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Cameron Parish, Louisiana: 14.5 Km Beach Nourishment Project Challenges and Solutions

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Cameron Parish, Louisiana: 14.5 Km Beach Nourishment Project Challenges and Solutions CAMERON PARISH, LOUISIANA: 14.5 KM BEACH NOURISHMENT PROJECT CHALLENGES AND SOLUTIONS V. Shepsis1, H. E. Bermudez2, J. D. Carter3, and W. Feazel4 ABSTRACT The Cameron Parish Gulf of Mexico shoreline of concern extends approximately 14.5 km (9 miles) westward from the west jetty of the Calcasieu Ship Channel and is bounded on the north by LA Highway 27/82. The beach (a relic chenier feature) and the highway are the only barriers between the Gulf of Mexico and an extensive wetland system covering more than 16,188 hectares (40,000 acres) north of the Gulf Shoreline. The Cameron Parish shoreline has experienced severe erosion resulting from a shortage of sediment supply, increased storm activities, and relative sea level rise with average shoreline erosion rates at 4.6 m/yr (15 ft/yr) between 1998 and 2009. Erosion has dramatically reduced the beach width which has increased the likelihood of breaching the beach barrier and LA Highway 27/82. The Louisiana Office of Coastal Protection and Restoration (OCPR) has undertaken and funded a project to restore approximately 14.5 kilometers (9 miles) of Cameron Parish shoreline by nourishing the beach in a manner that will delay further shoreline retreat and reduce breaching of the beach and highway barrier. Detailed coastal engineering analyses and numerical modeling were conducted to develop an understanding of coastal processes, determine the causes of shoreline erosion, and develop design criteria for beach nourishment alternatives. Based on this coastal engineering analysis, beach nourishment alternatives were developed and designed to provide different levels of protection depending on the availability of sand. A maximum required volume of sand was estimated to be approximately 3.1 million cubic meters (4.0 million cubic yards). A sand source investigation identified and delineated two offshore borrow sources with suitable sand material. These borrow sources are located at distances of 27 and 32 km (17 and 20 miles) offshore in depths of 6.1 to 7.6 m (20-25 ft) MLLW. Alternative sand delivery procedures including dredging, transportation, and beach placement were developed to identify the most cost-effective methodology. These alternatives were developed with and without nearshore sediment re-handling areas. The project is currently undergoing final design and permitting. Project construction is anticipated to begin at the end of 2010 or at the beginning of 2011. Keywords: Holly Beach, Louisiana, shoreline change, dredging, rehandling. INTRODUCTION The Cameron Parish project site shoreline, located in the chenier plain of southwestern Louisiana, extends 9 miles west from the Calcasieu Jetty to the most eastern breakwater at the Holly Beach-Constance Beach breakwater field as shown in Figure 1 and Figure 2a. The shoreline within the project site is retreating at a high rate, ranging from 1.5 m/yr (5 ft/yr) to as much as 9.1 m/yr (30 ft/yr) in some areas. Along much of the shoreline, the sand chenier, which acted as a barrier between the Gulf of Mexico and 16,188 hectares (40,000 acres) of freshwater wetlands and infrastructure, is severely or completely eroded as shown in Figure 2b and 2c. Because the chenier is now extensively eroded on much of the project site, Highway 82/27 is the only barrier that separates the Gulf from the uplands and wetlands for much of the project site. Currently both the wetlands and the highway are in danger of being severely damaged or destroyed if no action is taken. The Louisiana Office of Coastal Protection and Restoration (OCPR) undertook this project with the goals of increasing the Cameron Parish barrier headland’s (i.e., beach) longevity in a manner that will delay further shoreline 1 Principle, Coastal Engineer, Coast & Harbor Engineering, 110 Main St., Ste 103, Edmonds, WA 98020, USA, T: 425-778-6733, Fax: 425-778-6883, Email: [email protected]. 2 Principle, Coastal Engineer, Coast & Harbor Engineering, 3410 Far West Blvd. Ste 210, Austin, TX 78731, USA, T:512-342-9700, Fax: 512-342-9708, Email: [email protected]. 3 Coastal Engineer, Coast & Harbor Engineering, 3410 Far West Blvd. Ste 210, Austin, TX 78731, USA, T:512-342-9707, Fax: 512-342-9708, Email: [email protected]. 4 Project Manager, Louisiana Office of Coastal Protection and Restoration, 450 Laurel Street, Ste 1200, Baton Rouge, LA 70801, USA, T: 225-342-4641, Fax: 225-342-6801, Email: [email protected]. 