Document Type : Research Article
Authors
1
Associated Professor, Department of Physical Geography, Earth Sciences Faculty, Shahid Beheshti University, Tehran, Iran
2
Ph. D student in Geomorphology, Department of Physical Geography, Earth Sciences Faculty, Shahid Beheshti University, Tehran, Iran
Abstract
The Khoramroud River has undergone significant morphological changes with diverse consequences over the past few decades. Therefore, this study aims to investigate and analyze evolutionary trends, assess the changes that have occurred, and determine the relationship between these trends and their controlling factors across four time periods (2006–2025). The objective is to facilitate effective river management and mitigate adverse impacts in two distinct reaches: the first reach spans 16,700 meters and is divided into six sub-reaches (17 cross-sections), while the second reach extends 17,212 meters and is subdivided into seven sub-reaches (18 cross-sections). For this study, hydrometric data from two stations—Bahram Jo (Section 1) and Cham Anjir (Section 2)—were used, along with topographic maps, Google Earth satellite imagery, field observations, and surveys conducted in these sections to analyze channel dynamics and planform changes. The results indicated that the highest average periodic variations in centerline and channel displacement for both sections occurred during the second time interval (2017–2019). The findings from the first section (2006–2025) revealed a reduction in channel width due to river erosion, whereas the second section exhibited not only erosion but also bank erosion, leading to floodplain formation and subsequent river widening. A comparative analysis of the two sections demonstrated varying degrees of vulnerability in agricultural lands and rural settlements along both river segments. However, the vulnerability level in the first section was significantly higher and more pronounced than in the southern portion of the study area, primarily due to the downstream location of Khorramabad City.
Introduction
In recent decades, intensified human activities and alterations in hydrological regimes have induced significant transformations in river systems. Although natural variations in rivers are essential for maintaining ecological balance, the abrupt increase in anthropogenic interventions has led to extensive modifications in their natural courses. Consequently, most rivers today are undergoing substantial changes in width and depth. These transformations have posed considerable challenges for many countries, affecting policymakers, managers, and the communities dependent on these water bodies. Indeed, the long- and short-term impacts of river system alterations in densely populated regions—particularly in mountainous areas—are especially significant due to land scarcity and high population density. As a result, this issue remains a persistent challenge in management discussions.
Situated in a mountainous region, the Khoramroud River has undergone significant morphological changes over the past few decades, each with distinct consequences. To date, no comprehensive study has assessed the extent of these changes. Therefore, this research aims to (1) examine and analyze evolutionary trends, (2) quantify the magnitude of changes, and (3) identify the relationship between these trends and their controlling factors from 2006 to 2025. The findings will contribute to improved river management and the mitigation of adverse effects.
Material and Methods
The purpose of this study was to investigate morphological changes in the Khoramroud River channel between 2006 and 2025. To achieve this, data were initially collected through physical instruments, field observations, and surveys. The required maps were then generated and digitized using ArcGIS software. Given the length of the Khoramroud River (34 km) and the location of Khorramabad city in the middle of the study area, the research zone was divided into two sections: northern and southern. For a more comprehensive analysis, following the methodology outlined by Rinaldi (2013), the first section was subdivided into six reaches (comprising 17 cross-sections), while the second section was divided into seven reaches (consisting of 18 cross-sections). Subsequently, detailed maps of each section and its reaches were generated using ArcGIS software. Criteria for assessing transverse and vertical changes were then established using satellite imagery and field observations. To evaluate planform channel adjustments, the left and right banks of each river reach were delineated for different years. These delineations were then superimposed to calculate the displaced area, enabling the quantification of erosion and accretion rates for each reach using the River Network Change Index (RNCI).
A positive RNCI value indicates erosion dominance, whereas a negative value signifies sedimentation along the riverbank. To analyze channel dynamics (CA), the channel centerline was delineated for each section and reach across the study years. By aggregating these measurements, the displaced area of the channel was computed, and the extent of interannual change was determined. The data were then processed using Microsoft Excel to generate corresponding tables and graphs. Finally, riverbank changes and channel dynamics in both sections were compared across the study years, and an analysis of channel variations was conducted.
Results and Discussion
In the first phase of the study, based on multi-temporal imagery analysis and field observations, extensive and significant changes were observed in the river’s reaches over a 20-year period (2006–2025). Before major flood events, these reaches experienced erosion, accompanied by a notable reduction in channel width. However, in the post-flood period, a sudden increase in sediment supply and the presence of highly erodible banks led to substantial lateral channel expansion in addition to bed erosion. A comparative analysis across different time periods revealed that the six examined reaches underwent not only bed erosion but also a reduction in channel width of up to ten meters.
In the second phase, the results indicated that the river exhibited distinct behavioral patterns. Prior to major flood events, the reaches in this section experienced only minor erosion, with negligible channel widening. However, during the post-flood period, a combination of factors—including a sudden increase in sediment supply, the removal and thinning of riparian vegetation, and bed erosion—led to significant channel expansion. A temporal comparison of the results revealed that the seven studied reaches in this section underwent not only bed erosion but also a cumulative channel widening of eight meters.
Conclusions
Although channel banks, particularly in alluvial rivers, are seldom stable and naturally fluctuate over time due to channel compaction and erosion, the magnitude of the changes observed in the study reaches during the investigation period suggests an unnatural river condition. Lateral channel migration is a critical process that sustains river ecosystems and constitutes an integral part of normal river dynamics; however, the observed deviations imply potential anthropogenic influences or altered hydrological regimes.
However, given the high land value in this region due to its mountainous terrain, flooding can result in the loss of agricultural land and damage to infrastructure and residential areas. In addition to affecting agricultural land and rural settlements along both river sections, the extent of this vulnerability is significantly greater and more severe in the northern section than in the southern part of the study area. This disparity is primarily due to the downstream location of the city of Khorramabad. Therefore, to ensure more effective and efficient river management in this section, it is essential to identify and regulate areas prone to severe channel changes.
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