[Physics] Why do tidal waves appear so suddenly in some rivers

Question

As tide approaches in lower part of some rivers (e.g. Ganges), a several feet high tidal wave enters from the sea against the flow of the river (making a great noise), and the water level suddenly rises as the wave proceeds.

I would expect the water level to rise slowly and gradually, as the angle between the moon and the zenith continuously changes from $90^{\circ}$ to $0^{\circ}$ (or $180^{\circ}$).

Why does the tidal wave appear so suddenly?

I am unsure if this occurs in seas as well, but I have seen this phenomenon in rivers near the sea.

Answer 1

What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (typically more than 6 meters (20 ft) between high and low tide) and where incoming tides are funneled into a shallow, narrowing river or lake via a broad bay. The funnel-like shape not only increases the tidal range, but it can also decrease the duration of the flood tide, down to a point where the flood appears as a sudden increase in the water level. A tidal bore takes place during the flood tide and never during the ebb tide.

In the ocean the sea level rises quite slowly (only a few feet per hour). But in the shallow water of rivers (and also in funnel-like bays) this will result in a sudden wave-like rise of the water-level. Due to the small slope of the river the slow vertical rise (a few feet per hour) is converted to high horizontal speed (several kilometers per hour) of the wave-front. And due to the funnel-like shape (from a wide sea bay to a narrow river) the height of the wave front piles up on the way.

_{(image from Fisheries and Oceans Canada - Phenomena - Tidal bores)}

Here is a real image of the phenomenon. _{(image from Spectacular tidal bore surges up China river)}