Quantification of Geomorphological Changes of Sefidroud River and Investigation of Their Relationship with Hydraulic Characteristics of Flood Events

IntroductionRivers are dynamic geomorphological systems that continuously change due to various influencing parameters.Morphological changes in the river channel primarily involve adjustments in channel width, depth, slope, and river planform.The extent of lateral migration of the river channel depends on factors such as bank resistance to erosion, the duration and magnitude of flow, the curvature radius of the channel, and the flow capacity to transport sediments.Quantifying geomorphological changes in rivers with an emphasis on flood hydraulics requires detailed analysis and complex modeling of the dynamic interactions between water flow, sediment deposition, and riverbed alterations.Flood severity, characterized by high discharge, significant flow depth, and extreme flow velocities, can have substantial impacts on riverbed structures, including changes in depth, width, channel meanders, as well as sediment transport and deposition.The Sefidroud River is one of the largest rivers in Iran, directly and indirectly affecting the lives of a significant population.Throughout history, there has been a strong inclination to settle near this river, with numerous urban and rural settlements located along its course.The occurrence of frequent floods in the past and the river's tendency to alter its channel, especially in the lowland regions, make the delineation of floodplains and prediction of future river behavior essential aspects of this research.Therefore, identifying and quantifying the geomorphological changes of the Sefidroud River and investigating the relationship between these changes and flood severity during mid-term return periods are key objectives of this study. 2-ResultsIn the present study, four indices were calculated and used to quantify the river's morphological changes: channel braiding index (B), channel sinuosity index (P), lateral migration rate, and channel stability.The study reaches showed minimal variation in terms of the braiding index, with the Yasaval reach having the highest value, which is 0.07 higher than the Astaneh reach, which had the lowest value.The lower braiding indices of the Gilvan and Astaneh reaches, compared to Yasaval, can be attributed to the higher severity of human activities in the riverbed and the presence of the Sefidroud Dam upstream of the Astaneh branch.Regarding the channel sinuosity index (P), there is an inverse relationship between flow severity and shear stress with the sinuosity index.Reaches with lower flow severity and reduced shear stress tend to have higher sinuosity.Concerning the lateral migration rate, the results showed that this index is strongly influenced by the morphology and topography of the riverbanks.In the Yasaval and Gilvan reaches, which flow through mountainous regions, the lateral migration rate is lower (1.3 and 1.93, respectively) due to the rougher and more restrictive topography.The equations related to channel stability and migration rate indicate an inverse