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Thread: Impervious surface and stormwater management

  1. #1
    Unfrozen Caveman Planner mendelman's avatar
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    Impervious surface and stormwater management

    We've been getting many teardowns here recently (well, like 150 a year out of 17,000 single family pacels) and there have been many cries raised that the stormwater runoff from these properties, which usually involve more lot coverage than before, are affecting the infiltration into the soil and draining into neighboring properties.

    Now, I understand that stormwater drainage/management is mostly a product of engineering and grading, but I question is: Does the level of impervious surface at the individual lot scale really have an appreciable affect on poor versus acceptable management?

    Especially when one learns that typical lawn sod is not really very pervious and not mush better than a dirt road.

    What says cyburbia?
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  2. #2
    Cyburbian Coragus's avatar
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    Quote Originally posted by mendelman View post
    Does the level of impervious surface at the individual lot scale really have an appreciable affect on poor versus acceptable management?
    Yes. When calculating the amount of runoff a parcel will allow, engineers figure something called a runoff coefficient into their calculation. I don't have the book in my office that has the more complete list of coefficients, but in general, forest allows 10-20% runoff, so forest's coefficient is 0.1 to 0.2. This goes up depending on the intensity of land use until you get to an urban setting with a paved lot, which I've seen associated with a coefficient of 0.95.

    A good lawn will have a fairly low coefficient, but mostly bare, hard packed dirt will not be much lower than pavement. I can witness this in my own yard. The lawn doesn't allow any runoff, but my gravel driveway puddles and streams.
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  3. #3
    Cyburbian ludes98's avatar
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    Runoff will increase if you are adding more impervious surface. My guess is that it is compounding an already poor runoff/drainage system. Older developments likely weren't required to have the same pre/post calcs required today. If you allow on lot detention/retention you could require this as part of permitting a knockdown though it is only reasonable on large lot development.

  4. #4
    Cyburbian
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    The problem with stormwater detention at the individual residential lot level is the flows are so low, you would have to drain the lot through a drinking straw to get the required release rate. You would be much better off to collect several lots into one area, but in an allready developed area this may be hard.

    I would look for ways to slow runoff down without a detention area. One way is to make sure roof drains drain into the yard. If they drain to a concrete drive or are piped directly to the curb or connected directly to the storm sewer you will get a lot more water then if they have to run through 50 feet of grass first.

    Another way would be to recomend pervious surfaces for driveways. Look into pervious concrete or asphalt. Opinions are mixed on these, but it may be something to investigate.

  5. #5
    Unfrozen Caveman Planner mendelman's avatar
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    Thanks guys for the responses. I think in many of the already built areas, the problem is mainly poor existing drainage/management systems.

    But in an already built-out locality are the runoff coefficients really useful? I mean the increase of impervious surface coverage is so slight for teardowns that I don't really see an impact. Sometimes there is less impervious surface - old house with expanses of driveway pavement to detached garage replaced by larger footprint houes, but with front loaded attached garages and much less driveway pavement.

    I do agree that the coefficients are useful for new subdivisions when the increase of impervious surface can go from practically zero to 60-70% of development site (with roads, sidewalks, driveways, houses, etc.)
    I'm sorry. Is my bias showing?

    Let's not be didactic in this profession, because that is a path to disillusion and irrelevancy.

    Six seasons and a movie!

  6. #6
    Cyburbian Jeff's avatar
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    Is there not a pre vs post reduction requirement?

    Its pretty standard SWM practice to not allow an increase in runoff, and most places actually require you to reduce the post to a fraction of the pre.

    In theory, even with a bare bones SWM ordinance, your surface runoff should be less.

    How are they calculating the pre-developed hydrology? With the exising building before they tear it down? if so, this could be your problem. for a conservative number, never allow impervious surface into the computation of the pre-developed CN.

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