The shifting sands of Managaha Island

Quick, put your thinking caps on! Here’s a pop quiz to see who was paying attention to last week’s environmental page article. Who can tell me what the article was about? For those of you who said “Managaha’s wedge-tail shearwater bird community,” give yourselves a pat on the back. Now it’s time for a harder question. What is the main threat to the survival of these precious birds? If you said “habitat destruction,” right on! The shearwaters are indeed facing the loss of habitat due to erosion occurring at their nesting site on Managaha.

Now, a lot of people have been worried about Managaha’s erosion problems over the last few years. Anyone who has walked around the island has probably seen large ironwood trees that have toppled into the water, leaving their roots exposed and creating an obstacle course for beachcombers! So, we know erosion is occurring. But how serious is the problem?

In order to find out, I started my investigation with Rob Carruth, geologist at the U.S. Geological Survey. Actually, Rob told me, erosion of these types of islands is a completely natural process. Islands like Managaha are formed when bits of coral and sand, crushed by storms, settle in the calm lagoon waters inside the reef. Where they settle depends on the wind and current patterns in the area, but over time, more and more sand ends up in the same location, and soon an island forms above the water. In the case of Managaha, the sand was most likely deposited on a reef that was higher than the surrounding bottom. Eventually coconut or mangrove seeds washed up on the beach, and before you know it, we had a vegetated island!

The thing is, those waves and currents that deposited the sand haven’t gone away. They are still working their magic, and they will tend to pick up sand from one side of the island and deposit it on the other. Over long periods of time, the island will creep along. Imagine the way a tank moves on its treads, and that’s sort of like the way the island migrates. Rob assures me that we can see this process occurring on Managaha. As we lose ground on the southeastern portion, the sand is re-deposited on the northwestern side, creating a larger beach.

Islands all over the world are moving, and have been for millions of years. As the vegetation on one side erodes, it grows on the other side, and the animals just move right along with it. The problem comes when the natural process of island migration butts heads with the human element. Humans build “permanent” structures, and make roads on islands, none of which can migrate with the island. Then humans want to control the natural process.

“So, Rob,” I ask. “Does this mean that the erosion on Managaha is completely natural and everyone should quit complaining about it?” “Not necessarily,” he answers. “It depends on the rate of the erosion. If we humans have done something to increase that rate, then the vegetation and animals might not be able to keep up. You’ll have to investigate the history of the island to see what other processes might be happening.”

So I leave USGS and head down to the Coastal Resources Management Office to speak to Kathy Yuknavage, natural resources planner. “So Kathy,” I ask. “What can you tell me about the erosion on Managaha?” Kathy pulls out a long document for me to review. “The roots of the problem go back to 1996,” she says. She goes on to tell me that ever since World War II, there was a barge wrecked on the southern side of the island. CRM and the Marianas Visitors Authority were worried that people might be injured on the sharp shards of metal, so they requested the removal of the barge. Almost immediately thereafter, CRM staff saw evidence of erosion. Kathy and others believe that the barge was acting as an erosion protector, unnaturally keeping the sand in place. Once the barge was removed, the beach was exposed and Managaha resumed its natural process of island migration. In addition, Kathy suggests that the barge removal and natural migration might not be the only factors affecting the erosion rate. According to ex-CRM director Peter Barlas, renewed dredging of the channel and the Saipan Harbor may have contributed to further erosion.

Wow, my head is spinning now! “OK, Kathy,” I say. “I understand the history now, and I see that there are many factors contributing to what’s going on on Managaha, but do we have any facts about the rate of erosion? Hasn’t anybody done any studies about it?” Kathy smiles. “Yes, CRM has! Let’s talk to Ana Agulto, over in the Permit Section.” At least this time I don’t have to leave the building!

We sit down in front of the computer and pull up a bunch of files with all sorts of graphs. Ana explains, “these are the beach profiles that we’ve been doing of Managaha over the last three years. We go out every couple of months, and measure how the beach has changed over time in eight different locations, or transects.” She explains that at each transect, CRM staff measure the height of the beach every meter, relative to a reference point high on land. They monitor the distance from the reference point to the berm, or the spot on the beach where waves deposit rubble or sand in a pile. The location of the berm and how it changes over time shows whether the island is eroding or growing at any one place. Ana shows me that on most parts of the island, the beach grows and shrinks, but overall, there isn’t much change. There are two exceptions to that general rule. She shows me a photo of Managaha with the 1945, 2003, and 2005 shorelines drawn on it (see photo on this page). In the southeast side of the island, (site C on the photo), we have lost between 5-9 meters of beach over the last three years. But, on the northwest side (site A on the photo) we have gained nearly 20 meters!

Well, now I’m more confused than ever. I’ve learned that shifting islands are natural, but that in this case the rate of the movement might be increased by human activities. I’ve learned that in some parts of the island, sand is increasing, but at other points, it’s eroding. But what does this all mean for the management of the island, and the fate of the birds? To answer that question, I hike over to DFW and speak with Shelly Kremer. “Shelly, what does this all mean? What can we do about the erosion, and are all the birds going to die?”

Shelly assures me that I don’t need to worry TOO much. Fortunately, at this point, the main nesting sites for the shearwater birds are in the northeast side of the island, around site B. The erosion in this area is pretty much under control. The major erosion is occurring at the southern end of the nesting area. “Also, the birds are pretty flexible,” she says. If the island continues to move, they can probably shift their nesting sites and adapt.” The problem is, she says, that people are using many of the areas on the island that the birds could nest in. Tourists, pala palas, and the concession areas take up a lot of useable habitat. So…we’re back to the same problem. The available habitat is being eroded. What are our options?

Shelly slaps a folder onto the desk in front of her. “Time to do some reading,” she says. I pick up a document produced by the US Army Corps of Engineers in 2001, an erosion study of Managaha. The ACOE listed several possible alternatives to take, including 1) continued study of the area, 2) taking no action, and 3) several different construction alternatives, meant to stabilize the erosion problem. The report states that beach systems exist naturally in a state of “dynamic equilibrium.” In other words, they shift and change depending on the state of the environment surrounding them. If we take no action on Managaha, the erosion will probably continue until it reaches a hard point. That hard point might be the beach rock that you’ve seen in places like Wing Beach. When fresh groundwater comes into contact with the sand, a chemical reaction takes place. Over time it cements the sand and forms a solid beach rock. If there is hard beach rock under the ground on Managaha near the erosion, it might stop erosion in its tracks. But, if there is no hard point, it may continue. The other options listed in the ACOE report are a little more drastic, involving major construction of groins or breakwaters, structures meant to limit erosion. Not only are these expensive, but we can’t necessarily predict the outcome of building them. They would protect the beach in one area, but might cause further erosion in other areas. The ACOE recommended that the best courses of action are continued study and not building hard structures.

So, if we don’t build anything, can we still protect the birds? Fortunately, other management options exist. We can, for example, revegetate the new spit of land at the north side of the island, encouraging the tourists to move further out, and creating new habitat for the birds.

Whew! By now it should be pretty clear that the erosion problem at Managaha is not a simple matter. We have to think about tourism, which is a major source of revenue for the CNMI. We have to think about the shearwater birds, which, as far as we know, don’t nest anywhere else in the CNMI. We have to think about our economy, and continue to maintain an active port. We have to think about science, and keep track of the movement of the island. And, we have to think about nature, which has been creating and destroying and moving islands for millions of years. At the end of the day, there are many factors to consider, and we’re all doing our best to balance them. In the meantime, when you go out to Managaha, take some time to appreciate what we have, and remember we shouldn’t always feel like we need to control nature.

CoCo