Hydrologic Criteria and Drainage Design Manual - CCRFCD

CLARK COUNTY REGIONAL FLOOD CONTROL DISTRICT
HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL

SECTION 500
RAINFALL

TABLE OF CONTENTS

Page

501 INTRODUCTION 502

502 RAINFALL DEPTH-DURATION-FREQUENCY RELATIONS 502
502.1 - Rainfall Depth-Duration-Frequency Maps 502
502.2 - Rainfall Depths for Durations from 1- to 6-Hours 503
502.3 - Adjustments to NOAA Atlas 2 504

503 DEPTH-AREA REDUCTION FACTORS 504

504 DESIGN STORMS 505
504.1 - General 505
504.2 - 6-Hour Design Storm Distribution 505

505 TIME-INTENSITY-FREQUENCY CURVES FOR RATIONAL METHOD 505

505.1 - General 505

505.2 - Time-Intensity-Frequency Curves 506

506 RAINFALL DATA FOR McCARRAN AIRPORT RAINFALL AREA 506
506.1 - General 506
506.2 - Rainfall Depth-Duration-Frequency 506
506.3 - Time-Intensity-Frequency Data 506

507 EXAMPLE APPLICATIONS 506
507.1 - Introduction 506
507.2 - Example: 6-Hour Design Storm Distribution 507
507.3 - Example: Time-Intensity-Frequency Curves 508

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 500

LIST OF TABLES

501 PRECIPITATION ADJUSTMENT RATIOS
502 SIX-HOUR DEPTH-AREA REDUCTION FACTORS
503 SIX-HOUR STORM DISTRIBUTIONS
504 FACTORS FOR DURATIONS OF LESS THAN ONE-HOUR
505 DEPTH-DURATION-FREQUENCY VALUES FOR McCARRAN AIRPORT

RAINFALL AREA
506 TIME-INTENSITY-FREQUENCY VALUES FOR McCARRAN AIRPORT RAINFALL

AREA
507 TIME-INTENSITY-FREQUENCY VALUES FOR EXAMPLE IN SECTION 507.3

LIST OF FIGURES

501 RAINFALL DEPTH-DURATION-FREQUENCY 2-YEAR 6-HOUR
502 RAINFALL DEPTH-DURATION-FREQUENCY 5-YEAR 6-HOUR
503 RAINFALL DEPTH-DURATION-FREQUENCY 10-YEAR 6-HOUR
504 RAINFALL DEPTH-DURATION-FREQUENCY 25-YEAR 6-HOUR
505 RAINFALL DEPTH-DURATION-FREQUENCY 50-YEAR 6-HOUR
506 RAINFALL DEPTH-DURATION-FREQUENCY 100-YEAR 6-HOUR
507 RAINFALL DEPTH-DURATION-FREQUENCY 2-YEAR 24-HOUR
508 RAINFALL DEPTH-DURATION-FREQUENCY 5-YEAR 24-HOUR
509 RAINFALL DEPTH-DURATION-FREQUENCY 10-YEAR 24-HOUR
510 RAINFALL DEPTH-DURATION-FREQUENCY 25-YEAR 24-HOUR
511 RAINFALL DEPTH-DURATION-FREQUENCY 50-YEAR 24-HOUR
512 RAINFALL DEPTH-DURATION-FREQUENCY 100-YEAR 24-HOUR
513 McCARRAN AIRPORT RAINFALL AREA
514 DEPTH-AREA REDUCTION CURVES
515 6-HOUR DESIGN STORM DISTRIBUTIONS
516 DEPTH-DURATION-FREQUENCY CURVES FOR McCARRAN AIRPORT

RAINFALL AREA
517 TIME-INTENSITY-FREQUENCY CURVES FOR McCARRAN AIRPORT RAINFALL

AREA
518 HYPOTHETICAL BASIN FOR NON-URBAN, LARGE BASIN EXAMPLES
519 HYPOTHETICAL BASIN FOR URBAN, SMALL BASIN EXAMPLES
520 PRECIPITATION DEPTH VERSUS RETURN PERIOD FOR EXAMPLE IN SECTION

507.2
521 TIME-INTENSITY-FREQUENCY CURVE FOR EXAMPLE IN SECTION 507.3

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 501

Section 500
Rainfall

501 INTRODUCTION
502
Presented in this section are the design rainfall data for the minor and major
storm events as designated in Section 304.2 of this MANUAL. This data is
used to determine storm runoff in conjunction with the runoff models
designated in Section 304.3. All hydrologic analyses within the jurisdiction
of this MANUAL shall utilize the rainfall data presented herein for
calculating storm runoff.

