Location of the Rhine plume front by airborne remote sensing

Pergamon

Continental Shelf Research, Vol. 14, No. 4, pp. 325-332, 1994

Copyright© 1994 ElsevierScienceLtd Printed in Great Britain. All rights reserved 02784343/94 $6.00+ 0.00

Location of the Rhine plume front by airborne remote sensing K. G. RUDDICK,*L. LAHOUSSEand E. DONNAY (First received 29 May 1992; in revised fi)rm 15 December 1992; accepted 23 February 1993)

Abstract--The aim of this study was to determine the feasibilityof using airborne remote sensing to locate the Rhine plume front. Interest in fronts arises from the desire to predict the /ate of pollutants and biological nutrients discharged from rivers into the open sea. Observations were made during flights over the Dutch coastal waters using a vertically-mounted video camera and a side-looking airborne radar (SEAR) designed for oil slick detection. Comparison of radar images with visual observations of the sea colour discontinuity and foam line establish that fronts can indeed be detected by SLAR because of high radar backscatter along the convergence line, where the fresh water jet impinges on saltier water. This provides a sound basis for future investigations using Synthetic Aperture Radar as mounted on ERS-1. An estimation of errors is given, identifying priorities for improvement of the technique. The accuracy achieved is considered sufficient for the validation of hydrodynamic models.

1. I N T R O D U C T I O N

THIS report describes a study performed to determine the feasibility of airborne remote sensing for the location of the Rhine plume front. Heavy metals and hydrocarbons originating from industry in the Rhine and Meuse basins reach the North Sea via the R o t t e r d a m w a t e r w a y . It is e s t i m a t e d by HAINBUCHER et al. (1987) t h a t this d i s c h a r g e r e p r e s e n t s t h e l a r g e s t s o u r c e ( 5 5 % ) of river p o l l u t i o n to the s o u t h e r n N o r t h Sea. In a d d i t i o n to toxic c h e m i c a l s , t h e s u p p l y o f b i o l o g i c a l n u t r i e n t s , e s p e c i a l l y n i t r a t e s and p h o s p h a t e s , affects s t r o n g l y the e c o s y s t e m o f the s o u t h e r n N o r t h Sea; SCHAUB a n d GIESKES (1991) r e p o r t a high c o r r e l a t i o n b e t w e e n R h i n e d i s c h a r g e a n d p h y t o p l a n k t o n b i o m a s s in the D u t c h c o a s t a l z o n e . A m a t h e m a t i c a l m o d e l has b e e n d e v e l o p e d b y DE KOK (1992) to p r e d i c t the t r a n s p o r t a n d fate of s u s p e n d e d s e d i m e n t s a n d , thus, p r o v i d e a tool for m a n a g e m e n t o f the m a r i n e e n v i r o n m e n t . It is p r o p o s e d t h a t a i r b o r n e r e m o t e sensing offers a s u i t a b l e s o u r c e o f d a t a for m o d e l v a l i d a t i o n , c o m p l e m e n t i n g the s e a - g o i n g c a m p a i g n s o f VAN DER GIESSEN et al. (1990) a n d SIMPSON et al. (1993). S i d e - l o o k i n g a i r b o r n e r a d a r ( S L A R ) is u s e d o p e r a t i o n a l l y b y m a n y c o u n t r i e s for the d e t e c t i o n o f m a r i n e p o l l u t i o n by h y d r o c a r b o n slicks (e.g. LOOSTROM (1987)). I n s t a n c e s of front d e t e c t i o n b y S L A R a r e r e p o r t e d by ArrEMA a n d HOOGEBOOM (1978), DE LOOR (1981), JOHANNESSEN et al. (1991) a n d MATrHEWS et al. (1992).

*Author to whom correspondence should be addressed at: Management Unit of the Mathematical Models of the North Sea and Scheldt Estuary, (MUMM), Institute of Hygiene and Epidemiology, 100 Gulledelle, 1200 Brussels, Belgium. 325

326

K. Ca. RUDDICKet al. Table 1. Technical specification of the SLAR

Antennae Horizontal beamwidth Vertical beamwidth Gain Polarisation Transceiver Frequency Peak output Pulse repetition frequency Pulse width Noise factor Amplifier Along track screen resolution Cross track screen resolution

2 0.5 37 31

9.4 10 1.0 0.5 8

o ° dBi Vertical GHz kW kHz l,IS

dB Logarithmic

60

m

75

m

2. METHODS

2.1. A i r b o r n e p l a t f o r m

The aircraft used is a Britten N o r m a n Islander with a cruising speed of 220 km h i and autonomy of about 5 h. Aircraft position was determined by a Decca A P (air) system and altitude by a T R T radio altimeter.

