Cashman removed sediment from the bottom of the Intracoastal Waterway with a 7.6-cubic-meter (10-cubic-yard) Liebherr 994 excavator and placed it into hopper barges.
Editor’s Note: This article is an excerpt from a paper (“Permitting and Constructing the Intracoastal Waterway Deepening Project in Broward County Florida”) published at the Dredging Summit & Expo in June 2017. It is copyright 2017 WEDA and is reproduced here with prior permission.
From May 2016 until June 2017 Cashman Dredging and Marine Contracting Company LLC is dredging the Intracoastal Waterway (ICWW) — part of the federally authorized navigation channel along Florida’s East Coast. The Florida Inland Navigation District (FIND) funded a project to deepen the ICWW channel to -4.5 meters (-15 feet) MLW in a ±3.4 kilometer (±2-mile) section from the 17th Street Causeway to just north of the Las Olas Bridge in Fort Lauderdale Broward County Florida. FIND formulated the project to
provide larger vessels (including mega-yachts and commercial traffic) safer and deeper access to the boatyards and shipping facilities.
Existing depths were -3 meters (about 10 feet) Mean Low Water (MLW) and material that was mechanically dredged from the channel went to a hopper barge. The material was dewatered and temporarily placed at a 24300 cubic meter (±32000 cubic yard) dredged material management area (DMMA) located at Port Everglades. Solely brokered by the contractor the final disposal of material occurred at local commercial-zoned construction projects.
The project removed approximately 137620 cubic meters (±180000 cubic yards) of weathered limestone via a convention- al bucket and limited hand-held hydraulic dredge along a 3.4 kilometer (x±2-mile) sec- tion of the ICWW.
PERMITTING AND CONSTRUCTION
Taylor Engineering on behalf of FIND initiated permitting of the project in 2007 and acquired the Broward County License and the Florida Department of Environmental Protection and U.S. Army Corps of Engineers permits for the project between August 2011 and May 2014. The major permitting challenges which resulted in a nearly seven-year permit- ting timeframe stemmed from (1) adjusting for significant navigational restrictions due to natural channels and existing infrastructure; (2) securing a temporary placement area for dewatering; (3) handling sediments with elevated contaminants; and (4) avoiding sub- merged aquatic vegetation impacts specifically Halophila johnsonii (Johnson’s seagrass) a federally listed threatened species under the Endangered Species Act.
The final submerged natural resources survey and dredging template identified 7400 square meters (±1.8 acres) of submerged environmental resources (seagrass) within the project footprint. Due to the FIND’s commitment to minimize (and in this case completely avoid) environmental resource impacts the FIND reduced the average bottom width of the channel from 38 meters (125 feet) to 33.5 meters (110 feet). The dredging template required a mini- mum buffer from seagrass beds and reflects a compromise between navigational requirements and minimization of environmental impacts.
Construction on the project began May 2016. Cashman removed material from the bot- tom of the ICWW with a conventional open bucket (7.6 cubic meter or 10 cubic yard) excavator and placed it into hopper barges. The 50-meter by 15-meter by 3-meter (164 feet by 50 feet by 10 feet) mechanical dredge — a Liebherr 994 excavator — provided a shallow draft (1.5 meter or 5 feet) and was powerful enough to break through the weathered limestone material identified in the pre-construction geotechnical borings. When filled to capacity the 2200 metric ton (2400 U.S. ton) capacity hopper barges (70 meters by 13 meters by 3.6 meters or 230 feet x 43 feet by 12 feet) would transport the material from the dredge site to the temporary DMMA located on Port Everglades property ap- proximately 4.8 kilometers (±3 miles) south of the southerly project limit.
