Rock-Mass Characteristics - Geotechnical Engineering Investigation - page 409

TABLE 6.2
( Continued )

Geologic Condition Predominant Geologic Condition Predominant

Drainage Pattern Drainage Pattern

Schist Impure limestone

Medium to fine Medium angular

rectangular dendritic dendritic

III. Controlled by Regional Rock Form

Tilted interbedded Basalt flows over

sedimentary large areas

Trellis Parallel, coarse

Regional warping and uplift Domes, volcanoes

Barbed Radial

Domes with some joint control Soluble limestone;

glaciatedcrystalline
shields

Annular Deranged with

intermittent
drainage

Basin Coral No surface

drainage

Centripedal

a

Drainage patterns given are for humid climate conditions (categories I and II). Arid climates generally pro-
duce a pattern one level coarser.

b

Coarse: First-order streams over 2 in. (5 cm) apart (on map scale of 1:20,000) and carry relatively little runoff.

c

Medium: First-order stream ¼ to 2 in. (5 mm to 5 cm) apart.

d

Fine: Spacing between tributaries and first-order stream less than ¼ in. (5 mm).