A ‘break' of the ageing dam at tremor-prone Mullaperiyar would send out flood waters rising up to 9.4 metres just below the reservoir, 12.41 metres a little further down at Vallakkadavu and 8.5 metres at Idukki.
According to the first part of a report commissioned by the State Government, experts at IIT-Roorkee have said that as much as half of the century-old structure would give way within 12 minutes of the dam-break.
The ensuing flood situation would send the water levels rising to a height of 886 metres above sea level (asl) 50 metres downstream in the Periyar river; 854 metres asl at Vallakkadavu, the nearest human settlement; and 767.26 asl metres at the Idukki dam.
If the dam-break were to occur at the maximum water level of 136 ft at Mullaperiyar, it will cause the water level at the much larger Idukki reservoir situated 36 km away to rise by 20.85 metres.
Flood water from the Mullaperiyar dam would reach Vallakkadavu within 26 minutes; Vandiperiyar in 31 minutes; and Idukki dam within 128 minutes.
The State Government had asked the dam-break analysis to be conducted along a string of dams Mullaperiyar, Idukki, Cheruthoni, Kulamavu, Lower Periyar and Bhuthathankettu.
Contiguous regions bounded by the Periyar as it empties several hundred km through the midlands into the Arabian Sea too were brought under the focus of the study.
IIT-Roorkee has submitted only the first part of the detailed report pertaining to an area extending from Mullaperiyar to the Idukki reservoir. The second part of the report is expected to be submitted before May.
Usually a dam-break occurs when the dam overflows either due to flooding heavy rains, weak structural conditions, or due to earthquakes and landslips triggering strong waves which apply immense pressure against the dam wall.
A dam-break along the upstream of the main river could have a cascading impact on the structures downstream, the IIT report said.
Intensity and impact of a dam-break could be reduced if preventive steps are initiated, taking into consideration the quantity and force of the water expected to rush down, the density of population along the flood path and probable damage to life and property.
The report suggested that hydro-dynamics stimulation modules be set up for the purpose. This will help analyse the flood hydro graph, the expected water levels in various places and the likely flow path in downstream.
The dam-break analysis was carried out using one-dimensional hydro dynamic modelling. The HEC- RAS version 4.1.0 developed by the Federal Energy Regulatory Commission of the US was used for the purpose.
Breach guide lines provided by the Energy Regulatory Commission and the American Bureau of Reclamation were also consulted in the matter.