First, very few tarps are going to hold up in a good wind storm. They are just not strong enough (or the ones that are cost an arm and a leg !).
Second, even a light shower, can cause a huge puddle in a suspended tarp. Try to set your tarp so that there is a natural tendency for rain to run off or be prepared to add extra supports under the sags when it rains.
The biggest problem with poles, is finding ones that are strong enough and long/tall enough and can be easily transported. 1" galvanized metallic electrical conduit is excellent, except for the transportation (8'-10' are hard to deal with). If you can find some place like a custom muffler shop that has the tools/dies to swage (compress/squeeze down) 1" pipe, you could cut the conduit in to more manageable sizes and just slip them together.
