Concluding Report“Roadmap toward effective Flood Hazard Mapping
In Viet Nam”
1. Name : Nguyen Huu Phu
2. Country: Vietnam
3. Organization: Department of Water Resources, Ministry of Agriculture and Rural
Development, Viet Nam.
4. The course: Flood Hazard Mapping, 2005.
Contents
A) The role of flood hazard maps to mitigate flood damages in Viet Nam• The flood status in Viet Nam
Vietnam is located in the Southeast Asia area with its population of 80 million inhabitants(figure1). Its weather is influenced by the tropical monsoon climate. The result is very
high rainfall; the mean annual precipitation is about 2,000 mm. The availability of wateris also very abundant, the total annual runoff generated over the country is 928 billion m3,
of which the surface water resources are 880 billion m3, and the ground water resources
are 48 billion m3. There are 14 major river basins in Vietnam; the biggest rivers are Red
River and Mekong River (figure 2).
Vietnam exists in a tropical monsoon region and near one of the five of the biggest storm
outbreaks in the world. River and streams often have high slopes leading to fast flood.
Rainy season coincides with stormy ones so we always confront natural disasters of floodand storms. Runoff at high-flow season accounts for about 60-85% of the total annual
runoff. Total runoff in high-flow season in the central and southern littoral areas may be
60-70% of total annual runoff despite its short 3-4 month period. Highest flow levels
occur in July and August in the north and north-central area, September and October inthe south-central and central highlands, and October in November in the southern area.
The flow in those months is about 20-30% of the total annual flow. The highest flooding
also occurs in those months.

Flooding in small rivers rises and falls quickly, from 1-5 m3/s/km2 to over 20 m3/s/km2.
Flashfloods in mid-size and small rivers often occurs in some mountainous provinces in
the central region. High and extreme floods resulted in a high loss of life and extensive
property damage by inundation in downstream lowlands in the Red River in 1945 and1971, in the central littorals in 1964 and 1999, in the Dong Nai River in 1952, and in the
Cuu Long River delta in 1961, 1966, 1978, 1994, 1996, 2000.
Floods happen at different times every year in different rivers, but tend to be later towards
the south, and the interval between areas is about 1 month. Rivers in the north and northof Thanh Hoa province usually flood from June to October. Rivers in the littorals fromsouth of Thanh Hoa province to Ha Tinh province see floods from July to November, andrivers in the littorals from Quang Binh province to Ninh Thuan province flood from
September to November. Rivers in the south and plateau regions have floods from July to
November.
Northern Lowlands and Mid-Lands In spite of a large system of dykes and flood
control structures such as Hoa Binh and Thac Ba reservoirs, Day dam for flood diversion,and flood retarding zones, the flood control infrastructure is not capable of eliminating thedanger of flood destruction in the north. The northern river dyke system has been in
existence for hundreds of years. However as the construction was mainly made manually,and because it has been in and out of operation for long time periods with very little
consolidation, the system is starting to weaken, and the strength of structures and theirfoundations has become degraded. Settlement, cracking of structures, leakage, seepage,and boilers exist in a many important dyke sections of various rivers. Dyke consolidationand strengthening are carried out annually, including the treatment of potentially risky
phenomena like leakage, boilers, enlarging of dyke cross sections, and revetment of
upstream side slopes. Preparing human resources and materials for warnings andemergency actions, coordinating between rescue forces of various sectors for a rapid
response in case of emergency or disaster are also undertaken annually.
Although the dyke level in Hanoi and corresponding locations downstream has beenheightened to cope with historical levels of 13.30 m and 7.21 m at Hanoi for the Red
River dyke and Pha Lai for Thai Binh River dyke respectively. When very high flows
occur at the same time as a flood tide and strong typhoons, dykes may be broken anddevastating damage may result. The flood in 1945 caused nearly 2 million deaths fromstarvation as the result of the flooding of 312,000 ha of rice production area. During thehistorical flood in 1971, dyke failure caused serious inundation of 250,000 ha in many
Northern provinces, with consequences for 2.7 million people.
Cuu Long Delta Flooding occurs over a three to five month period and often inundates
over two-thirds of the delta area. Great floods happened in 1961, 1966, 1978, 2000, 2001,

