Morphological Assessment of Givi Chay River Channel Using the Rosgen Classification System

Extent Abstract



Introduction

The significance of rivers has increased due to their role in understanding the water cycle, ecology, and interactions with groundwater across local to global scales. Rivers are now recognized as dynamic environments where significant interactions and changes occur (Chakraborty & Datta, 2013). In river engineering projects, an understanding of fluvial geomorphology principles and channel processes allows researchers to understand the form-process relationship within the landscape. Geomorphic assessment generally comprises data collection, field investigations and channel stability evaluation, establishing a foundation for analysis and design (The Federal Interagency Stream Restoration Working Group, 2001). Therefore, to achieve sustainable and integrated river management, a comprehensive evaluation of river planform and channel morphology must be conducted.In this context, one of the most important aspects involves analyses related to the classification of river planforms and channel patterns.Two primary purposes can be identified for river classification: (1) to advance scientific understanding of fluvial processes and enable channel clustering, and (2) to provide management recommendations for decisions regarding channel restoration or conservation.in this regard, geomorphic criteria may be integrated with those from other Fields (e.g., ecology, water chemistry) (Kandulfo & Pigi, 2003). River classification is one of the fundamental and important subjects in geography, hydrology, and ecology that focuses on understanding their diverse characteristics and functions. Describing hydraulic and hydrological processes requires an understanding of river behavior. Comprehensive identification of river systems and classification of natural rivers are critically important for resolving environmental and ecological challenges and achieving sustainable development. Since the late 19th century, numerous classification systems for rivers have been developed. Most of these classifications have been developed based on tectonic activities (Powell, 1875; Davis, 1895), the stage of river development (Davis, 1899), fluvial planforms (Leopold and Wolman, 1957; Miall, 1977; Rust, 1977; Qian, 1985), river sediment transport (Schumm, 1963; Wang, 1999), fluvial processes and the geomorphic characterization of riverbeds (Montgomery and Buffington, 1997) (Li et al., 2024). Despite these numerous classifications, most of these methods cannot be directly applied in river engineering. This is because, firstly, most of these classifications were developed with an academic and scientific approach and Few of them are practical. Secondly, very few of these classifications can provide a comprehensive perspective of fluvial geomorphology and its hydrological components (Li et al., 2024). Among these, the Rosgen classification provides a practical tool for predicting river behavior based on geomorphological characteristics. This method establishes a direct linkage between fluvial geomorphology and ecological restoration assessment, while contributing to improved integrated river management. Rosgen (1994, 1996) classified rivers based on parameters such as single-thread channels or multiple channels, entrenchment ratio, width/depth ratio and sinuosity, categorizing them into nine major types. He further subdivides these types into 41 subclasses and 94 sub-types based on gradient variability and channel deposits. In this study, The Givi-Chay River channel morphology was evaluated through integrated application of the Rosgen classification system and HEC-RAS hydrodynamic modeling.







Materials and methods

In this study, the main data include 1:2000 scale topographic maps of the Givi-Chay Riverbed (Ardebil Regional Water Authority), 1:50,000 topographic maps (National Geographical Organization), 1:100,000 geological maps of the Khalkhal, Givi, and Hashtchin sheets (Geological Survey & Mineral Explorations of Iran), a digital elevation model (DEM) with a resolution of 12.5 meters from the ALOS-PALSAR satellite, Sentinel2 images (2024) with a resolution of 12 meters, google earth images, and hydrometric data from Istisou and Firozabad stations. Data processing was performed with HEC-RAS, ArcGIS with HEC-GeoRAS and ENVI software. The Rosgen classification system was used for geomorphological analysis and the HEC-RAS hydrodynamic model was used to optimize it. The Rosgen model includes four analytical scales from landform to physical and biological processes (Shroder, 2013) and often emphasizes general geomorphic surfaces and morphological description (levels 1 and 2) (Rosgen, 1994). This system has six key indicators including entrenchment ratio, width-to-depth ratio, sinuosity, channel number, slope, and bed grain size, and divides rivers into eight main classes and 90 types (The Federal Interagency Stream Restoration Working Group, 2001). The entrenchment ratio is defined as the ratio of flood-prone width to bankfull width, which was simulated using HEC-RAS and GIS (Kheirizadeh et al., 2018). The width-to-depth ratio represents the channel width at bankfull discharge divided by the mean depth, calculated for a 2-year return period (Rosgen, 1994). Sinuosity is determined as the ratio of stream length to valley length. Bed material grain size was assessed through Wolman pebble count methodology and volumetric sampling.



