FLOOD MONITORING and FORECASTING IN THE CENTRAL VIET NAM

1. FLOOD MONITORING and FORECASTINGIN THE CENTRAL VIET NAM Dr. Nguyen Thi Tan Thanh, National Hydro-Meteorological Service (NHMS) of the Ministry of National Resources and Environment (MONRE) E-mail: ktcaokhong@hn.vnn.vn; nttanthanh@yahoo.com

2. Viet Nam has around 331,000 km2 of natural land. • Located in monsoon humid tropics, Viet Nam is affected by both oceanic and continental climates, especially flood and inundation. • There are 2,360 rivers and streams that have lengths longer than 10 km, in the central of VietNam: 740. • The coastal plains are not large but densely populated (60% of population) with many important and rather developed political, socio-economic centers. Most plains are with low elevation, easy to be flooded or inundated. 1. Introduction

3. Rainfall is unevenly distributed in time and space. More than 70% of annual rainfall occurs in wet season. • Total flow of rivers in Viet Nam reaches 650 km3/year, equivalent to a flow depth of 960 mm. • Ratio of flood and low flow varies from 1.5 to 30. • Severe floods were recorded in 1945, 1969, 1971, 1978, 1983, 1993, 1990, 1996, 1998, 1999 and 2003. • In period of 1971 - 2001, death toll of 13,300 people. • Flood rising rate, amplitude and peaks are high. Flood rising intensity in mountainous rivers may reach 2-5m/hour; downstream plains 20-50cm/hour. Flood amplitude in mountainous areas may reach 10-20m, higher than 25m in some places. Introduction

4. High Flood and inundation last for about 2- 7 days in the Central Viet Nam, 7-15 days in the North Viet Nam, and 3-4 months in the Mekong River delta. • Depth of inundation is about 2-4m, in some places it would reach 5-8m. • Combination of typhoon, storm surge, heavy rain and high tidal level would cause the flood more severe. 1. Introduction

5. Often associated with heavy rains, typhoons, tropical depressions, occur in a large area covering some provinces or almost the whole region. • Rivers are steep, times of concentration are short, and downstream areas have low drainage capacity. • Somehow can be classified as flash floods. • Tide and storm surge play a certain role in increasing inundation. • Flood and inundation in the Central Viet Nam is ranged as the most severe. It occurs with high frequency causing prolonged inundation in small, narrow coastal areas where socio-economic centers and densely populated areas are located. 2. FLOOD IN THE CENTRAL VIET NAM

7. 2.1. Data information collection, processing for operational forecast • -International Station Network • National Telegraphic Station Network consisting of: • + 127 synoptic stations. • + 76 rainfall stations. • +182 hydrologic stations • +21 ocean-meteorological stations.

8. + 5 weather radar stations. + 3 low resolution satellite stations + 1 high resolution GMS and NOOA satellite stations.

9. 2.2. RAINFALL • Large amount of rainfall with high intensity. • Maximum daily rainfall: 500 - 1000 mm. • 24 hours max rainfall reached the highest value in Vietnam: • 1,384 mm at Hue, and • 1,009 mm at Son Giang.

10. Rainfall 7 hr 2nd to 7 h 3rd /11/1999

11. rainfall isohyetal map (from 19h 30.X to 7h 6.XI.1999)

12. 2.3. Flood season on main rivers in the Central VietNam

13. FLOOD IN 1999

14. FLOOD IN 1999

15. 3. INUNDATIONin the Central Vietnam

16. Inundation In historical floods in 1999 in the Central Vietnam

17. Inundation Map September 1978 flood in Nghe An - Ha Tinh Plain

30. Inundation Map November 1987 flood in Kon River Plain (Binh Dinh Province)

31. Inundation Map October 1983 flood in Da Rang River Plain (Phu Yen Province)

32. Inundation Map XI.1978 flood in Cai Nha Trang River Plain (Khanh Hoa Province)

33. Flood Warning Tower

34. 4. Hydrological Forecasting • Forecasting is made year-round in form of three types: short-term, medium-term, and long-term forecast. • 4.1. Short-range forecast • -Rainfall forecasts with lead time of 2-3 days are made and used as inputs for hydrological forecasts • During rainy season from June to December river flow and stage of main rivers are made daily with lead time of 24 to 48 hrs for rivers in the North, 6 to 24 hrs for rivers in central part.

35. 4.1. Short-range forecast (cont.) • - In case river stage exceeds the alert level II, information on flood situation and flood forecasting bulletins are disseminated twice a day and three times per day if it exceeds the alert level III • In emergency 8 to 24 forecasts are made within a day

36. 4.2- Medium and long-range forecasting • Medium-range hydro-meteorological forecasting bulletin is regularly disseminated 5 times per day during rainy season and 10 times per day during dry months • Long-range HM forecasting bulletin is released monthly or every three month in a year - Forecasting parameters are water discharge and stage as well as possible drought and water shortage in main river systems

37. 4.3. Dissemination of forecasting bulletin • - Flood - forecasting is provided to the TV and Radio broadcasting agencies, some central daily newspapers as well as posted on the website ofwww.nchmf.gov.vn • RHMC and PHMFC daily provide the forecasting to the Provincial Flood and Storm Control Committees (PFSCCs), the local TV and radio broadcasting agencies with the hydromet forecasting bulletin. • Information bulletin on flooding is delivered directly to the Leading Communist Party Bodies, National Assembly, Government and relief providing organizations

38. 5. Forecasting techniques • - Rainfall-runoff models, method of corresponding stage and multivariable regression have been used to produce short-term river forecasts. • - In recent years, different models like SSARR, TANK, NAM have been applied in flood forecasting of small and medium size rivers. Hydrological and Hydraulic models have also been used in forecasting storm for central part and lower parts of the big river basins like Hong and Mekong

39. 5. Forecasting techniques (cont.) - Medium-term and long-term hydromet forecasts are made mainly using statistical methods like time series analysis, multivariable regression, analogue etc. methods. - Forecasting operation has been automated in term of data/information collection, processing, updating and inputting to models as well as preparation of forecasting bulletin. - Forecasting accuracy is 75-85% for short-term, and 65-70% medium term and long- term forecasts respectively

40. 6. Recommendations • Establishment of flood and flash flood warning and forecasting system on the Thu Bon river system (Quang Nam Province) or in Quang Ngai Provice (Tra Khuc and Song Ve rivers), where flood and flash flood frequently occur: • Improvement of the hydromet station network (automatical stations); • Rainfall estimation: conventional method, Satellite, Rada.

41. 6. Recommendations • More efficient use of hydrologic and hydraulic models (e.g. DEM based distributed models); • Research on flood mapping and flash flood occurrence areas; • - Seting up Flood Warning Tower • Training; • Community awareness on flood and flash flood.

42. Map Thu Bon river system (Area:10,350km2)

43. Thu Bon river system

45. Map rivers of Quang Ngai Province (Area: Trµ Khóc: 3,240km2, s«ng VÖ: 1,250km2)

46. Tra Khuc River Ve River Son Giang An Chi 4 h Song Ve Tra Khuc 6 h

48. Thank you