FLOOD00281

• County: Larimer
• Community: Greeley
• Title: Design Criteria and Construction Specifications volume 2 of 5 Storm Drainage Criteria
• Date: 10/1/1997
• Prepared For: Greeley
• Prepared By: Department of Public Works
• Floodplain - Doc Type: Educational/Technical/Reference Information

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SECTION 4.0 RUNOFF ANALYSIS

4.1 Introduction

This section presents the criteria and methodology for determining
the storm runoff design peaks and volumes to be used in the city of
Greeley in the preparation of storm drainage studies, plans, and
facility design. Further details and discussion of each of these
rainfall/runoff models are presented in the Urban storm Drainaqe
criteria Manual (USDCM).

4.2 Rational Method

The Rational Method may be utilized for the sizing of storm sewers
and for determining the amount of runoff from undeveloped areas.
The limit of application of the Rational Method is approximately
160 acres. It has been concluded that, for tributary basins in
excess of 160 acres, the cost of the drainage works justifies
significantly more study, thought, and judgement on the part of the
engineer than is permitted by the Rational Method. When the urban
drainage basin exceeds 160 acres, the CUHP Method represents better
practice and should be used.

The procedures for the Rational Method, as explained in the USDCM,
Volume 1 "Runoff" shall be followed in the preparation of drainage
reports and storm drainage facility designs in the City.

standard forms for the calculation of Time of Concentration and
storm Drainage System Design are provided in Tables 4-1 and 4-2.

4.3 Colorado Urban Hydrograph Procedure (CORP)

The application of the Colorado Urban Hydrograph Procedure (CORP)
is generally applicable to basins greater than 90 acres, and is
required for drainage basins larger than 160 acres. The procedures
for the CUHP, as explained in the USDCM, Volume 1 "Runoff", shall
be followed in the preparation of drainage reports and storm
drainage facility designs in the city. The design storms to be
used with the CUHP method are presented in Section 3 of these
Criteria. Applicable infiltration depths are outlined in Table 2-
~, Section 2.4 of the USDCM, Volume 1, "Runoff".

A computer program has been developed to calculate hydrographs
using the CUHP Method. This program is available for use on both
large mainframe computers and on the smaller IBM-PC XT or IBM-PC AT
personal computers. In addition, the personal computer version has
an added capability of using CUHP to compute runoff hydrographs for
basins as small as 10 acres. For the City, only the personal
computer version with the capability to model basins as small as 10

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