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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.
DETERMINATION CRITERIA OF FLOOD LEVEL ALARM
CLASSIFICATION
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.
Classification criteria of flood level alarm in Vietnam
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).