الفهرس 
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Abstract
Determining peak discharge needed to run hydrological models requires the use of rainfall time distribution hyetographof the design storm. To estimate the design flood for a project by the use of a unit hydrograph, one needs the design storm. The Design Storm could be defined as critical rainfall event that is used for assessing the runoff hydrograph for a certain return period.
This research is aimed to develop design storm profiles representative of arid and hyper-arid areas, such as Oman. Two hundred thirty six rainfall storms were collected from seventeen rainfall gauges that covered the coastal zone of Oman facing the Gulf of Oman for the period 1993 to 2007. Storms have been grouped into five groups according to their total durations. The groups comprised event durations from 5min-1 hour, 1-3 hours, 3-6 hours, 6-12 hours and 12-24 hours. The dimensionless cumulative hyetograph is a plot between the dimensionless cumulative duration (i.e. percentage of the cumulative duration to the total storm duration), on the x-axis, and the dimensionless cumulative rainfall depth (i.e. percentage of the cumulative rainfall depth to the total rainfall depth), on the y-axis.For each group, cumulative dimensionless hyetograph of all storms have been compiled in one graph to observe the storm distribution. It was found that there was a great variation in hyetograph shape in each group of storms. Storms have no definitive clear pattern with peak intensities being in the beginning, middle or the end of the storm. This variation is the main reason to search for a design (critical) storm that will produce higher runoff discharges.
Design storm hyetographswerederived from the statistical analysis of observed rainfalls for all five groups using the Alternating Block Method (ABM) whether derived from individual storms or from the IDF (Intensity–Duration–Frequency) curves.
The methodology of the Alternating Block Method based on calculating the rainfall depth for different storm durations, with a user-defined analysis time step (usually in the range of 5 to 10 minutes) and arranging the incremental rainfall depth blocks alternatively at each side of the center of the storm hyetograph. The Alternating Block Method derived from individual storms was used for each group of storms. Rainfall depths for different time increments are obtained from the individual storms. To allow averaging storms with different durations and depths, dimensionless hyetographs are first derived by dividing each depth by the total storm depth and each duration by the total storm duration. The averages of the dimensionless profiles are computed at each time increment to obtain the design storm hyetographs.
The Alternating Block Method derived from the IDF curves is depended on calculating the maximum value of the 5, 10, 15, 30, 60, 120, 180, 360, 720 and 1440 minutes incremental precipitation depths for every year for all rainfall gauges. These values will be used in the frequency analysis. Four common statistical distributions are tested to fit the rainfall data: Lognormal, Gumbel, Pearson Type III and Gamma statistical distributions. The distribution that best fits the data is selected based on the ordinary moments diagram, the log-log plots and the mean excess function (Adlouni, et al., 2008) and the Akaike Information Criterion (Akaike, 1974) and Bayesian Information Criterion (Schwarz, 1978) using HYFRAN (Hydrological Frequency Analyses) (INRS-ETE, 2008) software. Gamma distribution was found to give the best fit on all durations based on the previously mentioned selection criteria.The 5, 10, 25, 50 and 100-year rainfall depths are calculated using Gamma for each duration. The five minutes incremental precipitation depths were computed. The ratio between the sum of five minutes incremental precipitation depths for 1, 3, 6, 12 and 24 hours and the depth of the first five minutes were computed to compare this ratio with the values which were obtained previously from the Alternating Block Method on individual storms. The five minutes incremental precipitation depths are arranged with the maximum rainfall depth assumed to occur in the middle of the storm duration (24 hours). The next largest five-minute incremental depth occurs just after the maximum depth; the third largest rainfall depth occurs just prior to the maximum depth, etc. Then the total cumulative of these values was computed.
The design storm derived using the Alternating Block Method on individual storms was compared to the design storm derived usingthe Alternating Block Method which calculated using the Intensity - Duration - Frequency (IDF) curves. from this comparison, there was a great similarity in the design storm values between these two rainfall distributions.
The developed dimensionless rainfall distribution by using the Alternating Block Method using individual storms and using the IDF curveare compared to the standard SCS type II storm distributions and the UK50 summer storm profiles. The standard SCS type II storm distributions are used in Egypt, Saudi Arabia and Oman while the UK50 summer storm profiles (FSR curves) are used in Qatar, Kuwait and Dubai.
The results of the developed dimensionless rainfall distribution by using the Alternating Block Method derived using individual storms or using the IDF curves are more critical than the results of the standard (SCS) type II and the UK50 summer storm profiles. Therefore, the study recommends using the newly developed dimensionless storm profiles derived from the actual records for arid and hyper arid regions.