MORPHOLOGY AND LONGSHORE SEDIMENT ... - CiteSeerX

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Yohida, Y. 1985. Shape of river mouth delta and shoreline around Nagase River. Annual Report,. Hideyo Noguchi Memorial Museum, 7-2: 2-4. (in Japanese)
MORPHOLOGY AND LONGSHORE SEDIMENT TRANSPORT IN LAKE INAWASHIRO, JAPAN Hitoshi Tanaka,1 Yutaka Fujita2 and Naoya Okajima 3 Abstract: Long-term shoreline change around the mouth of Nagase River in Lake Inawashiro, Japan is investigated by analyzing topographical maps as well as aerial photographs. Furthermore, short-term topography change is also discussed on the basis of bed material characteristics. In summer, there is a large amount of discharge from Nagase River into the lake, whereas high waves are generated in winter because of seasonal wind. Thus, the natural forces predominantly acting at the river mouth are clearly separated in a year. Therefore, the process of the shoreline change is much simpler in the lake, as compared with that on coastal beach facing the ocean. The main conclusion in the present study is that the sediment supplied from the river moves to the east, because the seasonal wind from northwest direction causes strong waves propagating in eastern direction in winter, resulting in longshore sediment to the east. INTRODUCTION

Several studies have already been conducted on shoreline change in a lake in Japan. These studies have focused on, for example, Lake Biwa (Tsuchiya et al., 1984), Lake Kasumigaura (Uda et al., 1998), and Lake Hinuma (Uda et al., 1985). These investigations described interesting characteristics of shoreline change, and are effective for understanding the basic processes of sand movement, because lakes are not influenced by tides and the external forces acting on lake shore are relatively simplified as compared to those on coast influenced by ocean waves, although the magnitude of the shoreline change is smaller in a lake, apart from coast facing a big lake with sufficient fetch length (e.g., Hands, 1979 ; Lorang and Stanford 1993). In addition, these studies are also extremely effective in facilitating an investigation of similarity law for coastal sedimentation, because the scale of topographical change and external forces are smaller in a lake than those encountered along seashore, but sufficiently bigger than those in a laboratory flume. In Lake Inawashiro, river discharge is predominant in summer, while waves prevail in winter due to strong seasonal wind. Since these external forces are dominating in different season, the process of sediment movement is simplified, and this area is more suitable for studying sand movement mechanisms. In the present study, the process of shoreline change around the mouth of Nagase River, flowing into Lake Inawashiro, is investigated based on aerial photographs, topographical maps, and sand grain characteristics along the shoreline.

1) 2) 3)

Department of Civil Engineering, Tohoku University, 06 Aoba, Sendai 980-8579, Japan. [email protected] Department of Civil Engineering, Nihon University, 1 Nakagawara, Tokusada, Tamura-machi, Koriyama, Fukushima 963-8642, Japan. [email protected] Department of Civil Engineering, Tohoku University, 06 Aoba, Sendai 980-8579, Japan. [email protected]

STUDY AREA

Figure 1 shows the geometry of Lake Inawashiro. Nagase River originates in the Urabandai Highlands and pours to the lake from the north, as shown in Figure 1. Nagase River is a class A river with the length of 25.5 km and the catchment area of 291 km2. At the southern end of the Shida-hama beach, there is an inlet of Asaka Channel, as illustrated in Figure 1. In recent years, deposit of sediment in the inlet pond of the channel is a big problem for the irrigation office. Clarifying the mechanism of considerable sediment intrusion into the pond is another objective in the present study. In the analysis of shoreline change, three topographical maps published by the Geographical Survey Institute during the Meiji, Taisho, and Showa Eras were used. Further, seven aerial photographs were also used, most of which have been issued by the Geographical Survey Institute, while the others were taken independently. The data source used in this study is summarized in Table 1. Because the scale of the topographical maps and aerial photographs are different each other, they were adjusted using an Affin transformation method to show the maps and the photographs on a common coordinate system.

Tenjin-hama Beach Shida-hama Beach Nag ase Ri ver 10m

60m

Inl et of Asaka Channel ●

North

9 0m

Lake Inawasiro



Lake Inawasiro JAPAN

0

1k m

East

Fig. 1. Lake Inawashiro and inlet pond of Asaka Channel On October 27, 2000, bed material on the shoreline was collected from the river mouth area. The measurement points from No.1 to No.13 were on the east side of the river mouth, while points from No.14 to No.17 were located along the Tenjin-hama Beach seen in Figure 1. The size distribution of the samples was measured by sieve analysis. Since measured wave data is not available in this lake, the wind data measured by a wind gauge, installed near the mouth of the river, was analyzed and used for examining the relationship with the direction of sand transport.

