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<br />6 <br /> <br />Theodore S. Me/is et at. <br /> <br />threshold, the instrument electronically enables the user-defmed programme of its <br />counterpart ISCOt 6712 sampler. Once activated, point samples are then collected <br />from an intake located near the LISST deployment at pre-defmed intervals. Automated <br />sampling continues until either the laser transmission threshold is again exceeded, or <br />the supply of sample bottles is exhausted. The ability of the LISST to control the pump <br />sampler ensures that additional concentration and grain-size data are collected during <br />periods when multiple scattering errors are most likely to occur. Low-transmission <br />periods are of obvious interest since they represent times of greatest sediment <br />enrichment, as well as peak export from Grand Canyon. This protocol also allows for <br />the 24 sample bottles to be conserved in the sampler's carousel - a critical factor at <br />remote measurement locations in Grand Canyon where daily maintenance is not <br />possible. <br /> <br />Monitoring channel-bed sediment supply using LISST and Il <br /> <br />Previous work has shown that suspended-sediment concentration and grain-size data <br />can be used to calculate grain size of sediment on the bed upstream (Rubin & Topping <br />2001). A dimensionless measure of grain size of sediment on the bed, p, is defmed as: <br /> <br />p= Db, <br />Dbm <br /> <br />(1) <br /> <br />where Db is the median grain diameter of bed sediment at an instant in time and Dbm is <br />the average of a sequence of median diameters at the same location at different times. <br />p is thus a measure of the relative coarseness of sediment on the bed, Expressed in <br />terms of the LISST -observable variables (concentration and grain size of suspended <br />sediment), p is given by: <br /> <br />p = (~)-o.1(~)O.2 <br />em Dsm <br /> <br />(2) <br /> <br />where C and D., are the concentration and median diameter of suspended sediment at <br />an instant in time, and em and Dsm are their time respective averages through time, In <br />equation (2), the exponent of the concentration ratio is negative, whereas tile-exponent <br />of the grain-size ratio is positive. An increase in suspended-sediment grain size is <br />accompanied by a decrease in concentration, indicating a coarsening of the bed <br />upstream. Suspended-sediment data from the 1996 controlled flood released from Glen <br />Canyon Dam (a flow treatment in which a fixed sand supply was exposed to a constant <br />discharge of 1,275 m3 S.1 for 7 days) reveals rapidly increasing values of P during the <br />first 72 h of that experiment (indicating bed winnowing). The P values derived from <br />the 1996 beach-building experiment provide an example of how LISST data might be <br />monitored in real time during artificial floods to identify the onset of sand depletion <br />during future sand-bar restoration tests. By this means, flood duration might be <br />optimized. <br />Because the 13 value, derived by the above method, is a surrogate for how enriched <br />a river segment is in fine sediment, it can thus provide an indirect and rapid reach-inte- <br />grated measure of a river's fine-sediment mass balance (in non-armoured conditions). <br />For example, within a period of less than 24 h on 11 January, 2002, the LISST -I OOB <br /> <br />02358 <br />