Maybe not. Total shade is not good when it happens just when the rhododendrons are developing new flower buds. That happens to be around now. If there is dense shade, flower production might not be at its best. Indirect sunlight might help (off cement surfaces, etc) but dense shade would not be good for overall health and flower production.

The rhododendron root system is shallow and contains many tiny roots that may not take kindly to flooding episodes. I am concerned about these floods moving the top layers of soil around and exposing the rhododendron roots above the soil. That would make them dry out and die. When you say that the area floods but drains quickly, how many inches of water are we talking about and how fast in minutes or hours does it drain?

And yes, there may be problems from competition for moisture and nutrients. The Eastern Redbud that is common here and some of its other varieties tend to become "less" shade tolerant as they get older and do not tolerate competition. I am aware that they develop a deep taproot but do not know how much the secondary roots will grow in the same area that the rhododendron roots will be found (top 4"). These secondary roots begin to grow from a depth of 3" or so, whish is with the area that rhodies also grow roots in.

After a heavy rain there'll be 1"-2" of standing water which will drain away in a couple of hours although the ground remains squishy for another 4-6 hours after that.

But your comment about heavy rains moving the top layer of soil reminded me that a rhododendron would not be happy in the existing heavy clay and improving a sufficient area of soil might be difficult to impossible because of the existing tree roots. This spring I tried to dig a short run of French drain there to improve drainage but the roots got the best of me.

I think I might have just talked myself out of this. Thanks for your input, luis pr. Between us we may have spared the life of a lovely rhododendron. :-)

I think it's definitely worth trying. If you plant on top of the existing soil rather than digging any kind of depression, the potential drainage problem would be avoided. Spread out the rhododendron roots and plant in coarse, well decomposed soft wood bark. Mulch well over the roots with the same stuff, which will probably have to be renewed periodically. Your comment about rhododendrons growing in forest duff is right on. A coarse, totally organic planting medium is exactly what rhododendrons need to thrive; it provides lots of the essential oxygen, retains moisture but not at an excessive level and does not readily compact over time.
Some rhodendrons grow and bloom better in dense shade than others - the old classic Lee's Dark Purple, for example. I'd also think the shade cast by a redbud would be less dense that that from many other trees and may well be an asset in the heat your climate.

If you can't improve the soil because of existing roots, forget it. Rhododendrons do not compete well with existing roots. Mulching is very important but in an area that floods, the mulch won't stay around long.

In the wild rhododendrons are usually growing near water, but high beside where it is moving rapidly, not where it is sitting. Creeks are frequently lined with rhododendrons. Also, cuts for highways are lined with rhododendrons due to the sun and drainage.

The combination of 1-2" of sanding water, clay soil, root competition, dense shade and poor drainage are all major problems.