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<br />~ <br />~ <br />I <br /> <br />, Gobi Block - Precast cellular concrete block often used as a <br />substitute for riprap. <br /> <br />, <br /> <br />. <br /> <br />f <br />~ <br />, <br /> <br />, <br />, <br /> <br />~ <br />r <br /> <br />~ <br /> <br />~ <br />r <br /> <br />Geomorphology - That branch of both physiography and geology that <br />deals with the form of the earth, the general configuration of <br />its surface, and the changes that take place due to erosion of <br />the primary elements and in the bui Idup of erosional debris. <br /> <br />Grade-control structure (sill, check dam> - Structure placed bank to <br />bank across a stream channel (usually with its central axis <br />perpendicular to flow) for the purpose of control I ing bed slope <br />and preventing scour or head-cutting. <br /> <br />Gravel - Rounded or semi rounded particles of stone. <br /> <br />Grout - A fluid mixture of cement and water or of cement, sand, and <br />water used to fi I I joints and voids. <br /> <br />Hard point - A streambank protection technique whereby "soft" or <br />erodible materials are removed from a bank and replaced by stone <br />or compacted clay. Some hard points protrude a short distance <br />into the channel to direct erosive currents away from the bank. <br />Hard points also occur naturally along streambanks as passing <br />currents remove erodible materials leaving nonerodible materials <br />exposed. <br /> <br />Head-cutting - Channel bottom erosion moving upstream through a basin <br />indicating that a readjustment of the basin's slope and its <br />stream discharge and sediment load characteristics is taking <br />place. Head-cutting is evidenced by the presence of waterfal Is <br />or rapidly moving water through an otherwise placid stream. <br />Head-cutting often leaves streambanks in an unstable condition as <br />it progresses through a reach. <br /> <br />Helical flow - Three-dimensional movement of water particles along a <br />spiral path in the general direction of flow. These secondary- <br />type currents are of most significance as flow passes through a <br />bend; their net effect is to remove soil particles from the cut <br />bank and deposit this material on the point bar. <br /> <br />Hydraulic radius - The cross-sectional area of a stream divided by its <br />wetted perimeter. <br /> <br />Jack (jackstraw, Kellner jack> - A component of a river training <br />structure consisting of wire or cable strung on three mutually <br />perpendicular metal, wooden, or concrete struts. <br /> <br />Launching - Release of undercut material (stone riprap, rubble, slag, <br />etc.> downslope; if sufficient material accumulates on the <br />streambank face, the slope can become effectively armored. <br /> <br />77 <br />