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새만금 지역을 살리기 위한 한·독 공동 심포지엄 – 새만금 염하구의 갯벌: 과거, 현재와 미래?

자료목차

– 새만금 염하구 갯벌?
– 주변지질과 지형
– 조류와 바람
– 연안퇴적물, 갯벌과 염습지

글 : 전승수 (전남대 지구환경과학부)sschun@chonnam.ac.kr
자료출처 :시민환경연구소

[영문]
Tidal Flats of Saemangeum Estuary:
Past, Present and Future?

Seung-Soo Chun (Chonnam National University)
sschun@chonnam.ac.kr

Wide (max. 28 km in width and 33 km in length) and two-funnel shape estuarine zone
formed by coastal merging of both Mankyung and Dongjin river mouths is called
as “Saemangeum Estuary.” This productive environment has been reclaiming for agricultural
and industrial purposes and so now is being faced with complete and permanent
destruction. The reclamation project was planed to make rice farm of 283 ㎢ and lake for
agricultural water supply of 118 ㎢ resulting from complete destruction of Saemangeum
Estuary by the construction of 33 km long dike in the nearly offshore area. This dike is
already finished about 75%. About 50% of the inner dike has been filled with sand which
is pumped up from adjacent subtidal areas, resulting in severe environmental problems in
wide, adjacent coastal zones. Total budget of the reclamation program is expected to be
over 4 billion US dollars.

