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

9 of 46 sheets Page 33 to 36 of 115 03/03/2025 06:36:47 AM 33 like that.1 MS. MCGOVERN: Okay.2 All right. Now, time of concentration is, as I3 A. said, how water flows through to get to the end4 discharge location. Water flows in different ways.5 This sheet flow is the shallow concentrated flow. Sheet6 flows is, as you can imagine, if there's a field and it7 starts to rain, kind of the water moves like a sheet8 along the ground. Now, as the water progresses and it9 builds up it will actually start to consolidate and it's10 called, something called shallow concentrated flow.11 Now, the DEP has rules on how you calculate that.12 That's all outlined out the Chapter 5 in the DEP manual.13 And what he found is when we looked at the rate the14 applicant calculated was, unfortunately it seemed like15 they had used an older version of how to calculate time16 of concentration. The rules and methods that we're17 supposed to use to calculate time of concentration has18 changed over time and unfortunately from what we can19 tell is that they used the older version. And this20 older version generated a lower time of concentration.21 So let me give you, I want to use the shallow22 concentrated flow as an example, and kind of a dead23 giveaway is you see here the shallow concentrated flow,24 unpaved. Now, it wasn't that long ago but there was an25 34 unpaved option where you would basically take the slope1 of the land and you'd say it was unpaved because it's2 forest and it would give you a flow rate. They don't do3 that now. Now they have a more complicated chart and4 like it tells you exactly which land it is. Is it a5 forest, is it grass, is it farm field. And based on the6 slope they will give you a specific flow rate.7 If I can ask you one question. So it's not a8 Q. generalized unpaved area flow rate. There's a specific9 flow rate based on the slope and the specific type of10 topography?11 Yes.12 A. Okay. Thank you.13 Q. Yes.14 A. All right. So what I did was I went back and I15 went to the manual and I decided to recalculate the16 shallow concentrated time. Their shallow concentrated17 time for this area was 7.2 minutes. I went back and I18 used the charts and I said, all right, this is a19 forested area with heavy litter, it's got the same20 slope, 5.4 percent, and using the chart that says it's21 got a velocity of 0.55 feet per second. And you divide22 1,625 feet

all right, this is a19 forested area with heavy litter, it's got the same20 slope, 5.4 percent, and using the chart that says it's21 got a velocity of 0.55 feet per second. And you divide22 1,625 feet by 0.55 feet per second, you get 2,95423 seconds, or 49 minutes. Now, what they're saying is24 that it's 7 minutes, but based on the new method it's25 35 taking 49 minutes. Which means it's about seven times1 longer than what they're saying.2 Now, I did this again for POA-2 -- sorry. Let me3 take a step back.4 There's this drainage area and then there's5 another drainage area over here. You can tell by the6 distinction of the two different blue lines which is7 your time of concentration. We also did it for POA-28 using the shallow concentrated flow.9 And that's on the right side of the chart?10 Q. That's on the right side of the chart. Of the11 A. plan.12 And so it's, they have 564 linear feet at unpaved13 9.2 -- 9.2 percent slope. According to their model and14 their calculation that takes 1.9 minutes. Again, I went15 back to the chart and the manual, I recalculated that16 velocity for a heavily forested area, add 9.2 percent17 slope and what I got was 12 and a half minutes. Again,18 that's about a seven fold increase in time of19 concentration.20 All right. Now, you're probably wondering why21 time of concentration is different. Now, what does --22 how does that impact the design. The shorter the time23 of concentration typically the higher the peak flow rate24 you have. The water is traveling a certain amount of25 36 time, so the longer it is, the lower your peak flow1 rates are. Now, it's really important to establish the2 correct peak flow rates of the existing conditions,3 because that's going to set the benchmark for what your4 designs going to be. Because the DEP will require you5 to reduce your runoff. Your peak flow rate from your6 design -- sorry. DEP is going to require two year peak,7 flow runoff rate is going to be 50 percent less than8 what is existing. For the 10-year storm it's going to9 be 25 percent less. For the 100-year storm it's going10 to have to be -- 10-year storm is 25 percent less.11 100-year storm is 20 percent less. So now, thing about12 peak flow rate it's easier to meet those reductions the13 higher your rates are, because it's just more water.14 You don't have to move much water, you don't have to15 have a big basin. I always

out12 peak flow rate it's easier to meet those reductions the13 higher your rates are, because it's just more water.14 You don't have to move much water, you don't have to15 have a big basin. I always hope, if you do the math16 right, the bigger the existing flow, it makes my life17 easier. But see here because they're miscalculating18 we're already getting miscalculation peak flows in19 existing conditions. So we're risking not reducing the20 flow as much as we should be.21 And what does that translate to in terms of22 Q. controlled outcome?23 It means more water is going to come off the site24 A. than it does and you can risk flooding further25