We are excited to share that the City of Concord is replacing the culvert next to our parking lot for the Merrimack River Floodplain trails. The Mill Brook flows through this culvert, under Portsmouth St., and then out to to the Merrimack River. This City knew this site was going to require some creativity in its new culvert construction. The Mill Brook frequently floods in the spring, and backs up around (and overtops) the existing culvert. In addition, the soil here is deep glacial till which doesn't hold structures very well. The soil, being any and all sediment deposited by the long-ago glacier, is a jumble of grains that range from tiny clay particles to large boulders. And our Portsmouth Street till is saturated with moisture, meaning that it is ultimately an unstable surface if differing amounts of pressure are applied to different areas. Traditional culverts do just that: they are a three-sided structure with two vertical sides and a roof (on top of which runs the road). Under each of the sides are placed large, flat, heavy footings to prevent the culvert from slipping in the soil. Here, those footings just weren't going to work, given our soil and moisture conditions.
So, the City and DES put their thinking caps on and designed a new type of culvert. In essence, it is a traditional culvert structure, just flipped upside-down so it looks like a U. (A "U" with a bottom that is 16' long and sides that are 8' high.) Mill Brook will be temporarily diverted, and the streambed will be carefully excavated, with all the materials saved for later. The culvert will be placed into this hole, and much of the bottom will be covered with the exact streambed materials that had been removed. In addition, the City will be placing local stones that are carefully selected for flood control and aquatic wildlife habitat to add in to this new streambed. Lastly, the new culvert, which comes pre-made, contains a small wildlife shelf upon which animals like raccoons, turtles, skunks, and foxes, can safely cross the road. This culvert design joins a larger effort across New England to make better wildlife culvert crossings, and we are very pleased to have it here. Once the upside-down U is in place, a flat, strong surface will be placed on top to cap it off and serve as the road.
If you are interested to learn more about the City's process and design, you can check out the documents included here. Construction will begin on August 12 and continue through at least the first week of October. During this time, the road around the culvert area will be closed. Visitors will still be able to access the trail parking lot from the East Side Dr. side, and they will also be able to reach the trails through the access road entrance on the Portsmouth St. side. If you are able, please consider parking at the SPNHF lot and walking down to the river trails from there, to minimize vehicle traffic next to the construction. Please leash all dogs in the parking lot for their safety.
If you have questions or concerns, you may contact Carl Murphy at the Forest Society, 224-9945, ext. 339. We look forward to having this new culvert installed, and we are very grateful to the City of Concord and the Department of Environmental Services for taking such care in the creation of this culvert design!