Coastal Engineer IV, Mott MacDonald, Morado Circle, Suite 300, Austin, TX 78759, USA, T: ,

Transcription

1 LA QUINTA TERMINAL MITIGATION PROJECT: LARGE SCALE DREDGED MATERIAL BENEFICIAL RE-USE FACILITY FOR ESTUARINE HABITAT CREATION IN CORPUS CHRISTI BAY, TX Luis M. Maristany, P.E. 1, Aaron G. Horine, P.E. 2 and Paul D. Carangelo, REM, CESM, PWS 3 ABSTRACT The US Army Corps of Engineers (USACE) has adopted, as a part of the 404-permit process, a policy which allows offset of project impacts through in-kind mitigation. To address these requirements, the Port of Corpus Christi Authority (PCCA) were tasked with creating 6.6-acres of smooth cordgrass (Spartina alterniflora) habitat and acres of Shoal Grass (Halodule wrightii) habitat as mitigation for the PCCA La Quinta Terminal Project. Meeting these mitigation requirements in semi-arid South Texas was a challenge which required careful planning, design and construction by PCCA and Mott MacDonald. This challenge was approached by constructing the 200-acre Beneficial Use Site 6 (BUS-6) using new work (non-maintenance) dredged material through several phases that included: placement of dredge materials into BUS-6, the reworking of placed materials, the re-use of dredged material from an upland dredged material placement area (DMPA), and planting 12.6 acres of marsh vegetation and 25.3 acres of Shoal Grass; one of the largest known actively planted seagrass mitigation project in Texas. BUS-6 was constructed in the Corpus Christi Bay in 2012/13 as a part of the 39 foot La Quinta Channel Extension Project via hydraulic placement with 24 cutter suction dredges. BUS-6 construction utilized new work material to construct an earthen levee approximately 9 high and 9,200 long for beneficial use purposes which included wave protection for the planned deep-water terminal, creation of habitat conducive to the propagation of seagrass and fringe estuarine marsh habitat, addressing mitigation requirements associated with the Corpus Christi Ship Channel 52-foot project, and to provide a suitable location for implementing future mitigation projects. In 2014/15 as part of the deepening of the adjacent channel, additional new work materials were strategically placed in BUS-6 for creation of estuarine marsh habitat. In 2016 as part of the La Quinta Aquatic Habitat Mitigation Phase I project, PCCA and Mott MacDonald re-used these materials to create Habitat Berms constructed and planted at elevations conducive to survival and propagation of smooth cordgrass. In 2017, during Phase II, material from an upland DMPA was beneficially reused by hydraulically pumping it across the adjacent navigation channel to construct additional Habitat Berms and a geotextile-covered earthen Protection Berm to shelter the new marsh habitat from erosive waves and currents. Hurricane Harvey occurred immediately after construction, far exceeding design conditions for the berms, but the Protection Berm withstood the storm, which incurred no loss of new marsh habitat, with minor and easily reparable damage. Keywords: Dredging, beneficial use, dredged material disposal, mitigation, marsh habitat, seagrass. INTRODUCTION The Port of Corpus Christi is the fourth-largest port in the United States in total tonnage. All cargo transported through the Port travels along the Corpus Christi Ship Channel which extends from the Gulf of Mexico, through the jetties in Port Aransas, Texas and across the Corpus Christi Bay. As with any major port, expansion is a necessity to allow for increasing cargo demand and increasing vessel sizes. The Corpus Christi Channel was deepened to 47 in 1989; immediately following this deepening, the Port applied for authorization to deepen and widen the channel to 530 ; this request set in motion the Corpus Christi Ship Channel Improvement Project. The Port was Congressionally authorized to deepen and widen the 47 channel to a depth of 54 from the Gulf of Mexico to Viola Turning Basin in the Inner Harbor, and to extend the existing 47 La Quinta channel 1.4 miles at a depth of 39 to the Port s La Quinta Multi- Purpose Facility. Extension of the La Quinta channel was completed in 2013; the extended channel was then deepened 1 Coastal Engineer IV, Mott MacDonald, Morado Circle, Suite 300, Austin, TX 78759, USA, T: , luis.maristany@mottmac.com. 2 Principal Coastal Engineer, Mott MacDonald, 711 North Carancahua, Suite 1610, Corpus Christi, TX 78401, USA, T: , aaron.horine@mottmac.com. 3 Coastal Environmental Planner, Port of Corpus Christi Authority, 222 Power Street, Corpus Christi, TX, 78401, USA, T: , paul@pocca.com. 437

