The biggest problem with raised beds is keeping the soil moisture level high in times of intense heat and high wind. The raised beds also increase the soil temperature which is a bonus in the spring and fall but a definite minus during the late summer months. My beds are only 8" tall made with raw cut boards of cypress that are a full 2 inches thick so they act as some insulation from the heat but still the edges tend to dry out first and fastest. I have found that using a very heavy layer of cypress mulch makes all the difference in keeping my moisture levels adequate and keeping the soil cooler during the hottest months of the year.
My 8 inch tall beds are adequate because I had already amended the soil very aggressively for several years before I went to the raised beds. It sounds like you will need at least twice that depth with your underlying soil conditions. Made sure when you fill the beds that you use a very high amount of organic matter and enough organic matter that won't breakdown too fast so that the soil will not pack down too fast.
Bill
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