This paper was written as part of the 2003 Alaska Ocean Sciences Bowl high school competition. The conclusions in this report are solely those of the student authors.
Seawall construction in Homer, Alaska
Well-built seawalls have long been used as an inexpensive way to control coastal erosion. Seawalls are associated with reduced aesthetic value, and increased erosion at the ends and in front of the seawall. In Homer, where bluff erosion is rapidly reducing many beachfront properties, local homeowners have banded together to build a seawall to protect their land. A seawall was chosen as the least expensive approach to protecting the property and it has increased the value of the properties it protects. Design flaws have allowed one section of the seawall to collapse within a month of its completion and caused a reinforcing wall to be added. The seawall is in the high tidal zone above a sandy beach environment. Expected erosion will reduce the sandy environment near the wall causing the ecology to shift towards rocky shore ecology. The main impact of the seawall will be economic, but the true value to the property and ecology should be considered before building such structures.
Brief History of Seawalls
People in Alaska and elsewhere have been trying for decades to prevent waves from eroding away beach and bluff properties (Mason et al. 1997). Several different kinds of beach armor have been developed to do this. These include seawalls, bulkheads, revetments, groins and jetties (Dean, 1999). Seawalls are the largest of these structures. They are designed to withstand the full force of the ocean's waves in order to protect endangered property. Many seawalls are very effective. They save homes and other buildings that otherwise would have been destroyed. However, there are some important negative consequences of building seawalls. Over time, they narrow and sometimes even destroy the beaches in front of them. A seawall is a constant and tides are variables. After high tides meet this wall, the waves retreat, carrying with them the sand from the beach in front of the wall. Often, the speed at which a beach erodes will increase when a seawall is put in (Bush et al., 1996). Many beach-front property owners consider the loss of the beach a reasonable sacrifice for the survival of their buildings. However, when an entire beach is wiped out by the use of seawalls, residents have regretted some of the beach armor development.
New Jersey is an example of a state that has used seawalls and other beach armor to excess. New Jersey has been experimenting with shoreline stabilization methods for over 125 years, making it the state with the nation's oldest beach development. New Jersey's first seawalls were built in the early twentieth century to protect the beaches in front of roads and buildings from erosion. The seawall built between Sea Bright and Monmounth Beach in New Jersey is one of the longest and tallest in the United States. The seawall is in some places so tall that the ocean can only be seen from second and third story windows (Kaufman and Pilkey, 1983). Construction on this wall was begun in the mid to late 1880's. By 1994, tens of millions of dollars had already been spent on maintaining the wall and the property it is protecting. Tons of sand have been dumped at the foot of the wall to replace lost beaches. Despite the wall and all the money spent on it, the beach is in places less than 200 yards wide. In some areas, there is almost no sand left at all.
Although most seawalls all over the world have a negative effect on the beaches in front of them, they still do their job overall. The purpose of beach armor is usually to protect the buildings, not the beaches, and well-built seawalls do this.
Local Erosion and Beach Armor History
The city of Homer, Alaska, has had trouble with both beach and bluff erosion in the past. Many residential homes are built on bluffs in Homer. The famous Homer Spit is naturally more vulnerable to erosion. It is almost completely surrounded by water. Only the base is not. The City of Homer and private landowners both have taken measures to protect buildings and all of the Homer Spit.
In the past, Homer has used revetments as protection against erosion (Mason et al., 1997). Revetments are non-freestanding structures consisting of some kind of armor that covers a sloped land facing the ocean. The Homer Spit Road is protected by a type of revetment, called rip-rap. Rip-rap consists of large boulders or blocks of broken rock. This rip-rap is along the right side of the road for approximately one half to three quarters of its length. This armored rock, as it is also called, has been in place for many years, and most of it remains stable. Before the armored rocks were put into place, steel sheet piling was used. It failed to protect the Homer Spit Road.
Many of Homer's bluff properties have some kind of defense against erosion. Often these defenses are simple, such as nets laid across bluff to minimize to amount of dirt rolling down. Some bluff property owners use more advanced and expensive methods of erosion protection. For example, a bluff property at the base of spit has built a bulwark, a small wall, against the bluff to protect it. A bluff property along the shoreline of East Homer is guarded against erosion by gabions and rip-rap. Gabions are rectangular wire containers filled with gravel. Although gabion has been successful on other Alaskan shorelines, this bluff continues to erode.
