Measurements of shoreline positions and intertidal foreshore slopes ...

August 16, 2005 11:44 WSPC/101-CEJ 00120

Coastal Engineering Journal, Vol. 47, Nos. 2 & 3 (2005) 91–107c© World Scientific Publishing Company and Japan Society of Civil Engineers

MEASUREMENTS OF SHORELINE POSITIONS AND

INTERTIDAL FORESHORE SLOPES

WITH X-BAND MARINE RADAR SYSTEM

SATOSHI TAKEWAKA

Department of Engineering Mechanics and Energy,Graduate School of Systems and Information Engineering,University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan

[email protected]

Received 2 August 2004Revised 27 April 2005

An X-band marine radar system has been employed to determine shoreline positions andintertidal foreshore slopes over an area 1.9 km in the longshore direction at the researchpier HORS in Hasaki, Japan. The X-band marine radar is an imaging radar that providesinstantaneous distributions of wave crests and shorelines along the shore. Time-averagedradar images were analyzed to estimate the horizontal positions of shorelines. Simultane-ously, the water surface level was measured at the pier and at the fishery port nearby todetermine the elevation of the shoreline. Radar measurements were conducted from highto low tide or vice versa to trace the bottom profile and estimate the foreshore slope in theintertidal range. The horizontal positions of the shoreline were measured within an error of10 m. The change of shoreline positions and intertidal foreshore slopes after attacks of highwaves is depicted to demonstrate the potential of the radar measurements in capturingessential characteristics of coastal morphology.

Keywords: X-band radar; shoreline position; foreshore slope; intertidal morphology; coastalremote sensing.

1. Introduction

1.1. Aim of the study

Morphological data are essential to evaluating and understanding the short- and

long-term behavior of a sandy coast. Traditional in situ surveying, such as leveling

and echo sounding provides precise position data at measured points. It is, how-

ever, costly, time-consuming and therefore provides only infrequent and low-density

measurements. An alternative to the traditional survey is remote sensing. Aerial

91

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92 S. Takewaka

photography is one of the earliest and most basic forms of remote sensing used to

image the wave field over an area. Nowadays, video cameras and radars are employed

for temporal coastal imaging supported by digital technologies.

This paper describes the use of an X-band marine radar to map intertidal

bathymetry over an area, i.e. the longshore distribution of shoreline positions and

foreshore slopes. The methodology and accuracy of the mapping are described and

then a seasonal change of the coastal morphology is shown to demonstrate the po-

tential of the radar measurements.

1.2. Previous studies

Remote sensing of coastal areas, providing both spatial and temporal data, at the

present utilizes visible light and the backscatter of emitted radar.

In the visible light range, several video techniques have been used [e.g. Lippmann

and Holman, 1989; Holland and Holman, 1997; Plant and Holman, 1997; Aarninkhof

et al., 2003]. Video cameras were mounted on a tower on or close to the shore in

these studies, providing slanted views, and rectifying and combining video images

from different cameras enabled analyses of wave and current dynamics and morpho-

logical processes. The author has also tried to observe surf zones with video cameras

attached to a moored balloon allowing for less slanted views [Takewaka et al., 2003],

and aircraft-mounted video images have been also analyzed in the same way [Pi-

otrowski and Dugan, 2002]. Video can provide several color images a second at rates

fast enough to detect wave breaking and suspension of foams and sediments, and to

trace their temporal and spatial variation. One severe shortcoming is that the video

cameras cannot capture images during the night and have difficulties in rainy and

stormy conditions.

An X-band marine radar is an imaging radar that is capable of tracking the

movements of wave crests over an area spanning several kilometers, and is becoming

popular in coastal studies these days. Bell [1999] tried to trace the motion of wave

crests, and estimated the distribution of wave phase speeds and water depths using

a linear dispersion relationship. Borge and Soares [2000] estimated the wave spectra

of wind waves and swells along the Spanish coast. Ruessink et al. [2002] reported

on the detection of coastal bars using time averaged radar images. X-band radar

provides distortionless wave field images of a broad area at intervals of 2 ∼ 3 seconds.

