EROSION OF A COAST NEARBY THE RIVER MOUTH

Vietnam -Japan Estuary Workshop 2006
August 22 nd -24 th , Hanoi, Vietnam
Abstract
EROSION OF A COAST NEARBY THE RIVER MOUTH
TAKAO SASAKI 1,*) , TOSHIMITSU TAKAGI 2) , TAKAHIRO HORIGUCHI 3) ,
and KHANH TOAN DAM 4)
1, 2, 3, 4) Department of Coastal Engineering, INA Corporation
1-44-10 Sekiguchi, Bunkyo-ku, Tokyo 112-8668, Japan
*) E-mail: ta-sasak@ina-eng.co.jp
Recently, severe erosion has occurred in the Yokosuka coast (Miyagi Pref., Japan). The aerial
photos shown the landward invasion progress of the shoreline is correspondingly to the reduction of
sandbars in the surf zone (these sandbars are formed by the sediment from Kitakami River). In order to
examine the mechanisms of sediment transport at the Yokosuka coast, a tracer investigation method is
carried out. It has been confirmed that existence of estuary-bound sand drift from the area between
surf zone and shoreline of the estuary.
Keywords: Beach erosion, Estuary sandspits, Sediment transportation, Tracer investigation
1. INTRODUCTION
The Yokosuka coast is a 1km long sandy pocket beach located in the Oppa bay in the eastern part of
Miyagi Prefecture, Japan (Fig.1). The coast is laid between the mouth of Kitakami River in the north
and the Nagatsura Inlet in the south (Fig.2). Without any coastal or port facilities on it, Yokosuka
coast is one of few shores where artificial structures are not installed in Japan. Therefore, people love
to both visits the ocean and to enjoy the natural coast of Yokosuka, especially during the summer time.
Sea of
Japan
SENDAI
Pacific
Ocean
OPPA BAY
Fig.1 Oppa bay in Miyagi Pref. Fig.2 Location of Yokosuka coast
Suga et al. (1987) recognized that the outflow sand is composes from Kitakami River, and then
forms the offshore sandbars in the Yokosuka coast. On the other hand, the periodical cycle of the
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Kitakami River
OPPA BAY
Yokosuka Coast
Nagatsura Inlet

shoreline changes is a conventional process in this coast. The erosion period stretches from winter to
early spring due to the high wave’s attack to the coast, and then the coast will be regained during the
summer (accretion period). However, after a severely invaded (as much as 100m into the land) during
the winter of 2002 the shoreline does not recover in summer as usual. Hence, a series of investigations
have been conducted recently to examine the mechanism of sediment movement, predict the erosion
tendency, and then seeking for the appropriate solution.
At a beginning stage, the works at Yokosuka coast is mainly focus on data collection and analysis of
existing data, such as aerial photographs, bathymetry, wave and current data, and gain/loss sediment
quantity. To determine the net patterns of sediment movement and their dynamic behavior, a part of
“Sediment Trend Analysis” process named as tracer investigation also have been carried out.
2. CHANGE TENDENCY OF SHORELINE
Fig. 3 illustrates the shoreline evolution of Yokosuka coast from 1963 to present, with the reference
shoreline as of the year of 1963 (the oldest available aerial photo of this coast). It is can be seen from
the figure that the shoreline is progressed from 1963 until 1975, since then the erosion became its
steady trend.
CHANGE OF SHORELINE(m)
No.1 3
5 7 9
BASED ON May.1963
1975.Nov
2001.Aug
2002.Jul
2004.Oct
2006.Jun
:SHORELINE(1963)
ACCRETION
EROSION
1963.May.12
1975.Nov.25
2001.Aug.3
2002.Jul.12
2004.Oct.7
2006.Jun.29
100 200 -100 基準時期:1963年 5月 水深: 0m 0
Fig.4 Cross section of depth changes
-200 -300 -400
DISTANCE OF ALONGSHORE(m)
Fig.3 Shoreline evolution of Yokosuka coast Fig.5 Sand volume changes
(Based on May 1963) (Based on March 2005)
-80 -40 40 080 160 2005.Mar~2005.Nov
-80 -40 40 80
-160 -80 080 160 VOLUME(10 3m 3)2005.Mar~2006.May
1 3 5 7 9 LINE(No.) TOTAL TOTAL VOLUME(10 3m 3) :NET
-
) 2000 1500 1000 500 0
180 -
DISTANCE(m)
ELEVATION(T.P.m)
+4.0
+2.0
+0
+4.0
+2.0
0
-2.0
-4.0
-6.0
-8.0
ACCRETION
No.1
EROSION
No.7
2005.Mar
-2005.Nov
2005.Mar
-2006.May
0 500 0 500
OFF SHORE DISTANCE (m)
:T.P.-4m≦h :T.P.-9m≦h

