FLOOD05658

• County: Weld
• Basin: South Platte
• Title: Project Notebook for I-25 Corridor Master Drainage Plan South Weld County: Volume 1 of 2
• Date: 12/6/1999
• Prepared For: Weld County Public Works
• Prepared By: Anderson Consulting Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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throughout the stonn. If the next two entries are made, then this value will be used as the
initial infiltration rate in Horton's equation.
10. Decay - Exponential decay coefficient in Horton's equation in "per second" units.
11. Final infiltration - FutaI infiltration rate in Horton's equation in inches per hour.
12. Nullifier of Rainfall Loss Adjustment Factors Enteredwith Rainfall - To be used if
rainfall loss adjustment factors entered with rainfall data are to be nullified (optional).
Ootional Inout Parameters For Hydrolmloh Shaping
The unit hydrograph is developed by the computer using the algorithm used in the 1984 revision
of the USDCM As noted before, the coefficients Ct and Cp are computed by the program;
however, they can be specified by the user as an option.

The shaping of the unit hydrograph also relies on proportioning the widths at 50% and 75% of
the unit hydrograph peak. The proportioning is based on 0.35 of the width at 50% of peak
being ahead of the "time to peak" and 0.45 of the width at 75% of peak being ahead of the
"time to peak". These proportioning W:tors were selected after observing a number of unit
hydrographs derived from the rainflillIrunotr data collected by the U.S.G.S. for the District. It is
possible for the user to override the unit hydrograph widths and the proportioning of these
widths built into the program. For drainage and flood studies within the District, the program
values shall be used. If the user has derived unit hydrographs from reliable rainfaII-runotr data
for a study catchment and can develop a "calibrated" unit hydrograph for this catchment, this
option permits reshaping the unit hydrograph accordingly.
Optional Parameters For Modelinll of Minimized DCIA
The following catchment parameters are optional inputs far adjustins the perviouslimpervious
area relationships.
1. DClA - Specifies the DCIA Level of practice as defined in USDCM, Vol. 3, Structural
BMPs. Input is 1, 2, or 3.
2. D - Defines the fraction of the total impervious area directly COMected to the drainage
system. Values from 0.01 to 1.0.
3. R - Defines the fraction of total pervious area receiving runotrfrom the "discoMected"
impervious areas. Values from 0.01 to 1.0.

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