Thursday, December 03, 2015

Bowman Bay

Another beach restoration project, not far from the last one (Ala Spit). But what an entirely different flavor of beach! Bowman is a pocket beach, facing the Strait. Waves action can be significant, but its incidence is highly constrained by the rocky headlands that frame the entrance of the bay, so we get the typical crescent form of a swash-aligned beach, broken only by the offset against the boat ramp in the middle.

Here's a previous post:  Bowman Bay: March 2012


The old ramp remains and the base of the pier still needed to be protected, but the revetment that had been here for decades was removed a few weeks ago. The basic idea was pretty simple, pull out the rock and excavate the old fill that was too high and too far waterward. On so many of these sites, the erosion problem is entirely one of our making - simply because we pushed the land seaward, crossing a well-defended border - and therefore the solution is simply to pull back to friendlier territory.

Ideally, we want to remove all of the fill and restore the entire profile to something similar to what existed prior to the fill.  Any fill or extra elevation that gets left behind will remain subject to unnaturally high rates of erosion and often a scarp will develop.

In this case, there were constraints on how much of the old fill could be removed, which left a higher profile than might have been preferred. That, combined with a strong high tide storm a week after the project was wrapped up, resulted in a significant scarp at the top of the beach. What's missing from the restored beach is the natural berms that mark the unaltered beach north of the boat ramp.  With time, this should remedy itself and a broad berm (or series of stepped berms) should form. A few good high tide storms might actually speed this recovery.

When the backshore is left too high, we essentially create a low eroding bluff (Keystone 2012). But natural berms are actually depositional features, so where it's possible, it may be be best to design to a lower profile, and then let nature build the berm where it wants to. This may require more excavation, the placement of more sediment lower on the profile (to supply that berm rebuilding process), and a tolerance of possible overwashing of the initial berm before it gets established.

Conceptually, the restoration at Bowman was pretty simple, but it provides a nice illustration that even simple projects can be difficult. Fortunately, restored beaches are inherently more malleable than what they replaced, so given time and some room, nature can sort things out. It may just take a little longer than we might have hoped.

You would think from many of my recent posts that taking rocks and concrete off of beaches has become pretty popular on Puget Sound!

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