When Hurricane Katrina barreled into the Gulf Coast nearly 10 years ago, the storm surge caused a catastrophic failure of the levee system in New Orleans, Louisiana, and left roughly 80 percent of the city under water and in ruin. The failure of these levees prompted the U.S. Congress to pass the National Levee Safety Act in 2007, which directed the U.S. Army Corps of Engineers (USACE) to reconsider its approach to levee safety across the country, including south Florida’s East Coast Protective Levee.
Gannett Fleming conducted extensive subsurface investigations and compiled volumes of data to support improvements to this levee system so that it would meet federal guidelines, increase the safety and security of people and wildlife, and protect homeowners in at-risk areas from flood damage and increased insurance costs.
The East Coast Protective Levee (ECPL) system stretches 105 miles and prevents hurricane-driven storm surges in the vast Everglades Water Conservation Areas from flooding some of the state’s most populous areas, including Palm Beach, Broward, and Miami-Dade counties. Following its construction by the USACE in the early 1950s, ownership of the ECPL system was transferred to the South Florida Water Management District (District), the largest water management district in the state. The District includes 16 counties from Orlando to the Florida Keys, and serves more than 8 million residents.
Joint annual inspections by the District and the USACE revealed that the 38-mile Broward County portion of the ECPL did not comply with Federal Emergency Management Agency (FEMA) certification standards. These standards detail the minimum design, operation, and maintenance standards that levees must meet to provide protection from the base flood on flood insurance rate maps.
Tapping into the specialized expertise of its geophysics team, Gannett Fleming applied the multichannel analysis of surface waves (MASW) method to evaluate five levees along the 38-mile stretch. MASW is a non-invasive, non-destructive, seismic survey method that evaluates the elastic condition (or stiffness) of the ground for geotechnical engineering purposes. MASW can explore to depths of more than 100 feet and determine the vertical distribution of shear wave velocities.
Below the levee body, the MASW survey provided a subsurface profile that revealed varying levels of stability, including an organic layer that was nearly continuous in some locations and intermittent in others. Below the organic layer, MASW found limestone of variable stiffness, and/or sand, and under the levee body discovered limestone and weathered limestone with cemented shell and sand of varying stiffness.
The variable stiffness signaled to the geophysics team that additional investigation was needed to confirm the stability of the levee in certain areas. As part of Gannett Fleming’s integrated project approach, the firm’s geotechnical professionals took the helm to extract soil borings from strategically selected locations along the levee. Portions of the levee that failed to meet acceptable standards were noted for additional detailed engineering analyses in order to develop the appropriate rehabilitation measures.
MASW also was instrumental in identifying anomalous conditions that traditional exploratory drilling may have missed – including a paleo-collapse. A paleo-collapse is formed by the dissolution of underlying sedimentary rock, which can collapse intact rock above, form extensive fractures, and open caverns that can result in an irregular top of rock (TOR) surface. A paleo-collapse in this area also can yield significant challenges, as collapse zones can be extremely permeable and allow for the passing of groundwater from the Everglades. In an extreme case, a paleo-collapse can open the door to a levee breach, leading to the potential loss of life and property.
Two specific borings raised concerns that a paleo-collapse along one segment was possible. One boring reported the TOR at 24 feet; the other at 33 feet. These TOR depths were approximately twice as deep as those observed elsewhere along the ECPL, making a strong case for the possibility of a paleo-collapse.
Gannett Fleming’s technical evaluation of the ECPL paved the way for an $18 million improvement project designed to strengthen the Broward County portion of the levee system. From its technical investigations and findings, Gannett Fleming prepared a comprehensive geophysical report that the District used to supplement the design of the levee rehabilitation and to secure accreditation from FEMA.
At the completion of the rehabilitation project, the District’s then Director of Operations, Maintenance & Construction Division, Tommy Strowd, remarked: “These upgrades build on 60 years of effective flood protection from the levee, ensuring it continues to operate as designed for many more years to come. The work meets FEMA certification criteria, which benefits residents and businesses that may pay for more accurate, risk-based flood insurance.”