Criteria based on correlation of flood
warning in the same river basin or river system
- For flood affected areas, the flood alarm levels of the downstream
hydrological stations is lower than those at the upstream stations;
- For areas strongly affected by coastal tides and surges, flood alarms
at downstream stations are often greater than upstream stations.
- Flood alarm levels at hydrological stations with hydrological and
hydraulic interconnection the alarm levels are need to be considered and
adjusted to ensure synchronized and systematic features.
Criteria based on regional
characteristics
-
For hydrological stations in upstream river of large river systems such
as Da, Thao, Lo, Ma, Ca and the Central Highlands region, flood
intensity and flood water derivation are paid attention. Intervals
between flood alarm level are defined to ensure suitable mantainance
duration to deploy the necessary response measures.
- For hydrological stations in detals of the Northern area, Thanh Hoa
and Nghe An provinceswith protection dykes, the following
characteristics should be considered: (1) designed water level to
prevent floods; (2) Height of dyke surface; (3) Dyke quality.
- For hydrological stations in river basins in the Central coastal
region, attention should be paid to the characteristics of flood
derivation and the level of flood impacts on people’s livelihoods and
socio-economy of the region.
- For floodplains in the Mekong Delta, the magnitude of the flood and
the maintenance duration of the flood at a given alert and flood damage
are important features to consider the feasibility of flood alarm
levels.
- For tidal stations, the considered characteristics are flood peak and
tidal peak that frequently impact on protection dykes causing damages in
agriculture, transportation, livelihoods and environment.
Criteria based on local
characteristics of hydrological station
- Height of riverbank and flood level;
- Levels of roads, national highways, intense population areas and
characteristics of flooding such as water depth and flooded duration
- Level of key locations in downtowns where the hydrological station
located, flood levels, flooding risk and flood damage.
- For the area with water works such as dykes and drainage culverts,
alarm level I is assigned for water level starting impact on dyke
footprint. Alarm level II is assigned for water level causing presure on
dyke body. Water level at two of thirds of sideslope alarm level III.
Criteria based on flood prevention
strategy
The national strategy on natural disaster mitigation until 2020 is
approved by the Vietnamese Government, which sets out common tasks and
solutions for each region. The strategy will be considered in the
process of determining flood water levels corresponding to flood alarm
levels. In addition, each province may have local strategy based on
critical features, especially in the Mekong Delta.
Local tradition and experience in
flood prevention
The criteria of experience and recommendations of local flood prevention
agencies is an important basis for assessing the reasonable level of
flood alarming levels at local hydrological.
The alarm level is then rounded based on fluctuation magnitude of water
level in multiple years and level intervals between alarm levels.
METHODOLOGY TO DETERMINE AND ADJUST FLOOD WARNING
LEVELS
Major steps
Step 1: Studying natural,
socio-economic and flood characteristics of major river systems;
Step 2: Preliminary assessment of
flood alarming levels based on the basis characteristic of topographic,
flood, field survey and impact level of floods to socio-economic
activities;
Step 3: Studying and developing a system of criteria based on the views
on flood alarm classification, technical bases and practices of flood
alarm levels;
Step 4: Assessing the resonable level of flood alarming levels, based on
basic criteria;
Step 5: Analyzing causes of reasonableness of flood alarm levels to
propose or adjust;
Step 6: Summary and analysis of proposals from research and survey;
Step 7: Evaluate options on the basis of criteria;
Step 8: Proposing or adjusting flood alarm level.
Description of flood alarm level determination
As mentioned in Section 2.2, the the flooding risk analysis and flood
fluctuation assessment of local stations are taken. The analyses are
then combined with large scale topographic maps including information of
population density, economic, cultural, defense facilities, etc.. Then,
flood alarm levels are preliminary determined as following (Step 1-Step
3):
Alarm Level I: When a flood risk exists, that mean flooding occurs in
certain residential areas or farming areas. Local authorities should
concern, support and guide the implementation of preventive and adaptive
measures.
Alarm Level II: When floods have spread to many local areas, likely
cause property damage, affecting normal economic activities, especially
affecting agricultural production, starting affect the lives of people
in protected areas.
Alarm Level III: When the flood situation has become serious,
threatening flood prevention structures or may cause structures
failures. For examples, in cases of nearly full reservoirs,
inefficiently drainage systems, high water level in river and so on.
These situations may strongly impact on social-economic activities and
cause damages in property and lives. .
After preliminary determining , the alarm levels are verified using
flood intensity to define the flooding classes of each alarm level (Step
4). The criteria used to verify the flood alarms can be average and
maximun flood derivation, flooding duration at each flood class,
frequency of the most severe flood, etc. The criteria of structures such
as protection dykes and transportation also are considered to validate
the reasonality of alarm levels.
The field survey is then implemented to check the proposed alarm levels
in historical flood events (Step 6). The impact assessment of alarm
level on local livelihoods is done in this step too. The adjustment will
be developed after integrated analysis of current alarm levels with
impact assessement.
From the proposed alarm levels, the authorities will select suitable
points that locate in high areas to timely implement guidances of
evacuation and migration during flood rising period between high flood
levels.
APPLICATION FOR REGIONS IN VIET
NAM
From the methodology developed in Section 2, the determination and
adjustment of flood alarm level are implemented for major river basins
located in regions of Vietnam. In general, there are 121 hydrological
stations determined in Decision 632 (2010), in which six stations are
proposed to remove from alarm system; flood alarm level at 85 stations
will be the same; flood alarm level at 30 stations is proposed to
change. In each region, flood alarms are proposed to assign at many
stations too.
In the North, there are 38 stations in the region defined in Decision
632 (2010); three of them are remove because of reservoir effects, 32
stations are remained and three stations in mountainous areas are
assigned with lower flood alarm level. The study also proposes to add 54
stations to the alarm system.
In Central region, 37 stations are in alarm system (Decision 632, 2010).
A station (Binh Tuong station) located in Kon River basin is removed
from the alarm system. Six stations are proposed to increase flood
alalrm level and one station proposed for decrease. The study suggests
to include 39 stations to the alarm system.
For the Highland region, in sixteen stations currently involved in alarm
system (Decision 632, 2010), there are three stations supposed for
higher alarm level and fours stations for lower alarm level.
In 30 stations in alarm system of the South, flood alarm water level at
six stations is supposed to increase while that at seven stations will
be decreased. The more 41 stations in this regions will be added to the
alarm system. The flood alarm at critical stations in regions are
summarized in Table 3.
Table 3. Flood alarm level at critical stations in regions of Vietnam