well it depend on the situation . its practical to made such system in cities and to some extent to the villages but what about people who live in desert ? are you gonna to made such system in all of the mountain and deserts of KSA ?
and contrary to peoples beliefs KSA may be a wealthy country bt like anybody else their resources are limited , they can only spend to some extent on each project.
Exactly. It is extremely difficult to built it all over the country. This cannot happen with countries that are as big as KSA. Neither with India, USA (even them), China, Brazil, Mexico, Argentina and other very big countries in the Middle East and outside of it.
The mountain areas are of a very big size in total. To make drainage systems in every single village would take A LOT of time and a lot of cost. There are already over 250 dams in KSA, especially in the Western, Southern and mountainous parts of the country and more are getting built.
The deserts in KSA are not flat but all largely hilly or even have mountainous landscape. Even the Empty Quarter has large sand dunes. The Empty Quarter is the only true sand desert but nobody lives there apart from a tiny population of Bedouins. There are no roads there either or villages. The remaining "desert areas" all have hilly, mountainous landscapes and have often volcanic soils and hundreds of villages and oasis. Flooding there is not a problem unless the village is situated near mountains/hills that could move heavy rainfall downwards and create flooding.
Villages as seen below are often situated on the hills rather than in valleys exactly due to the fear of flooding which has been known to occur since ancient times.
As seen in Ha'il which is in Northwestern KSA and has a altitude of over 1000 m. It was impacted exactly by that and all surrounding villages.
So what must be done and be obligatory is to make drainage systems in all major cities. This is affordable and in a country of KSA wealth and infrastructure it should have been done much earlier.
The weather is only going to get more extreme in the Middle East so the infrastructure should reflect that everywhere.