Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
consulted when using S-graphs are contained in Design of Small Dams (USBR, 1987) and the
USBR Flood Hydrology Manual (Cudworth, 1989).
Table 5.6
S-GRAPHS AND K n VALUES
K n
S-Graph Type Description Min Avg Max Description
Phoenix Valley Very shallow
slopes and/or
partially urbanized
0.015 --- 0.15 Variations dependent upon
slope, degree of urbanization
and connected impervious areas
and development of organized
drainage improvements;
extreme high values may be
appropriate in very flat areas
with little or no drainage network
Phoenix Mountain Mountain 0.045 0.05 0.055 Quite rugged, with sharp ridges
and narrow, steep canyons
through which watercourses
meander around sharp bends,
over large boulders, and
considerable debris obstruction;
ground cover, excluding small
areas of rock outcrops, includes
many trees and considerable
underbrush; no drainage
improvements
Foothills 0.027 0.03 0.033 Gently rolling, with rounded
ridges and moderate side
slopes; watercourses meander
in fairly straight channels with
some boulders and lodged
debris; ground cover includes
scattered brush, cactus and
grasses; no drainage
improvements
Desert/Rangeland
Agricultural
Gently sloping
natural areas
including
distributary flow
areas
Actively cultivated
areas with crops
0.020 0.025 0.03 Variations from minimum to
maximum roughness due to
degree of definition of
watercourses, extent of
vegetation, and land surface
hydraulic condition
0.06 0.10 0.15 Variations from minimum to
maximum dependent upon
slope, crop type and density
Note:
The majority of K n data upon which these values are based come from rainfall runoff events of magnitude
less than the 100-year event. Therefore, selected K n values for a given design storm need to be evaluated
for the purposes of modeling a particular watershed response to that design storm.
August 15, 2013 5-29