SEGMENTED BREAKWATERS AND THEIR USE IN COASTAL LOUISIANA Prepared by: Louisiana Coastal Protection and Restoration Authority Engineering Division March 2016
The geology of Louisiana s coastal zone is intimately tied to the history of the Mississippi River. Throughout time, the Mississippi River has shifted is course approximately once every 1,000 years in order to become a more hydraulically efficient system. These shifts have resulted in the complex morphology of Louisiana s coastal zone. This shifting process builds new delta lobes, while abandoning the old delta lobes, and establishes the framework necessary for barrier island development. The abandoned delta lobe is deprived of its sediment supply which results in the transgression of the delta. During this period the delta lobe is subjected to the erosional forces of waves, wind, and tide. These forces cause the delta lobe to elongate into barrier headlands through the transport of sediment along the shoreline. As shown in the figure below, the erosional forces, subsidence, and sea level rise cause the headland to detach from the mainland and become a barrier island. This transgression of the delta lobe is what ultimately formed the barrier island chains in Louisiana from Pointe au Fer to Hewes Point of the Chandeleur Islands (Louisiana Barrier Island Erosion Study, Coast 2050). In the early 1900s, the Mississippi River levees were completed in order to reduce the number of overbank flooding events and protect people. These actions allowed the increase in economic development but at the cost of disturbing the natural deltaic cycles. This disturbance deprived the delta lobes of the needed sediment, which contributed to an increase in subsidence and landloss. The continued landloss of the delta lobes caused the formation of the barrier island chains as described above. These barrier island chains are now sediment starved and rely on beach restoration projects and potential renourishment cycles for their survival. Restoration of Barrier Islands and Headlands Restoration projects on barrier islands protect the shoreline from waves, storm surge, and the resulting coastal erosion. These restoration projects can consist of either soft protection or hard structural protection, or a combination of the two. Soft protection projects, also known as beach nourishment projects, utilize large volumes of beach quality sand from outside sources to restore an eroding shoreline. Hard structures are built either parallel or perpendicular to the shoreline in order to prevent waves and currents from causing excessive erosion. These structures typically include breakwaters, seawalls, groins, CPRA Page 2 3/15/16
and jetties. It is critical that the chosen method of restoration be based on sound coastal engineering principles to address the coastal processes, and that all possible alternatives have been considered, along with potential impacts to updrift and downdrift shoreline areas. Use of Segmented Breakwaters With the lack of sediment supply, one of the biggest threats to the Louisiana barrier islands is erosion. Shoreline protection systems, including breakwaters, can be used to intercept and dissipate the energy of waves and currents and associated sand transport to protect the shoreline against erosion. Shoreline erosion from waves and currents account for only one of the major processes impacting the sustainability of Louisiana s barrier shoreline. Various hardened structural options, such as rock breakwaters, affect longshore and cross shore sediment transport processes by changing the wave and current properties near or on the shoreline. However, changes in waves and currents, while potentially reducing movements of sediment along and across the shoreline in one location, can cause increased erosion in other areas due to reduction of sediments being transported toward another stretch of shoreline. This negative impact is known as an erosional shadow. For example the western shoreline of Raccoon Island is eroding at double the historic erosion rate, now being in the erosional shadow of breakwaters. There are many variants to the design of rock breakwaters, including single or segmented, emerged or submerged, and narrow or broad-crested. All local coastal processes, both up-drift and down-drift, need to be considered when choosing the type of rock breakwater to be used in order to maximize the prevention of erosion on the shoreline (Leo van Rign, 2013). While developing the design criteria for segmented breakwaters, the desired beach response must first be identified. Local beach response can be categorized into five different classifications, shown below: 1) Permanent Tombolos, 2) Periodic tombolos, 3) Well-Developed Salients, 4) Subdued Salients, or 5) No Sinuosity. These beach responses are a result of the amount of wave energy reaching the lee side of the breakwater segment. Figure 1: Formation of salients and tombolos as a response to breakwater construction CPRA Page 3 3/15/16
