A culvert is a structure that channels water past an obstacle or to a subterranean waterway․ Typically embedded so as to be surrounded by soil‚ a culvert may be made from a pipe‚ reinforced concrete or other material․ Culverts are commonly used both as cross-drains to relieve drainage of ditches at the roadside‚ and to pass water under a road at natural drainage and stream crossings․ When they are found beneath roads‚ they are frequently empty․ A culvert may also be a bridge-like structure designed to allow vehicle or pedestrian traffic to cross over the waterway while allowing adequate passage for the water․ Culverts come in many sizes and shapes including round‚ elliptical‚ flat-bottomed‚ open-bottomed‚ pear-shaped‚ and box-like constructions․ The culvert type and shape selection is based on a number of factors including requirements for hydraulic performance‚ limitations on upstream water surface elevation‚ and roadway embankment height․