Sampling techniques for suspended sediment
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SAMPLING TECHNIQUES FOR SUSPENDED SEDIMENT. Introduction to Sediment Sampling USGS Technical training in Support of Native American Relations (TESNAR) 2013 Siletz, Coquille, Umatilla, and Cowlitz Tribes Siletz, OR May 20-23, 2013. SAMPLING TECHNIQUES. Why a suspended-sediment sampler?

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SAMPLING TECHNIQUES FORSUSPENDED SEDIMENT

Introduction to Sediment Sampling

USGS Technical training in Support of Native American Relations (TESNAR) 2013

Siletz, Coquille, Umatilla, and Cowlitz Tribes

Siletz, OR May 20-23, 2013


SAMPLING TECHNIQUES

  • Why a suspended-sediment sampler?

    • Standardization of instrumentation among investigators

    • Tool for obtaining a representative sample


The Colorado River SamplerUsed until the mid-1940’s


SAMPLING TECHNIQUESStandardization

Jaukowsky Suspended-Sediment Sampler (FISP, 1940, p. 131)


Deploying the Jaukowsky

Suspended-Sediment

Sampler from a boat

in the

Yellow River

near Zhengzhou, China

May 17, 2002

(FISP, 1940, p. 131)


Deploying the Jaukowsky Suspended-Sediment Sampler

(FISP, 1940, p. 131)

in the Yellow River near Zhengzhou, China

Zhang L. dumping sample


SAMPLING TECHNIQUES

  • Standardized instruments

    • Isokinetic sample collection

    • Velocity- or discharge-weighted samples


SAMPLING TECHNIQUES

  • Representative sample

    • Site selection -- characteristic of reach?

    • Temporal variability

    • Spatial variability

    • Sampling frequency


Streamflow Constrictions


SAMPLING TECHNIQUES

  • Site selection

    • Uniform flow in x-sectoin

    • Well-mixed flow

    • Ability to sample range of flows, most importantly medium-and-high flows

    • Availability of historical data


SAMPLING TECHNIQUES

  • Temporal variability

    • Sediment transport variability over time

      • Event hydrograph

      • Seasonal hydrograph

      • Annual hydrograph


SAMPLING TECHNIQUES

  • Spatial variability

    • Site specific

      • Variability over the width of the channel cross section

      • Variability in each sampled vertical

    • Basin characteristic

      • Higher versus lower elevation sites

      • Northern versus southern exposures


SAMPLING TECHNIQUES

  • Sampling frequency

    • Generally dictated by the study approach and level of funding

    • Less critical to collection of a discrete representative sample than site selection, and temporal and spatial variability


SAMPLING TECHNIQUES

  • Sample collection methods

    • Depth-integrated sampling

    • Point-integrated sampling

    • Point sampling

    • Grab or dip sampling

    • Pumped samples *

    • Single-stage samples *

      *will be covered in separate lecture


SAMPLING TECHNIQUES

  • Depth-integration techniques

    • Equal-width increment (EWI) method

    • Equal-discharge increment (EDI) method

    • Single-vertical sample (Box sample)


Key for Previous Chart

1) US DH-48 2) US DH-59

3) US DH-76 4) US DH-81

5) US DH-95 6) US DH-2

7) US D-74 8) US D-74AL

9) US D-95 10) US D-96

11) US D-96A1 12) US D-99

13) US P-61A1 14) US P-63

15) US P-72


SAMPLING TECHNIQUES

  • Equal-width increment (EWI) method

    • Cross section divided into (~8-20) equal-width increments

    • Sampling transit rate for all sample verticals determined at the deepest/fastest vertical

    • Collect velocity-weighted sub-samples from the mid-point vertical in each width increment

    • Composite all sub-samples for analysis


EWI


Transit Rate Ratios for Nozzle/Bottle Combinations


SAMPLING TECHNIQUES

  • Equal-discharge increment (EDI) method

    • Stream discharge divided into (4-9) equal increments of discharge

    • Collect velocity- and discharge-weighted samples from the centroid vertical of each discharge increment

    • Vary the transit rate among verticals to obtain equal sample volumes

    • Analyze samples individually or composited


EDI


EDI Centroids


Why the different number of verticals

  • Why sometimes 5 verticals

  • Why sometimes 20 verticals

  • Assumption made

    • Sample collected at the center of the centroid represents the mean concentration for that centroid

    • If, not, must increase number of verticals


SAMPLING TECHNIQUES

  • Single-vertical sample

    • Use EDI method to determine sample vertical location

    • Sample vertical should be at the point in the cross section where the mean sediment concentration occurs over the largest range in stage

    • Use the slowest transit rate possible without overfilling the sample bottle

    • Duplicate samples are typically collected


SAMPLING TECHNIQUES

  • Point-integration technique

    • Necessary in depths >15 feet (total sampler traverse >30 feet)

    • A point-integrating sampler is necessary

    • Used in conjunction with EWI or EDI method

    • Transit rates determined by EWI or EDI method

    • Descending traverse matched by ascending traverse

    • Single-vertical sub-samples composited for analysis


SAMPLING TECHNIQUES

  • Point sampling

    • Use a point-integrating sampler

    • Samples represent discrete points in the sampled cross section and/or verticals

    • Samples may be velocity-weighted over an interval equal to the fill time for the nozzle/bottle combination

    • Samples may be instantaneous snap shots of sediment transport at a point without consideration for velocity


SAMPLING TECHNIQUES

  • Grab or dip sampling

    • Flow conditions or other unusual circumstances generally render standard samplers unusable

    • Sediment is well mixed spacially

    • Samples are not integrated

    • Samples are seldom representative


Grab or Dip Sampling


Site selection- characteristic of reach, uniform flow, well mixed, sample full range of flows, H/L elev, N/S exposure, any historical data

Temporal variability- event, seasonal, annual

Variations in X-sec and w/i sampling vertical, sediment sizes (coarse,sand,silt,clay) will vary

How frequent sample- depends on objectives

EWI- equal width & transit rate, volume not equal, first set transit rate at thalweg

EDI- volume equal, width and transit rate not equal; need Q meas, divide by 10,30,50,70,90 %

Technique Points to Remember


A Good Reference

Field Methods for the Measurement of Fluvial Sediment

http://pubs.usgs.gov/twri/twri3-c2/


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