Flume Experiments to Assess the Effect of Bottom Roughness, Temperature, Water Velocity and Oil Condition on Critical Shear Stress Estimates of Heavy Fuel Oil

Abstract

Non-floating oil is challenging to detect, track, and recover due to limited visibility inhibiting verification of the oil's location and subsurface movement. Oil that sinks to the bottom (i.e., sunken oil) can form large mats or small agglomerates on the bottom, mix into sediments, or remobilize into the water column and move with currents potentially impacting shorelines, benthic and pelagic organisms, intakes for drinking water, and power plants. Trajectory models exist that predict movement of floating and submerged oil; however, many models cannot accurately address sunken oil movement because the bed shear stress (BSS) necessary to mobilize oil (i.e., critical shear stress (CSS)), neglects the effects of bottom roughness and assumes an immobile bed. The goal of this research is to provide responders and modelers with more precise CSS estimates that include the effect of bottom roughness and incorporate results into a response tool to predict sunken oil movement. The transport of oil depends upon in-situ environmental conditions and oil properties. This research used the Coastal Response Research Center's (CRRC) 2180-liter straight flume to test the effects of water velocity, water temperature, oil mass, and bottom friction on fresh and weathered No. 6 Heavy Fuel Oil (HFO) on an immobile boundary. The flume's test section provided a uniform, one-dimensional flow field measured in 3D by an acoustic Doppler velocimeter (ADV), a Nortek AS (Norway) Vectrino II Profiling Velocimeter. The fresh or weathered (%Ev=5) HFO was mixed with kaolinite clay as a sinking agent, and 100 grams of the mixture was injected into static water via subsurface injection. The water velocity was incrementally increased in a stepwise manner by 0.07 m/s intervals and held for 15 minutes at each velocity. This occurred until: (1) oil had stopped eroding or was completely eroded from the substrate, or (2) the maximum velocity of 1.04 m/s was reached. Bottom roughness was evaluated using the velocity profile and bed shear stress (BSS) was calculated using multiple methods applicable to lab and field conditions. The oil's behavior was documented by downward- and side-facing GoPro cameras and reviewed to estimate mass loss per velocity interval, the distance the oil migrated along the bottom, and the corresponding CSS. In the case of an oil spill, responders can compare CSS estimates, determined through this research, with in-situ BSS estimates predicting under what conditions the sunken oil will become mobile.