It's an interesting question, but unfortunately one that's not going to have a definitive or single answer. The short version is that you expect a lot of variability in flow velocity during a flood where the velocity in any given area (or with depth) will depend on the discharge (i.e., the volume of water moving through the system) but also critically the "roughness" of the contacted area. For a bit of a treatment on this from a fluid dynamics perspective, you could consider things like Pang, 1998. Roughness always play a role in flow velocity in rivers, but compared to periods where the flow is constrained below "bankfull width", during floods roughness can vary a lot both from natural elements (i.e., as flow moves up the walls of the bank and potentially over natural/manmade levees) but also from flows encountering all sorts of "objects", i.e., trees, other vegetation, and a whole host of things in the built environment (roads, poles, houses, etc.). You can see some of the extreme variations in relationships between depth and velocity estimated from modeling flood flows in built environments in papers like Kreibich et al, 2009. Another aspect of this is that this actually something that's pretty challenging to measure because floods are not necessarily the easiest or safest time to go measure flow velocity (and it may often destroy or damage more autonomous instrumentation deployed to track flow velocity). This is discussed a bit in papers like Tauro et al., 2016, which is attempting to use image techniques to estimate flow velocities during floods. This highlights some of the challenges in empirical measures of flood velocities, but also demonstrates the significant spatial variability in velocities during floods (as estimated from their techniques).

Wouldn’t the slope of the region also play in a role in the velocity?

It depends mostly on the topology of the area around the river that floods. If the area that floods is very wide and flat the average flow velocity will stay nearly the smae as pre flood, where as if the river is in a narrow steep walled valley the flow velocity will increase.

The main channel of the river speeds up. Basically, as the river floods, the cross sectional area of the river increases, eventually flooding the areas adjacent to the main channel. While these move slower than the main channel, they a still moving faster than they did before.

While gravity and mass of water produce the flow, friction with the riverbed slow it down. Thus, deeper channels move faster. As they deepen, they speed up.

Yes, if you look at the shallow water equations for river flow, the right hand side will basically be predominated by a) slope driving flow and b) friction resisting flow. The friction resisting flow will be determined by the water velocity, the characteristics of the bed material, and the water depth. As water depth increases (e.g. during a flood) the frictional forces decrease as h^(4/3) where h is water depth.

Depends how you're measuring and where in the cross section. Assuming no tailwater downstream, you'll see an increase in velocity in the main channel/floodway. As water levels increase upstream, your hydraulic gradient increases, and per Mannings equation, increases your velocity.

In the wider floodplain, water will typically be mostly stagnant as more trees, vegetation, and buildings increase the Mannings roughness.

Therefore, average velocity of the entire cross section could be significantly slower if the floodplain is significantly wide.

Tailwater effects also play a significant role. If your downstream water is high, say due to flooding downstream or a reservoir, the main channel may not actually increase much at all until the downstream levels recede. You see this often near deltas where storm surge or tides provide resistance to flow coming off fairly flat topography.

I see. So the characteristics of the bed material will factor in the role of sediments in river flows? Due to flooding, there should be an increase in loose sediments that the water would carry/get hindered by.

Faster every time. Large rivers only look like they are moving slowly because of their size. Streams only look fast because there are stationary objects visible. Water moves faster when it’s sliding against more water as opposed to objects.

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The water is moving due to gravity and it is braked due to frictional forces, with the land roughness at the bottom of the water column and with other water molecules at the top. Not hard to guess in what direction those two frictional terms are modified during a flood.

There is increases in sediment load with increased water depth and hydraulic gradient (2 of the 3 major components to shear stress at the channel bottom), but that suspended sediment is marginally going to affect water velocity, and in ways that are terribly difficult to quantify. Larger waterborne debris could have a larger effect as trees or boulders deposit temporarily causing local scour or eddy currents.

Bed characteristics such as sediment type can be an identifier of what roughness (ie friction) the water will face at the boundary. The scoured bottom, a function of grain size, may also have localized effects to velocity, similar to the debris mentioned above. Small pockets form holes or preferred paths along the bed.

Probably both. Water flows slowest right next to the bottom or sides of the river and faster out in the central. deeper parts, farther from contact with the ground. So in a flooded river, the middle part is even deeper and therefore faster than usual, but the parts that have spread out onto the shallow flood plain move much slower than usual

Great question! The answer is both, and it depends on the geometry of the specific river - not to mention how it interconnects to branching/merging paths of other river segments to really complicate what's going on.

I've done a fair amount of white water kayaking, and we always pay attention to current rainfall conditions, and even if an upstream dam has been recently released. High water conditions can be very fun as some stretches flow faster and make certain rapids larger. But I've also seen high water conditions completely wash out a drop to feel like a speed bump.

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I argue that under normal conditions the velocity in the shallow flood plain is zero and during a flood it is non-zero. :)

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It totally depends on the river and specific local conditions.

It is complicated because of how kinetic energy can manifest.

You can have an increase in velocity, an increase in mass, or a combination of both.

The exact combination is dependent on a massive amount of variables like LWD, Manning roughness coefficient, channel longitudinal profile, channel cross-section geometry, amount of precipitation, amount of time precipitation occurs over, antecedent soil conditions (saturation excess vs infiltration excess flow), aquifer and other groundwater dynamics, hyporheic exchange rates… the list goes on and on.

:)

Seriously though, in normal conditions there isn't much water in the normal river bed that is standing fairly still, although of course there is some.

In flood conditions, in broad flood plains, there may be a vast expanse of water that is barely moving.

Correct me if I am wrong, but the distance from the bed increases velocity due to lack of friction. As such a flooding river should expect higher velocity at the thalweg?

Im envisioning a lazy looking river with serious undertow.

Stream charecterization plays a role, I have always envisioned water finding the easiest path. Perhaps in a braided environment velocity could theoretically be decreased as the flow hops channels?

You're correct for the most part. Turbulence mixing can create localized areas of high velocity, but the free surface (more specifically, just below the free surface) is the highest average flow due to lack of friction.

Normally if you're seeing large current, the top will be reflective of that, with chops, waves, or dune/anti-dune shape.

And stream characterization does play a role, however very difficult to define accurately. My comment above was more related to sediment characteristics, but very much a braided gravel bed is going to be different than a braided river delta than a singular sand bed.

As far as braided being lower, that depends a lot on topography. You may see those with lower depth but higher velocity. Those braids exist partially from the large tractive forces cutting paths through the floodplain. They typically have larger sediment (gravel, cobble, boulders) that may also add friction. Sand braided rivers typically have fewer paths because the braids can cut deeper more easily, creating a greater cross sectional area to balance the increase in flow.

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