Surface runoff

“ The effect of this under natural conditions is readily seen in the fact that after the first, rapid absorption of falling rain by the soil when in good tilth, there is a gradual slackening of the process even when the rain is fine and slow, causing a perceptible increase of the runoff until, should the rain continue for some time, the absorption becomes so slow as to cause all, or nearly all the water to drain off the surface. ”

“ It is accomplished chiefly by solution, changes in temperature, the wedge-work of ice and roots, the borings of animals, and such chemical changes as surface water and air effect. The products of weathering are transported by the direct action of gravity, by glaciers, by winds, and by running water. Of these the last is the most important.
Corrasion is accomplished chiefly by the mechanical wear of streams, aided by the hard fragments such as sand, gravel and bowlders, which they carry. The solution effected by the waters of a stream may also be regarded as a part of corrasion. ”

“ When, too, in flood time, the hurrying current seems to respond more sensitively to the shape of the shallows and the banks beneath, there boils up from below a ceaseless succession of irregular circles as if the water there expanded from a centre, marking the verge of its outflow with bubbles and raised lines upon the surface. ”

“ In reply to this objection, we may point out that these accumulations of loose materials are of such a nature as to be capable of easy removal by denuding agents, and that as they are formed upon the land they will, if not already washed away by the action of rain, floods, rivers, &c., run great risk of having their materials distributed, when the land sinks beneath the waters of the ocean and the surface is covered by new deposits. ”

“ When, therefore, we attempt to study them from the valleys, or from gaps and openings of the forest, we are confounded by a multitude of separate and apparently antagonistic impressions. The bottom of the storm is broken up into innumerable waves and currents that surge against the hillsides like sea-waves against a shore, and these, reacting on the nether surface of the storm, erode immense cavernous hollows and cañons, and sweep forward the resulting detritus in long trains, like the moraines of glaciers. ”

“ The rocks were shattered by changes of temperature, and at times at least crumbled by the freezing of absorbed water and also underwent chemical changes which softened and disintegrated them. The rains beat upon them, and streams flowing to the sea cut channels and carried away the material forming the land. These processes of disintegration and erosion have been in progress since islands and continents first appeared on the earth, and every mountain range now giving diversity to the surface of the land represents the net result of elevation over denudation. ”

“ When the deposit is not very deep, and the adjacent land lying to the leeward of the prevailing winds is covered with water, or otherwise worthless, the surface is sometimes freed from the drifts by repeated harrowings, which loosen the sand, so that the wind takes it up and transports it to grounds where accumulations of it are less injurious. ”

“ What, then, were the agents of denudation and transportation which operated in the formation of these great depressions? Under the existing arid conditions the surface rocks, unprotected by vegetation, are rapidly disintegrated or weathered as the result of the great diurnal variations of temperature to which they are subjected (insolation) . The weathered material, however, does not accumulate and form a protective soil-cap, but is carried away by the wind (deflation) , the heavier siliceous grains effecting an immense amount of abrasion of the exposed rock-surfaces over which they are swept. ”

“ It acts chemically upon the rock substance of the earth, causing induration in some instances, but more often inducing disintegration and change of composition by means of which rock is reduced to soil, or soil-like material, and rendered susceptible of easy removal by winds and waters. When in motion the atmosphere acts mechanically on the surface of the earth, transporting dust and sand, and by the friction of these it abrades the surface. ”

“ By carefully watching the effect of heavy rain 66upon the slopes and the formation, the places where water amasses can be traced, and means used to restore the surface to the same condition as the other portions of a cutting or embankment. As water is the chief cause of slips, the friction and cohesion of earth being impaired and, perhaps, destroyed by it, it is obvious that at the time percolation, which varies greatly with the seasons, is at its maximum, i.e. ”

“ As a rule, upon railway works, serious slips, except in very unstable soil, such as drift earth, or those quickly decomposed by atmospheric influences, seldom occur, vide Chapter II., until a year or two has elapsed, the process of disintegration from water being necessarily slower in its action, the material having an opportunity to return to its normal condition, and the surface being more equally exposed. ”

“ Surface cooling under these conditions should give only feeble thrusts, developed and eased nearly constantly. Such movements should be admirably adapted to give those gentle, nearly constant subsidences that furnish the nice adjustments of water-depth required for the accumulation of thick strata in shallow water, and those slow upward warpings that renew the feeding-grounds of erosion, the necessary complement of the deposition. ”

“ When, under the vertical rays of the never-clouded sun, the carbonized turfy covering falls into dust, the indurated soil cracks asunder as if from the shock of an earthquake. If at such times two opposing currents of air, whose conflict produces a rotary motion, come in contact with the soil, the plain assumes a strange and singular aspect. Like conical-shaped clouds, the points of which descend to the earth, the sand rises through the rarefied air on the electrically-charged centre of the whirling current, resembling the loud water-spout, dreaded by the experienced mariner. ”

“ If, after the storm, we examine again the face of the bluff, we find it striated with numberless tiny channels, down which have just poured little rivulets of water hardly so much as wetting the surface, while from top to bottom the erosion has been about the same, the slightly increased density of the upper layer enabling them to sustain the brunt of the storm, and yet suffer no more wear than the softer strata beneath. ”

“ Corrasion likewise is favored by high declivity, for the abrasive power of a stream increases as the square of its velocity. With corrasive power increased, corrasion will also be increased if the water124 has tools to work with. Since high declivity greatly increases both the transporting and the corrasive power of running water, and favors certain elements of weathering, it is clear that the aggregate effect of high declivity is to favor erosion, whether in the channel of the stream or on the general surface of its drainage basin.
The Influence of Rock. ”

