Flood vulnerability assessment of donstream area in Mono basin in Yoto district, south-eastern Togo
par Abravi Essenam KISSI
University of Lome - Master 2014
Trend analysis of the downstream part of the Mono River basin has been done also with 40 years of river discharge data from 1971 to 2010. Statistical properties of the annual and monthly flow series were tested and presented in "annexe 3". The results show positive skewness which means the data were normally distributed. According to the results, all the individual months show the largest CV representing similar variation during the study period. The annual average discharge for these 40 years is 114.985 m3/s. During the record period,
the maximum discharge occurred in the year 2001 with the total discharge of 262.408 m3/s approximately and a minimum discharge in the year 1984 with the total of around 19.09 m3/s.
On running the Mann-Kendall test on river discharge data, the Sen's slope shows an evidence of a positive trend in annual series. The rate of annual rainfall change is about 2.462m3/s/year. The result indicates that the null hypothesis was rejected for the annual discharge trend (p-value= 0.002) "annexe 4". Thus, statistically significant positive trend is found for annual river discharge over the time period.
On plotting the linear trend line for the 40 years river discharge data, the following results in "Figure 9 " were obtained.
Annual discharge plot
Y= 2.625x + 61.173
R2 = 0.2706
The "figure 10" shows the monthly discharge distribution of 40 years. It shows one peak in September (333.442 m3/s)
Monthly Average Discharge
In the non-parametric Mann-Kendall test, monthly trends of river flow for 40 years have been calculated for each month individually together with the Sen's magnitude of slope (Q). The Sen's slope reveals the trend of the series for 40 years for individual 12 months from January to December which are 2.653, 2.13, 2.233, 2.031, 2.289, 0.910, -1.354, -3.391, -4.697, -0.047, 2.295, and 3.433 respectively. While July, August, September and October show an evidence of negative trend, the others months show evidence of positive trend. The Null hypothesis was accepted for July, August and October months and rejected for the others months "annexe 4".
Annual Discharge Cumulative Deviation
The "figure 11" shows the cumulative deviation from mean that reveals a cyclic pattern of variations with alternating low and high discharge years. Three main phases of different lengths were detected (1971-1978, 1979-1997 and 1998-2010). The first phase shows negative anomalies that let to conclude a low discharge phase. It is followed by a period (1979-1997) characterized by variations with alternating low and high discharge years. The last period (1998-2010) shows a phase of high discharge years. This result explains river discharge variability over the study area during the period of record.
The application of the trend analysis reveals an overall upward trend in annual rainfall and river discharge. It is well known that rainfall is one of the major inputs into runoff processes while river discharge shows a composite response of the whole basin. The upward trend in the two variables may show a causal effect of rainfall on river discharge and an evidence of climate variability. The evidence of positive trend in the river discharge and the rainfall characteristics suggest that the study area may be exposed to either river flood or flash flood.
The result equally shows that even though there is an evidence of upward trend in annual rainfall and river discharge in the study area , the trend in the rainfall series is not significant compared to the one of the river discharge. This may be explained by these studies (Rossi 1996, Klassou 1996, Amoussou and al.2012, ) which show that the Mono River is under the influence of downstream and upstream rainfall and besides, under the effect of the Nangbéto dam put in service since 1988 for hydropower production reasons which confers to the river an artificial character at the downstream part. According to the interview carried out in the scope of this study, the increase of flood hazards in the area is not only due to change in precipitation patterns causing overflow of the river but also to man-made actions such as: the regular opening of Nangbeto dam at the upstream of the basin. Clearly, flood hazard in the study area is a natural phenomenon which was exacerbated by anthropogenic factors.
From the result, river flood peaks may occur in September. The increase of river discharge causing flood hazards in the study area calls up the need to describe past floods magnitudes in order to predict design floods for the targeted area. To this end, calculation of the return period as well as the probability of occurrence of past flood magnitude to estimate the likely values of discharges to expect in the river at various recurrence intervals based on the available historical record was carried out.