106 retreat and to prevent breaching of the sandy chenier barrier in the next 20 years. Beach nourishment is proposed as the measure to achieve these goals. The engineering for beach nourishment of this project site is composed of identifying a sand borrow source, determining the volume available for nourishment by determining the most efficient method of sand delivery from borrow site to the beach, and a coastal engineering and analysis which includes predictions of shoreline position and beach nourishment design configurations. Project Site Figure 1. Project vicinity: Southwest Louisiana Chenier Plain. a b c Figure 2. (a) Project extents and reaches along the Cameron Parish shoreline, (b) Reach 2 shoreline looking northward at Highway 82/27, and (c) Reach 2 shoreline looking eastward. BORROW SITE INVESTIGATION A borrow source investigation was undertaken during June of 2009. A review of literature and previous data identified eight potential sites for further field investigation. These included four nearshore sites within 10 km (6 miles) of the shoreline where a possible buried river channel may be located and four offshore sites within 30-40 km (20-25 miles) of the shoreline on sandy shoals along the Sabine Banks. These sites are shown in Figure 3. Sites A, C, D, E are the nearshore sites and F, H, J, K, are the offshore sites. Field data collection for the investigation included 395 km (245 miles) of geophysical data collection (sub-bottom profiler, sidescan sonar, fathometer, and magnetometer) and 79 vibracore borings along with 307 grain size analysis tests. 107 The investigation identified two borrow sites along the Sabine Banks that together contain at least 6.9 million m3 (9 million cy) of high quality, fine to medium sized sand (d50 = 0.22 to 0.25mm), with a very low percentage (less than 2% on average) of silt and clay. The sand layer is between 1.5-4.6 m thick and 3 m on average (5-15 ft thick and 10 ft on average), and has no overburden to be removed prior to dredging. The borrow sites are located approximately 32 km (20 miles) offshore along the eastern portion of the Sabine Banks, shown in Figure 4, labeled HF and JF. The total volume of sand available in Site HF (Figure 5, top) is between 3.7 and 4.4 million m3 (4.8 and 5.8 million cy), and the total volume of sand in Site JF (Figure 5, bottom) is between 4 and 4.3 million m3 (5.3 and 5.6 million cy). Typical longitudinal fence diagrams of both borrow areas are shown in Figure 6. The sand thickness on site HF ranges from 1.5 to 3 m (5 to 10 ft) on average, while the sand thickness in site JF is on average about 1.5 m (5 ft thick). Details of the borrow source investigation are available in the Borrow Source Investigation Report (CHE, 2009). D E C A H J F K Figure 3. Locations of borrow site investigation search blocks. 108 Figure 4. Location of proposed borrow sites HF and JF. 109 Figure 5. Proposed borrow sites HF and JF. 110 a b Figure 6. Fence diagrams of borrow sites (a) HF and (b) JF. SAND DELIVERY METHODS The configuration of sediment placement alternatives (beach nourishment alternatives) depends on the volume of material available for placement, which in turn depends on the construction budget and the overall unit cost of sand delivered to the project site. The unit cost depends on the sediment delivery methodology, and therefore, the sediment delivery method is developed first to provide a reasonable estimate of the volume of material that can be used for the alternatives. After the available volume of sediment is determined, configurations for shoreline restoration alternatives are developed. The sediment delivery system that will be utilized for this project will ultimately be determined by the selected dredging contractor to meet project conditions and requirements. For the purpose of estimating the sand volume that will be available for nourishment, several methodologies for dredging and placement of sand material are considered: Several alternative sand delivery methods were developed and included pump-out from a hopper dredge from within Calcasieu Pass (P-C), rehandling from a temporary disposal site within Calcasieu Pass (R-C), and pump-out from a hopper dredge offshore the project site (P-O). The R-C method consists of dredging the borrow source by trailing suction hopper dredge(s) and transporting the dredged material to the designated rehandling area inside Calcasieu Pass. There, the material in the hopper dredge will be bottom dumped in the rehandling area, and the hopper will return to the borrow site for continuation of dredging. A cutterhead dredge will re-dredge the material placed in the rehandling area and pump sand to the beach for placement through a pipeline with booster pumps (as needed). For the P-C method, a trailing suction hopper dredge(s) will dredge sand from the borrow site and transport the dredged material to the designated pump-out station inside Calcasieu Pass. The hopper dredge will hook up to a pipeline and pump out the dredged material directly from the hopper onto the beach through a pipeline with booster pumps (as needed). For the P-O method, a trailing suction hopper dredge(s) will dredge sand from the borrow site and transport the dredged material to a designated pump-out station offshore to the project site.
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