The rainfall data published by the National Oceanic and Atmospheric
Administration (NOAA) in the NOAA Atlas 2, "Precipitation - Frequency
Atlas of the Western United States, Volume Vll - Nevada" (NOAA, 1973)
and their subsequent modification by the United States Army Corps of
Engineers (USACE), Los Angeles District (1988) shall be used to develop
point rainfall values for Clark County. The depth-area ratios developed by
the USACE, Los Angeles District (1988) based on NOAA Technical
Memorandum NWS HYDRO40 (NOAA, 1984) and area data are used to
reduce point rainfall to area rainfall. The distribution of design rainfall is
based on studies conducted by the USACE, Los Angeles District (1988).

Calculation methods and procedures are presented herein to compute
rainfall depths and intensities for return frequencies of 2-, 5-, 10-, 25-, 50-,
and 100-year and durations of 5-, 10-, 15-, and 30-minutes, and 1-, 2-, 3-,
6-, and 24-hours. Some of these values are not used to determine runoff
for the analysis required by this MANUAL, but are included for informational
purposes.

In cases where probable maximum precipitation analyses are required,
methodology outlined in a publication by NOAA and the USACE entitled
“Hydrometeorological Report No. 49, Probable Maximum Precipitation
Estimates, Colorado River and Great Basin Drainages” (NOAA and
USACE, 1977) shall be used.

The information presented in this section is the state-of-the-art information
available at the time of preparation of this MANUAL. The information
should be updated as better techniques and data become available in the
future.

RAINFALL DEPTH-DURATION-FREQUENCY RELATIONS

502.1 Rainfall Depth - Duration - Frequency Maps

The NOAA Atlas 2 Rainfall Depth - Duration - Frequency Maps are
reproduced for the Clark County area at the end of this section. Maps are
presented for the 6- and 24-hour durations for the 2-, 5-, 10-, 25-, 50-, and

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 502

Section 500 - Rainfall

100-year recurrence frequencies as shown in Figures 501 through 512.
The data obtained from these maps shall be modified according to
procedure stated in Section 502.3 (except for SCS TR-55 as described in
502.3).

502.2 Rainfall Depths for Durations From 1- to 6-Hours

The refined rainfall values for the 6- and the 24-hour durations for 2- and
100-year recurrence intervals are utilized in calculation of rainfall values for
1-hour duration and 2- and 100-year recurrence intervals. The following
equations shall be used to derive the 2-year, 1-hour and 100-year, 1-hour
rainfall:

Y2 = -0.011 + 0.942 [ (x1) (x1 / x2) ] (501)
Y100 = 0.494 + 0.755 [ (x3) (x3 / x4) ] (502)
Where:

Y2 = 2-yr, 1-hr estimated value (in)
Y100 = 100-yr, 1-hr estimated value (in)
x1 = 2-yr, 6-hr value from Standard Form 3 (in)
x2 = 2-yr, 24-hr value from Standard Form 3 (in)
x3 = 100-yr, 6-hr value from Standard Form 3 (in)
x4 = 100-yr, 24-hr value from Standard Form 3 (in)

The 2- and 100-year, 1-hour rainfall (Y2 and Y100) values are then plotted on
Standard Form 3 and a straight line connecting these points is drawn.
The 5-, 10-, 25-, and 50-year, 1-hour rainfall values are then read from the
graph.

The 2- and 3-hour duration rainfall for the various recurrence intervals may
then be calculated using the following equations:

(2-hr) = 0.341 (6-hr) + 0.659 (1-hr) (503)
(3-hr) = 0.569 (6-hr) + 0.431 (1-hr) (504)

Where:

2-hr = 2-hr 'x'-yr estimated value (in)
3-hr = 3-hr 'x'-yr estimated value (in)
1-hr = 1-hr 'x'-yr previously determined (in)
6-hr = 6-hr 'x'-yr previously determined (in)

These point rainfall values shall be modified as stated in the following
Section 502.3.