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Table 1. List of data source for morphological change analysis No. Year Data source Topographical map Aerial photograph 1 1908 O 2 1931 O 3 1947 O 4 1960 O 5 1963 O 6 1976 O 7 1982 O 8 1994 O 9 2000 O 10 2001 O

LONG-TERM TOPOGRAPHICAL CHANGE

Figure 2 shows a topographical map in 1908. It is seen that there is a river channel flowing into the lake from the north, similar to the current river mouth (hereafter denoted river mouth (A)). Furthermore, another river mouth (B), which does not exist today, can also be seen at the southern end of the Tenjinhama Beach. In 1888, a huge eruption of Mt. Bandai located in the river basin has occurred, which caused large amount of sediment yield in the mountainous region. Due to dam up caused by deposit of debris, a lot of lakes was formed in Urabandai area. Some of the natural dams, which had temporarily filled with water, could not withstand the water pressure, and they broke several times, allowing sand to flow down into the river (Yoshida, 1985). This had caused a significant amount of sediment supply into the lake through the river mouths. This is the reason why remarkable sediment deposit can be seen at the river entrance in Fig.2.

River Mouth (B)

River Mouth (A) Fig. 2. Topographical map (1908) 3

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N 6000

NNW

N NE NE

NW

4000 N WW

NE E

2000

W

E

SWW

SE E SW

SE SSW

S

SSE

Fig. 3. Frequency of wind direction (November 1999-Feruary 2000) At Lake Inawashiro, westerly wind prevails in the winter (see Figure 3) and the strong wind generates high waves. Because these waves attacked river mouth (B) directly, a part of sediment discharged from the mountainous region was pushed into the river mouth, while the rest was transported to the north to form long sand spit. The sand spit at the southern end of the Tenjin-hama Beach cannot be observed in the later topographical maps and aerial photographs. A topographical map in 1931 is shown in Figure 4. It is noted that the sand spit at the river mouth (B) has disappeared and a noticeable advancement of Tenjin-hama Beach can be observed. This topography change has occurred as the sand that had accumulated in front of river mouth (B) gradually accumulated along the southern part of Tenjin-hama Beach over a long period of time under the action of westerly wind in winter. On the eastern side of river mouth (A), in contrast, a sand spit had been formed. This was also generated by the high waves caused by the prevailing westerly winds in winter.

River Mouth (B)

River Mouth (A) Fig. 4. Topographical map (1931) 4

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Photo 1 Aerial photograph (November 11, 1947) The river mouth morphology in 1947 seen in Photo 1 is similar to one observed in Figure 4, but it is noted that river mouth (B) was completely closed. Furthermore, sand deposit in front of river mouth (B) can be seen. High waves caused by westerly wind pushed back sediment, resulting in the closure of river mouth (B). In the area of the triangular sandbar bounded two river mouths, no vegetation can be seen, suggesting that sediment movement is so active that the vegetation cannot survive. Figure 5 shows a topographical map issued in 1960. River mouth (B) can be seen again, similar to Figure 4. On the eastern side of river mouth (A), a series of sand spit were formed, and their shape and direction indicates that they were generated by predominant waves caused by westerly winds.

River Mouth (B)

River Mouth (A) Fig. 5. Topographical map (1960)

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Photo 2. Aerial photograph (April 27, 1963) Photo 2 shows an aerial photograph taken on April 27, 1963. River mouth (B) was again completely closed. Two series of sand spit were formed on the eastern side of river mouth (A). Between them, the sand spit on the right was remarkably elongated. As described earlier, no vegetation can be seen on the sandbar at the river entrance in 1947, though, slight vegetation can be observed in Photo 2, indicating that there had been no large floods to cause considerable topographical changes since the previous photograph. In Photo 3 on October 18, 1976, the revetments have been built on the both sides of the shoreline at the river entrance, although they are not so visible in Photo 3. Compared with the river mouth morphology seen Photo 2, the area of the triangular sand bar between both river mouths was clearly reduced. The objective of the revetment construction was to prevent further erosion in this area. Further east area up to Shida-hama Beach is covered in Photo 4 taken on May 24, 1982. As compared with Photo 3, no significant topographical changes can be seen in the area near river mouth (A), because of the protection of the waterline by the revetments, but the formation of a long sand spit from the eastern side of the river mouth is especially noticeable. It is seen that the tip of the sand spit almost attached the shoreline of Shida-hama Beach located on the right side end in the photograph.

