Field measurements may not always provide the absolute truth about measured quantities like water levels and waves. Redundant field measurements using different instruments often disagree, indicating measurement errors. The accuracy of field measurements depends on factors like the measurement technique, interactions with infrastructure like measurement poles, and environmental conditions. During storms, measurement errors can be very large, over 10 centimeters for water levels and 0.5 seconds for wave periods. To improve accuracy, the measurement configuration should be optimized by considering instrument properties, placement relative to infrastructure, and local hydrodynamics.

1. 30 June 2015
Field measurements –
do they provide the ‘absolute truth’?
Ivo Wenneker and Niels G. Jacobsen

2. Key questions
Rijkswaterstaat (Netherlands):
“How accurate are field measurements of:
• Water levels from measurement poles?
• Waves in the surf zone?”
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3. 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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4. 1. Introduction
Let Qtrue be the correct value (‘absolute truth’) of quantity Q at some
location 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 location
simultaneously with different instruments, yielding datasets Q1 and Q2
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5. 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 of
quantity Q and some quantity R
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6. 1. Introduction
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Example Van de Casteele plot
DLM-radar = hDLM - hradar.
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7. 30 June 2015
2. Case study I: water levels from measurement poles
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8. 2. Case study I: water levels from measurement poles
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9. 2. Case study I: water levels from measurement poles
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10. 2. Case study I: water levels from measurement poles
Stepgauge
DLM: Float gauge in
stilling well
Radar
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11. 2. Case study I: water levels from measurement poles
Indicative values, applicable for typical Dutch conditions (tidal range of a
few meters; wave heights up to a few meters) and Rijkswaterstaat
measurement poles (diameter 1.5 m; 2 DLM openings; stepgauge and radar
at 40 cm resp. a few meters from pole).
DLM Stepgauge Radar
A. Properties of measurement technique/instrument
Phase 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 cm
Water surface outside measurement range Seldom Seldom Seldom
Specular reflection due to finite radar footprint --- --- < few cm
B. Presence of measurement pole on local hydrodynamics
Wave diffraction (run-up/down against pole) < 30 cm < 30 cm < 10 cm
Bernoulli effect < 5 cm < 5 cm < 0.5 cm
C. Determination of vertical reference level (NAP)
Determination of vertical reference level (NAP) Possible Possible Possible
D. Miscellaneous
Closure of DLM openings Possible --- ---
Ice, algae, sediment, damage, floating debris, etc. Possible Possible Possible
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12. 2. Case study I: water levels from measurement poles
Wave diffraction around pole Bernoulli effect due to currents
Depends on: wave height, wave
period, spectral shape, wave
direction, directional spreading,
local depth
Depends on: current velocity
and direction
Regular wave: H = 2 m; T = 5, d = 8m
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13. 2. Case study I: water levels from measurement poles
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14. 2. Case study I: water levels from measurement poles
Conclusions for well-calibrated water level measurements from measurement
poles
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 + measurement
pole):
Interaction between measurement pole and local hydrodynamics:
Wave diffraction
Currents
Properties of instrument and measurement technique
Measurement 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

15. 3. Case study II: wave measurements in the surf zone
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16. 3. Case study II: wave measurements in the surf zone
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17. 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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18. 3. Case study II: wave measurements in the surf zone
Pressure transformation lim lim max
coshˆ
ˆ , min , ,
cosh
kdp
a K K K K K
g kz
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19. 3. Case study II: wave measurements in the surf zone
Indicative values for situation at Petten
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Pressure
sensor
Stepgauge
A. Properties of measurement technique/instrument
Pressure transformation: cut-off frequency to avoid noise
blow-up
Hm0: 5%
Tm-10: 5%
Tm02: significant
and scatter
---
Pressure transformation: nonlinear wave effects Hm0: small
Tm-10: small
Tm02: some scatter
---
Pressure transformation: wave-current interaction Small ---
B. Presence of measurement pole on local hydrodynamics
Wave diffraction (run-up/down against pole) Small Hm0: 15%
Tm-10: small
Tm02: small
C. Instrument gauging
Instrument gauge Small Small
D. Miscellaneous
Ice, algae, sediment, damage, floating debris, etc. Possible Possible

20. Conclusions for well-calibrated wave measurements in the surfzone
General:
During mild conditions, the errors are small
During storm conditions, the errors can be very large (~ 40 cm in wave
height; ~ 0.5 s in wave period)
Accuracy depends on measurement configuration (= instrument +
measurement pole):
Pressure transformation
Wave 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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21. Conclusions for well-calibrated water level and wave measurements
General
During mild conditions: errors can be small
During storm conditions: errors can be large
Accuracy depends on measurement configuration
A. Properties of measurement technique/instrument, including analysis
B. Presence of measurement pole on local hydrodynamics (waves and
currents)
C. Determination of vertical reference level / Gauging
D. Miscellaneous
Recommendation to improve accuracy
Acquire a thorough quantitative understanding of the measurement
configuration
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4. Overall conclusions
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22. Thank you
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