Series B No 13-14 1999 Special issuekjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 'PROCEEDINGS AND ABSTRACTS OF THE INTERNATIONAL WORKSHOP GPA'99 TECTONICS, GEODYNAMICS AND NATURAL HAZARDS IN WEST PACIFIC - ASIA HA NO!, NOVEMBER 6-15,1999 Organised by Institute of Geological Sciences, NCNST of Viet Nam and IGCP Project 383 In collaboration with Vier Nam National Committee for IGCP, Geological Association of Viet Nam, Viet Nam Society of Rock Mechanics and Society of Tectonics (GA of Viet Nam) DEPARTMENT OF GEOLOGY A & MINERALS HA NQJ OF VI1!:TNAM 70 Qiao Sen, Chen Qiongying, Zeng Xiangfeng Study on maximum entropy principle for earthquake prediction and determining potential seismic source area 228 71 Panza G.F., Vaccari F., Romanelli F • The lUGS-UNESCO rGCP Project 4f4: Realistic modeling of seismic input for megacities and large urban areas 228 Part IV NATURAL C onvenors: T rdn Van T ri HAZARDS SRQPONMLKJIHGFEDCBA & I C h e sn o k o v a 72 Nguyen Trong Yern, Le Thi Nghinh, Doan Dmh Lam, Dao Thi Mien, Nguyen Xuan Huyen, Phan Dong Pha Flash flood in Northwest Viet Nam 233 73 Uromeihy A Evaluation of slope instability by the use of landslide zonation maps with reference to landslide cases of Iran 241 hazard 74 Nghiem Hiru Hanh, Tran Minh Dan, Ngo Van Sj, Le Xuan Thu Landslide in some open pit coal mines in Vi~t Nam 244 75 Novikova T.I., Wen K.L., Huang B.S Numerical modeling of generation and propagation processes of tsunamis and oceanic rayleigh waves excited by seismic sources 25 AI 76 Nguyen Dich Dj, Do Van Tu, Dinh Van Thuan, Mai Thanh Tan Relationship between Quaternary sedimentation and landslide-rnudflow formation in Nothwest vier Narn 256 77 Tran D(lC Thanh 262a Risk of Tu Hien Wet closure in Tam Giang - Cau Hai Lagoon 78 Chesnokova I., Koff G Assessment Asian part of the Russian Federation of risk of natural processes occuring in the 262 79 Nguyen Ttr Dan, Le Tram Using remote-sensing data and GIS technology in modeling geological hazard in mountainous region: the study case of M~ Dire, Hoa Bmh 263 80 Ozer A., Phan Trung Trinh, Jaspar A., Risack S • Study on the Hai Phong Bay coastal evolution over a sixty year period using remote sensing data 266 81 Aurelio M.A On-going studies in tectonics, geodynamics the Philippines 267 and geologic hazards in 82 Doan Minh Tam A review of landslide processes on the road network of Viet Narn : 268 83 D*u Hlen I andslide in Viet N am in the view of weathering research 268 84 Doan Van T u y e n , Tran Canh The application geological hazard studies at engineering sites of electrical tomography in 85 Nguyen Van Ph6, Hoang Tuyet Nga, Doan Thu Tra, Study on the stability of Thac Ba hydropower dam by using nuclear track detector method 269 270 Part V PALEOEARTHQUAKE A H u ll & Y K in u g a s a C onveners: 86 Takashima I., Charusiri P., Kosuwan S., Won-in K TL-age dating results on the Mae Chan segment of the Mae Chan fault, Northern Thailand: constraints for Quaternary faulting (*-) 275 Proceedings and abstracts of the International W orkshop GPA'99 Series Journal B, No.13-14/1999, of GEOLOGY p.262a-262b RISK OF TUHlEN INLET CLOSURE -IN TAM GIANG - CAU HAl LAGOON A mAN H d i P O O n g In s titu te o f O c e a n o lo g y , (F a x : -7 , D O C 'l1 W f f i SRQPONMLKJIHGFEDCBA 246 Dd N a n g S t r e e t , H d i P O O n g , VItI' N A M E m a il: td th a n h @ h io a c v n ) I n t r o d u c t io n With the area of 216 sq.km and the length of 70 km, the Tam Giang - c a u Hai (TO CH) Lagoon occupies the most coastal length of Thira Thien - Hue' Province: Its great values of natural resources have been used for the local sectors of fishery, agriculture, waterway- harbor and tourism - service The TO - CH Lagoon also takes a very important function in coastal ecology and environment that influences deeply to the survival of 300 thousand inhabitants in surrounding It is connected to the sea by two inlets of which Tir Hien Inlet in the south sometime is suddenly closed It is a heavy coastal risk that needs to be controlled In fact, Tu Hien Inlet consists of two subinlets, named as Vinh Hien larger in the north, and LQc Thuy very smaller in the south Two subinlets are separated by a sand beach ridge km long and from 2-2.5m high The lagoon is connected to the sea by the larger subinlet directly or by the smaller subinlet and small channel behind the sand beach ridge Mentioning the closure of Tir Hien Inlet it means that of the larger subinlet When the larger subinlet is opened, the smaller one is closed by sedimentation in general When the larger inlet is closed, the people have to dredge the smaller one to get the water way for smaller ships and boats to the sea The historical document shows some evidences for the status of Ttr Hi~n Inlet closure and opening on the table S ta tu s Table S t a t u s o f T I J H ie n I n le t c lo s u r e Year Before 1404 Before 1811 1823 1844 Before 1953 1959 1979 1984 1990 1994 - 1404 1811 1953 Larger one Unique inlet of the lagoon Existed Gosed Opened Closed Opened O ,£ened Closed Opened