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.
  1. METHODOLOGY TO DETERMINE AND ADJUST FLOOD WARNING LEVELS

    1. 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