APPLICATION OF RADAR GAUGES TO MEASURE THE WATER LEVEL AND THE STATE OF THE SEA

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1 APPLICATION OF RADAR GAUGES TO MEASURE THE WATER LEVEL AND THE STATE OF THE SEA Jens Wilhelmi 1 and Dr. Ulrich Barjenbruch 1 The sea state in coastal waters and on the open sea used to be measured with pressure sensors, acoustic (ADCP) systems, or wave-following buoys. These measuring techniques are both costly and labour intensive and require regular maintenance. A novel device for operative wave-height measurements, using a low-cost and low-maintenance radar sensor, was developed and tested by the German Federal Institute of Hydrology (BfG), Department M1 Hydrometry and Hydrological Survey. Liquid-level radar sensor based water-level gauges on inland waters Water-level gauges based on liquid-level radar sensors for industrial applications have been used on inland waters for a couple of years now. Their main advantages lie in the easy installation, the information gained in relatively high measuring frequency, their reliability, and their low costs. In 2002, the BfG began to test whether such commercial radar liquid-level sensors are also suitable to measure the water level and, particularly, the sea state in coastal waters. Wave-flume experiments In the first step, the radar sensors were tested under laboratory conditions in wave-flume experiments (Wellenkanal Schneiderberg WKS) at the Franzius Institute of the University Hannover and in the large wave channel (Großer Wellenkanal GWK) of the Coastal Research of the universities Hannover and Braunschweig. Reference measurements make it possible to determine the waterlevel oscillations very exactly by these experiments. First, the sensors were tested with very slow water-level variations, that means without any waves at all, in order to verify the accuracy of the measured distance between sensor and water surface. The next steps were test runs with regular and irregular waves. 1 Department: M1 Hydrometry and Hydrological Survey, Federal Institute of Hydrology, Am Mainzer Tor 1, Koblenz, 56068, Germany 1

2 2 Table 1. Selection of results from the wave-channel experiments (good results are denoted by grey fields) Endress & Hauser Krohne VEGA Siemens Enraf Slow water level variation Δ < 0.5 cm Δ < 0.5 cm Δ < 0.5 cm Δ < 0.5 cm Δ < 0.5 cm Regular waves H m0 x σ T 01 x σ Irregular waves H m0 x σ T 01 x σ Four of the tested five sensors seemed to be suitable so far; the best results were obtained with the sensor VEGAPULS 42. The firmware of the VEGA radar sensor had to be adjusted, in order to achieve the desired 2-Hz sampling rate, for instance, by removing software components for internal validly checks. Hence, these checks have to be performed in post-processing. More details of the results from the wave-channel experiments can be found in the reports published by the Franzius Institute. Field tests After the completion of the laboratory experiments, the radar sensors were tested in the field in a full-year routine operation to study their practicability and to verify the results from the laboratory research and development. At the gauging station BORKUM SÜDSTRAND on the North-Sea island of Borkum, a measuring station was build for this purpose in the year At this site, the wind blows predominantly from North-west, and the sea state is not influenced by obstacles like islands or shoals, while passing ships may generate waves there.

3 3 Figure 1. Test set-up with the four field-tested radar sensors at the gauging station BORKUM SÜDSTRAND. The radar raw data, i.e. the measured water surface oscillations, were transmitted to the office along with supplementary environmental data (such as temperature and wind velocity/direction). The post processing of the data was accomplished with the mathematical-software Matlab. In a very important first step, outliers had to be removed, taking the possible maxima of velocity and acceleration of water surfaces as criteria. Moreover, a statistical outlier test procedure was applied. Figure 2. Example of a data set over one hour duration. (Identified outliers are marked by circles)

4 4 All gaps left by eliminated data had to be filled with interpolated values, where a hermitic polynomial was applied. In contrast to a spline interpolation, a hermitic polynomial does not add artificial extremes. After band-pass filtering, which removes artificial noise, the sea-state parameters were calculated with the aid of the WAFO Matlab-library, developed by the University Lund/Sweden. The water level was computed by applying an exponential smoothing procedure. The derived results were compared with the measurements from a very accurate magnetostrictive reference sensor and those of the official float gauge at this site. Figure 3. Error distribution [in cm], between water-level data obtained by radar sensors and the reference sensor (left) and the official float gauge (right). From the error-distribution plots shown above, one can see that the results are rather good. Another aspect examined in the context of this project was the influence of icing on the quality of radar measurements of the water level and on the uncertainty and outlier rate of these measurements. Significant deviations from the hydrograph of the official gauge were observed only in the measurements of one of the radar sensors, while the others responded merely by a slight increases in the measuring uncertainty. Other possible influences examined were changing salinity of the water in the course of the tidal cycle, on the one hand, and the influence of the height of the sensor above the water surface and the associated footprint size of the radar beam on the other hand. No influences were observed in this regard in the studies of this project.

