an impermeable covering of the bottom of a reservoir adjacent to a dam or some other water-retaining hydraulic-engineering structure. An upstream apron is used to lengthen the path of the water that is seeping through beneath the structure and to reduce the uplift on the bottom of the structure. If an upstream apron is provided, the velocity and flow rate of the water that seeps through are reduced; consequently, the danger of destruction of the base of the structure by the seepage flow is minimized.
The materials used to build upstream aprons are clay, gravel-clay mix, asphalts, peat, concrete, and reinforced concrete. Upstream aprons made of reinforced-concrete slabs, with the reinforcement anchored in the body of the dam, are capable of withstanding some of the horizontal stresses that act on the structure. Aprons of this type, called anchored aprons, increase the shear resistance of the structure.