Comparison and Evaluation of NBS and BEHI Model's Results in Bank Erosion of Hashtrood Qaranqoo Chai River

Introduction River bank erosion has an important role in the instability and damage to agricultural lands and facilities around the rivers and it is a significant source of sediment in fluvial systems. The river bank protection against the erosion is considered as one of the main objectives of river improvement in sustainable development of water resources. The erosion of the river banks is causing damage to agricultural land, damage to structures such as bridges and roads, widening of stream channel and environmental issues considerably. Bank erosion is a major cause of non-point pollution of water resources and increased sediment load in many rivers. Not only increasing banks erosion does increase the sediment load but it also causes river instability and changing flow and channel pattern. Thus, during the recent decades, sediment load and river banks have been created a large concern in the world and the large sums have been spent to stabilize banks. Hence, river bank erosion and channel changes are social, environmental and economic issues that often cause irreparable damage to people and infrastructure in the river banks.
Material and Methods Qaranqoo chai catchment (upstream of Sahand dam) is located on the southwestern of Hashtrood city (located in 37° 43 to 37° 20 northern latitude and 46 58 to 46 28eastern longitude). The catchment is located on the east Azerbaijan province.In this reach, we used Bank Erosion Hazard Index (BEHI) and Near-Bank Stress (NBS) to predict annual bank erosion in QaranqooChai River. Therefore, nine cross-sections were selected and some parameters were measured for BEHI, parameters such as bank full width, average bank full height, root depth, root density, bank angle, surface protection, and bank material and stratification bank were measured. Bank full height is the distance from bank toe to bank full stage elevation. Bank height divided by bank full height gives a bank-height ratio. Rooting depth is measured from the top of the bank to the bottom of vegetal rooting. Rooting depth is then divided by bank height to get root-depth ratio. The weighted root density calculation begins with a visual estimate of root mass, per unit volume of soil. Bank angle is the angle of the bank face along the elevation plane of the bank. Surface protection is estimated as the percent of bank covered by vegetation, woody debris, boulders or manmade materials. An open bank face has 0% protection while a fully vegetated bank has 100% surface protection. Bank material may affect a bank’s susceptibility to erosion. If bank material is medium or large cobble, ten points are subtracted from the total BEHI score. Five to ten points are added for gravel, a mix of gravel and small cobble, or a mix of gravel and sand. Sand, or a predominantly sand mixture requires the addition of ten points. No adjustment is made for cohesive silt or clay bank material. Banks of bedrock or boulder are always scored as to be very low. Indeed BEHI incorporates bank variables that are factors in entrainment, surface erosion and mass erosion. These variables are bank–height ratio, root–depth ratio, weighted root density, bank angle and surface protection. Variables have empirical values that are in turn, converted to index values and summed for a total BEHI score. Scores are adjusted by bank material and bank material stratification. BEHI scores are then categorized by erosion potentials. A greater score indicates greater erodibility.
2- NBS: Near-Bank Stress (NBS) assessment is important in predicting erosion so that it is associated with energy distribution in channel cross-section spatially stream banks and this disproportionate distribution of flow energy can lead to bank erosion. In NBS method, two levels were used (level 2 and level 5).Ratio of radius curvature to bankfull width (Rc/Wbkf), ratio of near-bank maximum depth to bankfull mean depth (dnb/dbkf).
Results and Discussion According to NBS method and level2, the erosion ranged from moderate to extreme in all of cross-sections except the cross-section 7 (low). Also according to level 5, the erosion was very low in the cross-section one and was extreme in the cross-section two and was low to moderate in the other cross-sections. The results of BEHI method showed that both of right and left banks have erosion so that the erosion risk was moderate to very high in all of the right banks of the cross-sections and was only very low in the cross-section 4 and also the erosion risk of the left banks have been estimated to be very low to extreme in the cross-sections.
Conclusion According to the results of both models, in general erosion in the period under study is the important factor at sediment production in QaranqooChai river and bank erosion risk. Also, both methods used to analyze and evaluate this type of erosion show that in general, due to the exacerbation of the hydraulic stress on the external side the amount and risk of erosion for the outer sides of the arches in the area of the meandering river pattern is more than the opposite side.
Similarly, the degree of risk of erosion is reduced on both sides of the banks in the middle section of the studied reach. In the last section of the studied reach, the risk of erosion is increased by both models. This study showed that the river pattern, vegetation, and marginal materials play a crucial role in maintaining the bank stability in this area. Based on field observations, the results of the model of the Bank Erosion Hazard Index (BEHI) in the QaranqooChai River is more consistent with reality.