تحلیل مورفولوژیکی مجرای رودخانۀ زرینه ‏رود با استفاده از مدل رزگن

Introduction
A basic belief in geomorphology says that ‘form implies process.’ Thus, numerous geomorphic classifications have been developed for landscapes, hillslopes, and rivers. The form–process paradigm is a potentially powerful tool for conducting quantitative geomorphic investigations (Shroder, 2013: 730). Streams typically have similar suites of channel morphologies, with repeatable patterns of occurrence, which resulted in numerous classification efforts (Roper et al., 2008: 417-427). Recent approaches for river classification focus on watershed analysis related to land management and stream restoration, using a hierarchical approach that nests successive scales of physical and biological conditions and allows a more holistic understanding of basin processes (Shroder, 2013: 739). One of the most widely used hierarchical channel classification systems was developed by Rosgen (1985, 1994, 1996, Shroder, 2013: 742). In the current study, Zarrineh Roud river channel planform are studied by using Rosgen geomorphological model in combination with HEC-RAS model.
Materials and Methods
This study is based on fieldworks and topographic maps of scale 1: 2000 (West Azerbaijan Regional Water Authority). The data from Sari-Qamish and Nezam-Abad hydrometric stations in the main stream and Qureh-Chay and Janaqa stations on the tributaries were used for calculation of return periods and discharge–stage relation. To determine the friction coefficient distribution of channel and floodplain, land cover maps was generated using Google Earth satellite imagery. Laboratory equipment including Van Veen Grab- Bottom Sampler, shakers, digital scales, and caliper were used for Sediment particle size analysis (granulometry). Rosgen (1985, 1994, 1996) hierarchical system was used to analyze river channel morphology. The Rosgen system uses six morphological measurements for classifying a stream reach-entrenchment, width/depth ratio, sinuosity, number of channels, slope, and bed material particle size. These criteria are used to define eight major stream classes with about 100 individual stream types (The Federal Interagency Stream Restoration Working Group, 2001, chapter 7: 29). In this research, some of these parameters were calculated using HEC-RAS hydrodynamic model. For steady, gradually varied flow, the primary procedure for computing water surface profiles between cross-sections is called the direct step method. The basic computational procedure is based on the iterative solution of the energy equation. Given the flow and water surface elevation at one cross-section, the goal of the standard step method is to compute the water surface elevation at the adjacent cross-section. The flow data for HEC-RAS consists of flow regime, discharge information, initial conditions and boundary conditions (HEC, 2010).
Results and Discussion
Zarrineh Roud River based on different variables, such as channel planform, geological controls, bed material and anthropogenic effects, was divided into five different reaches: Reach (1) from start point to Shahindejh industrial town; reach (2) from industrial town to Norouzlu Dam; reach (3) from this dam to Miandoab city; reach (4) from Miandoab to Chelik village; reach (5) from Chelik village to Urmia Lake. Reach (1) is type C3, which according to the slope and bed material divided into two sub-reaches: sub-reach C3b and sub-reach C3. Type C3 streams have a good recovery potential, moderate sediment supply, moderate streambank erosion potential and very high vegetation controlling. In this reach, the lateral dynamics of channel is low due to the geological control and low erodibility of bank material. Also, the bed incision is largely limited due to the presence of coarse material and bed armoring. Therefore, this reach is relatively stable. From starting point of reach (2) significant changes can be observed in the geomorphological characteristics of the Zarrineh Roud River. Floodplain width significantly developed and geological controls reduced. In this reach, Zarrineh Roud River becomes specific example of gravel Bed Rivers that extends to the downstream Norouzlu Dam. At the upstream of reach (2), the river is transformed into a type D4 stream. According to the field studies, high erodibility of bank materials along with poor control of vegetation are the main reasons to create the type D in some parts of this reach. Other parts of reach (2) are very specific example of type C stream that according to the slope and bed material, belong to the type C4. This reach shows complete characteristics of type C4 streams, so that, it has a high sediment supply, very high streambank erosion potential and very high vegetation control. In fact, vegetation cover combined with the erodibility of the streambanks determines the degree of lateral adjustment and stability of this reach. In this reach, most meanders are active. Lateral migration of meanders is accompanied by the mass failure in elevated clay banks. Reach (3), from downstream of Norouzlu dam to Miandoab city, in most sections, are converted into type G and type F streams. In fact, in this reach, a conversion has taken place from type C (C4 in upstream and C5 in downstream) to type G and type F due to very extensive anthropogenic interference. In this reach, sand and gravel mining is widely uncontrolled. River bed entrenched up to several meters (confined) and often isolated by steep and vertical banks from floodplain. Type G5, type F4 and type F5 streams have very poor recovery potential, very high sediment supply, very high streambank erosion potential and high vegetation control. This reach in terms of natural lateral dynamics is inactive. Based on the delineative criteria of Rosgen model, reach (4) have characteristics of the type C5b streams. In this reach, flood prone areas are reduced due to the human interference. Flood prone areas are mainly in accordance with scroll bars which developed in the convex sides of meanders. This reach due to the anthropogenic disturbances does not have typical characteristics of type C streams and is better considered as a reach converted from type C to type F. Reach (5) is converted to a type E that according to substrate material and slope, divided into two sub-reaches: sub-reach type E5b upstream and sub-reach type E6b downstream. In this reach, floodplain is very broad and developed. Creation of a developed meandering pattern is related to the low stream power and cohesive bank material. This reach is also relatively stable.
Conclusion
Efficiency of Rosgen model upstream the reaches of Norouzlu Dam and final reach is relatively good and almost all sections are compatible with a type of Rosgen streams. In these reaches, river form and pattern largely indicate the processes governing river channel and river morphology controlled by variations of stream power and variability of bank and riparian conditions. In the both reaches (3) and (4), due to high control of anthropogenic factors, explanation ability of the Rosgen model is limited and incompatibilities are observed in determining the type of Rosgen streams. These two reaches are critical reaches along the Zarrineh Roud River. Rosgen (1997) proposed four priority in its geomorphological approach to restoration of incised rivers that prioritized as follows: Conversion of G and/or F stream types to C or E in previous elevation w/floodplain; Conversion of F and/or G stream types to C or E; reestablishment of floodplain at existing level or higher but not at original level; Conversion into a new stream type without an active floodplain but containing flood prone area. Conversion of G to B stream type or F to Bc; Stabilization of channel in place. Given that rehabilitation and restoration of type G and type F are difficult, it is better to apply restrictions in relation to sand mining regardless of the conversion the reach (4) (downstream Miandoab) to type G or type F. For type F and type G reaches is recommended, because of the high population density in the region and increase in the probability of flood event, the second priority, conversion of F and/or G stream types to C or E and reestablishment of floodplain at existing level or higher but not at original level.