The pilot of the tundra tire, tailwheel-equipped airplane reported that, during the landing touchdown, he discovered that the left emergency brake was partially engaged. He added that, the left wheel was locked and the airplane exited the left side of the runway. Subsequently, the right landing gear collapsed and the right wingtip struck the ground. He added that, before the previous takeoff, from a grass airstrip, he had performed his preflight checklist, which was normal. But, "upon reflection", he realized that he had changed his normal routine by shutting down the engine, setting the parking brakes and exiting the airplane after completing the checklist. After re-entering the airplane, he only did a "visual left to right flow check". He then, pressed and released the brakes and applied power holding the left rudder and brake down to turn the airplane in the direction of the departure, and departed after a "very short ground roll downhill". He further added, that by taking an immediate turn to the left, during the takeoff roll, he "may not have released enough pressure on the parking brake to turn it completely off." He believed that, due to the combination of the large smooth tire, wet grass and downslope hill he was unable to determine that the emergency parking brake was engaged. The airplane sustained substantial damage to the fuselage and right wing. The pilot did report that the left parking brake did not disengage.