a. Running Water. Streams and rivers, which are forms of running water, are referred to as fluvial
systems. Of all the transportation agents, fluvial systems are the most effective and, consequently, the most
important to engineers. Structures have been made totally useless after a relatively short time by stream deposits
or even completely destroyed by stream erosion. From an engineering standpoint, no two rivers are exactly alike;
therefore, each one requires complete investigation. However, there are some general erosional and depositional
features common to all fluvial systems.
(1) Erosional Features of Running Water. Although there are numerous types of erosional features
associated with fluvial systems, only those most likely to be encountered will be discussed here.
(a) Gullies. A gully is a ravinelike erosional feature that does not permanently contain water and
cannot be crossed by a wheeled vehicle nor covered by a farmer's plowing. These types of erosional features
form when surface runoff becomes channelized. As gullies evolve, they take on characteristic cross-sectional
shapes that reflect the textural composition and cohesiveness of the underlying soils; therefore, the cross-sectional
shape of a gully can be used as an important indicator of the soil type and thus the parent rock material. Four
general gully types may be distinguished on the basis of the cross-sectional shape (see figure 2-1).
Shallow, Rounded Gullies. Gullies that have developed on cohesive, relatively impermeable
materials, such as clays or silty clays, tend to be smoothly rounded in cross section.
U-Shaped Gullies. The cross-sectional shape of a gully developed on weakly cemented
sandy clay soils resembles a "U."
Box-Shaped Gullies. Box-shaped gullies, or gullies containing nearly vertical sides with flat
bottoms, form in areas underlain by silty soils.
V-Shaped Gullies. Gullies formed in areas underlain by granular soils, such as gravel or
sand, take on a characteristic "V" shape when viewed in cross section.
(b) Stream and River Valleys. The primary features of fluvial erosion are the easily recognizable
valleys of streams and rivers. Collectively, the valleys make up the drainage pattern and affect the type of
topography that will develop over an area. Several different types of drainage patterns are possible, depending on
the type of underlying geologic material that is present. See figure 2-2, pages 2-6 and 2-7, for some of the more
common types of drainage patterns and their associated geologic materials.
Information may be gleaned not only from the shape of the drainage system but also from its density, or texture.
For example, any type of flat-lying, homogeneous sedimentary rock unit may display a dendritic or rectangular
drainage pattern. The problem is to determine which specific type of sedimentary rock is represented.
Sandstones, conglomerates, and other coarse-grained rock types are usually permeable; that is, most precipitation
will percolate downward through the soil and underlying rock. Therefore, the amount of surface runoff will be
minimal, and a relatively small number of streams will develop in a precise area. This sparse stream development
is referred to as coarse-textured drainage (see figure 2-3, page 2-8).