Retention & Detention Ponds Concepts

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    David Kaplan

    My first recommendation is to read about them in the Site Planning & Design Handbook if you have that source available.  They have good explanations, pictures, etc. 

    In a nutshell:

    -Your definitions are basically correct.  Retention ponds keep water in and utilize evaporation over time to get rid of it.  Detention holds it in place temporarily until the rest of the designed management system can assist with its removal.

    -They're typically used when you have a large development project, like a big shopping complex, maybe a big residential complex - anything where the built environment is significantly taking place of existing site areas, so much so that you need to manage the stormwater runoff in this manner.  I.E. just doing your typical catch basin design isn't going to cut it anymore because you've done such extensive building and site improvements.  Lots of stormwater to manage from building roofs, no where for it to go really.  That's the watered-down definition - pun intended.

    -With any project, the goal of successful site planning is to minimize the amount of site disruption, so in theory, locating these in areas that are already near low points in the topography would make sense because you then utilize the natural drainage design of the property and locate these where water is already flowing.  They can require a good amount of excavation, so you wouldn't want to locate them where bad soil is or bedrock. 

    Best I can offer.

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    Darguin Fortuna (Edited )

    Thanks a lot David.

     

    Here is the thing. I have read a lot on the Site Plan and Design Handbook (I have all of the resources for all ARE exams thought it would be worth it and it has until PA came along.) The handbook says that putting them at the bottom might be a huge issue for the neighbors. Also the images you mention are black and white pictures that tell me little. See images. I have noticed that many books group detention and retention basins or ponds under the same sections and even the same sentences which is not entirely how it works. You very well put together reply also makes their location be relatively the same but my assumption is retention ponds can be at the bottom of our watershed but detentions should not be given that they might get overcharged. I am completely confused on this maybe so please David help me on this. Thanks a ton for your reply. Exam day is just a few days away for me.

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    David Kaplan

    I guess my thoughts on this are - this level of minutiae is not going to be required for the exam.  Know what they are and how they are used in projects, and you should be good to go.

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    Darguin Fortuna (Edited )

    I clearly understand the concepts but I am not as skilled into how to locate them and so forth. In my previous experiences with PA (Multiple rounds) I vaguely recall more depth than just the concepts. They indeed want us to be a pond experts. :) I love ponds tho.

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    David Kaplan

    I hear you.  My experience on the PA test was not that, at least in terms of this specific issue on ponds. Sorry to hear that you've had trouble with the PA exam, hopefully you are not letting it get you down too much and are continuing to plow forward.  I think you want to go into this exam with the main concept in mind that the goal of successful site planning is to disturb the existing conditions as little as possible and to use what's existing to your advantage.  This is reinforced throughout the Site Handbook ad nauseum, and I found that questions I received on the exam were answered with this guideline in mind.  The excerpt you provided above basically says that there may be limitations with where it can go, so that doesn't really help you.  To that point, I mentioned above that putting it at the low point of a site might be good, but maybe that's where bad soil conditions are or buildings are.  There's limits to any situation.

    I might recommend talking to a civil engineer, or even googling "retention pond design" to see if you can find more tips on this.  I personally did not experience significant coverage on this topic for my PA test, but I know there's multiple versions. 

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    Justin Pelland

    Darguin,

    I think some of the specifics discussed in that excerpt are a little too detailed for the knowledge you would be expected to have on the exam. Where exactly a detention or retention pond gets located is based on a number of very specific factors. I think what the book is trying to say about low vs high, is that you have to be careful where you site them so that the rate of discharge matches whatever it was before the development was in place. That's really the key there. On a rural or suburban site with no municipal storm system, you would be worried primarily with the runoff of stormwater from  your site onto an adjacent property. If, for example, your site was sloped towards a neighbor, then there would be some volume of discharge that has been present prior to development. If you were to come in and re-grade the site in a way that now channels significantly more storm water towards that property, you might need to consider placement of a detention or retention pond so that someone standing on the other side of the property line would see no significant difference in the volume of stormwater being discharged onto their property.

    In an urban environment, the problem is often much different. Where municipal storm systems are provided, many times you'll find these systems have a maximum capacity of storm water that they can handle at any given time (typically based on pipe diameter, age of the system, and the volume of storm water discharge coming from upstream). If you were to develop an urban site and maximize the lot area with the building, parking, and other impervious surfaces, you would now need to capture more stormwater than would have existed previously. The city might tell you the system at your location can handle a maximum discharge rate of 1X, but your site produces 2X on a 20-year, 50-year, or 100-year storm (whichever you're required to design to). Since your site would be putting twice the stormwater into the system per hour than it can handle, you would need to incorporate a retention or detention system. At that point, it's just a holding tank with a fixed discharge rate that ensures you can capture all the water and put it into the municipal system at a rate of 1X.

    I would be surprised if you were given a question that asked you to choose a location for a detention pond or retention pond as there are a lot of factors that go into it. You might get a question showing an existing site topo and a new building site topo that asks you to identify the area where drainage will now be a concern, or to identify when stormwater management strategies might be necessary, but I don't think you'll be expected to know how to design them specifically.

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    Darguin Fortuna

    Thanks Justin and David. I am pushing as far as I can. You guys are correct this shouldn't be in the exam but yet it is. :( I have spoken with multiple engineers and landscape architects. They seem to disagree on some concepts as well. Hence me being puzzled. 

     

    I am not discouraged by my 3 fails. I am actually closer to a pass than ever so in a few days we will see how much closer I can get the Football. I might score a touchdown. 

     

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    David Kaplan

    I agree with Justin's comments entirely and too, would be surprised if you get a higher level of question.  Hang in there!!!

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    Mary Dietsch

    Hi Darguin!

    I can't really add to the helpful comments already posted - but just wanted to take a minute to wish you luck!  I took PA yesterday and happily did pass but I also came out of it feeling like I needed to be an expert in a few areas that I definitely am not!  I have quite a bit of experience but not much of it is site related.  I think what saved me was just staying calm and moving through the exam without panicking. There will be plenty of questions that you do know - trust in that and focus on getting all of those right!

    I did review the ARE 4.0 site planning handbook section prior to my test and thought it was helpful.

    Best of luck!

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    Darguin Fortuna

    I took PA yesterday and passed. I think the most important feedback I can give people is to READ THE QUESTIONS MANY TIMES and then answer them carefully. The answers tend to become easy after you clearly understand what is being asked. This exam experience was by far my most enjoyable. I felt like it was an Allstar game and I was ready for the show. I must say the community deserves most of the credit. Folks like David, Mary, and Justin among many others make this often times lonely process a bit more fun. Onto my last exam now. :)

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    Mary Dietsch

    Nice job Darguin! Glad to hear you passed!  Which test is your last? I am studying for PPD now and hope to have both that and PDD done by March 28th - just booked a dive trip to Cozumel leaving March 29th hopefully to reward myself for completing this process!

    Good luck!

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    Mary Dietsch

    Nice job Darguin! Glad to hear you passed!  Which test is your last? I am studying for PPD now and hope to have both that and PDD done by March 28th - just booked a dive trip to Cozumel leaving March 29th hopefully to reward myself for completing this process!

    Good luck!

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    Kurt Fanderclai

    Darguin --  CONGRATULATIONS!

    I shot you an email.

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