2 to 47 in 2014 to match the rest of the La Quinta channel depth. As part of this expansion and deepening, Beneficial Use Site 6 (BUS-6) was constructed for the placement and beneficial use of the new work dredge material produced during expansion and deepening operations. BUS-6 CONSTRUCTION BUS-6 was constructed using the new work dredge material produced from the dredging of the La Quinta channel extension, which consisted of Fat/Lean Clay & Clayey Sand. The area serves multiple purposes including: a placement area for dredged materials, providing shelter from waves due to prevailing winds for the adjacent multi-purpose terminal, and creating shallow water marsh and seagrass habitat for mitigation related to the adjacent multi-purpose terminal construction. Figure 1. BUS-6 conceptual drawing. BUS-6 Construction began with the placement of material in the open bay with an approximate depth of 11 along the footprint of the approximately 9,200 long offshore emergent levee. Material was dredged using 24 suction cutter dredges to create the adjacent La Quinta channel extension and placed in lifts to create the offshore emergent levee which is a stone riprap armored structure constructed to shelter the adjacent La Quinta channel from predominant wave conditions. The nearly completed first pass of the offshore emergent levee is shown in Figure

3 Figure 2. Offshore emergent levee material placement, first pass. Once the first pass was completed, a second and final pass on the emergent levee was made to bring the levee crest elevation up to +9 Mean Low Tide (MLT), making a total height of approximately 20 off the bay bottom. After placement of the levee material was completed, earthwork began to shape the material and armor the bayward side of the levee using stone riprap. Nearing completion of the emergent levee armoring activity, construction of the nearshore submerged levee began. Totaling a length of over 7,500, the nearshore submerged levee serves as a retaining levee during the fill operation within BUS-6. The nearshore submerged levee also serves as temporary sheltering from waves impacting the area. After completion of both levees, filling operations within BUS-6 began. Material was carefully placed to elevations conducive to the propagation of seagrass. Fill operations and dredge material placement were closely monitored during construction to ensure the proper elevations were met. Additional material was placed on the north end of BUS-6 for the future construction of estuarine marsh habitat. An aerial of the completed BUS-6 can be found in Figure 3. Over 3 million cubic yards of material was placed within BUS-6 during this filling operation. In total, over 7.7 million cubic yards of material was dredged as part of the La Quinta Channel extension and deepening projects, of which most of that material was used in the construction of BUS-6 and its levees. Figure 3. BUS-6 aerial immediately after completion. 439

