cross-section overview cross-section instructions …surveying skills are required for the...

Stream Habitat Measurement Techniques Day 2: Field Day

Cross-Section

Cross-section Overview

Cross-Section Instructions

Sample Cross-section Survey Notes

Cross-section Survey Forms

Copyright © 2018 Wildland Hydrology

Stream Habitat Measurement Techniques

Cross-SectionsCross-sectiondataatrifflelocationsprovidesamajorityofthemorphologicalparametersrequiredforstreamclassification.Bankfull cross-sectional area,bankfull width,bankfull mean depth,bankfull maximum depth,bankfull width/depth ratio,andentrenchment ratioaredeterminedfrom the cross-section.Calculationofentrenchment ratio isillustratedin Figure 9.Basicsurveyingskillsarerequiredforthecross-sectionalsurveyandaredescribedinHarrelsonet al.(1994).Asummaryofinstructionsandcalculationsareprovidedinthefollowingsection.

Inner Berm (IB)An inner bermisadepositionalfeatureinalluvialchannelsthatrepresentsthelowflowchannelboundary(Figure 6).Itisoftenrelatedtomeanannualdischarge.

Bankfull Cross-Sectional Area (Abkf)Thecross-sectionisdividedintomultipletrapezoidsandtheareaofeachindividualtrapezoidiscomputed.Thetotalcross-sectional areaisdeterminedbyaddingtheareaofalltheindividualtrapezoids(Figure 11).

Bankfull Width (Wbkf)Bankfull widthisthesurfacewidthofthestreammeasuredatthebankfullstage(Figure 7).

Bankfull Mean Depth (dbkf)Bankfull mean depthiscomputedbydividingthebankfull cross-sectional areabythebankfull width (Figure 8).

Bankfull Maximum Depth (dmax)Bankfull maximum depth isthemeasurementofthedepthofthethalwegtothebankfullstage(Figure 8).

Bankfull Width/Depth Ratio (Wbkf / dbkf)Bankfull Width/depth ratio is the bankfull widthdividedbythe bankfull mean depth.Thewidth/depthratiodescribesthechannelshape(largenumberindicateswideandshallow;smallnum-berindicatesnarrowanddeep).

Entrenchment Ratio (ER)Entrenchment istheverticalcontainmentofariverandisquantitativelydefinedastheflood-prone area widthdividedbythebankfull width. The Flood-prone area width (Wfpa)isthewidthofthechannelatanelevationthatistwicethebankfullmaximumriffledepth(Figure 9). Flood-prone area widthmustbemeasuredperpendiculartothefalllineofthevalley.



Figure 5. Recommended cross-section locations for bankfull cross-sectional area measurements.

Riffle / Pool SYSTEM

Point BarFLOW

FLOW

Cross-Sections

Cutbank / Outside Turn Area

Step / Pool SYSTEM

POOL

STEP

RUN

POOL

1

2

3

4

43

21

PLAN VIEW CROSS-SECTION VIEWS

PROFILE VIEW

The location of cross-section 3 is recommended for “bankfull” measurements.

Cross-Section LocationsAppropriatelocationstomeasurebankfullwidthscorrespondingtoaconsistentnormalhighflow(bankfull)stageareindicatedforriffle/poolandstep/poolchannelsinFigure 5.



Figure 7. Bankfull width and bankfull cross-sectional area obtained from the riffle cross-section.

Figure 6. Typical channel features for a C Stream Type in an unconfined terraced, alluvial valley (U-AL-FD).

Figure 8. Bankfull mean depth and bankfull maximum depth obtained from the riffle cross-section.



1

5

44

2

6

SURVEYOR’S ROD

FLOOD-PRONE AREA

BANKFULL STAGEMAX DEPTH

2 x MAX DEPTH

SURVEYOR’S LEVEL

Elevation / Width

STEPS: 1. Obtain a ROD READING for an Elevation at the “MAX DEPTH” Location.2. Obtain a ROD READING for an Elevation at the “BANKFULL STAGE” Location.

3. Subtract the “Step 2” reading from the “Step 1” reading to obtain a “MAX DEPTH” value.Multiply the “MAX DEPTH” value by 2 for the “2 x MAX DEPTH” value.

4. Subtract the “2 x MAX DEPTH” value from the “Step 1 Rod Reading” for theFLOOD-PRONE AREA Location Rod Reading. Move the rod upslope, online withthe cross-section until a Rod Reading for the Flood-Prone Area Location is obtained.

5. Mark the Flood-Prone Area (FPA) locations on each bank. Measure the DISTANCE between the two “FPA” locations.6. Determine the DISTANCE between the two BANKFULL Stage locations.

7. Divide the FPA WIDTH by the BANKFULL WIDTH to calculate the ENTRENCHMENT RATIO.Figure 9. Determining Entrenchment Ratio.

Figure 10. Determining Flood-Prone Area Width (Wfpa).



