June 4, 2025, Planning Board Regular Meeting Transcript - WEHI

7 of 41 sheets Page 25 to 28 of 101 06/20/2025 06:55:03 AM 25 eastern drainage area, the lighter green, is that1 correct?2 THE WITNESS: That's correct.3 So with the development what's happening, this4 A. blue indicates the current surface flow patterns towards5 the wetlands. Pretty much follows the topography high6 to low and the wetlands occupy the lower landscape7 position on the site. And this is a typical headwater8 wetland system where this is the beginning of Canoe9 Brook. Both streams are at the headwater of Canoe10 Brook. But with the development you see that that11 current pattern now is modified by man, it's designed to12 collect stormwater and be directed towards stormwater13 facilities or best management practices. This features14 stormwater basins, whatever it might be for treatment,15 and ultimately discharged. You can see these pink16 arrows are collecting and ultimately discharging well17 below the wetlands on the site. So it is modifying that18 flow path of surface flow to the wetlands. You could19 see in the western wetlands about 32 acres, it's reduced20 by nine acres, roughly 28 percent of the flow that goes21 there.22 And as I was saying before, these are forested23 wetlands that tend to be a dryer end of a wetland24 contiguum. So the reduction in surface water flow has25 26 an impact.1 The eastern wetlands are similar. 48.4 acres,2 it's going to be reduced by 16 acres. So 33 percent of3 the existing drainage area.4 Just another depiction of the post development5 drainage area. You could see where the drainage area6 will be reduced, the same with the eastern drainage7 area.8 In addition to surface flow paths, the9 groundwater flow paths are also indicated. And10 generally, as I was saying, that groundwater hydrology11 is moving over those restrictive horizons, in this case12 the fragipan, and it's moving down slope towards the13 wetland towards the stream. And at a minimum it's being14 daylighted or it's discharging to the surface at or near15 the stream.16 And here's an example of one of the impacts. So17 this is the wetland, this is Basin 7, one of the18 infiltration basins. So the basin's bottom is proposed19 at 532. The wetland elevation is 540. So the20 infiltration basin is eight feet lower than the adjacent21 wetland. And one of the key issues here is that22 groundwater flows towards the wetland. With this part23 of their

elevation is 540. So the20 infiltration basin is eight feet lower than the adjacent21 wetland. And one of the key issues here is that22 groundwater flows towards the wetland. With this part23 of their development it's changing the hydraulic24 gradient from the water going to the wetland to the25 27 water in the wetland draining towards the basin. And1 there's two things here that are important. One, and we2 said it before when Sean Walsh testified, somehow their3 testimony missed the fragipan. Missed the restrictive4 horizon. But this is an issue defined and well looked5 at here.6 Here's the same basin.7 And the groundwater elevation, they have the8 estimated seasonal high elevation at 519. Well, go9 back, their's 540, wetland elevation 536, it's like 1510 feet lower than the wetlands. One of the things to11 remember in those sites is that wetland hydrology should12 make sense. That groundwater flow path that follows the13 contours, follows the topography towards the wetlands,14 it's just a natural way that these wetlands function.15 To find a seasonal high water table below the elevation16 of a wetland, and not by feet by more than ten feet,17 makes no sense. That water here from that seasonal high18 water table is not flowing uphill. So this basin does19 two things, one, it makes no sense. Partly because it20 is lower than the wetland for a seasonal high water21 table.22 But two, the designer would have to take into23 account the water from the wetland and the stream are24 now being redirected towards the wetland. That's an25 28 additional amount of water in the basin that impacts1 function.2 Again, you can see this is the high water table3 here, 528. 522. 519.4 Now, whose seasonal high measures are those?5 Q. These are the applicant's soil investigation.6 A. Thank you.7 Q. And these are just from the applicant's plan.8 A. The red is what I added to make it easier for you and9 the audience to see.10 Same basin, this is Basin 7. Again, here's the11 wetland, 534. This stormwater basin estimate seasonal12 high is 519. It's only 50 feet away and it should be up13 slope. The water should be flowing towards the wetland14 and in this case it is indicating that the seasonal high15 water table is 15 feet lower than the wetland. Once16 again, it just doesn't make sense.17 Basin 7, same idea. Basin bottom, 544. Wetland,18 546. Here is 548. 550 contour. Basin bottom,

easonal high15 water table is 15 feet lower than the wetland. Once16 again, it just doesn't make sense.17 Basin 7, same idea. Basin bottom, 544. Wetland,18 546. Here is 548. 550 contour. Basin bottom, once19 again, is below the elevation of the wetland.20 And the test pits show a seasonal high water21 table of anywhere from 537 to 542. And we'll go back,22 sorry, and look at the wetland elevation. 546. 544.23 So, again, the natural path of water in the groundwater24 should be towards the wetland. It should be a gradient25