Turbidity Current Measurements in the Congo Canyon

Abstract

Abstract Turbidity currents are known to occur frequently in the Congo Canyon which is within a few kilometers of blocks being explored and developed by oil companies. Given the potential strength of turbidity currents, design engineers are keen to know their strength, frequency, and vertical extent. Unfortunately few of these variables have been measured directly in strong turbidity currents because the equipment is often buried or otherwise damaged during the event. This paper describes observations from two moorings which were deployed in the main channel of the Congo Canyon at 2000 m for 7 months. These moorings were designed specifically to withstand strong turbidity currents using insights from previous unsuccessful attempts in the same region. The most important mooring contained a downward looking ADCP (Acoustic Doppler Current Profiler) located at 85 m above the canyon floor which was capable of measuring within 5 m of the bottom. The ADCPs captured 11 distinct turbidity current events with durations from 1 to 6 days and peak velocities of 0.6 (1.1 kt) to 2.5 m/s (4.9 kt). These measurements appear to be the strongest turbidity currents ever directly measured anywhere. Analysis of the measurements shows that the duration of the event correlates with the severity. The vertical extent of the turbidity current is also correlated to severity although it appears to reach an asymptotic limit around 150 m. A preliminary look at turbulence within the events suggests that it goes down as the velocity increases. One limitation of the ADCPs proved to be loss of data near the seabed during the strongest events when the water became so turbid that it prevented a sufficient acoustic echo. However, further analysis suggests that the ADCP was able to capture the highest velocity in a given event. Despite this limitation, ADCPs appear to hold great promise in taking direct velocity measurements in turbidity currents. Introduction Turbidity currents (TCs) are basically underwater avalanches of soil that can generate high velocities. Because of their high density, TCs can be self sustaining even with modest sea floor slopes of a few degrees and they can propagate for hundreds of kilometers. Given their ferocity, it is not surprising that there have been few direct measurements of TCs with current meters - the equipment is typically buried in mud or badly damaged. Most TC velocity estimates have been inferred from indirect methods such as cable breaks and these suggest they can reach velocities of 25 m/s (50 kt) (Heezen and Ewing, 1952). Because of these high velocities and high densities, TCs can impose impressive loads on pipelines, flow lines, risers, and moorings. Indeed, it may be economically infeasible to design them to withstand the loads. TCs are the force that maintains the Congo Canyon (Figure 1), one of the largest submarine canyons in the world. It extends about 350 km from its origins 30 km inland to the 3000 meter isobath where it forms a huge subsidiary fan or delta. The Canyon has a gradient of about 5° and is about 0.5 km wide with steep walls hundreds of meters high reaching slopes of 30°. The existence of TCs in the Canyon was first documented by Heezen et al. (1964) who examined the time history of breaks in a telegraph cable. Repair crews reported that the cable breaks were in tension and the cable was often deeply buried in sediment. Since the breaks occurred on nearly an annual basis and were costly to repair, the cable company tried relocating its cable crossing but the breaks continued until the company finally gave up in the 1930s, about 40 years after laying their first cable through the Canyon. Regardless of location, Heezen et al. found that the time of the cable breaks correlated strongly to the period just following the maximum River outflow. They conjectured that sediment built up at the head of the Canyon during the peak outflow and eventually became unstable resulting in a turbidity flow.