4 LA QUINTA AQUATIC HABITAT TERMINAL MITIGATION PROJECT Upon completion of the La Quinta channel expansion and construction of BUS-6, the next phase of the BUS-6 lifecycle began; the creation of estuarine habitat. After allowing sufficient time for partial consolidation of the dredged material, earthwork could begin. The La Quinta Aquatic Habitat Terminal Mitigation Project involved the creation of estuarine marsh habitat within BUS-6 for mitigation of habitat impacts from the adjacent La Quinta Multi-Purpose Terminal construction. The project would involve planting 12.6 acres of marsh vegetation and 25.3 acres of Shoal Grass and was separated into two phases. Design Mott MacDonald was contracted by the PCCA to design the Aquatic Habitat Mitigation Berms and protection structures in early Smooth Cordgrass often grows as a narrow strip of vegetation (fringe) extending from the higher marsh to the water, but can grow in large fields near the heads of tidal creeks, the latter being more common in the Corpus Christi area. The mitigation berm design was optimized to allow large fields of Smooth Cordgrass to grow onsite. Design of the mitigation berms involved evaluating the optimal elevations for the survival of Spartina alterniflora. These were identified based on surveys of existing vegetation in the vicinity of the project site. Tidal channels were incorporated into the design which were designed to provide the maximum circulation throughout the project site while not resulting in erosion of the berm material. Relative sea level rise was also evaluated over the next 20 years to ensure enough sacrificial upland material was incorporated in the design assumed necessary to maintain elevations of the berms within the Spartina survival ranges. Finally, wave conditions at the site were evaluated based on return period events to ensure wave conditions within the mitigation berms remain below the threshold for healthy smooth cordgrass growth. Several alternatives were developed and evaluated during the preliminary engineering phase based on the criteria specified. Alternatives were also evaluated to maximize the plantable surface within the project site while minimizing the quantity of fill material required. Renderings of preliminary conceptual alternatives are shown in Figure 4. Variations between alternatives included adjustment in mitigation berm dimensions and channel depths and configuration. 440

5 Figure 4. Phase I preliminary mitigation berm alternative renderings. Phase 1 Phase I of this project began in June of 2015 and would involve the construction of 10 acres of Aquatic Habitat Mitigation berms and 1,394 linear feet of protection berm. The mitigation berms were platforms designed at elevations conducive to the propagation of estuarine marsh habitat, within final constructed tolerances of +/ The protection berm consisted of an earthen berm constructed along the centerline of the nearshore submerged levee to protect the newly created marsh habitat from erosive wave forces. Aquatic Habitat Mitigation Berms The aquatic habitat mitigation berms are the platforms upon which the Smooth Cordgrass can be planted. These berms are designed to be partially inundated throughout each tidal cycle as is necessary for Smooth Cordgrass to survive. The center of the berms included a crest designed to be emergent throughout the current life cycle. The main purpose of the crests is to create a platform where the Smooth Cordgrass can continue to grow as the sea level rises. The mitigation berms were surrounded by channels, as shown in Figure 5, to promote water circulation throughout the site. 441

6 Figure 5. Aquatic Habitat Mitigation Berms. The mitigation berms were constructed using three marsh excavators. Material that was previously placed within the site during filling operations was excavated and redistributed as necessary to construct the project features as designed. Excavated material was stockpiled and used to construct mitigation berms in lower lying areas as necessary. Figure 6. Phase I excavation. Additional material was harvested from the adjacent offshore emergent levee. Excess material from construction of the riprap levee protection had been stockpiled on the backside of the levee, this excess material was then harvested during La Quinta Phase I using an excavator and tracked dump trucks with a heaped capacity of approximately 7 cubic yards (Figure 7). This material was transported along the emergent levee by the trucks to the shore where the marsh excavators would collect the material and place it at the required locations. 442

7 Figure 7. Levee material harvesting. Excess material was used to construct additional mitigation berms. Earthwork for Phase I was completed October of A comparison of the pre-construction and post construction surveys for Phase I is shown in Figure 8. Figure 8. Phase I pre and post construction survey comparison. Smooth Cordgrass Planting Planting began upon completion of the mitigation berms. In order to ensure the survival of the transplants, planting and harvesting of transplants was limited to between February and May or between September and November. Spartina alterniflora was harvested from the adjacent shoreline where the La Quinta Multi-Purpose Terminal is to be constructed. 443

