Tumbling Bay

This is a construction detail in drainage engineering.

In sewer design it has been long recognised that you cannot build a sewer pipe with the wrong gradient; specifically not too steep. Without going into too much detail a steep sewer is very likely to block and be troublesome. 

Gradients, so called self cleansing, around 1 in 60 or 1 in 200 work away very nicely.

The land that we build on does not neatly lend itself to such tight design constraints. The site here is steeply sloping but the contractor has managed to build it well. Sections of drainage run between inspection chambers. The pipe may start off deep underground but the land may be sloping more steeply than the drain, thus it gets nearer and nearer to the surface. Before it gets too shallow you build another chamber with a "tumbling bay". Before, the now shallow pipe, enters the chamber, a tee junction is introduced where the flow is obliged to tumble down a vertical section of pipe then around a 90 degree bend and so enter the chamber channel at the flloor of the manhole. The high level pipe continues straight on in to the chamber too and presents a convenient access point if it ever needs "rodded".
To the uninitiated the perception of two pipes arriving in to the chamber one above the other can cause confusion.
With the pipe now deep under ground again the process can continue and be repeated per the dictates of the terrain.

Does anyone else have a glamour job?