Flood Generated from Tana Lake Basin

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Addis Ababa University


The computation of runoff usually consists of applying a stage- discharge relation to daily gage heights to determine mean daily discharges. However, for this particular study a hydrologic model, HEC-HMS is applied to compute the hourly base runoff generated from Lake Tana basin. The basin was sub divided in to eleven sub basins with a total area of 15046.32 square kilometers including the area of the lake. The Basin has many streams of which the seven have daily gauged data. The gauged data from six streams are used for parameter estimation for the ungauged sub catchments. Since the study focuses on the computation of flood particularly peak flood, these daily discharge data are hourly distributed using the formula QT = M*T ½.Where T is time in hours and M is a coefficient of which value is determined from known discharges and respective time or return period. The Probable Maximum Flood (PMF) entering Lake Tana from the sub basins would be determined based on the convolution of the Unit Hydrograph (UH) with the Probable Maximum Precipitation (PMP) using the storm frequency meteorological model built in HEC-HMS. The Bahir Dar 24 hours maximum annual rain fall depth values were the basis for the evaluation of 24 hours PMF values for the return period 25-years, 50-years, 100- years.The direct runoff contributed by each sub-basin is computed for return period of 10-, 25-, 50- and 100-years.Finally the calculated direct runoffs are combined to know the magnitude of peak runoff coming into Tana Lake. Flood generated from Lake Tana basin is determined in such a way that adding the rivers base flow to the direct runoff and summing the floods from sub basins assuming all sub basins are active at a time. The highest combined incoming flood from all sub catchments to the Lake are 4,053 m3/sec ,4,241 m3/sec,5,040.0 m3/sec and 25372 m3/sec. While the corresponding total inflow are 235,474 m3,250,687m3,235,474.2 m3,292,258 m3,643611m3 for v the respective return periods. The water surface elevation at the time of this highest incoming flood ranges from 1787.9 m a.m.s.l for 10 – years return period to 1789 m a.m.s.l for 100 –years return period. The probable maximum flood (PMF) computed using the model is 56,526m3/s(or 9,522,011 m3) and the corresponding lake water surface elevation is 1789.3m, too. This shows that the Lake level is raised by more than two meters. Hence, low laying area of Bahirdar and Fogera and Dembia flood plain will be affected.



Tana; Lake Basin