fluid dynamics of rivers (physicists, canoeists?)

So,

we swim in a river once a week.

Right now the river is very low. We were expecting it to be slower than previously, and to be able to swim a decent distance upstream yesterday. In practice it was not much slower if at all than when it was two feet higher.

I was under the impression that water flow rate was proportional to water depth in some way.

It is about 500m downstream from a weir with a bunch of sluice gates for flood control and a small hydro-electric plant, but not so close that I can see that doing anything to the water other than changing the level.

So how come it isn’t nice and slow?

Joe

it depends on a lot of things but a key factor for flow rate is the height drop along the river (or the slope of the river). This is probably more important than water depth

In summer, I am pretty sure it is somewhat slower than currently, I can swim noticeable amounts upstream in summer, so it can’t purely be the slope of the river.

Could it be something to do with the sluices and big weir up at the mill 500m upstream affecting it?

Could it be something to do with the sluices and big weir up at the mill 500m upstream affecting it?

Could be this, im an angler and i sometimes have these problems (albeit trotting a float downriver to catch fish). Sometimes after heavy rain or if lots of debris block various parts of the river they will open the sluice gates to prevent flooding. Also for spring tides etc (the moon also affects the tide).

so it can’t purely be the slope of the river.

no, that’s right, there are lots of factors like the cross-sectional shape of the river bed and how that affects where, and how deep and fast the flow is

Did my dissertation on this.

Basically. Its the hydraulic geometry of the river that affects the flow rate (Discharge). Hydraulic geometry relates to surface roughness, slope gradient, and BANKFULL cross sectional area. The actual speed of the water doesn’t change hugely until it starts dropping into the cracks in the bottom of the river, putting it simply.

Been a while since I did this so probably sounds a bit garbled.