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SEA-LEVEL AND SEA-STATE MEASUREMENTS WITH RADAR LEVEL SENSORS

Dr. Ulrich Barjenbruch1 and Jens Wilhelmi2

The German Federal Institute of Hydrology (BfG) developed a cost-efficient method to monitor the water level and the wave condition in coastal waters. This measuring method has proved its func-tionality for several years in practice. The technology introduced here needs hardly any maintenance and its results show very good correlation in measuring accuracy with those of the conventional methods used so far in sea-state monitoring, such as the wave-rider buoy.

INTRODUCTION The dimensioning and the safety of coastal-defence structures and port facilities presuppose good knowledge about the state of the adjacent sea areas. This knowledge also allows to derive data for the precise modelling of extreme sea-state situations. Moreover, in harbours incoming and leaving ships need exact information about the wind sea and the prevailing swell. These requirements are confronted with shrinking budgets that are available for the monitoring of the sea state.

RADAR SENSOR FOR SEA STATE MEASUREMENTS Against this background, the German Federal Institute of Hydrology (BfG) de-veloped a cost-efficient method of measuring the sea level and the sea state on the basis of a commercial radar level sensor that is normally used to indicate the filling-level in tanks. Such systems are at present in operation e.g. in the North Sea at the lighthouse "Alte Weser", at the gauge "Borkum", on the research platform „FINO“ and in Italy in the lagoon of Venice in conjunction with the MOSE flood-defence system (see Figure 1).

1,2 Department: M1 “Hydrometry and Hydrological Survey”, Federal Institute of Hydrology, Am Mainzer Tor 1,

Koblenz, 56068, Germany

Figure 1: Radar wave gauges (from the lef)t: Gauge “Borkum”, Lighthouse “Alte Weser”, re-search platform “FINO”, and the wave gauge in the lagoon of Venice (Italy)

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Radar

Reference

Figure 2: Radar-Sensor test in wave flume

The radar level sensor was tested intensively in a wave flume for its measuring un-certainty. Figure 2 shows the measuring device and below the hydrographs of the radar sensor and the reference with a breaking wave of nearly 4 metres height. The principle of the radar wave gauge is the rapid measurement (scanning rate 2Hz) of the distance between the sensor (mounted on a building in the sea, such as a lighthouse) and the water surface. A post-processing of the raw data from the

radar sensor is strictly recommended for the exact monitoring of the sea state. For this purpose, the BfG developed special routines. First, the data should be checked for outliers. In the outlier test, all measured data are deleted which de-

viate too much in speed and in accel-eration of the water body. Then, interpo-lation of the deleted outliers and resam-pling of the data fol-low. Finally, the data are subjected to a bandpass filter to exclude artefacts. Then the microcon-

troller can compute in real-time from these measurements the sea-state variables "water level", "significant wave height" (HS), "highest wave", and "mean wave period". Post-processing routines allow to determine additional characteristics of the sea state, such as the "spectral energy density" (Sf) or a spectrogram of the sea state as it is shown in Figure 6. An example of the spectral energy density (Sf) for a 15-minute period is given in Figure 4. On the left-hand side, the swell and the

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Figure 3: Resampling of the raw data with hermite interpola-tion and band-pass filter with a 2nd order Chebyshev filter

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S(f

) (m

2 s)

frequency [Hz]

fp1 = 0.096fp2 = 0.17fp3 = 0.19

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S(f

) (m

2 s)

frequency [Hz]

fp1 = 0.21fp2 = 0.088fp3 = 0.23

Figure 4: Spectral energy density on 19 Oct. and 1 Nov. 2006. In the left-hand graph a clear distinction can be made between swell and wind sea, while the right-hand graph highlights the prevailing intensive wind sea.

wind sea are clearly separated, whereas the right-hand graph shows a prevailing strong wind-sea situation.

In Figure 6, one can clearly recog-nise the further temporal develop-ment of the sea-state spectrum. In the lower frequency range, close to 1 Hz, one sees the temporal devel-opment of the swell. On 24, 29, and 30 September, a strong sea state was measured, which is also seen in the recorded wave deflec-tion in Figure 5. The spectrogram is computed with a 15-minute wide sliding window of a Fast Fourier Transformation (FFT).

