Morphological changes in alluvial and coastal plains (Case example: Bahmanshir River)

Introduction

The river system and its environment is one of the most productive areas with various fertile morphological units, including alluvial and coastal plains, which are subject to changes from time to time. An alluvial plain is a large flat land formed by the deposition of sediments over a long period by rivers flowing from the highlands. Due to the fertility of the soil (Sarkar et al., 2022), most of the alluvial plains around the rivers are used for cultivation activities. However, in recent years, the conversion of traditional agricultural land into economic activities has become common around the world. Development activities are a threat to the geomorphic dynamics of the river

After reaching Khorramshahr, the Karun River divides into 2 branches, the first branch flows into Arvandrud, then Arvandrud also flows into the Persian Gulf. The second branch forms the Bahmanshir River, which is about 90 kilometers long and carries the water of Karun towards the Persian Gulf. It has been the place of production of the best dates in Iran. In recent years, due to unprincipled dam building, drought and traditional agriculture, and the entry of salty and toxic wastewater from some agricultural fields upstream of Karun, there have been many problems, including the decrease of the water level in Behmanshir and the dominance of sea salt water over the river water level, and as a result The high salinity of this river has a negative impact on agriculture and groves in the region. Due to the sharp decrease in the water entering this river and the decrease in water flow and lack of timely and continuous dredging, the sediments have caused the depth of the river to decrease for several years.





Methodology

This study has adopted the widely used approach of using temporal satellite images of the region to map morphological landforms and their changes through geospatial techniques. The dynamics of the river bank was studied using the DSAS model to determine the locations of erosion and sedimentation.

The morphological features interpreted with details such as geographic locations, unique identifiers, area in square kilometers and landform class were determined and prepared as a spatial database in GIS format. Then, the spatial extent of different classes' characteristics was calculated and the changes in their spatial extents were evaluated for different periods of time by human-caused activities. In this study, transects were considered at every 50-meter distance. Since the meandering of Bahmanshir River is high, close transect distance was considered for better river line change data. The model runs the rate of change of the historical data set from the baseline and provides various parameters such as net shoreline motion (NSM), linear regression rate (LRR) and endpoint rate (EPR). The obtained statistics were calculated for classification of erosion and accretion.

Based on this, the erosion and sedimentation and the stable places of the river bank during the studied period were obtained. Changes in the spatial area of the river bank were quantified for further analysis of the study.

Results and Discussion

The pre-processed satellite images of 1973, 1986, 1999, 2012 and 2024 were classified using maximum likelihood classifier (MLC) and other morphological features were mapped in the alluvial plain and coastal plain around Bahmanshir river. These features include catchments, swamps, raised lands, deltas, salt marshes (active and abandoned), floodplains, and alluvial areas, as shown in Figure 2. The morphological characteristics identified in different time periods in the alluvial and coastal plains of the river have been determined. The total spatial extent of the alluvial plains around Bahmanshir River is about 857.4 square kilometers, as shown by the research. Alluvial plains were mainly used for agricultural lands, settlements, and other natural ecosystems such as salt marshes, mudflats, and morphological features associated with rivers such as floodplains and deltas. Figures 3 and 4 show the disproportionate changes in geomorphological forms and forms in the alluvial and coastal plains of Bahmanshir River in the studied time periods. The rapid conversion of alluvial plains to salt marshes from 3.3% in 2012 to 19.1% in 2024 or 13.5% in 1973 to 27.5% in 1999 is among the notable cases. In all time periods, the swollen lands have the largest area in the region.



Conclusion

In this study, the changes in the alluvial and coastal plains of Bahmanshir River for the period from 1973 to 2024 have been analyzed using Landsat satellite images in the autumn season. This study showed that both alluvial and coastal geomorphic landforms have changed under the pressure of human activities over the years, especially after 2012. So that the volume of alluvial sediments has increased from 97.3 square kilometers in 1973 to 108.8 square kilometers in 2024. And the volume of flood sediments has increased from 73.7 square kilometers in 1973 to 84.3 square kilometers in 2024. The results of the study showed that the banks of the Bahmanshir River in the alluvial plain have faced significant erosion in recent years due to the increase in agriculture and built-up areas such as settlements and industries. While in the coastal plain, the place of sedimentation has increased significantly in recent years due to the increase in the suspended load of the river and the retreat of the sea water. The erosion/sedimentation situation of the river bank showed that the right bank of Bahmanshir river is significantly eroding due to relatively more human activities than its left bank. Regarding the impact of alluvial and coastal plain changes on nearby villages, the study showed that 4 out of 7 villages have lost their alluvial lands due to human activities with more than 40%. This study is a combination of evaluating morphological changes of fertile landforms caused by human activities, including river bank dynamics. The result of this study can be used as an indicator to evaluate the socio-economic effects of dependent local residents and can also be used as a key input to draw up effective programs to reduce the deformation of river-based fertile lands to ensure their sustainability