culvert n 1: a transverse and totally enclosed drain under a road or railway
Culvert \Cul"vert\ (k?l"v?rt), n. [Prob. from OF. coulouere, F. couloir, channel, gutter, gallery, fr. couler to flow. See {Cullis}.] A transverse drain or waterway of masonry under a road, railroad, canal, etc.; a small bridge. [1913 Webster]
35 Moby Thesaurus words for "culvert": aqueduct, arroyo, bed, canal, creek bed, donga, dry bed, flume, gulch, gully, gullyhole, headrace, irrigation ditch, nullah, race, river bed, riverway, runnel, sluice, spillbox, spillway, stream bed, streamway, swash, swash channel, tailrace, wadi, water carrier, water channel, water furrow, water gap, water gate, watercourse, waterway, waterworks