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Do artificial reefs work? Vol 5. Erosion control

November 05 2009 | Artificial reefs, Jim's Blog,
by Jim

This is the number five in a series asking the question "do artificial (surfing) reefs work? The rest of the series is here.

A series on artificial surfing reefs would not be complete without a post on erosion control.

When we think of artificial surfing reefs many assume they are being put in to enhance (or introduce) a surfable wave. To be blunt, that assumption is increasingly misguided. The economics don’t support that sole value proposition (it takes millions of dollars to build a reef and its unlikely that it will generate enough use to justify the cost/benefit ratio).

Even though a good portion of the existing artificial reefs were designed solely to create a surfable wave, I believe most of the future artificial reefs will serve another primary purpose; erosion control. This because the cost of an artificial surfing reef is small relative to the budgets of many erosion response projects. It goes something like this “a few million dollars to build a reef is a good deal if it protects millions of dollars of property or slows down the need to import millions of dollars of sand (also aimed at controlling erosion)."

So with this post I will to explore the core issues involved in artificial reefs for erosion control. Like all the other posts in this series, I’m seeking your views and thoughts.

Wikipedia defines this concept as "the mass of a closed system will remain constant over time, regardless of the processes acting inside the system."

In layman’s terms; sand isn’t created, it’s simply moved.

As illustrated in Figure 1, this says that if your are seeking to protect property A and believe you’ll do this building reef B all you’ll really do is steal sand from location C and put property D at risk.

Another way to look at this is that artificial reefs don’t “solve erosion” challenges, they merely move them elsewhere.

Look at Figure 2 and think of a seawall or groin that is perpendicular to the beach. To protect building A, wall B is put in and causes erosion C, thus putting property D at risk. We see this day-in and day-out, all over the globe. An artificial reef isn’t some magic coastal structure that is immune to the laws of physics, it is simply another category of coastal infrastructure that is submerged.

This brings us to the next point, the size of the erosion issue.



The caveat to this is if you add sand via a beach dredge and fill project (euphemistically called beach “nourishment”), artificial surfing reefs may act to reduce wave energy in the lee of the structure and slow the erosion of this sand in that area. I talk about this more in point four later in this post.

There is a massive difference between a regional erosion issue and an hot-spot erosion issue. In the graphics above there is a systemic erosion problem that is causing both properties A and D to be at risk. In both of the examples above it was treated as if it were a hot spot issue, property owner A built a structure (artificial reef or a seawall). This solution never works as it puts other people's property at risk.

Artificial reefs cannot solve regional erosion problems because these erosion problems are simply too large for any single structure to address.

Artificial reefs may be able to address smaller, hot spot issues.

We’re not saying artificial reefs WILL solve hot spot erosion issues. We’re saying they may be able to solve these issues. We’re further saying that this is the best case scenario and that anything larger than a small hot spot cannot be solved by constructing an artificial reef.

Beach dredge and fill, euphemistically known as “beach nourishment”, is the process of taking sand from somewhere and dumping it somewhere else. I won’t go into the usual negative consequences of beach fill, you can find that here. I’m simply acknowledging that many times there are other variables at play when considering artificial reefs.

Beach fill is expensive, very expensive.

It literally means that sand is collected from one area, brought to another and deposited. Sometimes these areas are far from one another (Florida has imported sand from the Bahamas, Hawai’i has imported sand from Mexico). Sand isn’t easy or lightweight to transfer; it’s similar to transferring concrete. In addition sand, as well all know, washes away. A town could spend millions of dollars on new sand and see it washed away a week later due to an unplanned storm event. After the sand is washed away, more sand needs to be brought in which means more money must be spent.

Beach fill is man’s belief he can control nature. It’s folly. Expensive folly.

The way this relates to artificial reefs is that in many cases the true equation involves sand that doesn’t belong there in the first place. In Figure 3 sand X has been added to the mix. Therefore property A was already way too close to the coastline but buffered by new sand X. An artificial reef simply moves all the sand around (the natural. existing sand and imported, purchased sand). This is relevant as towns with beach fill programs have a very challenging time of understanding where nature’s lines would be naturally and therefore they tend to make financial investments (imported sand and artificial reefs) in areas that are beach monsters with man-made sections "bolted on."

Beach fill with an artificial surfing reef will not solve an erosion problem. The combination may potentially slow down the erosion process or increase the time period between beach fills.

I’m going to take a run at the seven artificial reefs that exist in the world and suggest why they were built. If you disagree with me, please leave your comments.

Even though most of these reefs were built for surfing, my sense is that they’ve failed at that objective (read the larger series for that perspective and detail). Building artificial reefs, while seemingly popular, I believe will become increasingly challenging.

Specifically, I believe that due to the horrible track record of existing artificial reefs based on their ability of producing waves when compared with the high quality waves that surfers actually seek out... artificial reefs won't continue to be able to be sold with unrealistic sales pitches (as has been the case in the past).

After decades and numerous implementations, it's impossible to point to one decent wave on the planet created via an artificial reef. That's a problem for artificial reef sales pitches that stress good, consistent waves.

Furthermore due to the very high cost of creating a reef for a relatively small number of recreational surfers that will benefit, I believe we will see a shift for artificial reefs to be sold to towns, cities (and any other entity that can write million dollar checks) with the business case primarily focusing on erosion control. If this specific hypothesis is true we will see even less success with artificial reefs ability to create a meaningful wave that surfers seek out (because they will be built with an emphasis on erosion control and not creating a surfable wave).

To summarize this post, artificial reefs are not that different than any other coastal structure. Perhaps the only difference is that they are under water. We know from the conservation of mass principle (and from instances all over the world) that sand isn’t created; it’s simply moved around. Artificial reefs, like all their coastal structure brethren move sand from one place to another. The best case scenario is for a coastal structure to have a single, primary purpose. When the purpose is anything less than 100% focus on creating good wave forms that are sought after, we will see worse results than we’ve seen to date. This all points to a challenging future for artificial reefs.
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