كليدواژه :
سيلاب , دشت لوت , راه آهن , IFAS
چكيده لاتين :
Introduction
The floods particularly in arid regions of Iran damage railroads each year. Most of the damages are in the rail bridges on stream channels because of the channel bed replacements, sudden rainfalls and disjointed bed materials. Dry channels of the streams, the flat surface of the ground, and lack of vegetation cover result in seasonal and annual channel bed replacements. These channel replacements, destruction of viaducts, collapse of embankment, blockage of the channel under the viaducts by sediments and inundation destroy railroads each year with many losses. Thus, flood forecasting in arid and desert regions can, a large extent, help reducing the losses. The sudden rise and fall in river discharge, transportation of a huge amount of sediments, changes in river sections, lack of control sections and the sudden rainfalls make it impossible in arid regions to measure properly surplus flows by hydrometric stations. On the other hand, development of integrated flood analysis system is necessary to reduce flood hazard and to minimize the loss of life in different countries, particularly in the regions where the rivers have bed modifications. Developing countries have difficulties in these systems including establishment of monitoring stations and maintenance of equipment for forecasting and warning systems in that the systems need to collect upstream rainfall data and to have access to real time information. The use of Earth Observation Satellite (EOS) and runoff calculation and flood forecasting without ground observation make it possible to create the flood forecasting and warning system.
Study Area
The study area is located in the northern part of Lout Plain Basin and a portion of Yazd Province. The length of existing railroads in the study area is up to 356.37 Km that is situated in an area of 20251.98 Km2. Lout Plain, in topography, is a vast area with low elevation in central plateau of Iran and from the north and northwest it is separated from Kavir-Namak by dispersed mountains and some hills. The low slope plain with average elevation of 500 m is covered of saline areas and the railroad network passes from its northern part. There is no perennial drainage network in the area and only the seasonal and incidental streams convey the runoff from northern and western mountains toward the railroad. The incidental drainage networks cross the railroad through 930 viaducts. There are a lot of watercourses with annual channel changes that make it difficult to control runoffs over the area.
Material and Method
Two groups of data were used for the research: in the first group we used real time information from international websites related to Integrated Flood Analysis System (IFAS) and in the second group some of the data were replaced by topographic map, 1:25000 at scale, and Aster elevation data, 30 meters in pixel size, in order to enhance spatial resolution in the data. The real time information include rainfall data derived from Cover-Area GSMaP (JAXA), elevation data from GTOPO30 (USGS) with 30second resolution, land use data from GLCC (USGS), hydrology data from Hydro Spatial Resolution with pixel size of 1 Km, and Global Map with spatial resolution of 30second.
The research has been conducted in two stages. In the first stage, the Surface Tank Model of the series of hydrological distributed models in IFAS was applied. The surface tank model consists of the ratio of rainfall to land surface, rapid intermediate and ground infiltration flows. In the model the ratio of ground surface to the surface saturated of flow is estimated as a fraction (3/5) of storage capacity based on Manning Law. To calculate the Manning Roughness Coefficient in watercourses and stream channels, 17 samples were collected along the railway from the bed materials of channels crossing the rail. After grain size measuring, the channels were classified based on 50% diameter particle size (D50) into 8 different classes that the roughness coefficient according to Alridge and Garret Method was calculated for them. The underground flow, not rapidly saturated, is also estimated as a fraction of storage capacity and the ground infiltration as a fraction of storage capacity based on the Darcy Law.
Results and Discussion
The results from IFAS have indicated that out of the total length of 356.37 Km railroad that are crossed by watercourses and stream channels through 930 bridges and viaducts, up to 47.21 Km between the stations of Tabas and Namakzar, RobatPoshtBadam and Khenj, and also a part between Ramel and Jandagh, all including 107 bridges, are exposed to flooding and inundation; up to 49.75 Km of this total also in the distance between the stations of Shirgasht and after DehShour as well as Tal Hamid and Robat Posht Badam including 111 bridges are exposed to the possible danger of flooding. A comparison between the results and the amount of runoff from I.C.A.R. Model has revealed that the dangerous area has the maximum height of runoff and the total of annual runoff is more than 100 cm. This volume of annual runoff is high in these regions because it does not have a smooth temporal distribution and does receive the annual precipitation seasonally and accidentally in short duration.
The results of grain size analysis of channel bed samples indicated that the D50 in this bed is about 1.3 mm that is typical of intensive floods. During sedimentation by flood the solid material are separated from the soluble and suspended load and the deposits from floods have sorting with distinct boundaries. By t-Trask Index the flood deposits can be distinguished from other sediments; it changes from 1.8 to 2.7 for floods. The index for 17 samples of the study ranged from 2 to 2.45 that is representative of the fact that the accumulated deposits below the bridges were conducted by flood discharges.
Conclusion
The study area is located in the course of railroad network of the east of Iran that is very important in rail transportation of the country. The area as one of the most important parts of Iran rail transportation network experience many damages in flooding seasons. These damages include deposition of flood sediments on the railway, danger of foundation destruction in the railway, a disruption in rail traffic, and possible loss of life. In some areas, the replacements in channel bed and building construction and orchard developments diverted the runoff from its natural network. The IFAS program using different satellite data have been applied to recognize the flooding and damages by river channels in the basin, and to determine the vulnerable parts of rail networks. Some inputs in the program were replaced by base map and field observation data. The analysis of the results of the research has represented that the northern part of Lout Plain across which the east railroad network passes along an arid region have uneven distribution in precipitation so that it sometimes receive the total amount of annual precipitation just in 2 hours. This causes the extensive floods that their volume and velocity are increased due to lack of vegetation and presence of disjointed materials on the surface. The field observation revealed that the railway crosses floodplains of many watercourses. These watercourses do not have a stable channel so that they change their channels almost in every rainfall event. Because of these shifts in the place of channel bed, in flood events these channels are incompetent for drainage and there would be an oblique invasion by flow to the bridges and make their piers susceptible to destruction.