Cross-Section Instructions1. Setupthesurveyinginstrumentinalocationwheretheentirecross-sectioncanbe

viewed.Theinstrumentshouldbeplacedatanelevationhigherthanthehighestfeaturerequiredforthesurvey.Ideally,onlyoneinstrumentsetupwillberequiredtosurveytheentirecross-section;however,determiningtheflood-prone area width mayrequiremultipleinstrumentsetupsduetodensefoliage.

2. Stretchthetapeacrossthechannel(zero on left bank)makingsurethetapeisperpendiculartothebankfulldischargeflow–notthebaseflow.

3. Backsight(BS)abenchmarkorpermanentfeatureusedforrelocationorresurveyofcross-section.

4. CalculateHeightofInstrument(HI);HI=BS+Elevation(knownorrelative).5. Obtainrodreadings(Fore-Sights)atmajorbreaksinbedelevationandkeyfeatures,

suchasleftbankfull(LBKF),leftedgewater(LEW),Thalweg(THL),rightedgewater(REW)andrightbankfull(RBKF).

6. Recordthedistanceonthestationingtape,thecorrespondingrodheightandfeaturenotesinthecross-sectionforms(seeTable 2forexample).

7. Plotthecross-section(Figure 10b);calculatethebankfull cross-sectional area,bankfullwidth,bankfull mean depth,bankfull maximum depth,andbankfull width/depth ratio(Figure11,Table 3).Measuretheinnerbermdimensions.

8. Measuretheflood-prone area width(widthofthechannelatanelevationthatistwotimesthemaximumbankfulldepth)(Figure 10)‒besuretomeasureperpendiculartothefalllineofthevalley.Calculateentrenchment ratio (Figure 9).

9. Forrifflecross-sections,checktomakesurethecross-sectionalareaisreasonableusingtheappropriateregionalcurves;makesurethebankfullmeanvelocityisreasonable(mean velocity=bankfull discharge/bankfull cross-sectional area).

11. Recordalldataintheappropriatesurvey data forms.

Day 2: Field Day



BM

20171270

363228262322

5.0 1056.00

9.9510.009.509.008.808.258.007.907.706.60

100

98.40

95.0595.0095.5096.0096.2096.7597.0097.1097.30

99.00Benchmark

LBKF

IB

THL

Lower Nevada Creek nr Helmville, MT

Team #4

8/8/06nr USGS Gauge # 12335500

10 20 00 02

5045424139

75706256

9.50

6.006.106.507.607.808.008.459.00 96.00

99.0098.9098.5097.4097.2097.0096.55

95.50 REW

RBKF

LEW

SITE:Location:

Date:

Observers:Distance,

Backsight Fore-Sight Height ofInstrumentPoint, or

STATION B S H IItem

F S Elevationftft ftftft

1

2

3

4

7

10

14

18

5

8

11

15

19

21

25

23

27

6

9

13

17

12

16

20

22

26

24

28

NOTES

REMARKSCOMMENTS

CROSS - SECTION 1SURVEY DATA

HUC: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

105

Table 2. Form to record cross-section notes with example data.

Riffle Cross-SectionGroundPoints

BankfullIndicators

WaterSurfacePoints

Inner BermIndicators

Ele

vatio

n (ft

)

Horizontal Distance (ft)

94

95

96

97

98

99

100

0 10 20 30 40 50 60 70 80

Wbkf = 25 Dbkf = 1.2 Abkf = 30Wib = 15 Dib = .67 Aib = 10.1

Ri�e Cross-Section

Horizontal Distance (ft)

Bankfull Indicators

Water Surface Points

Ground Points

Width of Flood-Prone Area

Wbkf = 25 dbkf = 1.2 Abkf = 30.0

Inner Berm Indicators

Wib = 15 dib = 0.67 Aib = 10.1

Bankfull Stage

Inner Berm

Figure 10b. Plotted cross-section using measurements in Table 2

(graph from RIVERMorph).

Day 2: Field Day



SITE:Location:

Date:

Observers:Distance,

Back-Sight Fore-Sight Height ofInstrumentPoint, or

STATION B S H IItem


1

2

3

4

7

10

14

18

5

8

11

15

19

21

25

23

27

6

9

13

17

12

16

20

22

26

24

28

NOTES

REMARKSCOMMENTS

CROSS - SECTION SURVEY DATA

HUC: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Day 2: Field Day



SITE:Location:

Date:

Party / Notes:Distance,

Back-Sight Fore-Sight Height ofInstrumentPoint, or

STATION B S H IItem


1

2

3

4

7

10

14

18

5

8

11

15

19

21

25

23

27

6

9

13

17

12

16

20

22

26

24

28

NOTES

REMARKSCOMMENTS

CROSS - SECTION 1

SURVEY DATA

HUC: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Day 2: Field Day

cross-section overview cross-section instructions …surveying skills are required for the...

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