The intensity of a pixel in the radar image corresponds to the relative amount of

backscatter signal from the sea surface of the emitted radar beam and hence it is

usable during the night and under rainy and stormy conditions. However, one defect

of the radar system is the difficulty it has in detecting wave breaking state and

suspended materials.

Severe erosion of the coast occurs under high-wave conditions lasting for several

days usually accompanied by bad weather. The main advantage in using an X-band

radar system is its ability to collect data on coastal processes, continuously and

August 16, 2005 11:44 WSPC/101-CEJ 00120

Measurements of Shoreline Positions and Intertidal Foreshore Slopes 93

remotely, in bad weather that typically accompanying erosive high-wave condition.

In this context, this paper describes a radar imaging survey of intertidal foreshore

morphology to demonstrate the potential of radar measurements in capturing the

key features of coastal morphology.

2. Observation Site and Experimental Setup

2.1. Research pier HORS

X-band radar measurements were conducted at the research pier HORS, of the Port

and Airport Research Institute located in Hasaki, Japan (Fig. 1). The main facilities

are a 400-m pier facing the Pacific Ocean and a research building on the backshore,

which is located approximately 100 m backwards from the mean shoreline position

as shown in Photo 1.

HORS is located on an almost straight sandy coast, stretching 17 km from north

to south. The Port of Kashima is at the north end of the coast, and the mouth of

the Tone River and the Choshi Fishery Port are located at the south end. The pier

is approximately 4 km from Kashima Port. The coast has been almost stable in the

past decade except for the southern part close to the river mouth and the fishery

port where it is suffering from erosion.

Figure 2 shows the coordinate system used in this study. The x-axis corresponds

to the longshore extent and the y-axis coincides with the pier. A radar echo image

is overlaid on the figure, and will be described in more detail afterwards.

Water level variations are measured with several wave gauges mounted along the

pier, at y = 40, 80, 145, 230 and 380 m. The bottom profile along the pier is surveyed

Pacific Ocean

HORSChoshiFishery Port

Pacific Ocean

Port of Kashima

HORS4 km

13 km

Tone River

Fig. 1. Location of HORS.

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94 S. Takewaka

Research buildingPier

Pacific OceanPo

rt o

f K

ashi

ma

Choshi Fishery Port

Photo 1. Aerial photo of HORS.

pier

radar108 m

51 m

385 m

Choshi Fishery PortPort of Kashima

- 875 m

x

y818 msize of a radar image

977 m

(x, y) = (0, 0)

Fig. 2. Coordinate system.

August 16, 2005 11:44 WSPC/101-CEJ 00120


Figure 3

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14 16 18 20 22 24

2002/Aug./09 HORS (D.L.)

2003/Jan./16 HORS (D.L.)

Mea

n w

ater

leve

l at H

OR

ST

ide

leve

l at C

hosh

i Fis

hery

Por

t

D.L. [m]

time [h]

2002/Aug./09

2003/Jan./16

Fig. 3. Mean water level variation measuremd at HORS and tide level variation measured at ChoshiFishery Port. D. L.: Datum Level. D. L. 0 m is 0.687 m below mean sea level of Tokyo Bay (TokyoPeil, T.P.).

almost daily on weekdays. Tide levels are measured by the Meteorological Agency

at the Choshi Fishery Port approximately 13 km south of HORS. The mean water

level measured at y = 380 m during calm wave conditions differs by an amount of a

few to several centimeters from the tide level measured at the Choshi Fishery Port,

as shown in Fig. 3. Depth survey around the pier was conducted on August 11, 2002,

which is used to validate the radar estimation of foreshore morphology. Tide levels

and bottom elevations are expressed with Datum Level (D. L.). D. L. 0 m locates

0.687 m below Mean sea level of Tokyo Bay (Tokyo Peil, T. P.).

