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Flood vulnerability assessment of donstream area in Mono basin in Yoto district, south-eastern Togo

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par Abravi Essenam KISSI
University of Lome - Master 2014
  

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4.2. Determinants of Communities' Vulnerability to Floods

4.2.1. Flood Frequency and Magnitude Analysis

The maximum instantaneous flow of 736.70 m3/s was recorded at Athieme during the 1999/ 2000 hydrological year while the lowest flood flow of 69.16m3/s was recorded in 1983/1984 hydrological year. The 40-years mean instantaneous maximum flood flow is 372.34m3/s with a CV of 40.6% and a standard variate of 182.757m3/s "table 5". The coefficient of variation was applied to measure the consistency and the steepness for the frequency curves in the river flow data. The CV value obtained indicated that the distribution of flood flows was not highly variable.

Table 5: Annual Maximum flow basic statistics

Basic statistics

Values

Mean

372.34

Maximum

736.70

Minimum

69.16

Std. Deviation

182.757

Coef. of Variation (CV)

0.491

Skewness

0.406

The return period and the probability of occurrence for each observation of the Mono River have been computed, using Weibull's formula, for the period 1971- 2010 which generally starts to peak in the month of July with the maximum in the month of September. according to flood hydrology data of Athieme station.

The Mono River discharge at the downstream reveals that the study area has been affected 22 times by low intensity flood with return periods of 1 or 2 years with high probability of occurrence. The low intensity is ranged between (69.16-372.33m3/s). The study area experienced nine times moderate intensity flood with return period of 2, 3 or 4 years with probability of occurrence less than 50% and magnitude between (372.34-549.64 m3/s). The study area was challenged with high flood event nine times ranging between (549.65-736.70 m3/s) "figure 18" and "table 6".

The latest more severe flood for the downstream part of the Mono basin is the one that occurred in 2010. Its return period is 5 years and the probability of occurrence of the 2010 flood (as a same magnitude) would be once in five years (Probability=0.22) "annexe 5". During the period, the recurrence interval of high flood based on the 2010 flood magnitude has ranged from 5 to 41 years. There are eight recurrence intervals covering a total period of 40 years between the first and the last occurrence of high flood events.

Low hazard

High Hazard

Moderate hazard

Discharge

Frequency

Magnitude of flood

549.65-736.70

9

High

372.34-549.64

9

Moderate

69.16-372.33

22

Low

Figure 12:Flood Frequency and Flood Magnitude

4.2.2 Flood Duration and Flood Water Level Assessment

The water level as well as flood duration were different for the targeted villages "table 7". The result reveals that the higher the flood water level, the higher the flood duration.

It was noticed that villages such as Kpodji, Tchakponou and Logokpo have the highest flood level and highest flood duration although they are the most distant of the Mono River. This can be explained by the fact that these villages are surrounded by Mono River's sub-branches. They are not directly inundated by the Mono river itself but rather by the Mono's sub-branches. Then, when water comes from all the sub-branches, the total areas is highly inundated. In addition, the area is made up by heavy soil which can decrease the flood water infiltration capacity and increase the duration of flood water in the area.What about the soil types in the affected villages?????

Table 6: Depths of flood water (2010) as revealed by marks on building walls and average flood duration (2010) from household

Flooded areas

Depth of flood marks on walls (2010)

Proximity to the river body (m)

Flood duration (days) (2010)

Djrekpon

63- 99cm

470.35

58

Batoe

50-118 cm

303.79

82

Logokpo

70-100cm

909.28

88

Tchakponou

50-106cm

626.13

80

Kpodji

57-170 cm

1874.22

95

Tofacope

30-45cm

112.54

41

Atikpatafo

65- 70 cm

165.55

71

Mawussou

45-65cm

435.55

51

In addition, all the eight surveyed villages lie in the low altitude level of the Yoto district comprised between (12-58m) which makes them to be highly exposed to flood hazards "Map 2".

Map 2: Map of Yoto district showing the surveyed villages in low altitude

4.2.3. Elements at Risks

The study focuses on two main elements at risks: households and their agriculture croplands. The total population from the surveyed sample is about 2124 composed of children, young, elderly, and adult. 42.74% of the total population are children, 4.61% are elderly and 17.14% are women. As it is shown in various studies and confirmed by the majority of the respondents 72.36%, women, children and elderly are considered as the most exposed to flood due to their fragility. The little high proportion of children under 15 years in the study area may increase the communities' vulnerability to flood disasters.

Based on information collected through the simple random survey, it was found that those surveyed households were having agriculture croplands of about 506 ha of which the majority is very closer to the river body (less or equal to 200m). The crop production activity in the study area depends on rainfall and practices which is still traditional with rudimentary tools. Farmers are faced with problems of storage and preservation of harvested crops. Thus, when flood came, before and even after harvesting period, the majority of crops are destroyed.

Photo 1: Oil palm farm under water since the 2010 flood, Photograph taken during field work

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