ISLAMABAD: A new hydrology study conducted by the army’s Directorate of Design and Consultancy (DD&C) has revealed that natural streams had narrowed down in E-11, which caused urban flooding last year.

The study, however, proposed interventions and a mitigation plan to avoid such flooding in future.

The report of the study released on August 16 stated: “The simulations revealed that every drainage channels in the area was overflowing its bank in the built-up area. However, no stream overflowed its banks outside the developed area.”

In July last year, because of the narrowing down of a stream which enters the sector from E-12 side saw urban flooding in Pakistan Medical Cooperative Housing Society (PMCHS) leaving a woman and her child dead besides inflicting loss to property and several vehicles.

The sector - which was exempted from land acquisition by former president Ayub Khan due to the Golra Shrine -houses some unauthorised societies, operating without having valid no-objection certificates and over 70 high-rise unauthorised buildings. Most of these buildings were built during the last one decade, but the Capital Development Authority (CDA) failed to stop their construction as in most of the cases it confined itself to issuance of notices only.

Army’s directorate suggests various interventions to reduce risk of overflowing water channels

Following the flooding and death of the woman and child, the CDA had decided to engage DD&C for carrying out a hydrology study in the sector.

The analysis revealed that natural hydrology and natural flow paths were extensively modified during the development of PMCHS as well as across sector E-11. By year 2011, one of the major streams, Jod Khas, passing through PMCHS was narrowed down to three metres, the study said.

Similarly, another stream was straightened and narrowed down to almost two metres, with its surface having been paved and contained with a stone masonry wall on either side. A third stream passing through the society was also confined to a deep channel lined up with stone masonry and paved at its base. A box channel ran through the built-up area with different cross-sections and made many right angle bends on its way.

“It was cleared from old data that developed model applied in PMCHS and E-11 had invariably narrowed, straightened and paved the natural drainage paths. Buffer zones were completely missing and properties were built right up to the edges of these narrowed channels,” the study revealed.

“It is obvious that natural drainage network has been significantly narrowed over the entire sector,” it added.

The study further said the narrowed right of way (ROW) of the natural drains was the biggest structural shortcoming to handle maximum flooding in E-11.

“On top of it, narrowed ROW has been further encroached upon by some properties which seems to cause backwater ponding – resulting in overflow of culvert on double road in the north-western perimeter of sector E-11,” it said.

Conversely, if ROW of natural streams is restored in the sector to the extent of the estimated on Map 19 and 20, the flood hazard would likely be eliminated altogether. However, since properties have already been built in reclaimed ROW of natural drainage, restoration of ROW to its original limits may be expensive (for builders and property owners) and may involve other intricacies.

It said that as long as the sector kept functioning with narrowed ROW of natural drains the flood hazard would loom for the residents of E-11 in the flood hazard zone.

The study proposed interventions, “which are deemed effective and implementable within the current development with minimal requirements of demolishing existing building and or infrastructure”.

The suggested interventions are: water sensitive urban design, introduction of drainage swales, intervention of holding ponds, creation of drainage network, drainage swales in main boulevards, recharges trenches in residential streets and drainage swale through PMCHS.

Recommending for flood-risk management in flood-prone areas in general and E-11 in particular, the study stated that flood hazard zones be evaluated and mapped and residents be communicated with flood hazard maps. Monsoon be declared as flood emergency season for vulnerable zones, basements should not be rented/used for living and any structural interventions planned for flood mitigation should be proceeded by a holistic hydrological model.

The study also recommended that streams within E-11 should be improved by removal of concrete/masonry lining and replaced with boulders, gravel and sand.

This will improve groundwater recharge. It said all sections of streams covered with concrete slabs must be demolished and this practice should be strictly banned.

The study also called for widening of narrowed streams, where possible, adding some properties had further encroached on narrowed streams which should be identified and removed. It also called for a sustainable development model for the sector besides updating bylaws of the area with modern tools.

The CDA officials said the recommendations would be implemented and meetings of the stakeholders would be held in this regard soon.

Published in Dawn, August 24th, 2022