Streambank erosion hazard analysis by BEHI method, case study: Sajadroud stream, Mazandaran province

Streambank erosion hazard analysis by BEHI method, case study: Sajadroud stream, Mazandaran province Extended Abstract Introduction River bank erosion is a complex natural process and plays an important role in the dynamic equilibrium of the river. The amount of river bank erosion affects the river plan, cross section and dimensions of the river and is the main factor controlling channel migration and the evolution of the river planform. This research was conducted with two main objectives, which are: 1. River bank erosion susceptibility analysis in incised rivers in mountainous areas, 2. Comparison of river bank erodibility with two methods original BEHI and modified BEHI. Method In this study, Bank Erosion Hazard Index (BEHI) and the modified BEHI method along a part of Sajadrood Stream in Mazandaran province have been investigated. The original BEHI (Rosgen, 1996, 2001) evaluates the river bank erosion field measurements. In this method, several parameters are measured, including bank height, Bankfull height, bank angle, root depth, root density, surface protection, bank material structure and stratification. From this parameters, the bank angle and height, root depth can be measured, but indicators such as plant root density and surface protection are visually estimated as a percentage. The score of each index varies from 1 to 10. The total scores of all indicators are classified into 6 groups: very low, low, medium, high, very high and extreme. Newton and Drenten (2015) Based on the modified BEHI proposed a protocol for estimating the river bank erosion. In this protocol, the lengths of similar riverbanks are first defined as a uniform section of bank. These uniform sections can be identified due to differences in bank slope, differences in bank material and a break in vegetation. The characteristics of uniform sections are then evaluated in a pre-screening questionnaire consisting of six questions. If the answer to two or more questions is "yes", there is a high probability of erosion and the BEHI evaluation will be performed. Otherwise, the measurement will not be taken because the erosion is low or very low. Result and discussion The study area was divided into four reaches and 36 sites were surveyed. These river reaches have a deep bed (incised), high slope, low sinuosity, low width to depth ratio and predominant cobble sediments in the bed. They are in type A3 according to the Rosgen River classification. The height of the banks of the stream is high and its average is 2.6 meters with a minimum and a maximum of 0.4 to 9.7 meters. The average height of bankfull was 0.44 meters and varied from 0.15 to 0.85 meters. Hence, the ratio of bank height to bankfull height has been high. The root depth of plants was low and their average was 0.34 meters. Root density of plants was also low and averaged 5.7% The stream bank angle is measured from 31 to 90 degrees and an average of 51degree. The percentage of surface protection varied between 25 and 65% and averaged 42%. From a total of 36 sites, 8% are in the medium group, 39% in the high erosion category, 45% in the very high erosion group and 8% in the extreme erosion category. Among the various variables, the β coefficients of the surface protection index and the ratio of bank height to bankfull height were -0.62 and 0.51, respectively. To evaluate the modified BEHI method, all reaches were first examined according to the pre-screening table and more than two "yes" answers were confirmed for each reach. In the modified BEHI method, 26 out of 36 sites were in the erosion group. But in the original BEHI method, 16 sites are in a very high class. The total score of the studied sites was evaluated by two BEHI methods with Pearson correlation coefficients, which obtained a coefficient of 0.21 and shows a relatively low correlation. Conclusion Sajadrood stream has high and steep banks due to the incision created in the channel bed. The deposition of large boulders at the toe of the streambank has caused its protection and the flood currents of bankfull are not able to carry this piece of rock. Under these conditions, calculating the ratio of bank height to bankfull height cannot indicate the erodibility of the streambank in the bankfull stage. Nevertheless, the erodible potential of the bank for larger flood currents is confirmed. Comparison of the original BEHI with the modified BEHI showed that the modified method has an overestimate than the original BEHI. Key word: river bank erosion, BEHI, Sajadroud, Mazandaran