Assessment of Geomorphologic equilibrium by Rosgen and River Style Framework methods (a case study of Tarwal River, Kurdistan)

1- Rivers, under influences of different factors, change in terms of size, shape, direction and pattern. These changes indicate that rivers tend to equilibrate. Tarwal River, as the main branches of SefidRud, is located in the eastern part of Kurdistan province. Equilibrium, stability and the role of environmental variables on Tarwal River have not studied yet as there is no proper knowledge of its actions and reactions. Therefore, this case study is trying to find out the answers the questions related to such issues in Tarwal basin. The balance between erosion and deposition in a river defined as equilibrium (Davis 1902) while The stability of stream morphology defined as the ability to preserve existing conditions for a long time (Bureau of technical Vice Presidency for Strategic Planning and Supervision, 2012). Doyle and Harbor (2003) showed that the type of bed sediments has a great impact on the equilibrium, so that time needed for a channel to equilibrate with sand bed is almost half the time needed for a channel with gravel bed. It’s because sediment transport more rapidly in channels with a sandy bed and these channels equilibrate more rapidly than those with fine sediments. One way to evaluate river stability is Rozgen classification system. Savery et al (2007) declared that Rozgen classification system is applicable for flat and low steep areas. This method is usable in the engineering designs, management issues and stream restoration (Rustaei et al. 2013). River Style Framework method is also another applicable method to evaluate river stability.
2- For each style of Tarwal River network, equilibrium capacity and geomorphic condition was determined by river style framework using three parameters of channel properties, channel planform and bed characteristics. In the next step, stability of Tarwal river was determined based on Rozgen classification system using 15 parameters (bank vegetation, channel capacity, section cuts, aggradation, degradation, sediment and …). Therefore, in addition to extensive field campaigns, topographic maps, aerial photos and Google Earth software used to determine cross section dimensions and vegetation condition as well as trenches and terraces in the study areas. During field campaigns, cross sections dimensions token by a laser meter Leica D5 and locations of each sections and trenches recorded by GPS. In addition, collections of photos token from different features. Then data digitized to Arc GIS software.
3– Based on river style framework, styles of low sinuosity with fine grained, ada clay bed and meandering sand bed show a local equilibrium capacity. These rivers styles have limited vertical and lateral adjustment and sorts sediments well, while styles of low sinuosity gravel bed, multichannel sand bed in Ozon Darreh River, low sinuosity sand bed in Tarwal River and meandering fine grained in Sang Siah River show a high equilibrium capacity because of vertical and lateral adjustment and non-homogeneous sediments. In meandering fine grained style of Sang Siah River, equilibrium capacity is increased because of vertical (bed incision) and lateral adjustments in the form of channel contraction (Alluvial terraces) where it dominated by vertical adjustment. These results support the results of Nayyeri and Rezaei moghadam (2005) in meandering river of Simineh rood where bed equilibrium reported in the form of bed incision. In River Style Framework method, the geomorphic conditions of river assessed through river characteristics and behavior. Styles of low sinuosity gravel bed in Sis River and a small tributary (in the northeast of Tarwal River), low sinuosity sand bed in Tarwal River and multichannel sand bed in Ozon Darreh River showed a relatively high width to depth ratio, low sinuosity, compound and irregular channel shapes and erosional banks. These styles show poor geomorphic conditions because of non-homogenous, poor-sorting sediments and bed erosion. Styles low sinuosity fine-grained bed in Esmail Jamal, Jorvandi, Ozon Darreh and Tarwal Rivers, ada clay bed in Sang Siah and Jorvandi Rivers and meandering sand bed in Tarwal River show a good geomorphic condition because of well sorting sediments, considerable vegetation cover and lack of erosion bank.
The stability of all sections analyzed based on Rozgen classification system. Therefore, in each reach style, 1-3 sections and totally 34 sections picked up. Styles of ada clay bed in Sang Siah River and meandering sand bed, multichannel sand bed, low sinuosity fine grained and meandering fine grained in Tarwal Main River are stable while styles of low sinuosity gravel bed in Sis and a small tributary (in the northeast of Tarwal River), low sinuosity sand bed in Tarwal River and multichannel sand bed in Ozon Darreh River are unstable. This instability is because of significant incision, high roundness of sediments, poor sorting and filled pools. .
4– Section instability in Downstream of Jorvandi and Ozon Darreh Rivers and in upstream of Tarwal and Sang Siah Rivers can be resulted from numerous faults in some sections as the slope has increased in current period results in increasing stream power and unstable reaches. Increasing particle size is a good evidence of activity of faults in the section. Faults have increased stream power and causes extensive move of particles. Fine particles removed and only coarse particles remain in current periods. Such evidences clarify that the river bed has affected by a tectonic activity in a small scale (Ramesht, 2012). In this case study, Tarwal River were analyzed by River Style Framework method, Rozgen classification system and field campaigns. Due to non steepness of the study area, Rozgen classification system well fitted to the field data which support Savery et al (2007) recommendation about applicability of Rozgen classification system in flat areas. The results of this study can help to improve the evaluation of watershed management activities, hydrological designs and river adjacent activities such as land use changes, sand and gravel mining, flood plain management, flow adjustment by storage and diversion dams and river rehabilitation.