A reclaimed lake, a 1988 drain, and two floods in four years
Ward 7 of a city of 1.1 million lies on the left bank of a river and flooded in 2019 and again in 2022. Both events exceeded 140 mm of rain in 24 hours; the storm-water drain was designed in 1988 for 100 mm in 24 hours. Built-up area in the ward rose from 22 per cent in 1991 to 61 per cent, and a 14-hectare lake was partly reclaimed in 1998 for a bus depot and a vegetable market. The main nala carries about 12 cubic metres per second against an estimated peak of 25. The corporation proposes Rs 90 crore for 1.6 km of concrete-lined widening downstream of the market, plus a riverfront promenade on the same stretch. Consultants propose instead Rs 60 crore of distributed works: 400 recharge pits, revival of 12 hectares of the lake, three playgrounds converted to retention basins, and a rain gauge network with SMS-based early warning. Residents downstream of the widening say they will be worse off.
1. Why might widening the nala make flooding worse for the residents downstream of it, and who gains?
Read the worked answer
Widening increases conveyance and velocity, so the same volume of runoff arrives downstream sooner and in a sharper peak. A stretch that previously spilled and stored water in its floodplain was, accidentally, attenuating the flood; lining and confining it removes that storage. If the widened section ends at a bridge, culvert or the river outfall that is still narrow, the bottleneck simply moves downstream and the peak reaching it is higher. Lining also cuts infiltration, so recharge falls and the next event runs off faster.
The 14-hectare lake matters as much as the drain. Reclaimed detention storage has to go somewhere, and it goes downstream. Set against those losses, 1.6 km of new concrete may reduce local ponding near the market while increasing depths at the outfall.
The gainers are the traders and property owners around the market and along the promenade, where land values rise and the view improves. The losers are the households at the downstream constriction, who may sit in a different ward or even a different local body and have no vote in the decision, plus the informal settlements that promenades typically displace.
2. Compare the two packages against at least four criteria and recommend one, with conditions.
Read the worked answer
Criteria: cost, speed of benefit, co-benefits, land requirement, and performance in an event larger than design.
On cost the distributed package is cheaper (Rs 60 crore against Rs 90 crore). On speed it is also quick if the pits and playground basins are simple works, while a 1.6 km lined channel needs utility shifting and traffic diversion. On co-benefits it wins clearly: recharge raises the ward's falling water table, revived lake and basins cut local heat, and early warning saves lives even when no works are finished. On land it requires the bus depot and market question to be reopened, which is politically hard; the nala option requires demolition along a narrower corridor. On extreme-event performance, neither is sufficient: the drain is designed for 100 mm while events exceed 140 mm, and storage is limited. Twelve hectares of lake at one metre depth holds about 0.12 million cubic metres, which is roughly an hour and a half of a 25 cubic metre per second peak. Useful for the first flush and for delay, not a cure.
Recommendation: the distributed package plus the early warning system now, and a widened channel only after a full hydraulic model shows the bottleneck is removed all the way to the river, without lining where infiltration matters, with a setback that keeps the floodplain.
Conditions: no further building on the lake bed; no promenade until the drain and storage questions are settled; publish the model; and re-examine the 100 mm design standard.
3. What single measure would you use to judge whether the intervention worked, and what makes flood claims hard to evaluate?
Read the worked answer
The measure should be the number of properties inundated above floor level per event, recorded against the rainfall that caused it, at fixed points along the nala. Absolute flood damage will not do, because it varies with rainfall and with what has been built since.
That choice already exposes the difficulty. There is no reliable baseline: 1991 and 2022 have different buildings on the same plots, and informal dwellings are often not counted at all. Rainfall is highly variable, so a dry year after the works will look like success. Compensation and insurance create an incentive to overstate loss, while households without title have an incentive to stay quiet. Peak stage at a gauge is more objective than damage estimates, but gauges are few and often sit outside the affected stretch.
So I would insist on three things before any money is released: fixed gauges with published readings, a pre-project property-level survey of floor levels and inundation depth in the two worst streets, and a rainfall record from the new rain gauge network. Without these, whichever side loses the argument will simply say the other flood was bigger.