Result and discussion

Evaluating channel morphology is one of the most essential components of river management. In recent years, river classification and morphological assessment have increasingly focused on watershed management and river restoration, with the Rosgen hierarchical classification system now widely recognized as one of the most important and effective approaches. This study evaluates the geomorphological characteristics of Givi-Chay river channel through an integrated approach combining Rosgen’s classification model and HEC-RAS hydrodynamic modeling. Givi-Chay is one of the most important sub basins of the Ghezel Ozan River, playing a significant role in water supply for the counties of Khalkhal and Givi in Ardabil Province. Based on geomorphological characteristics, this river was divided into four reaches in the downstream direction: Khalkhal, Givi Dam, Givi, and Firozabad. The Khalkhal reach has an extensive floodplain with developed meandering patterns. Based on the Rosgen classification system, this reach was categorized into two main types (C and E) and four sub-types (C5b, E6b, E5b, and C4). All river types in this system exhibit high vulnerability to both natural and anthropogenic disturbance. Anthropogenic modifications to channel form and fluvial processes have been extensive near Khalkhal city, resulting in multiple disturbances to the river's natural functioning. In the Givi Dam reach (Reach 2), the river was classified into two main types (B and C) and four sub-types (B4, C5b, B3, and B4) based on the Rosgen classification system. The C5b-type reach, extending approximately 4.4 km near Anaviz village, displays a relatively wide floodplain and well-developed meandering patterns, characterized by high width-to-depth ratios and significant entrenchment. The C5b-type reach, extending approximately 4.4 km near Anaviz village, displays a moderately wide floodplain and well-developed meandering morphology, featuring high width-to-depth ratios and significant entrenchment characteristics. The remaining sections of this reach are classified as primary Type B. Two dominant controls explain this reach's morphology: canyon confinement and non-existent floodplain development. The banks of this river type exhibit stable to moderately stable conditions. The Givi reach contains three primary channel types (E, C, and D) under the Rosgen classification system, with downstream sub-types occurring in the following sequence: E5, C5, D4b, C4, D4, and C4. This reach features an extensive floodplain with quaternary young alluvial deposits. D4b and D4 types are classified as braided rivers, characterized by wide channels, abundant bars, high erodibility, and unstable banks. The extensive floodplain, confluence of multiple high-discharge rivers, and presence of erodible bank materials constitute the primary controlling factors for Type D channel formation in this reach of the Givi-Chay River system. Ultimately, the entire Firozabad reach is classified as Type B, which downstream differentiates into three subtypes: B4, B3a, and B3, formed in response to geological structure controls. In this reach, the river valley is narrow and confined by resistant volcanic rocks.



Conclusion

In this study, the Givi-Chay River was analyzed using Rosgen's hierarchical classification system and the HEC-RAS hydraulic model. The river was divided into four reaches: 1) Khalkhal, 2) Givi Dam, 3) Givi, and 4) Firozabad. The Khalkhal reach (Reach 1) is situated within a developed floodplain and exhibits a meandering pattern, though lateral channel mobility has been constrained by anthropogenic modifications. This reach was classified into Rosgen Classes C and E, specifically including types C5b, E6b, E5b, and C4. The C4 and C5 channels exhibit high sensitivity to disturbance but demonstrate adequate recovery potential, with vegetation cover playing a critical role in their stability. In this reach, establishing riparian parks could significantly contribute to channel restoration. The Givi Dam reach (Reach 2) is predominantly mountainous with narrow valleys, and was classified into Rosgen Classes B and C (types B4, B3, and C5b). Type B3 channels exhibit low sensitivity to disturbance and high recovery potential. The Givi reach (Reach 3) features an extensive floodplain and is classified into three primary Rosgen classes (E, C, D), including the following subtypes: E5, C5, D4b, C4, and D4. The presence of braided patterns (types D4 and D4b) in this reach indicates high channel mobility and high sediment supply. This reach exhibits high sensitivity to disturbance and requires riparian zone management. The Firozabad reach (Reach 4) was classified as Rosgen Type B, including subtypes B4, B3a, and B3. In summary, the results demonstrate that the Givi-Chay River system exhibits disturbance-sensitive channel classes with variable recovery potential. The integrated application of the Rosgen classification system and HEC-RAS modeling proves effective for analyzing morphological conditions and developing river conservation/restoration management plans. By understanding how these factors interact and change river morphology, we can predict future river development trends and provide a scientific basis for river management and conservation.



Keywords: Fluvial Geomorphology, Rosgen stream classification, Geometric Channel Parameters, The Givi Chay River