Estimating the peak discharge by means of boulder size of river beds of Zirabe and Qoordanloo catchments

Rivers، based on their power، erode loose materials from higher elevations، transport and finally deposit them in lowlands. Extensive parts of earth surface are influences by surface runoff and their erosional operations. These dynamic factors، based on other characteristics of land surface، affect differentially different parts of land surfaces. Many researchers have tried to establish some methods to estimate peak discharges of rivers without relying on precise observed data of rainfall. These methods include combination of geomorphological factors (i. e. high water mark) and sediment logical data (i. e. boulder size). The lack or shortage of hydrometric formations and statistics in drainage basins of country necessitate the use of suitable experimental methods for estimating maximum flood. The aim of this research is to estimate the rate of peak flood by means of boulder size of river bed (at catchments outlet) and to evaluate the effects of geological formations and geomorphological landforms on flood of Zirabe and Qoordanloo catchments.

In this research، required maps such as height classes، slope، geology and drainage system were extracted based on geological and topographical maps in ARC Map software. The analysis of morphometric characteristics of studied basins has been carried out based on above-mentioned maps. To estimate peak flood at basins outlets، first، data associated with channel morphometry such as area، cross section، wetted perimeter، hydraulic radius and channel were measured. Then water velocity in channel were estimated based on Costa (1983) and Jarrett (1990) methods. The Costa method of velocity estimation is based on the measurement of boulder size of river bed. To estimate flood velocity based on Costa method، the 24 boulders of the largest ones in every river bed at catchment outlet were obtained. Then the mean of b axis (width) of boulders were calculated. Then the flood velocity was obtained based on the following equation: V: 0. 18 (d) 0. 487 Where V is velocity in cubic meter per second، d is the mean of b axis of boulders. The velocity in Jarrett method is calculated based on the following

V: 3. 17 R0. 83 S0. 12 Where V is velocity in cubic meter per second، s is slope of channel I percent، R is hydraulic radius in meter. The peak discharge of flood was calculated from multiplication of channel cross section area and velocity.

Results of this study reveal that peak discharges estimated from Costa method are respectively 17. 29 and 78. 04 m3/s in Zirabe and Qoordanloo catchments. The peak discharges estimated from Jarrett method are respectively 35. 68 and 49. 64 m3/s in Zirabe and Qoordanloo catchments. Data show that the estimated peak discharge obtained from Costa method are closer to observed data implying that Costa method (estimation of peak discharge based of boulder size) has adequate performance. Analyzing geology of studied basins show that 60% of Zirabe catchment’s area is composed of Tirgan calcareous formation while the mentioned formation comprise only 7. 2% of Qoordanloo catchment’s area. Formationss of Qoordanloo catchment are mainly impermeable rocks such as shale، marl. Due to the higher area of calcareous formations capable of karstic landforms development in Zirabe catchment، the surface runoff and hence the drainage density decrease. The lower drainage density of Zirabe catchment (2. 5) compared to Qoordanloo catchment (3. 69) reveals the lower runoff generation and the more infiltration of water into ground in Zirabe catchment whereas the drainage density is higher in Qoordanloo catchment with impermeable formations implying the higher runoff and hence flood peak generation in mentioned catchment. The mean topographic slope of Qoordanloo and Zirabe catchments are 20. 7% and 12. 2% respectively. Evaluation of catchment’s shape data show that the Qoordanloo catchment is more circular in shape than Zirabe one. It seems that the steeper slope and relatively circular shape of Qoordanloo catchment have resulted in higher flood peak of mentioned catchment.

Data analysis reveal that، in spite of larger size of Zirabe catchment than Qoordanloo one، the mean of b axis of boulders is larger in Qoordanloo river bed than Zirabe one. The larger size of boulders in Qoordanloo river bed is indicator of higher water strength and velocity. The higher rate of estimated and observed peak discharge in Qoordanloo catchment compared to Zirabe one demonstrate that Qoordanloo catchment is more h vulnerable to flood damage. The presence of impermeable formations in Qoordanloo catchment has resulted in the formation of badland erosion that is indicator of higher flood vulnerability and runoff generation while the karstic features are dominant landforms of Zirabe catchment which has played an important role in the reduction of runoff generation. Overall، it can be concluded that firstly، the Costa method has adequate performance in peak flood prediction in studied catchments and secondly، the differences in geological، geomorphological and morphometrical characteristics of catchments have strong effects on flood differences of catchments.