4/27/2026 – I recently read Bill King’s fascinating new book, Depopulation: Our New Demographics Reality. The demographic trends he discusses have huge implications for flooding and flood control.

It may seem insane to talk about depopulation in most places. The world has grown from 1 to 8 billion people in the last two centuries. And it may seem even more insane in a region like Houston.

“We’ve built our entire world—our economies, cities, institutions, and expectations—on the assumption that growth never ends,” says King. “In 63 countries, home to 28 percent of the world’s people, populations are shrinking right now. China’s population is collapsing. Europe is aging into irrelevance. Global fertility has plummeted to barely above replacement level—and it’s still falling.” He continues…

“Yet while policymakers scramble and nations face demographic catastrophe, most people remain blissfully unaware.”

Bill King

Suspend Disbelief Momentarily

So, let’s suspend disbelief for a few minutes and discuss what the implications of shrinking population growth could be for flood control – both negative and positive.

Shrinking population growth would change hydrologic, financial, and planning assumptions behind flood control. The impacts would be seen over time in: 

• Land-use change
• Infrastructure financing
• Risk distribution.

Slower Population Growth Would Slow Increases in Runoff

Population growth typically increases flooding risk because it increases impervious cover (roofs, streets, parking lots). Impervious surfaces reduce infiltration and increase peak discharge.

If growth slows, so would peak discharges. Flood infrastructure may end up over-sized if development slows enough. People in danger of flooding may consider that good news. It lowers their risk and gives them more time to implement solutions.

Depopulation could make it easier for flood infrastructure to keep up with development. Agencies could shift from reactive to preventive flood management. Infrastructure projects could catch up with development. However, it’s not all good news.

Increasing Financial Pressure

On the other hand, slower population growth would make it more difficult to fund large, capital-intensive flood-mitigation projects.

We usually fund such projects through some combination of property taxes, impact fees and/or bond programs. When property taxes grow more slowly, impact fees decline sharply, and developer-built detention capacity decreases, flood control agencies may struggle. This is reportedly a common problem in shrinking or no-growth cities.

At the same time, long-term infrastructure liabilities grow. When population stagnates or shrinks, as it did in Rust Belt Cities for decades, so does the tax base. But maintenance costs remain, increasing the per-capita infrastructure burden. Fewer people must support aging drainage systems.

Land Conservation Becomes Easier

On the positive side again, slower growth increases opportunities for floodplain preservation. With less pressure to build subdivisions in floodplains and over wetlands, there’s more room for detention space and green infrastructure. This can dramatically reduce downstream flood peaks.

Preservation is already much more cost-effective than flood mitigation. In a low-demand, cost-constrained environment, preservation becomes even more financially attractive.

Sediment and Channel Impacts Decline

Another potential positive. Since Hurricane Harvey, I’ve researched hundreds of articles showing how development drives:

• Erosion from construction sites
• Sand mining
• Channel instability

Slower growth typically means:

• Fewer disturbed soils
• Less sediment entering rivers
• Slower aggradation of channels and reservoirs

This could potentially reduce flooding impacts and dredging requirements downstream.

Change in Planning Horizons

Planners usually design flood infrastructure for a service life of 50 to 100 years. If population growth slows, that infrastructure may appear overbuilt initially, but the extra capacity could support the surrounding population over a longer period.

Planning Assumptions May Become Obsolete

Many flood models reportedly assume steady population and urban growth. If demographic reality changes, agencies may need to update/debate:

• Watershed build-out assumptions
• Impervious cover forecasts
• Detention requirements

These will likely become hotly contested public debates fueled by shrinking demand for the services of contractors and homebuilders eager to maintain their profitability. Competition will become cutthroat in shrinking industries. And pressure on legislators and regulators to “cut us some slack” may become irresistible.

If regional population declines, companies will struggle to survive by offering higher quality. This could continue to fuel population growth in suburban submarkets. We saw this in northern rust-belt cities such as Detroit and Cleveland during the last century. Consumers will always look for better quality, especially as the aging infrastructure in urban cores deteriorates.

Strategic Opportunities

With less development pressure, governments can shift from parcel-level detention toward regional systems, such as:

• Large, regional detention basins
• Floodplain buyouts
• Restored wetlands
• Green corridors

Hydrologists tell me these are usually more effective and more economical than small, distributed detention ponds.

Conclusion

While shrinking or slower population growth could generally reduce future flood-risk, it will also reduce funding capacity for mitigation.

Shrinking population should reduce growth of impervious cover, runoff, sedimentation, and revenue. But it will also increase the maintenance burden per capita.

The net result will depend on whether the hydrologic benefits of slower development outweigh the fiscal constraints on flood-mitigation investment.

Changes will not happen overnight. Developers will keep building because they believe buyers will trade up from older housing.

In other cities, it has taken 20-30 years for reality to catch up after population decline begins. During that time, superior new housing will capture demand from older neighborhoods. I just pray the new housing is not in risky, flood-prone areas.

Posted by Bob Rehak on 4/27/26 with help from Bill King, Several Hydrologists, and ChatGPT

3163 Days since Harvey