Measurements at sea using shore based radar

Herman Peters, Rijkswaterstaat

SeadarQ meeting

Leidschendam

January, 12th, 2011

Information need for waves, currents, water level, depth etc.

• There are many users like the Storm Surge Warning Organization, harbor authorities, coastal management, etc...

• Wave modeling problems: wave penetration, wave-current interaction, wave growth etc.

• Spatial information and forecasts needed about the wave and current field.

• Actual information about sea bed topography (fluctuating during storm conditions) very desirable.

Overview measuring methods

• Fixed multi-parameter measuring poles (waves, currents, wind etc.)

• Anchored wave measuring buoys

• Frames at the seabed (ADCP)

• Ship borne bathymetry surveys

• Remote Sensing techniques

Measuring poles near the sea defence (WRW and UHW)

• On tidal flat near the dike (ca 100 m)

• More robust and reliable than a buoy in extreme conditions

• Many parameters (including wind, waves and water level) at one location

• Suitable for long term wave statistics

Wave measuring buoy

ADCP measuring frames

Depth surveys for measuring bottom topography

– Ship borne echo soundings

– Airborne laser altimetry

over dry flats

SBW measuring campaignWadden Sea

• Waves (26 buoys mostly Directional Waverider and 3 measuring poles)

• Wind (11 measuring poles)• Current (3 ADCP bottom frames and 3 measuring

poles)• Data telemetry and processing (mostly in real-time

except ADCP frames)• Quality control (Operational buoy surveillance and

on-line validation)• Data storage and standard reports

Borkum

Schiermonnikoog

Lauwersoog

Nes

Nes

Ferwert

Leeuwarden

Drachten

Groningen

Veendam

Nieuwe Statenzijl

Dollard

Eems

Uithuizerwad

PieterburenwadWierumer wad

Ameland

West-Terschelling

Oost-Vlieland

Texel

Oude Schild

Den HelderDen Oever

Breezanddijk

Makkum

Kornwerderzand

Harlingen

Delfzijl

Amelander Zeegat

Stavoren

BRKN1

WEO1

RZGN1

WEW1

SMN1

UHW1

PBW1

WATN?

WATZ?

SMWG

WRW1

AZB12

AZB42

AZB22

AZB32

AZB52

AZB62

AZB31

AZB21

AZB11

AZB41

AZB51

AZB61STM1

PNG1

BRS1

KWZ1

BRZ1

OWN?

OWZ?

ELD1

MZW1

SBW Wadden Sea measuring configuration season 2008/2009

Advantages of shore based radar Remote Sensing

• Synoptic coverage of (very) large areas (hundreds square km).

• Equivalent with a multitude (thousands!) of point sensors: hence very cost-effective and much less logistic effort.

• No disturbance of the wave field by large water piercing constructions (e.g. measuring poles).

• Shore based installations are almost invulnerable; hence very reliable, also during storms.

• Radar is an all-weather observation technique: thus shore based systems are very suitable for continuous (day and night) wave and current monitoring and also very suitable for capturing rare (e.g. storm) events.

Oceanographic applications of Microwave imaging radar

• Shore based radar system primarily used for detecting and tracking ships at sea.

• The rough sea surface also produces a radar echo; the so-called “sea clutter” (one man’s noise is sometimes another man’s signal….)

• Microwave imaging radar can (also) be used for measuring waves, currents and water depths at sea.

History of shore based radar for oceanography in the Netherlands

• Pioneering phase using photos of the PPI screen of a radar in Zeeland in the sixties by Rijkswaterstaat (Oudshoorn).

• Fundamental and practical research at TNO (Paul de Loor) and TU Delft in the seventies and eighties.

• Growing towards operational applications in the 21st century by the Dutch companies Tech5 and SeadarQ (Jan Kleijweg).

Signal processing with the Dutch SeadarQ system

• SeadarQ is basically a software package operating on a PC, which is connected to the raw signal output (analog video, sync pulse, azimuth, north reset etc.) from the microwave radar.

• Often data from an existing navigation radar can be used, but sometimes a separate radar has to be installed (much more effort).

• A (short) reconnaissance survey is necessary.• Installation is relatively simple and takes a few

days/weeks.

Current and depth information derived from the wave dispersion relationship

• In case of an undisturbed (deep water, no current) wave situation the phase velocity (different at each frequency) of the waves is well-known.

• The wave velocity calculated from the series of radar images deviates in practice from the undisturbed one due to the influence of currents (especially the short waves) and finite depth (mainly the long waves).

• From the measured deviations the current and depth can both be estimated separately.

• In fact the waves function as a kind of Lagrangian drifters or floats; carried along by the ambient current and retarded by the bottom.

VV short pulse (70 ns) Radar on Ameland lighthouse

Time average (3 min) radar image at Ameland

Wave patterns clearly visible in the raw radar image

Ebb model results (black) compared with radar (red)

Flood model results (black) compared with radar (red)

Flow improvement in hydrodynamic model

Evaluation of Ameland data• Definition of different kind of products based

on the radar images.• Raw radar images seem to be most useful for

analyzing wave patterns (refraction, penetration of long North Sea waves).

• Time averaged radar images (sequence over many tidal cycles or years) useful for monitoring morphology and tracking the breaking wave areas. Interesting during and shortly after a storm.

• Dispersion relationship in small sub areas useful for current and depth measurements.

Plans for further activities • Monitoring more different and more severe

storms at the Ameland tidal inlet.• Achieving a better estimation of absolute wave

height from the images; quite a challenge.• Investigate the use of these data for other

applications (ship guidance, coast guard etc.).• Move to another location (possibly the Eems-

Dollard region?) or deploy more SeadarQ systems spread out over the Wadden Sea area.

• Combination with a large range HF radar.

Applicability SeadarQ-increases the capabilities (added value) of a standard navigation

radar by giving a spatial distribution of oceanographic features (e.g.

waves).

-the many capabilities (oil, depth, current etc.) make the system

interesting for different users (port authorities, coast guard, modelers,

coastal zone managers).

-output data has to be carefully calibrated and validated before an

acceptable and comparable absolute accuracy is reached.

-no information available in unfavorable weather conditions (no

wind/small waves)

-not applicable in all areas, depending on features / local physical

conditions