Criteris aplicats

An understanding of the overall risk context requires, according to the conceptual framework of this study (see section 2.4.2), firstly, an understanding of the most prevalent hazards on site. A hazard is one of the two components forming the risk context; it is defined as a “potentially damaging phenomenon” (see definition in section 2.4 based on UN-ISDR 2004; Turner et al. 2003). In the Coastal Zone, where the research area is located, drought, whirlwinds, heavy rainfall and most importantly tidal flooding and saline intrusion are the most prevalent water-related hazards (see section 3.2). Tidal flooding and saline intrusion will therefore be analysed more in-depth in the subsequent paragraphs. Both hazards stem from three main sides: firstly, lying at the low end of the large Mekong River, coastal flooding and saline intrusion are determined by the hydrological regimes upstream which influence water levels in the rivers and canals downstream. Secondly, being also located close to the sea, the research area is influenced by the tide of the South China Sea. And thirdly, local rainfall patterns shape the water regime and accordingly also tidal flooding and salinity (see sections 3.1.1 and 3.2 for more details). In the following section, these hazards will be presented against the background of both past events and future scenarios.

5.1.1 History of tidal flooding and saline intrusion

Tidal flooding occurs most commonly between September and December, i.e. in the late rainy and early dry season. At that time, high precipitation often coincides with a large inflow of water from upstream. High tides contribute to even higher water levels leading to a maximum at the beginning and middle of the lunar month. In late January, February and March of 2011 and 2012 some rather unusual peaks in water levels occurred, as Figure 5.2 depicts for Cau Quan station44 close to the research area. This led to unusual flood events in the dry season. The highest water levels were recorded in 2011. In that year, precipitation was exceptionally high and the water discharge from upstream was far above normal. In this rainy season the northern Mekong Delta provinces experienced the most severe flooding since the large flood in 2000 (see section 3.2.1). Data from the Hydro-Meteorological Centre (HMI) in Tra Vinh show a maximum water level of 184 cm – the highest value at Cau Quan station of the previous 30 years (HMI Tra Vinh 2012a, 2012b). Moreover, the HMI data reveal that there were more days of flooding45 than in previous years, i.e. 18 days in 2011 compared to, for instance, 14 days in 2010.

Saline intrusion typically occurs in the dry season between February and June when precipitation and water discharge from upstream are at their lowest. This allows saline sea water to intrude further inland than in the other seasons46. The highest salinity levels are commonly measured in April (HMI Tra Vinh 2012b; DONRE Tra Vinh 2012; Le Anh Tuan et al. 2007). Being also under the influence of the tidal regime, salinity reaches its peaks at the beginning and middle of the lunar months in this season. The spring tide can also be reinforced by the Gio Chuong, a south-western wind from the South China Sea, which occurs from December to January (see section 3.2.1). The HMI records of the average daily salinity levels for the years 2010-2012, for example, show that the salinity levels are higher than the defined threshold for irrigation water47 throughout most of the dry season. These data also reveal that variations in timing and intensity can be significant (see Figure 5.3). In 2011, for instance, it was

44

Cau Quan station in Tieu Can district is a gauging station close to the Can Chong gate which is one of the most relevant sluice gates for the research area.

45

The Hydro-Meteorological Centre defines flooding in Tra Vinh as water levels of more than 160 cm (AI-P-HMI- 0417).

46

Regular measurements of salinity levels were undertaken in these months only (February to May) until 2012. 47

Drinking water threshold: < 0.5 g/l (non-saline); irrigation water: 0,5-1,5 g/l (slightly saline); primary drainage water and groundwater: 1,5-7 g/l (moderately saline); secondary drainage and groundwater: 7-15 g/l (highly saline); very saline groundwater: 15-35 g/l (very highly saline); seawater: > 35 g/l (brine) (Omani 2005: 6).

reported that salinity was already at a high level in January (AI-P-HMI-041748)49. The salinity levels in 2012 were, in contrast, low until April (mainly due to high water availability from upstream) and revealed only two comparably small peaks until June. On average, the salinity levels were highest in 2010 (compared to the last six years). The average and maximum records were higher than in 2011 and 2012 for the vast majority of days. In March 2010, the highest salinity level of the last six years (11.8 g/l) was measured at Cau Quan station.

Figure 5.2: Daily maximum water levels at Cau Quan Station for the years 2010, 2011 and 201250 (Source:

author, based on data from HMI Tra Vinh 2012b)

Figure 5.3: Average daily salinity levels at Cau Quan Station between March and June for the years 2010, 2011

and 2012 and the defined threshold for irrigation water (Source: author, based on HMI Tra Vinh 2012a; Omani 2005: 6)

48 _{The interviews on province level are coded as followed: “AI”: Authority Interview; “P”: Province level; “XXX”:} name of the institution; “mmdd”: date of interview.

49

For that reason, the Hydro-Meteorological Centre started to take records from January onwards in 2012. 50 _{For the year 2012, only measurement data until May was available at the HMITra Vinh.}

0 20 40 60 80 100 120 140 160 180 200 in cm 2010 2011 2012 0 1 2 3 4 5 6 7 8 9 10 in g/l 2010 2011 2012

5.1.2 Future Scenarios

Against the background of climate change, tidal flooding and saline intrusion in the research area are expected to increase in intensity, frequency and variability. Being a low-lying coastal region, sea level rise is a major concern in the region (see Map 5.2). In Tra Cu district, a sea level rise of 65 cm51 is predicted to inundate nearly 13% of the entire district area (DONRE Tra Vinh 2012). Storm surges, which are anticipated to increase in intensity and frequency, can periodically raise the sea level even further (ibid.). Moreover, precipitation patterns will presumably change. In the medium emission scenario for 2100, Tra Vinh is predicted to experience in the dry season between December and February nearly 11.5% less rain (20% in the A1FI high emission scenario), whereas in the rainy season precipitation is expected to increase by 12.7% between June and August in the medium emission scenario (21.6% in A1FI scenario) (DONRE Tra Vinh 2012). The temperature in Tra Vinh province is predicted to increase in all seasons by a yearly average of 2.2°C in the B2 scenario (MONRE 2012). Furthermore, the variability of both precipitation and temperature is likely to increase.

In conclusion, higher temperatures, less precipitation in the dry season and sea level rise will most likely increase the danger of saline intrusion for the research area. The peril of tidal flooding is also expected to be reinforced due to sea level rise and increased precipitation in the rainy season. Both of these hazards will increase not only in intensity but also in variability (DONRE Tra Vinh 2012).