The Rock Creek bridges

MTA briefed the CCCT board on the Purple Line trail design plans at the board’s Sept. 13 meeting. I reported at Close, but still waiting that the briefing came up short on plans for the trail at the Silver Spring Transit Center. But there was much new information presented about the latest plans for other parts of the future trail. This post will address what’s new in the plan for the trail crossing of Rock Creek.

The old trail plan for this area is discussed at Access to a real park and is shown on the MTA website in the engineering drawing LPA-08.pdf. In summary that plan shows the trail as being 10′ wide and approaching from Bethesda on the north side of the Purple Line tracks, but at approx. 800′ west of Jones Mill Road the trail would be elevated and cross the Purple Line tracks on a trail bridge to the south side. The trail would then drop in elevation to run alongside the Purple Line on the south side, with both transit and trail crossing under Jones Mill Road in an underpass. The existing trail trestle at Rock Creek would be removed and the existing berm that holds the trail high above the Park today would be “shaved down” by about 15′, so that the top of the berm would become wide enough to carry the two Purple Line tracks across the Rock Creek stream valley. The trail would be held on the south side of the berm with retaining walls, about 20′ below the elevation of the Purple Line tracks. Both transit and trail would cross the Rock Creek stream on their own new bridges, then rise slowly on the berm as they continue east, to meet the current trail elevation at the east side of the park. There would be a long switchback trail to connect the future CCT to the existing Rock Creek Trail on the south side of the berm.

The major changes in the plan is that the newer plan has a 12′ wide trail and removes the trail crossover bridge that was to be west of Jones Mill Road. Instead, the trail would remain on the north side of the transit tracks, and would pass under Jones Mill Road and drop down to about 20′ below the Purple Line much as before except now the trail would be on the north side of the berm instead of the south side.

Under this new plan the trail will cross to the south side of the Purple Line directly over Rock Creek, at the trail and transit bridges. The trail bridge will be on a diagonal alignment below the transit bridge, so trail users will go directly under the bottom of the transit bridge. Once across Rock Creek and on the south side of the Purple Line, the trail would continue east on the south side of the berm as in the older plan. For this newer plan, the trail would be approx. 35′-40′ above Rock Creek at the bridges (compared to approx. 70′ above Rock Creek for the trail on the trestle now.) The Purple Line tracks would be about 20′ above the trail.

An image taken from the plans distributed by MTA at the meeting is below – my apologies for such a crude image. The trail is shown as dashed where it is directly beneath the transit bridge. The switchback ramp down to the Rock Creek Trail is shown on the lower right.

Plan view of future CCT at Rock Creek
(click on image for a large image)
source: MTA Plan and Profile – Trail, dated September 2010

MTA stated that a major reason for the change in plan was to eliminate the trail crossover bridge approx. 800′ west of Jones Mill Road. They did not elaborate. I believe it most likely the elevation changes required to bring the trail high over the Purple Line for that crossover, then bring the trail low enough to pass under Jones Mill Road, would be very difficult to achieve with an acceptible trail grade within that distance. The new plan reduces the change in trail elevation required in this area.

Guests at the briefing expressed concern that passing directly under the transit bridge would be unpleasant for trail users. MTA responded that they were committed to developing an attractive transit bridge design that would have a cover on the bridge understructure. The bridges that we see in lower Rock Creek Park when on the Rock Creek Trail come to mind.

An outdated MTA concept drawing for the Purple Line at Rock Creek.
(source: www.purplelinemd.com)

The concept drawing MTA presented for the older plan, shown above, might give us some idea what MTA thinks is visually appropriate for the Rock Creek Park crossing. This drawing is out of proportion in the vertical scale and does not show the trail bridge as being on a diagonal directly under the transit bridge, but it does give us an idea of what a covered, arch span bridge can look like in this location.

The devil is often in the details of execution for a concept, and that will be especially true here. The details of the design will determine if MTA can succeed in providing visually attractive bridges, and whether the trail can pass on a diagonal directly under the transit bridge while maintaining both good trail overhead room and good trail elevation above Rock Creek.

MTA also indicated they are considering a second major change for this area, i.e. removing the switchback ramp connector between the future CCT and the Rock Creek Trail. That is a big deal and deserves its own post, to follow on this blog soon.

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