It was the decision of the Homer City Council to build a composite fiberglass seawall to protect to bluff properties of Ocean Drive Loop (Figure 1). This seawall is the main component of the Ocean Drive Loop Bluff Erosion Control Project overseen by the City of Homer. The city is paying for the wall, but the property owners who will be protected by the wall will reimburse them in time. Some of the residents support the construction of the seawall, while others are very much opposed to it. Both sides have valid reasons backing their opinions. Most of the residents of Ocean Drive Loop, however, realize that the seawall was designed poorly in order to save money. Weep holes were not installed in the wall until after it had collapsed once. Excess water that finds its way behind the wall exits through these holes. Because there were not any weep holes installed initially, there also was not a filter cloth installed in the wall. Filter cloths are necessary for the success of a seawall. At of the time the wall collapse, there was no toe protection at the foot of the wall. Improvements were afterwards made on the design and construction of the Ocean Drive Loop seawall in Homer.
Property Owners Viewpoint
This is a compilation of information obtained in an interview with Mr. Christopher Newby, a resident of Ocean Drive Loop, Homer, Alaska, where the seawall is currently being constructed.
Mr. Newby and many of his neighbors supported the building of the seawall. They decided that a seawall was the best protection for their property that they could get for their money. However, some of Mr. Newby's neighbors were against the building of a seawall.
The property owners needing protection from the encroaching erosion by the tidal action wanted the seawall built because the bluff was eroding from the top and the bottom. The seawall is approximately 2,000 feet in length. Overall, the cost will be approximately one million dollars. The residents believe by stopping erosion they will, in a way, get their money back by saving the land and not having to move to a different house, or move the house they live in now. Over the past ten years most of the residents have lost about 10 feet of bluff. More of the bluff is lost each year. Lake Street has lost an approximate 100 feet in the last ten years (Newby pers. comm. 2002).
Since having built the seawall, the value of the property has increased. This is good for the property owners. Though they know that this seawall is probably not the best or most efficient way to protect the bluff, it is the cheapest. The best protection would cost almost three times more than the seawall.
According to Mr. Newby, the bluff on the sides of the seawall will erode very slowly, if at all. Research shows that erosion on the sides of seawalls is often very severe. He knows that the beach consists of a very compact sand. Rocks and other debris have been taken from the beach to build many different things in Homer. This causes even faster erosion.
Even though the top and bottom of the bluff are eroding, the seawall will only protect the bottom. Each property owner will need to build a secondary retaining wall to protect the top of their bluff property. Mr. Newby also believes that the sand the seawall is built on is hard enough, and the water level low enough, that the sand will not erode. The erosion of this sand would cause the seawall to collapse.
According to some people at city hall and the Kachemak Bay Research Reserve the seawall was built incorrectly. It is missing some very important parts. Unless these are added, the seawall will eventually collapse. The seawall has already, in part, collapsed because of these missing elements (Figure 2).
Most of the property owners behind the seawall are very glad to have it. They feel it is the best they could get with the amount of money they had available.
Effects of Seawalls and Alternatives
A seawall temporarily protects coastline property from erosion by the sea and may temporarily prevent shoreline retreat by blocking and dissipating wave energy. That is an engineer's goal when he/she builds a seawall, but there is also a negative side to a seawall that must also be taken into account. A seawall exposes adjacent unprotected property, destroying it faster than wave action would normally. Wave action is deflected off the wall and concentrated on the sides, quickly eroding neighboring property and reducing the sand supply to these beaches. The beach is also depleted in front of the wall, which is good for boaters, but bad for those who use the beach (Bush et al., 1996).
Another negative factor of building a seawall is maintenance costs. A seawall may have low maintenance costs if it is properly constructed, but becomes expensive if not. Some possible maintenance costs are:
Although there are several different ways of constructing a seawall, there are three main methods used. The first is thin, interlocking sheet piles driven deeply into the ground. The second method of seawall construction is individual piles used to support an above-ground structure. The third method is a massive gravity construction resting on the shore bottom or imbedded slightly in it. This construction is supported by its own weight rather than by piling.
