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The tunnel will be kept as dry as possible through a combination of stormwater management, raingardens/bioswales, and pervious pavers in the shoulder area of the tunnel. The stormwater management involves grading slopes to capture storm water runoff before it gets to the tunnel and redirecting it to the raingardens or to storm drains that pipe water under Manchester Road. Additionally, the trail pavement has ADA-compliant trench drains on both ends of the tunnel that will help capture stormwater before it enters the tunnel/underpass.
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The pedestrian tunnel will be located at the intersection of Manchester Road and Mary Avenue.
The tunnel will be 14’ wide and 9’4” tall at the highest point. The walking surface inside the tunnel is approximately 12 feet below the ground level of Manchester Road.
The grade of land at Mary Avenue and Manchester Road is too high of a slope for a street-level crossing. The addition of a pedestrian tunnel allows for better ADA (Americans with Disabilities Act) access and improves safety by allowing those using the trails and paths to cross under Manchester Road. Not adding a street-level crossing also will help the flow of traffic on Manchester Road.
Manchester Road will be raised approximately 5 feet from the existing ground elevation to accommodate the addition of the pedestrian tunnel.
LED light fixtures will extend the length of the tunnel on both sides and will be lit 24 hours a day. Exterior lighting will lead into the tunnel at both entrances. Safety features in the tunnel also include at least one camera at the east entrance of the tunnel (closest to the new Brentwood Park); consultants and staff continue to discuss a way to add a second camera at the tunnel access closest to Manchester Road. The tunnel will be closed for use during any significant rain/flood event.
While one of the objectives of the Brentwood Bound Plan is to mitigate the base flood event (1% annual chance) along Deer Creek south of Manchester Road, the pedestrian tunnel will be approximately 12 feet lower than Manchester Road at the point of crossing, and it is possible for a flood event to exceed project design constraints. Check valves have been incorporated into the flood mitigation design to prevent storm sewer backups from the flood mitigation area into the pedestrian tunnel; however, a significant area tributary to Deer Creek resides north of Manchester Road. Storm sewers are typically designed to convey more frequent events, and when larger events occur, flood waters can sometimes be conveyed overland to their downstream most point. Because of the low elevation of the tunnel, it may accumulate some of this water during large storm events. Flooding can be extremely unpredictable, and because the pedestrian tunnel is connected to the new Brentwood Park and the flood mitigation area of the trail system – which will be subject to more frequent flooding – the tunnel will be closed off during any significant storm event.
More than 70% of the $4.8 million tunnel is being funded through grants from MoDOT, Great Rivers Greenway and the East-West Gateway Council of Governments.