Monday, December 31, 2012
The west side of San Francisco slopes down to the ocean in rolling sand dunes, now stabilized by the endless residential blocks of the Richmond and Sunset neighborhoods. The sandy beach is all that remains of the original system, cut off from the city by the Great Highway and the impressive recurved seawall built to keep the Great Highway where it was by built.
This is a complicated beach that has experienced long cycles of erosion at one end or the other for the past century. Climate and wind patterns shift sediment back and forth as they do on most beaches, but there are also interesting dynamics related to the availability of sand from offshore deposits at the mouth of the bay (USGS Field Trip Report, ASBPA 2005). The beach is doing well at the north end but there have been serious erosion problems at the south end down near Sloat Avenue for many years and there have been recent efforts to nourish the beach at that area.
The O'Shaugnessy Seawall at the north end is often held up as an example of a successful seawall - although I suspect what it really shows is that seawalls work best, and have the least impact, where the beach in front of them is stable or building. It is a substantially built structure which means it may continue to protect the Great Highway even if the beach were to go away or sea level were to rise. For the time being, it provides a nice promenade with easy access to the wonderful beach.