and 2002. The “Co-existence with Flood” solution is applied in addition to structural andnon-structural measures to minimize the damages from floods and maximize its
advantages. High floods, floods combining with high tides and water level rise due totyphoons, are events of great concern. To mitigate and eliminate the negative impacts
from floods, the sea dyke system in the Cuu Long River delta needs to be upgraded tosecure salinity prevention and to withstand strong winds. The embankment systemshould be capable of withstanding early floods to protect summer and autumn crops.
Flood protective areas should be established in both shallow flooded and deep flooded
regions.
Central Littorals the center of Vietnam is constantly faced with natural disasters, themost frequent of which is floods. Floods occur after heavy rains, typhoons, and tropicaldepressions. They are often extensive and repeatedly inundate vast areas, even the whole
region. Floods in central rivers are mainly flash floods and extreme floods that occur
rapidly, rising and falling quickly. They are largely due to short rivers with steep
gradients, and deforestation of watershed forests which deteriorates the ability of flow
regulating as of heavy rains. In this region, the frequency of high floods and flash floods
is higher than in other locations. Great and extreme floods occurred in 1964, 1969, 1975,
1978, 1998, 1990, 1992, 1993, 1995, and 1999. In addition to the construction ofreservoirs upstream, sea dykes in the north-central region must be upgraded to withstandstrong winds combined with high tides.
Mountainous and Central Highlands Areas Great floods in major rivers of these areas
often caused inundation in river lowlands which last for several days. Rivers affected bysuch floods are the Tuyen Quang, Yen Bai, Ha Giang, Kon Tum, and Hoa Binh. Floodprevention and flood-related disaster mitigation should be comprehensive and include
afforestation, forest protection, and prevention against water concentration downstream.
Reservoirs should be built upstream for flood control. A number of national programs
have been implemented such as 5 million ha reforestation and a bare hills afforestation.
Flood diversion projects should be developed, along with an improvement of dykes.
Protection of densely populated residential areas and concentrated economic zones shouldbe implemented. Another recommended measure is the clearing of river channels for theeasy discharge of the flood flow to the sea. Natural disaster and flood are serious issue fora long time. They have caused bad effects to the socio- economic development andenvironment in particular and the sustainable development of the Viet Nam. The flood
can occur in any place, at any time in the catchments of our rives and springs. Together
with the global climate changing, the situation of natural disasters, heavy rain and flood isgetting more complex, more difficult to deal with, and the consequence caused is moreheavy not only is our country but also in many countries in the Asian region and theworld.

• The outline of the present countermeasures for mitigating flood damages in Viet
Nama. Non-structural measures

A forestation and forest protection; rearrange crop season and cropping pattern, andapply new kind of seeds that can be able to resist drought, inundation, salinity intrusion,and insects; reorganise for aquatic product catching activities, improve capacity of boats
and ships operating at the deep sea, equip communication means, and life jacket;
rearrange residential areas where frequently affected by disasters, particularly flash floodand mud flood; organise and establish observation network to monitor disasters anddamage caused by disasters; establish information and alarming network from centrallevel to local level; establish policy and mechanism for disaster prevention and mitigation
activities, as well as rescue, aid relief, and compensate activities.
b. Structural measures

Participating in structural measures, the role of the whole community is very
necessary. For simple measures, localities can mobilise all forces of local people to carryout, with partial support from the State. The four-on-the-spot guideline is applied for
responding to not only flood and storm but also other types of disaster, including fire,
environment problems, reservoir problems, landslide, forest fire, inundation, drought, etc.
Structural measures are as follows:- For mitigating damage caused by typhoon or heavy storm: build storm shelter for
medium-size and small-size boats and ships in coastal area and islands (refer to theexample of the Bach Long Vi Island); build storm resistant houses (or trenches) forcoastal inhabitant according to models proposed by program 06 B. Storm resistant houses
or trenches should be constructed while considering storm surge with probability of 1 to
5 %.
- For mitigating damage caused by floods: build dyke system in the populous plain toresist floods with 5 to 10% of probability; build flood-release way by combining dyke
system and river flat; build upstream reservoirs for multiple use like flood retention andflow regulation for dry season; build large scale reservoirs with design flood probabilityof less than 1% (as case of North and Central Vietnam).
+ In Central Vietnam, reservoirs should be multiple-used to control flood and regulate
flow in dry season and generate electricity. Proper reservoir-operating process should be
prepared to effectively control flood and fully store water for generating electricity (inCentral Vietnam).
+ Build big reservoirs upstream to retain flood water for keeping flood water level under
the design flood level for dyke construction; and dredge river bed and river flat forstrengthening capacity of flood-release way.

