TR-72 : Flood plain zoning for downstream area of Macchu dam-II

Abstract

This report presents a typical study for flood plain zoning of downstream area of Machhu Dam-II in Gujarat State. This study uses the National Weather Services DAMBRK Model developed by Fread (1984)to compute the maximum flood elevation due to dam break flood of Machhu Dam-II downstream of the dam. The routing option of. DAMBRK model using dynamic wave theory is used to compute the maximum flood elevations due to floods with different peaks, which includes 100, 200, 500 and 1000 years return period floods, 1979 flood, design flood and dam break flood from Machhu Dam-II and floods with peak 8765.3, 9734.9, 10956.3 and 11879.7 cumecs, representing earlier estimates of 100, 200, 500 and 1000 year floods. The maximum flood elevations due to floods as mentioned above are computed at about half km interval along the reach of 39.65 kms downstream of the dam and the same have been marked on contour map on both the sides of river reach at corresponding locations. These maximum flood elevation marks at different downstream location have been joined by straight lines indicating the limits of the inundated areas due to the corresponding flood. The inundated areas within these limits have also been measured to assess the total inundated area due to the corresponding flood downstream of the dam. It is seen that dam break flood affects the maximum area (160.40 sq. km.) along the reach and 100 years unrevised flood affects the minimum area (37.287 sq. km.) along the reach. The quantitative results are however dependent upon accuracy of available information regarding river cross sections and contours in the flood plain, and assumptions made for routing of flood wave.