2.2. Radar system

The radar employed in this study is a conventional marine X-band radar for commer-

cial use (JMA-3925-9 Japan Radio Co. Ltd., 3 cm wavelength, transmitting power

25 kw, HH-polarization, radar pulse length 0.08 µs), which is usually installed on

fishery or pleasure boats. The 2.8 m antenna (Photo 2) rotates with a period of

approximately 2.6 seconds and transmits with a beamwidth of 0.8◦ in the horizontal

and 25◦ in the vertical. The radar was installed on the roof of the research building

at a height of approximately 17 m from the mean sea level as shown in Photo 2.

Backscatter or echo signals from the sea surface, so-called sea clutter, are grabbed

with a specially designed A/D-board with a sampling rate of 20 MHz, installed on a

Windows PC. The echo signals are sampled with an 8-bit accuracy along the radial

direction and then converted to a rectangular image of 1,024 pixels in the horizontal

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96 S. Takewaka

Photo 2. Radar antenna (width ∼ 2.8 m) on the roof of the research building at HORS.

and 512 pixels in the vertical. Each pixel corresponds to a square of 1.8 m, which is

smaller than the theoretical spatial resolution 7.5 m of the radar system determined

from the pulse length of the emitted beam. Figure 4 shows samples of the radar echo

for stormy and calm conditions. The radar is located at the centers of the bottoms of

the diagrams. The horizontal extent of the image is 1,852 m, or 1 nautical mile (NM),

and the vertical extent is 926 m. The gray images have pixel intensities between 0

and 255, with brighter pixels corresponding to a point with higher signal returns.

The meaning of pixel intensities is discussed in the next section.

These image samplings are done at 2-second intervals; part of the image is not

renewed since the imaging intervals are shorter than the rotation time of the antenna.

This may generate high frequency noise in the time domain but does not affect the

analyses for the wave motions, since they have lower dominant temporal frequencies.

2.3. Echo signals

There are two main scattering mechanisms providing backscatter or echo signals

from the sea surface [e.g. Skolnik, 1990]. The first is Bragg scattering from capillary

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pier position of the radar1852 m

926

m

wave crest

shor

elin

e po

sitio

n

(a)

pier position of the radar1852 m

926

m

shor

elin

e po

sitio

n

(b)

Fig. 4. Examples of radar echo images: (a) Stormy condition (2002/July/11, 03:00. Significantwave height H1/3 = 2.4 m, significant wave period T1/3 = 5.7 s measured at y = 145 m). (b) Calm

condition (2002/Aug./09, 08:00. H1/3 = 0.5 m, T1/3 = 5.8 s measured at y = 380 m).

roughness on sea gravity waves. Bragg scatters occur when the length of the rough-

ness is half the wavelength of the radar beam, which is 1.5 cm in this study. The

second is specular spikes that come from steep and breaking waves. In a horizontally

polarized radar, or an HH-polarization radar, sea spikes cause stronger backscatter

signals than the Brag scattering.

A comparison between echo signals and water level variations measured at the

pier under a stormy wave condition is shown in Fig. 5. The variation of the raw

echo signal at (x, y) = (15 m, 145 m) is shown in Fig. 5(a), where frequent signal

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98 S. Takewaka

0

50

100

150

200

250

200 220 240 260 280 300 320elapsed time [s]

echo

(0-

255)

inte

nsity

var

iatio

n at

y =

145

m

(a)(a)

200 220 240 260 280 300 320

1

2

3

4

200 220 240 260 280 300 320

water surface radar echo

-1

-0.5

0

0.5

1

wat

er s

urfa

ce v

aria

tion

at y

= 1

45 m

filt

ered

and

nor

mal

ized

rad

ar e

cho

inte

nsity

elapsed time [s]

D.L. [m]

(b)

Fig. 5. Variation of echo signal in stormy condition. (2002/July/11, 03:00) (a) Raw data (x = 15 m,y = 145 m). (b) Bandpass filtered (passband: 0.002–0.2 Hz) echo signal and water surface elevation.

saturation is observed. The same radar signal after being filtered through a bandpass

filter (passband: 0.002–0.2 Hz) and normalized with the difference of maximum and

minimum signals, is shown in Fig. 5(b), displayed along with the water surface

elevation measured at y = 145 m. It shows that echo signals follow reasonably well

the passage of wave crests whose periods are approximately from 6 to 12 seconds. It

is, however, hard to find a systematic relationship between the wave height and the

echo signal intensity. This implies that we can find location of wave crests from the

echo signal variation, but that it is hard to estimate the wave height of individual

waves.