Figure 6: Spectrogram of the sea state for 10 days, calculated with a sliding win-dow FFT

BfG wave gauge / time [days] from: 20.09.2008

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Figure 5: Wave deflection corresponding to the event shown in Figure 6

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BfG wave gauge / time [days] from: 01.05.2008

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wave rider / time from: 01.05.2008

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Figure 9: Comparison of the spectrograms of the BfG wave gauge and the wave-rider buoy.

A commercial wave-rider buoy (see Figure 7) is an-chored in the immediate vi-cinity of the research plat-form FINO 1. A comparison of the data from these two systems is given in Figures 8 and 9. There is good agree-ment between the measure-ments regarding the basic structures of the significant wave height HS. The data gap in the middle of the Figure 8 originated from a power breakdown on the research

platform.

Figure 8: Comparison of measurements of the BfG wave gauge and the wave-rider buoy.

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Figure 7: The research platform FINO 1 with a wave buoy in approximately 100 metres distance.

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Figure 9 imparts an im-pressive overview of the temporal characteristics of the sea state over more than 15 days. At a closer look, one can also recog-nize that the BfG method has a somewhat lower signal-to-noise ratio (S/N) compared with the wave-rider buoy method. This is perhaps related to the sup-porting structure on which the radar sensor is mounted. Nevertheless,

the spectral energy density values measured by both systems show the good correlation between the data of the two systems. In Figure 9 one can identify three separate sea-state condition at the end of 12 May 2008. This is also visible in the diagram of the spectral energy density of this time period (Figure 10). The spectrum shows a double peak swell and a slight wind sea (0.43 Hz). The match of the measuring data is very good, par-ticularly as both test points are 100 metres distant. In conclusion, we can state that both systems have their advantages and draw-backs: � The wave buoy is an established and approved system. The BfG system is a

relatively new development. � The wave-rider buoy can measure the wave direction. With the BfG system

this is not yet possible in this stage of development. � The measuring accuracy is quite the same with both the systems. � With its low total cost of ownership and maintenance requirements, the BfG

system has decisive advantages. The investment for the wave-rider buoy amounts to approximately €80,000; that for the BfG system to about €5,000.

� In contrast to the wave-rider buoy, the radar sensor is able to measure the water level.

� The BfG system needs a supporting structure. � The wave-rider buoy is at higher risk of getting lost totally during storm.

ACKNOWLEDGMENTS

We thank the German Federal Maritime and Hydrographic Agency (BSH) for the good cooperation in testing of our measurement technology on the research platform FINO 1.

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Spectral density

BfG w ave gauge / frequency[ Hz]

S(f

) [m

2 s]

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Spectral density

w ave r ide r / Frequency [Hz]

S(f

) [m

2 s]

fp1 = 0.082 [Hz]

fp2 = 0.13 [Hz]fp3 = 0.44 [Hz]

fp4 = 0.25 [Hz]

fp1 = 0.084 [Hz]

fp2 = 0.13 [Hz]fp3 = 0.43 [Hz]

fp4 = 0.25 [Hz]

Figure 10: Spectral energy density with double peak swell and a slight wind sea.

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REFERENCES Kranz, S., Zenz, T. and U. Barjenbruch (2001): Radar: Is it a New Technology application to water

level gauging, Phys. Chem. Earth (C), Vol. 26, No. 10-12, pp.751-754 Barjenbruch, Mai, Ohle, Mertinatis, 2002. Proc. of the Hydro 2002 Conference, 328-337, Germany-

Kiel Monitoring Water Level, Waves and Ice with Radar Gauges Blasi, C.J., and U. Barjenbruch, 2006. Comparison of Radar Devices in Germany. In: Manual on

Sea-level Measurements and Interpretation, Vol. IV: JCOMM Technical Report No. 31, WMO/TD. No. 1339, UNESCO 2006