Quantitative Assessment of Bank Erosion Hazard in Tarwal River(The upstream area of Hassan Khan gauging station)

Rivers, as a dynamic system, constantly change their morphological characteristics over time to achieve their equilibrium. Stream bank erosion can be a major source of sediment to a stream and possibly threaten nearby lands, bridges, and other facilities. Numerous watershed management practices are introduced to mitigate such potential damages. By the way, to select proper strategies and appropriate activities, it is necessary to select the best practices using the improvement of our knowledge on river condition, morphological changes and trends, and stream bank erosion and sedimentation. Therefore, this study tries to put a figure on the stream bank erosion in Tarwal River.

Tarwal River, one of the main tributaries of Sefidrud basin, is located in the eastern Kurdistan province. In this case study, the upstream area of Hassan Khan gauging station in Tarwal watershed is considered as the study area. It is located between 47 6' 38״ to 47 46' 21״ E and 35 2' 22״ to 35 35' 55״ N. Several methods have so far been introduced for assessing stream bank erosion, but most of them are too complex and it is not easy to apply them in the field. BEHI (The Bank Erosion Hazard Index) as a well-known method introduced by Rosgen (2001) uses stream cross section geometry and field observations, to evaluate stream bank erosion. In this study, stream bank erosion was assessed by complete and modified BEHI in 67 cross sections. These two versions of BEHI index were calculated by assigning point values to some aspects of bank condition using standard table, and an overall score of BEHI index were calculated by summing the scores for each individual metric. BEHI values of all assessed cross sections can be used to prioritize eroding banks for remedial actions, etc. The complete BEHI procedure consists of the following metrics:1. Ratio of bank height to bankfull height 2. Ratio of root depth to bank height 3. Root density, in percent 4. Bank angle, in degrees 5. Surface protection, in percent Modified BEHI procedure uses four of five above-mentioned metrics and does not involve “Ratio of bank height to bankfull height“.

Standard table of BEHI consists of six hazard categories for stream bank erosion; vary from “very low“ to “extreme”. The results of our study show that most stream reaches of Tarwal watershed were taken place in the erosion hazard category of “moderate” and “high”. Stream banks located in the upstream tributaries tend to show higher bank erosion rates while stream banks located in the lower part, where the stream is mainly stable, tend to have the lowest erosion rates. In other words, hot spot reaches for stream bank erosion are mainly located in the upstream areas. These results are in the line with the previous studies of Khaledian and Khedri tarzhan (2005), Ahmadi and Khanjani (2013), Zandi (2015), and Osmani (2015). Vegetation rehabilitation and improvement using native plants with strong roots in stream banks and streambeds are highly recommended to enhance stream bank stabilization and mitigate stream bank erosion. It also could improve stream water quality by reducing the concentration of pollutants and sediments.Analysis of variance (ANOVA) of five metrics used to calculate complete BEHI indices proved that the means of metric scores for “ratio of bank height to bankfull height”, “ratio of root depth to bank height”, “root density” and “surface protection” were significantly different between BEHI erosion hazard categories (p< 0.01) while it was not statistically significant for “bank angle” values.

In this study, we assessed stream bank erosion by complete and modified BEHI procedures in 67 cross sections. Most of the reaches are subjected to moderate-high stream bank erosion hazards. The results of our study showed that complete and modified BEHI indices are useful procedures to quantify stream bank erosion. These procedures can be used for zoning vulnerable reaches of the Tarwal River on the way to take urgent remediation measures for controlling stream bank erosion.Accurate determination of bankfull indicators, as the main challenge of using complete BEHI procedure, requires great experience. Therefore, the modified BEHI is proposed for use by researchers who do not have enough experience in identifying bankfull dimensions.hese procedures can be used for zoning vulnerable reaches of the Tarwal River on the way to take urgent remediation measures for controlling stream bank erosion.Accurate determination of bankfull indicators, as the main challenge of using complete BEHI procedure, requires great experience. Therefore, the modified BEHI is proposed for use by researchers who do not have enough experience in identifying bankfull dimensions