§Æng Huy R»m

Geomorphologic hazards are those related directly with the relief and geomorphologic processes. They are also geologic hazards in particular and natural hazards in general. Geomorphologic hazard study is an important item in the environmental geomorphologic study. This paper presents the results of the environmental geomorphologic studies of the author during the recent years, mainly geomorphologic hazards and the risks of damages caused by them along the §µ N½ng - Qu¶ng Ng·i coastal zone.

The study area is a coastal lowland area, therefore the geomorphologic hazards occurring here are mostly those characterizing coastal lowland areas such as flood, river bank erosion, coastal erosion and sand drift [1,2].

Flood hazard: is the hazard that may cause greatest damages than other hazards such as erosion and sedimentation. The flood study has been approached from a completely new point of view (Fig. 1). It includes the prediction of flood hazards on the basis of combining the data on flood hydrology (the maximum flood height and flood frequency) with landform surfaces of various geneses presented on the geomorphologic map of the study area (Table 1). The results of this study have been showing various areas subjected to flood hazard as presented in Fig. 2.

- Areas with high flood hazard: are the areas where floods may occur most frequently and seriously, i. e. low-lying alluvial-marine accumulation plain and river flood plains, especially areas near the Cöa §¹i, Trµ Bång river estuaries, areas around Tr­êng Giang bay.

- Areas with moderate flood hazard: are the alluvial - marine accumulation plains with moderate height, having the risk of relatively frequent and serious floods.

- Areas with low flood hazard: high alluvial - marine and lagoonal accumulation surface (with elevation of 4-6 m) which are flooded only by big flood with low degree of seriousness.

- Areas with sand dune surface (with elevation of 8-20 m), not flooded.

- Mountain and hill areas surrounding the plain not subjected to flood: are areas with elevation over 10m, nearly never flooded, only flash flood or local flood may occur.

Table 1. Flood hazard degrees

River bank erosion: areas with different hazard degrees are also delineated on the map based on the results of present status investigation mainly by remote sensing and map superposition methods and the results of studying the factors controlling these phenomena. The high river bank erosion degree is mainly concentrated in strongly meandering reaches of the Thu Bån River and in strongly branching reaches near the mouth of Thu Bån River with the average erosion rate of 4-16 m/ year.

Table 2. River bank erosion hazard degrees

Coastal erosion hazards: degrees within the study area can be also determined based on the present status and the main factor controlling the coastal erosion process, such as: the composition of the soils and rocks building up the coast, the slope of the sea floor, the shape and spatial location of the coast sections (Table 3). It is notable that the coastal sections with high erosion potential have been identified as open coast sections,

composed of unconsolidated sand sediments, extending from S¬n Trµ Peninsula to the north of Cöa §¹i Estuary and the coast section from the south of Cöa §¹i Estuary (7 - 8 km from it) to An Hßa Cape; whereas the coast section close to Cöa §¹i northwards and southwards are coast section vulnerable to relatively strong erosion and deposition or only deposition .

Table 3. Coastal erosion hazard degrees

Sand drift hazard: may occur in an area of about 546 km², occupying 20 % of the coastal plain area with different hazard degrees. These hazard degrees have been also determined based on the present status of sand drift occurring on the surface of sand dunes and the factors affecting directly this process, such as wind regime, water content, grain size and specific gravity of the sand, as well as the ruggedness of the surface, wind tempo, vegetation coverage, etc. (Table 4).

Table 4. Sand drift hazard degrees

In short, typical geomorphologic hazards of the study area are those of low lying coastal areas such as flood, river bank and coastal erosion and sand drift. However, these hazards occur seriously only in some areas: the low lying alluvial - marine sediments accumulation surfaces near Cöa §¹i estuary (the mouth of Thu Bån River), the sand bars in Cöa §¹i estuary and flood plains along the lower course of Thu Bån and Vu Gia rivers.

These results of hazard classification are the scientific basis necessary for evaluating the risk of damages and setting forth measures for effective mitigation of the damages.

Table 5. Integrated evaluation of vulnerability of population areas

After this evaluation principle, different risks of damages have been determined not only for population areas, but also for transport lines, agricultural areas, urban and industrial areas, etc, of which the most notable are some elements and land uses of high risk of damage, such as: population and urban areas in the lower course area of Thu Bån River belonging to the territory of §iÖn Bµn District and Héi An Town, some sections of the North-South Railways and of the National Highway No 1, agricultural cultivation areas on the alluvial-marine sediment accumulation surface near the mouth of the Thu Bån River, etc.

From the results of study on geomorphologic hazards and the risk of damages caused by them, some main measures for prevention and control of damages may be set forth as follows:

- Working out and implementing the long-term plan for moving the population from the areas with high risk of damages such as the low-lying flood plains and alluvial-marine sediment accumulation surfaces near the mouth of the Thu Bån River.

- Building flood detaining dikes for concentrated population areas on the high and moderate alluvial and marine sediment accumulation surfaces

- Giving the priority to the plantation of sand detaining forest, limiting or stopping the building on the surface of the coastal sand dunes

1. The flood, river bank erosion, coastal erosion and sand drift are typical geomorphologic hazards of the study area.

 They have been investigated, evaluated and classified according to different hazard degrees: high, moderate, low, etc., on the basis of studying on their current status and factors controlling them.

2. The above mentioned geomorphologic hazards cause different risks of damages for urban and rural population areas, important transport infrastructures and agricultural areas, as well as the existing and planned urban and industrial areas in the study area, where the risk of damages caused by flood hazard are the most important. The areas affected most seriously are low-lying areas near Cöa §¹i estuary, sand bars in the river mouths, sea beaches and flood plain areas in the lower course and along the branches of the Thu Bån River. These areas are not suitable for arranging population settlements and important infrastructures. At the same time, there must be plans for moving the population areas from these dangerous areas to safer ones.

1. §Æng Huy R»m, 2000. Geomorphologic study and delineation of forecast flood areas in the Thu Bån river delta. Proc. of the 14th Sci. Conf. of the Univ. of Mining and Geology, Hµ Néi (in Vietnamese).

2. §Æng Huy R»m, 2002. Coastal erosion hazard in the §µ N½ng - Qu¶ng Ng·i coastal zone (from Liªn ChiÓu to Dung QuÊt). Scientific J. of the Univ. of Sci., Hµ Néi Nat. Univ.

3. Hå V­¬ng BÝnh, Lª V¨n HiÒn, §Æng Huy R»m, et al., 1995. Urban geology of §µ N½ng - Héi An area. Proc. of the 3rd Geosci. Conf. of ViÖt Nam : 60 - 67.