Protection From Propwash: Erosion Control on the Panama Canal’s Pedro Miguel Saddle Dam

Constructed in the early 20^th century, the Panama Canal is an impressive feat of human ingenuity and engineering. Each day, around 36-38 ships make the eight- to 10-hour journey though the canal’s three sets of locks – the Gatun, the Miraflores, and the Pedro Miguel. For each ship, a total of 52 million gal. of fresh water flows into these locks, raising the vessel 85 ft. above sea level.

Propwash from tugboat viewed from western canal bankThe locks have two shipping lanes permitting boats to go both directions, toward either the Pacific or Atlantic Ocean. Near the locks, a center wall runs parallel to the canal’s banks, separating the shipping lanes. As boats travel south through the smallest set of locks, the Pedro Miguel, they are secured to this center wall by powerful tugboats. The tugboats create propwash – the spiral jet of water from each propeller, which thrusts out toward the western canal bank where the manmade Pedro Miguel Saddle Dam is located. Steve Maher of Tensar Earth Technologies Inc. describes how the tugboats’ propwash travels at high speeds, “churning the entire area like a huge mixing bowl with a great deal of turbulence and some locally high velocities.” This constant disruption has caused erosion, forming 15-ft.-high bluffs at the canal bank and leading the Panama Canal Authority (ACP) to worry about the dam’s condition. Since the 300-m-long Pedro Miguel Saddle Dam protects the adjoining valley from being flooded by the waters of the Panama Canal, erosion control measures were urgently needed.The ACP faced several obstacles in January 2001 when looking for a remedy to the Saddle Dam’s erosion problem. The most complicated restriction involved workspace. Because of the canal’s important function and high vessel traffic, construction was prohibited close to the shipping lane. Maher explains the significance: “The erosion solution could not extend to the canal bottom and be toed in; it would need to be terminated midslope, flexible to tolerate potential undermining, and stable to resist hydraulic uplift along the exposed edge.” In addition to space limitations, the erosion control system needed to extend 30 ft. underwater. And because of poor water visibility and safety issues, the ACP wanted to use as few divers as possible.Burdened with these complicating elements, the ACP needed to decide between several erosion control options. Available methods included geotubes, grout-filled mattresses, or concrete block systems. In the end, the agency decided to use a combination of Triton marine mattresses and riprap. The ACP chose the mattresses-a relatively new form of erosion control – for their durability and strength, stability on the exposed edge, and constructibility, as well as their successful track record in such extreme conditions as steep riverbanks and coastal revetments. The units are made of inert, durable materials: UV-stabilized polymer grids and connectors and crushed stone fill. Each 18-in.-long compartment contains a 12-in. thickness of stone fill, making the mattresses flexible while maintaining a uniform thickness. This composition is important at subgrade irregularities and along the exposed edges, especially where the installation ended in midslope on the Saddle Dam project.

Filled marine mattress unit, with pre-attached geotextile, lowered into canalThe mattresses are large – typically 10 m long x 1.5 m wide – and any small gaps between mattresses can be accommodated by a layer of rock fill. To provide redundant protection, a layer of riprap was placed over the mattresses and a special mattress configuration was used to form a toe buttress that helped prevent the riprap from sliding. For this project, Maher explains, “Filter fabric was preattached to the bottom of each mattress unit to facilitate a single-step installation.” The nonwoven fabric extended beyond the end of each unit and overlapped when the units were placed, allowing continuous coverage. To use as few divers as possible, the contractor, Intercoastal Marine of Panama City, employed a remote-release system and side-scanning sonar equipment that showed the placement of the mattresses.“The Triton marine mattress system saved at least 25% more time than other methods. It also allowed installation without stopping operation of the canal. This would not have been possible with other systems,” notes Eduardo Parajon, general manager of Tensar’s International Division. “We sent two engineers [to Panama] to train the local crew of the contractor in everything from fabrication to installation. We also helped the ACP with designing the project.” The Saddle Dam project began in January with a deadline of 120 working days for completion and is on schedule.