KARACHI: The NED University of Engineering and Technology has submitted a report in the Sindh High Court about evaluation of drainage infrastructure in the Defence Housing Authority (DHA) stating that existing design may be inadequate if stormwater drainage system is also used for sewerage flows.

The report suggested that sewerage must be drained out separately to get maximum benefit from the existing and proposed drainage system in future.

It further said that sewer water and deposition of solid waste had reduced the capacity of stormwater drains, and warned that more inundation was possible at hotspots and overall in the areas of DHA and Cantonment Board Clifton.

When a two-judge bench headed by Justice Syed Hasan Azhar Rizvi on Thursday took up two petitions, filed about the sewerage infrastructure that caused flooding during the torrential rains in DHA and CBC in 2020, the NED University filed its report in the light of the SHC orders.

It is further recommended in the report that the major drains were clogged, due to sewage flow and solid waste, required regular cleaning and maintenance to ensure drainage of stormwater in future.

“It is also important to mention that the simulations made in this vetting study and in the design report, the amount of inundation is still underestimated because model has the assumption that drains are empty at the start of rainfall; therefore, the amount of drained water from the area could be a lot less than shown as discharge. Consequently, more inundation is possible at the hotspots and overall in the area,” it added.

The report further said that even at a 10-year return period, with no sewage flow in the existing drainage system, a total of 15 manholes were found inundated for more than one hour with the spread of water at pavement greater than standard six feet and depth at curb exceeding six inches.

The current rainfall warrants the use of a return period larger than 10-year as Karachi is witnessing a recurrence of rainfall events like 2020 and 2022, it added, and suggested to use at least a 50-year return period rainfall intensity for redesigning of the stormwater drainage system which is also supported by international design criteria and guidelines.

“Part 4 of SWD Plan 2007, which was the Sewerage Master Plan for DHA, must be implemented by authorities to ensure no [flow of] sewage into the existing drainage system (SWD Plan 2007),” the report further recommended.

It said that the extreme flooding conditions in 2020 and 2022 in Karachi in general and DHA in particular were the recent examples of flooding due to lack of planning and maintenance of stormwater drainage system of a cosmopolitan city.

The NED varsity in its report said that floodwater accumulation was also related to solid waste management and it had tendency to clog the drainage system if not removed accordingly while another possible problem was that Karachi was using same infrastructure for both sewerage and stormwater drainage.

It stated that the main objective of this study was to vet the “Construction of leftover stormwater drainage network in Phase-I to VIII, DHA Karachi design validation” for improvement of existing/suggested drainage system.

The bench adjourned the hearing till Sept 14 as both sides sought time to examine the report.

It may be recalled that in September last year, the SHC had directed the NED University to conduct the evaluation of the drainage infrastructure within the jurisdictions CBC and DHA and also asked the two institutions and the petitioners to provide requisite information and documents to the vice chancellor of the university.

Published in Dawn, August 26th, 2022