8 Planting was completed on January 2016, upon completion of Phase I, a total of 7.9 acres of Smooth Cordgrass (Spartina alterniflora) was planted throughout the site. Figure 9. Panorama of completed Smooth Cordgrass planting Phase II Due to project constraints during Phase I, a second Phase of the La Quinta Terminal Aquatic Habitat Mitigation Project was necessary. For this phase, a protection berm design was incorporated to consist of a 1500 long earthen protection berm covered with a geotextile fabric apron and ultraviolet light protection shroud to increase the durability of the structure. The project also included construction of an additional 2 acres of Aquatic Habitat Mitigation Berm for planting of Smooth Cordgrass and planting of 20 acres of Shoal Grass within BUS-6. Protection Berm and Mitigation Berm Construction The design of the protection berm incorporated several borrow sources which the contractor could select from for construction of the structure. The sources included: harvesting excess material from the adjacent emergent offshore levee, dredging material from the slope of the adjacent La Quinta Channel turning basin, and harvesting and transporting material from the adjacent upland dredge material disposal area 14 (PA-14). The contractor chose to harvest material from PA-14 and hydraulically pump it through approximately 1.5 miles of pipeline to the project site as shown in Figure 10. Material in PA-14 had been previously dewatered and consisted of clays and sands, but an area within PA-14 was delineated based on geotechnical sampling to target fine to coarse sands, which are more suitable for the construction of the protection berm. 444

9 Figure 10. PA-14 location and pipeline corridor. Construction operations began on October 2016 with the installation of approximately 1.5 miles of 10 High Density Polyethylene (HDPE) dredge pipeline leading from PA-14 to the protection berm template within BUS-6. Additional pipeline was also installed to pump water from the Corpus Christi Bay up to PA-14 to be used to mix the material into a slurry to be pumped to the site. The water pumps needed to provide a steady supply of water for dredging operations while overcoming an approximate head difference of 50. Two 475hp diesel pumps coupled with centrifugal pumps with 17.3 impeller diameters and maximum capacities of 5,390 GPM were used. Two additional pumps of the same specifications connected to two hydraulic submersible dredge pumps attached to cranes were used to dredge and pump slurry over 1.5 miles to BUS-6 (Figure 11). Figure 11. PA-14 harvest material pumping operation. Due to the adjacent seagrass and marsh habitat, turbidity control measures were critical to this project. The contractor enclosed the entire work area with floating turbidity barriers to ensure any excess turbidity from placement operations did not impact adjacent habitats (Figure 12). Once installed, silt curtains were monitored and maintained as necessary throughout construction. 445

10 Material was pumped along the mitigation berm and protection berm templates to their approximate design elevation. Once enough material had built up and consolidated, excavators were deployed to the area to help redistribute material as necessary throughout the project site. The contractor pumped additional material within the protection and mitigation berm templates to account for any potential losses of material due to erosion during construction. Figure 12. Slurry outfall at project site; note silt curtains in background. Upon completion of pumping operations, work began on installation of the geotextile scour apron over the protection berm. The purpose of the scour apron is to contain the protection berm material to reduce erosion of the structure and increase its design life. The scour apron consisted of a geotextile fabric attached to anchor tubes to anchor the fabric in place, as shown in Figure 13. The geotextile also included a UV shroud to shield the scour apron from ultraviolet radiation which accelerates degradation of geotextile fabrics. Figure 13. Protection berm design cross section. Scour apron installation required special coordination and planning to ensure the design template was achieved. The scour apron needed to be installed as soon as possible after the underlying material was graded to the design template to minimize erosion of the unprotected protection berm due to currents and waves. The scour apron was delivered in rolls of 100 sections which could be installed individually, these were installed immediately after the underlying material was graded as shown. Adjacent sections of scour apron were overlapped to ensure even coverage of the scour apron. After the fabric was installed, the anchor tubes were filled by dredging excess material within the protection berm template into the tubes using a small slurry pump with a 4 intake and outfall hose. A water jet was attached to the intake hose to help direct material into the pump while the contractor directed material into the anchor tube. Each anchor tube included several fill ports, which the contractor would fill individually as necessary to ensure the anchor tube is uniformly filled. After the tubes were filled, the landward tubes were buried as necessary to construct the habitat bench. Installation of the scour apron proceeded at an approximate rate of 1 to 2 446

