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<br />Gobi Block - Precast cellular concrete t'!ock often used as a <br />substitute for riprap, <br /> <br />Geomorphology - That branch of both physiography and geology that <br />deals with the form of the earth, t~e general configuration of <br />its surface, and the changes that take place due to erosion of <br />the primary elements and in the buildup of erosional debris. <br /> <br />Grade-control structure (si I I, check dam) - Structure placed bank to <br />bank across a stream channe I (usua II y ., i th its centra I ax i S <br />perpend i cu I ar to f I 0"') for the pur:JOS" of contl'o II i ng b"d s lope <br />and preventing scour or head-cutti~g. <br /> <br />Gravel - Rounded or semi rounded particl IS of stone. <br /> <br />Grout - A fluid mixture of cement and w.ter 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 passi ng <br />currents remove erodible materials leaving nonerodible materials <br />exposed. <br /> <br />Head-cutting - Channel bottom erosion meving 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 pn~sence of water.falls <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 />Hel ical flow - Three-dimensional movement of water partic'es along a <br />spiral path in the general direction of flow, These secondary- <br />type currents are of most significance as f low passes through a <br />bend; their net effect is to removt! ;oil par.ticles ir"om the cut <br />bank and deposit this material on the point bar, <br /> <br />Hydraulic radius - The cross-sectional ,;,r..a of a stream divided by It:s <br />~etted perimeter. <br /> <br />Jack (jackstra~, Kellner jack) - A component of a river training <br />structure consisting of wire or cabl. strung on three mutual y <br />pe'-pendicular metal, wooden, or cone-ete st."uts, <br /> <br />Launching - Release of undercut material (stone riprap, rubble, s:ag, <br />at".) downslope; if sufficient matr"r-ial accumulates on the <br />straambank face, the slope can bee om. effectively armored. <br /> <br />67 <br />