Thursday, March 15, 2012

Notes From Another Shore

I recently spent some time exploring a non Washington shoreline. The principles are the same as many sites I visit. Erosion causes bluffs to become steep and slope failures follow. Property owners want to protect their property, but protection efforts pose a risk to public resources and neighboring properties. The range of protection efforts, slope stability and policy development along this reach of California coast kept me occupied.
Concrete plugs along eroded joints or vertical clastic dikes in Eocene sandstone

Eroded clastic dike

Concrete covers erodable silty unit along base of bluff to the left and right of uncovered silty unit

Concrete fills of overhangs and shallow caves

Unfilled cave

In general the concrete plugs and cave fills consisted of what is called erodable concrete. Concrete that will erode at a similar rate as that of the adjoining bedrock. However, it was not clear that this has been the case as monitoring is not well documented. However, is a requirement that the concrete appear at least similar in color to the adjoining bedrock. In places it took a bit of examination to tell that the concrete was not rock as cross bedding by scraping was done and even fake concretion were added to the concrete in places.

The California Coastal Commission requires that the home above the concrete fill areas must be at risk with no options in order for concrete to be approved. Preemptive concrete fills are not allowed. Based on observations of the concrete sites, this approach has not been consistently applied and it may be earlier works were permitted with a lower threshold of need demonstration.

Sand berm placed in front of soft erodable sediment

Two sections of the shore reach were missing the somewhat resistant Eocene sandstone and contained much younger alluvial deposits that were subject to higher erosion rates. The above site was protected with sand berm. The local city was working on a scheme at this area to retain sand on the beach with a submerged structure along the shore. There is a bit of a debate on how well a proposed structure would work with a peer review report claiming the structure as designed would not retain sand and would increase erosion. A third consultant was going to look at the issue.

This site had a more erodable unit that was somewhat elevated above the beach with a very resistant unit at the beach level. This lowermost resistant unit consisted of mix of sand and mudstone with ample oyster fossils.

On some stretches the entire lower cliff was covered with concrete

Most of the bluff along this reach of shore consisted of relatively resistant Eocene sandstone from the base of the slope to a height of 25 feet or so topped by poorly consolidated sediments that were on the order of 100,000 years old. Part of the full lower wall approach was to maintain the slope angle of the upper slope which is very subject to erosion from wind, water run off and wetting and drying.
This section of bluff utilized concrete sand bags to protect the lower slope

A tiered prefabricated concrete wall protecting the lower and upper slopes

Rock rip-rap section. This was the only rip-rap section on this reach

Concrete wall bulkhead with concrete cribbing wall on upper slope and slope engineering to maintain upper slope stability

Two soil engineered slopes with an apparently untreated slope in between

Over steep upper slope showing recent failures with endangered building near slope edge

Full concrete wall covers lower slope and provides support for upper soil engineered slopes

Fresh sand (brown) blown off upper slope onto the beach with gray sand

This shoreline reach is exposed to open ocean waves. However, the waves here are not nearly as large as the outer coast of Washington due to not being subject to as intense storms and the presence of some off shore islands. But regardless this is an actively eroding shoreline which is much more active than shorelines in the Salish Sea. And the same principles apply. Stopping erosion of the bluff reduces sediment to the beaches with the consequence of ultimately increasing the overall rate of erosion and the loss of public beaches. Another reduction in beach sand is due to the construction of dams across many of the rivers that flow into this shoreline drift sector.

This particular beach as well as several others on this drift sector are slated to have sand imported to the beaches. The beach I was checking out will have 142,000 cubic yards of sand deposited onto the beach later this spring. Portions of the beach will be 7 feet higher than current levels. Some of those cave sites will be buried at least for a while.

This approach was pushed for a long time by the California Coastal Commission and required the coordination of a lot of governments and government agencies as well as the citizens impacted. The sand kept off the beach from the various erosion control schemes as well as the construction of dams on the rivers will be replaced by another agent, hydraulic dredging from sandy areas off shore.

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