**8. Conclusion**

*Rainfall - Extremes, Distribution and Properties*

**7. The relation of the storms to the depth of the rain calculated for** 

*Cumulative dimensionless hyetographs: (a) for storms above 50 mm and (b) for storms below 50 mm.*

Rainfall depth of return periods 5–100 years was estimated by the Saudi Geological Survey and AECOM company (**Table 2**). The values are due to the analysis of the historical records of all rain stations located in and around the Jeddah area. An average value for each return period was calculated and used in

*(a) Dimensionless histogram for storms above 50 mm. (b) Dimensionless histogram for storms below 50 mm.*

By comparing the average data of **Table 2** with the rainfall amounts recorded in the recent storms, it was found that the storms of the first group, which exceeded the amount of rainfall of 55 mm, fall within the rainfall depth above the 5-year return period. The storm recorded on January 26, 2011, 111.6 mm, falls below the 50-year return period; the storm recorded on November 21, 2017, 88 mm, falls below the rainfall value of the 20-year return period; the storm recorded on November 17, 2015, in the Wadi Qaws, 61 mm, falls under the value of the 10-year return period; and the storm recorded on November 17, 2011, at the J134 station, 79 mm, falls under the storms of the 12-year return period, while in the second group storms, they did not exceed the amount of rainfall of 50 mm, which falls below the return period of 5 years.

SGS 2016 (mm) 53.1 71.1 93.4 110.0 125.0 AECOM 2011 (mm) 57.6 75.6 98.4 115.0 132.0 Average (mm) **55.4 73.4 95.9 112.5 128.5**

**5 10 25 50 100**

**Source Return period**

*Rainfall analysis of the historical records for the Jeddah area.*

**different return periods in the Jeddah area**

**28**

**Table 2.**

the current study.

**Figure 12.**

**Figure 11.**

Our findings indicated that Jeddah rainfall is characterized by two patterns: one for the rain values less than 50 mm and the other one for the rain values above 50 mm. The study shows that dealing with rainfall data in the Jeddah area required the following points to be considered: (1) it is necessary to pay attention to the details of rainstorms (intensity values) and not only to the recorded daily values. (2) There is a convergence of the nature of storms with impact (greater than 30 mm), which begins to rise after the onset of the storm almost immediately and continue the same tendency to the end of the storm. (3) There is a slight difference between the nature of these storms and the SCS-type II distribution, which assumes that about 60% of the depth of the rain falls in a fraction of the duration of the storm ranging from 8 to 20% and the rest distributed over the rest of the storm. (4) The average length of storms should be considered within the study areas. (5) In short-term storms of less than 2 h or even (with some reservation) up to 3 h, the distribution of SCS type II can be used safely but with duration limited to the same span of the storm rather than 24 h.
