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 3.0 RAINFALL

3.1 Introduction

Presented in this section is the design rainfall data to be used
with the Colorado Urban Hydrograph Procedure (CUHP) and the
Rational Method. All hydrological analysis within the jurisdiction
of the criteria shall use the rainfall data presented herein for
calculating storm runoff.

The design storms and intensity-duration-frequency curves for the
City were developed using the rainfall data as presented in the
NOAA Atlas for Colorado and the procedures presented in the Urban
storm Drainage Criteria Manual (USDCM).

3.2 Selection of Design Storm Frequencies

All drainage systems for new development have to take into
consideration two separate and distinct drainage situations. The
first is the initial storm, which occurs at fairly regular
intervals and is based on the two- to ten-year storm event,
depending on land use. The runoff from the initial storm is
usually not the cause of extensive damage, but can represent higher
costs in maintenance, repair and replacement of public facilities
if not handled correctly.

The second drainage situation that must be considered is the
planning and design of facilities to convey the major storm, which
occurs at less frequent intervals, and is based on the laO-year
storm event. The runoff from this type of storm event can cause
catastrophic property damage and personal injury or loss of life.

The purpose of the major drainage path criteria is to establish
guidelines to protect the major paths from encroachment. No
development shall be allowed along the major drainage path except
by specific approval of the city. The major drainage paths are
shown on exhibits in the Comprehensive Drainage Plans for each
major drainage basin within the City. All development that occurs
along the major paths will be required to meet the guidelines
herein. The guidelines herein are intended to be supplements to
the specific recommendations made for each drainage basin analyzed
in the Comprehensive Drainage Plans.

The major drainage paths are a part of the major drainage system
and all facilities along said paths will be required to be designed
using a lOa-year design storm frequency. The only exceptions are
those specific design storm frequencies recommend for the drainage
basin investigated by the Comprehensive Drainage Plans.

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