That's a tall order. Even if every house in the city has a rain garden by the downspout, that barely scratches the surface. We need rain gardens everywhere--especially for streets and parking lots. Not just rain gardens, but all manner of tricks for preventing runoff--solutions like green roofs and porous pavement.
This may seem like some tree hugger's dream--way too radical. That's what I thought, until I heard about Philadelphia's plan to spend 1.6 billion dollars, transforming the city in the next 20 years. But the benefits will be considerable. Green infrastructure is going to be a lot more aesthetic.
That's why I chose a stormwater pond for my banner photo (below).
So, if we need a multitude of rain gardens, why not start in the greenway?
Nice idea, but it's not that easy. In an urban stream or stormwater channel, you see three different kinds of flow during a year.
- You have cloudbursts or spring runoff, when there's a big, destructive flood. The banks get eroded, and any rain garden in the channel could easily be destroyed.
- Next, there's the normal rain. The stream runs like a normal stream. Lots of nice rushing sounds. A day after the rain stops, there's just a trickle, with some pools for wildlife. All those pools serve as rain gardens. Since there are scores of normal rains for every flood, the rain gardens are important.
- During a dry spell, the stream completely dries up. Vegetation along the banks dies, making it more vulnerable to erosion in the next flooding cycle.
Cherokee creek, near Thoreau School, after cloudburst.
To return the greenway to health, we have to handle the floods, and keep the plants healthy during dry spells.
Who said life was easy? We flew to the moon. We built planes invisible to radar. We can figure out how to make natural-looking stormwater channels that work--and look good, and support wildlife. It just takes some research.
Our proposal--two pathways for the water
At the east end of the greenway, where the stream is buried, I propose we keep the old stormwater pipe, and also build a chain of connected rain gardens beside it.* We build a side pipe at the far east end of the greenway that directs modest flows into the chain of rain garden, but switches big floods through the existing pipe.
In other words, the stream is uncovered in the sense that low flows run throught the chain of open rain gardens, to the north of the existing pipe. But the existing pipe remains buried, and carries (for several years) the overflow floodwaters. If the rain garden route proves robust enough, then ALL the flow (including floods) could be directed into the rain gardens. (details at end).
In the series of rain gardens next to the pipe, we can experiment with different plants and different linings to the bottom. Once we find several good designs, with vigorous plant growth, we can then switch the flow so floods go through the rain gardens. Next, refine the designs until we have one that can withstand a full flood.
At the same time, for the rest of the ravine downstream--the middle and western parts--we can experiment with various designs, including, I hope, many pools or rain gardens. If the upper (eastern) rain gardens demonstrate the best design and plantings, the lower pools might later be changed to that best design.
And what about the dry spells? Rain barrels, connected to soaker hoses, can keep the plants healty and better able to resist the next flood. Imagine all those lovely woodland plants--ferns and flowers. Finally for the access required to maintain the hoses, rain gardens, and to do some weeding, there should some reinforced pathways and steps.
Lean and mean
The problem with riprap (as usually practiced) is that it's over design The huge piles of rubble discourage plant growth and wildlife, and have all the charm of a strip mine.
We don't build airliners with steel and concrete. Sure, they'd be sturdy--the geese would just bounce off of them. But they'd never get off the ground. In the modern world, "lean and mean" makes for better performance. That's what our greenway needs. Just enough reinforcement and smart design to withstand the floods--followed up with sufficient maintenance to keep it ready for next year's flood.
And our greenway is a good place to figure out--for the whole city--what that smart design entails.
* * ** The series of rain gardens at the eastern end should follow the old streambed, in a natural curve--not just parallel to the buried pipe. These rain gardens have to be lower than the pipe, so their water won't just seep back into the pipe.
Madison deserves credit for it's excellent research on rain gardens.
Details of rain gardens at east end of greenway
Some excavation is required north of the existing stormwater pipe, to create a channel for the chain of rain gardens. But excavation is needed anyway, to repair the sanitary sewer, which now crosses under the stormwater pipe.
The "switch," to direct stormwater from the pipe into the rain gardens, does not have to be actively controlled. It is just built so the water automatically does what we want it to do. For example, if you burrowed into the stormwater pipe near owen drive, and inserted a pipe 1 foot in diameter, connecting with the bottom of the existing stormwater pipe, then an amount of water that could flow through the 1' side pipe would always go into the rain gardens. If a flood exceeded the capacity of that 1' pipe, then the excess floodwaters would go down the existing stormwater pipe, bypassing the rain gardens, and sparing them from damage.
Why don't we just leave the stormwater pipe as it is, and experiment with rain gardens further downstream?
Because the stormwater pipe allows us to keep floodwaters out of the rain gardens. It's not going to be easy to design rain gardens that won't be destroyed by a flood. This gives us more freedom to experiment with or without floodwaters. In addition, there's much more room at the east end of the greenway, the slope is more gradual, and there is less water volume here. The rain gardens would be more visible and enhance that end of the greenway.
Won't the property to the North, on Owen Drive, be threatened again by floods?
No, because at first, the existing pipe will handle the bulk of the floodwaters. If, later, floodwaters are directed into the rain gardens, this change can be reversed if problems occur. The rain gardens will be more robust that the uncontrolled stream used to be.
In the city's Option 1, Lisa Coleman proposed one or more rain gardens near the stormwater pipe. How do the rain gardens proposed here differ?
- I'm proposing a chain of connected rain gardens that connect to the stream channel downstream, almost like a natural stream. Lisa was proposing one or two isolated rain gardens.
- In my proposal, more water goes into the rain gardens. The side pipe is larger. The rain gardens have more capacity, and so will require some reinforcement.
- The gardens in my proposal are experimental, and may be modified. The water flow can be modified.
- I'm proposing a variety of approaches to channel protection downstream, for experimental purposes.