There is also backscatter from dry shore and bodies on the shore like vehicles,

driftwoods, coastal forest, etc. Their echo signals have sometimes strong intensities,

but are stationary, so the distinction between water surface and motionless obstacles

is easy.

3. Measurement of Shore Positions and Foreshore Slopes

3.1. Time-averaged image

Individual echo images are averaged yielding a “time-averaged image” or so-called

“time exposure”. Figure 6 shows images averaged over 15 minutes at a high tide

and low tide observed during calm conditions on August 9, 2002. The incident

waves were small during the day, and wave breaking occurred only in the vicinity of

the shoreline. The mean water level variation measured at the pier (y = 380 m) is

shown in Fig. 3.

August 16, 2005 11:44 WSPC/101-CEJ 00120


off shore shift of shoreline position due to ebb tide

00

500m

0

-500 500 m -500 500 m

high tide low tide

close-up view

Fig. 6. Averaged images at high (left, 06:00) and low (right, 10:00) tide. (2002/Aug./09).

x-300 m 300 m

y

0

400 m

0 m

D.L. 0 m

D.L. -3 m

D.L. 2 m

Fig. 7. Overlay of averaged image (2002/Aug./09, 10:00) and depth contour map. Interval betweencontour lines is 0.5 m.

August 16, 2005 11:44 WSPC/101-CEJ 00120

100 S. Takewaka

The vertical streaks close to the center of the images in Fig. 6 are the pier.

Individual waves vanish in the time-averaged image and an edge extending in the

longshore direction becomes visible. As this edge moves offshore-wards with the fall

of the tide, the shoreline must be located in the vicinity of this edge. An overlay of

an averaged image and a depth survey, which was conducted on August 11, 2002, is

shown in Fig. 7. As an isoline of the contour map follows the longshore distribution

of the bright pattern in the averaged radar image, the high return region must

mark some bathymetric feature, most likely, the shoreline and iso-depths lines of

the breaking zone at the time of the measurement.

3.2. Determination of shoreline position and foreshore slope

As shown in Fig. 6, the horizontal edge appearing in a time-averaged image may be

related to the instantaneous shorelines corresponding to the tide level. Pixel inten-

sities extracted along a cross-shore line from time-averaged images, the mean water

level measured at y = 380 m, and bottom profile are shown in Fig. 8. The location

of the broad peak in echo intensity in the figure coincides with the intersection of

the mean water level and the bottom profile. Thus, we can determine the shore-

lines by locating peaks in the cross-shore pixel intensity distributions. Local fittings

of parabolic curves to the intensity distributions were done to determine the peak

locations, but manual manipulation was sometimes necessary to correct the result.

There is no physical interpretation available currently to explain the link between

instantaneous shoreline position and peak in averaged radar echo; the relationship

explained here is heuristics.

Shorelines at different tide levels and intertidal foreshore slopes are estimated

as shown schematically in Fig. 9, where intertidal refers to the region between high

and low tide levels. The horizontal positions of the shoreline are determined from

the radar measurements and a common vertical position is estimated from direct

-1

0

1

2

3

0

50

100

150

200

250

-50 0 50 100 150

bott

om p

rofi

le,

mea

n w

ater

leve

l (D

.L.)

[m

]

echo

inte

nsity

y [m]

MWL06:00

shoreline position 06:00

echo intensity

bottom profile -1

0

1

2

3

0

50

100

150

200

250

-50 0 50 100 150

echo

inte

nsity

bott

om p

rofi

le,

mea

n w

ater

leve

l (D

.L.)

[m

]

shoreline position 10:00

MWL10:00

echo intensity

bottom profile

y [m]

Fig. 8. Cross-shore pixel intensities extracted from averaged images at different mean water levelsdisplayed with bottom profile at x = 200 m. (2002/Aug./09).

