GUWAHATI, Oct 6 � Guwahatians� flash flood-related woes are not connected with a change in the rainfall pattern towards excess downpour. Rather, the rainfall rates as well as amount over the city have gone down over the years. Despite this, Guwahatians are made to reel under knee-deep, thigh-deep, waist-deep, breast-deep or neck-deep water at places, whenever it rains in a significant manner.
This is the observation made by Dr Diganta Goswami, an Associate Professor of the Civil Engineering Department of Assam Engineering College (AEC).
Dr Goswami told this correspondent that there has been a significant decrease in the city�s water retention capacity and the Bharalumukh pumping station has not been designed to pump out the stormwater from river Bharalu even in its present form.
Moreover, the present discharge capacity of the Bharalu is not at all adequate and the river requires re-sectioning, he said.
Dr Goswami said between 1931 and 1960, the maximum daily rainfall in Guwahati was recorded to be above 300 mm. In 1985, there was a record of 179.6 mm of maximum daily rainfall and it was the highest during the 1961-2014 period.
Against the above picture, the Chandmari headquarters of the Water Resources Department recorded a rainfall of only 84.6 mm and its Beltola Chariali River Research Station recorded a rainfall of only 88 mm on June 27, 2014. And these led to a record flash flood in Guwahati, he said.
He maintained that when the Brahmaputra water level is low and the sluice gate at Bharalu pumping station is open, minimum 1.6 metre of flooding is required on the streets near the Bhangagarh pump station to run the Bharalumukh pumps.
When the Brahmaputra water level is high and the sluice gates on the Bharalu are closed, pumping capacity of the pumps measuring 9,800 litres per second (LPS), is not at all adequate for a discharge of even 14,812.5 LPS, which is the maximum discharge of the present Bharalu river, said Dr Goswami.
With proper re-sectioning, the Bharalu can be made to discharge 72,610 LPS under gravity and the travelling time of the stormwater from the Bhangagarh point to Bharalumukh could also be reduced to one hour 13 minutes, when the Brahmaputra water level remains low. This will make the flood walls along the Bharalu and the intermediary pumps within Guwahati redundant, he said.
But when the Brahmaputra water level remains high, the sluice gates on the Bharalu must remain closed and an entirely new pumping station of adequate capacity and with intake wells, trash racks etc., will require to be installed, said Dr Goswami.
However, even with its re-sectioned channel, the peak discharge of the Bharalu will remain restricted within 6, 56,000 LPS, against the total volume of 11, 90,000 cubic metres of stormwater (excluding 15 per cent of infiltration) generated by a storm event with a rainfall intensity of 50 mm per hour for a period of 30 minutes and time of concentration as less than 30 minutes.
Therefore, storage of stormwater is a must and this could be done with the provision of dry sump in every household, conversion of filled up areas into semi-pervious areas and making stormwater management plan compulsory for every household.
The wetlands like the Borsola beel, Silsako beel should be developed to enable them retain stormwater respectively of the volumes of 2, 25,750 cubic metres and 5, 25,000 cubic metres etc. Besides, the Bonda sluice gate should be provided with a pumping station of the capacity of 50,000 LPS, among others, said Dr Goswami.