The pilot of the tailwheel- and tundra tire-equipped airplane reported that, during takeoff from an off-airport field, the airplane climbed to about 15 to 20 ft above ground level but then began to descend. He reported that a wind gust "pushed" the airplane down. He then verified that the engine was at full power and attempted to climb, but the left wheel struck the ground, and the airplane then veered left and came to rest in a ditch. The airplane sustained substantial damage to the fuselage and both wings. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot reported that the wind was light and variable, gusting to between 8 and 10 knots. The pilot added that, when the airplane encountered the wind gust, the notch of flaps he was using "dropped out." After verifying that the engine was at full power, he attempted to "grab [the] notch of flaps but [he] wasn't fast enough."