Water is in constant motion on this planet. It’s always raining somewhere, it’s always traveling somewhere. Naturally flowing through streams, rivers, lakes, oceans and groundwater reservoirs. Somewhere along its path we animals grab a small of portion of it, ingest it into our bodies, and put it back into nature just ever so slightly more yellow where it can then go back on its merry way. Fun fact for the day: every glass of water you and I drink has dinosaur pee in it.
On average 0.08 inches of water in the form of rain hits the surface of the planet every single day. That might not sound like a whole lot but when you consider that the surface of the Earth is 197 million square miles. This equates to 274,000,000,000,000 gallons of water constantly raining down upon this planet every single day. Once a drop of water hits the ground surface, that drop can take almost a near infinite number of possible paths in its journey. Many drops will coalesce into small mountain brooks, then into streams, eventually into rivers, then into oceans, then evaporate into a rain cloud, and start the process over again. The process is truly amazing when you think about it on an existential level. The water cycle is like a machine that keeps on turning and pumping, around and around. Like coolant in a car but never ever stopping.
As the water spins and churns on our planet, the formations of the land will determine its course. The Earth itself under our feet charts the course for the rain over our head. Both the land and the water are mutually linked side by side. Water will always move over the land following what is called the “path of least resistance.” As water moves across the surface of our planet it really does not have time for small talk. Please don’t stop rain water and try to talk to it about the weather. It is the weather. Rain water has one mission, and one mission only, to get downhill and get there as fast as possible. Every single time for the past 3.8 billion years since this planet has had water, rain when it hits the ground, will take the most direct path for the least amount of effort. Take for example the Colorado River and the Grand Canyon. The Grand Canyon has been the path of least resistance for the water in the Colorado River for the past 7-8 million years. During those years, not once has one single water droplet said to itself, “You know, on second thought, today, I think I’m going to go around the Canyon today instead.”
Water is constantly spinning and circulating on this amazing planet, constantly looking for and seeking out the smoothest possible downhill ride with the least amount of resistance. What happens on the surface of the Earth dictates the path the water will eventually take. And if there’s one thing we humans are the grand masters at, probably better than any other known species, it’s manipulating the surface of this planet. The Earth in many ways is like our own little toy sandbox. How we manipulate the contents of that sandbox determines the eventual path water will take in its journey. We cut down swaths of forests, create massive dams, dig giant canals, construct massive underground water storage vessels. All of these things, and everything we do inside our sandbox, impacts the course water will eventually take.
Flooding happens when we put too much of the wrong contents inside our sandbox.
I’ve been writing about the modern history of the decline of our forests for the past few months on the blog. Why you may ask? Because there is no better natural occurring surface better at preventing flooding than a healthy thriving forest. And over the past two centuries we’ve gone and cut down most of them (see the blog posts here). Many of the forests that are remaining, especially in urban areas, are dwindling and often in poor health.
Take for example a healthy forest versus an agricultural meadow. In each of these sandboxes the water encounters a smoother surface with less obstacles, thereby speeding up the water. Remember water seeks the path of least resistance, always, every single time.
Suppose 1.5 inches of rain water falls upon each of these surfaces? What will happen?
For a healthy forest, on average 1.4 inches of rainwater will be captured and absorbed by the forest. The surface is very rough and has many obstacles (stumps, logs, and fallen branches). The water must take many paths to get to where it’s going. The more paths the water takes, the more chances it has to soak into the ground and absorb.
For a healthy agricultural grassy meadow, on average 1.2 inches of rainwater will be absorbed. Meadow grass, while still rough, provides fewer obstacles and is smoother than a forest. Also when it rains exceptionally hard, grasses can often get compacted and matted down, especially if trucks have driven over the grass before it rains.
If there’s one thing we humans really love to do in our sandbox it’s to create smooth surfaces. We all love to live in single family homes with a smooth roof, nice outdoor slate patio with a grill, and long asphalt driveway to park our 1.8 cars. Just like water, we humans also love to get to places fast using highways and roads. We love to buy stuff and park our cars in convenient shopping center parking lots. We call all of these surfaces “impermeable surfaces”, because rain water cannot permeate (i.e. pass) through them. If there is a complete opposite sandbox to a forest for rain water, it’s a smooth impermeable surface. Impermeable surfaces send water downhill very fast and have no obstacles for the water to encounter. Impermeable surfaces are the Webster’s Dictionary definition of the “path of least resistance.” (Don’t fact check me on that one, by the way. Hey, but if they aren’t they should be, Mr. Webster, whoever you are.)
Suppose 1.5 inches of rain water falls upon each of these types of surfaces? What will happen?
For a single family home in a dense residential neighborhood such as Pittsburgh, on average 0.8 inches of rainwater will be absorbed. Most water will hit the roof, enter a gutter, go into a pipe under the yard, and enter the common sewer pipe in the street. Water will sheet flow off the driveway and the street and into the curbside catchbasin where it will enter the common pipe. The rain water within that pipe will eventually make its way to the stream and it will get it there fast.
For a typical parking lot, on average 0.2 inches of rainwater will be absorbed by puddles and cracks. The rest will sheet flow over the surface, find the nearest pipe, and enter the stream. It will travel there even faster than the single family home.
Maybe instead of “Impermeable surfaces” we should start calling these types of surfaces “flood out your neighbor” surfaces. Because in reality that’s what they are doing, especially if the rainwater that hits them is not properly controlled. All of these surfaces add up within a watershed. Urban flooding is a death by thousands of individuals cuts. Take for example Saw Mill Run or Four Mile Run in Pittsburgh, two of the more prominent flood prone watersheds in our region. We’ve heavily developed these watersheds, with large buildings, homes, and parking lots. Over the decades, very little has been done to counter act the increased amount of water that is no longer absorbed.
In a lot of ways, the Earth’s surface is humanity’s toy sandbox. Even more so over the past century now that we have all these amazing combusting engines at our disposal to make it super easy to manipulate the land.
Our actions inside our individual sandboxes matter, especially when people and towns get flooded.
In a lot of ways, all of us are responsible for flooding.
P.S. A big huge thank you to my awesome graphic artist big sister, Karen, who helped me with creating these amazing sandbox graphics!
P.P.S. In part two we are going to talk about ways to counter act “flood out your neighbor” surfaces and what you can do to help reduce flooding.
The Pittsburgh Urban Flooding Journal 