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Yuse Lajiminmuhip: Hello and welcome to the AGRU America podcast. My name is Yuse, and joining me again today is Chris Richgels, a civil engineer with over 24 years of experience in solid waste engineering. In this episode, we’ll be exploring part two of our series on risk-based design of geosynthetics.
Announcer: Every month, the plastics experts at AGRU explore how geosynthetics are changing the world around us. Discover more about AGRU at agruamerica.com or by calling 1 (800) 373-2478.
Yuse Lajiminmuhip: Hi Chris. I know we touched on this subject in our last podcast, but could you go over what it means to design for a 100-year event?
Chris Richgels: A 100-year event, statistically speaking, is an event that has a 1% chance of happening in any given year. I try not to call it a 100-year event because that tends to confuse people. They think, “Oh, the 100-year storm happened and I don’t got to worry about it anymore.” Not the case. And the reason being is, if you look at the hydrologic history of a rain station/weather station that goes back hopefully 100 years (Not all here in the States do that), you will see some pretty significant events can happen within a couple years of each other. Regardless, you take all that storm weather data from that rain gauge and go through what’s called a log pearson analysis to generate statistical predictions of when a storm of a certain magnitude will happen or has a 1% chance of happening in any given year. It’s what you require basically as data, and hopefully a lot of it. The more you have, the more reliable the prediction is.
Yuse Lajiminmuhip: How does the design process typically start? So once you have the data, what are some of the initial steps?
Chris Richgels: It depends on the site location: stormwater issues, runoff, stormwater control, run-on, that kind of thing. Once you have that location, then you start searching around in that general area for weather stations that had been recording rainfall data. Not all of them have a lot of years of data. Tipping-bucket gauges will record rainfall data every five minutes, and from that you can start getting peak concentrations within a five-minute period which are usually pretty high. Rain will fall in bursts — If you have ever been out the rainstorm walking through it or driving through it or whatever — It’ll be sprinkling then, all of the sudden, it’ll dump and then go back to kind of a nominal rainfall. It’s those quick bursts that can cause a lot of damage and that’s what the engineer has to design for.
Yuse Lajiminmuhip: How do you design for this? So, now you know that you have an idea of how much rain is going to fall and you have an idea of the temperature and the maximum wind speed in that area. How did these items affect the design?
Chris Richgels: That peak rainfall (it’s called 5-minute intensity) will dictate how big your drainage infrastructure needs to be (or ditches, oversight drainage, channels, outlets to stormwater basins). Those kinds of things are dictated by that burst intensity and another thing that’s called time of concentration. Basically, it (time of concentration) describes how long it takes from that burst of rainfall to run from the top of the landfill to a drainage ditch, then along the drainage ditch to a catch basin, and the catch basin through a pipe to a canal retention basin, for example. We estimate time to run along items like that will determine what the peak water flow is at any point along the drainage infrastructure. Other items, like you said, are wind. In an exposed geomembrane, wind can be very critical. It starts blowing along and air collects underneath the geomembrane and will start to form pockets, or pillows, and then the wind will just start to rip those apart. You need to know what the maximum wind speed is in your area. A lot of that has already been established, as far as peak wind gusts, by the American Society of Civil Engineers. It’s a design manual just called ASCE 7. There’s that basis that you use for wind design and rainfall intensity.
Yuse Lajiminmuhip: Speaking of geomembrane, how do the findings — how do the data — from calculating and analyzing the past weather events inform product selection?
Chris Richgels: For product selection, depending on the intensity of the rainfall and the runoff or the resulting runoff, it will dictate what products you will have in your drainage ditches: Whether it’s grass-covered soil, or if you need to go to something more robust like a concrete lining or an artificial turf with sand-cement lining. Those last two products take quite a bit of water flow. High velocity water flow and hydraulic shear stress: That’s two of the items that comes out of that design. Having known what the rainfall intensity is and how high the flows are going to be through those channels.
Yuse Lajiminmuhip: I understand. AGRU offers specification sheets for all of its products. As an engineer, how would you use this information to determine whether or not that particular product meets your design specifications?
Chris Richgels: If you were to use AGRU Drainliner or Super Gripnet on the slope of the landfill, for example, there are specifications that are on the available on that particular product line, called the integrated drainage system (IDS), that provides index transmisivity values. From that, you can start calculating how long your side slopes can be; how it would handle infiltration through the soil cover into the IDS matrix. Before you reach the maximum capacity of the IDS system, we’ll tell you how long the slope needs to be. The other side to that is, you need to know what kind of soil that you’re going to put on top of the IDS system, because adding on the permeability of the soil also dictates how much water is flowing into the IDS drainage matrix and how much surface flow is on top of that soil cover. So there’s items that you need be aware of there.
Yuse Lajiminmuhip: That’s very much site specific.
Chris Richgels: Very. When it comes to issues like that. This is a very site-specific design. There is no one-cookbook design that you can apply to any site anywhere. You’ve got to know what it’s going to be exposed to.