عنوان مقاله [English]
Fluvial geomorphology is the study of the interactions between river channel forms and processes at a range of space and time scales (Charlton, 2008). The last two decades or so have seen a major focus in fluvial geomorphology on developing topographic monitoring and modelling techniques to better quantify channel and floodplain morphology and morphological change in three dimensions (Rumsby, 2007). Morphological change in river channels primarily consists of adjustments to channel width, depth, local channel slope, and planform (Labbe, 2011). Several studies have documented the complexity and variation in the causes and rates of lateral movement in alluvial rivers. Depending on the input conditions and planform geometry, lateral movement can take different forms including meander migration Hooke l980; Bradley and Smith 1984; Nanson and Hickin 1986; Thorne 1991; Lawler 1993; Richardson 2002, width changes Surian 1999; Winterbottom 2000; Buhman et al. 2002; Chitale 2003, and wandering, avulsion, and cutoffs in the case of braided rivers Coleman 1969; Klaassen and Masselink 1992; Warburton et al. 1993; Xu 1996; Cao et al. 2002. During the past tens or hundreds of years, in many fluvial systems, river dynamics have been significantly affected by human disturbances such as land use changes, urbanization, channelization, dams, diversions, gravel and sand mining (Surian, 2002). In this study, lateral changes of the Gara-Sou river channel have been investigated in during the last 12 years.
The most important data of the present study include topographic map scale of 1: 2000 (Ardabil Regional Water Authority), Topographic map scale of 1: 50,000 and 1: 25,000, geological maps scale of 1: 100,000, Satellite images of Sentinel (2017), IRS (2005, 2017) and Google Earth. In this study, GIS & RS software includes Google Earth, ENVI and Arc GIS software with HEC-GeoRAS and Planform Statistics extensions was used. In this research, field studies are basis for geomorphological analysis. The methodology and models used in this study can be summarized in processed satellite images to study changes river channel, the extraction of vegetation and land use; Geomorphometric indices for quantitative analysis of planform and lateral changes in the river channel.
Results and discussion
To measure the dynamics of the Gara-Sou river channel using quantitative Geomorphometric indicators of GIS techniques were used. Therefore 271 cross sections was drawn on 31 km from the Gara-Sou river channel. The four indices of lateral movement begin with a gross measurement of total bankline change, then break down the lateral movement into width change and migration. The indices are as follows: 1: Total bankline change, E (m/year) 2: Normalized lateral movement, N (% width/year) 3: Width change, dW (m/year) 4: True migration, M (m/year). Lateral stability is measured by quantifying how much the active channel area changes with time and how much of the active channel area remains in the same place. Total movement of the channel banks (E), incorporates width change and lateral migration of the channel. The index value (E) for the first reach, 0.86, second reach, 1 and for the three reach, 0.89 was calculated. The high value of the index (E) indicates the narrowing of the channel. The normalized lateral movement rates (N) were computed by dividing the average of the right and left bank change by the active channel width averaged over the time period. This value was then divided by the number of years in the time period to get an annual rate. This Index for the first reach, 0.048%, second period, 0.2% and for the three reach, 0.057%, was calculated. Narrowing of Gara-Sou River channel, which is primarily due to reduced lateral dynamics channel. This leads to the deposition processes of erosion processes have been overcome. Occupation and establish natural vegetation on the active point bar and encroaching agricultural land to near river land and river flood plains from other important factors are narrowing of Gara-Sou river. Also the morphology of Gara-Sou River are controlled by anthropogenic variables.
In this study, lateral dynamics of Gara-Sou river channel in the recent 12 years were investigated. For this purpose, a collection of Geomorphometric quantitative variables was combined with field results. High mobility in Total Bank Line Change (E) Normalized Lateral Movement Rates (N) Gara-Sou River channel is not related to the True migration but has a strong correlation with the changes in the width of channel. The true migration rate for Gara-Sou River during the last 12 years in the study area was about 0.36 meters per year. This study tries to quantify the changes through new approaches for studies fluvial Geomorphology.
Key words: Channel Lateral Change, Geomorphometry Indicators, Channel Lateral Migration, Gara-Sou River.