[MQP] Evaluation of the Erosion Control Methods Implemented by the Panama Canal Expansion Program

Executive Summary

The Panama Canal expansion project involves the excavation of massive amounts of soil for the third set of locks and the widening and deepening of the canal channel to accommodate PostPanamax vessels. The soil removed is relocated within the project area to other locations. Depending on the material excavated, it is either stored or disposed of at sites specified by the Panama Canal Authority (Autoridad del Canal de Panamá, ACP). Contractors working for the ACP, separate out materials such as basalt rocks (used to make concrete) and clay and unwanted material such as dredged soil. The soil disposal sites have to meet ACP approval with criteria such as site location, storm water management and erosion control.

Erosion is one of the most important environmental aspects being addressed by the ACP Environmental Management and Follow Up Section (Sección de Manejo y Seguimiento Ambiental, IARM) in compliance with the environmental impact study of the expansion project. The excavation and disposal sites are of major concern because the vegetation cover has been removed and the exposed soil is more susceptible to erosion. As such, erosion control practices have been implemented to mitigate the problem of accelerated erosion. These best management practices (BMPs) are classified as either temporary or permanent. Examples of temporary practices are silt fences and sedimentation basins, while permanent practices include hydroseeding and culverts. The contractors responsible for installing and monitoring these BMPs periodically submit progress reports to the ACP with qualitative evaluations of the effectiveness of the erosion control.

This project aimed to evaluate the soil erosion mitigation best management practices installed for temporary and permanent erosion controls for the Pacific sector of the Panama Canal expansion project with a specific focus on the disposal sites. In addition, this project provided recommendations for alternatives or improvements to these controls. In order to achieve these goals, three major objectives were completed. The first was to gather information on the current conditions at sites in the Pacific sector of the expansion project. The second objective was to determine soil erosion rates using historical data and erosion bridge measurements. The third objective was to compare the results for the different sites and different BMPs to assess the advantages and disadvantages of each. By completing these objectives, we were able to recommend an alternative design for temporary and permanent erosion control for the Panama Canal expansion project.

Site assessments consisted of visual inspections, slope measurements and interviews with ACP contractors. Erosion bridges, water quality testing and the Revised Universal Soil Loss Equation (RUSLE) were used as quantitative measures of erosion. Six erosion bridges were installed at each of six sites in the Pacific expansion area to study the change in soil height over time. The bridge’s sites were selected based on accessibility and soil types that would allow for the installation of the bridge supports. In addition, our sponsors provided suggestions for areas that would not be disturbed during the time frame of our study.

Of the six sites, four sites had BMPs (silt fencing; hydroseeding; terracing; and hydroseeding with silt fencing), one had natural vegetation (control area) and one had clay soil. Relative soil height at each of the erosion bridges was monitored over 18 days and extrapolated to yearly loss rates.

The sites with hydroseeding had a statistically lower soil loss rate than the site with silt fencing or the site with no BMP. Soil loss rates ranged from 180,400 tons/km2 /year for the hydroseeding site to 691,900 tons/km2 /year for the site with no BMPs. Estimation of soil erosion was also made using the Revised Universal Soil Loss Equation (RUSLE), which incorporates data on rainfall intensity and soil characteristics. These estimates showed the potential for mitigating soil loss using hydroseeding with or without silt fencing. However, both erosion bridges and RUSLE showed soil loss rates higher than typical for construction sites by one to two orders of magnitude. Longer term data collection is recommended to potentially improve erosion estimates. Further data analysis did not reveal trends with regards to erosion and rainfall intensity or proximity to blasting.

Erosion control BMPs were evaluated based on: short term effectiveness, applicability to different slope angles, applicability to different soil types, effectiveness without additional BMPs, estimated soil loss prevented, and cost of installation and maintenance. Silt fences are effective in the short term and have moderate costs. Hydroseeding is applicable to different slope angles and soil types, does not require additional BMPs to be effective, and prevents soil loss effectively but has high installation costs and is not effective immediately. Terracing prevents soil loss and can be applied without additional BMPs, but has limited applicability to different soil types and slope angles.

Interviews with contractors showed that a combination of erosion control measures is needed to best manage erosion in most cases. Our recommendation was a combination of terracing, silt fences and hydroseeding. The terraces would be approximately 33 m apart, with slopes that do not exceed 25% (14.4 degrees) and silt fences at the end of every terrace on the top of the hydroseeded slopes. This recommendation was for soil types such as the Pedro Miguel formation, which has a mix of sand, silt and clay, which can promote healthy vegetation growth. The estimated total cost for installation and maintenance on a 3,150 m2 hillside was $28,480, with majority of the cost based on the hydroseeding costs.