Discussion of “Influence of Cohesion on Scour under Submerged Circular Vertical Jets” by Sarfaraz A. Ansari, Umesh C. Kothyari, and Kittur G. Ranga Raju

Abstract

The authors present an important study for the understanding of scour in cohesive material. As the authors suggest, there are few studies that have systematically examined the influence of the cohesive soil properties on the size of scour hole produced by jets. Herein, previous data from scour tests with a turbulent submerged impinging vertical jet in cohesive soil Mazurek 2001; Mazurek et al. 2001 is used to discuss the presented equation for the temporal variation of the scour hole dimensions. In the tests of Mazurek 2001 , scour was produced in cohesive soil samples of approximately the same properties and the jet properties were varied. In the experiments, the soil eroded primarily by mass erosion. For a series of tests carried out under the same hydraulic conditions, Fig. 1 of this discussion shows the growth with time of the cube root of the scour hole volume Vsc, the maximum depth as measured from the original bed level dsc, and the depth at the jet centerline, dcc. The tests were carried out using a nozzle diameter d of 8 mm, a relative impingement height of 8.1 the ratio of the height of the jet above the sample surface, H, to d , and a velocity of the jet at the nozzle Uo of 9.0 m/s. Fig. 2 shows a series of three tests run at d=8 mm, H /d=14.5, and Uo=9.0 m/s. Note that for these tests there was some small variation in the water content of the samples, as indicated in Mazurek et al. 2001 . As is typically observed in mass erosion Moore and Masch 1962 , it is seen in Figs. 1 and 2 and in some of the experiments of the authors, there are sudden jumps in the growth of the scour hole, which represent the erosion of large chunks or lumps of soil. For the growth of the scour hole volume, it appears that when a large chunk of soil is removed early in a test, the subsequent rate of growth of the scour hole is slowed. Although the scour hole size at a particular time varies between each test, by the time the scour holes reached their equilibrium or asymptotic size they are approximately the same shape and size. The growth of the maximum depth of scour most often tended to depart from the typical linear variation of scour depth with the logarithm of time. After the erosion of a large chunk or lump of soil, dsc can remain constant for several hours 8.0/8.1/9.0/1 . This also happened for dcc, although to a lesser degree due to the erosion by chunks, the maximum scour depth was often not along the jet centerline . It is thought the dcc might provide a more reliable measure of growth of the depth of the scour hole.