Grey + Green instead of Grey vs Green?
What if we combined grey and green infrastructure in a grey to green transition model instead of pitting them against each other?
I was at a workshop in DC yesterday on nature and insurance where a panel was asked, “how do you deal with the temporal challenge of NBS investments?” Nature-based solutions/green infrastructure takes a lot longer to deliver benefits than grey infrastructure.
It wasn’t necessarily a direct solution, but Deb Halberstadt, said something so simple and yet so profound that really got me thinking.
"Green infrastructure appreciates, grey infrastructure depreciates.”
I’m guessing most of you have already had this realization, but for some reason I hadn’t and it hit me like a lightning bolt with some interesting implications.
Green infrastructure assets get more valuable and delivers more benefits over time while grey infrastructure assets get less valuable and deliver less benefits over time. But this still doesn’t get us over the temporal challenge. Because grey infrastructure benefits are felt immediately, but green infrastructure often takes time.
So, how do we square this circle?…
Put ‘em together - The Grey & Green Transition Model
**Caveat that this idea is very half-baked and I’m hoping you smart folks reading this can take it and run with it. Or take it and tell me all the reasons why it doesn’t make any sense.**
What if we leveraged the strengths of both grey and green by putting them together? I’ve seen a lot of pure green infrastructure pitches and a lot of grey infrastructure pitches, but I don’t think I’ve ever seen a strategy that intentionally combines green and grey infrastructure and wraps them together to go out and raise project finance.
Why together?
Grey infrastructure delivers immediate benefits, but not long-term.
Green infrastructure delivers long-term benefits, but not immediate.
Where we know green infrastructure is more cost effective than grey infrastructure in the long-term, what if we designed infrastructure projects that combined the grey and the green? What if we designed grey infrastructure to bridge a 10 year gap until the green infrastructure is fully functional? And what if we bundled the two together to go out to raise project finance?
The main challenge with raising project finance for NBS today is that the benefits occur too far in the future to make a case for investment today (whether from public or private sources). But if we combine grey and green infrastructure, grey transitioning to green, then we bring forward the benefits thanks to the grey infra but maintain the delivery of those benefits thanks to the green, theoretically in perpetuity.
All of a sudden, the economic case might start to pencil out. This is where I’d love to have infrastructure folks kick the tires on this model with potential use cases. Below are some examples after a brainstorm with ChatGPT and also an example use case centered on combined sewer overflow. Please do not take these numbers at face value. I have no idea how ChatGPT came up with them, but it is a nice illustrative example to start to conceptualize.
Again, I don’t know if this approach holds water, but I would be curious if folks have use cases where we could see if the numbers and dollars on this “Blended Grey/Green Infra Transition Model” pencil.
- Eric
🌧️ Urban Flooding / Stormwater Management
Grey (Immediate): Underground stormwater pipes, culverts, retention tanks.
Green (Long-Term): Green roofs, bioswales, rain gardens, permeable pavement, restored wetlands.
✅ Why it works: Pipes quickly move water away; green infrastructure slows, absorbs, and filters stormwater over time.
🌊 Coastal Erosion and Storm Surge Protection
Grey (Immediate): Seawalls, breakwaters, levees.
Green (Long-Term): Mangrove restoration, dune rebuilding, oyster reefs, salt marshes.
✅ Why it works: Grey structures provide immediate barrier protection; green solutions buffer waves, reduce erosion, and adapt over time.
🛣️ Highway or Urban Heat Island Mitigation
Grey (Immediate): Shade structures, ventilation systems, cooling centers.
Green (Long-Term): Urban tree canopy expansion, green corridors, reflective surfaces with vegetation.
✅ Why it works: Built systems cool specific spaces fast; vegetation cools ambient temperatures and improves air quality long-term.
🏭 Water Treatment / Pollution Control
Grey (Immediate): Water treatment plants, filtration systems, chemical treatments.
Green (Long-Term): Constructed wetlands, riparian buffers, vegetated swales.
✅ Why it works: Grey ensures clean water instantly; green maintains quality naturally and cost-effectively.
🚧 Infrastructure Resilience in Flood-Prone Areas
Grey (Immediate): Elevated roads, floodwalls, pumps.
Green (Long-Term): Floodplain restoration, wetland reconnection, reforestation.
✅ Why it works: Grey keeps services running today; green reduces future flood risks and restores ecosystems.
💧 Watershed Management for Water Quality & Quantity
Grey (Immediate): Water treatment plants, detention basins, storm drains, chemical dosing systems.
Green (Long-Term): Forest and wetland restoration, riparian buffers, soil health improvement, floodplain reconnection.
✅ Why it works: Grey provides quick control over polluted or excess water; green solutions rebuild the watershed’s natural ability to filter, store, and release water sustainably over time.
PROJECT FINANCE EXAMPLE - Combined Sewer Overflow (CSO) Management
Grey (Immediate): Underground storage tanks to prevent sewage overflow
Green (Long-Term): Green roofs, bioswales, and rain gardens to reduce inflow and future storage needs
Outcome: Green infrastructure reduces the long-term size/cost of grey expansion and cuts treatment costs
Total Project Budget: $100 million
Timeline: 20 years
Goal: Achieve regulatory CSO compliance and build long-term resilience through green stormwater management.
💵 Capital Stack
🧩 Deployment Strategy (Years 0–10)
Grey Components:
2 underground storage tanks
Pumping station upgrade
Smart monitoring systems
Cost: $70 million
O&M Costs (Annual): $2.5 million
Green Components:
300 rain gardens
50 blocks of permeable pavement
Green roofs on municipal buildings
Riparian buffer restoration
Cost: $30 million
O&M Costs (Annual): $400k (much lower than grey)
💸 Revenue & Repayment
✅ Why it Works:
Grey ensures immediate regulatory compliance + reliability.
Green reduces long-term system strain + unlocks new financing streams.
Joint financing reduces per-unit costs and improves creditworthiness.
Flexibility to scale or shift investment based on performance.
Oftentimes grey interferes with the function of green but that’s a design issue. To take municipal flooding as an example, Phoenix can use grey to shunt water as fast as out of the city or use green to become a sponge city and prevent massive drought down the road. But at least with conventional approaches, green and grey are very much at odds with each other in terms of what they are trying to accomplish.
“Green infrastructure appreciates, grey infrastructure depreciates.” - love this quote. So obvious it should smack you in the face, but it often gets lost.
Nice thought-provoking article.