Geology and Geomorphology

The coast of Saemangeum Estuary corresponds to typical ria-type coast formed during
Holocene sea-level rise. Very wide fluvial plain was distributed along the Mankyung and
Dongjin rivers although its most part has been reclaimed and used for rice farm. The
adjacent land of Saemangeum Estuary consists of various rocks from Precambrian
metamorphic rocks to Cretaceous volcanics including some sedimentary rocks (Fig. 2). In
the southern part of Dongjin River, relatively young (Late Cretaceous) explosive
volcaniclastics and volcanics are broadly distributed to provide an excellent scenery
with some exposures of sedimentary rocks, which became a famous national park
area. Two rivers meet and merge at the their river mouths since past 5,000
yrs to form a two-funnel shape estuary, Saemangeum Estuary. This merging would have
contributed wide distribution of giant sand bars at the mouth (Fig. 3), which would have
played the important roles for the invaluable bio- and geo-diversification of this
estuary. These tidal sand bars are variously submerging and emerging according to tidal
ranges and cycle, and are changed in shape, size and location severly with time. They
range from 4 to 15 km in length, 1-4 km in width and 5-20 m in height, and are composed
of fine sand (3-4 phi). Their surfaces during ebb tide are flat and are ornamented by
various ripple marks. If variable, their exposed area is about 90 ㎢ which corresponds to
about 44% of total intertidal flat distributed now still in this area. Four main tidal
channels and numerous tidal creeks were distributed under the natural state before 1992
(Fig. 3). The largest channel is located in the southern part of Shinsi-do(island) in
which it is 2 km wide and 26 m deep (Fig. 3). Most subtidal zones show a complex
bathymetry due to channel and bar movement but generally have a shallow depth and gentle
slope. The northern part shows relatively low-relief, very gentle (<1/1000) seafloor surface. The complex geomorphological feature is considerably different from relatively simple one occurring in the adjacent tidal flats, that would be one of reasons of excellent biodiversification here. This estuary has kept its reputation still as the most important natural site for clam spat generation, especially of large clam in Korea as well as for generation and living sites of other clams, shrimps and fishes. Tide and Wind Tides are semi-diurnal with a mean tidal range of 4.40 m (mean neap tide: 2.76 m, mean spring tide: 6.03 m). General tidal current shows NE-SW direction and ebb-tide dominant pattern with about max. 100 and 140 cm/s in flood and ebb tides, respectively. Monsoon in this area reveals dominantly northwesterly wind in winter and southerly wind in summer. Storms (defined as wind speed over 13.9 m/s) are relatively frequent in fall and winter as well as in early spring season. Spring wind shows N-NW direction with a mean speed of about 2 m/s (at 15 m above the MSL) and a significant wave height of about 1.5-2.0 m. Storms occur at a frequency of 4-6 days/month during spring period. In summer, wind direction is changed into SE-SW with much weaker wind speed (<2m/s), resulting in much subdued wave height, ranging from 0.5 to 1.0 m. In the fall season, wind speed and storm frequency increase suddenly and wind direction changes again to northwesterly wind. Mean wind speed and significant wave height are very similar to those of spring season. In this period, however, storms are more frequent, ranging from 5 to 7 days/month. In winter, wind is the strongest over the mean speed of 5-6 m/s, resulting in the significant wave height of 1.5-2.0 m. Storm frequency is up to 9 to 10 days/month. Coastal Sediment, Intertidal Flat and Salt Marsh Distribution of surface sediment in Saemangeum Estuary could not explain easily because the sediment type in this coastal area shows typical seasonal change by monsoon effect. In general, intertidal deposit is rich in silt content during summer season when wind speed is relatively weak and its direction is dominantly southerly. On the other hand, the silt content is diminished considerably during fall to spring seasons when storms are frequent and dominant wind direction is westerly to northwesterly. This very active, seasonal change of hydrodynamic condition in surface sediment could make this area healthy still in spite of considerable destruction of natural environment. Figure 4 shows the sediment type and mean grain size of surface sediment in summer season. Most sediments consist of muddy sand except in the inner river mouth and intertidal flat zones. Despite typical estuary, this mud-deficient character would be caused by the strong reworking process during winter season. Intertidal flats are broadly developed along the two-wedge shape coasts together with numerous moving and interconnecting sand bars. Individual tidal flats show slightly different depositional features with location, direction and sheltered conditions. Total area of intertidal flat is now approximately 235 ㎢. It was about 205 ㎢ in 1992. About 30 ㎢ of intertidal flat has been newly formed since starting of dike construction. It means that some tidal channels are filled rapidly with sediment (see Fig. 5). Salt marsh is developed in some limited, narrow zones mainly along Mankyung River with 10-100 m in width. Saemangeum Tidal Flat: Reclamation History and Future This area has a long history of reclamation (over 1,500 years?). It means that most salt marsh and uppermost intertidal flats had been changed already into rice farms. Kimje town, about 8 km landward far from coast, would have been built on the reclaimed tidal flat since 1,000 years ago. Rice has been main food to Korean people so the reclamation would play the most important role for providing large-scale new farm land. For making rice farm, a lot of large-scale reclamation project have been performed in Korean Peninsula also during 20 century. Saemangeum estuarine area has been a representative region to do large-scale reclamation program continuously even in 21 century. Figure 5 shows how the Saemangeum estuarine tidal flat has been changed during this century. In the northern part, near Kunsan City and along Mankyung River, about 45 ㎢ of intertidal flat was reclaimed until 1919. About 20 ㎢ was reclaimed further in the northern intertidal flat of Dongjin River estuary until 1930. During the period of 1964- 1979, about 40 ㎢ of southern intertidal flat along Dongjin River was changed into rice farm. Total 105 ㎢ of the intertidal area has been destroyed during the last 80 years. Fortunately, new intertidal flat of about 55 ㎢ was formed onshore during the same time so net decrease of tidal flat area was about 50 ㎢. However, the prograding rate of intertidal flat by estuarine sedimentation has decreased rapidly from 0.9 ㎢/yr to 0.09 ㎢/yr during the periods of 1916-1979 and 1979-1996, respectively. Even if the state of art is sustained, it would take over 500 years to recover the lost tidal flat of 50 ㎢ by natural process. Assumed that the 0.09 ㎢/yr rate could be applied, to recover the lost 283 ㎢ wetland in Saemangeum Estuary should have needed over 3,100 years. However, it is impossible permanently even to recover an estuary or simple intertidal flat because this coastal zone would be changed from estuary, most valuable environment in the Earth, to tidal beach by the giant dike built in offshore (not true reclamation project!).

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