Flood Generated From Tana Lake Basin

The computation of runoff usually consists of applying a stage- discharge relation to dailyrngage heights to determine mean daily discharges. However, for this particular study arnhydrologic model, HEC-HMS is applied to compute the hourly base runoff generated fromrnLake Tana basin. The basin was sub divided in to eleven sub basins with a total area ofrn15046.32 square kilometers including the area of the lake.rnThe Basin has many streams of which the seven have daily gauged data. The gauged datarnfrom six streams are used for parameter estimation for the ungauged sub catchments. Sincernthe study focuses on the computation of flood particularly peak flood, these daily dischargerndata are hourly distributed using the formula QT = M*T ½.Where T is time in hours and M is arncoefficient of which value is determined from known discharges and respective time orrnreturn period.rnThe Probable Maximum Flood (PMF) entering Lake Tana from the sub basins would berndetermined based on the convolution of the Unit Hydrograph (UH) with the ProbablernMaximum Precipitation (PMP) using the storm frequency meteorological model built inrnHEC-HMS. The Bahir Dar 24 hours maximum annual rain fall depth values were the basisrnfor the evaluation of 24 hours PMF values for the return period 25-years, 50-years, 100-rnyears.The direct runoff contributed by each sub-basin is computed for return period of 10-,rn25-, 50- and 100-years.Finally the calculated direct runoffs are combined to know thernmagnitude of peak runoff coming into Tana Lake.rnFlood generated from Lake Tana basin is determined in such a way that adding the riversrnbase flow to the direct runoff and summing the floods from sub basins assuming all subrnbasins are active at a time. The highest combined incoming flood from all sub catchments tornthe Lake are 4,053 m3/sec ,4,241 m3/sec,5,040.0 m3/sec and 25372 m3/sec. While therncorresponding total inflow are 235,474 m3,250,687m3,235,474.2 m3,292,258 m3,643611m3 forrnvrnthe respective return periods. The water surface elevation at the time of this highestrnincoming flood ranges from 1787.9 m a.m.s.l for 10 – years return period to 1789 m a.m.s.l forrn100 –years return period.rnThe probable maximum flood (PMF) computed using the model is 56,526m3/s(or 9,522,011rnm3) and the corresponding lake water surface elevation is 1789.3m, too. This shows that thernLake level is raised by more than two meters. Hence, low laying area of Bahirdar and Fogerarnand Dembia flood plain will be affected.

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