Taming Shoreline Erosion on Lake Pontchartrain: Engineerd Solutions at the Expense of the Environment

May 1, 2001
Southeastern Louisiana contains 40% of the United States’ wetlands. As such, this region, which includes the Mississippi Delta area, constitutes the largest single seafood production region in North America. Over the last several decades, the state has been losing these fertile wetlands at an alarming rate. Between 1932 and 1990, coastal Louisiana lost 1,250 mi.2 of land. Since 1990, vital wetlands have been eroding away there at the rate of 25 mi.2/yr.The largest lake in the region, Lake Pontchartrain–which contains a mixture of fresh and salt water and on whose southeast shore sits the city of New Orleans–has also suffered widespread destruction of wetlands along its extensive shoreline. An important factor in the destruction of valuable wetlands along Pontchartrain’s shores, according to Shea Penland, a professor of geology at the University of New Orleans, is the manner in which many owners of lakefront property have dealt with shoreline erosion, which ranges from 3 to 25 ft./yr.Engineered Erosion Control–and Wetlands Destruction
The beginning of the destruction of wetlands on the lake goes back to the early 1900s when Orleans Parish filled in 200 ac. of land along Pontchartrain’s southeast shore and proceeded to build homes on that filled-in area. As time went by, considerable erosion of the lakeshore occurred along both the filled-in area and other stretches of the southeast coast. To put an end to that erosion, Orleans Parish constructed a 6-ft.-high stepped concrete sea wall during the early 1930s. Although the sea wall solved the problem of shoreline erosion, Penland observes, it did so at the cost of destroying vital habitat for fish, shellfish, and wildlife.As the 20th century wore on, more and more development took place along Lake Pontchartrain’s shorelines, and with it came the destruction of many biologically fertile marshlands, especially on the southern shoreline. In the early 1970s, Slidell, on the northeastern shore, expanded rapidly with housing developments, but at the expense of more precious wetlands. Natural shorelines and beaches quickly disappeared and were replaced by bulkheads.The type of erosion control measures chosen depended on the particular location along the Lake Pontchartrain shoreline. Specifically, natural marshlands were replaced with a stepped concrete sea wall in the New Orleans area on the southeast shore of the lake; with riprap, especially popular in Jefferson Parish on the south coast; with timber or steel bulkheads, common in the Slidell area and with residential homeowners generally; or with vertical concrete sea walls, popular in the Mandeville area on the north coast.So what is wrong with all these engineered measures to prevent further eating away of lakefront property? In Penland’s view, and that of some others, there is an important issue here of real-estate conservation vs. natural resource conservation. “When people build hard structures along a waterfront to prevent further shoreline erosion,” Penland maintains, “the natural resource base goes away.” By destroying the natural sloped banks and marshlands along the lake shoreline, individual lakefront property owners are destroying an important regional and even national resource–the abundant fish, shellfish, and wildlife formerly nurtured by these fertile wetlands, he says. “Why not push out the natural shoreline again?” Penland asks. “Push it out to where it once was, far enough so that it won’t erode back for another 50 years or so. When an individual property owner installs a bulkhead along a lakefront, he protects his own property against further erosion–but at the expense of our society. This is a short-term solution that has a long-term cost to society. When a landowner installs a bulkhead rather than a coastal marshland, in effect he is sacrificing an important societal natural resource for the sake of protecting his own property. The problem is that each landowner is looking at his property individually–and no one is thinking of the collective good of the entire community.”Not all the destruction of Lake Pontchartrain wetlands is attributable to these erosion control techniques. Some is believed to be caused by excessive saltwater intrusion from the Gulf of Mexico. Another factor may be a deficiency of plant nutrients in some wetlands. Centuries ago, the Mississippi River regularly overflowed its banks, depositing its nutrient-rich sediments over the entire delta region. The construction of massive flood-control levees along the Mississippi River (these stretch for 900 mi. along the length of the lower Mississippi) has brought the benefit of flood control but has also deprived the Mississippi River Delta region, including Lake Pontchartrain, of the rich nutrients that spur growth of vegetation in wetlands. One proposed solution is to divert some of the sediment- and nutrient-rich waters of the Mississippi to wetlands areas that are in trouble. In an effort to bring new life to the delta, several projects have already been implemented, and others are planned. Diverting some Mississippi River waters into Lake Pontchartrain has also been proposed but not done; this