TR-34 : Development of dimensionless flood hydrographs from Machhu Dam-II failure using Dambrk model

Abstract

This report presents a methodology for the quick estimation of dam break flood wave and its characteristics such as peak flows, peak stages and their respective timings at the dam site and at specified locations downstream of Machhu dam-II, using the technique of dimensionless hydrographs of dam break flood wave developed based on different breach area criteria. These dam break flood waves were developed using U.S. National Weather Services DAMBRK model on the data of Machhu Dam-II which failed on11th August 1979 in Gujarat State. Preliminary investigations made using DAMBRK model showed insignificant impact on the dam break flood hydrograph characteristics relevant to flood warning due to variation in shape of the breach and due to the consideration of inflow hydrograph. Therefore, the dimensionless hydrographs were developed based on the consideration of trapezoidal breach with area varying from 50% to 250% with 100% area corresponding to the actual area of breach observed at Machhu Dam-II. The dimensionless hydrographs relate the time of hydrograph non-dimensionalised with reference to the time to peak flow and discharge non-dimensionalised with reference to the peak discharge. Relationships have been established for area of breach vs. the peak flow and the peak flow vs. the time to peak flow at the dam site. Similarly non-dimensional hydrographs were developed at the specific sites downstream of the dam, besides the relationship between peak flow of upstream site and the next downstream site, time to peak flow at upstream site and next downstream site and the peak flow and peak stage at the respective sites. Using these dimensionless hydrographs and relationships one can quickly estimate the peak flow and peak stage at specific sites knowing only the breach area at the time of disaster without the need for using the DAMBRK model. The usefulness of this approach has been demonstrated by developing the dam break flood wave hydrographs for a breach area which was not used for the development of dimensionless hydrographs and other relationships.