Catastrophic floods are exceptional phenomena that usually return periods longer than the other floods commonly do. The severe hydrological events that on August 2001 and 2002 occurred in Madarsoo river Basin produced extreme floods in the number of sub-basins and the main watercourse. Madarsoo river basin is a sub-basin of Gorganrood in Golestan province. This paper focuses on the types of flood that flow in this basin. These floods had important changes in morphology of Madarsoo basin. Because of the floods, about 300 passengers and residents died in the region and within a few hours important changes were created in the basin landscape. The discharge and the severity of this flood compared to the mean annual flood, and also to the maximum size of instantaneous discharge recorded in five decades for the Basin, were exceptional. The measures such as intensity, the sudden flood phenomenon and what has been left there, such as many pieces of stone with over 1 meter in diameter in the bottom and the sides of the river channels and surface of the fans are reasonable. On the other hand, some injuries on the trees as well as the long-term return periods placed these floods in the group of catastrophic floods. The return period of these floods has been calculated to be about 55 years to 200 years.
Materials and Methods
The methods used in this research are Descriptive-Analytic and causal researches. This research has been conducted using descriptive- analysis. The research tools were mainly topographic maps (scale 1:50000 and 1:100000), Geology maps and satellite images with high resolution (Quick Bird of 2004 and IRS 1998) and field work tools such as GPS were used. A lot of information in the field working and observations and measurements were collected as well as the previous studies. To check the conditions before the flood, aerial photographs (scale 1:55000 in 1955 and scale 1:20000 in 1975 and IRS images in 1998 with digital resolution analysis and software) were used for comparison. For digitalizing and transferring the data and also the required measures, ARC View and ARCGIS softwares were used. Analyzing the data is performed with argument methods and inference with correlation between related variables. The analysis of geologic layers, faults, drainages, the slope and the old zoning of landslides were used as original data. Finally, the analysis of the tools of data analysis was conducted using cases through the comparison of two variables and their influence on floods that raised the problem of research. Then, the method of the occurrence of torrential flow of Madarsoo River (in Golestan forest) in the form of a basin and data hydrometer station using hydrologic models is classified.
Results and Discussion
Occurrence of heavy rainfall in Madarsoo Basin on one of the driest months in 2001 and 2002 (September) increased the water volume pressure and created instable slopes on hills in forested areas of Golestan National Park. This problem caused large amounts of slippery material and old unstable material to move and to flow down the slopes, and this phenomenon could carry large pieces of stone and trunks of the trees. Within the semi-arid basin, the extensive formation of marl and conglomerate makes many Gullies and causes their development. The Creation of the material through intense rainfalls increased in these areas and it has led to the development of Gullies. As a result, the range based on the current review of 75 sub-basins was a little more separated, and then all of them were classified as general in the second group. 42 sub-basins were caused by normal flood. These floods in their own beds directions did not cause any main geomorphologic changes; while 33 sub-basins very heavy streams. Also most small basins densely covered by vegetation were caused by normal flood; while in 19 small basins in Golestan forest range of materials flow has occurred.
The recent catastrophic floods in Madarsoo basin had many geomorphic effects and made many changes. One of these effects is the original river bed being intensively profound by erosion. For the determination of the profound disruption of the bed, we took several profiles through the bed river and the status was compared to the status before the flood. The measurements on the profiles show the amount of profundity to be at least 3 meters and maximum 12 meters. For clarifying the factors contributing to the current occurrence of basin materials studied, the flow path length and the area of fan deposits at the end of it were measured by size. The measurements of the basin level with current materials indicated that 56% of the occurrence is in basins smaller than 5 kilometers. 28% of the basins had an area of 5 to 20 kilometers and 9% had an area of over 20 kilometers. These studies show that basins smaller than 5 kilometers have the highest frequency of occurrence.
The results show that numbers of small and steep sub-basins were invaded by debris and hyper concentrated Flows. To get the results, these sub-basins can be divided to 75 small basins. 42 of these experienced the water floods, 3 sub-basins invaded hyper concentrated flows and 30 of them invaded the debris flows. The debris and hyper concentrated flows volume and peak discharges are much more than the water floods that are usually estimated by traditional risk management on mountainous basins. This is one of the most important reasons for the failures in many of the flood control plans.