Data Availability Statement: Author elects to not share data

INTRODUCTION

Determining the flood alarm level in rivers is considered a very effective non-structural measure for flood prevention and damage mitigation. Therefore, flood warning levels need to be defined to ensure the effectiveness of services such as providing information on flood and inundation, warning and forecasting flood and inundation as well as efficicently support to guiding and preventing flood and inundation.
In the United States, the magnitude of floods in the United States is described by the concept of recurrence interval. The remarkable floods are 5-year, 50-year and 100-year return periods (J.A. Moreland, 2001).
In the Unite Kingdom, the Ministry of the Environment decides three levels of flood warning, including: (1) Flood Watch: flooding is very likely, it is necessary to prepare for floods; (2) Flood Warning:  houses, places of production, trade and major roads will be flooded; (3) Severe Flood Warning: Large-scale flooding can occur, causing life-threatening and major material damage (F. Farquharson, 1999).
In Australia, flood alert levels are defined in three levels depends on the flooding area, as follows: Major Flooding; Moderate Flooding; Minor flooding (Attorney-General’s Department, 2009).
For lowland rivers in Japan, the flood alert level is divided into 3 levels, of which the evacuation level is divided into 2 levels, as follows: Flood levels start to be warned (Danger level or warning level); Levels for starting flood protection must be implemented (Level for starting flood defense); Evacuation level: Evacuation Advice (Evacuation Advice); Evacuation Order (Kazuki Mori, 2000) is required.
For China, in the area protected by dykes, floods are classified into three levels (Lu Hongjun, Chen Yinchuan, 1999): moderate flood grade; serious flood grade; most serious flood grade.
Flood control standards may vary depending on the natural, socio-economic conditions of each country. Generally, flood alarm are classified into 3 levels, depending on the natural characteristics and conventional experience of each country. Corresponding water levels of flood alarm are determined on the basis of the magnitude of the flood and their impact on livelihoods, economic losses and environmental impacts.
In Vietnam, scientific and pratical fundaments and the criteria delineating flood alarm levels have been established well. The basic criteria are based on rainfall-runoff characteristics in watersheds and hydrological regime in rivers, the scale of floods, the frequency of occurrence, the duration maintaining at a critical flood level corresponding to the alarm levels, the time of information transfer from flood level alarm to communities. The criteria is to ensure the continuity and similarity of flood alarm levels in a river system as well as in a watershed. (Tran Thuc et al., 2008, 2010).
The suitable criteria classificating flood alarm levels are based on knowledge of hydrometeorology, topography, environment, economy and technology compositing with field survey and practical experience in applying flood level alarm. To update and improve the present flood level alarm system, a synthesis method that integrates statistics, modelling, field survey and expert-opinion is needed. In which, statistic and modelling tools are used to proposed primary levels. The fieldtrip and expert-opinion are then used to validate the flood level alarm.
  1. DETERMINATION CRITERIA OF FLOOD LEVEL ALARM CLASSIFICATION

    1. Flood type classification

Water levels is determined as follows:
- Warning level I: Flood water level of low floods occurring annually, affecting agricultural and fishery production that is needed to provide guidelines to prevent flooding and environmental pollution and to enhance information transfering process, material and labor preparing to serve flood prevention and opposition. The corresponding flood frequencies are from 70% to 90% (Decision No.18/2008/QD-BTNMT).
- Warning level II: Flood water level of moderate floods (frequency from 30% to 70%) (Decision No.18/2008/QD-BTNMT), having significant impacts on social and socio-economic activities. Information transfering, hydrometeorologcial warning and forecasting processes are maintained to strengthen patrolling, reinforcing, handling incidents regarding embankment; damaged assets, supplies, equipments, etc.
- Warning level III: Flood water level of high floods (frequency from 10% to 30%) (Decision No.18/2008/QD-BTNMT), starting cause people loss and significant property damages, requiring pointed attention in guiding natural disaster prevention and preparing conditions for evacuation and relief in case of particularly extreme floods or incidents such as flood diversion, flood discharge, dyke overflow, etc.
  1. Classification criteria of flood level alarm in Vietnam

