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Project Information > Natural Conditions
The Hai Van area is at the intersection of different climatic regions as evidenced by the large climatic differences between the northern and southern slopes. The Project area penetrates the Ngu Hanh Son mountain, at the interference between 4 areas of 2 different climatic regions of the country, therefore between the northern and southern slope there are large climatic differences. For example, the annual precipitation in Hue is approximately 2900mm, while it is about 2000mm in Da Nang. On the northern slope, the air is usually more humid and colder than on the southern slope. The long-term mean humidity on the north is about 2% higher. Also importantly, the northern slope is subjected directly to the action of the northeast monsoon, whereas the southern slope is not, due to the local topography. This results in a more uniform climate over the northern portions of the pass approach. This is important when comparing the potential visibility impacts of climate on the different route alternatives.
The overall climate of the region is a tropical monsoon climate. The annual maximum in precipitation occurs from September to December, and it is followed by a relatively dry period from February to April. During the latter period, the northeast monsoon dominates. The precipitation in Vietnam is largely caused by disturbances, which follow tracks near the polar front, situated not too far to the north of the study region. Orographic lifting locally increases rainfall from these disturbances. The importance of the orographic lifting, however, should not be overemphasized, since the main source of precipitation is from the travelling disturbances. Conditions in the northern portions of the region are often characterized by 3-5 day periods of fog, or heavily overcast skies, associated with light drizzle, and little precipitation accumulation. These conditions occur when a shallow depression is situated over the Gulf of Tunkin.
CLIMATE CHART OF TT-HUE AND DANANG IN 2001
The project area is topographically, hydrometeorologically and geologically different from the coastal areas, north and south of the Hai Van Pass area.
The Truong Son mountain range situated in the west of Central Vietnam stretches from the north to south forming a backbone of Indochina separating Vietnam from Laos. Along the southern border of Thua Thien Hue Province, the mountains protrude into the South China Sea. The Hai Van mountains with rolling summits of different shapes and different altitudes run into the sea. The highest mountain on the coast is 253 meters. The Hai Van Pass on National Highway No. 1 has an elevation of approximately 475 meters passing between two mountains with elevations of 724 meters on the east and Nui Hoi with an elevation of 1,192 meters on the west.
If two narrow parts in the Hoi Dua river valley in the north and the Cai river valley in the south area excluded, the whole of the Project area has a dissected mountain relief. It is a part of the Hai Van mountain range oriented in latitudinal direction running straight to the sea. The mountain ridge is narrow and complicated, oriented mainly in latitudinal direction. Some peaks exceed 1,000 m.
The northern and southern slopes are steep, with a complicated structure. The slopes are usually inclined 30-35 degrees at lower elevations, and in some places may exceed 40-45 degrees. Many vertical cliffs appear not only in the areas with high peaks, but also in low sloping areas along valleys, increasing the ruggedness of the terrain and serving as conditions susceptible of the development of landslides, rock-falls, slope erosion, etc.
The rivers and streams in the Project area are generally limited in length. On the northern slope is the Hoi Dua river with its streams. Most of the streams have narrow cross sections and with a very steep gradient in the longitudinal direction, resulting in many rapids and waterfalls with a very strong current during the rainy season. Many large boulders are carried as far as the outlet of the stream and onward toward the sea. On the southern slope are the streams of the Cai river. The valley structure and hydrographic characteristics of the streams here are similar to those on the northern slopes.
The valleys in the area are mainly in the NW-SE direction. Besides the capability of deep erosion and transportation of large boulders, the streams have great hydrographic variation, causing torrential and flash floods, possibly damaging many road sections and engineering structures.
The Hai Van massif is a granite batholith of approximate aerial extent in excess of 100 sq. km. A large number of faults, both known and inferred are indicated on geological plans of areas. Some of these faults have been positioned in areas of no outcrop and are presumably aerial photograph derived.
The underlying basement structure and geology of the area is reflected in the areas topography, which shows the development of two main drainage patterns, north northwest (NNW) south southeast (SSE) and radial. It is probable that these drainage patterns follow regional jointing (possibly cooling joints), faulting and/or compositional banding.
The radial drainage is well developed to the east of the Hai Van massif summit and east of the Hai Van Pass itself, suggesting that the granite there is more massive. West of the Hai Van massif summit, the drainage trends NNW and SSE, indicating the influence of basement features.
Though the tectonic history of the granite has not been confirmed, development of basement features which influence later drainage are considered to relate initially, to early, pre-solidification thermal/tectonic strain under the influence of which, parallel and sub-parallel jointing and faulting developed. Later tectonic events would be expected to modify pre-existing structures and fabrics as well as develop new trends. The variation in orientation of linears illustrated in an available linear analysis map might be explained in this way. The region has experienced significant past geological events that contributed to the formation of the local topography. The mountain range exhibits extensive lineation, jointing and possible faulting or folding which has contributed to advanced weathering and decomposition of the rock mass near the surface. These features tend dissipate as the mountain range extends to the west. The batholith is intrusive and is in faulted contact with Pre-Cambrian folded sediments to the southwest near the Cu De river. The Hai Van granitic massif is located directly on the boundary between two structural zones, which are different not only in the structural trend, but also in geophysical field and age of folding. In general, the eastern part of the granite appears to be more massive and to the west appears to be strained, demonstrating a penetrative and extensive gneissosity.
Throughout the Hai Van massif, deep weathering is primarily restricted to the lower valleys walls and floors, gullies and moderate slopes, and appears to diminish in magnitude to the west. In general, mountain side slopes above 300 meters in elevation are composed of relatively fresh granite and granitic talus characterized by the presence of large scale boulders and blocks resting on thin, poorly consolidated, immature soils containing abundant granitic detritus. Below this elevation, the weathering profile is thicker and large interfluvial, talus deposits occur. Fresh rock appears to be close to the surface at most localities and crops out commonly as hillside scarps, particularly at higher elevations whereas greater indications of weathering are more predominantly confined to areas lower in the valley walls. Where the rock possesses moderate to strong foliation, weathering appears to be more extensive.
The rock comprising the Hai Van massif is generally defined as a pale grey-mottled, course grained, variably xenolithic, biotite quartz feldspar granite. The texture ranges from massive, equigranular to gneissic. Darker phases of the rock reflect local increases in biotite and possibly hornblende and diorite.
Although the Hai Van massif is composed of massive granite origin, the rock mass is jointed and mountain valleys typically run NW to SE at regular intervals along the mountain range. These topographic features of the NW to SE valleys could be a result of the underlying geological structure and predominant lineation of the rock mass. The predominant joint sets are near-vertical and near-horizontal. The most prominent near-vertical joints appear to strike in a north to northwest direction with the secondary joint set striking in a more northeasterly direction. This results in an acute angle (600+/- wedge) intersections. Joint spacing of all three joint sets is typically 1 to 3 meters; although some are less than 0.5 meter in more fractured areas. Joints are typically tight with no infilling.
Water staining was observed in numerous joints. Wider joint openings may be observed at vertical exposures, where toppling failures are progressing. Evidence of water bearing joints is observed at some locations throughout the mountain range within the project area.
Talus deposits are typical along the slopes of the mountain range. The talus slopes may be classified into two basic categories; 1) Large boulders and blocks which have fallen from the steep slopes or carried down with mud slides to the valley floor, 2) Secondary deposits on gentle slopes consisting of finer material which has been washed out and separated from the main talus slope by surface and groundwater runoff. The larger talus deposits may be subject to massive discharge from ground/surface water runoff during or shortly following a heavy rain.
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