comparing of difference between hazard classes of model. Results show that model has higher efficiency in higher classes of hazard. Results show also measured chi square rate is meaningful at 99% of confidence interval، and there is suitable separation among landslide hazard classes.

One of the main objectives of hydrology is rainfall–runoff forecasting process for determination of flood hydrograph at the outlet of a watershed. The aim of this study was to investigate the consistency، accuracy and reliability of time–area and Clark instantaneous unit hydrograph models for estimating the shape، peak discharge، time to peak، base time and volume of flood resulting from a rainfall with certain intensity and duration. For this purpose، the topographic map of Bazoft watershed was digitized by using R2V package، the digital elevation model (DEM) and isochronal prepared with the ILWIS software. Finally، the flood hydrographs were estimated by Clark and time-area models for six selected storm events. The computed hydrographs then compared with those obtained by measured data an outlet of the basin. The results showed that while both methods have some errors، the Clark model gives a better estimation of outlet hydrograph in comparison with time-area model. The amount of relative error for peak discharge for Clark and Time- Area methods were 23. 3% and 32. 8%، respectively.

Rainfall erosivity is one of the effective factors on rainfall erosion which is very important in water erosion. Preparing rainfall erosivity mapping is an important issue in soil conservation، erosion control and land management. The objective of this research is evaluation of geostatistical methods for rainfall erosivity mapping in Fars province. First، 92 meteorological stations having at least 20 years data and relevant distribution over the region were selected and the modified Fournier index was calculated for each station. Then several interpolation methods including Ordinary Kriging (OK)، Simple Kriging (SP) and deterministic methods such as Inverse Distance Weights (IDW)، Radial Basis Function (RBF)، Local Polynomial Interpolation (LPI) and Global Polynomial Interpolation (GPI) were used in order to extend point data to spatial information in the process of preparing rainfall erosivity map. The evaluation of the results based on higher R value and lower MAE and RMSE values showed that among the studied methods، the OK method has higher R value and lower RMSE and MAE values (R=0. 87، MAE=13. 09، RMSE=18. 52)، then it was selected as the most appropriate method for rainfall erosivity mapping in Fars province. Also the results revealed that 0. 24% of the province has negligible erosivity، 27. 06% has low erosivity، 43. 62% has moderate erosivity، 14. 48 have high erosivity and 14. 6% has very high erosivity.

Flood estimation with certain frequency is one of the fundamental factors for designing hydraulic structures، floodplain planning، river coastal stabling، and watershed management. The Kushkabad watershed is one of the main sub-basins of Kashafroud basin located in the north-east of Iran، with an area of 87. 45 km2، and 11 kilometers far from Mashhad، which has always been facing severe damages due to flash floods in special climates and topography. In this research، HEC-HMS، flood frequency analysis، Creager، Dicken and Fanning rainfall-runoff hydrological empirical methods were compared. The statistical index of relative percent error was used for comparison of different methods. The results showed that HEC-HMS model has little difference with observed data، and empirical methods have a reliable approach with HEC-HMS model. On the other hand، regional flood frequency analysis has more significant difference with reality.