The design of an offshore breakwater, especially a series of segmented offshore breakwaters, is a complex problem involving a number of design parameters. The first step in the design process is for the coastal engineer to establish the goals of the breakwater, including the desired shoreline response. Based on the goals and site conditions, the engineer then needs to choose the length, spacing, offshore distance, crown elevation, crown width, side slopes, rock size, and orientation to the shoreline. The combination of these variables plus the local and system wide coastal sediment regime and coastal processes all contribute to the success, or failure, of the breakwater project. Benefits, Impacts and other Considerations The dissipation of wave energy and current has a direct effect on the sediment transport along the shoreline. In sediment starved systems, such as Louisiana s barrier islands, this disruption of sediment transport along the shoreline can have a detrimental effect on adjacent shorelines. Generally, segmented breakwaters cause a sediment deposition in the lee waters between the breakwater and the shoreline. While this appears to be a successful outcome, this deposition of sediment may have actually accelerated the erosion rate on the adjacent shoreline, shown as downdrift erosion in Figure 1. The use of rock breakwaters generally requires that the shoreline be maintained at a certain position over time in order for them to be effective. The overall transgression of the barrier islands, as discussed previously, severely limit the successful applications of these structures throughout coastal Louisiana. As the barrier island retreats, the distance between the breakwater and the shoreline increases, limiting the structure s ability to dissipate the wave energy. Eventually the structure is deemed ineffective and will require maintenance in the form of sediment additions, to restore the effectiveness of erosion prevention. In addition, rock breakwaters are large, heavy structures that require high strength soils for a stable foundation. These high strength soils are rarely found in coastal Louisiana. In order for rock breakwaters to be successful, a detailed geophysical/geotechnical investigation must be performed within the project area. This investigation will provide the information needed for the coastal engineer to properly design the breakwater structure. The breakwaters must be designed to an elevation that will also account for all settlement of the structure during the design life. If the breakwater settles more than anticipated, the structure will be ineffective and will require maintenance construction events. Conclusion The decision to use segmented breakwaters should be based on a thorough analysis of the shoreline developments in the past, a thorough understanding of the coastal processes, and a forecasted shoreline response for future developments. Hardened shorelines have historically shown impacts due to erosional shadows, and as breakwaters do not address rising sea level, subsidence, and the increasing tidal prism (larger volumes of water within the estuary due to wetland loss), the use of soft alternatives (i.e. beach renourishment) have proven to be a more cost effective approach, even understanding that those sediments will continue to mobilize. A complete understanding of the nearshore sediment regime and long-term pre-project sediment budget is essential. The techniques utilized in the design of a highly effective and efficient breakwater system are complicated due to the complexity of the problem as well as Louisiana s deltaic geology. Shoreline erosion can be caused by numerous factors which need to be properly identified prior to reaching the decision of using a segmented breakwater structure and during the design of the structure. In many cases, segmented breakwaters may seem to stabilize the shoreline, but they will not remedy the cause, and additionally could exacerbate erosion rates on adjacent shorelines. CPRA Page 4 3/15/16
References Louisiana Barrier Island Erosion Study: Atlas of Shoreline Changes in Louisiana From 1853 to 1989. United States Geological Survey. S. Jeffress Williams, Shea Penland, and Asbury H. Sallenger. 1992. http://www.coast2050.gov/reports/bia/barrierislandatlas11x17.pdf Leo C. van Rijn. Design of Hard Coastal Structures Against Erosion. www.leovanrijn sediment.com. March 2013. United States Army Corps of Engineers, Coastal & Hydraulics Laboratory, ERDC. EM 110 2 1100 Coastal Engineering Manual (CEM) Parts I through VI. Vicksburg, Department of the Army, 2003. CPRA Page 5 3/15/16