Nantucket people may come to expect snowstorms near the end of January now that we’ve had another. The relatively heavy snow and high winds brought us out to the Sankaty Bluff once more to see the effects of the weather on the geotextile container installation and the bank itself. The storm and the recent heavy rains have ravaged both. The SBPF has upped its efforts considerably, bringing in prodigious amounts of pit sand, and nature appears to have responded by easily taking away much of the increase.
As it has always been, the threats to the bluff are from the sea below and surface and subsurface runoff from above. When one looks above the sand envelope over the geotextile containers, one sees the effect of rain runoff over the edge and runout through the bluff face from the adjacent wetlands. It’s considerable and dramatic. The work to put a new cover of sand, netting and grass has been rewarded with shredded netting, disappearing grass and washouts huge and small. When one looks below on the beach, one sees tiers of geotextile containers exposed by the relentless pounding of the storm surf. In places these two forces work in concert, the runoff channeling great amounts of sacrificial sand from above directly to the waves.
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Ironically, the two locations for the erosion viewing pen have been at the points of greatest destruction. At both locations, they needed to be moved as the ground washed away. At the left of the photo, the netting, grass and underlying sand have slid downhill. None of this was prevented by the geotextile containers below.
Although a fourth tube has been added in front of the cottage “Swept Away”, the facelift is falling anyway. The bluff had been evenly covered, but soon began losing sand beneath the netting and sagging noticeably. Now the recent work has partially collapsed.
The loss of sand and grass from top to bottom in front of “Swept Away”.
The bank to the north of the pre-’78 structures is not faring much better, with more loss of sand and grass and heavily damaged netting.
Crevice or canyon? This impressive washout originates at the viewing pen and continues deeply through the sand envelope to the beach. As deep as it is, it has not reached the depth of the tubes. None of this washout is due to wave action. It’s entirely the result of runoff from the top.
A shot from inside the crevice to lend human scale to the washout.
The entire length of the geotextile container installation wiped clean by the ocean.
Purely from runoff.
The north end of the geotextile containers with the recently installed return bags angled toward the bank. Given that these all were completely covered by the applied sand envelope, it’s easy to imagine the enormous amount of material carried off by the ocean.