I have a bigger creek to play with, but the same kinds of flow issues.
A tube penstock is a big project, and unless the diameter is very large, the friction losses will cancel the gain of head.
I would be looking more at making a stream bed dam upstream, and then take a wood or concrete flume to a generating site adjacent to the creek. That way in flooding the equipment is out of direct harm. Doubling or tripling the head of your present setup would be a great gain, and could be done with the same wheel.
I am sure you already know about the sectional Banki turbines, you could tack on a smaller section for very low flow conditions, and with greater head probably still generate significant power.