The Effect of Physical and Mechanical Soil Properties  And Water movment on Bank Stability of Shatt

AL ـ Arab (Sundibad ـ Umm-Alrissass)

N. KH. M. Albahili

Marine Science Centre/ University of Basrah-Iraq

(E-mail: hartha 1955@ Yahoo.com)
 

Abstract

     The sigment of Shatt Al- Arab river, which lies between the Sundibad and Umm- Alrissass islands has been selected as a study area. Four stations are selected on the river to study the banks stability. The geometric properties of the banks are measured, besides the physical and mechanical properties of the bank soil at the depths of 0-50cm, 50-100cm, 100-150cm and 150-200cm. Nine river cross sections are selected in order to determine the annual erosion rates of the banks as well as some of the hydrological properties of Shatt Al- Arab river, wave character generated by river traffic, and vegetation.

    The results showed that there are variations in the soil properties between the main stations at different depths of each station, in which the soil texture ranges between clay and silty clay. It has been also found that there are variations in the values of the porosity , natural moisture content, wet bulk density, plasticity index and the shrinkage properties. The percentage of compaction did not exceed the maximum level, whereas, the natural moisture content exceeds the optimum moisture content for the compaction in all stations. At the same time, the over consolidation ratio values approached to one unit in those stations, and the consolidation of the soil banks is normal. The bank soil had been found to have medium compressibility.  Also the results proved that the vegetation have a clear effect on the cohesion of the bank soil and consequently  on its erodibility.

      There is a variance in the values of the flow shear stress, the maximum value has been remarked at Hamdan region, while the minimum value has been recorded at Abu Flus region. Some fluctuation in the ground water level of the banks depending on the hydrological system of Shatt Al- Arab river and other interference factors which have been remarked. Also this study has been proved that there are many factors affecting wave character generated by the river traffic, such as the boat geometry, boat speed, the distance between the sailing line and the river bank as well as the boat direction relative to the flow direction.

      The all sides of cross sections of the stations except the west side at Maqaal region have been exposed to sedimentation process. The highest degree of the bed erosion has occurred at Hamdan region, while the lowest degree has been shown at Abu Flus region. Also  there are some fluctuation in the bed level along the longitudinal section of the  studied river. The lateral linear erosion rates of the banks has been  varied, the highest value of these rates is at the period between May and June, while the lowest value at the period between December and February. The annual total rate which was calculated by the pin method is 0.26 m.year­¹  and 0.24 m.year­¹ by the field reference method. The bank failure of the block failure type is common, and the cantilever failures are widespread, whereas the rate of the soil volume removed from the banks in the form of blocks was 0.38 m ³.year . All of the bank slopes which are selected for this study are in the stable state, but they vary in the degree of stability.

    The cohesion property  plays  the main role in participation with the height of the bank slope to specify the degree of Shatt Al-Arab bank stability. This property has been effected by the flow stages of the river and the ground water level of the banks. Therefore, some prediction  equations have been found in this study in order to know the stability state under different conditions which effect the bank slope stability.

Unpublished Ph.D. Thesis, University of Basrah, College of Agriculture, 2006