WMS:Maricopa County Time of Concentration

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The Maricopa County, Arizona equation for computing time of concentration has also been included in WMS as a pre-defined equation.

 tc = 11.4 \sqrt {L}K_b ^{0.52}S^{-0.31}i^{-0.38}

where:

tc = Time of concentration.
L = Length of the flow path (ft).
Kb = Representative watershed resistance coefficients. Values are computed using the equation:
K_b = m \log (A) + b
where m and b are defined in the Table below of the Maricopa County drainage manual (shown below) and A is the drainage area in acres.
S = Ground slope of the flow segment (ft/ft).
i = Rainfall intensity (in/hr).
Parameters for Estimating K_b = m \log (A)+b in the Maricopa County Tc Equation
Type Description Typical Applications Parameters
m b
A Minimal roughness: Relatively smooth and/or well graded and uniform land surfaces. Surface 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 land
-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 flow to be diverted around roughness elements. Surface runoff is sheet flow for short distances draining into meandering drainage paths. Hillslopes
Brushy alluvial fans
Hilly rangelands
Disturbed land, mining, etc.
Forests with underbrush
-0.025 0.15
D Maximum roughness: Rough land surfaces with torturous flow paths. Surface runoff is concentrated in numerous short flow paths that are oblique to the main flow direction Mountains
Some wetlands
-0.030 0.20

Conversion of watershed area between square miles and acres may be required in order for this equation to compute the proper time of concentration.

Because the appropriate rainfall intensity value is a function of tc, it is necessary to define an IDF curve (equation). The IDF Curves dialog used as part of the rational method is used to set up equations relating i to tc.


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