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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
Table 5.3
EQUATION FOR ESTIMATING K b IN THE T c EQUATION
K b = m log A + b
Where A is drainage area, in acres
Equation
Parameters
Type Description Typical Applications
m
b
A
Minimal roughness: Relatively smooth
and/or well graded and uniform land
surfaces. Surfaces runoff is sheet
flow.
Commercial/industrial areas
Residential area
Parks and golf courses
-0.00625 0.04
B
Moderately low roughness: Land
surfaces have irregularly spaced
roughness elements that protrude from
the surface but the overall character of
the surface is relatively uniform.
Surface runoff is predominately sheet
flow around the roughness elements.
Agricultural fields
Pastures
Desert rangelands
Undeveloped urban lands
-0.01375 0.08
C
Moderately high roughness: Land
surfaces that have significant large to
medium-sized roughness elements
and/or poorly graded land surfaces
that cause the flow to be diverted
around the roughness elements.
Surface runoff is sheet flow for short
distances draining into meandering
drainage paths.
Hillslopes
Brushy alluvial fans
Hilly rangeland
Disturbed land, mining, etc.
Forests with underbrush
-0.025 0.15
D
Maximum roughness: Rough land
surfaces with tortuous flow paths. Surface
runoff is concentrated in numerous
short flow paths that are often
oblique to the main flow
direction.
Mountains
Some wetlands
-0.030 0.20
Note:
A is the area of the entire subbasin, not the area of the surface type A, B, C or D within the
subbasin.
5.5.2 Storage Coefficient
Very little literature exists on the estimation of the storage coefficient (R) for the Clark Unit Hydrograph.
Clark (1945) had originally proposed a relation between T c and R since they can both be
defined by locating the inflection point of a runoff hydrograph (refer to Figure 5.2). The Corps of
Engineers discuss the development of regionalized relations for T c and R as functions of watersheds
characteristics in Training Document No. 15 (U.S. Army Corps of Engineers, 1982b).
According to Corps procedures, T c and R are estimated from relations of T c + R and R / (T c + R)
August 15, 2013 5-17