Once at the DMMA the contractor tied the loaded barges to fender dolphins and used a long-reach excavator (Sennebogen 880) and hydraulic clamshell bucket to unload the barges from land. Due to permit requirement of zero- water discharge directly into the adjacent canal the contractor used a pump to remove and transfer any freestanding water from the barge into the DMMA. With the excess water removed the excavator offloaded the dredged material directly into the DMMA. To prevent material from spilling onto the shoreline and protected mangroves the contractor installed a steel spill plate directly over the shoreline and mangroves to collect and transfer any spilled material to an upland area. As the material began to build up at the discharge point the contractor used a front-end loader to move the material within the DMMA to facilitate dewatering. Once the material was mostly dewatered the contractor transferred the material into the “stockpile area”
located on the north end of the DMMA for final draining and loading into sealed 13.8 cubic meter (18 cubic yard) dump trucks.
Water that collected in the material-settling pond was gravity fed to two corrugated metal pipe flashboard weir risers connected to two 0.8-meter (30-inch) corrugated metal pipes that discharged into a decanter area. The DMMA design allowed suspended sediments to settle out of the water column by slowing the ow rate through the decanting via interior baffle berms. Water traveled from the decanter area through an elevated drain consisting of a single 0.8-meter (30-inch) corrugated metal pipe and across two trip dams for final discharge into the adjacent canal via an elevated drain/riser structure and a single 0.8-meter (30-inch) corrugated metal pipe. This process gave sediments time to settle and thus prevented excessive turbidity in the discharge from the DMMA. Before allowing initial discharge of water from the DMMA the permits required that the contractor collect and test water samples adjacent to the DMMA and background canal locations for contaminants. The contractor also collected and tested turbidity samples for compliance with the permit turbidity criterion. Once the contractor had obtained passing test results discharge began.
FIND and its engineer identified six utility crossings — independently verified by the contractor — within the dredging template prior to construction. Of the six utility crossings four were required to be relocated by the utility companies. The dredging template was altered to avoid the remaining two crossings. Identification and verification of the utility lines was a difficult and time-consuming process because most of the utility companies had inaccurate or missing as-built records of the actual utility line locations. Highly-specialized divers located utility crossings within the dredging template. The project plans required 15-meter buffers between energized lines and dredging operations.
The project area includes numerous marine-based industries that require continued access to the ICWW. With its ±30-meter (100-foot) wide equipment (dredge and adjacent hopper barge) the contractor expected to significantly reduce the 38-meter (125 feet) navigable channel width for vessel passage. Prior to project commencement FIND conducted an outreach meeting with the local industries U.S. Coast Guard (USCG) Port Everglades and the dredging contractor. The dredging contractor explained the project logistics including the operation schedule and dredging and offloading operations. To facilitate communication with marine operators strict vessel movement protocols were put in place and coordinated with Port Everglades. In addition the contractor provided daily equipment position reports that helped to notify local mariners of dredging location and duration.
One year of the seven-year permitting process was strictly dedicated to securing an access point through a newly created temporary haul road outside of the Port Everglades property so as not to disturb ongoing port related operations. The 4.5 meter (15 feet) wide x 550 meter (1800 feet) long one-way access road required authorization from landowners (Broward County Parks and Recreation and Florida Power & Light) permits from the U.S. Army Corps of Engineers and Broward County and an individual license from the Florida Power & Light that included a fee for use along with design mandates monthly usage fees and environmental monitoring reports from a third independent party. The contractor was also required to have its upland Maintenance of Traffic Plan approved by Port Ever- glades Broward County and Florida Power & Light prior to engaging in any hauling traffic.
On average the dredging contractor achieved a production rate of approximately 965 cubic meters per day (±1300 cubic yards per day) over the course of the 160-day working or 665 cubic meters per day (±900 cubic yards per day) over the 232-day total dredging period. With exception of one turbidity exceedance early in the construction period the project resulted in no environmental permit violations. The contractor achieved the design project depth through most of the project area.
Two of the required utility relocations were not completed in time for the dredging project and 4129 cubic meters (5400 cubic yards) of material remained post-construction. FIND will need to return eventually to remove that material once utilities are relocated.
Lori S. Brownell is the director of waterfront engineering at Taylor Engineering in Jacksonville Florida. She can be reached at 904-731-7040 or by email at lbrownell@tay- lorengineering.com.