2021-02-01-CONCOM-min - Laserfiche WebLink

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Alex Dohan—Aye Ruth Ladd—Aye David Langseth—Aye Holly Samuels —Aye Phil Hamilton—Aye 7:19 PM New Business/Pendine Matters, Continued Informal Meeting: Town Engineering Project, Drainage Improvements and Kiln Brook Daylighting Info Session- Environmental Partners Mr. Matthew Weisman, Senior Civil Engineer for the Town of Lexington, and Mr. Erik A. Kelley, Project Manager for Environmental Partners, presented before the Commission the Kiln Brook outfall and drainage improvements. There is roughly a 135 foot difference in the highest point on Massachusetts Avenue to the drainage outfall and currently the drainage system can handle a two-year storm event. The project team is proposing to use a daylighting approach by opening up a portion of the culverted stream to an open channel stream. By doing this it will create an increased carrying capacity and the opportunity to restore a portion of the stream and abutting wetland. They are also proposing to increase the size of some drains on Constitution Road to alleviate flow restriction and create a relief drain. Varoujan Hagopian, GEI Consultants, stated that the proposal is to use geotextile material and riprap stone to slow down the velocity of the flow and to prevent future erosion of the channel. Questions and Comments from the Commission: — How aggressively has the team searched for modifications to reduce flow upstream? — Would be nice to see more curvature in the design. — Is future maintenance considered in the permit submission? — How big is the box culvert? — How temporary is the construction easement and what is there now? — Big proponent of ecological restoration and recommend live stakes in the winter months. Would recommend more of a habitat within the rip rap channel. Response from the Representatives: — With the amount of overland flooding that the model is simulating, it would take roughly 5,300 drywell systems in order to contain that amount of flow leaving the drainage system. The topography is extremely challenging and the project team did look at upstream modifications to some degree but it is just not feasible. — The abutting properties being higher in grade limits the ability to go higher and are working with a 25 foot easement but will try to design more curvature. — Future maintenance is a top concern. Looking to put a distribution box in the culvert for sediment control and access for the Town within their easement will be easier. — The existing culvert is a 36 inch diameter reinforced concrete pipe. The proposed 3x5' box culvert has elements on the channel bottom to dissipate channel energy.