30 June 2015

Field measurements –do they provide the ‘absolute truth’?

Ivo Wenneker and Niels G. Jacobsen

Key questions

Rijkswaterstaat (Netherlands):

“How accurate are field measurements of:• Water levels from measurement poles?• Waves in the surf zone?”

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Overview

1. Introduction

2. Case study I: water level measurements from measurement poles

3. Case study II: wave measurements in the surf zone

4. Conclusions

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1. Introduction

Let Qtrue be the correct value (‘absolute truth’) of quantity Q at somelocation and some moment in time

Examples of quantity Q:• wave height Hm0• wave period Tm-10• wave period Tm02• water level h• tidal current velocities U and V• …

‘Redundant’ field measurement data: Q has been measured at one locationsimultaneously with different instruments, yielding datasets Q1 and Q2

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1. Introduction

If measurements would always provide the ‘absolute truth’, then:

Q1 = Q1 – Qtrue = 0 (measurement error in 1)Q2 = Q2 – Qtrue = 0 (measurement error in 2)

1-2 = Q1 – Q2 Q1 Q2 = 0 (measured difference between 1 and 2)

It appears that, often, 1-2 0. So, Q1 and/or Q2 deviate from the ‘absolute truth’

Van de Casteele plots (probability density plots of 1-2 versus some quantity R):indicate whether there exists a correlation between the measurement accuracy ofquantity Q and some quantity R

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1. Introduction

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Example Van de Casteele plot

DLM-radar = hDLM - hradar.

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2. Case study I: water levels from measurement poles

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2. Case study I: water levels from measurement poles

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2. Case study I: water levels from measurement poles

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2. Case study I: water levels from measurement poles

Stepgauge DLM: Float gauge instilling well

Radar

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2. Case study I: water levels from measurement polesIndicative values, applicable for typical Dutch conditions (tidal range of afew meters; wave heights up to a few meters) and Rijkswaterstaatmeasurement poles (diameter 1.5 m; 2 DLM openings; stepgauge and radarat 40 cm resp. a few meters from pole).

DLM Stepgauge RadarA. Properties of measurement technique/instrumentPhase lag between water level outside and inside pole < 1 cm --- ---Density differences outside and inside. meas.pole < 3 cm --- ---Wave diffraction and Bernoulli effect against instrument --- < few cm ---Distance (5 cm) between stepgauge electrodes --- < 1 to 2 cm ---Detection of water surface in rough circumstances --- < few cm < few cmWater surface outside measurement range Seldom Seldom SeldomSpecular reflection due to finite radar footprint --- --- < few cmB. Presence of measurement pole on local hydrodynamicsWave diffraction (run-up/down against pole) < 30 cm < 30 cm < 10 cmBernoulli effect < 5 cm < 5 cm < 0.5 cmC. Determination of vertical reference level (NAP)Determination of vertical reference level (NAP) Possible Possible PossibleD. MiscellaneousClosure of DLM openings Possible --- ---Ice, algae, sediment, damage, floating debris, etc. Possible Possible Possible

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2. Case study I: water levels from measurement poles

Wave diffraction around pole Bernoulli effect due to currents

Depends on: wave height, waveperiod, spectral shape, wave

direction, directional spreading,local depth

Depends on: current velocityand direction

Regular wave: H = 2 m; T = 5, d = 8m

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2. Case study I: water levels from measurement poles

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2. Case study I: water levels from measurement polesConclusions for well-calibrated water level measurements from measurementpoles

General:During mild conditions, the errors are small (up to a few cm)During storms, the errors can be very large (10 – 30 cm, and possibly larger).

Accuracy depends on measurement configuration (= instrument + measurementpole):

Interaction between measurement pole and local hydrodynamics:Wave diffractionCurrents

Properties of instrument and measurement techniqueMeasurement configuration with radar provides the most accurate water level data,because farthest away from pole

Improving accuracy:Optimising instrument position wrt measurement pole (in particular:increase the distance between them).30 June 2015 14

3. Case study II: wave measurements in the surf zone

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3. Case study II: wave measurements in the surf zone

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3. Case study II: wave measurements in the surf zone

Redundant wave observations in both MP6 and MP67• pressure instruments (pressure sensor and S4)• stepgauge

Under mild conditions (Hm0 < 0.3 m)• pressure data stepgauge data

Under storm conditions (Hm0: 2 – 3 m; Tm-10: 7 – 10 s; Tm02: 4 – 6 s)• pressure data stepgauge data

• Hm0(stepgauge) Hm0(pressure) + 0.4 m (~ 20 %)• Tm-10(stepgauge) Tm-10(pressure) – 0.5 s (~ 7 %)• Tm02 (stepgauge) Tm02(pressure) – 0.5 s (~11 %)

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3. Case study II: wave measurements in the surf zone

Pressure transformation lim lim max

coshˆˆ , min , ,cosh

kdpa K K K K Kg kz

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3. Case study II: wave measurements in the surf zoneIndicative values for situation at Petten

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Pressuresensor

Stepgauge

A. Properties of measurement technique/instrumentPressure transformation: cut-off frequency to avoid noiseblow-up

Hm0: 5%Tm-10: 5%

Tm02: significantand scatter

---

Pressure transformation: nonlinear wave effects Hm0: smallTm-10: small

Tm02: some scatter

---

Pressure transformation: wave-current interaction Small ---B. Presence of measurement pole on local hydrodynamicsWave diffraction (run-up/down against pole) Small Hm0: 15%

Tm-10: smallTm02: small

C. Instrument gaugingInstrument gauge Small SmallD. MiscellaneousIce, algae, sediment, damage, floating debris, etc. Possible Possible

Conclusions for well-calibrated wave measurements in the surfzone

General:During mild conditions, the errors are smallDuring storm conditions, the errors can be very large (~ 40 cm in waveheight; ~ 0.5 s in wave period)

Accuracy depends on measurement configuration (= instrument +measurement pole):

Pressure transformationWave diffraction around measurement pole

Improving accuracy:Optimising instrument position wrt measurement pole (in particular:increase the distance between them).Put pressure sensor high in water column

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3. Case study II: wave measurements in the surf zone

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Conclusions for well-calibrated water level and wave measurements

GeneralDuring mild conditions: errors can be smallDuring storm conditions: errors can be large

Accuracy depends on measurement configurationA. Properties of measurement technique/instrument, including analysisB. Presence of measurement pole on local hydrodynamics (waves and

currents)C. Determination of vertical reference level / GaugingD. Miscellaneous

Recommendation to improve accuracyAcquire a thorough quantitative understanding of the measurementconfiguration

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4. Overall conclusions

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Thank you

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