Bandaid or cure for Alewife erosion?

The eroded Cottage Ave. entryway to the Alewife Greenway gets some TLC on August 31. View is east from Cottage Ave.

Badly needed repairs have restored the Alewife Greenway’s Cottage Ave. entry way but fail to address the underlying problem of rainwater runoff that had eroded the entrance. (Click any photo for a close up.)

The fixes were made as part of a last minute push to get the path in good shape by Labor Day.

Cottage Ave, where I live, drops 10 or 15 feet down to its end at the Alewife Brook. It becomes quite the sluice way when it rains, and rainwater runoff had cut a deep ditch into the stone-dust entrance.

Runoff has pushed the stone dust into a river-delta-like flow and carved canyons into the coarser gravel substrait. View is west, facing Cottage Ave., on June 25.

Furthermore, runoff water collects where the smaller entry paths meet the Greenway. The standing pools can last for more than a week.

The problem is even worse than it looks in the photo above, which seems to show how water has messed up the stone-dust pavement of the entrance path. There are two of these pathlets, curving north and south from the entrance to join the Greenway: the photo shows the northern branch.

When new in November 2010, the pavement only curved south. The northern arm was left unpaved. ESE from the Greenway.

The truth however is that the northern path (shown) was never actually paved at all. The sandy “pavement” in the large photo above is actually washed off of the southern branch, from where it curves away from the entrance.

The chunks of gravel in the mix are similarly washed from the foundation layer of the northern path.

The smaller photo above  shows where the pavement actually was before the water moved it.

To solve this problem, path planners had considered a comprehensive solution that entailed directing the runoff into the two drainage swales to the northeast and southeast of the path entrance.

Unfortunately, on August 31 (first photo above) all that happened was that the area in question was repaved (or, in the case of the northern pathlet, finally paved for the first time.)

This time the stone dust was wetted before it was rolled. This procedure, which was not followed back in 2010, activates binders in the dust and causes it to harden. The result is a very firm surface, while the binders last.

The result is a complete repair of two years’ worth of damage, but not a remedy for the problem that is causing the damage. Consequently, it seems likely that the damage will recur.

The stone dust is firm and hard when flat and dry, but behaves in some respects like a liquid when wet.

Think of the path beds, excavated to below grade, as trenches filled to the brim with pavement. If the path is flat and its sides are intact, wet pavement will not move anywhere. If the path is inclined there will be erosion over time.

There are a few places where small flaws in the side of the trench are easy to spot, because of the lines of stone dust leaking down to the Alewife.

This blog is about Mass. Ave. but the story of the Alewife path near my house on Cottage Ave. has proven an irresistible detour for me.

If you are similarly distracted, you may enjoy the previous Alewife post or even the whole thing starting with the first post from July of 2010; the next report is here.

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