Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

STUDY ON ANNUAL SHORELINE CHANGEAROUND A HEADLAND
An Seunghyun1 and Takewaka Satoshi2
1Graduate School of System and Information Engineering, University of Tsukuba, an@surface.kz.tsukuba.ac.jp
2Graduate School of System and Information Engineering, University of Tsukuba, takewaka@kz.tsukuba.ac.jp

Headland is a coastal structure to control longshore sediment. This study focuses on the annual
variation of shoreline position around a headland at Kashima coast, Ibaraki. With results of radarmeasurement, shoreline position, intertidal slope, and cross-section area are estimated by time-stack dataprocessing. And their features are discussed in terms of wave condition and compared with computation
results of current field. To understand connection among variation of shoreline position, intertidal slope,cross-section area, and current field, wave deformation and current variation around the headland arecomputed by REF/DIF-1 and SHORECIRC model.
Key Words : headland, shoreline position, intertidal slope, current field, REF/DIF-1, SHORECIRC


1. INTRODUCTION

2. STUDY AREA AND RADAR OBSERVATION

Headland is a coastal structure to control lonshore
sediment transport, and a considerable number ofheadlands are constructed in Japan. This study
focuses on the annual variation of shoreline position
around a headland at Kashima coast, Ibaraki.
Wave climate at Kashima coast differs in seasons.
In winter season, wave heights are high and highwaves are incident from the northern. On the other
hand, in summer season, wave heights are low andprimary incident wave direction is southern.
Shoreline positions around a headland respond to
the wave climate: Northern sides of the headland are
generally deposited in winter season and are erodedin summer season, and at southern side of theheadland, the opposite is observed.
X-band radar has been installed to trace annual
variation of shoreline positions around a headland.
Variation of shoreline position, intertidal beachslope and beach volume are estimated and theirfeatures
arediscussed.
Then,numerical
computations on wave and current filed are done tounderstand the observed morphological behavior.
To compute the wave and current field, REF/DIF-1(Kirby et al., 1994) is used as wave deformation
model and SHORECIRC (Svendsen et al., 2000) is
used as circulation model.

(1) Study area and radar observation
Fig. 1 is an aerial photograph of the study area.
The area is approximately 3.8 km south form theresearch pier HORS, PARI (http://www.pari.go.jp).
The headland located at X=0 m. The X-band radar is
installed at (X, Y) = (120 m, -40 m) to trace thehourly shoreline distribution around the headland.
Radar data are collected and processed every hourto time averaged images (Takewaka, 2005). Data
collected for year 2010 are analyzed in this study.
Fig. 2 shows time averaged images for high and lowtide. It shows that shoreline positions shift seawards
by ebb tide.

- 36 -Fig.1 Study area
X-band radar is installed at (X, Y) = (120 m,-40 m)

off shore shift of shoreline position due to ebb tide
Fig.2 Time averaged images around headland(2) Data processing
Six locations, X=-200 m, -150m, -100 m, 100 m,150m, and 200 m, are selected to process time-stack
images to estimate shoreline position, intertidal
slope, and cross sectional area. Fig. 3 is a part oftime-stack image for 15 days, which shows hourly
instantaneous shoreline positions of X=-100 m and
X=100 m with tidal variation.
Time-stack image shows variation of shoreline
position in time. White lines in the top and mid
panel are shoreline positions marked by manual
inspection and bottom panel shows the tidal
variation. Fig. 3 shows that shoreline moves
seaward when the tide is ebbing and vice versa.
Variation of shoreline positions, intertidal beachslopes, cross sectional areas, and beach volumes are
estimated in the following manner: Intertidal beachslopes are calculated by using linear regression
method with the measured shoreline position andtide. Data length used for the regression is 7 days.
Mean shoreline position is estimated with the resultof regression. Cross sectional area is also estimatedfrom the result of regression for the height from -0.7
m to +0.7 m around mean sea level. Beach volumes
are then calculated with the averaged cross sectional
areas of both sides of the headland and distance
between measurement locations.

Fig.3 Time stack for X=-100 m and X=100 m, and tidelevel (bottom panel) for period of day 0 to 15 (Doy, 2010)

(3) Result of radar observation
Fig. 4 shows annual variation of shoreline
position, intertidal beach slope, beach volume, andsignificant wave height and wave direction
measured at Kashima Port. Positive wave direction
indicates northern incident wave, and 0° indicatesnormal incidence to shore.
Wave characteristics are different with seasons:
In winter season, wave heights are high and higher
incident waves arrive from the northern. On theother hand, in summer season, wave heights are low
and primary incident wave direction is southern.
Shoreline positions are fluctuating all the year.
Northern sides (X=-200 m, and X=-100 m) of theheadland are generally accumulating in winterseason and are eroding in summer season. Southernsides (X=100 m and X=200 m) of the headland havethe opposite trend.
The variations of the intertidal beach slopes ofnorthern sides (X=-200 m, and X=-100 m) andsouthern sides (X=200 m and X=100 m) show
significantly different features for the periods of doy(day of year) 200 to 250. For most of the periods,
variations of the intertidal beach slopes of X=-100m and X=-200 m, and that of X=100 m and X=200m show similar behavior, respectively. On the otherhand, for the period of doy 200 to 250, the slopes at
X=-100 m and 100 m are becoming milder whereas
the slopes at X=-200 m and 200 m show almost nochange.
Beach volumes can be used as an index
determining occurrence of erosion or accumulation.
It can be seen from the variation of beach volumes
that southern side of headland is eroding in winterseason and accumulating in summer season, and theopposite is observed at the northern side.

