Three-dimensional Effects on Formation of a "Tiger Tail" of Turbidity in Lake Biwa, Japan, following a Typhoon.

Lake Biwa, Japan, supplies water to 14.5 million in Kyoto and beyond. The North Basin's dimensions of roughly 20×40 km, and maximal depth of 104 m, facilitate geostrophic gyres during the stratified season. Our motivation is pollution by large-scale runoff, possibly combined with hypothetical earthquake-induced damage. This report focuses on the days following Typhoon Man-Yi, which dumped record rainfall in our area around September 16, 2013. Clear skies allowed unobstructed visible and IR satellite images during the period Sept 17-19. The image on the left of the attached file is a near-IR MODIS image taken at 03:54 UTC on Sept 17th, in which turbid water appears milky. There is a thin filament of turbidity extending from the Ado River delta on the western shore northeast toward Chikubu Island near the north shore. The structure of turbidity near the tip of this filament, taken together with a corresponding image take 97 minutes earlier, strongly suggests that the flow at the tip is decelerating as it approaches a saddlepoint-like structure to its north-east. This aspect, and the overall shape, are strongly reminiscent of the "Tiger Tail" of surface oil that was observed after the Deepwater Horizon disaster and analyzed mathematically in terms of Lagrangian Coherent Structures by Olascoaga et al (2012 ; PNAS 109 (13): 4738-4743).

We have applied the SUNTANS ocean model, in hydrostatic mode, to simulate the flow in the Lake in the period around Typhoon Man-Yi. Comparison of simulation results with temperatures measured in-situ at six depths every six hours near the Ado River mouth, and with available data from MODIS images (cf. right side of image file) lends credibility to the simulation results. Notably, an upwelling was hindcasted by the model to occur near the north shore during the night of Sept 16-17, and this matches MODIS estimates of SST distribution. We find that the upwelling water strongly limits the initially northward motion of the sediment from the Ado River, thus representing a transverse, strongly three-dimensional effect that accordingly forms this turbid river plume into a filament. Associated effects at this time, and during the following hours and days, will be discussed.