عنوان مقاله [English]
Erosion and sedimentation are highly important in geomorphological studies. Accelerated soil erosion causes adverse agronomic, ecologic, environmental, and economic effects both on-site and off-site. The main on-site consequences involve the reduction in soil productivity, while the off-site consequences are mostly due to the sediment and sediment-associated chemicals transported from the source into surface waters. Nowadays most researchers investigate the sediment particle size distribution in river systems from different standpoints in order to understand the hydrodynamic behavior of suspended sediments and to calibrate transfer models and sedimentation in flood plains and channels. Water spreading system is a mechanical method to control flood in the plain in order to recharge of aquifer and increase the groundwater level. Suspended materials in floods can be changed the sediment and soil texture and structure and increase the depth and fertility of soils on which these floods flow. Hence, it is necessary to investigate these changes and their process so that an appropriate method for increasing the efficiency of these plans can be presented. Despite the importance of sediment particle size distribution, no studies on this subject have been done in Iran. Therefore, the main objective of this study was to investigate the effect of water spreading system on sediment particle size distribution watershed, alluvial fan, and water spreading system of Zahab Plain.
In order to study spatial variation of sediments particle size distribution four samples were taken from the sediments of each of the 9 water spreading channels, and 5 samples were taken from the sediments of the main valley, active channel and inactive alluvial fan. The sediment particle size distributions were determined using dry sieve method. The particle sizes in different site sampling were compared using one-way ANOVA. Finally, the within comparison were done using Tukey-HSD method. test was used.
Results and discussion
The distribution of coarse sediment particles
Multiple comparisons with Tukey-HSD method indicated that the groups have a significant difference. The amount of sediment particles with diameters of 16, 4, and 2 mm in the flood distribution channels are approximately zero due to the existence of stilling basin prior to floodwater spreading channels. On the other hand, due to the smallness of the basin, the steep channel, the steep-walled valley, lack of vegetation, dry climate and the low depth of soil in the upstream basin can be considered as the main factors increasing erosion and coarse sediments’ particles in the main valley, active and inactive channels. The lack of significance for the diameter of 1 mm can be attributed to the increase of this diameter in floodwater spreading channels as a result of stilling basin and meeting the relative conditions for the sedimentation of particles in the channels. On the other hand, the decrease in the number of sediment particles in the main valley and the active basin can be attributed to the direct relation of particle size distribution with the portability of sediment flows.
The distribution of fine sediment particles
Multiple comparisons showed that the amount of sediments in diameters 0.5, 0.25, 0.063 mm was significant. Only in diameters larger than 0.063 mm it was not significant. The results showed the increase of average in floodwater spreading channels due to the existence of stilling basin prior to channels which led to the sedimentation of most coarse sediment particles in them and the movement of particles not sedimented in the stilling basin toward the channels. Since in the main valley and the active basin as the particles become smaller they are more easily transferred with the least amount of water force because of their small weight, they show no conspicuous change.
The present research was done with the aim of investigating the spatial variation of sediment particles in the distribution basin of Zahab Plain. It can be concluded from the results that particle size is due to the hydraulic force of floodwater. Considering the slope and the power of water flow in the main valley and the active channel and on the other hand the existence of stilling basin and diversion channel toward the spreading project, the sediments’ particle size will certainly vary in different parts. Hence, it can be said that floodwater spreading operations have led to the reformation and lightness of soil of the spreading project. Therefore, the spread of floodwater and sedimentation of suspended materials in floods in rocky and coarse sediment lands of alluvial fan can lead to changes in the tissue and structure and increase the depth and fertility of soils on which these floods flow turning them into suitable lands.