مجله مهندسی و مدیریت آبخیز
سال دوم شماره 3 (پاییز 1389)

When upstream water surface of an earth dam falls down rapidly and the upstream slope of the dam cannot be drained, the rapid drawdown analysis is necessary. There are several methods for rapid drawdown analysis including; Bishop, Ordinary, Janbu, Morgenstern-Price, Spencer, Lowe-Karafiath, United State Army Corps of engineers (USACE) and modified USACE. Modified USACE is presented by Lowe and Karafiath and is modified by Duncan, Wright and Wong. For simulation two models of SEEPE/W and SLOPE/W, are used. Drawdown velocity of 1, 2, 4 and 6 m/day are used in simulation. Simulations show that increasing drawdown velocity causes decreasing safety factor. Rapid drawdown analysis by Bishop, Morgenstern-Price, Spencer, Lowe- Karafiath, USACE and modified USACE give the same results for safety factor. Safety factor of rapid drawdown analysis will decrease 26% compared to steady state analysis on the average.

Effective discharge (Qeff) that transports the most of suspended sediment in rivers and controls the bed conditions is an important criterion for evaluating the quality of water. Qeff usually has 1.5 years recurrence interval on the yearly maximum flood data series. For examining similarity between Qeff and Q1.5 in ten stations of Daryacheh Namak Drainage Basin, at first the probability density function (pdf) of daily flows and sediment rating curves was plotted, after that probability density function of suspended sediment was constructed by multiplying the coefficient of sediment rating curves with pdf of daily flows. Then its recurrence interval was estimated by Vibul's method. The results showed that there are not any similarities between Qeff and Q1.5, and the ratio of Qeff to Q1.5 ranged from 1.03 to 16.8. Therefore more research is required to reach more certain result.

The intensity and type of soil erosion are functions of climatic, topographic, soil and land use characteristics. The role of land use is more than others due to the human impacts. In order to study the relationship between land use change and soil erosion, the Manderijan watershed located upstream of Zayandehrud dam basin, comprises 23000 ha, was selected. The satellite images of MSS1979, TM1990, TM1998 and ETM2002 were used to map land use as well as soil erosion features during the study periods. The relationship between the existence as well as areas of each particular soil erosion feature and land use was also investigated using regression and Chi-square analyses. The results of the study verified a significant relationship between areas of land uses and soilerosion features. The moderate sheet erosion was further related to the land use changes in particular in the form of cubic regression equation having the determination coefficient of less than 81% and relative error of below 39.65%. The Chi-square analysis was also certified the insignificant relationship between existence of each particular soil erosion feature and land use whereas the relationship between land use and soil erosion areas could be significantly approved. The results of the study therefore can be utilized for proper management of the area through devoting necessary attention into the land uses.

Rill erosion map is one of the basic maps in erosion and sediment studies and also watershed management programs. Some methodologies for preparing rill erosion map (1:250000) by using RS and GIS were compared in a research carried out in Kan- Sologhan and Jajrood sub-basins in North-west and North-east Tehran, Iran. At the first phase, four working unit's maps were prepared by integration of a) plant cover, geology and slope b) land use, geology and slope c) land use, rocks sensitivity to erosion and slope and d) land use, rocks sensitivity to erosion and land units’ layers in Kan- Sologhan basin. For comparing the efficiency of four working units’ maps, 53 ground control points were investigated. Results showed that map "d" was better than "a", "b" and "c" in providing rill erosion regarding economic and executive considerations. At the second stage, map "d" was compared with three maps of e) land units f) rocks sensitivity to erosion and g) satellite image photomorphic units (ETM+ images) in a broader basin of Jajroud. Rill erosion intensities in 314 ground control points were controlled and ground truth rill erosion map was prepared. Rill erosion map was crossed with different working unit maps where results showed that the highest accuracy is related to the images interpretation (81.0% accuracy). Accuracy was 78.4, 66.8 and 5. percent for methods of "d", "e" and "f", respectively. Root Mean Squared Error was the highest for map "f" and the least for integrated layers method (map "d"). The greatest precision is related to image interpretation and the images interpretation method is better than other methods in preparation of rill erosion map.

Planning and suitable management is necessary for optimal use of soil and for this; spatial variability of soil characteristics is important which may be edcarried out through geostatistical methods of parametric and non-parametric predictors such as TPSS, WMA, Kriging and Co-kriging. This research work was done in Southern part of Uromieh plain with 36690 ha surface area in order to study the spatial variability of soil lime, sand and saturation moisture percentage. Distance between soil profiles ranged 1300 to 4700 meters. For estimation and prediction of them in non-sampled points, the Kriging, Co- kriging and Weighted Moving Average were used in Geographic Information System environment. For selecting suitable interpolation method, Cross validation and MAE and MBE parameters were used. Selected method was also used for estimating and mapping of the selected soil characteristics. The Sturges rule was used for defining map classification. Results showed that the Kriging method has the highest accuracy with correlation coefficient of 0.83 and error of 3.98 percent for prediction of soil characteristics in non-sampled points.

One of the usual methods for river banks protection is using Gabion groyne structures. If this permeable structure is designed and constructed appropriately, it will control bank erosion and reclaims flood plain areas. Since there are some uncertainties in the determination of length, space and shape of Gabion groyne, this experimental project was performed. In this investigation a laboratory movable bed flume with 17m length, 1.5m width and 0.8m depth was used to study local scour near the head of unsubmerged Gabion groynes. The T shape and straight groynes were constructed perpendicular to the flume's wall. Groynes with two different lengths of 15.0 and 22.5 centimeter (i.e. two contraction ratios of 20% and 30%) and four different spaces (i.e. four ratios of space to length, S/L), were used with one constant bed material, one constant bed slope and three different discharges. Based on the experimental results, a dimensionless relationship was derived for estimation of Local Scour near the Head of straight and T shape Gabion groynes.

One of the common methods of river training and bank protection is using spur dikes to modify flow path. Because of the concentration of flow in the middle part of the river it causes the river side not to be washed out. This new river path makes the river bed next to the end of the spur dikes to be washed out. Usually the spur dikes are built in groups to extend the protected zone. The stability of these spur dikes against the river flow depends on stability of the first spur dike. The purpose of this laboratory research is to investigate different ways for reduction of erosion around the first spur dike. It is suggested to use a shorter spur dike at the upstream part perpendicular to the flow direction. In this research, it is intended to find the optimum distance between the shorter spur dike and the first spur dike in order to have minimum erosion around the first spur dike. From the hydraulics point of view, L’/L (the length of shorter spur dike to the length of first spur dike), X/L (the distance between first spur dike and shorter spur dike to the length of first spur dike) and H/L’ (the depth of water at upstream to the length of shorter spur dike) have been studied and results has been presented in the form of equations.