diversion issue is controversial. Proponents argue diversions are needed to give Lake Pontchartrain wetlands a much-needed nutrient boost; opponents claim such would spur the growth of algal blooms.Returning now to the issue of manmade destruction of Lake Pontchartrain wetlands, how bad is the situation today? Penland estimates that about 30% of the shoreline is currently developed, with much of that developed area accompanied by extensive destruction of wetlands. Another 20% of the shoreline is set aside by federal and state authorities as a wildlife refuge and, as such, is totally off-limits to any development in the foreseeable future. The remaining 50% still is in a natural state and undergoing natural erosion, but since it has not been set aside as a national or state wildlife refuge, it could be subject to development. Argues Penland, “We need to maintain what wetlands we still have on Lake Pontchartrain. Let’s not fill in any more of them.”Effects of Wetlands Destruction on Lake Ecosystem According to Michael Poirrier, a research professor in the Department of Biological Sciences at the University of New Orleans, the destruction of wetlands around Lake Pontchartrain has a significant impact on the ecology of the lake and of the estuary linking the lake with the Gulf of Mexico. The lake ecosystem is based in part on small particles of marsh plants and associated fungi and bacteria flaking off and dropping into marsh waters. These materials are then washed into Lake Pontchartrain. Poirrier says these small fragments from marsh plants, called detritus, are an important food source for fish and shellfish in the lake and estuary.A typical marsh along the shores of Lake Pontchartrain consists of grasses known as spartina, one variety being 2-4 ft. high, another 4-8 ft. high. These tidal marshes are flooded during high tide. The main species of fish in the lake are shrimp, crab, menhaden (pogy), catfish, red fish (red drum), and speckled trout. All these fish depend on detritus from the marshes. “If you decrease the marshes around Lake Pontchartrain or decrease the exchange of water between marsh and lake,” Poirrier points out, “you will decrease the production of these species.”Has the fish catch, then, declined in the Lake Pontchartrain area over the past decade or so? Poirrier says fishing is not as good as it used to be. The current harvest is the same as before, but fishermen have to work much harder and longer to bring in that catch–more boats trawling more often. The catch per fisherman is less.Poirrier is especially concerned about the destruction of marshlands on the southern shore of the lake, running from South Point, east of New Orleans, west to New Orleans and still farther west to Kenner, a total length along the southern lakeshore of about 20 mi. As recently as 30 years ago, there were still some marshes along this shoreline. Today they are nearly completely gone.This destruction of wetlands along the southern shore, Poirrier insists, has had a very significant impact on lake ecology. Instead of marshes lining this shoreline, now there is only a concrete sea wall (Orleans Parish) and limestone riprap or broken concrete. These marshlands were most important not only because they produced detritus for lake fish to feed on, but also because they provided a habitat for juvenile fish life. Shrimp, crab, menhaden, and other fish species spawn in the Gulf of Mexico. The young are then carried in by the tides, where they have traditionally used the marshes along the Lake Pontchartrain shoreline for habitat. As they mature, they migrate back to estuarine waters, then to the gulf.Bringing About a Rebirth of Destroyed WetlandsPenland is calling for a well-thought-out plan to fill in land along various stretches of the Lake Pontchartrain shoreline, to contour the banks with a gradual slope with no abrupt changes in elevation. His proposal is to create new wetlands at various stretches along the Lake Pontchartrain coast, areas of submerged vegetation, and new beaches and sand dunes, all of which would act as natural barriers against erosion while re-creating natural habitats.He believes private lakefront landowners should band together and work with the Lake Pontchartrain Foundation to jointly create a plan for lakewide restoration of wetlands habitat. It would call for filling in eroded areas along the shoreline with dredge material, thereby creating marshlands to protect the shoreline against accelerated erosion while providing a food supply and habitat for fish and wildlife. Penland believes with collective effort, local citizens’ groups could get funding through the Coastal Wetlands Protection, Preservation, and Restoration Act of 1990 to carry out natural lakeshore restoration. “We need to use that 1990 act to help to restore wetlands around Lake Pontchartrain,” he argues.Penland would give priority to enhancing wetlands at Labranche on the southwest corner of the lake and at Fernier on the south shore, to building breakwaters, and to beach nourishment projects (which might involve the pumping of dredge material to create new marshlands) along the lakefront in Orleans Parish and Jefferson Parish. He believes it would make sense to construct either new beaches or wetlands in front of the bulkheads in developed areas, to re-create a buffer of wetlands on the water side of those walls.Focusing on the 20-mi. stretch of shoreline along the southern shore that bounds greater New Orleans, Poirrier would like to completely re-create a wetlands there the way it was decades ago. A crucial first step would be to construct a breakwater to create a quiescent area between breakwater and shore where a new wetlands can be established. The force of the waves must be broken if new marshland vegetation is to have an opportunity to establish itself. Building the breakwater in 3-ft.