A quantitative study on channel-bed changes of the Alamut and Shahrud rivers (N Qazvin) in 1981-2015 period times

Document Type : مقالات پژوهشی

Authors

• Efat Pasban ¹
• Saeed Khodabakhsh ¹
• Mohammadreza Garibreza ²
• Shima Kabiri ³
• Naser Bahiraei ⁴

¹ Buali Sina

² Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization (AREEO),

³ Water Structures Eng., Qazvin Regional Water Authority

⁴ Islamic Azad University

10.22067/sed.facies.v11i1.64450

Abstract

Introduction
Changes in the geometric shape of the river channel can be studied in horizontal and transverse views; Plan view changes in river systems based on aerial photographs or satellite imagery (using remote sensing and geographic information systems) are tracked at specific time intervals and include: channel widening and narrowing, channel simplification, braiding, meander migration, sinuosity growth, chute cut-off, neck cut-off, avulsion, simple irregularities in the bank, as well as the combination of two or more of these processes (David et al., 2016; Li et al., 2017). River channel profile changes can be mapped through the time intervals, and the most important are degradation and aggradation (Ollero, 2010; Little et al., 2013). The main objectives of this study are: 1- Investigation of geometric variations of Alamut and Shahrood rivers on the basis of field visits, geological maps and plan view mapped from satellite images (1981, Landsat 3; 1991, Landsat 5; 2002,  Landsat 7; 2009, Landsat 5 and 2015, Landsat 8) in almost certain months; 2- Estimating the rate of aggradation and degradation of the channel bed based on the survey of the profile of the channel (based on mapping) in the years mentioned at the four selected stations (Khooban Baghkalayeh, Late and Rajaee Dasht stations). The most important features used to determine the geometric variations of the channel were the extension, width, branching index and sinuosity.  
Alamut-Shahrud catchment (49° 30' to 51° 10' E and 36° 07' to 36° 30' N; 4853.67 km2 area) is one of the two Sepidrudsub-catchments and is situated in the South Caspian Sea catchment. The main outcrops of this catchment is in the eastern (upstream) zone and are composed of the Miocene terrigenous rocks, Eocene volcaniclastic rocks and some Mesozoic carbonate rocks in mid zone and Eocene volcanic units in downstream zone (Annels et al., 1977).
Result and Discussion
The most significant changes from 1981 to 2015 in 130 locations have been recorded from upstream to the outlet of the basin and include the following: Channel migration (57 locations), meander cutoff (23 locations), sinuosity growth (28 locations) and branching (22 locations).
Based on the migration of the channel, the following are observed: The maximum displacement is in the eastern (upstream) and central regions; in the central-range (Razmian), the length of channel shift occurred  about 3 km and reaches up to 575 meters. The lateral displacement of the channel in the western part is less (161 m) than other intervals and is neck cut off. The river in the eastern and central part (Khooban to Bahram Abad) is on a nearly straight path. In terms of the sinuosity, the channel has open angles and the sinuosity index is between 1.1 to 1.2. While the sinuosity angle of the channel in the western part is steep and its range is much shorter than the eastern and central parts of the river. The reason for the difference in the sinuosity pattern is the existence of two fault systems with an angle of about 120 degrees along the river course in the western part. Increasing more than three times the amount of channel shift in the central region (Rajaei Dasht-Bahram Abad, Figure 8) is also evident with the decrease of the slope of the river.
This change was simultaneously happened by changing the single-channel river pattern to braided channel; the drop in water velocity as a result of slope reduction, caused the sediments to be deposited more like bars and the branching ratio has increased.
Channel profile change
Profile changes of rivers were measured in 4 hydraulic stations; Khooban (56° 38' 47.56″ ; 36° 23' 51.23″) and Baghkalayeh (56° 29' 40.50″ ; 36° 23' 35.25″) on the Alamut River, respectively in the period from 2005 to 2014 and 1984 to 2014; and Rajaee dasht (50° 16' 46.38″ ; 36° 27' 34.98″) and Lats (50° 03' 59.96″ ; 36° 36' 36.88″) in the Shahroud River; between the years 1984 to 2014 and 2005 to 2014. Mapping cross-sectional profile was carried out during the years mentioned by Qazvin Regional Water Company. The measurements were done from the right to the left of banks in the channel.
The main goal of the study is to examine cross section, diagnosis of aggradation and degradation channel. Effective factors in these two processes can be indicated by the discharge, sediment load, bed slope and human activity. These factors are in dynamic equilibrium with controlling the morphology of the river channel, and whenever one of them increases or decreases, the river channel changes to become stable under the new conditions.
By increasing flow rate or decreasing sediment load, the capacity of river transport sediment is more than that, and this led to degradation, while decreasing flow rates or increasing sediment load led to aggradation (David et al., 2016). At Baghkalayeh Station between 1988-1991, 2000-2001, 2007-2008 and 2012-2014, discharge decreasing led to aggradation process, between the years 1992-1993 and 2002-2007, with increasing flow rate; the bed degradation process has occurred. The highest rate of bed degradation between 2002 and 2007 was followed by the highest increase in bed aggradation in 2007-2008. In the Rajaee dasht station, the trend is almost identical. In the Lat station, from 2007 to 2011, changes in the flow rate and bed height of the channel were similar, and from 2011 to 2014, the bed aggradation was evident with decreasing flow rate.
Some important results are summarized as follows:
1- The most important changes in the plan view of the river channel are the channel abandont process, neck cutoff, sinuosity growth, increased branching index (ie bar migration, sedimentation and branching) and lateral migration.                                                        
2- The reasons for these changes are: migration of channel bars, changes in sediment load and flow discharge, flood, growth of the fans/mouth bar formation, changes in branching and sinuosity index, geology of the watershed.
3- The bed aggradation, degradation and channel width variation processes in the studied stations in a complex site and time unit under the influence of changes in flow rate and sediment load.
Keywords: Alamut; Shahrud; Aggradation; Degradation; GIS.           
References
Annells, R.N., Arthurton, R.S., Bazley, R.A.B., Davies, R.G., Hamedi, M.A.R., & Rahimzadeh, F., 1977. The Geology Map of SHAKRAN, 1:100000, No. 6162, Geological Survey of IRAN.
David, M., Labenne, A., Carozza, J.M., & Valette, P., 2016. Evolutionary trajectory of channel planforms in the middle Garonne River (Toulouse, SW France) over a 130-year period: Contribution of mixed multiple factor analysis (MFAmix). Geomorphology, 258: 21-39.
Li, Z., Yub, G.A., Brierley, G.J., Wang, Z., & Jia, Y., 2017. Migration and cutoff of meanders in the hyperarid environment of the middle Tarim River, Northwestern China. Geomorphology, 276: 116-124.
Little, P.J., Richardson, J.S., & Alila, Y., 2013. Channel and landscape dynamics in the alluvial forest mosaic of the Carmanah River valley, British Columbia, Canada. Geomorphology, 202: 86-100.
Ollero, A., 2010. Channel changes and floodplain management in the meandering middle Ebro River, Spain. Geomorphology, 117: 247-260.

Keywords

• Alamut
• Shahrud
• Channel change
• river
• Qazvin
• Iran

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