The current comes from the poles and is cold so it creates mist and does not carry or evaporate water so the area a few kilometres from the sea is dry. High pressure High pressure is when cold air sinks down and so can't evaporate the water that means that no clouds are formed meaning that there won't be any rainfall. 180,000 years ago the Sahara was much wetter than it is today. At that time rivers ran across the land carving out deep valley.
Today there are usually dry and are known as Wadis.
If a desert storm lasts 30 minutes draw a possible storm hydrograph for the wadi to show the water output. When there is a sudden rainstorm floodwaters flow down Wadis. They are short lived but carry sediments in large quantities. This is deposited as alluvial pans at the foot of steep slopes. Several fans join together to form a Bajada.