Writers : Rostamizadeh,Ghasem;Supervisor:Mousaviharami,Mahboubi
Refference : M.Sc.Thesis _Ferdowsi University,Mashhad
Publishing Year : 1383

Abstract :

The Bidvaz Watershed is a part of Central Iran drainage basin located east of the city of Esfarayen in Northern Khorasan Province. This watershed has an elongate to fan form and its surface area is more than 500 square kilometer. It is a part of eastern Alborz zone and geologicaly is composed of Dalichai, Lar, Shurijeh, Tirgan, Khangiran formation, as well as Neogen sediments and Quaternary alluviums.
Geomorphologic studies show that this watershed is within a synclinal and surrounded by Shah-Jahan Mountains in the south and to Allahdagh Mountains in the north. Structural and lithological variability have a very important role in this watershed formation; therefore, the watershed has the same trend by structures (NW-SE). Diversify geomorphologic forms such as folding, faulting, basalt mass, alluvial fans, sedimentary bars, channels, rills, gully and types of mass movement have been identified in this watershed.
Based on this study, the Bidvaz River has a braided pattern with gravelly bed.
Sedimentologic studies along rivers in two subbasins show that two basic factor (sudden changes in slope gradient and distributaries entrance) are the main reasons for changes in texture of bed load sediments within the channel as well as the break in sedimentary links. Textural variations in grain size parameters have a regular trend within every one of two sedimentary breaks. Average (mean) and median of grain size decrease along anyone of sedimentary links. More than 70 to 80 percentage of sediments weight in anyone samples is gravel and streamflow may have transported many finer grains (mud, sand and even granule to medium pebbles) toward downstream.
Therefore, selective transport or hydraulic sorting is the main factor that controls the fining trends. As well as, positive skewness can be related to this mechanism. The study of textural changes in grain size parameters along the main river (Bidvaz) shows a longitudinal linkage in these parameters. Changes in slope gradient are the most important factor that caused the dispersion in grain size distribution. Lithofacies that known in this watershed are gravelly (Gmm, Gmg. Gem), sandy (Sm, St) and muddy (Fr).
Study of erosion and sediment generation in Bidvaz watershed showed that the eastern parts of this watershed have a higher potential of water erosion than mass movements. Because these areas have the following characteristics: (1) a large expaosure of many profile of Lar Formation, (2) lack of herbaceous covering and (3) high slope gradient. In the higher gradient slope in central parts of this watershed, the erosion intensity is very low, because the more resistant massive limestones of Lar and Tirgan Formations are exposed. Western parts of Bidvaz watershed, near the exit point, are covered by many-conglomerate of Neogen and alluvial fans sediments. Therefore, these areas have a high potential of erosion and sediment generation. In conclusion, this study shows that 3 basis factor (lithology, gradient slope and human interferences) have the main role in sediment generation in this watershed.

Subject List : Geomorphology