Jonathan Munro: Urban wetlands key to water management

Last year, the city completed DigIndy, the $2 billion tunnel system increasing stormwater capacity and reducing waterway contamination events.

Our problem now? While the concrete has set, the world keeps changing: Our city expects higher overall precipitation in coming years and more extreme rain events. And while work took place below ground, the city changed above ground, too, as urban forests were cleared and replaced with parking lots and buildings. Marion County now has less canopy than non-porous surface area, and this deficit is growing. The result is that we are more vulnerable to flooding and contamination events than originally planned, despite DigIndy.

Let me wind back the clock and ask a hypothetical question: What if the city had spent a fraction of the DigIndy money to instead acquire strategic parcels of natural land from voluntary sellers in the late 20th century? I propose that each dollar spent on this “green infrastructure” would have more than paid for itself by scaling back the eventual size and cost of DigIndy.

The storage capacity of wetland is impressive. One acre of wetland can hold more than 1.5 million gallons of water. A wetland the size of one large golf course would therefore approach the 290-million-gallon capacity of DigIndy. But it’s not enough to have the capacity; the available land must be in useful locations. It happens that much of the undeveloped land in Marion County remains undeveloped precisely because it is wetland or floodplain or else follows creeks or rivers, nature’s own stormwater system. The result is a strong overlap in land that is both available and beneficial.

Let’s do some valuations. DigIndy captures 1/7 of a gallon of stormwater per dollar. At a current valuation of $25,000 per acre, wetland captures 60 gallons of water per dollar. In fairness, the DigIndy system processes stormwater and sewage much faster than wetland. Wetlands do their work more slowly — breaking down pollutants and absorbing nutrients while filtering water to recharge aquifers or streams. However, this capture and delay is actually complementary to a system that fails at times of rapid drainage. Trees, for their part, can mobilize large amounts of water to the air through evapotranspiration under the right conditions.

Cost-effectiveness alone underscores that natural land acquisition should have had a greater role in our stormwater strategy, but let us consider other bonus benefits. Urban forests provide air purification, a point of weakness for our city, which has been given a failing grade on air quality by the American Lung Association. Wooded natural areas also provide surrounding neighborhoods with heat mitigation and recreation opportunities. Finally, the mental health and developmental benefits of urban forests are increasingly recognized.

In a word, these urban forests would have appreciated as assets to the city aside from stormwater considerations. Concrete, by comparison, depreciates and cracks.

We can’t change the past, but we can refuse to repeat our mistakes. We should undertake now to buy remaining urban forests and suitable natural lands from willing sellers to reduce the scope of the next DigIndy. We need both green infrastructure and gray infrastructure and should follow the example of cities such as Milwaukee, a national leader in integrating green infrastructure into its flood-prevention strategy. That city’s Greenseams initiative has acquired more than 5,000 acres from voluntary sellers, land that collectively holds more than 3 billion gallons of water.

We should applaud Mayor Joe Hogsett’s administration and the City-County Council for taking meaningful steps forward with the acquisition of land in the Grassy Creek corridor for stormwater management in 2024 as well as funding other urban forest purchases. And yet, planned spending on gray infrastructure continues to dwarf spending on green infrastructure.

We implore the city to urgently address this imbalance in investment. We have no more time or forest to lose. If water has no place on natural land, it will continue to overflow our sewers and flood our basements.•

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Munro is director of Forests for Indy.