Cooling options: Heat and waterlogging
What to build or change, what keeps it working, the traps to avoid and rough costs.
Protects. Neighbourhoods that bake in summer and flood in the monsoon, and the groundwater both depend on.
Key caution. Stop sewage and rubbish flowing in, and fund the desilting, before calling it blue-green.
Options, and what keeps each working
| Option | What it does | Keeps working with | Avoid | Rough cost |
|---|---|---|---|---|
| Sponge parks | Small parks shaped to hold, soak up and slowly release stormwater, with plants that cool the area around them. | An overflow, mosquito control and desilting before each monsoon. | Standing water. | About US$8,400–US$18,000 per park (₹8 lakh–₹17 lakh at Indian prices) |
| Restored ponds and lakes | Desilted, protected water bodies with planted edges that store floodwater, recharge groundwater and cool their surroundings. | Sewage diverted away, and rubbish kept out. | Calling a polluted pond a cooling asset. | About US$38,000–US$320,000 per pond (₹36 lakh–₹3 crore at Indian prices) |
| Planted drains and rain gardens | Channels and hollows that slow, clean and soak up runoff from roofs and lanes. | Rubbish collection and an overflow. | Places where they will become open garbage drains. | About US$5.3–US$32 per m² (₹500–₹3,000 at Indian prices) |
| Permeable paving | Paving that lets rain soak in instead of running off. | Regular cleaning so it does not clog. | Claiming it cools unless there is shade and moist soil too. | About US$9.5–US$26 per m² (₹900–₹2,500 at Indian prices) |
| Recharge pits | Small pits that send roof and courtyard runoff into the ground. | A filter, so polluted water does not reach shallow wells. | Recharging dirty runoff. | About US$53–US$320 per pit (₹5,000–₹30,000 at Indian prices) |
What the evidence says
- Over four years, Greater Chennai Corporation restored 70 ponds for ₹144.34 crore and built 88 rainwater-harvesting parks for ₹14.74 crore.
- Bangkok’s Benjakitti Forest Park, a former tobacco factory turned wetland, is designed to hold 87 million litres of water.
- China’s sponge-city pilots in 30 cities (2014–2019) often built projects one at a time instead of as a city-wide system, struggled to get engineers and designers working together, and left cities worried about paying for more once pilot money ended.
- Jakarta’s green space was about 5.2–5.6% of the city in 2024–25, sources differ, against a legal target of 30%, now pushed back to 2045.
How you will know it works. Waterlogging hours after heavy rain, and summer temperature near the site, before and after
Five traps that waste cooling money: Buying before diagnosing · Built, with no one to keep it · Green without water · Promising degrees you haven’t measured · A list of activities, not a model.
Before you choose, ask: Does it need electricity, and is the supply reliable in the hottest weeks? · What happens in a power cut: does it stop, or keep cooling without power? · Does it need water, and where will that water come from in a dry spell? · Who services or repairs it, and how fast? · Who owns the running cost each year, and from which budget? · Who can use it: renters, women, older people, people with disabilities? Keep the outcomes apart: a cooler room, comfort, less heat strain, better health and kept income are different outcomes; measure the one you mean.
Costs are rough planning ranges for early budgeting, not quotes. They are Indian prices, from a practitioner’s planning workbook, shown in US dollars at about ₹95 to US$1 (mid-2026) with the rupee range alongside. India has some of the lowest building prices in Asia, so expect most Southeast Asian cities to pay more: see the regional prices below. Where a public figure exists, it is linked. Where no range is solid enough to share, the card says so: get local quotes.
Sources
- The News Minute. thenewsminute.com/tamil-nadu/chennai-civic-body-ramps-up-pond-restoration-rainwater-harvesting-parks-ahead-of-monsoon
- Mongabay, 2024. news.mongabay.com/2024/09/bangkok-turns-to-urban-forests-to-beat-worsening-floods
- McPhillips et al., chapter 7 in Nature-Based Solutions for Cities, 2023. doi.org/10.4337/9781800376762
- Metro TV News. metrotvnews.com/read/KXyCWwmW-target-30-ruang-terbuka-hijau-jakarta-dikejar-hingga-2045
- Chennai: 57 sponge parks commissioned for ₹7.67 crore, about ₹13.5 lakh each by our arithmetic. swarajyamag.com/amp/story/infrastructure%2Fchennais-transformation-into-sponge-city-gcc-to-set-up-57-sponge-parks-for-flood-prevention
- Chennai: one pond restored for ₹36 lakh. dtnext.in/news/chennai/after-50-years-siva-vishnu-pond-rejuvenated-for-rs-36-lakh-by-chennai-corporation-796892
- U.S. dollar–Indian rupee rates, 2026 (Pound Sterling Live). poundsterlinglive.com/history/USD-INR-2026