-deep water would minimize construction costs. Most submerged aquatic vegetation and wetlands grasses also require shallow, calm water. The breakwater would run parallel to the shore, anywhere from 50 to 300 yd. offshore. It could be built inexpensively, Poirrier believes, by using sausage tubes or geotubes (cylinders made of a geotextile typically 4 ft. in diameter and hundreds of feet long) filled with material from a nearby dredge (see sidebar). Poirrier is enthusiastic about using geotubes to construct breakwaters because they enable the ecological engineer to make use of abundant local material (i.e., dredge material). There is a compromise involved, though: A breakwater should do more than attenuate wave energy; it should also serve as a habitat for fish. That means the breakwater must have abundant nooks and crannies, as in a natural reef. Small fish will hide in these spaces, waiting to feed on numerous organisms that swim by. Larger fish will then also be attracted to the reeflike breakwater and eat the smaller fish flourishing there. Though a riprap breakwater would provide more nooks and crannies and would comprise a better habitat for fish, it would be considerably more expensive. It is crucial that there be not one continuous breakwater running for miles but, rather, numerous discrete breakwaters along a line roughly 200 yd. offshore, with gaps or overlaps between adjacent breakwaters. Such gaps, Poirrier explains, will ensure the biologically crucial interchange of water between new wetlands and the lake. Poirrier believes it would take about a year for grass beds to establish themselves in the region between breakwater and shoreline. Over several years, sediment would build up naturally as it settles out in this calmer region from water flowing in from the lake. Pumping in dredge material would allow a new marshland to be built up more quickly.Thomas Podany of the Army Corps of Engineers’ New Orleans District maintains that it is not always a case of engineered solutions vs. natural ones. Both are used on the same project–for instance, in some of the cases previously discussed, when a breakwater, an engineered structure, is used as part of a scheme to rebuild a wetlands. Another example of a mixed engineered and natural solution lies on the western shore of Lake Pontchartrain, north of Laplace, where erosion has been gouging into the shoreline at the high rate of 15 ft./yr. If that erosion continues unabated, the land bridge there traversed by Interstate 55 could be inundated. Just placing riprap along the shoreline there would have proved ineffective, for the riprap simply would have sunk into the wetlands. The best response in this case, Podany says, was to combine an engineered and natural solution. The corps plans to construct a breakwater off the western lakeshore, which would not only tame the waves but also trap sediment in the quiescent region between breakwater and shoreline, eventually extending that shoreline eastward, as that new wetlands developed. Podany says corps research has demonstrated that the difference between a stable and an eroding shoreline is very little. Small increases in wave energy can transform a stabilized shoreline into an eroding one. Planting vegetation along a shoreline subjected to high-energy waves does not work; one must first build a breakwater offshore to attenuate wave energy. Yet an alternative to building a costly breakwater, he suggests, is to grow an oyster reef on the bottom of a lake, in shallow waters near the shoreline, to act as a wave break.Although natural solutions work best in low-wave-energy environments–on small rather than large lakes–Podany points out that a mix of engineered and natural solutions can be used on the same lake. For instance, the prevailing wind over that lake might be blowing in one direction, creating intense wave action and severe erosion only on the northwest shoreline. One could use riprap there and natural solutions (marshes) on the remaining shoreline. Finally, he notes, it is much easier to grow wetlands vegetation in the warm south compared to the cold north.Podony is much less critical than Penland of Lake Pontchartrain property owners installing bulkheads because he says most bulkheads are installed in developed areas that have long ceased to be important habitats. Most corps efforts for establishing new wetlands are on lakes in remote, undeveloped areas. It is for those undeveloped areas that the corps has federal funds to initiate projects for protecting existing wetlands or restoring deteriorated ones. In brief, Podony does not believe it makes sense to re-create wetlands in developed areas.Bulkheads: Popular