Volume and Velocity. Why Streamside Buffers Are Pennsylvania's Best Flood Defense

Flooding is Pennsylvania’s costliest and deadliest weather hazard. Between 1996 and 2018, floods caused $3.2 billion in property damage here and took 89 lives. Flash floods, where too much rain falls too fast, account for nearly two-thirds of that toll. With storms getting more intense because of climate change, these impacts aren’t going to shrink.

During my time at the Pennsylvania Department of Environmental Protection, one lesson came up again and again. If you want to control flooding, you have to manage two things: volume and velocity. Volume is how much water ends up in a stream. Velocity is how fast it gets there. A flood happens when too much water arrives too quickly for a stream to carry. For anyone trying to reduce flooding, these are the two factors that matter the most.

Most of what we build works against us on both counts. Roofs, parking lots, and roads shed nearly every drop that hits them, sending water downhill fast and all at once. For homeowners, businesses, and communities, every acre of paved ground turns a one-inch rainstorm into roughly 27,000 gallons of runoff instead of water soaking into the ground.

Now picture the strip of trees, shrubs, and native plants along a creek. That streamside buffer tackles velocity first. Its rough, leafy, uneven ground slows the flow of stormwater and delivers it to the stream gradually instead of in a surge, allowing the stream, creek, or lake to handle the amount of stormwater without flooding or erosion.

Slowing water down also reduces volume. When runoff moves slowly, it has time to soak into the soil and recharge groundwater instead of running straight into the channel. The roots of trees and shrubs help too, opening small pathways in the soil that let even more water infiltrate. Water that goes into the ground doesn't show up in the stream during the storm. It seeps back out slowly over days and weeks, which also helps keep streams flowing during dry spells.

So, a healthy buffer along our waterways (known as a “riparian” buffer) manages both halves of the equation at once. And it is flood control that builds itself, maintains itself, and costs almost nothing.

When those buffers are cleared, the opposite happens. Water hits the stream faster and in greater amounts; banks erode, and channels get carved wider and deeper. That sends the next storm's water downstream even faster. The damage compounds.

Here's what concerns me: in most of Pennsylvania, despite our strong state water quality laws, these buffers have very little protection. The decisions that determine whether they survive are being made locally, one project and one permit at a time.

And that’s why I’m so excited that PennFuture is stepping up to help municipalities manage velocity and volume with our Model Riparian Buffer Ordinance and Guidebook. In a time when communities across the Commonwealth are being inundated by hyper-scale development, municipalities have the opportunity—and the power—to protect our waterways and communities from harmful impacts from warehouses, data centers, or whatever is next on the horizon.