Besides a seawall, there are two other alternate approaches to shoreline stabilization. The first is shore parallel structures offshore, such as breakwaters. The positive aspects of this method of shoreline stabilization are the same as the seawall. These are temporary protection of property, temporary prevention of shoreline retreat and low maintenance if properly constructed. There are also some disadvantages to breakwaters. These include eventual loss of recreational beach, increased erosion at the ends of the wall (Figure 3) and/or down drift, limited access to beach, aesthetics damage, high cost and regular maintenance requirements (Bush et al., 1996). Also, debris from the breakwater may make the beach hazardous, or destroy structures behind it. The second alternate method of shoreline stabilization is shore perpendicular structures, such as groins and jetties. There are several advantages to this method of stabilization. They trap sand to rebuild an eroding beach. They also retain sediment already on the beach, as well as new sand from beach nourishment projects. Of course there are several disadvantages to this method, as well. Down drift beaches are "starved" of sediment. There is increased erosion on the down drift side of the stabilization. Also, storms may detach groins from the shoreline.
Ecological Effects of the Seawall
At first glance the sand beach in front of the new seawall probably looks somewhat barren. Yet it is far from barren, all of the organisms that reside on a sandy beach usually remain hidden because of the pounding of the surf, and they also have a tendency to be small enough to go unnoticed. There are no large plants on most sand beaches; instead the plants tend to be single-celled dinoflagellates and blue-green algae.
Residing in the supra littoral zone are different species of isopods such as sand fleas. Moving down into the mid and lower tidal zone the diversity of animals increase. There are more macroalgae as well as many fast-burrowing clams that migrate up and down the beach as the tide changes. Large razor clams, cockles, snails, and small crab also tend to inhabit these areas along the beach in front of the new seawall. This sea life then is eaten by the shorebirds migrating through the region. In the lower tidal zone there are burrowing sea cucumbers, urchins and sand dollars (Lalli and Parsons, 1997).
Now that the seawall has gone up we expect that the sandy beach near the seawall will be eroded away and be replaced by a rocky environment. The inhabitants of rocky beaches are quite different from that of a sandy beach so we expect that there may be a large shift in the local ecosystem. Sand migrating through the area may not be enough to support a sandy bottom ecosystem yet also prevent a rocky shore ecosystem from developing.
Properly constructed seawalls can provide protection to the property behind them at a relatively low price. For the homeowners who are ultimately paying for the seawall the price is important and the presence of the seawall is needed to protect their property. However, the normal flow of sand along the beach is altered by the seawall causing additional erosion at the sides of the seawall and in front of it. The seawall may need to be lengthened in the future to protect surrounding property. The seawall is far enough up the beach that it isn't exposed to seawater except at high tides and the additional erosion in front of the seawall may only affect the creatures of the supra-littoral zone. While not having a large ecological impact on the region the seawall does have a large economic impact on the beachfront properties that it does protect. Only the future can fully show the value of the seawall that was constructed. Its partial collapse within a month of completion and the many post-construction modifications do not bode well for this seawall providing a truly inexpensive solution to bluff erosion in the area.
Bush, D. M., Pilkey, O. H. Jr., and Neal, W. J., 1996, Living by the rules of the sea, Duke University Press, Durham N. C., pp. 179.
Dean, C., 1999, Against the tide: The battle for America's beaches, Columbia University Press, New York, pp. 279.
Kaufman, W., and Pilkey, O. H. Jr., 1983, The beaches are moving: The drowning of America's shoreline, Duke University Press, Durham NC, pp. 336.
Lalli, C. M., and Parsons T. R., 1997, Biological oceanography: an introduction, Butterworth Heinemann, Oxford, pp. 314.
Mason, O., Neal, W. J., Pilkey, O. H., Bullock, J., Fathauer, T., Pilkey, D., and Swanston D., 1997, Living with the coast of Alaska, Duke University Press, Durham N. C., pp. 348.
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