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Photo 3. Aerial photograph (October 18, 1976)

Photo 4. Aerial photograph (May 24, 1982) Although Photo 5 is an oblique one taken on November 5, 1994, an arc-shaped sand bar is clearly observed on the east side of river mouth (A), indicating significant amount of sediment had discharged into the lake before this photo was taken. Considering that this sedimentary formation was not visible in Photo 4, this sediment deposit is attributed to a big flood occurred in early August 1989 during Typhoon No.8913. According to the reports of the Meteorological Agency, the rainfall was concentrated in the region of Mt. Azuma, located in the northeast area of the Nagase River basin. At the Mt. Azuma station of the Meteorological Agency, they recorded 433 mm of rainfall between 12:00am on August 5 and 9:00am on August 7. Such a large amount of precipitation probably caused a lot of sediment intrusion into the lake, resulting in bar formation shown in Photo 5. 7

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Photo 5. Oblique photograph (November 5, 1994)

Photo 6. Aerial photograph (August 4, 2000) Photo 6 was taken six years after Photo 5, and it demonstrates more noticeable sediment deposit on the left hand side of the river entrance. Slight deposit can also been seen on the right of the mouth. In August 1998, there was another flood caused by Typhoon No.9804. The cumulated amount of rain from August 26 to 31 in the mountainous area of the Nagase River basin was more than 400mm (Mano et al., 2000), being similar to the previous one in 1989 that presumably caused sediment deposit in Photo 5. Hondo et al. (2000) reported the formation of sand terrace around the river mouth due to this flood. 8

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Photo 7. Aerial photograph (January 2001) Photo 7 shows an aerial photograph of the river mouth morphology covered with snow, taken in January 2001. Owing to the snow coverage of the land area, the position of waterline is more clearly recognized, as compared with other photographs. The sediment deposit in front of the concrete revetment on the left side of the river mouth can still be observed, while the small sand bar formed on the right hand side of the mouth (see Photo 6) disappeared due to predominant waves. Superimposition of the shorelines from these maps and photographs yields Figures 6, 7, and 8. Figure 6 obviously shows that the shorelines on both the east and north sides of the river mouth advanced gradually due to sediment supply from the mouth. Figure 7 shows the process of shoreline change in 17 years from 1947 to 1963. It is seen that shoreline change on Tenjin-hama Beach is fairly stable, whereas much more active shore evolution can be seen on the eastern side of the river mouth. Figure 8 shows the shoreline change in 39 years spanning from 1963 to 2001. The western side of the river mouth was eroding until the revetments were built in 1976. In the eastern part of the river mouth, morphological change is still distinct, such as development and migration of the sand spit and sediment deposit observed in 2001, while Tenjin-hama Beach seems to have reached equilibrium in response to incident waves from the west.

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18000

18000

1908 1931

Tenjin-hama Beach

1947 1960

1947

1963

16000

16000

y(m)

14000

14000

Nagase River

Nagase River

Shida-hama Beach 12000

12000 sand spit

River mou th (A)

R iver mo uth (A) sand spit Lake Inawashiro

Lake In awashiro 10000 800 0

10000

12000

1400 0

10000 8000

10000

12000

14000

x( m)

x( m)

Fig. 6. Shoreline change from 1908 to 1947

Fig. 7. Shoreline change from 1947 to 1963

18000

1963 1976 1982 16000

y(m)

y(m)

R iver mouth (B)

2001

14000 Nagase Riv er

12000 River mou th (A) sand spit Lake Inawashiro 10000 800 0

10000

12000

1400 0

x( m)

Fig. 8. Shoreline change from 1963 to 2001 10

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LONGSHORE VARIATION OF BED MATERIAL CHARACTERISTICS

Figure 9 shows the distribution of bed material characteristics, such as median diameter d50, and the uniformity coefficient Uc, at each measurement point. The uniformity coefficient in this figure shows maximum value at points No.3 and No.7, indicating a poor sorting of the grain diameter distribution. Point No.3 is in the area of new sediment deposit caused by 1998 flood, while point No.7 may be located in the area formed by sediment supplied during the flood in 1989. At points No.5 and No.6, smaller sand grains have been selectively transported to this area by the longshore current caused by waves. In contrast these results, the spatial variation of the sediment characteristics is not so distinct at points from No.14 to No.17 along Tenjin-hama Beach, indicating this area is not affected by sediment supply from the river mouth in recent years.

No.1 7 No. 16 Tenjin -h ama Beach No.1 5

No.1 4

No. 1 No.6 No. 11 N o.9 No.1 2 N o.7 No.4 No. 8 No .10 No .13

No.5

No. 2 No. 3

Eastern side of Nagase River mo uth

d50(mm), Uc

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Tenjin-hama Beach

Uc

4

2

d 50

0

1

2

3

4

5

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7

8

9

10 11 12 13 14 15 16 17

Meas. Point No.

