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| :::Hydrogeological Investigation Study, Groundwater Observation Net |
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Analyze The Sedimentary Environments of The Various Groundwater Regions
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| (1) Hydrogeological Framework of the Lanyan Plain |
The Lanyan Plain Groundwater Sub-Region is situated north east of Taipei. To its north there is the
Hsuehshan Mountain Range; to its south, the Central Mountain Rang; on the east there is the western
part of the Okinawa Trough. The Okinawa Trough is presently undergoing persistent active opening.
The Lanyan Plain is a basin that owes its origin to opening of the trough. The Lanyan Plain is covered
by Recent Alluvial Deposits. Sediment sources for the Recent Alluvial Deposits mainly came from the
Hsuehshan Mountain Range and the Central Mountain Range. The major river on the LanyanPlain is the
Lanyanhsi River. The coast line of Lanyan Plain is subject to erosion, and is arcuate in shape. On
this arcuate-shaped plain, most estuaries of rivers are concave towards the land, the Lanyanhsi River
is the only exception, it is convex towards the sea, due to silting at the estuary. Except the
Tehtzukouhsi, the Lanyanhsi and the Hsinchenghsi, at the estuaries of all of the streams on Lanyan
Plain there is an elongated sand spit. These sand spits all elongate in the same direction parallel
to the coast line, and all show elevation of under 20 m, with a width of 800 m. The Tungshanho and
the Ilanho Rivers are both blocked by such sand pits, and with their direct entry to the sea denied,
these rivers pour into the Lanyanhsi River and enter the sea.
The Lanyan Plain is situated at the westernmost margin of the Okinawa Trough. Land subsidence rates
within the basin varied greatly. In addition a drop of 120 m in sea level in the last 10,000 years
resulted in many sedimentary depositional facies on the Lanyan Plain. These sedimentary depositional
facies include alluvial fans, alluvial plains, estuaries, near-shore zone, tidal zone, and off-shore
zone. The over-all sea-level for the entire world back 18,000 years ago stood at 120 m lower than the
presentsea-level. As a result, the majority of the lower part of the Lanyan Plain comprised an
unconformity. The deposition environments following the unconformity first became deepening upward and
then shallowing upward in sea water depth. Deposition near the sea shifted from an estuarine or near
shore environment to that of an off-shore environment, and later shifted to shore face to alluvial
plain environments. Deposition near the hill changed from a downstream alluvial plain to alluvial fan
environment. The global sea-level attained its maximum height around 8,000 years ago. The depositional
environment did not become any deeper, however, vast quantity of sediments was deposited onto the
basin, and the depositional environment rapidly became shallow. Coarse-grained sediments were rapidly
deposited on the seaward side, and formed the present day Lanyan Plain, an alluvial plain larger in
extent than the past. Under these depositional environments, factors affting the hydrogeological
framework of the Lanyan Plain include the mian course of the Lanyanhsi River, tributaries originating
from the Hsuehshan Mountain Range as well as from the Central Mountain Range, and the sea-level change.
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| (2) Brief Summary of Map Sheets Mailiao, Hsilo, Taihsi, and Peikang |
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The total area of the Mailiao, Hsilo, Taihsi and Peikang 1:50,000 scale geological map sheets covers
most of the extent of the Chuoshuihsi Alluvial Plain, and part of the Touliu Hills and Terraces
regions. Sediments on the area mainly originated from the catchment areas of the Chuoshuihsi River.
During deposition of the sedimentary sequences a number of transgression and regression of the sea
occurred, shifting of coast lines resulted in intertonguing of the terrestrial facies with marine
facies strata. Preparation of these map sheets made references to existing geological maps as well
as to lithologies of formations within a subsurface depth of 300 m below ground surface. Explanatory
text also made reference of the deep well boring results.
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| (3) Preliminary Conclusion Regarding the Meinung Reservoir Issue |
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In September 2002 the Central Geological Survey accepted a project entrusted by the Bureau of Water
Conservancy, MOEF, to execute "Discussion Sessions on Topics of Debate on Regional Geology of the
Meinung Reservoir, Phase 1". After fifteen months, following scheduled discussions on sessional
topics of debate: "Stratigraphy and assigning", "Explanation on regional geological structures",
"Extent and distribution of faults" and " Fault activities characteristics" a preliminary conclusion
had been achieved. Thirty five experts on Meinung Reservoir were invited to attend, and reference
materials for the topics came from 25 reports was furnished. With the understanding of unity among
diversification, and difference among unity, most key issues of debate had been answered, and a
general consensus was attained.
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