Obstacles to Flow.
Action of Irregu-larities in theBeJ.
Action of Spurs.
Action of Floods.
232 RIVER AND INLAND NAVIGATION.
Speed per second,ft. in.
Coarse ballast 2 0
Sea shingle, about 1 inch diameter . . . . 2 2
Large shingle.3 0
Angular flints, size of a hen’s egg . . . . 3 3
Broken stones.4 0
„ agglomerated, or soft schistose rocks . 4 4
Rocks with distinct layers, ‘ flakey’rocks ... 6 0
Hard rocks.10 0
It may be observed that very moderate speed enables the water to carry off thelighter soils, such as are usually found in the beds of rivers. Hence, to a greatextent, the frequent changes they are subject to. The destructive action of thecurrent is augmented by the beating of the waves, the abrasion by floating ice, thealternatives of wetness and dryness, and especially by frosts. Aquatic plants protectthe beds in a very sensible manner, however, when the depth of water does notexceed much more than 6 feet.
11. Any unusual obstacle opposed to a current, whether naturally or artificially,acts by heaping up the waters on the up-stream side, and by accelerating their speedon the down-stream side. If the bottom be of a nature to yield to this increasedspeed, an excavation is necessarily formed. When one of the banks of a river yieldsto the action of the current, the elbow thus formed has a tendency to increase rapidly.The depth of the water augments, and the currents deposit the matters they hold insuspension upon the opposite side, which becomes gradually convex whilst thussilting up. The curves or bends thus formed advance gradually from the up-streamdirection to the down-stream.
M. Defontaine observed that upon the Rhine , a river with a bottom of sand andgravel, exposed to sudden floods of remarkable violence, the natural bed of the riverwas not affected by the formation of new elbows, when the radius of its curvaturewas above 8300 feet, at least to any serious extent. His experience led him to adoptone-half of that radius for the works in masonry he constructed to defend anyadvanced parts of the banks, in order to protect them against the corroding action ofthe current at their feet.
12. Any sudden depression, any abrupt projection, either upon the banks or evenin the bottom, produces a species of whirlpool and backwater, attributed by Venturito the lateral movement of the waters. If the bottom is susceptible of being carriedaway, there are excavations formed in some places, the materials of which are depo-sited at distances, which of course vary according to circumstances. We find alsothat the meeting of two rivers, of the same volume and speed, at an angle nearly of90°, not only produces an oblique final movement compared with the direction of theprimitive current, but also a series of these whirlpools.
A spur projecting into the river also determines a backwater both upon the up-stream and the down-stream side. If the bottom be soft, the point is liable to beundermined and carried away; a silting up takes place in the down-stream angleof the spur, which also sometimes takes place on the up-stream side. This, how-ever, more properly belongs to that portion of the subject which treats of the defenceof the banks of rivers.
13. Another very important cause of perturbations in the ordinary course orflow of rivers is found in floods, which, from their velocity and the immense volumeof water they bring down, often-affect the bed and the sides of rivers in a dangerousmanner. Sir C. Lyell mentions one flood in which the waters attained a velocity of