+ Create new flood-release ways, flood retardation areas in case of that floods are higher
than design floods (probability of 0.5% and 0.2%); in case of extreme floods (probabilityof 0.1%), it need to open more emergency flood-release ways (particularly in the Red
River Delta).
+ Build sea dyke with proper height to resist tide water levels and storm surge caused bystorm with the strongest wind force measured at Beaufort Scale 10 and above. For
important sections of dyke system, revetment should be made so that when flood water
overflow dyke surface, dyke will not be breached and as a result, area intruded by salt
water will reduce (particularly in Middle and Southern Central Vietnam).
+ Build more reservoirs to supply water in dry season for the southern central region tocontrol drought and desertification.
+ Build more electrically-operated drainage pumping stations in hardly gravity-drainage
areas, particularly in the areas that have surrounding dykes (in Northern and Southern
regions).
+ Build salinity flow regulators, particularly in the Mekong River Delta, downstream
Dong Nai River, and on some rivers in the Centre. Salinity flow regulators should notaffect capacity of flood release and inundation-water drainage.
+ Upgrade reservoirs, strengthen spillways, upgrade flood release sluices on the NorthSouth traffic ways both railway and road.
+ Establish forest fire prevention belts. Build water stores, or water canals for forest fire
suppression.
+ Improve slope-side of mountains and hills where landslides probably occur and threattransportation roads (reduce slope degree, or cover slope-side by stone). Build ditches andsluices to drain rainwater.
+ Build revetment in coastal area in the Red River Delta, the North Centre, the South
Centre, and some estuaries (Da Nang, Cai Nha Trang, Cai Phan Rang)+ Dredge estuaries in the Red River Delta, the Centre, and the South to prevent fromsedimentation.
• Flood hazard maps will be useful in Viet nam
Natural disaster and flood are serious issue for a long time. They have caused bad effects
to the socio-economic development and environment in particular and sustainable
development of the country in general. The flood can occur in any place, at any time inthe catchments of our rivers and springs. To gather with the go BAL climate changing,
the situation of natural disasters, heavy rain consequence caused is heavier not only in ourcountry but also in many countries in the region and the world.

In order to effective flood control, the role of flood hazard maps to mitigate flood
damages in Viet Nam is an importanｔ issue. Only on basic, we can have proper andreasonable access to flood control in each region, river catchments and the way to collectnecessary information for following, accurate and timely forecasting the situation,
ensuring the implementation of the flood control measure effectively.
B) The allocation of roles in making flood hazard maps
• Organization should hold the main responsibility for making a fundamental map
such as an anticipated inundation area map- Department for Dike Management and Flood Control of Ministry of Agriculture andRural Development.
• Organization should hold the main responsibility for flood hazard maps,
including:- Locals government.
C) The “Action Plan “of making flood hazard maps
• Which area do you choose for target river basin area? Why?
The Kone River originates on the eastern slope of Truong Son Range, flows down to thesoutheastern direction, changes the river course to the east in Tay Son, and discharges to
the South China Sea in the north of Quy Nhon city. The catchments area of the Kone
River is 3,640 km2 and the river length is 160 km.
The existing discharge caring capacity of the Kone River in the downstream reaches is
estimated at about 500m3/s only at bankfull capacity.
Flood often hit the Binh Dinh province, causing loss of production and living condition.
In 1999 flood, 73 people were killed, 21 people were injured, 50,000 houses were affected,
schools and clinics were inundated, 12,230 ha of paddy field were damaged, and crops,
domestic animals and fertilizers were destroyed. Earth dams, gates, weirs and otherirrigation works were destroyed, and canals, embankments, bridges and roads weredamaged. Fishing boats sunk, shrimp ponds were destroyed, and stores of shrimps, fishand salt were swept away. Total loss is estimated at US$ 22million.
• What do you think is necessary to make flood hazard maps in the chose area?
Preparation of a flood hazard map is also an inevitable part of flood control management
plan. The role of flood hazard maps to mitigate flood damages in Kone river basin is an
important. Only on basic, we can have proper and reasonable access to flood control andthe way to collect necessary information for following, accurate and timely forecasting
the situation, ensuring the implementation of the flood control measure effectively.
• We have some basis data in order to establish flood hazard maps.
+ Topographic maps.

+ Land use maps.
+ Existing situation of urban central (their map and area in comparison with basin area)+ Data on existing irrigation systems and reservoirs.
+ Traffic systems.
+ Planned scheme of province on village, regrouping zones for flooding avoidance and+ Regrouping zones for rescue in urgent circumstances.
+ Data on hydrometeorology+ Rainfall data
+ Data on flooding traces.
• What seems a problem in making flood hazard maps in Viet Nam- Knowledgeable and experience flood hazard maps of people are very limited.
- Flood area very big, flood duration very long and so much effected by sea waterlevel, so that difficult to making flood hazard maps.
- Lack of data base and construction flood control.
- Limited budgets necessary for making flood hazard maps, training and transferring
flooding hazard maps.

D) My advice/ suggestion for making this training course more meaningful?
+ This course is very comprehensive and allows the participants to understand everythingthat is involved in producing flood hazard maps.
+ The organization of course is very and equipped with training materials, facilities andvery competent staffs.
+ The lecturers are very knowledgeable and experience flood hazard maps.
+ A survey to be carried out in the next five years to find out the progress of producing
hazard maps for each countries and identify problems in carrying out this activity.
Finally, on behalf of my works and my Department and I knew more about Japan,
japanese …
I would like to thanks to JICA and PWRI in allowing us to participate in this course
and making ours stay very fruitful and enjoyable.
I would like to thanks Mss Shibata for taking good care of us.

Figure 1: Located of Viet Nam

Figure 2: