# What is the purpose of these "steps" in rivers?

I have seen a number of structures within rivers, which resemble steps, and which allow the water to cascade down them instead of flowing naturally down the course of the river.

Example 1: River Avon in Bath

Example 2: River Seine in Paris (from Les Miserables (2012))

What is the purpose of these structures?

• Implicit in some of the answers is a feature known as "hydraulic jump" (and other names) whereby a fast flowing water stream at a given depth is "tripped" into a mode where it decreases velocity and increases depth to match. This is the effect where you see a "stopper" wave at the bottom of a chute or rapid (designed to make life fun for kayakers :-) ). There is energy loss in the transition (as depth x V = constant but mv^2 drops) and the downstream impact is thereby reduced. Presumably the water is somewhat warmer after the jump, but you'd probably not notice. Commented Jul 3, 2015 at 21:59
• @RussellMcMahon I would generally reserve the word "hydraulic jump" for those cases in which the water surface level is higher downstream of the disturbance than at the disturbance. I can't see it in these pictures but you can see it here: youtube.com/watch?v=cRnIsqSTX7Q The best example is when water exits below a gate. youtube.com/watch?v=v5gXfyViGIE Commented Jul 4, 2015 at 8:17
• @steveverrill - We don't disagree in fundamental principles - as above " ... where it decreases velocity and increases depth to match." So yes, certainly across the "jump" height will increase, and in cases of low fall it will be a substantial distance past the jump (say a number of "jump heights") before the level is again as low as it was before the jump. However, and I imagine you will agree, that's a matter of typical topography and the key thing is the triggering the transition from super-critical to subcritical flow. As you note, at low flow rates the Pulteney Weir doesn't .... Commented Jul 4, 2015 at 10:56
• @steveverrill ... achieve that. At higher flow rates, in a rather untidy way, it does. Here it is in reasonable flood, with hydraulic jump all around the 'horseshoe' but most visible at left due to camera angle, and here is a video of it with a small and untidy but visible jump occurring (evidenced as much by velocity and turbulence change as by jump height), In all cases the distance that height exceeds original height is small due to the layout. And, just for fun, here it is a bit past capacity. Commented Jul 4, 2015 at 11:07

The two photos in the post show the same structure: Pulteney Weir, downstream of Pulteney Bridge on the River Avon in Bath. The shot of the "Seine" in Les Miserables was filmed on location in Bath.

Pulteney Weir was designed by architect Neville Conder, and built between 1968 and 1972. It's one component of the Bath Flood Prevention Scheme, which was carried out after the disastrous flood of December 1960. The Bath in Time website has photos of the old weir (which descended in a single step) and of the new weir under construction (1968, 1972).

An upstream view of the weir shows that there's more to the structure than the horseshoe-shaped steps: these meet a mid-river artificial island, and a channel to the left of the island terminates in a sluice gate.

(Photo by GeodesyMike from Wikimapia, licensed under CC-BY-SA.)

As with any weir, it has multiple purposes: to control the level of the river above the weir for navigation, fishing, and irrigation; to preserve water in long periods of drought; to hold flood waters back and release them gradually downstream; and to control the speed and force of the water, preventing the river from scouring the banks and damaging the foundations of the bridge and nearby buildings.

This video shows the weir in operation during flood. You can see that the shape of the weir directs the force of the water to the centre of the river, preventing it from damaging the banks.

• Wow nice, wasn't expecting this! Commented Jul 3, 2015 at 17:07
• You may be right that this type of weir avoids erosion of both the banks and the weir by dissipating energy gradually, but that doesn't explain why this type of weir isn't the only design in use. I suspect that besides its aesthetic appeal, this type has an advantage of providing a large surface area of white water, improving oxygenation for the fish in cases where this is desired. Commented Jul 4, 2015 at 8:24
• Wait, so the city's called "bath" and it got flooded? :P Commented Jul 5, 2015 at 14:48

The 'steps' are called weirs, and they are used for a number of flow control reasons in rivers. In the case of the images you posted, they are probably being used to stabilize the grade (slope) of the river. When used for this application they are sometimes called drop structures which "pass water to a lower elevation while controlling the energy and velocity of the water as it passes over."

In rather simplified terms, they slow down the flow of a river by dissipating the energy picked up from travelling downhill as heat and sound instead of allowing it to speed up the flow of the water.

• Another name for a weir is a "drowning machine". Commented Jul 3, 2015 at 6:11
• Added benefit is increased aeration which is great for river life. Also a series of weirs instead of a larger dam allows some fish to continue to migrate. Commented Jul 3, 2015 at 12:38
• @RossV can that cause effects similar to eutrophication?
– Tim
Commented Jul 3, 2015 at 20:36
• @Tim - aeration reduces eutrophication by oxygenating water and degrading bio materials therein. Commented Jul 3, 2015 at 21:55
• @Tim- Russell is right. Although I wouldn't conclude that installing weirs would solve a eutrophication problem. You have to address the source of nutrients foremost. Commented Jul 5, 2015 at 15:17