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Real-time Flash Flood Forecasting Method Case Study Medjerda River

Congress: 2015
Author(s): Sahar ABIDI (Beja)

Keyword(s): Sub-theme 11: Key vulnerabilities and security risks,
AbstractIntroduction When it rains in a catchment area, all the difficulty lies in the definition of rainwater division between its various possible destinations (evaporation, infiltration or streaming...) and in the definition of the concerned physical processes to realize these tasks. The Flash flood are definite by Gaume (2002) as being suddenly appear, often not easily foreseeable, fast climb time and had relatively important specific discharge. These floods are usually associated with intense rainfall events and occur in basins of moderate size. The tributary flows and dams' releases aggravate the situation. This is the case of the plain of Bou Salem which became a flood stage. It is crossed by the river Medjerda receiving water from Jendouba station, tributaries Mellègue, Tessa and Bouheurtma. Plain Bou Salem has experienced devastating floods that have caused serious human and material damage. Materiels & Methods Forecasting methods The method used in this study for flow forecasting is based on the coefficients resulting from the reconstruction of flood hydrographs with propagation models. These coefficients will be taken, for each method and for each period of prediction, from a previous flood haven same degree of humidity and same season like the flood to be predicted. Prediction delay varies from 2 to 8 hours with a pitch of 2 hours. Reconstitution methods The reconstitution is an operation consisted to calculate the simulation coefficients of each flood, by each method and for each calculation period. In this study, we will use the results of reconstitution of 26 flood of Bou Salem station in Medjerda River (Abidi, 2011). Two models were used to predict the flash flood of Bou Salem: - Muskingum model, - Regression model. Results & discussion Study area Medjerda is the most important river of Tunisia by the size of its watershed (23,500 km2) and the importance of annual contributions which represent on average a third of the surface water resources of the country (915 million m3 in average). Plain Bou Salem is located in northern Tunisia and high valley of Medjerda. It is limited by the hydrological station of Jendouba (upstream) and Bou Salem (downstream). The plain is crossed from west to east by the Medjerda River over a length of 10 km and a catchment area of 6808 km2. Historic flooding in Bou Salem We recovered thirty flood of the station Bou Salem during the period 1973-2013. These floods occur mainly during the spring and winter seasons. From these floods, we noted the existence of five major floods where the runoff volume for a few days can reach the average annual volume. The following table summarizes some characteristics of main floods: The flood of March 1973 is characterized by a single high peak inflow and considerable rainfall in the entire basin during 6 days. The volume spilled from Mellegue dam, Jendouba basin and Tessa tributary reached 155, 259 and 75 million m3 respectively. The flood of Mai 2000 had a high speed entering with a single point, and localized rainfall. Precipitation focused on the sub-basins Tess and Mellegue. The volume spilled reached from Mellegue dam 93 106 million m3 and Jendouba basin 56 million m3. The flood of January 2003 had unique inflow and sudden rainfall peak. The dam releases Mellegue (82 million m3) and Bouheurtma (36 million m3) and contribution of Jendouba basin (108 million m3) and Tessa tributary (18 million m3) participated in the aggravation of this flood. The flood of November 2004 had a flat unique inflow. The precipitation was located at Mellegue dam with moderate intensities for 4 days. The dam releases Mellegue (3.6 million m3) and Bouheurtma (12 million m3) and contribution of Tessa tributary (5 million m3) and Jendouba basin (42 million m3) caused this flood. The flood of April 2009 caused by sudden and significant rainfall, releases from the dams of Mellegue (14 million m3) and Bouheurtma (69 million m3) and contribution of Tessa (15 million m3 and Jendouba basin (54 million m3). The flood of February 2012 provoked by sudden and significant rainfall, releases from Bouheurtma dams (68 million m3) and contribution of Jendouba watershed (196 million m3). Choice of flash floods in Bou Salem Several definitions of flash floods have been found in the literature. According to Estupina 2004, we selec three flash floods: the one of January 2003, April 2009 and February 2012. In the next chapter, we will precede to the prediction of these three floods with the models of Muskingum and Regression. Based on the criteria sited above, the propagation models gave satisfactory results. We can conclude that the satisfaction of the three numerical criteria at once is not possible to predict the flood. The lower value of Nash coefficient and relative peak error were furnished by Regression model. The error between calculated and observed hydrographs was lower in the reconstitution by Regression model. Concerning the reconstitution of Bou Salem hydrograph, the best results were donated in using the Jendouba flow upstream. . 1. Cunge J. A. (1969) On the subject of a flood propagation computation method (Muskingum Method), J. Hydraulic Res. 2. Habaieb H. (1992) Numerical Comparison of models of flood forecast, application to watershed Belgian, French and Tunisian. Doctoral thesis at the INP Toulouse - France. 3. Bentura P. (1996) Development of a propagation method of flood (Method "Delay-storage"). Thesis in environmental science and technology, University of Paris XII Val de Marne. 4. Gaume E. (2002) Elements of analysis on flash floods, Paris. Brief submitted for the grant of the title of Doctor of the National School of the Rural Engineering of waters and forests and Doctor of the National Institute of Scientific Research. 5. Estupina B.V. (2004) toward a hydrological modeling adapted to the operational forecasting of flash floods, Application to small watersheds of the south of France. Thesis in Science of the Universe, the environment and space, National Polytechnic Institute of Toulouse. 6. Abidi S. (2011) Test of reconstitution and flood-forecasting to the main station of the Medjerda to upstream of the dam Sidi Salem (Ghardimaou - Jendouba - Bou Salem). Masters in research in hydrology and hydrogeology, Higher School of the engineers of the Rural Equipment of Medjez El Bab, Tunisia.
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