Two prototype fluvial suspended sediment samplers evaluated in an instream flume

Fluvial suspended sediment (FSS) is a measured physical component of hydrologic and environmental watershed studies. A variety of tools and strategies are employed to collect a FSS sample that is representative of the source water FSS. The discrete point-in-time automatic fixed speed pump sampler (DPS) is a commonly used tool in FSS studies. During May-October 2006, two prototype FSS samplers, a time integrated passive sampler (TIPS) and a continuous flow proportional variable speed pump sampler (CPS), were evaluated using an instream flume and a DPS sampler, placed in a small, low energy, second order stream in the Southern Interior of British Columbia, Canada. The time integrated passive sampler consisted of a passive instream sampler, and collected FSS continuously over a 24-hour period from a point in the stream cross-section. The CPS consisted of a combined instream suction pipe, pressure differentially controlled variable speed peristaltic pump, removable inclined pipe expansion, and filter bed system. The CPS pumped and filtered water for FSS continuously over a 24-hour period from a point in the stream cross-section. The instream flume consisted of an immersed 1 m diameter by 5 m long half pipe section on adjustable legs. Results indicated that all three samplers were correlated in the assessment of FSS silt/clay (mineral particle size fractions < 53 μm), sand (mineral particle size fractions > 53 μm), and organic matter based on weight, concentration, and percent proportions. Regression results demonstrated that there was disagreement between the DPS and the other two samplers for the assessment of organic fractions. The CPS- and TIPS-measured blend of percent organic and percent mineral fractions were in agreement, particularly if the TIPS measured percent sand was less than 35%. The TIPS collected FSS variables were most responsive to changes in flume flow velocity (log-log relationship), particularly the measurement of FSS total weight. The TIPS weight variables were predictors of flume FSS load values when calibrated to CPS or DPS load values. The study of the TIPS operational characteristics revealed that the inlet velocity was less than the ambient flume velocity of 400 mm ⁻¹ by a factor of 3.9. The proportion of inlet FSS retained in the TIPS body was determined by studying the ratio of expelled to retained FSS proportions (E:R). The E:R ratio was stable at 1:1 for flume velocities between 300 mm ⁻¹ and 400 mm s⁻¹. The E:R ratio varied predictably (linearly) according to the flume velocity, but less predictably according to changing proportions of organic matter, mineral particle size < 53 μm, and mineral particle size> 53 μm. The TIPS and CPS were capable of collecting enough FSS to assess a mineral particle size < 53 μm and> 53 μm. DPS assessment was restricted by the collected sample size to the assessment of organic matter and total mineral fractions only. The DPS was capable of partitioning the 24-hour hydrograph into 2-hour segments, whereas the CPS and TIPS collected a time integrated composite sample over the 24-hour period. The DPS pump speed was fixed, whereas the CPS pump speed was variable, according to the pressure differential readings on a venturi at the TIPS sampler outlet that responded to flume flow velocity. The instream flume provided a stable 24-hour sediment supply, linearized flow and isolated the FSS supply from streambed sediments. The flume was also a convenient, adjustable working platform above the watercourse. The CPS and TIPS prototypes represent successful initial steps toward designing an alternative sampling device to the automatic pump sampler.

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