pebble count overview - usfws€¦ · pebble count overview . representative . reach pebble count...

Stream Habitat Measurement Techniques Day 3: Substrate/Sediment Measurements

Pebble Counts

Pebble Count Overview

Representative Reach Pebble Count Form

Riffle Pebble Count and Largest Particle on Bar Count Form


Pebble CountsTwotypesofpebblecountsarerequiredinthemorphologicaldescription.Arepresentative pebble countcharacterizesthechannelandbedmaterialpresentthroughagivenstudyreachtoclassifythestreamtype(e.g.,C3vs.C4);anactive bed riffle pebble countisusedforhydrauliccalculations(R/D84)toestimatevelocity(onrifflebed)andtocalculatesedimentcompetence.

Representative Pebble CountTherepresentativepebblecountprocedureisastratified,systematicmethodtoproportionallysampleallthebedfeaturespresentwithinthebankfullchannelthroughadesignatedreachandisusedtodeterminethestreamtype.Thedesignatedreachisdividedintotwocategories:pools and non-pools. Non-pools include riffles, runs and glides.Thetotaldistanceofthereachisdividedintototalpoollengthandtotalnon-poollength(Figure 17). Forexample,assumethetotalreachlengthis1,000ft.Tostratifythesample,collectaminimumof100observationsproportionallybasedonbedfeatures.If300ft(30%)ofthereachiscomposedofpoolsandtheother700ft(70%)iscomposedofriffles, runs and glides,then30particles(or30%)aremeasuredwithinpoolsand70particles(or70%)aremeasuredwithinthe non-pool features.Tocompleteasystematicsample,10particlesacrossthreedifferentpoolcross-sectionsand10particlesacross7differentnon-pool cross-sectionswouldbesampled.Non-pool dataisenteredseparatelyfrompooldataonthepebblecountform(Table 6).Thisallowsaseparateanalysisofriffledataforspecificbiologicalandsedimentologicalpurposes.Particlesarecollectedatevenly-spacedintervalsacrosstheentirebankfullchannelateachoftheselectedcross-sections.Thesamplingintervalwillvarybasedonthebankfullwidth.Theintermediateaxis(B-axis)ofeachparticleismeasuredwitharulerinthefield(Figure 18)andisgenerallyrecordedinmillimeters.However,inthecaseoflong,linear-shapedparticles,the“intermediateaxis”datapointiscalculatedfromtheaverageoftheA-,B-,andC-axes.Theobservermustdeterminethesurfaceareaofthestreambankscomparedtothestreambedtoobtainaproportionalbankfullareasample.Topreventover-samplingbankmaterial,if10observationspertransectareobtained,generallyonlyonebanksampleeveryothertransectistaken.Thisgivesa5%representativesample.

Active Bed Riffle Pebble CountTheactivebedrifflepebblecountcharacterizesthebedmaterialatthesurveyedrifflecross-section.Onehundredparticlesaremeasuredatevenly-spacedintervalsacrosstheactivebedofthesurveyedrifflecross-section.Ifthestreamwidthissmall,thenmorethanonetransectmaybetakentoobtain100observationsaslongasthevaluesrepresentthebedofthesurveyedrifflecross-section.Theactivebedrifflepebblecountdataisusedforhydrauliccalculations(R/D84)toestimatevelocityandforsedimentcompetencecalculations.

Copyright © 2018 Wildland Hydrology

Copyright © 2018 Wildland Hydrology


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are

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iffle

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A

B

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Figure 18. Comparing the three axes of a particle. The B-axis, or intermediate axis, is selected for data entry (the narrowest portion that may fit through or be retained on a sieve tray of a given size), unless the particle shape is long and linear – then the average of the A-, B-, and C-axes is recorded.

Representative Pebble Count Instructions1. Pacetheentirestudyreach;recordpoollengthsandnon-pool lengthsusingthepebblecount form(seeTable 6foranexample).

2. Calculatethepercentofthereachcomposedofpoolandnon-poolbedfeatures.3. Determinethenumberofpoolcross-sectionsandnon-pool cross-sectionsneeded(to simplifythecalculations,measure10particlesat10totalcross-sections).

4. Identifybankfullonbothsidesofthechannelatyourfirstcross-sectionlocationanddetermine thesamplinginterval(sampleatequalincrementsacrosstheentirebankfullchannel).

5. Beginthepebblecountbelowthebankfullelevation.Toavoidbiasofselectinglarger particles,theobservershouldlookawayfromthechannelbedandselectthefirst particletouchedbythetipofindexfingeratobserver’stoe.Often,a5%banksampleis taken(onesampleeveryothertransect)toavoidskewingtheparticlesthatmakeupthe boundaryofthechannel.

6. MeasurethelengthoftheB-axisinmillimetersandmarkadotinthecorrectcolumn androwasinTable 6.Iftheparticleislinear-shaped,averagetheA-,B-,andC-axes(Figure 18).

7. Continueuntil10particlesat10differentcross-sectionshavebeenmeasuredin proportiontothebedfeaturesofthereach.

8. FollowtheexamplecalculationsinTable 6.9. Plottheupper limitofeachsizeclassandthecorrespondingcumulativepercentfiner thanvaluesonthex-axisandprimaryy-axisrespectivelyforpool,non-pool,andtotal(Figure 19).

10. Plottheupper limitofeachsizeclassandthecorrespondingvaluefromthetotal# columnonthex-axisandthesecondaryy-axisforthecombinednon-poolandpooltotal(Figure 19).

11. Record the D16,D35,D50,D84,D95,andD100ontheappropriateforms.

Active Bed Riffle Pebble Count Instructions1. RepeatSteps6–11,onlysamplingontheactivebedoftherifflecross-section(100particlecounts).

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pebble count overview - usfws€¦ · pebble count overview . representative . reach pebble count...

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