Page 436 - Planning and Design of Airports

376 Airp o r t D e sign

n
Pipe
Clay and concrete
Good alignment, smooth joints, smooth transitions 0.013
Less favorable flow conditions 0.015
Corrugated metal
100% of periphery smoothly lined 0.013
Paved invert, 50% of periphery paved 0.018
Paved invert, 25% of periphery paved 0.021
Unpaved, bituminous-coated or noncoated 0.024
Open channels
Paved 0.015–0.020
Unpaved
Bare earth, shallow flow 0.020–0.025
Bare earth, depth of flow over 1 ft 0.015–0.020
Turf, shallow flow 0.06–0.08
Turf, depth of flow over 1 ft 0.04–0.06

Source: Federal Aviation Administration [2].
TABLE 9-10 Coefficients of Roughness n

As a guide for the design of storm drains, the coefficient of
roughness n for various types of pipes and open channels is listed in
Table 9-10.
Subsurface Drainage
The functions of subsurface drainage are to (1) remove water from a
base course, (2) remove water from the subgrade beneath a pave-
ment, and (3) intercept, collect, and remove water flowing from
springs or pervious strata.
Base drainage is normally required (1) where frost action occurs
in the subgrade beneath a pavement, (2) where the groundwater is
expected to rise to the level of the base course, and (3) where the
pavement is subject to frequent inundation and the subgrade is highly
impervious.
Subgrade drainage is desirable at locations where the water
may rise beneath the pavement to less than 1 ft below the base
course.
Intercepting drainage is highly desirable where it is known that
subsurface waters from adjacent areas are seeping toward the airport
pavements.