2.2. S i d e - l o o k i n g airborne radar The technical specification of the Ericsson S L A R used is given in Table 1. S L A R visualises the sea surface roughness by emitting pulses of electromagnetic radiation and measuring the backscattered signal. Periodic structures in the surface wave field with wavelength in the viewing direction close to half the wavelength of emitted radiation produce resonant Bragg scattering. Solid objects such as boats, moorings and coastal structures produce a strong reflected signal, while a reduction in surface roughness caused by hydrocarbon slicks or sand banks gives particularly low backscattering intensity. ROmNSON (1985) suggests that river plume fronts could be detected by microwave radar either from the difference in capillary wave field of the two water masses or because of the accumulation of foam and surfactants at the front itself--the siome convergence phenomenon described by, for example, KLEMAS and POLIS (1977) or SIMPSONand JAMES (1986). S L A R operation requires straight and level flight with optimum altitude of about 500 m. This S L A R has an optimum range between about 2 and 15 km: at larger distances resolution degrades, while at shorter range direct reflection occurs. The return signal is processed automatically to produce an image on a V D U monitor with vertical axis as along-track distance and horizontal axis as distance perpendicular to aircraft heading.

Fig. 1. S L A R image at 12:41:50 G M T on 22 S e p t e m b e r 1992. T h e aircraft track is up the centre of the picture, with current aircraft position m a r k e d as a star in the middle at the top. Scale is given by crosses on the screen at 10 km intervals, T h e "target" ( T G T ) selected by light p e n is m a r k e d by O and its position is given in the legend.

t~ 1",,9 "--d

=_

~=

7:

2.

a"

~r ©

328

K. (3. RUDDI(:K et al.

Fig. 2.

Hasselblad photo showing a 85 m length of the front. The water colour discontinuity and foam line are clearly visible.

Airborne remote sensing of the Rhine plume

329

2.3. Visual observations Visual observations were made using a Panasonic WV-F250 A E video camera, mounted vertically or hand-held, a single lens reflex (SLR) camera with a 35 mm lens, and a Hasselblad camera with an 80 mm lens. The S L A R and video camera images and intercom soundtrack were recorded on a Super-VHS video recorder with annotation giving the time in GMT, aircraft position (POS) in latitude and longitude, ground track direction (HD) in °, altitude (ALT) in feet and mission number ( R E G ) . 2.4. Test conditions The observations relate to flights made on 29 January and 22 September 1992 in the region of the Rotterdam waterway outlet. Mean water depth in the region of interest (5 km offshore of the waterway mouth) is approximately 20 m. In both flights the observations were made a few hours before low water neap, and weather conditions were calm. 3. RESULTS 3.1. Observations In both flights, S L A R images was recorded indicating a line of high back scatter about 10 km long, approximately 5 km west of the waterway mouth. Fig. 1 shows the S L A R output for a pass parallel to the Dutch coast, which can be seen clearly along the right of the image. The continuous white line, whose ends are marked by the light pen indicator "O" and the Europort, was targetted as a possible front. This line separates regions of different back scatter intensity, possibly arising from the effect of different surface currents on the capillary wave field. Boats appear as white spots, sometimes with attached wake. The white haze close to the aircraft track results from S L A R reflection at low incidence angle. Visual observations were then made to corroborate or falsify the hypothesis that this S L A R target represents a river plume front. The line of high backscatter was indeed found to correspond to a sharp discontinuity in water colour (suspended matter content), coinciding with a foam line as shown in Fig. 2. Figure 3 shows a comparison of S L A R observations with the combinations of sightings reconstructed from vertical and oblique video images, SLR photos and the eyewitness account recorded on the video tape. 3.2. Error estimation The front location is deduced by measuring the perpendicular distance from the current aircraft location on a number of S L A R images; thus, using the aircraft location and heading the geographical coordinates of the front can be determined. Since only the aircraft groundtrack is recorded, an error arises from the difference between heading and groundtrack, the "drift" angle. This can be minimized by flying with the wind or could be eliminated by improved instrumentation giving directly the aircraft heading from compass readings suitably corrected. An estimation of the errors in front location found from S L A R is given in Table 2 for the 29 January 1992 flight. Errors arising from S L A R internal processing, including aircraft pitch and roll have not been analysed. Errors of map

330

K.G. RUDD1CKet al.

Airborne

MAST-O050-C

i"

I

0

VISUAL

•

sl~n

remote

sensing

[

-52"

4" i

F r o n t l o c a t i o n f r o m SLAR Flighl 29/1/92

and

v i s u a l observations

o

5 km

0

2 Nm

Fig. 3. Comparison of the front location as measured from visual and SLAR observations on 29 January 1992.

Table 2.

Resolution of a/c location Resolution of a/c groundtrack Drift angle Measurement of SLAR image Total relative to a/c Total