5 As to measurements of the sea state it is essential that the radar sensors operate with a high scanning rate and a low noise ratio. In our tests, the VEGAPULS 42 was the only sensor to meet these criteria. A more comprehensive description of the field test and its results is given in the report Waves and Ice With Radar Gauges. 5 Comparison with Waverider buoy Another field test, that compared the performance of a radar sensor with the output of a Waverider buoy, was undertaken on the research platform FINO1 which is also situated in the North Sea. The radar sensor is mounted directly on the platform, while the Waverider buoy is anchored in a distance of 100 m. 6 8 Hm0 / m WaveRider buoy slope = coeff. of corellation = T01 / s WaveRider buoy slope = 1.02 coeff. of corellation = H m0 / m radar system T 01 / s radar system Figure 4. Comparison of significant wave height H m0 (left) and the mean wave period T 01 (right) between radar system and Waverider buoy over a measuring campaign of three months. The three-month measurements of the radar system and the Waverrider buoy shown in Figure 5 deviate only slightly. The standard deviation in the differences of the significant wave height is merely 8 cm, and that of the mean wave period is less than ¼ sec.

6 6 Figure 5. Amplitude spectra of water-surface oscillations over a period of four days. The spectra shown above illustrate the temporal variations of the swell (frequency ~ 0.1 Hz) and the wind sea (frequency ~ 0.3 Hz). The upper image refers to the radar data and the lower one to the output of the Waverider buoy. The results are in rather good agreement. The Waverider buoy has an upper cutoff frequency of 0.65 Hz, whereas the radar sensor operates at 2Hz, what allows to detect water-surface oscillations up to 1 Hz. Table 2. Comparison of assets and drawbacks between the radar system and the Waverider buoy. radar system Waverider buoy well-established o ++ wave direction - + accuracy + + costs ++ - maintenance ++ - water level + - mounting requirements - o risk of loss during storm o - The comparison in Table 2 identifies the individual strengths and weaknesses of both systems. The well-established Waverider system is also able to measure the wave directions; a radar system might achieve this by using multiple sensors.

7 The radar system is able to determine the sea state and the water level. The main advantage of the radar system is its low maintenance requirement. 7 Operational radar wave gauge Because of the good test results the BfG decided to develop in 2006 an operational radar wave gauge for the Waterways and Shipping Office Bremerhaven (Figure 6). The software is running on a small ruggedized PC with embedded Linux and makes use of the software Octave, which is in fact a freeware version of Matlab. Water-level and sea-state parameters are computed in real time and transmitted to the coastal communication network which distributes the data instantaneously. The total costs of the radar-based measuring system amount to about Euro To date, two such systems have been installed. One at the lighthouse Alte Weser and a second one in the lagoon of Venice (Italy) where it is used in conjunction with the flood defence project Mose. Figure 6. Operative radar wave-gauge system at the lighthouse Alte Weser (left) and in the lagoon of Venice (right)

8 8 Figure 7. Evaluation of the sea-state data recorded during a storm surge on 1 November 2006 at the lighthouse Alte Weser" Conclusion and Outlook Industrial radar liquid-level sensors are available on the market that offer a cost-efficient alternative in sea-state monitoring. The data derived from the radar system are in good agreement with those of the well-established Waverider buoy. It is envisaged to provide the existing coastal radar-based water-level measuring systems with the ability to determine simultaneously the sea-state parameters.

9 9 REFERNCES Barjenbruch, Mai, Ohle, Mertinatis Proceedings. of the Hydro 2002 Conference, , Germany-Kiel Monitoring Water Level, Waves and Ice With Radar Gauges. Brodtkrob, Johannessson, and Lindgren, Rychlik, Rjydén, Ande. Sjö WAFO toolbox wave analysis for fatigue and oceanography, tech. rep., Dept of Mathematical Statistics, Lund. Zimmermann, Mai, Irschik Grundlagenuntersuchung zur Signalbildung von kommerziellenradar-füllstandssensoren im Bereich der schnellen Wasserstandsmessung(Seegangsmessung) im Küstenpegelwesen unter Einbeziehung der Eisbildungauf der Oberfläche, Franzius Institut, Hannover.

10 10 KEYWORDS ICCE 2008 PAPER TITLE APPLICATION OF RADAR GAUGES TO MEASURE THE WATER LEVEL AND THE STATE OF THE SEA Authors Jens Wilhelmi and Dr. Ulrich Barjenbruch Department: M1 Hydrometry and Hydrological Survey, Federal Institute of Hydrology, Am Mainzer Tor 1, Koblenz, 56068, Germany Abstract number 151 radar sensor wave gauge Waverider buoy sea-state swell wind-sea