11 panels per day. Photos of the scour apron installation and anchor tube filling are shown in Figure 14, the completed scour apron is shown in Figure 15. Figure 14. Scour apron installation and anchor tube filling. Figure 15. Installed scour apron. While the scour apron installation proceeded, a bulldozer simultaneously graded the aquatic habitat mitigation berms to their target elevations and an excavator excavated any necessary channels. The last section of scour apron was installed in March 2017, marking the end of the earthwork portion of this project. Shoal Grass Planting (Halodule wrightii) Because Shoal Grass planting was independent of earthwork activities, Shoal Grass planting began prior to construction on October The planting contractor began activities by surveying BUS-6 and identifying an area suitable for installation of the 20 acres of Shoal Grass. Upon primary investigation of the site, the contractor found that several areas within BUS-6 had been naturally colonized with seagrass. These areas were avoided to not impact existing seagrass and the Shoal Grass planting within BUS-6 was adjusted and increased to 23.3 Acres. An additional 2 acres of Shoal Grass were planted around the aquatic habitat mitigation berms constructed during Phase 1. To ensure 447

12 the quality of the harvested units and the success of the transplanted units, stringent transplant requirements were incorporated in the design. These requirements included a minimum plug diameter of 6 for each transplant unit (Figure 16), visual inspection of the transplants to ensure proper green tissue, and sampling of the borrow units to ensure the unit had the specified minimum quantity of apical meristems, an indicator of overall health and growing potential of each unit. Once harvested, units were kept covered with burlap which was periodically soaked (Figure 17) to ensure the Shoal Grass did not dry out before it was transplanted. Each unit was transplanted on the same day it was harvested to minimize stress to the plant. Figure 16. Harvested Shoal Grass transplant unit (Halodule wrightii). Figure 17. Contractor soaking units prior to planting. Planting of the initial 18 acres of Shoal Grass was completed by late fall of The planting contractor returned to the site in March 2017 to install the remaining 2 acres of Shoal Grass and to sample previously installed 18 acres of 448

13 Shoal Grass for survival. The survival surveys indicated that well over 50% of the transplanted units had survived the winter and were flourishing. The remaining 2 acres of Shoal Grass planted around the mitigation berms were sampled in July 2017, these surveys also indicated that the units were surviving and flourishing within the site. Upon final completion of planting, a total of 25.3 Acres of Shoal Grass had been planted throughout BUS-6. Surveys also indicated that several additional species of seagrass, including Halodule wrightii, had naturally established throughout BUS-6. Smooth Cordgrass Planting (Spartina alterniflora) Smooth Cordgrass (Spartina alterniflora) planting began after the earthwork was complete on March acres of Spartina alterniflora (Smooth Cordgrass) were planted on the newly constructed aquatic habitat mitigation berms. As in Phase I, transplants were harvested from the adjacent La Quinta Terminal shoreline. In order to ensure the survival of the transplants, units were planted on the same day that they were harvested. Transplants were also inspected for quality immediately after installation. 60 days after completion of Smooth Cordgrass planting, the units were inspected and sampled to determine survival of the transplants. The Smooth Cordgrass Survival Survey showed that the units exceeded 50% survival and were uniformly distributed throughout the site. The installed Smooth cordgrass units within the Phase II mitigation berms are shown in Figure 18. Hurricane Harvey Figure 18. Planted Phase II Smooth Cordgrass (Spartina alterniflora). On August 26, 2017, Hurricane Harvey made landfall as a Category 4 hurricane on San Jose Island, approximately 25 miles East of BUS-6. Winds in the area exceeded 100 miles per hour and storm surges in excess of 5 were observed. Due to the extreme surges, the protection berm was overtopped during the storm. These conditions far exceeded the design conditions for the protection berm. Despite this, the berm dissipated storm waves successfully and no damage to the aquatic habitat mitigation berms and plants was recorded after the storm. The storm waves and overtopping resulted in some erosion of the front slope of the protection berm in some areas, but that material remains contained under the scour apron. While some sections of scour apron had been partially displaced, the geotextile fabric remained intact and the protection berm had survived the storm with only minimal damage. The Smooth Cordgrass was unaffected and continued to flourish onsite largely due to the success of the protection berm design. An aerial of the project site shortly after the passing of Hurricane Harvey is shown in Figure

