Flood defence dam structure for providing a flood defence dam (94,194) comprising one or more sub-structures (96,196 a,b), wherein each sub-structure comprising: - a floor element (95,208) having a length and a width defining the position of use of the flood defence dam; - a first side (93,212) extending in a lengthwise direction of the dam structure and being permanent connected with a foldable connection (97,214) to the floor element (95,208), and a second side (93,212) extending in a lengthwise direction of the dam and being permanent connected to the floor element (95,208) or the first side, with at least one of the two sides and/or the floor element (95,208) comprising a sealable edge, wherein the foldable connections (97,214) between floor element (95,208), first side (93,212) and second side (93,212) extend in a substantial longitudinal direction of the dam; - two side elements being permanent connected with a foldable connection to the floor element or the first side and comprising at least one sealable edge arranged for connecting to the sides, wherein the side elements having a dam shape defining configuration; and - an inlet for a liquid, wherein the dam structure is of a flexible material, characterized in that - the dam structure comprises two sub-structures each defining a U-shaped chamber (96a,96b), the two U-shaped chambers (96a,96b) with the open sides directed towards each other, and a partition wall (98,198) extending in longitudinal direction of the dam between the two chambers (96a,96b) and separating them, wherein the individual floor elements of the two chambers (96a,96b) together define the floor element of the dam structure, and wherein the foldable connections, the floor element and the first and second sides are arranged such that after sealing the sealed connections that extend in longitudinal direction of the dam structure are at or close to the substantial vertical plane of symmetry seen in longitudinal direction when the dam structure is in use and are positioned at a central position at some distance away from the longitudinal edges of the structure; and wherein the vertical plane of symmetry is the partition wall between the two chambers. - a floor element (95,208) having a length and a width defining the position of use of the flood defence dam; - a first side (93,212) extending in a lengthwise direction of the dam structure and being permanent connected with a foldable connection (97,214) to the floor element (95,208), and a second side (93,212) extending in a lengthwise direction of the dam and being permanent connected to the floor element (95,208) or the first side, with at least one of the two sides and/or the floor element (95,208) comprising a sealable edge, wherein the foldable connections (97,214) between floor element (95,208), first side (93,212) and second side (93,212) extend in a substantial longitudinal direction of the dam; - two side elements being permanent connected with a foldable connection to the floor element or the first side and comprising at least one sealable edge arranged for connecting to the sides, wherein the side elements having a dam shape defining configuration; and - an inlet for a liquid, wherein the dam structure is of a flexible material, - the dam structure comprises two sub-structures each defining a U-shaped chamber (96a,96b), the two U-shaped chambers (96a,96b) with the open sides directed towards each other, and a partition wall (98,198) extending in longitudinal direction of the dam between the two chambers (96a,96b) and separating them, wherein the individual floor elements of the two chambers (96a,96b) together define the floor element of the dam structure, and wherein the foldable connections, the floor element and the first and second sides are arranged such that after sealing the sealed connections that extend in longitudinal direction of the dam structure are at or close to the substantial vertical plane of symmetry seen in longitudinal direction when the dam structure is in use and are positioned at a central position at some distance away from the longitudinal edges of the structure; and wherein the vertical plane of symmetry is the partition wall between the two chambers.