عنوان مقاله [English]
There are many different factors changing rivers in terms of size, shape, direction and pattern. These changes highlight that rivers tend to equilibrate. The balance between erosion and deposition in a river is defined as equilibrium while the stability of stream morphology is defined as the ability to resist morphological changes for a long time. Doyle and Harbor (2003) showed that bed sediments type has a great impact on the equilibrium, so that the time needed for a sandy-bed channel to equilibrate is almost half of that needed for a gravel-bed channel. It’s because sediments transport more rapidly in sandy-bed channels and such channels equilibrate faster than those having ﬁne sediments. A well-known method to evaluate river stability is Rosgen stream classification system. Savery et al (2007) reported that Rosgen stream classification system is applicable for flat and low steep areas. This method is applicable in river engineering designs, management purposes and stream restoration. River Style Framework method is another applicable method to evaluate river stability. Equilibrium, stability and the role of environmental variables on Tarwal River have not been studied yet. As a result, there is no proper knowledge of its equilibrium and stability. Therefore, this case study is trying to study the equilibrium and stability issues in Tarwal basin.
Materials and methods
This case study demonstrates the capabilities of Rosgen and River Style Framework methods in Tarwal River, the main tributary of Sefid Rud. The tributary is located in the eastern part of Kurdistan province. For each river style segment of Tarwal River network, equilibrium capacity and geomorphic conditions have been determined by river style framework using three parameters of channel properties, channel planform and bed characteristics. In the next step, stability of Tarwal River has been determined by Rosgen stream classification system using 15 parameters (bank vegetation, channel capacity, section cuts, aggradation, degradation, sediment and etc.). In addition to extensive field campaigns, topographic maps, aerial photos and Google Earth software have been used to determine cross section dimensions and vegetation condition as well as trenches and terraces. During field campaigns, cross sections dimensions have been measured by a laser meter Leica D5 and locations of sections and trenches have been recorded by GPS. Then, the data have been digitized by ArcGIS. Furthermore, required photos have also been collected from different features in situ.
Results and discussion
Based on river style framework method, river style segments of fine-grained bed having low sinuosity, clay-bed and sandy-bed meandering have represented a local equilibrium capacity. These rivers style segments have indicated limited vertical and lateral movements and their sediments sorted well while river style segments of gravel-bed having low sinuosity or sandy-bed multichannel in Ozon Darreh River, sandy-bed with low sinuosity in Tarwal River and fine-grained meandering in Sang Siah River all have indicated a high equilibrium capacity. This is because of vertical and lateral movements and non-homogeneous sediments. In fine-grained meandering river style segments of Sang Siah River, equilibrium capacity was increased because of vertical (bed incision) and lateral movements in the form of channel contraction (alluvial terraces). This was dominated by vertical movements. These results support Nayyeri and Rezaei Moghadam (2005) findings in meandering river of Siminehrood where bed equilibrium reported as the form of bed incision. By River Style Framework method, the geomorphic conditions of the river were assessed through river characteristics and behavior. River style segments of gravel-bed river with low sinuosity in Sis River and a small tributary (in the northeast part of Tarwal River), a sandy-bed river with low sinuosity in Tarwal River and sandy-bed multichannel in Ozon Darreh River have represented a relatively high width to depth ratio, low sinuosity, compound and irregular channel shapes and erosional banks. These river style segments have represented poor geomorphic conditions because of non-homogenous poor-sorting sediments and bed erosion. River style segments of fine-grained bed with low sinuosity in Esmail Jamal, Jorvandi, Ozon Darreh and Tarwal Rivers, clay-bed ada in Sang Siah and Jorvandi Rivers and sandy-bed meandering in Tarwal River have also indicated a good geomorphic condition because of well-sorted sediments, considerable vegetation cover and lack of erosional banks.
The stability of all sections has been analyzed by Rosgen stream classification system. For this purpose, each river style segment, 1-3 sections (totally 34 sections) have been studied and their dimensions measured in situ. River style segments of clay-bed ada in Sang Siah River with sandy-bed meandering, sandy-bed multichannel, fine-grained river with low sinuosity and fine-grained meanders in Tarwal Main River are stable while river style segments of gravel-bed river with low sinuosity in Sis and a small tributary (in the northeast of Tarwal River), sandy-bed with low sinuosity in Tarwal River and sandy-bed multichannel in Ozon Darreh River are unstable. This instability is because of significant incision, high roundness of sediments grain distribution and their poor sorting as well as filled pools.
Instability of River style segment in downstream Jorvandi and Ozon Darreh Rivers and upstream Tarwal and Sang Siah Rivers might be related to numerous faults in some sections. Increase in sediment size is a good evidence of recent faulting activities in these sections. Faults possibly increase stream channel slope that results in increased velocity of streamflow and creating unstable reaches. Increasing streamflow velocity causes considerable movements of sediments so as fine sediments washed out and only coarse sediments remain in the streambed. Such evidences supports that the riverbed of these segments has affected small-scale tectonic activities. Due to non-steepness of the study area, Rosgen stream classification system is suitable for the field data very well. This supports Savery et al (2007)’s recommendation about benefits of Rosgen method in flat areas. The results of this study can help us improve integrated evaluation of watershed management activities, hydrological designs and river rehabilitation projects.