Tidal Bores

We huddled together on the bank of the river in the cool spring night, drinking tea and warming our hands at the fire. The full moon shone intermittently through the clouds, illuminating the river below. Suddenly from downstream came the cry "Flood-O!" We ran higher up the bank with a low, rumbling sound in our ears. As we reached the top of the bank a wall of water four feet high passed be­ low us, moving faster than we could run. The wave was moving upstream, carry­ ing with it pieces of wood and other de­ bris. In the few minutes it took for the wave to move out of range of our hear­ ing the water level rose another two feet. Gradually the turbulence subsided into gentle eddies, and after an hour the river began flowing toward the sea again. Thus in 1978 did I get my first sight of the tidal bore on the River Severn in southwestern Britain. A tidal bore is a remarkable hydrodynamic phenome­ non: it is the incoming tide in the form of a wave going up a river that empties into the sea. After the bore passes, the river flows upstream for some time before the current reverses again. For a river to have a bore two conditions must be met. The tides in the adjoining tidal body must be exceptionally high; a difference of more than 20 feet between high and low water is generally required. In addi­ tion the river must be shallow with a gently sloping bottom and a broad, fun­ nel-shaped estuary. As the incoming tide is forced into the narrowing river mouth it builds in height until a single wave materializes and moves upstream. A bore is a member of the class of solitary waves. Such waves appear in many forms and in many mediums be­ sides water; they are even observed in the atmosphere of Mars. Their most in­ triguing property is that they do not dis­ perse. Like all waves, a tidal bore is made up of many sinusoidal waves with many different wavelengths. In deep wa­ ter the component waves move with dif­ ferent speeds and separate from one an­ other. Because of the way shallow-water waves interact with the bottom in the