Self closing flood barrier for protecting a hinterland against floods, said flood barrier comprising: - a chamber having a water side wall and a hinterland side wall (21; - a single support block with an inclined surface; - a water inlet for letting in water; - a dam wall member (4,5a,5b,16,18) up and down movably arranged in said chamber, said dam wall member comprising: - a retaining wall for avoiding water to flow to the hinterland; - a forcing element (4,5a,5b) for causing an upward movement of the dam wall member, and - a blocking element for providing a blockade for limiting said upward movement of the dam wall member, the blocking element comprising an inclined surface, which acts against the inclined surface of the single support block in a closed state of the flood barrier, - wherein the single support block is connected to said water side wall; and - said dam wall member further comprises a substantially flat surface facing said hinterland side wall allowing abutment of the substantially flat surface of the dam wall member against said hinterland side wall, characterized by: - the hinterland side wall having a substantially vertical flat surface over its height from the bot-tom of the chamber up to its upper level, which upper level is arranged at the same height as the upper level of the single support block, - the abutment of the substantially flat surface of the dam wall member against said hinterland side wall being caused by the inclined surface of the blocking element being forced by water in the chamber against the inclined surface of the support block resulting in a sideward movement of the dam wall member until its substantially flat surface abuts against the substantially vertical flat surface of the hinterland side wall of the chamber. - a chamber having a water side wall and a hinterland side wall (21; - a single support block with an inclined surface; - a water inlet for letting in water; - a dam wall member (4,5a,5b,16,18) up and down movably arranged in said chamber, said dam wall member comprising: - a retaining wall for avoiding water to flow to the hinterland; - a forcing element (4,5a,5b) for causing an upward movement of the dam wall member, and - a blocking element for providing a blockade for limiting said upward movement of the dam wall member, the blocking element comprising an inclined surface, which acts against the inclined surface of the single support block in a closed state of the flood barrier, - a retaining wall for avoiding water to flow to the hinterland; - a forcing element (4,5a,5b) for causing an upward movement of the dam wall member, and - a blocking element for providing a blockade for limiting said upward movement of the dam wall member, the blocking element comprising an inclined surface, which acts against the inclined surface of the single support block in a closed state of the flood barrier, - wherein the single support block is connected to said water side wall; and - said dam wall member further comprises a substantially flat surface facing said hinterland side wall allowing abutment of the substantially flat surface of the dam wall member against said hinterland side wall, - the hinterland side wall having a substantially vertical flat surface over its height from the bot-tom of the chamber up to its upper level, which upper level is arranged at the same height as the upper level of the single support block, - the abutment of the substantially flat surface of the dam wall member against said hinterland side wall being caused by the inclined surface of the blocking element being forced by water in the chamber against the inclined surface of the support block resulting in a sideward movement of the dam wall member until its substantially flat surface abuts against the substantially vertical flat surface of the hinterland side wall of the chamber.