3. NUMERICAL COMPUTATIONAs discussed in the previous chapter, trends ofintertidal beach slope variations differ from doy 200
to 250. The slopes at X=-100 m and X=100 m arebecoming milder whereas there are no considerable
changes of the slopes at X=-200 m and X=200 m.
To infer the reason of this difference, numericalcomputations of the wave and current are carriedout.
In order to compute the current field, wave filed
data is necessary. To estimate the incident wave
field, a backward propagation computation isperformed with wave data measure at Kashima port
wave station. Backward propagation computation isdone by a refraction computation. Once the offshore
wave data from the backward propagation isdetermined, forward computations are carried by
REF/DIF-1 (Kirby et al.,1994). Results of wave

- 37 -computation by REF/DIF-1 are verified with thewave data measured at Kashima wave station, andthen wave data at -10 m water depth in front of theheadland are employed as incident wave data for thedetailed computation around the headland.
As mentioned previously, variations of theintertidal beach slopes of northern and southern of
the headland show significant different show for theperiod of doy 200 to 250. To understand causes fordifferent features of variations of the intertidal beachslopes for the period of doy 200 to 250, two types ofnumerical computation for the period of doy 200 to250, and the period of doy 261 to 311 are carriedout, respectively. Wave heights for the period of doy
261 to 311 are higher than the period of doy 200 to250.

Fig.4 Annual variation of shoreline position, intertidal slope, beach volume, wave height and direction of year 2010.

- 38 -

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Wave conditions for the computation are incident
wave height H=0.84 m, wave period T=8.7 sec, andincident wave angle θ=-36.7° (average of the periodof doy 200 to 250) and H=2.1 m, T=8.0 sec, andθ=-14.5° (average of the period of doy 261 to 311),
respectively.
Through the numerical computations, the reasonof variations of the intertidal beach slopes for theperiod doy of 200 to 250 may be understood anddifferences of current field both periods (doy 200 to250 and doy 261 to 311) are discussed.
To compute the wave and current field,
REF/DIF-1 is used for the wave deformation modeland SHORECIRC is used for the circulation model.
REF/DIF-1 is developed by Kirby et al. (1994) anduses a nonlinear parabolic mild slope equation asgoverning equation. SHORECIRC is developed bySvendsen et al. (2000) and uses a Quasi-3Dequation as governing equation.
Fig.5 Bathymetry of study area

Fig.6 Current distribution (H=0.84 m, T=8.7 sec, and θ=-36.7°)

Fig.7 Current distribution (H=2.1 m, T=8.0 sec, and θ=-14.5°)

- 39 -

Bathymetry of the study area is shown in Fig. 5.,
and results of the computations are shown in Fig. 6and Fig. 7. Fig. 6 and Fig. 7 show current
distribution and wave breaking distributions. Notethat current vectors are skipped to improvereadability.
Fig. 6 is the result of computation for the periodof doy 200 to 250. It shows that waves are breaking
between depth of -2 m and -1 m. Longshore currents
are by passing the headland from south to north andisolated circulations are formed around the headlandwith flow separation and attachment.
As mentioned previously, intertidal beach slopesat X=-100 m and X=100 m are becoming milder forthe period of doy 200 to 250, Fig. 6 shows thatcurrent separates at the vicinity of the headland at
X=100 m. Then, the flows attaches the shore at
X=-100m. This flow pattern could make intertidal
slopes milder at X=-100 m and X=100 m.
Fig. 7 is the result of computation for the periodof doy 261 to 311. It shows that waves are breaking
along the depth line of -3 m. Longshore currents are
flowing beyond the headland from south to northand there are no the isolated circulations around theheadland, and no flow separation and attachment.
The headland is totally immersed in the surf zone.
For the period of doy 260 to 310, variation ofintertidal slopes of northern sides (X=-200 m, and
X=-100 m) and southern sides (X=100 m and
X=200 m) were synchronizing. Fig. 7 shows noformation of detached vortices at the headland andthis kind of flow pattern may not significantly affect

the configuration of the shoreline, so there were no
un-synchronizing changes for intertidal beach slopes
for this period.

4. CONCLUSION
In this study, study on annual shoreline changearound the headland at Kashima coast is performed.
Through time averaged image (Fig. 2) and timestack image (Fig. 3), annual variation of shoreline
position, intertidal slope, and beach volume areestimated. They show a seasonal change according
to seasonal wave characteristics.
From the numerical results of the current field, it
is suggested that morphological changes around theheadland are closely related with the longshore
current pattern.
REFERENCES
Kirby J.T, Dalrymple R.A, and Shi F. (1994): REF/DIF 1
Version 3.0, Center for Applied Coastal Research
Department of Civil and Environmental Engineering
University of Delaware, Res. NO. CACR-02-02.
Svendsen I.A, Haas K., and Zhao Q. (2000): Quasi-3D Nearshore
Circulation Model SHORECIRC Version 2.0, Center for
Applied Coastal Research University of Delaware.
Takewaka S. (2005): Measurements of Shoreline Positions andIntertidal Foreshore Slopes with X-BAND Marine Radar
System. Coastal Engineering Journal, Vol. 47, Issues:2-3
pp. 91-107.
(Received June 15, 2012)- 40 -