FLOOD MONITORING and FORECASTING IN THE CENTRAL VIET NAM

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FLOOD MONITORING and FORECASTING IN 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_at_hn.vnn.vn
nttanthanh_at_yahoo.com
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1. Introduction

• 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.

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Introduction

• 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.

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1. Introduction

• 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.

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2. FLOOD IN THE CENTRAL VIET NAM

• 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.

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• 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.

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

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Rainfall 7 hr 2nd to 7 h 3rd /11/1999
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rainfall isohyetal map (from 19h 30.X to 7h
6.XI.1999)
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2.3. Flood season on main rivers in the Central
VietNam

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FLOOD IN 1999
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FLOOD IN 1999
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3. INUNDATIONin the Central Vietnam
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Inundation In historical floods in 1999 in the
Central Vietnam
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Inundation Map September 1978 flood in Nghe An -
Ha Tinh Plain
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Inundation Map November 1987 flood in Kon River
Plain (Binh Dinh Province)
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Inundation Map October 1983 flood in Da Rang
River Plain (Phu Yen Province)
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Inundation Map XI.1978 flood in Cai Nha Trang
River Plain (Khanh Hoa Province)
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Flood Warning Tower
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• 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.

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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

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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

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• 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 of
  www.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

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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

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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

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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.

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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.

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Map Thu Bon river system (Area10,350km2)
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Thu Bon river system
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Map rivers of Quang Ngai Province (Area Trµ
Khóc 3,240km2, sng VÖ 1,250km2)
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6 h
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Thank you