Assessment of The Effect of Dam Construction River pattern And Pass changes Using GIS & RS (Case Study: AHAR CHAI River)


1 qom governer

2 tabriz univercity


Extended abstract
Introduction :
Rivers are one of the most important sources of water supply in the world. Reduction of water supply and resources reveals the necessity of investigation of water character changes. The catchment area of AHARCHAI is in northwest of Iran (longitude: 46˚20ˈ to 47˚40 east, latitude: 38˚13ˈ to 38˚45ˈ north). This catchment that extends to the tributary of river GHARESOU and covering almost 3168KM2 is very important, because it contains massive portion of Aras catchment area. Morphologically its shape is close to oval and AHARCHAI is the main tributary of this catchment that emanate from GHASABE mountain (2952 m high) moving from west to east. This river on its way is crossing AHAR & VARZEGHAN cities and finally joins to GHARESOU and makes valley. VARZEGHAN CHAI, KASIN, ALIREZA CHAI, KORGIGH, SIAH KALAN and … are its important tributaries. SATARKHAN dam is constructed on the river AHARCHAI in 1377, 120 km far from north east of Tabriz city, 15 km far from AHAR city and is the most important water construction in the area. This dam is constructed with the aim of controlling the ground streams, supplying water of 12450 hectares of farms and AHAR citizen and to avoid wasting of 92 million cubic meter of water. The SATARKHAN reservoir dam is made of soil and its core is full with clay. It is 350 m high, 11 m width on the top and has 480 m Embankment on the base of the dam. Igneous rocks and volcano sedimentary rocks such as Andesite, Andesite tuff, Agglomerate, lapilli tuff, sandy tuff, sandstone, siltstone covers the basin of the dam .Construction of SATARKHAN dam changed morphological parameters of river AHAR CHAI such as stream velocity and sedimentary process therefor identifying these changes of the river morphology is very important to avoid disasters in future. Some villages, forms and communication lines sunk because of dam construction and also because of river bed changes and flood control, lots of environmental and human problems happened. In this paper the pre dam construction morphology of AHARCHAI river and post dam construction changes of river bed are studied and compared quantitatively.
In this paper the study area was identified by aerial photograph (scale: 1:20000) and topographic map (scale: 1: 50000). Then two series of data of pre and post dam construction were collected from hydrometric stations and field work .To forecast the annual sediment for each station, the measured sediment and water supply during 16 years were used. Statistical procedures were used to compute the annual sediment. Finally the annual sediment per ton was obtained using annual water supply. Three reach were selected to compare the changes and movements of the river during pre and post dam. To digitize the river pass, two series of aerial photos (1967- 1995), IRS satellite data (2008), Google Earth and topographic map (scale 1:1000) were used. The river pattern in the three selected reach were studied by using cornice angel and river sinuosity.
In KASIN hydrometric station, the mean annual sediment and water supply is decreased respectively 20.64% and 26.3% after dam construction. Also in ORANG hydrometric station, the mean annual sediment is decreased 39.39% and the annual water supply is about 1.866m3/s which is 40% less than pre dam situation .Assessment of river movement from 1995 to 2008 showed that the changes are decreased respectively. But from 1967 to 1995, the river meanders are expanded, and braided river is turned into meandering river .Before dam construction, the sinuosity was low and the river pattern was straight. But after dam construction, the sinuosity increased and the river pattern turned into meandering river.
Conclusion :
In KASIN hydrometric station, the peak water supply intensification did not continue after dam construction. In ORANG hydrometric station, the mean annual peak water supply is 38c3/s. In post dam condition, the rain is decreased and, the mean annual peak water supply must have been decreased about 32.5c3/s but the real score was 29c3/s which shows the effect of dam on water supply. Computing the cornice angel and river sinuosity showed that the river pattern at first was braided, but it turned in to meandering and finally to straight shape. In the first reach, from 1967 to 1995, the river bed extremely changed and braided shapes are modified to meandering shapes with huge curves. From 1995 to 2008, the changes are declined and the river. In the second reach, from 1967 to 1995, braided shapes are modified to meandering shapes and meanders are expanded relocated but after dam construction, sediment and water supply are decreased there for the river flowed straightforward. In the third reach, from 1967 to 1995, the same situation occurred but in some portions the sinuosity increased and meanders are lagged. After dam construction, the sinuosity is reduced and the river flowed straightforward progressively.