February 26, 2025, Planning Board Special Meeting Transcript - WEHI

5 of 46 sheets Page 17 to 20 of 115 03/03/2025 06:36:47 AM 17 guide they provide to engineers to design stormwater1 structures according to the standards. They have rules2 about how many test pits you're supposed to dig for3 every basin or underground structure you're going to4 build. It's mostly based on size of the structure. So5 you have minimal of two and then the larger the6 structure gets the more they want you to dig test pits7 for. As you can see in the first example I gave with8 porous pavement number 4 there could be a lot of9 variation. Lots of different test pits to make sure you10 capture the entire picture. You're not missing11 anything.12 We did note that not all of the -- not all the13 test pits were dug for all areas. For instance five,14 six, eight, nine and ten there were test pits for it but15 then 7 they should've dug three test pits, they only dug16 one, that isn't compliant. And then you're thinking17 Sean Walsh is being a stickler, there's only one, but18 again porous pavement number 4 there can be a lot of19 variations. Some of these are very large basins. It's20 important to be able to capture the entire story of21 what's going on underneath the ground for these basins22 so you don't have a mistake and you don't -- you make23 sure you have the capacity that you're designing for.24 So that actually notes that six of the 12 basins25 18 do not have the sufficient number of test pits they're1 supposed to have.2 All right. Next I would like to talk about3 groundwater mounding. Groundwater mounding is the4 process of, you'll have the seasonal high water table on5 this level here. During a rain event -- I'm sorry, I'm6 pointing using my hands. It's on a level, right. It7 rains, when you have a concentrated area of infiltration8 like a basin, it will mound a little bit. It's a normal9 process, the DEP has a form and it's a calculation you10 have to fill out to make sure that the mound doesn't get11 to far or perhaps get into someone's basement, I'm sure12 everyone's familiar with that, when it rains the mound13 rises. So they make you -- they provide a form, you14 have to fill it out and demonstrate that you're not15 mounding anywhere to cause any issues. Now, typically16 you have to do this for the water quality storm and, you17 know, the applicant provided all the forms for that and18 it was compliant. But what the applicant does with

use any issues. Now, typically16 you have to do this for the water quality storm and, you17 know, the applicant provided all the forms for that and18 it was compliant. But what the applicant does with some19 of these basins is they use infiltration as a way to20 keep the flow -- reducing the peak flow runoff rate. So21 that's, they're not just passing the water through pipe,22 they're pushing the water through the ground. So in23 those situations you also have to do groundwater24 mounding for a 100 year storm. The DEP wants to see not25 19 just a small storm but they want to see what happens1 during the larger storms. To give you guys a2 perspective, the water quality storm is about an inch3 and a quarter. The ten-year storm -- sorry. The4 100-year storm is eight inches of rain. So it's a much5 more significant amount of rainfall, so mounding can be6 significantly different.7 All right. One other section, porous pavement 38 from the applicants design plan.9 For the record, this is a diagram headed, Basin10 Q. Outlet Structure Number 3.11 It's a profile section of porous pavement number12 A. 3, sorry. So it's a cross section or a profile of the13 porous pavement system.14 So when you have a history of pavement or porous15 core stone, you have stone storage and now your soil16 replacement permeability and that soil replacement17 permeability ends at the seasonal high water table, what18 happens --19 When you say, ends, do you mean the bottom line20 Q. of the soil section or the top line?21 The bottom. The bottom of the soil replacement22 A. ends at the elevation of the seasonal high water table.23 Thank you.24 Q. Okay.25 A. 20 So what they did here was, when they did the soil1 borings they did all the soil tests, they measured2 infiltration rates as well, because they needed to know3 how quickly the water would go into the ground. And4 what they found was that this ground doesn't have great5 infiltration rates, which is not unusual or unexpected6 for this area. So what they chose to do is they chose7 to replace that soil. They dig deeper in the bottom of8 the basin and replace that soil with sand material9 that's a much higher infiltration rate. Allows the10 basin to go, to go down quicker and get rid of the water11 quality storm. Here's the problem, it wasn't the water12 quality storm, and that's why mounding is fine if you13 didn't have any issues, but the

in to go, to go down quicker and get rid of the water11 quality storm. Here's the problem, it wasn't the water12 quality storm, and that's why mounding is fine if you13 didn't have any issues, but the issue here is -- so the14 sand here you can see it says eight-inches per hour15 infiltration rate, which is fantastic. But when you get16 down to this soil that's when the soil infiltration rate17 becomes 0.25 inches per hour. This is effectively a18 wall. So what you need to demonstrate is that when19 you're infiltrating the 100-year storm you need to show20 whatever you're promising to infiltrate you can hold21 that water in here. That you don't have any, the22 groundwater mounding is not supposed to go into your23 storage. It's supposed to get down and not interfere24 with the storage of your basins.25