ABSTRACT
ALLUVIAL DEPOSITS Alluvium: river-deposited sediment; sorted and bedded, but any grain size from clay to boulders; laterally and vertically variable, with wide range of engineering properties. Floodplain: zone of alluvial deposition along valley floor, subject to periodic flooding. The alluvium builds up over time, much of it formed as over-bank flood deposits, which are mostly fine grained and horizontally bedded. Meander scrolls: cross-bedded, crescentic lenses of sediment, mostly sand or gravel, left on insides of migrating river bends or meanders. Channel fills: abandoned river channels filled with sediment, commonly clay or peat. Alluvial fans: coarser, poorly sorted sediment (including fanglomerate) on steeper slopes and at mouths of hillside gullies and tributary streams. River terraces: remnants of any older, higher floodplains, abandoned when river cut to lower level; formed of alluvium, though may be rock-cored; eroded away as modern floodplain enlarges. Tufa and travertine: weak, porous calcite deposit, forming thin layer or cementing a gravel; may overlie uncemented alluvium and can be confused with rockhead. Les Cheurfas dam, Algeria, was built in 1885 on tufa crust, and failed by piping on first impoundment. Peat: black organic soil, formed in small lenses or large areas of upland bog or lowland fen; extremely weak and compressible (section 27). Lake deposits: similar to fine alluvium (section 15).
