عنوان مقاله [English]
نویسنده [English]چکیده [English]
Frequency studies are, essentially, an application of the theories of probability and are based on available records of flood experiences. Several methods of analysis, based on the mathematics of probabilities, have been proposed which, when applied to the same series of data, yield frequency curves which are in reasonable agreement throughout a useful range of probabilities. A widely used method for generalizing flood characteristics is to prepare a regional frequency curve in which discharge is expressed as a factor times the mean annual flood, or the median annual flood. To do this, a common period of record is chosen for all stations in the region and individual flood frequency curves prepared, then from this set of curves, for each of several return periods, the median of the individual station factors were determined. This analysis provides two curves: the first expresses the flood discharge time relation, showing variation of peak discharge, expressed as a ratio to the mean annual flood, with recurrence interval. The second relates the mean annual flood to the size of the drainage area. A frequency curve may be defined for any place in the region by use of these two curves. To estimate the mean annual flood for ungauaged points in a homogeneous region, a satisfactory relation may be used in which available mean annual flood values have been plotted against size of catchment area. Such a plot is commonly represented by a fairly straight line on log-log paper.
If the scatter is excessive the region may not be homogenous, or factors other than catchment area are significant.
In addition flood having any return periods is estimated by multiple regression based watershed characteristics such as size of catchment, annual precipitation, channel slope, shape factor, density drainage etc. As a final results, several models presented for estimating design
floods, also somes aspects were proposed for further investigations.