Among the Engineered SolutionsRed Eye Crossing soft dike demo projectProperty owners often move to have a bulkhead installed when they fear that the erosion occurring on their lakefront property will shortly begin to undermine houses or other buildings. Brett Audibert of Gill’s Crane Service, a bulkhead-construction contractor in New Orleans, explains that some waterfront property owners also have a groin built in conjunction with their bulkhead. A groin is similar to a breakwater, only this structure of rocks is built perpendicular to the bulkhead. It extends from the bulkhead out into the lake. Its purpose is to trap sediment from the lake waters, resulting in the buildup of a beach in front of the bulkhead.Until recently, wood was the most popular choice of materials for bulkheads because it was the most economical. A typical wood bulkhead along Lake Pontchartrain is constructed by driving 8- to 10-in.-diameter wooden piles into the ground every 5 ft., then connecting them together by fastening horizontally a 6- x 6-in. wooden wale near the top of the piles and another near the mud line. Next, 2-ft.- x 8-ft.- x 2-in.-thick wooden sheets, with tongue-in-groove design, are driven into the ground on the land side of the pile-wale framework and nailed to it. In Lake Pontchartrain, these treated wooden bulkheads last about 25 years. During that time, the Toledo worm is busy eating the wood.Over the years the vast majority of wooden bulkheads have been constructed of creosote-treated wood, says Audibert. Recently, though, contractors have moved away from this type of treated wood because the creosote seeps into the water, producing a film on the water’s surface that can cause skin burns on swimmers. Instead, makers of wooden bulkheads more commonly use wood treated with chromated copper arsenate. Some states, such as Florida, ban the use of creosote for bulkheads, but elsewhere it remains widely used, including by many government agencies, such as the Corps of Engineers.Today most of the bulkheads Audibert installs make use of vinyl sheet piling fastened to the usual wooden piling-wale framework. He started to use interlocking vinyl sheet piling about five years ago, mainly to extend the longevity of new bulkheads. Unlike wooden bulkheads, Audibert maintains, vinyl bulkheads resist attack by worms, other marine organisms, and sunlight. The vinyl supplier, Materials International of Atlanta, GA, warrantees the material for 50 years, but government tests suggest that a vinyl bulkhead should have a lifetime of at least 100 years. According to Audibert, the installed cost of a vinyl bulkhead ranges from $85 to $200/lin. ft., depending on the thickness (0.25-0.75 in.) of the vinyl sheeting. Audibert likes vinyl sheeting because it can be used as is during construction: There is no need to paint it (as with steel) before installing.Only a relatively small fraction of bulkheads, Audibert believes, are built using steel sheet piling (0.25-0.5 in. thick). The installed cost of a steel bulkhead is in the $300 to $500/lin. ft. range. The steel sheets must be sandblasted and coated with coal-tar epoxy in the field. If the exposed surfaces are repainted every 10-15 years, the steel bulkhead should have a lifetime of at least 50 years. Steel bulkheads are used only where great strength is needed, such as in places of intense wave action.The Future of Lake PontchartrainWill the dream of Penland and Poirrier and other environmentalists to reverse the decades-long process of wetlands destruction on Lake Pontchartrain become a reality? That remains to be seen. One difficulty is that many citizens do not perceive a problem. They look at the sea walls, riprap, and bulkheads along Lake Pontchartrain and conclude that these structures are doing their job: They are preventing further erosion of the lake shoreline, and that seems good. What they do not see is the significant environmental costs of preventing shoreline erosion in that manner.Another difficulty is that the lakefront along the southern shore in the greater New Orleans area has low visibility and low priority. A major reason is the 26-ft.-high levee (built in the 1970s to provide flood protection during hurricanes) that runs along the waterfront for 20 mi., largely isolating the waterfront from the rest of New Orleans. Other factors that downgrade the value of the waterfront in the eyes of Louisianans are the pollution from city stormwater outfalls, with a consequent ban on swimming, and the presence of railroad tracks. The best hope for fulfilling the dream of restoring Lake Pontchartrain’s wetlands is stepped-up public education concerning the ecological gravity of this issue. Quite clearly, Penland and Poirrier feel strongly about the need for wetlands restoration because they view the issue from a historical and an ecological perspective. They are keenly aware of the amount of wetlands destruction over the last several decades and of how things used to be. Further, they understand how critical those wetlands are to the entire region’s ecology. If Louisianans hope to still have a robust fisheries industry in the decades ahead, they would do well to listen more closely today to what some thoughtful and farseeing environmentalists have been saying.