A Stroll to the Oosterschelde Oyster Reefs

[Nerding, not kayaking]

My first impression of tidal flats in the Oosterschelde (Eastern Scheldt), a major river in South Holland, was the ease with which you could walk across them. In San Francisco Bay, where the tidal flats are muddy (rather than sandy), each step sank deep into the mud. The crossing to the oyster restoration site for monthly monitoring was always an exhausting adventure. Tuesday's visit to the tidal flats of the Oosterschelde brought back fond memories of tromping through the mud in California.

The EMERGO project team: Bram, Lodewijk, Tom, and Oliver

Some background (skip if you're just here for the pretty pictures)...
This was the first field visit for the EMERGO project, a project by TU Delft and NIOZ (the Netherlands Institute of Sea Research) that I'll be working on during my time in the Netherlands. The overarching goal is to understand the morphological and ecological responses of tidal flats to different restoration measures. The project was sparked, in part, by the ongoing loss of intertidal habitats (tidal flats, oyster reefs, marshes, sea grasses) in the Oosterschelde. Between 1976 and 1986, a storm surge barrier was constructed across the mouth of the river. This barrier was the largest component of the 50 year Delta Works flood management project to protect the Netherlands from flooding after devastating floods in 1953. Stay tuned for a future post about this barrier - an epic bike trip is in the works that will involve biking across many of the storm surge barriers.

Anyways, the barrier reduced the tide range in the river (i.e. the tide does not go as high or as low as it used to). Since intertidal habitats depend on being dry and wet for a certain amount of time, this smaller tide range has lead to rapid shrinking and loss of tidal flat and marsh habitat. Many pilot projects are in the ground already to test ways to slow/stop this degradation, including artificial oyster reefs and tidal flat sand replenishments. The EMERGO project will look at how well these (and other) actions address the unraveling of intertidal habitats.

Let's go on a fieldtrip!
This week, two professors (Bram, my advisor from TU Delft and Tom, a prof from NIOZ), two new PhD students (Lodewijk and Oliver), and I visited a few artificial oyster reefs and a massive tidal flat nourishment project.

Lugworm, or "wadpier" in Dutch. The worms live under the sand and eat+poop piles of sand, which cover the entire mudflat.

Worm poop piles everywhere

In 2009, small test oyster reefs (2 of 12x4m) were constructed at the mudflats of Viane. The artificial reefs begin as metal cages filled with dead oyster shells. This is similar to the San Francisco Bay project, where bags of oyster shells were used. New oysters like to colonize on the dead oyster shells. Over time, the live oysters stick together and make the structure sturdier, removing the need for the metal casing, which disintegrates away. A year later, this project was deemed successful because the reefs were stable, some oysters had colonized, and sedimentation was occurring behind them. Larger reefs (200x10m, 2 at Viane and another at De Val) were constructed in 2010. We visited these three larger reefs, as well as some vibrant natural reefs nearby. The map below marks the artificial reef locations. Source

Artificial reef at de Val. The reef begins at the bottom of the levee and extends out into the mudflat.

At de Val. As is the case in most of the Dutch estuaries, a large levee runs along the back of the intertidal habitats.

Our walk across the mudflat at Viane was a long one - it took a couple hours and ~6 km to visit the natural and artificial reefs at this site. We first visited a natural oyster reef. 

A natural bed of oysters near the Viane artificial reefs.

Happy beds of natural oysters

Of all three reefs, the westernmost reef at Viane showed the most growth. Clumps of happy oysters pointed their shells up towards the sky. The second reef at Viane was less promising, which may be because it was built higher on the mudflat. Oysters don't like to be dry for long periods of time, so they usually don't grow past a certain elevation "ceiling," relative to the tide. Another reason may be that a bar of sand has been moving slowly across the area, filling in the gaps in the reef where oysters could otherwise grow. 

The EMERGO team checking out one of the artificial oyster reefs at Viane.

Oyster reef at Viane, the most successful of the three artificial reefs

Of course, you can find many other interesting things on the mudflat, a few of which I've shared above (worms) and in the photos below...

Mud waves, caused by waves over shallow water covering the mudflat.

Some areas were covered in seaweed

... and shells

Maeslantkering: Testing a Storm Surge Barrier

[Nerding, not kayaking]

Once a year, Rijkswaterstaat does a test closing of the Maeslantkering, a massive storm surge barrier on the waterway that connects Rotterdam to the ocean (the Nieuwe Waterweg/the Scheur). Check out the structure in Google Maps. Rijkswaterstaat is the government agency that oversees flood protection in the Netherlands by building and maintaining the complex and highly managed system of rivers, lakes, coastlines, pumps, and flood protection structures. This is necessary because more than 25% of the country is below sea level (protected by dikes). This was one of the last projects within the 50-year Delta Works, a massive flood infrastructure project triggered by the devastating 1953 floods that killed almost 2000 people.

We (Elizabeth, Toni, Sander, Maria, and I) biked the 20+ km from Delft to the Maeslantkering on Sunday afternoon to attend the annual test closure. It's quite the event, with europop music and hoards of politicians in fancy suits. Civil engineering students give guided tours every 10 minutes throughout the afternoon. We took a tour and picnicked on the adjacent hillside with the rest of the crowd.

Panorama of the partially closed barrier, and the hillside of observers. Click to see larger!

Elizabeth and Sander enjoying some good Dutch beer before the closing

Here are some fun facts that we learned during our tour of the Maeslantkering:

• It's painted white so it doesn't expand as much in the hot sun (14-16 cm when white vs ~ 40 cm when black)
• Dimensions: ~25 m high and ~240 m long (like laying the Eiffel tower on its side)
• It rotates on a 10 m "joint" sphere, which is manufactured to 2mm accuracy and lubricated by a plastic polymer (after it started getting stuck on the original teflon layer).
• Constructed between 1991 and 1997
• It still uses same software as when it was built - very simple.
• A computer decides when to close the barrier, and closes when water levels reach 3m above normal levels. Therefore, the closures are triggered automatically to prevent emotions/politics from interfering in its function.
• The closure trigger level (3m) is based on an agreement between many parties, and is expected to result in a closure roughly once every 10 years. With 50 years of sea level rise, this will increase to once every 5 years. So far, the barrier has been closed due to storm surge once, in 2007.
• Every day that the barrier is closed it costs the economy ~$15M in lost income from shipping

How does it work?
The two arms rotate from each side of the river, at a rate of 3 m/minute, until they are ~1 m apart. This small gap just considered a leak. Water is pumped into both arms to fill them with water, lowering them towards the river floor. Then, it pauses for ~1 hour before lowering completely, to allow flow to flush out the track underneath (water moves faster when most of the flow is blocked). This is part of the maintenance, to make sure the tracks don't fill up with sediment permanently. Then, more water is pumped in and the arms lower to the bottom of the river, stopping almost all flow. This animation makes it a bit clearer:

Photo credit

This structure is an improvement on many of the other storm surge barriers in the Netherlands because it lets the river be completely open most of the time. This allows tides and salt water to come in and out, which allow coastal and wetland habitats to exist. However, it still requires having a very controlled mouth of the river, rather than giving the river space to meander/evolve naturally. This works in a place like Rotterdam, which is already highly developed and engineered, but would be a detriment to more natural systems. 

Looking down the north arm, towards the ocean.

View of the closed barrier, from the back.

View of the closed barrier, from the front

We took a much longer route home, making a stop in Hoek van Holland on the coast and biking through the dunes until we reached the town of Monster (yes....) and turned inland towards Delft. Soreness ensued.