Sunday, December 06, 2015

Perego's Lagoon

Here's the third of my Ebey's Landing posts from Black Friday, a week and a half ago.

The 3 1/2 mile loop from Ebey's around Perego's Lagoon is the best coastal hike on Puget Sound. Yes, there are other nice ones, but this is more special than the rest. Especially on a day like this one! Most people hike the loop counterclockwise, hiking up along the edge of the bluff and then returning along the beach.

The last time I hiked the whole loop was also right after Thanksgiving, but on a very foggy day! 
Perego's Lagoon: 2008 (or for a different perspective on the same trip, see hshipman)

Perego's Lagoon is formed behind a narrow barrier beach - a looped barrier is one term I've heard to describe this kind of feature, a closed spit might be another. The general pattern of longshore transport is from north to south, although it's probably not quite that simple along here.


In the previous post I mentioned the perched dunes on the top of the bluff, which you traverse as you walk the crest of the slope to the north, with the grassy angle-of-repose bluff falling off to your left and the forest to your right. The bluff trail provides great views down at the lagoon and the spit, including some very nice overwash fans (I believe washover fans also works). The backside of the barrier is marked by several of these features, which mark occasions when waves overtopped the berm during a high tide storm and spread sand and gravel into the lagoon.

The northernmost is interesting for several reasons. First, it is fairly recent. It appeared sometime between the 1970s and the early 1990s. I'd put my money on the 1979 storm, which walloped this part of the Sound, but am in the process of seeing if I can find out more. Second, it completely crosses the lagoon, isolating a small portion to the north. I don't know whether these fans represent an event contained within a single storm and tide cycle, or whether they may have remained open for longer. I've also wondered how this feature would record a tsunami coming down the Strait!

Ebey's Landing

Ebey's Landing lies below a low spot on the bluff on the west edge of Ebey's Prairie, a little west of Coupeville. The road drops down to a stretch of broad backshore terrace where the original pier had been built (it's a pretty exposed location for a pier). It's now a popular taking off spot for beach hikes, including the classic loop to the north.

I've posted about Ebey's Landing before on Gravel Beach, but here's one from my other blog (hshipman).


I mentioned in the previous post that the beach along this reach often diverges from the bluff. Here, the beach has filled in a curve in the coastline, creating a fairly wide low area. A little farther north of here, the beach takes an even wider excursion seaward of the original bluff line, creating mile-long Perego's Lagoon (next post).

Ebey's Prairie is a good place to think about the last stages of the Vashon glacier. Glacial marine drift, exposed in the upper portions of bluffs near here, records sediment deposited immediately after the ice left and marine water reoccupied the area. What you don't see without LIDAR or perhaps, really good low-light conditions, is the old strandlines that follow contours around Ebey's Prairie, marking the rapid retreat of the sea as glacial rebound returned Whidbey to its preglacial elevations. Imagery shows dozens of faint shoreline traces on the gradual slopes of the prairie.

Something you can definitely see, looking north along the top of the bluffs, is the dunes perched above the bluff itself. They are stable and largely forested now, but indicate conditions when wind blowing over the Strait carried fine sand up the bluffs and deposited it above the crest. Not surprisingly, these bluff-top dunes are called perched dunes and can be found elsewhere on the northwest side of Whidbey (West Beach 2012), on Protection Island farther west along the Strait, and on Lake Superior (Grand Sable Dunes 2010), among other places.

Ebey's Landing - south

This is the first of three posts from the area around Ebey's Landing, all visited on the Friday after Thanksgiving. I'll try to keep this pretty short, since I've had too many long posts recently and the next couple are also likely to drag on.

The whole stretch north and south of Ebey's is backed by bluffs, but one of the many things that makes the reach interesting is that while in some places the beach runs right up to the base of the slope, in others the beach has built much farther seaward, isolating the bluff toe from wave action. It's interesting on the air photos to compare the line of the bluffs with the line of the beach and speculate about why they diverge.


The photos in this post are from the beach south of Ebey's Landing, which consists of high bluffs all the way to Camp Casey, two miles farther south. Although the width of the berm varies - and this is reflected in the amount of erosion at the bluff toe - the bluffs in this southern reach generally rise pretty directly from the beach. 

Typical of Puget Sound's glacial stratigraphy, there is a great deal of lateral variability in the exposed units. Till is a relatively minor component of the bluff above (difficult to distinguish in the above photo, it lies below the glacial marine drift that caps the sequence and above the thick sandy unit mid-bluff), but dominates the upper portion of the bluffs farther south (below).

Keystone Spit

Keystone Spit is one of the larger barrier beaches in the central Sound, maybe even the largest, depending on how you measure these things. It's a south facing beach that forms a long crescent separating Crockett Lake from the rest of Puget Sound. The lake itself is actually a freshwater (maybe a little brackish in places) lagoon.


The well-sorted gravel results in a very steep beach. Breaking waves disappear into the permeable gravel with a whoosh, followed by the rattle of gravel rolling back down the beachface. Storms pile gravel into high storm berms. And as should be expected of any large south-facing beach on Puget Sound, this one collects logs, lots and lots of them, along with plastic and styrofoam and anything else that floats.

Keystone Spit is a beneficiary of large amounts of sediment converge from two different directions -- and that then have no where else to go. Sand and gravel deposited by landslides in the Ledgewood area (Ledgewood 2013) probably ends up here. And sediment from farther north on the island (see subsequent posts from Ebey's Landing) is transported southward around Admiralty Head. This material is held up briefly by Keystone Harbor at the west end of the spit, but is eventually bypassed (Keystone 2012) and gets blended on this beach with the gravel from the south.

A couple of ponds mark borrow pits form when gravel was removed from the spit decades ago. The two concrete posts were associated with a small building (Navy, I think) that was removed in the last couple of years, but I'm not sure just how they functioned.

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!