In July 2002, an extremely large volume of discharge from the Kitakami River flushed off the sandspits
of the estuary; as a result, the shoreline eroded dramatically. Although the estuary sandspits
began to form again after the flush, the shoreline eroded 140m as of June of 2006 (the average erosion
amount is 72m from 1963 to 2006).
It was recognized from survey results (conducted at three different times) that a massive sandbars
had been developed from March to November of 2005, at 500m offshore area. However, the sandbars
reduced rapidly and moved toward the estuary (Fig.4). Volume changes calculation results shown that
an amount of 120,000m3 of sediment volume is lost during a period of 8 months (from March to
November 2005). In the next 6 months (from November 2005 to May 2006), 60,000m3 of sediment
was recovered. (Fig.5)
3. ANALYSIS OF EROSION FACTORS
Fig. 6 illustrates the observation location of river flow
discharge (at Tome) and the wave data station (at Enoshima
Island). Exerting forces that influences topographic changes of
Yokosuka Coast are summarized in Fig.7
According to the figure, the erosion accelerated in June 2002
coincides with large discharge from Kitakami River. On the
other hand, high significant wave height (which exceeds 4m) is
frequently occurs at the same time. It convinced that the severe
erosion, which began in 2002 is caused by either unusually
large volume of discharge from the Kitakami River or/and high
waves, because there is almost no sand dredging had been done.
KITAKAMI
RIVER
Fig.6 Observation location
20 40 60 80 100WAVE(ENOSHIMA,H1/3≧4m)
TOTAL TIME(HOUR) 0
100 200SAND VOLUME(RIVERMOUTH,1996 ~) :-0.8~-0.6km,:-0.6~-0.4km,:-0.4~-0.2km 0 20 40 60 80 100 120 140FLOWING DISCHARGE(TOME,Q≧2000m3/s) :8m

4. TRACER INVESTIGATION
A tracer investigation method was conducted in order to examine the net patterns of sand movement
at the shoreline of Yokosuka coast. 1m3 of color sand with D50=0.55mm is used (Fig.8). The sample
color sands are dumped at the “injection point” at the shoreline on March 22, 2006. Sampling sands
were collected at 31 points in a range of 2,000m along the shoreline in every two weeks. Fig.9 shows
the wave condition during the investigation (at Enoshima Island), and the timeline of sampling pickup.
Color sand was found at the north side of the dumping point at the first and the second samplings
(Fig.10). Since there was no color sand found from the areas of south side of the dumping point, it was
assumed that the littoral drift is moving toward the North.
WAVE HEIGHT(m)
Fig.8 Sampling color sand (D50=0.55mm) 1 2cm 0
Injection 1 st time 2 nd time 3 rd time
Fig .9 Wave condition
:H1/3 :T1/3
during
WAVE(ENOSHIMA)
investigation 1 20 301 10 20 1 10 20 30 30 10 012345
16
2006.Mar Apl May 3104812
10FIRST(2006.Apr.5 INJECTION 5 ) ) POINT(2006.Mar.22) 10SECOND(2006.Apr.20 :COLOR SAND
1200 05 10THIRD(2006.May.13 ) NUMBER OF PARTICLE :PICK UP POINT
05
FROM INJECTION POINT (m) NOT DETECTED 800 400 0 -400 -800 DISTANCE
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INJECTION POINT
Fig.10 Layout and results of tracer investigation
WAVE PERIOD(s)

5. CONCLUSIONS
From the above-mentioned investigations, the mechanism of erosion of Yokosuka coast after the
year 2002 can be summarized as follows:
• Flood of June 2002 flushed sandspits and adjacent sediment from the estuary.
• By high waves, this occurred frequently after the year 2002, sediment was flown out to the
offshore area and formed submerged sandbars. The high waves also smoothed the crest of
sandspits at the estuary, and moved the sandspits into the estuary.
• Reformed estuary’s sandspits was consisted of extracted sand from the adjacent area of
Yokosuka coast, and it accelerated the erosion of the shoreline.
6. ACKNOWLEGEMENTS
This work is a part of a contract project titled “Countermeasure of Erosion in Yokosuka Coast”,
managed by Ishinomaki Office, Miyagi Prefecture. The authors would like to express their
appreciation to Professor Tanaka Hitoshi for his advices and guidance.
7. REFERENCES
Suga K., Ishikawa T., Nadaoka K., and Tanaka H.(1987). Formation of sand terrace in front of a
river mouth and its decline. Journal of the Japan Society of Civil Engineering (JSCE), No.381/II-7,
227-230.
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