Closed oresent days Opened Go sed The Thuan An Inlet was opened in 1404 and the feudal government decided to block it 2.62 a a n d o p e n in g Subinlets Smaller one Not existed Closed Closed Opened Closed Opened (dredged) , and to maintain Tir Hien Inlet in 1469 It roves the fact that Tir Hien Inlet began to be closed after the opening of Thuan An Inlet Consequences Falling in the years of Tu Hien Inlet closure, the flood - inundations are higher in number and intensity and their consequences are more heavy During last 50 years, there were six very heavy floods - inundations of which five have felt in the closed duration of the Tv Hien Inlet The floods inundations have made serious consequences such as the loss of human life, livestocks, crops, damage of infrastructure (houses, roads, gates, bridges, boats etc ), coastal erosion and pollution For example, there were nearly A th o u s a n d dead people in the flood inundation generated from a typhoon in October, 1985 Due to the closure of Tu Hien Inlet the waterway for marine transportation and fishery is blocked Hundreds small ships and boats have had to pass an additional distance of 40 km for sailing to the sea through Thuan An Inlet in the north of lagoon This risk has damaged seriously the sea fishery economy and impacted negatively to the life of fishmen The closure of Tu Hien Inlet has freshened the lagoon water, degraded the lagoon ecosystem, decreased biodiversity, damaged brackish aquaculture and changed the catching patters in the lagoon This event has also decreased the water quality, promoted the pollution, especially oil and organic matters by restricting the water circulation Every year, there are about 1.1 million tons of suspended matters discharged into the lagoon From them, 30 percent are transported to the sea through the inlets The closure of Tu Hien Inlet strengthens the sedimentation and shallows the water body that leads to quick filling of the lagoon Finally, the deep changes in coastal environment, ecology and natural resources concerning the closure of Ttr Hi en Inlet make unsustainable the development of local socio-economy Cause The intrinsic cause of Ttr Hien Inlet concerns the natural evaluation of TG -CH Lagoon Previously, it was the unique inlet, and Phii Cam River was the main one running into the south part of lagoon Later, Phii Cam River was ruined, and Huong River running into the north part of lagoon become the largest one Based on the new dynamic balance, a new inlet named Thuan An was opened in 1404 in front of Hirong River mouth Since this time, T tr Hien has been being the secondary inlet and sometime is closed by sedimentation The direct cause of Tu Hien Inlet closure is the weakening of the flow passing the inlet and the violent concentration of longshore sandy drift in the inlet area The mathematical model calculation shows that a total number of 500,000 ton sand/year are transported to Tir Hien - Chao May coastal part by the eastsouthward wave longshore drift Meanwhile a westnorthward local longshore drift transports about 900,000 ton sand/year from Chao May site to Tu Hien Inlet This local concentration makes the closure of T tr Hien Inlet in the late stage of northeast wind season from December to April, generally, It falls in the time of ending the flood flow and changing the prevailing wave direction After a closing duration from 6-11 years, the inlet can be opened in the coincidence with a heavy flood and violent coastal erosion in the early stage of northeast wind season The opening duration is from to 20 years Conclusion As a heavy geological hazard, the closure of Ttr men Inlet makes a negative impact to coastal environment and ecology, and the great losses of economy and people's life This hazard happens also in some other coastal lagoons in Central Viet Nam Its intrinsic cause is related to the geological evolution of the lagoon created by the combination of endogenous and exogenous factors The direct cause concerns the interaction between the longshore sand drift transported by wave and the eroded tidal and flood current through the inlet The control of Tir Hien Inlet closure must be regarded as a prior issue of TO CH Lagoon management REFERENCE Trdn H(re Th~nh, 1997 Environment impact of the closure and displacement of inlets of the Tarn Giang - Cau Hai Lagoon.SRQPONMLKJI M a r in e resources and e n v ir o n m e n t 196 Sci and T ech P ub H ouse, I V - - H d ¢ i 262b ... faulting (*-) 275 Proceedings and abstracts of the International W orkshop GPA'99 Series Journal B, No.13-14/1999, of GEOLOGY p.262a-262b RISK OF TUHlEN INLET CLOSURE -IN TAM GIANG - CAU HAl LAGOON. .. concerning the closure of Ttr Hi en Inlet make unsustainable the development of local socio-economy Cause The intrinsic cause of Ttr Hien Inlet concerns the natural evaluation of TG -CH Lagoon. .. opening of Thuan An Inlet Consequences Falling in the years of Tu Hien Inlet closure, the flood - inundations are higher in number and intensity and their consequences are more heavy During last