At the last board of commissioners meeting, there was a lengthy discussion of the water runoff problems during major storms in parts of the township. A combination of issues leads to excessive water runoff in places – the amount of impervious surfaces such as driveways that don’t capture rainwater, grass lawns which capture little water and underlying geography that allows little water capture.
Many residents have asked for storm drains, which is both an expensive and not environmentally friendly solution to the problem. Ideally, the water should be captured and not allowed to easily flow into the nearby creeks. The board has asked the township engineer to present the options. But, Nether Providence is primarily a residential area and little land is owned by the township itself so its options are limited. There are actions residents could take on their own initiative, however.
Some starting numbers. Philadelphia gets precipitation an average of about 120 days /year and an average of 44 inches of precipitation. Combining those numbers, the average rainfall is 0.36 inches. The EPA has detailed information on local soil conditions throughout the US in their stormwater calculator.
Rain barrels collect runoff falling on impervious surfaces like roofs from gutters and retain it so it can be released after the storm. Each square foot of roof collects 0.6 gallons of water per inch of rainfall. A 40×30 roof is 1200 square feet so would collect 260 gallons in an average Philadelphia rainfall. Many rain barrels capture 50 gallons, so you would need multiple ones to capture all the water from a roof this size. As an added benefit, the water can then be used later for watering the garden. Its helpful to elevate the rain barrel a foot or two so you can use gravity to empty it. Its possible to take this even further. When we visited New Zealand, my aunt had an elevated rain tank with a filtration system that provided all of the water for herself and her farm.
Use permeable landscaping. Asphalt driveways are a major offender, but patios and other surfaces cause water to run off instead of being absorbed. The older style driveways were better – gravel, cobblestones and pavers all allow water to be absorbed into the ground. There are more modern solutions like permeable concrete or plastics (which have their own issues), but they tend to be significantly more expensive. Each 10’x10′ paved area generates 22 gallons of water runoff in the average Philadelphia rainstorm.
Trees also reduce stormwater runoff. Governments have a lot of articles about using trees to manage stormwater, as they tend to solve lots of other problems as well. The USDA forest service has a handout that notes that a medium size tree can absorb 2,380 gallons of rainwater a year. Using the numbers above for Philadelphia’s average rainfall, this medium tree would then absorb approximately all the rainfall from a 10’x10′ area. So, one medium tree for each 10’x10′ area of other impervious surface will allow you to break even.
Other vegetation can help as well. The extension service has a page on rain gardens, which are designed to absorb rainwater. This is a list of some plant suggestions (not all of these are natives). We’re in zone 7 here. There are low-lying areas which naturally collect water during storms that may be ideal for a rain garden. Here’s a walkthrough of building one. Hedges, shrubs, garden beds – almost any other planting than turf grass will slow water runoff more than a grass lawn.
More information is available at the following sources:
- Absorb the Storm from the University of Vermont
- Woody Shrubs for Stormwater Retention Practices from Cornell
- Plants for rain gardens by the Penn State Extension Master Gardener program