Estimate o f S L A R errors

0.1' latitude, 0.1' longitude 1° in 360° 4° in 360° 1 mm in 30 mm 5 km* x / ~ %

230 m 0.3% along track 1.1% along track 3.3% cross track 180 m 410 m

p r o j e c t i o n , a n d video r e s o l u t i o n can be c o n s i d e r e d negligible. W h i l e i n t r o d u c t i o n of a m o r e precise m e t h o d for m e a s u r i n g the p e r p e n d i c u l a r distance from the aircraft to a S L A R f e a t u r e m a y i m p r o v e the crosstrack accuracy, the m a i n l i m i t a t i o n is the r e s o l u t i o n of the aircraft n a v i g a t i o n system. High accuracy is o b t a i n e d with a vertically m o u n t e d video c a m e r a ; the principal source of e r r o r is the finite p r e c i s i o n (roughly 230 m) of aircraft location. O b l i q u e video a n d S L R p h o t o s have a n a d d i t i o n a l e r r o r of a b o u t 140 m (e.g. i n c l i n a t i o n of 35 ° at 200 m a l t i t u d e ) , giving a total e r r o r of 370 m. A n a d d i t i o n a l d i s c r e p a n c y b e t w e e n S L A R a n d visual images m a y arise from m o v e m e n t of the front d u r i n g the time difference (20 rain) b e t w e e n these observations. C o m p a r i s o n of the p o s i t i o n of a fixed p o i n t in the E u r o p o o r t area as e s t i m a t e d by S L A R

Airborne remote sensing of the Rhine plume

331

and by overflying with the video camera mounted vertically suggests an absolute accuracy of about 300 m for these techniques. The scatter of points in Fig. 3 reflects this.

4. DISCUSSION This study has established the feasibility, at least in good weather conditions, of using side-looking airborne radar for the detection of the outflow front separating the turbid waters of the Rhine plume from the waters of the southern North Sea, The front appears on the S L A R as a line of high backscatter less than 100 m wide. The p h e n o m e n o n producing this backscatter seems to be related to either wavebreaking or an accumulation of foam and floating debris (J. P. Matthews and J. Vogelzang, personal communication) at the strong surface convergence, where the surface jet of fresh water impinges on the tidal stream of salt water. Visual observations were made using a video camera and an SLR camera to corroborate the hypothesis that the S L A R feature corresponds to a front. An estimation of measurement error from theoretical considerations, practical evaluation of a fixed reference point and the scatter of points locating the front suggest an accuracy for S L A R and visual observations of the order of 400 m, with the main irreducible error arising from the finite precision (one tenth of a minute) of aircraft location. A limitation of this technique is that flight safety precludes bad weather operations, and, thus, study of the effect of strong wind mixing on front formation. In conclusion, it is suggested that S L A R provides a useful additional tool for investigating river plumes, complementing sea-going measurement campaigns by giving a synoptic view. Conversely, airborne remote sensing relies on ships and/or fixed moorings to provide data for calibration and verification. The mesoscale temporal resolution and control of the observation period represent advantages over satellite remote sensing, which is limited to predefined, periodic sampling. Identification of the Rhine plume front sea surface roughness signature provides a sound basis for future investigation using E R S - I ' s Synthetic Aperture Radar. Finally, the quantity measured, the front location, is particularly useful for the validation of mathematical models, and is important in determining the horizontal extent of pollutants and nutrients from the river discharge. Aeknowledgements--K. G. Ruddick was funded by the European Community under contract MAST-900064. The pilots of the Belgian Army Light Aviation are acknowledgedfor their expert assistance. The authors thank colleagues in the BELMEC (Belgian Marine Environmental Control), CAMME (Computer Assisted Management of the Marine Environment) and the MAST-0050-C"'PROFILE" (Processes in Regions Of Freshwater Influence) teams for many helpful suggestions concerning airborne remote sensing, image processing and interpretation of results.

REFERENCES ATTEMAE. P. W. and P. HOOGEBOOM(1978) Microwave measurements over sea in the Netherlands. in: Surveillance (~f environmental pollution and resources by electromagnetic waves, T. LUND,editor, pp. 291298, D. Reidel. 1)E KOK J. M. (1992) A 3D finite difference model for the computation of near- and far-field transport of suspended matter near a river mouth. Continental Shelf Research, 12,625~542. DELool~G. P. (1981) The observation of tidal patterns, currents, and bathymetrywith SLAR imageryof the sea. IEEE Journal of Oceanic Engineering, OE-6, 123--129. HAINBUCHERD., T. POHLMANNand J. BACKHAUS(1987) Transport of conservative passive tracers in the North Sea: first results of a circulation and transport model. Continental Shelf Research, 7, 1161-1180.

332

K . G . RUDDICK et al.

JOHANNESSEN J. A . , R. A. SCHUCHMAN,0 . M. JOHANNESSEN,K. L. DAVIDSON and D. R. LYZENGA (1991) Synthetic Aperture R a d a r imaging of upper ocean circulation features and wind fronts. Journal of Geophysical Research, 96, 10411-10422. KLEMAS V. and D. F. POLLS (1977) A study of density fronts and their effects on coastal pollutants. Remote Sensing of the Environment, 6, 95-126. LOOSTR~)M B. (1987) The Swedish airborne remote sensing system for maritime surveillance. Oil and chemical pollution, 3, 209-230. MAUlHEWS J. P., V. R. WISMAN, K. LWIZA, R. ROMEISER, I, HENNINGS and G. P. DE LOOR (In press) A study of frontal boundaries near the Rhine plume by radar scatterometer, airborne thematic m a p p e r and in situ techniques. Journal of Geophysical Research. ROBINSON I. S. (1985) Satellite oceanography. Ellis Horwood. SCHAUB B. E. M. and W. W. C. GIESI