WSP06982 - Laserfiche WebLink

Home Browse Search

01960 6 Theodore S. Me/is et al. .\ threshold, the instrument electronically enables the user-defmed programme of its counterpart ISCO' 6712 sampler. Once activated, point samples are then collected from an intake located near the LlSST deployment at pre-defmed intetvals. Automated sampling continues until either the laser transmission threshold is again exceeded. or the supply of sample bottles is exhausted. The ability of the LlSST to control the pump sampler ensures that additional concentration and grain-size data are collected during periods when multiple scattering errors are most likely to occur. Low-transmission periods are of obvious interest since they represent times of greatest sediment enrichment, as well as peak export from Grand Canyon. This protocol also allows for the 24 sample bottles to be consetved in the sampler's carousel - a critical factor at remote measurement locations in Grand Canyon where daily maintenance is not possible. Monitoring channel-bed sediment supply using LISST and II Previous work has shown that suspended-sediment concentration and grain-size data can be used to calculate grain size of sediment on the bed upstream (Rubin & Topping 200 I). A dimensionless measure of grain size of sediment on the bed, p, is defmed as: p= Db. Dbm (I) where Db is the median grain diameter of bed sediment at an instant in time and Dhm is the average of a sequence of median diameters at the same location at different times. j3 is thus a measure of the relative coarseness of sediment on the bed. Expressed in terms of the LlSST -obsetvable variables (concentration and grain size of suspended sediment), j3 is given by: p = (~)-o '(ll)O 2 e", Ds", (2) where C and Ds are the concentration and median diameter of suspended sediment at an instant in time, and Cm and Dsm are their time respective averages through time. In equation (2), the exponent of the concentration ratio is negative, whereas the exponent of the grain-size ratio is positive. An increase in suspended-sediment grain size is accompanied by a decrease in concentration. indicating a coarsening of the bed upstream. Suspended-sediment data from the 1996 controlled flood released from Glen Canyon Dam (a flow treatment in which a fIXed sand supply was exposed to a constant discharge of 1,275 m3 s" for 7 days) reveals rapidly increasing values of p during the first 72 h of that experiment (indicating bed winnowing). The p values derived from the 1996 beach-building experiment provide an example of how LlSST data might be monitored in real time during artificial floods to identify the onset of sand depletion during future sand-bar restoration tests. By this means, flood duration might be optimized. Because the p value, derived by the above method, is a surrogate for how enriched a river segment is in fine sediment. it can thus provide an indirect and rapid reach-inte- grated measure of a river's fine-sediment mass balance (in non-armoured conditions), For example, within a period of less than 24 h on II January, 2002, the LlSST-IOOB