Streambank stability analysis using geomorphic rapid assessment techniques, case study: Saymareh river, Ilam province, Darreh Shar county

Stream bank erosion can have many geomorphic effects in the river. The erosion of stream bank causes destruction of the river banks, increase the sediment load, river pattern changes, deposition in channel, reduce water quality and decrease of river habitants. In fluvial geomorphology, a set of objective indicators are used for the rapid measurement of river geomorphological characteristics which are known rapid ggeomorphic assessment (RGA). Using these techniques, Effective factors in the occurrence of geomorphic forms on the ground are scored And the sum of the points of all factors describes the conditions. In this research, the stability and instability of the stream bank using RGA methods in the Seymare River have been studied. The study area is part of the Seymareh River with a length of 18 km that located downstream of the Seymare Dam (Dare Shahr county in Ilam province). One of the most commonly used RGA techniques is the channel stability index (CSI). The CSI was proposed by Simon and Klimetz (2008) and requires measuring the bank height, bank face length, average diameter of streambed sediment, bed and bank protection, degree of constriction, streamnank erosion, evidence of mass movement, riparian vegetation, deposition in near bank and stage of channel evolution model. Each index is scored and sum of score is used to categorize each stability category of stream reach include: 20 is considered highly unstable.
The Oklahoma Ozark streambank erosion potential index (OSEPI) was developed by Heeren et al (2012). In this method, eight factors, including the bank height, bank angle, Percentage of Bank Height with a Bank Angle Greater than 80o, evidence of mass movement, bank Unconsolidated Material, streambank protection, riparian woody-vegetation and Stream Curvature are scored. According to OSEPI, the sum scores of 0-25 are highly stable, 35-26 moderately stability, 45-36 stable, 55-46 unstable, 65-56 Moderately Unstable and 85-66 highly unstable. In the study area, 14 reaches were investigated and indicators of both methods were measured.
Results Substrate sediments at all reaches were in gravel size and the bed and bank had no artificial protection. The height of the critical banks has varied from 6 to 34 meters due to its proximity to the to the alluvial terraces. It is observed evidence of mass movement landslide in most reaches. Riparian vegetation (tree and shrub)was found in 25 to 75 percent of the river corridor. Bank attach sedimentary forms have been found in 10 to 75 percent of the length of the reaches. in terms of the channel evolutionary model, the most of reaches are in stage 5.
In the CSI method, the total score of the river reaches varies from 14 to 18.5 and all reaches are falled in the moderately instability group. The total score of indicators in the OSEPI method varied from 47 to 60, Which 86% of the reaches are being in unstable group and 14% fall in Moderately Unstable class. In order to investigate the factors affecting the instability of river bank, the scores of each index were calculated in relative score in both methods. Based on the average relative scores of each index in the CSI method, the most influential factors on the bank instability are the bed incision, evidence of mass movements, bank erosion and channel evolution model index with scores of 0.82, 0.77, 0.75 And 0.75 respectively ranked first to fourth. In the OSEPI method, bank heights, bank Unconsolidated sediments, evidence of mass movements and riparian vegetation with scores 1, 0.64, 0.6 and 0.56 were the most effective indexes. Relationships were calculated between CSI and OSEPI based on all 13 reaches with an R2 (determination coefficient) of 0.71. The results of the scoring of the indexes and relative scores of factors in the streambank instability show that the OSEPI method provides more suitable indicators and its descriptive division is better than the CSI method.
These methods show the current conditions of the reaches in terms of stability and instability and they aand they can not predict instability in the future. The instability of river bank is different in regions depending on the geological, climatic and hydrological conditions. Therefore, they can be used to modify some of the indicators.