    1. Flood peak criteria

Water level is one of important characteristics to quantify and identify the water level corresponding ​​to the flood alarm levels.
(a) For hydrological stations located in the mountainous area of North and North Central regions without dyke system, high flood fluctuation magnitude, low population density, only extreme floods impact significantly on livelihood and socio-economic activities, therefore: (1) Alarm level I is assigned to small and moderate floods (50% to 90% frequencies); (2) Alarm level II is assigned to moderate and high floods (30% to 70% frequencies); (3) Alarm level III is assigned to floods with frequency smaller than 30%.
(b) For area with dyke protection in the Red River and Thai Binh River deltas and high population density, alarm levels are determined as follows: (1) Alarming level I: small floods; (2) Alarm level II: moderate flood; (3) Alarm level III: 20%-30% frequency floods .
(c) For the Ma and Ca River deltas, where are protected by dykes with faster flood climbing and shorterwater level maintenance time compared to Red River deltas, flood control standards is higher than that of Red River Delta at Alarm level III: floods with frequency of 25-10%.
(d) For the area without dyke protection in the Central Coast region, due to rapid flooding, often cause inundation on a large scale, large impact of floods on livelihood and economic activities, so the flood frequency corresponding to the flood alarm levels is determined lower than in other regions of the country: (1) Alarm level I: very small flood - small flood; (2) Alarm level II: small flood - moderate flood; (3) Alarm level III: flood frequency is about 45%-35%.
(e) For the Central Highlands region, without dyke protection, since fast flood climbing and distribution of population and economy activities mainly along river valleys, the alert level III in this region, thus, is at 35% to 25% frequency .
(f) For flooded areas in the Mekong Delta region, floods often cause widespread flooding but due to slow flood clibming, responding activities can be takeninitiatively.The corresponding flood frequency to the flood alarming levels is assigned similar to that of the hydrological stations in the Red and Thai Binh river deltas. Alarm level III of flood frequency is 25%.
(h) For tidal-flooded areas of the Red - Thai Binh River Delta - and Mekong River Delta, since insignificant amplitude of water level fluctuationthat causes flooding at medium depth, flood level alarm III at frequency of 25-10%.
(k) For the tidal-flooded areas of the Red River Delta - Thai Binh, Sai Gon- Dong Nai River deltas and Mekong Delta, since insignificant water level fluctuation still causes difficulties in social-economic activities and environmental protection leading damages of agriculture and aquaculture, flood prevention standards in the area are higher than other areas. The alarm level III at peak water level of 10% frequency (or lower) or around highest observed peak water level.

Criteria based on other flood characteristics

The other flood characteristics are considered to determine alarm criteria of flood level are:
- Flood derivation: the change of flood level in time. The flood characteristic closely related to flood damage in areas without dykes and embankments.Flood derivation can be used to estimate the reasonable time intervals of alarm levels.
- Time to maintain flood water level: to show the scale and duration of flood impacts.
+ For rivers in The Red-Thai Binh River system, the mantainance period of alarm level II is about 10 to 20 days, medium in 15 days (2 weeks). For alarm level III is 1-2 days.
+ For river system in northern and northern central (Da, Thao, Lo, Ca and Ma rivers), the mantainance time for alarm level I is 5-7 days, for alarm II is 2-3 days and for alarm III is about ½ days (12 hours) to 1-2 days.
+ For rivers in the Central region, floods rise faster and goes down faster than those in the Northern Rivers. However, the number of floods in the year is more than rivers in the North, so the reasonable mantainance time is similar to that of Northern mountainous and North Central areas.
+ For flooded areas in the Mekong Delta, the mantainance time for flood alarm level I is from 1 to 2 months, for alarm level II is from 20 days to 1 month, and for alarm level III is about 7 to 15 days.

Criteria based on flooding impacts

The levels of flood alarm also depend on damages of flooding on the area. The risk of flood on areas with different dyke systems are different.
(a) Area with dyke and flood retaining walls:
For the areas, the impacts of flood are shown through protection dyke failures. The flood alert level is considered in the correlation among flood levels, maintenance duration and the number of dyke failures to make appropriate adjustments (Table 1).