Rock-Mass Characteristics - Geotechnical Engineering Investigation - page 407

Stream Forms
Stream channels are classified with respect to shape as straight, crooked, braided, or mean-
dering (also classified as young, mature, old age, or rejuvenated). Although influenced by
rock-mass conditions, stream forms are described under Section 7.4.1 because of their
characteristic channel and valley soil deposits.

Drainage Patterns
Formed by erosion channels on the land surface, the drainage pattern is controlled by or
related to the geologic conditions. Patterns are readily apparent on topographic maps and
remote-sensing imagery. When traced onto an overlay, a clear picture is provided for inter-
pretation. Pattern classes (dendritic, rectangular, etc.) are described in Table 6.1, and typi-
cal patterns for various geologic conditions are illustrated in Table 6.2 ( see Table 7.3 for soil
formations).
Texture refers to pattern intensity and is given by Way (1978) as fine, medium, and
coarse, as apparent on vertical aerial photos at a scale of 1:20,000 (1667 ft/in.) and defined
in terms of the distance between streams measured at this scale (see Table 6.2) . Texture is

TABLE 6.1
Classes of Drainage Patterns in Rock Formations

Class Associated Formations Characteristics

Dendritic Sedimentary rocks (except limestone) Tributaries join the gently curving

and uniform, homogeneous soil mainstream at acute angles and rock

formations. Other patterns are structure control is weak. The more

modifications of the basic dendritic impervious the material, the finer the

pattern and are characteristic of texture. Intrusive granite domes cause

other rock conditions curvilinear alignments

Rectangular Controlled by rock structure: A strongly modified dendritic pattern with

primarily joints and foliations tributaries connecting in a regular pattern

at right angles. The stronger the pattern,
the thinner the soil cover

Angulate Controlled by rock structure that A strongly modified dendritic pattern that

includes major faults and joints. Also is highly irregular

gneiss and impure limestone

Trellis Tilted, interbedded sedimentary A regular, parallel pattern. The main

rocks tributaries follow the strike of the beds

and the branches follow the dip
The mainstream cuts across ridges to
form gaps during rejuvenation

Barbed Regional uplift and warping changes Reverse dendritic pattern

flow direction

Parallel Gentle, uniform slopes of basalt Modified dendritic with parallel branches

flows: also mature coastal plains entering the mainstream

Radial Domes, volcanoes Patterns radiate out from central high

ground to connect with mainstream

Annular Domes with some joint control Radial pattern with cross tributaries

Centripedal Basins, or ends of anticlines or Radial drainage toward a central

synclines connecting stream

Deranged Young landforms (floodplains and Lack of pattern development. Area contains

thick till plains) and karst lakes, ponds, and marshes. In karst,

channels end on surface where runoff
enters limestone through cavities and joints

Note : See Table 7.3 for forms in soil formations. Include pinnate, meandering, radial braided, parallel braided,

and thermokarst as well as dendritic, parallel, and deranged.