Comprehensive Numerical Modeling of Erosive Forces for Remedial Design

Abstract: Evaluating the integrity and effectiveness of sediment caps during remedial design is critical. Caps function as protective barriers, isolating hazardous substances and preventing their migration into the environment. Erosive hydrodynamic forces impact stability of the cap; therefore, an erosion protection layer is generally needed to safeguard against short-term and long-term damage to the cap’s chemical isolation layer. This paper discusses the numerical modeling approach used for evaluating cap erosion protection requirements for remedial design in a major industrial area within Portland Harbor, Portland Harbor, Oregon.

Comprehensive numerical modeling was performed for the assessment of erosive hydrodynamic forces (natural and anthropogenic) to determine the characteristics of the erosion protection layer potentially needed to prevent erosion of the cap. The erosive design forces evaluated included river currents, wind-waves, rainfall runoff, stormwater outfall discharges, drydock activities, propeller wash, and vessel-generated waves. The following numerical modeling tasks were conducted for the assessment: (1) River flow and tides were simulated using the 3D hydrodynamic model Delft3D-FLOW for a range of river flows. The model consisted of a large-scale model for boundary forcing, and a nested model to resolve the finer-scale processes in the Project area; (2) Flood modeling was performed using the nested Delft3D model for 100-year flood event conditions; (3) Wave conditions generated by winds during 100-year storm conditions were predicted using the SWAN model. Modeled wave conditions were extracted at transects around the Project area and wave runup velocities on the slopes were computed using the SWASH model; (4) Rainfall runoff was modeled for a scenario with peak rainfall rate (in past 5 years) using a depth-averaged (2DH) model that simulates surface water flow, sediment transport, and morphological changes; (5) Computational Fluid Dynamics (CFD) modeling was performed to evaluate drydock operations that generate bottom velocities, including lowering and raising the drydock (hull movement displacing water) and ballast water intake and discharge (localized water jets); (6) Propeller wash was simulated for each vessel transit during a three-month period using a 3D empirical model for bottom velocities. Individual vessel transits for the propeller wash modeling were constructed using Automatic Information System (AIS) data; and (7) CFD modeling was conducted to simulate outfall hydrodynamics for 100-year outfall discharge rates for multiple outfalls in the Project area.

Using the modeling results, peak bed shear stresses from each hydrodynamic process were overlaid onto a fine grid, and maximum values were taken from each process to identify stable grain sizes for cap based on the USGS guidelines. Results show that in most of the Project area, gravel is large enough to provide static material stability against erosion. In limited quiescent areas, medium-to-coarse sand used for the chemical isolation layer is likely to be stable (i.e., no erosion protection layer may be required). This paper provides details regarding the analysis methodology and results.

How does this project / topic contribute to the advancement of the industry and profession?: The effects of urbanization, port development and operations, and industrial and agricultural activities have significantly contributed to coastal pollution. Hazardous substances and pollutants have been found in sediment of many ports and harbors. Remedial design mostly involves protective caps for isolating hazardous substances and preventing their migration into the environment. This paper provides details regarding a comprehensive numerical modeling approach to determine the characteristics of the erosion protection layer potentially needed to prevent erosion of the cap. The analysis approach employed here is applicable to other sites and should assist decision-makers in making informed choices during the remedial design phase.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The project uses state-of-the-art numerical modeling tools to simulate erosive design forces including river currents, wind-waves, rainfall runoff, stormwater outfall discharges, drydock activities, propeller wash, and vessel-generated waves. Since the studies related to cap evaluation for the remedial design are limited, innovative techniques were used and explored as part of this study.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect of our project was the interdisciplinary collaboration. We were working with experts from diverse fields such as coastal engineering, geotechnical engineering, and environmental science. Each field has its own unique methodologies, and perspectives, which made the collaboration complex. However, we organized regular meetings and workshops to ensure clear communication and mutual understanding among all team members. We also encouraged everyone to share their unique insights and perspectives, which enriched our analysis and led to innovative solutions.

Who is the target audience for this paper?: The target audience for this paper include port, coastal and environmental engineers; port operators and planners; scientists; and policy and decision makers.