The pilot reported that he was making an approach to an airstrip for a brief stop. He overflew the field and determined the wind was variable from the north and started his approach. On final approach he noticed an above average loss of altitude and speed due to a large mountain wave coming over the foothills to the west. He added power to compensate for the downdraft and continued on his landing approach. Once clear of some trees at the approach end of the runway he reduced the power to idle and was again pushed down by another downdraft. The pilot then added full power to arrest the airplane's descent but he did not have enough speed at that time to avoid a hard landing. The airplane slid off the runway and into a guard railing. The fuselage tubular frame sustained substantial damage. The pilot reported no mechanical problems that would have precluded normal operation.