عنوان مقاله [English]
River bank instability, erosion and sediment transportation are natural processes in rivers, causing destruction of agricultural fields and human infrastructures. The channel bank erosion is one of the sources of sediment production in rivers. The bank erosion causes not only increase in the sediment yield but also the river instability as well as change in the flow regime and channel pattern. During recent decades, sediment load and river bank instability have raised major global concerns, and a lot of expenses have been allocated to the river bank stability. The occurrence of occasional floods and the erosion of channel bed and banks cause great damages to the residential areas around streams every year. Therefore, the river bank protection against erosion is of great importance in environmental planning. Destruction and failure of river banks is associated not only with the type and intensity of erosion, but also with the bank properties such as the shape of structures in banks as well as the mechanical characteristics of the constituent materials of banks. So far, some researches have evaluated the stability and instability of river banks based on different parameters. The aim of this paper is study the potential of river bank erosion risk and also instability in the Bidvaz River. The Bidvaz River is a branch of Kalshor River located in the North Khorasan Province.
In this study, topthe 1:50,000 topographic maps of Iranian National Geography Organization were used to extract topographic data, and the 1:100,000 geological maps of Geological Survey & Mineral Exploration of Iran were used to obtain geological data. The identification of geomorphological landforms of the study area was carried out by the Google Earth™ images and fieldworks. Evaluation of the river bank erosion was determined by the Johnson (2006) method. In this research, a segment of Bidvaz River with a length of 6 km was selected and 9 cross-sections were surveyed to analyze the instability of the channel. In the Johnson method, 13 indicators were used to determine the stability of river channel stability: (1) watershed and flood plain activity and characteristics; (2) flow habit; (3) channel pattern; (4) entrenchment/ channel confinement; (5) bed material; (6) bar development; (7) obstructions; (8) bank soil texture and coherence; (9) average bank slope angle; (10) vegetative or engineered bank protection; (11) bank cutting; (12) mass wasting or bank failure; (13) upstream distance to bridge from meander impact point. The score (ranging from 1 to 12) for each indicator was determined and, based on the sum of scores from all 13 indicators, the overall stability of the channel was obtained in four classes: excellent, good, fair, and poor. The scoring to parameters was based on the fieldworks, satellite imagery, laboratory tests and on the expert’s knowledge and experiences. In order to evaluate the river morphology, a total of 9 topographic cross-sections were prepared using a theodolite. Finally, the rate of channel instability of 9 sections was analyzed.
Results and discussion
The result of this study shows that, in section 1, the flow habit (parameter 2) and watershed and flood plain activity and characteristics (parameter 1) are the most influential factors on the channel instability, whereas the upstream distance to bridge from meander impact point (parameter 13) has the lowest effect on channel instability. Overall, parameters 1 and 2 are effective factors in channel instability in all sections. Nevertheless, the two mentioned parameters are not the most important in all sections. For example, in section 2, vegetative or engineered bank protection (parameter 10) is the most important factor in channel instability, whereas the most effective parameter in channel instability in section 3 is the entrenchment/ channel confinement (parameter 4). Result of this study reveal that some parameters had less or without effect on channel instability. For instance, parameter 13 had the minimum effect in channel instability in sections 1, 2, 3, 4, 5, 8 and 9. In section 6, parameters 4, 5, 8, and 12 are not important factors in channel instability. In section 7, vegetative or engineered bank protection (parameter 10) had the lowest impact on channel instability. Overall, the results of Johnson method and soil texture test show that the Bidvas River is inherently susceptible to bank erosion. Nevertheless, the rates of channel banks and bed erosion increase from section 1 (upstream) towards section 9 (downstream).
The study area has an arid and dry climate in which the irregular and intense precipitations prevail. The Bidvas is a seasonal river and the occasional flooding is one the marked characteristic of this river. The occurrence of occasional floods and the significant variations of its discharge in Bidvas River have resulted in the erosion of river banks and bed. Data show that the instability of Bidvaz channel varies in its different sections. Results of Johnson method demonstrate that the rates of channel banks and bed erosion increase from upstream downstream. The higher rate of instability in downstream sections is attributed to the higher slopes of channel banks and their lower soil coherence. Among 13 parameters evaluated in the Johnson method, the flow habit (parameter 2) and watershed and flood plain activity and characteristics (parameter 1) had more effective effects in channel instability than other parameters, in all sections. The parameter 13 (upstream distance to bridge from meander impact point) had the lowest impact in channel instability.
Keywords:channel instability, Johnson Model, Bidvas River