“ The intricate pattern of river etchings.—The attack of the weather upon the solid lithosphere destroys the integrity of its surface layer, and through reducing it to rock débris makes it the natural prey of any agent competent to carry it along the surface. We have seen how, for short distances, gravity unaided may pile up the débris in accumulations of talus, and how, when assisted by thaw water which has soaked into the material, it may accomplish a slow migration by a peculiar type of soil flow. ”

“ Many of them spring from the ordinary incidents of the surface, and claim attention chiefly as obstacles to the study of more significant oscillations. Winds, waves, waterfalls, the tread of animals, the rumble of traffic, the blasts of mines, the changes of temperature, the variations in atmospheric pressure, the weighting of rainfall and the lightening of evaporation, the rupture of rock or ice or frozen earth, and many other processes, make their contributions to local and minute movements. ”

“ The water of subterranean currents and reservoirs, as well as that of springs and common wells, is doubtless principally furnished by infiltration, and hence its quantity must vary with every change of natural surface which tends to accelerate or to retard the drainage of the surface-soil. ”

“ Sometimes, the rain which falls upon the high land is collected together by fissures in the rocks, or by seams or ruptures in the impervious strata below the surface, and finds vent in a gushing spring on the hill-side. ”

“ The inequalities of the torrent bed and walls cause its eddyings, and the eddies fashion the concavities. The more profound the latter become, the more disturbance is caused in the water. The destruction of the rocks proceeds at an ever-increasing rate, for the larger the amount of surface that is exposed, the greater are the opportunities for the assaults of heat and cold. ”

“ Not only does the drifting sand wear the surface over which it passes and against which it strikes, but the grains themselves are worn in the process. They are liable to be broken as they strike rock surfaces, and they are likely to strike one another in the atmosphere. In both cases they are subject to wear, and so to reduction to a finer and finer state.
The erosion accomplished by the wind is therefore of various sorts. The impact of the wind itself picks up the fine materials which are already loosened, thus wearing down the surface from which they are removed ”

“ The mechanical work of the rain.—In falling the rain washes the atmosphere, taking from it much of the dust, spores, etc., which the winds have lifted from the surface of the dry land. Not only this, but in passing through the atmosphere the water dissolves some of its gases, and perhaps particles of soluble solid matter. When therefore the falling water reaches the surface of the land it is no longer pure, and some of the gases it has taken up in its descent enable it to dissolve various mineral matters on which pure water has little effect. ”

“ Sometimes, the mode of action has been the transmission of undulations through the crust, producing earthquakes, cracking and forcing apart strata already petrified, and bending and variously contorting those that have but partially become solid. Sometimes, the fiery impulse is sufficiently concentrated to break through the superincumbent materials, forcing a passage for the molten and incandescent rock, which then flows forth from the surface, penetrates into the cracks and fissures of the fractured strata, and frequently spreads into the hollows and over the summits of the latest formations. ”

“ The trickling of water down fissures forms a slimy and easy-sliding surface most difficult to treat or prevent, and so long as the natural contour of the ground does not offer resistance to movement, 39a slip may extend for a long distance, either in deep or shallow cuttings, and there may be considerable hydrostatic pressure. ”

“ The surface of the road is designed to turn water rapidly to the ditches, but when the material is the natural soil, there is always considerable absorption of storm water. Surfaces such as sandclay, gravel and macadam do not [Pg 32] absorb to exceed 10 per cent of the precipitation during short showers. Bituminous surfaces, brick and concrete pavements, do not absorb an appreciable amount of storm water. ”

“ Weathering may be possible only upon the surface or may gradually extend downwards, and as it can hardly be called a slip, the point to ascertain is not so much that it is sound and weather-resisting, as to know that there is no chance of any portion becoming detached or sliding, through the cohesion of the joints being impaired or destroyed by water, frost, or other agencies ”

“ The rain which falls on plains or gently undulating surfaces, although after a while it dries up, yet to some degree aids in levelling the land, partly by washing down particles of earth, however slowly, to lower levels, partly by soaking the earth and preparing a thin stratum of its upper surface to be converted into dust, and blown away by the wind. But it is when very heavy storms occur that the levelling action of rain over widely extending regions can be most readily recognized. ”

“ This it does, partly by the help of some chemical power, from carbonic acid, and other admixtures, but principally by the grinding agency of the sand, pebbles, &c. which it moves along. In times of flood, these act with violence like so many hammers on the rocks, ploughing long channels on their surface, or whirling round and round in deep pits, especially beneath a fall, or where the current breaks into eddies over an uneven floor of stone. ”

“ Whenever the surface of the ground bulges or upheaves upon earth being tipped upon it or pressure applied, it shows that the disturbing harbingers of instability are in action and that its sustaining power has been reached, its surface ruptured, and any more soil placed thereon will only sink and displace the earth until it meets the solid ground beneath, or the material becomes sufficiently compressed and dense for it to sustain the load; such a condition of unrest generally proceeds from over-pressure and will loosen the soil, increase percolation, and aid disintegration. ”

“ Wind erodes the Earth’s surface by deflation, the removal of loose, fine-grained particles by the turbulent eddy action of the wind, and by abrasion, the wearing down of surfaces by the grinding action and sand blasting of windborne particles.
Most eolian deflation zones are composed of desert pavement, a sheetlike surface of rock fragments that remains after wind and water have removed the fine particles. Almost half of the Earth’s desert surfaces are stony deflation zones. The rock mantle in desert pavements protects the underlying material from deflation. ”