August 16, 2005 11:44 WSPC/101-CEJ 00120


radar

bottom profiletide level

shoreline position

Fig. 9. Mapping of intertidal bathymetry.

measurement of the mean sea level. After measuring the shoreline positions at dif-

ferent tide levels, the mean foreshore slope of the concave beach profile is defined

here as the slope given by the linear regression of the shorelines from high to low

tide. The idea introduced here is basically used by Plant and Holman [1997], and

Aarninkhof et al. [2003] in the mapping of intertidal beach bathymetry using video

cameras.

3.3. Validation of the estimation

Figure 10 shows comparisons of intertidal morphology derived from radar measure-

ments during an ebb tide on August 9, 2002, from high tide (6 hours) to low tide

(10 hours) as shown in Fig. 3, and surveyed bottom profiles on August 11, 2002. The

symbols in the figure are positions of the shorelines at different times whose horizon-

tal locations are determined from radar data, and vertical locations from the mean

water level measured by wave gauges at the pier. The symbols follow reasonably well

the surveyed bottom profile: the horizontal locations of shorelines vary more widely

where the foreshore slope is relatively mild than where the foreshore slope is steep.

The shoreline positions in the area just in front of the radar (−100 < x < 100 m)

were hard to define due to the strong backscatter resulting in saturated echo sig-

nal distributions. Improvement of the A/D system by changing the gain of signal

conversion may avoid this problem.

Determination of shoreline positions using remote sensing encounters the prob-

lem of correcting for wave set-up, which shifts the shoreline position landwards,

especially for stormy high-wave conditions. There are formulas for predicting the

amount of set-up at shoreline positions; however, to use these, data on the wave pe-

riod, height and other features of the incoming waves are required, and they are

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102 S. Takewaka

-50 0 50 100 150

x = -300 mx = -250 mx = -200 mx = -150 m

x = 150 mx = 200 mx = 250 mx = 300 m

300 m

250 m

200 m

150 m

-250 m

-200 m

-150 m

2 m

y [m]

bott

om e

leva

tion x = -300 m ebb tide (2002/Aug./09)

06:00 - 10:00

Fig. 10. Comparison between radar measurements and surveyed results at different longshorepositions.

sometimes more difficult to measure than the mean water level or tide level. The

error introduced by the wave set-up effect is lessened when radar measurements are

conducted under calm wave conditions to determine the shoreline positions.

The accuracies of the estimations of shoreline positions shown in Fig. 10 are

summarized in Fig. 11 by plotting the horizontal locations in the cross-shore di-

rection from radar measurements against those from the survey, assuming that the

mean water level measured at the pier coincides with the level of shoreline and ne-

glecting the effect of wave-setup. The scatter is within a range of 10 m, which is

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0

20

40

60

80

100

120

0 20 40 60 80 100 120

x = - 300 m

x = - 200 mx = - 250 m

x = - 150 m

x = 300 m

x = 150 m

x = 250 mx = 200 m

shor

elin

e po

sitio

n: r

adar

est

imat

ion

[m]

shoreline position: survey data [m]

mean error of radar estimation = -1.4 m standard deviation = 11.1 m

Fig. 11. Accuracy check: Estimation of shoreline position.

almost the same as the spatial resolution of the radar measurements as described

before. Estimated and surveyed foreshore slopes are compared in Fig. 12 in the same

manner as in Fig. 11. There is an apparent discrepancy for steeper cross sections,

where a slight deviation in the estimation of shoreline position may lead to a large

error. Another cause of errors in foreshore slope estimation may arise from the use

of linear regression for a concave surface.

3.4. Seasonal deformation of the beach

Results from two different dates, August 9, 2002 and January 16, 2003, are compared

here to demonstrate the potential of the radar measurements. In September, October

and November of 2002, several high wave conditions lasted for days and the coast

was eroded extensively. Figure 13 shows the bottom profiles along the pier in August

2002 and January 2003. The berm on the foreshore is totally eroded away and the

bottom elevation decreased by over 1 m at the foreshore and in the adjacent offshore

part.

