Environmental Engineering Reference
In-Depth Information
TABLE 7.3
Classes of Drainage Patterns in Soil Formations
Class
Associated
Characteristics
Formations
Dendritic
Uniform, homogeneous
Tributaries join the gently curving mainstream at acute angles.
formations
The more impervious the materials, the finer the texture
Pinnate
Loess and other easily
Intense pattern of branches enters the tributaries almost at right
eroded materials
angles, or slightly upstream
Parallel
Mature coastal plains
Modified dendritic with parallel branches entering the mainstream
Deranged
Young landforms:
Lack of pattern development. Area contains lakes, ponds, and
floodplains and thick
marshes in which channels terminate
till plains
Meandering
Floodplains, lake beds,
Sinuous, curving mainstream with cutoffs and oxbow lakes
swamps
Radial
Alluvial fans
Mainstream terminating in numerous off-parallel branches radiating
braided
outward
Parallel
Sheet wash, coalescing
Parallel and subparallel streams in fine-textured pattern
braided
alluvial fans
Thermokarst
Poorly drained soils in
Ground freezing and heaving causes hexagonal patterns: subsequent
thermofrost regions
thawing results in a sequence of lakes, giving a beaded
appearance along the stream
Note
:
Patterns in rock formations (see
Table 6.1)
include rectangular, angulate, trellis, barbed, radial, annular,
and centripedal as well as dendritic, parallel, and deranged.
is gradual, as is the case with most rocks. In some rocks, such as limestone, the transition
is abrupt.
Chemical decomposition produces the most significant residual soil deposits.
Mechanical weathering produces primarily granular particles of limited thickness, except
in marine shales.
The depth and type of soil cover that develop are often erratic, since they are a function
of the mineral constituents of the parent rock, climate, the time span of weathering expo-
sure, orientation of weakness planes (permitting the entry of water), and topography (see
Section 6.7.2).
Rock-Type Relationships
Igneous and metamorphic rocks composed of silicates and oxides produce thick, predom-
inantly clay soils.
Sandstones and shales are composed chiefly of stable minerals (quartz and clay), which
undergo very little additional alteration. It is the impurities (unweathered particles and
cementing agents) that decompose to form the relatively thin soil cover.
Carbonates and sulfates generally go into solution before they decompose. The rela-
tively thin soil cover that develops results from impurities.
Marine and clay shales generally undergo mechanical weathering from swelling with
some additional chemical decomposition.
Climate
Soil profile development is related primarily to rainfall, but temperature is an important
factor. Very little soil cover develops in either a cool-dry or hot-dry climate. Cool-wet
zones produce relatively thick soil cover, but tropical climates with combinations of high
temperatures and high rainfall produce the greatest thicknesses.
