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!

Sunday, November 29, 2015

Ala Spit

Here's my most recent update from Ala Spit from a visit a week and a half ago.

In 2011, rock was pulled from the neck of the spit and the berm was reshaped, with hopes of restoring both the original spit and the processes that maintained it (see link to previous posts below). But once those processes were restored, they went to work reshaping the spit in ways that weren't exactly planned. A few quick storms eroded through the restored berm, spreading gravel into a broad deposit on the back side and leaving the berm crest low enough that high tides regularly washed over it. Unfortunately, the geometry of the site and the volume of available sediment have been inadequate to allow the berm to rebuild in the intervening years.


Earlier this fall, the County took another swing at it. The groin to the south has been partly dismantled. Essentially, the central portion of the groin was removed, a compromise that restores some sediment movement while also trying to maintain the wider beach that had built up on the southern (updrift) side of the structure. I suspect some of the structure may also have been left simply because a few of the rocks were enormous and would have been very difficult to remove.

The concrete bulkhead that ran from the groin to base of the spit is also gone, except for a short section in front of the parking lot. Whether erosion will be a problem at all in this area remains to be seen, but if nothing else, the remnant wall will provide a reference against which to compare future changes.

The big change is that the berm along the neck has been rebuilt. Right now it's still a little raw and looks more like a gravel causeway than a spit, but hopefully time and vegetation will help the site heal. My guess is that there is more than sufficient height and volume to make overwash unlikely. Ironically, one of the original processes to be restored was overwash, but finding a balance between too much and not enough is difficult.

It's been hard to write about Ala Spit, since there are strong opinions and much at stake. And because the more I watch it, the more I realize I don't understand. I'm one of those folks who thinks if the answer is simple or elegant, it is probably wrong!

Previous posts mentioning Ala Spit - most from Ala, but some from other places, too.

My attraction to Ala Spit has been that it illustrates some of the challenges of restoring dynamic geomorphic environments (not that we shouldn't keep trying). For example, different processes play out at different timescales. High tides and waves can wash over a berm in a single storm. The movement of sediment from updrift (south) occurs over seasons or years.  And significant changes to the shallow bars offshore of the spit may take many decades.

Ala Spit may have been changing rapidly, even in the absence of 20th-century human modifications. The morphology of the spit is probably influenced by the building or shifting of those bars offshore, which influence sediment availability to the spit itself and which modify the wave environment along the base of the spit. And just to complicate things, Skagit Bay is subject to changes in currents as the Skagit Delta presses towards North Whidbey Island and is subject to potential elevation changes related to nearby faults with Holocene activity, both of which would have ramifications for the morphology of Ala Spit over longer time scales.

Thursday, October 29, 2015

Tarboo Bay

Tarboo Bay is located at the northernmost end of much larger Dabob Bay, sheltered behind an elegant series of spits.


This small site at the very head of the bay was recently restored by the Northwest Watershed Institute, the local organization that has spearheaded so much restoration work in this area. A year ago, there was a building built on piles over the water and an old timber bulkhead, but the structures have since been removed and the bank and the old driveway have been replanted with native trees and shrubs. In a few years, it will blend perfectly with the adjacent landscape.

The now unprotected bank is eroding, supplying coarse gravel to build a small beach and sand that is reshaping a tiny beach nearby. It will be interesting to see how long it takes for the marsh to recolonize the beach surface - wave energy is sufficiently low here that vegetation is the natural end point rather than a beach, something seen by looking at the adjacent shoreline to the north where marsh grass obscures the gravel beach beneath.

Dabob Bay

I don't get to this remote corner of Puget Sound very often, and not many others do either, so it remains a relatively pristine landscape. The last time I visited was 8 years ago, on a day a little bleaker than this one (Broad Spit: October 2007).


One of the things that struck me on that trip was how difficult it is to find small stream mouths on Puget Sound - even way back here - that haven't been significantly modified. They were the natural place to sluice logs out of the hills, to homestead, to construct shingle mills and oyster farms, and later, to build vacation homes. Valley bottoms were cleared, streams channels were relocated, and spits and small estuaries were buried. Reference sites for restoration projects are hard to find.

Once riprap, now a beach
Local groups, along with DNR and the Nature Conservancy and others, have patched together a large mosaic of forested uplands, beaches, and tidelands here in northern Dabob Bay and are in the process of restoring the more disturbed sites. Some work has already been done here - a bulkhead was removed and some roads were taken out from an earlier development effort on the hillside. But now there's an opportunity to do more and I'm looking forward to coming back in a few years to see what it looks like.

Thursday, October 08, 2015

Point No Point

Point No Point is an important landmark - the southernmost of three prominent points that mark the western entrance into Admiralty Inlet and Puget Sound. Point Wilson in Port Townsend. Marrowstone Point. And Point No Point. Each has a small lighthouse. Each is a cuspate foreland.


Cuspate forelands are triangular landforms that typically form where wave action approaches from two different directions. Longshore transport moves sediment toward the point from each direction, resulting in an accumulation of material and the growth of the feature. Sand and gravel may accrete on one limb or the other of the landform, or may be lost off the tip into deep water.

The old t-sheets show a tidal inlet on the north side, serving a large wetland behind the beach homes along Point No Point and Norwegian Point (which is basically a western continuation of the same barrier beach). The wetland is now broken up by development and drainage systems and the open tidal channel to the north has been replaced by a pipe and tide gate on the east.

On this visit, there was a distinct scarp high on the beach where recent wave action had dropped the sandy beachface half a foot or so.

The Point offers great views of Puget Sound to both north and south. Mount Rainier and the high rises of Seattle are visible to the south. Mount Baker rises over Whidbey Island to the northeast.