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<br />r <br /> <br />GLOSSARY <br /> <br />A I isting of terms commonly used to describe streambank erosion and <br />instabi I ity mechanisms, as wel I as terms related to streambank <br />protection and river mechanics, is provided below: <br /> <br />Abrasion - Removal of streambank material due to entrained sediment, <br />ice or debris rubbing against the bank. <br /> <br />Angle of repose - The maximum angle (as measured from the horizontal) <br />at which gravel or sand particles can stand. <br /> <br />Aggradation (bed) - A progressive bui Idup or raising of the channel <br />bed due to sediment deposition. Aggradation is an indicator that <br />a change in the stream's discharge and sediment load <br />characteristics is taking place. <br /> <br />Alluvial fan - A cone-shaped deposit of sediment formed at the <br />confluence of a stream and its tributary. If the sediment load <br />of the tributary cannot be carried away by the stream, an <br />alluvial fan forms. <br /> <br />Armoring - (a) Natural process whereby an erosion-resistant layer of <br />relatively large particles is formed on a streambank due to the <br />removal of finer products by streamflow. (b) Placement of a <br />covering on a streambank or fi Iter to prevent erosion. <br /> <br />Articulated concrete mattress - Rigid concrete slabs usually hinged <br />together with corosion-resistant wire fasteners; primari Iy placed <br />for lower bank protection. <br /> <br />Asphalt block - Precast or broken pieces of asphalt that can be hand- <br />placed or dumped on a streambank or fi Iter for protection against <br />erosion. <br /> <br />Asphalt (bulk) - Mass uncompacted asphalt usually dumped from a truck <br />(upper bank protection) or a barge (lower bank protection) that <br />is placed to protect the bank against erosion. <br /> <br />Avulsion - A change in channel course that occurs when a stream <br />sUddenly breaks through its banks; usually associated with a <br />catastrophic event. <br /> <br />Backfill - The material used to refi I I a ditch or other excavation, or <br />the process of doing so. <br /> <br />Backwater area - The low-lying lands adjacent to a stream that may <br />become flooded during periods of high water. <br /> <br />Bank - The side slopes of a channel between which the streamflow is <br />norma I I Y conf i ned. <br /> <br />Bed - The bottom of a channel <br /> <br />73 <br />