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104 S. Takewaka

0.01

0.02

0.03

0.04

0.05

0.01 0.02 0.03 0.04 0.05

x = - 300 mx = - 250 mx = - 200 mx = - 150 mx = 150 mx = 200 mx = 250 mx = 300 m

fore

shor

e sl

ope:

rad

ar e

stim

atio

n

foreshore slope: survey data

Fig. 12. Accuracy check: Estimation of intertidal foreshore slope.

-6

-4

-2

0

2

4

6

-100 0 100 200 300 400

2002/Aug./09 2003/Jan./16

bott

om e

leva

tion

(D

.L.)

[m

]

y [m]

Fig. 13. Variation of bottom profile along the pier from August 2002 to January 2003.

August 16, 2005 11:44 WSPC/101-CEJ 00120


(lower panel) from August 2002

0

20

40

60

80

100

120

-1000 -500 0 500 1000

2002/Aug./09, 09:00 2003/Jan./16, 19:00sh

orel

ine

posi

tion

[m

]

x [m]

0

0.01

0.02

0.03

0.04

0.05

-1000 -500 0 500 1000

2003/Jan./16 2002/Aug./09

fore

shor

e sl

ope

x [m]

Fig. 14. Variation of longshore distribution of shoreline positions measured at mean water levelD. L. ∼ 0 m (upper panel) and foreshore slopes (lower panel) from August 2002 to January 2003.

Shoreline positions defined with almost the same mean water (D. L. ∼ 0 m) level

for August 9 (9 hours), 2002 and January 16 (19 hours), 2003, and the intertidal

foreshore slope distributions are shown in Fig. 14. Shoreline positions retreated

landwards from August to January. Overall, some morphological features, however,

are preserved during the recession; the curve for the shoreline positions at x <

−200 m retreated almost uniformly due to storm events, indicating the shape of the

coastline as well as the foreshore slopes were preserved during this recession. On the

other hand, foreshore slopes for x > 0 changed in a complex manner. Some part of

the foreshore has flattened and some part has steepened.

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106 S. Takewaka

A drain is installed at the vicinity of x = −250 m to discharge inland water from

the coastal residential area as shown in Photo 1. The formation of a hump in the

shoreline profile at x ∼ −300 m in Fig. 14 may be due to the discharge from this

drain.

4. Concluding Remarks

An X-band marine radar system has been employed to determine shoreline positions

and intertidal foreshore slopes over an area spanning 1.9 km in the longshore direc-

tion at the research pier HORS at Hasaki, Japan. An X-band marine radar is an

imaging radar that provides instantaneous distributions of wave crests and shore-

lines along the shore. Ensembles of radar images were processed to time-averaged

radar images, which were analyzed to estimate the horizontal positions of shore-

lines or shorelines. Simultaneously, the water surface level was measured at the pier

and at the fishery port nearby to determine the vertical position of the shoreline.

Radar measurements were conducted from high to low tide, or which may be done

vice versa, to trace the bottom profiles and estimate the foreshore slopes in the in-

tertidal range. Horizontal positions of the shoreline were measured within an error

of 10 m, which is close to the spatial resolution of the radar measurements. The

change in the shoreline positions and intertidal foreshore slopes after attacks of high

waves is reported in order to demonstrate the potential of the radar measurements

in capturing key characteristics of coastal morphology. Since radar measurements

are applicable during stormy conditions and night, the X-band radar system should

prove to be a powerful tool for continuous tracing of morphological features on sandy

coastlines.

Acknowledgments

The author is grateful to the members of the Littoral Drift Division, PARI, who

provided assistance in radar measurements and results of depths surveys and wave

measurements. The author appreciates the efforts from former graduate students,

Mr. Toru Inaba and Mr. Isamu Gotoh who cooperated in the radar measurements.

This study was financially supported by the Grants-in Aid of the Japan Society for

the Promotion of Science and by a grant of the Japanese Institute of Technology on

Fishing Ports and Communities.

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Bell, P. S. [1999] “Shallow water bathymetry derived from an analysis of X-band marine radarimages of waves,” Coastal Engineering, Vol. 37, pp. 513–527.

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