Gash river flood risks modeling

Abstract

Kassala Town and the Gash Scheme are the gifts and victims of the Gash River waters. It is paradoxical that while the Gash River, which flow about 80 to 100 days per year, provides prosperity and survival it is also the main source of disasters to Kassala Town and its neighboring areas including its delta. Kassala Town and its neighboring areas are subjected to flooding problems every four to five years from the Gash River. The ground water which is used during the remaining part of the year is not fully known. The objective is to solve the problem of Kassala Town and its neighboring areas from the Gash River flooding on one hand and appraise the surface and ground water potential on the other. An Empirical Correlation Model applied at El Gira, the most upstream gauging station with three other downstream stations including K.1.5 Kassala indicated that Kassala is continuously subjected to high floods risks. Application of the well known Gumbel Distribution Model supplemented with digitized map of the Gash River catchment area gave the same degree of risk. The four to five years flood return period was found to be about 600 m3/sec with a probability of occurance of 25% and 20% at K.1.5 Kassala Station respectively and a corresponding discharge at El Gira of about 750m3/sec. Only 5 % of the surface water passing Kassala Town is consumed irrigating an area of 20 000 feddans per year. It was found that the agricultural area can be reclaimed through a buy pass upstream Kassala Town, making Kassala safe on one hand and using the excess water in cultivation on the other. Using HEC-HMS Model at the confluence of Khor Abu Alaga with the Gash River a hydrograph was obtained. The hydrograph indicated that the rainfall loss has a maximum depth of 80 mm and a minimum depth of 10 mm while the rainfall maximum depth is 145 mm with a minimum less than 60 mm. Further interpretation of the hydrograph indicated that the base flow has a maximum of 400m3/sec. It also revealed that the observed surface flow has a maximum of 900m3/sec. The outflow reading has a maximum of 2900 m3/sec and then it merges to be equal to the observed flow. These results indicate that the flood of 2007 was significantly high. They also indicate that the ground water as well as the surface water are basically fed from the Gash River. Hence the HEC-HMS Model besides indicating ground water base verified the results of the surface flow upstream K.1.5 Kassala gauging station obtained by both the Empirical and Gumbel Distribution Models. The observed surface flow being 900m3/sec is logical and consistent with the result obtained at K.1.5 Kassala which was found to be a maximum of 750m3/sec, which is less than that upstream K.1.5 Kassala due to the flood of Abu Alaga at the application point. Using G.I.S. available facilities contour maps were plotted, and ground water potential being both quantitatively and qualitatively excellent were found to be 3897.6 Mm3. The study recommended cooperation between Sudan, Eritrea and Ethiopia in the management of the Gash River Basin to avoid floods hazards. It also recommended establishment of an early warning system to reduce the risk hazards suffered by Kassala and its neighboring areas