Tuesday, August 13, 2013

Why Norristown Floods

During this morning's storm, 10 inches of water swept along and over the curbs of my street. I assume Elmwood Park flooded--Stony Creek always does in this kind of weather.

Norristown was built on hills, with two creeks--Stony and Saw Mill Run--at the bottom. Much of the hill between the 2 streams is paved and developed. Rainwater is only absorbed by lawns and gardens. So when we get a torrential downpour, much of the rain that falls on that hill drains down the side streets from Dekalb to each creek.

I got to thinking about that proposed development at 1202 Dekalb, where 24 housing units with no yards would replace 2/3 of an acre of grass and trees. I wondered how much rainwater that parcel now absorbed.

I found a formula online: Every 1000 square feet of grass absorbs about 600 gallons per one-inch rainfall. The parcel at 1202 Dekalb is over 29,000 sq. ft. That's about 18,000 gallons of runoff per one-inch of rain.

The storm this morning lasted over 2 hours. I don't know the official measure yet, but I'd guess we had about 2 inches of rain. That means, if the houses at 1202 Dekalb had already been built, 36,000 extra gallons of water would have drained down the hill into one of our creeks. Or into the basements of residences along the way.

Remember the house on Penn St. that collapsed earlier this year? Saw Mill Run goes underground in that area. Before the dam on Fornance was built, East Penn, Main and Lafayette Sts. flooded in every big storm. The houses down there always have basement flooding problems. A wet season was enough to undermine the foundation of one house this time. Keep paving uphill from there, and you'll have more collapses.

Ironically, Norristown's website has a page about Storm Water Management. Here's a quote: "Direct all downspouts away from pervious surfaces and onto lawns." (The word "pervious" is actually not correct here, but we'll assume they mean "nonabsorbent.")  Here's another quote: "Rain Gardens or Grassy Swales: These specially designed gardens can be planted with native vegetation to provide an area for rainwater to collect and soak into the ground. Stormwater from rooftop drains and pavement areas can be directed to these vegetated areas."  Except that, lately, zoning variances are doing away with lawns and vegetation.

And so, another plea for people to show up to the Planning Commission meeting tonight. (Municipal Hall, 7 pm).  If you let the rain keep you away, the next flood could be on your property. Even if you live at the top of the hill, they'll use your tax money to fix the damage.

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