Investigation of Root Tensile Strength of riparian TAMARIX Trees in direct River flow and beech Slope in Order to Use in Soil Bioengineering

Changing the morphology of rivers, associated with erosion and sedimentation, has resulted in damaging riverside structures and loss of valuable agricultural lands. This process occurs in long length of rivers, which structural protection is virtually impossible due to high costs. Today, the use of bioengineering techniques (non-structural) is increasingly growing, due to economic and environmental sustainability. Lack of technical knowledge in biological characteristic of plant species is one of the important limitations of the bioengineering techniques in the protection of riversides. The matrix root and soil increase the soil shear resistance. The Wu (1976) model was employed to calculate the increase shear resistance. To apply this model, determination of the root tensile strength is required. The pilot of research was located 12 Km from Darashahr city, inside of Vahdatabad village, on the Seimareh riverbank, Ilam Province, Iran. Six trees (from Tamarix species) were randomly selected on the river bank by a distance about one km. The annual precipitation, temperature, evapotranspiration, relative humidity and wind speed in this region are 442 mm, 20.2 Cₒ, 1874.2 mm, 61.7% , 3.4 m/se and 122 m3/se, respectively. This zone is located in the semi-arid climate. The area of watershed is about 28.5 km2. Circle profile trenching method was employed to obtain characteristics of root system in direction of river flow and the riverbank slope. The trees were selected on the left riverbank. Circular trenches were dug at a distance of half a meter from the tree stem. The maximum depth of each tranches was 1.5 meters. Surface of the trenches were divided into four equal quadrants. To define the quadrants, X-axis was selected in the direction of flow and Y-axis in perpendicular to the river flow. Then, the quadrants were named in trigonometric direction. In this way, the first and second quarters were located on the upslope and the third and fourth quarters on the downslope. As well as, the second and third quarters were located in upstream and the first and fourth quarters in downstream. For testing tensile force, the sample roots were selected from each quadrant. The roots were cut in 20 cm length, and were kept in a solution of 15% alcohol. Before testing, root diameter in three points of the beginning, end and middle were measured. Using the arithmetic average diameter, cross-sectional areas of roots were calculated. Tensile force was applied on roots sample with 10 mm/m velocity. Root tensile strength was calculated through dividing the amount of tensile force at the moment of rupture (on the cross-section of root,) by the area (according to the mean root diameter). Hundred and forty eight root tensile tests were performed. Seventy three percent of them were successful. The main problem in these tests was the root slip and rupture in between the clamps. For the larger dimeter of roots likelihood of slipping is increasing. The minimum and maximum root diameters were tested, which were 1 and 8.2 mm. The minimum and maximum tensile forces were 16.1 and 1063 N. The relationship between the diameter and rupture force was a positive power function. Its coefficient of determination (R2) was equal to 0.92. In other words, by increasing the diameter of the root, the amount of force needed to rupture is increasing. In this way, the minimum and maximum root tensile strength was 6.1 and 44.7 MPa, respectively. The relationship between the diameter and tensile strength can be described as a negative power function. Its coefficient of determination (R2) was equal to 0.39. SPSS software was used for data analysis. Comparing the tensile strength of roots in the upstream and downstream slope and flow, Mann-Whitney and Kruskal-Wallis test were used. The result showed that average root tensile strength was not significantly different on the up and downslope and also by direction of the flow. Although the effect of flow and slope on root's tensile strength was slight, the effect of slope was more than flow. However, the average roots tensile for each tree was significantly different. The root tensile strength values decreased with diameter according to a negative power equation. In conclusion, the results suggest that it is likely the condition of environment and genetic characteristics of trees affects tensile strength. These findings may help to improve the application of soil bioengineering techniques in the protection of riverbanks.