The tide was ebbing fast at Silloth tonight. There was a clear flow out to sea from right to left. Look at what happens when it crosses the submerged pipeline in the middle of the picture. It changes from laminar flow to turbulent flow. This means that the Reynold's number must have exceeded a critical value. Reynold's number is a ratio that depends on several key factors. Speed is one of them and size is another. One way of reading this is that the flow rate must be maintained but the pipeline decreases the cross-sectional area of flow so the speed increases. Another approach would be to say that the size changes. This is taken as the hydraulic diameter if it's flow inside a pipe - this isn't. But Reynold's number is proportional to size and saying the depth decreases might suggest that Reynold's number has decreased. It clearly hasn't. https://en.wikipedia.org/wiki/Reynolds_number