14 Figure 19. La Quinta Terminal Aquatic Habitat Mitigation Project site post hurricane Harvey. A preliminary inspection of the seagrass showed no observable impacts from the storm, and it was also undamaged by the storm as evidenced by the photograph shown in Figure 20. Figure 20. Post hurricane Harvey Phase I Plants. SUMMARY The 200-acre Beneficial Use Site-6 (BUS-6) was constructed to address the needs of the Port of Corpus Christi for disposal of dredged material associated with the La Quinta Channel expansion and deepening projects as well as addressing aquatic habitat mitigation requirements for future projects. BUS-6 also serves to protect the shoreline adjacent to the La Quinta Channel from predominant wave conditions. BUS-6 was carefully constructed using 24 suction cutter dredges to create an armored emergent levee for wave protection, a submerged nearshore levee to 450

15 contain additional dredged material, placement of fill material at elevations conducive to seagrass propagation, and placement of select fill to be used for creation of Marsh Habitat for the La Quinta Terminal Aquatic Habitat Mitigation Project. After completion of BUS-6 construction, work began on the La Quinta Terminal Aquatic Habitat Mitigation Project. This work was completed in two phases. Phase I consisted of construction of Aquatic Habitat Mitigation berms to be Planted with Smooth Cordgrass (Spartina alterniflora). Upon completion of Phase I over 7.9 acres of Smooth cordgrass habitat had been successfully created using the in-situ material and additional material harvested from the adjacent emergent offshore levee. Phase II involved creation of a 1500 long earthen protection berm structure covered with a geotextile scour apron for erosion protection. Phase II also involved creation of an additional 2 acres of estuarine marsh habitat and over 20 acres of Shoal Grass habitat. The protection berm and marsh habitat were constructed through the beneficial use of previously dredged material stored in an adjacent dredged material placement area. Material was hydraulically pumped over 1.5 miles from the placement area to the project site. Upon completion of Phases I and II, a total of over 12 acres of estuarine marsh habitat had been created, over 25 acres of Shoal Grass planted within BUS-6 (The largest known actively planted seagrass project in Texas), and 1500 of protection berm constructed. On August 26, 2017, Hurricane Harvey impacted the site, resulting in conditions that far exceeded the design of the earthen protection berm. Despite this, the structure remained intact and no loss of marsh or seagrass habitat was observed. Although the protection berm suffered minor damage, the structure remains intact and continues to protect the newly created marsh habitat. REFERENCES Coyle, Mark, La Quinta Channel Expansion. Presentation from 2015 Dredging Your Docks, CRA, Corpus Christi, USA. US Department of Commerce, NOAA, National Weather Service. Major Hurricane Harvey - August 25-29, National Weather Service, NOAA's National Weather Service, 22 Jan. 2018, CITATION Maristany, L.M., Horine, A., Carangelo, P., La Quinta Terminal Mitigation Project: Large Scale Dredged Material Beneficial Re-Use Facility for Estuarine Habitat Creation in Corpus Christi Bay, TX. Proceedings of the Western Dredging Association Dredging Summit & Expo 18, Norfolk, VA, USA, June 25 28, ACKNOWLEDGEMENTS We thank Kyle McElroy, Eileen Mink, Troy Barton, Brice Tadlock, Kirsten McElhinney, Arpit Agarwal, and Josh Carter for their contribution to the planning, design, and construction of the La Quinta Phase I and II projects. 451