Thursday, October 07, 2010
The road was probably built on the back of the beach a century ago. Sea level is probably 20-25 cms higher since then, and the big house on the artificial point just north has trapped any sediment originally destined to rebuild this beach, so the road was getting undermined.
They could have dumped rock - such an easy, cheap, ugly, and often not completely successful solution. But instead, they tried anchoring logs on the beach face and reinforcing the soils on the bank with geofabric. I don't know the whole story here, but I wish we had the resources to monitor more of these (other than brief visits while racing to the late afternoon ferry). We could actually start to learn something. And then we could tweak the design, not just bury it under riprap next winter when the storms roll in.
I believe the logs are intended to act like sills and help perch a slightly higher beach behind them, reducing wave action at the bank. Sometimes this approach works, but often it does not, with beaches rising and falling as if they are oblivious to the logs' intended role. Here, I don't think the logs have hurt, I'm just not sure they've added much.
The geotextile is "softer" than riprap and allows for planting if it doesn't unravel, but it has neither the resistance to erosion (and floating logs) of something more durable, nor the ability to act like a natural gravel berm. I suppose a more substantial gravel berm might have been worked a little better here, although there might still have been need for some structural elements and some occasional maintenance - in the form of a truckload of sandy gravel every couple of years. But ultimately, there just isn't much room to work where the beach is narrowest. These sites are never easy.