Fig. 9. Median sand diameter and uniformity coefficient at the measurement points

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As shown in Figure 3, approximately 60% of wind during the winter season is from either west or northwest with strong magnitude. After the closure of river mouth (B) and the construction of revetments around the mouth, sediment can no longer be transported from the river mouth area to Tenjin-hama Beach. For this reason, this beach remains stable shoreline, maintaining equilibrium against the incident waves. At the shoreline on the east side of the current river mouth, however, the waves attack the shore with a large incident angle. In this regard, wave conditions are most unlike those found along coastal shore facing the ocean. Due to these wave conditions and the effects of the frequent sediment supply from the river basin during a big flood occurred in 1989 and 1998, active shoreline change can be seen in the eastern area of the current river mouth. CAUSE OF SEDIMENT DEPOSIT IN THE INTAKE POND OF ASAKA CHANNEL

The Intake of Asaka Channel is located at the southern end of Shida-hama Beach as denoted in Figure 1. In these several years, the irrigation office has encountered a problem of sediment deposit in the intake pond as seen in Photo 8, although there had been no sedimentation problem for long years until then. Figure 10 denotes the annual variation of volume of dredged sediment from the intake pond of the Asaka Channel. It is seen that the volume shows abrupt increase in recent years, especially after 1996. The average of the dredged volume from 1996 to 2000 attains to 1,700m3/year. Judging from the results of lakeshore morphology analysis described earlier, this indicates that sediment supplied from the Nagase River mouth has reached here several years ago, after filling the area in between the mouth and the intake.

(a) March 6, 1993. Photo 8. Sediment deposit in the intake pond of Asaka Channel

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(b) March 11, 1996. Photo 8. (Continued) Sediment deposit in the intake pond of Asaka Channel 5000

3

Dredged volume(m )

4000

3000

2000

Average: 1,700m3/y 1000

0 1992

1988 & 1993

1993

1994

1995

1996

1997

1998

1999

2000

2001

Fig. 10. Dredging in the intake pond of Asaka Channel

CONCLUSIONS

The process sediment movement around the mouth of Nagase River in Lake Inawashiro has been studied by analyzing aerial photographs and morphological maps. The following results were obtained. 13

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(1) It is found that after the closure of the previously existed river mouth, Tenjin-hama Beach located on the north side of the river mouth remained stable, while erosion occurred in the area around the mouth of Nagase River. (2) After the closure of the mouth at the southern end of Tenjin-hama Beach, sediment transported from the Nagase River into the lake has been affected by the westerly wind that prevails during winter, and has been transported into the eastward direction. (3) The results of bed material analysis show correspondence to the occurrence of big flood in 1989 and 1998. (4) Sediment deposit in the intake pond of Asaka Channel is remarkable for these several years. This indicates that massive sediment supplied from the Nagase River mouth during extremely big floods has reached here, after filling the area in between the mouth and the intake. ACKNOWLEGMENTS

The authors would like to express their grateful thanks to the Fukushima Prefectural Government and the Asaka Channel & Land Improvement Office for their kind offer of the field data used in the present study. A part of this study has been supported by a Grant from the Ministry of Education, Culture, Sports, Science, and Technology to promote multidisciplinary research projects on “Study of ecological life cycles in local cities and middle grade mountain areas and the information and communication technology indispensable for their support” at Nihon University, College of Engineering ( head investigator: Professor Motohisa Onozawa ). The same ministry has provided another support for Scientific Research (B), No.14350262 (head investigator: Hitoshi Tanaka). REFERENCES

Hands, E. 1979. Changes in Rates of Shore Retreat, Lake Michigan, 1967-76, Technical Paper 794 December 1979, U.S. Army Corps of Engineers Coastal Engineering Research Center. Hondo, M., Izumi, N. and Tanaka, H. 2000. Sedimentation characteristics and numerical calculations of sediment transport at a river mouth. Proceedings of Japanese Conference on Coastal Engineering. 47: 536-540. (in Japanese) Lorang, M. and Stanford, J. 1993. Variability of shoreline erosion and accretion within a beach compartment of Flathead Lake Montana, Limnol. Oceonogr 38-8: 1783-1795. Mano, A., Takahashi, M., Imamura, F. and Nagabayshi, H. 2000. The August 1998 flood disaster in the Abukuma River basin, Journal of Natural Disaster Science 22-1: 1-11. Tsuchiya, Y., Kawata, Y., Shibano, T. and Yamashita, T. 1984. Study on beach processes on the Biwa Shoreline and the Haginohama Beach. Kyoto University Disaster Prevention Research Institute Annual Report. 27, B-2: 641-681. (in Japanese) Uda, T. and Yamamoto, K. 1985. Shoreline change on Oyasawabana Beach in Lake Hinuma. Proceedings of Japanese Conference on Coastal Engineering. 32: 370-374. (in Japanese) Uda, T., Ako, S. and Otani, Y. 1998. Littoral drift characteristics in Lake Kasumigaura. Proceedings of Japanese Conference on Coastal Engineering. 35: 477-481. (in Japanese) Yohida, Y. 1985. Shape of river mouth delta and shoreline around Nagase River. Annual Report, Hideyo Noguchi Memorial Museum, 7-2: 2-4. (in Japanese)

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