A structure (110; 400) comprising: a first layer having an externally-facing side and an internally-facing side opposing the externally-facing side, wherein the first layer is formed from a material that is substantially impermeable to water; and a second layer attached to the internally-facing side of the first layer, wherein the second layer is water-swellable, and wherein the second layer comprises an elastomeric material that is formed of a mixture of a polymeric hydrogel and a cured rubber; wherein: the first layer comprises one or more channels (16; 402, 402') that extend through an entire thickness of the first layer to the second layer, the one or more channels (16; 402, 402') expose a region of the second layer, the one or more channels (16; 402, 402') allow water to contact the exposed region of the second layer, each of the one or more channels (16; 402, 402') has a first channel sidewall in the first layer and an opposing second channel sidewall in the first layer, and the first sidewall has a first position relative to the second sidewall when the exposed region of the second layer is substantially dry, and a second position relative to the second sidewall when the exposed region takes up water and swells, wherein the second position differs from the first position in a measurable distance along at least a portion of a length, a width (a, a', c, c', c", e, e', e"; g, g', h, h'), a height, or any combination thereof, of the at least one channel (16; 402, 402'). a first layer having an externally-facing side and an internally-facing side opposing the externally-facing side, wherein the first layer is formed from a material that is substantially impermeable to water; and a second layer attached to the internally-facing side of the first layer, wherein the second layer is water-swellable, and wherein the second layer comprises an elastomeric material that is formed of a mixture of a polymeric hydrogel and a cured rubber; wherein: the first layer comprises one or more channels (16; 402, 402') that extend through an entire thickness of the first layer to the second layer, the one or more channels (16; 402, 402') expose a region of the second layer, the one or more channels (16; 402, 402') allow water to contact the exposed region of the second layer, each of the one or more channels (16; 402, 402') has a first channel sidewall in the first layer and an opposing second channel sidewall in the first layer, and the first sidewall has a first position relative to the second sidewall when the exposed region of the second layer is substantially dry, and a second position relative to the second sidewall when the exposed region takes up water and swells, wherein the second position differs from the first position in a measurable distance along at least a portion of a length, a width (a, a', c, c', c", e, e', e"; g, g', h, h'), a height, or any combination thereof, of the at least one channel (16; 402, 402'). the first layer comprises one or more channels (16; 402, 402') that extend through an entire thickness of the first layer to the second layer, the one or more channels (16; 402, 402') expose a region of the second layer, the one or more channels (16; 402, 402') allow water to contact the exposed region of the second layer, each of the one or more channels (16; 402, 402') has a first channel sidewall in the first layer and an opposing second channel sidewall in the first layer, and the first sidewall has a first position relative to the second sidewall when the exposed region of the second layer is substantially dry, and a second position relative to the second sidewall when the exposed region takes up water and swells, wherein the second position differs from the first position in a measurable distance along at least a portion of a length, a width (a, a', c, c', c", e, e', e"; g, g', h, h'), a height, or any combination thereof, of the at least one channel (16; 402, 402').