Water: Surface and Subsurface - Geotechnical Engineering Investigation - page 620

Runoff and Infiltration
Land erosion from runoff results in gullies, which grow to streams and finally to rivers,
developing into a regional drainage system carrying the runoff into lakes and seas.
Drainage basins consist of the rivers and their systems of branches and tributaries. The
boundaries of a drainage basin are divides, ridge lines, or other strong topographic fea-
tures separating the basin from adjacent basins.
Channel represents the volume within the river banks with the capacity to carry flow.
Floodplain is that portion of a river valley with a reasonable probability of being inun-
dated during periods of high flow exceeding channel capacity.
River stage is the elevation of the water surface at a specific gaging station above some
arbitrary zero datum.
Discharge , the runoff within the stream channel, is equal to the cross-sectional area times
the average velocity (ft 3 /sec or m 3 /sec, etc.). It is an important element in the determina-
tion of the time required to fill a reservoir, in the evaluation of surface erosion, in the eval-
uation of flood potential, and in the design of flood-protection and drainage-control
structures. When discharge quantities are computed, the cross-sectional area is usually
based on preflood data, which do not account for the channel deepening that occurs dur-
ing flooding. Discharge quantities, therefore, are often underestimated.
Infiltration occurs as runoff entering the subsurface through pore spaces in soils and
openings in rocks. It occurs most readily in porous sand and gravel, through cavities in
soluble rocks, and through heavily fractured zones in all rock types. Water moves down-
ward and through the subsurface under gravitational forces.
Groundwater results primarily from infiltration.
Effluent streams are characterized by the flow of groundwater to the stream, and the
stream represents the interception of the surface drainage with groundwater flow as
shown in Figure 8.3a. They are characteristic of moist climates.
Influent streams supply water to the ground (Figure 8.3b) and are characteristic of inter-
mittent streams in any climate, and most or all streams in arid climates.

8.2.2 Erosion

Causes
Natural agents causing erosion include running water, groundwater, waves and currents,
wind (see Section 7.5.1), glaciers (see Section 7.6.1), and gravity acting on slopes. Erosion
from running water and gravity are the most significant with respect to construction and
land development, since such activities often result in an increase in erosional processes.
Human causes result from any activity that permits an increase in the velocity of water,
thereby increasing its erosional capacity, especially on unprotected slopes. Removal of
trees and other vegetation from slopes to clear land for construction, farming, or ranching

(a)

(b)

FIGURE 8.3
Stream type and water infiltration relationships: (a) effluent stream; (b) influent stream.