Semipervious

Semipervious, also written semipermeable or semi-pervious, is an adjective describing a material that allows a limited amount of fluid to pass through it. It lies between fully permeable (pervious) materials and impervious ones. The degree of permeability depends on pore structure, porosity, connectivity, and thickness, as well as the fluid's properties and the driving gradient.

In civil engineering and hydrology, semipervious materials are used to balance drainage and runoff control. Semipervious pavements and soils permit some infiltration of rainwater, reducing surface runoff and promoting groundwater recharge, while still providing load-bearing performance and reducing the risk of hydroplaning.

In membrane science, semipervious refers to membranes that are selective: they permit passage of certain solvents or solutes while restricting others. Biological membranes and dialysis membranes are semipermeable; inorganic or polymeric membranes are designed to achieve specific selectivity via pore size, charge, or chemical affinity. Pore size is often described by MWCO.

Measurement and design: Permeability is often quantified by hydraulic conductivity or infiltration rate for soils, with tests such as constant-head or falling-head permeametry. On membranes, performance is characterized by flux and selectivity (MWCO, rejection rate) and operating conditions such as pressure and temperature. Material choice, compaction, moisture, and contaminants affect performance.

Related concepts include permeability, perviousness, imperviousness, graded porous media, and semi-permeable membranes.