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

DRAINAGE CRITERIA MANUAL

RUNOFF

The CUHP model is sensitive to the "percent impervious" parameter. Since
e the value of this parameter depends, to a degree, on the density of the
~sidential development, Figure 2-1 is provided to assist in estimating the
.percent impervious values for low to medium density residential areas. It is
based on the observed trends in the urbanized portions of the Urban Drainage
and Flood Control District (39), and reflects the densities found in built-up
areas. Figure 2-1 does not account for parks or large open areas.

2.2 Depression Losses
Rain water that is collected and held in small depressions and does not
become part of the general surface runoff is called depression storage. Most
of this water eventually infiltrates or is evaporated. Depression losses also
include water intercepted by trees and bushes and water that is detained on
the surface and does not run off. The CUHP Method in Chapter 4, requires
numerical values of retention loss as input in calculating the effective
rainfall. Table 2-1 can be used as a guide in estimating the amount of
depression retention storage to be used with CUHP.

.

TABLE 2-1

TYPICAL DEPRESSION RETENTION FOR VARIOUS LAND COVERS
(all values in inches)
(For use with CUHP Method)
Depress i on &
Land Cover Detention Recorrmended
Impervious:
Large Paved Areas
Roofs - Flat
Roofs - Sloped

0.05 - 0.15
0.1 - 0.3
0.05 - 0.1
0.2 - 0.5
0.2 - 0.6

0.1
0.1
0.05

Pervious
Lawn Grass
Wooded Area and
Open Fields

0.35
0.4

e

When an area is analyzed for depression retention storage, the pervious
and impervious storage values for all parts of the basin must be considered

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5-1-84