The objectives of the study were to: 1) demonstrate and validate innovative methods to estimate erosion potential by propeller wash in two DoD harbors (source term); and 2) characterize, map and predict fate and transport of sediment plumes and contamination by propeller wash (fate and transport).
To achieve the first objective, both laboratory and field studies were conducted to measure the parameters that govern propeller wash and its erosion potential, and then the Graphic Maynord’s (1984) model was refined and validated for evaluation of erosion potential. For the second objective, field studies were conducted to measure masses of different sediment particle sizes and the associated metal loading, in propeller wash plumes in San Diego Bay, and Pearl Harbor. Sediment traps were deployed and sediment depositions were measured at Pier 7 in Sinclair Inlet. The linked CH3D+TICKET was successfully implemented and validated for simulation of fate/transport and re-deposition of the sediment plumes from propeller wash in San Diego Bay.
A fortuitous event resulted in validation of the Finite Analytical Navier-Stokes Solve (FANS) model for prediction of sediment resuspension by a deep draft vessel. While working on the resuspension event in Bravo Pier, Pearl Harbor, information on sediment resuspended during the transit of the USS Chafee from Bravo Pier led to the application of the FANS model for that specific case. The FANS model successfully predicted the plume patterns observed during the transit of the deep-drafted vessel.
In general, all quantitative performance objectives were successfully met. The water velocity produced by the propeller thrust, and the associated shear stress was successfully estimated for 92% of the data and all four propeller speeds. The erosion rate produced was successfully estimated for 7 out of 9 data sets. Those two data sets that were not estimated were measured after the propeller stopped. Resuspended sediment load was successfully predicted for 89% of the measured data, and metal load was predicted correctly for 86% of the dissolved copper data, and 81% of the total (dissolved and particulate) copper data. Copper partitioning estimated with the CH3D+TICKET model was correctly estimated in 23 of the 24 field data sets, or 96% of the time. Copper loads in sand, silt and clay were successfully predicted for 83% of the data.
Similarly, qualitative objectives were successfully met. As evidenced from the quantitative objectives results, linking CH3D with TICKET was successfully accomplished, providing data that met quantitative objectives. Also, application of CH3D to estimate loads of total suspended solids was successful for the Pearl Harbor demonstrations. FANS correctly predicted the resuspension of bottom sediments by the USS Chafee, which was measured incidentally during the demonstration in Pearl Harbor.
Lessons learned during the demonstration study are as follows:
- Collection of field data of propeller wash is challenging, due to the highly turbulent flow and dynamic boat and propeller movements during the study
- Good coordination and cooperation with the boat crew, in particular, the driver of the boat is important so that the tug wash experiment can be conducted under controlled conditions
- Good logistical support and coordination are needed for field study
- Field data are important and costly and collection and analysis are laborious. It is necessary to plan well and identify the types of data based on priorities and budget
- Models can be effective, if calibrated and validated against field data
- Make graphic user interfaces for easy model input and model output
- Provide users’ manuals for the models
- Further research is needed for long term impacts with and without propeller wash on sediment dynamics and remediation options in DoD harbors
- Maynord’s model is based on the theory of conservation of momentum and implemented for propellers with a single engine (Maynord 1984) and twin propellers (Maynord 2000). While convenient, Maynord’s model has its application limitation – namely, the ratio of propeller diameter to propeller-to-bottom distance, Dp/Hp, should be less than 1.2. Specifically, Maynord’s model is applicable for propeller wash studies for tugboats and may not be applicable for deep-draft vessels, such as aircraft carriers and DDGs.