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 503

502.3 Section 500 - Rainfall

503 Adjustments to NOAA Atlas 2

Recent analysis of rainfall data in the Clark County area (WRC, 1989 and
USACE, 1988) indicates that the NOAA Atlas 2 values do not necessarily
reflect the trend of observed and recorded rainfall values which have
occurred following publication of the Atlas in 1973. Therefore, the rainfall
values in Section 502.2 are to be adjusted to reflect the current trend of
rainfall values based on the latest available information for the Clark County
area. This adjustment consists of increasing the rainfall depths for
durations of 6-hours or less by multiplying the values previously obtained by
the appropriate factors presented in Table 501.

The said adjustment shall not be used when developing the design rainfall
for use with SCS TR-55. The 24-hour design rainfall for TR-55 shall be
used directly as developed in Section 502.1.

DEPTH-AREA REDUCTION FACTORS

The NOAA Atlas 2 precipitation depths are related to rainfall frequency at
an isolated point. Storms, however, cause rainfall to occur over extensive
areas simultaneously, with more intense rainfall typically occurring near the
center of the storm. Standard precipitation analysis methods require
adjusting point precipitation depths downward in order to estimate the
average depth of rainfall over the entire storm area. This is normally
performed using depth-area reduction curves relating to a point precipitation
reduction factor to storm area and duration.

In previous hydrologic studies in Southern Nevada, three methods have
been used for adjusting point-precipitation depths to areally-averaged
depths. All early studies used the depth-area reduction curves presented
in the NOAA Atlas 2 (NOAA, 1973). These curves were developed through
investigations of storms throughout the Western United States. In fact, the
NOAA Atlas 2 for each state in the West contains the same family of
depth-area reduction curves. Most of the recent studies have adopted
depth-area reduction factors from a relatively new publication known as
"Hydro 40" (NOAA, 1984), which developed factors applicable specifically to
Arizona and New Mexico.

The USACE, Los Angeles District (1988) used slightly different depth-area
reduction factors than those presented in "Hydro 40" for areas greater than
30 square miles. These factors were based on analysis of thunderstorms
in the greater Las Vegas area. For areas up to 30 square miles the
depth-area reduction factors are almost the same as those in "Hydro 40".

The 6-hour USACE, Los Angeles District (1988) depth-area reduction
factors are to be used for all rainfall analysis in the Clark County area. The
USACE, Los Angeles District depth-area reduction curve is shown in Figure
514. The depth-area reduction factors for the 6-hour storm are also
tabulated in Table 502.

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 504

504 Section 500 - Rainfall

504.1 For areas greater than 200 square miles, the ability of the thunderstorm
generating mechanisms (i.e., available moisture, strong convective
504.2 currents, etc.) to sustain a thunderstorm much greater than 200 square
miles in diameter is greatly reduced. Therefore, only a portion of an entire
505 drainage basin could be subject to precipitation from the thunderstorm
event. Analysis of this effect on runoff peaks and volumes is complicated
505.1 by the necessity to determine the "storm centering" which produces the
greatest peak flow and/or volume at the selected design point. In order to
obtain a consistent method of analysis for these areas, the designer shall
consult the local entity (and/or the CCRFCD if suggested by the local entity)
to determine the appropriate method of analysis and design rainfall area
reduction factors for the specific location and basin under consideration.

DESIGN STORMS

General

The design storm within the jurisdiction of the MANUAL shall be a 6-hour
duration storm. The 6-hour duration storm is to be used for all HEC-1
runoff modeling in the Clark County area. The exception to the 6-hour
design storm duration is when the SCS TR-55 method is used to compute
runoff values. For SCS TR-55, a Type ll rainfall distribution shall be used
in conjunction with the 24-hour rainfall depth as described in TR-55.

6-Hour Design Storm Distribution

Three different 6-hour storm distributions are to be used as design storms
in the Clark County area. The three design storm distributions, labeled
SDN 3, SDN 4, and SDN 5, are graphically presented in Figure 515 and
tabularized in Table 503. For drainage areas less than 8 square miles in
size, use SDN 3. For drainage areas greater than or equal to 8 and less
than 12 square miles in size, use SDN 4. For drainage areas greater than
or equal to 12 square miles in size, use SDN 5.

TIME-INTENSITY-FREQUENCY CURVES FOR
RATIONAL METHOD

General

Procedures stated in Section 502 to obtain and modify the NOAA Atlas 2
rainfall depths must first be done before proceeding with development of
time-intensity-frequency curves to be used with Rational Method.

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 505

505.2 Section 500 - Rainfall

506 Time-Intensity-Frequency Curves

To develop time-intensity-frequency curves for the Rational Method of
runoff analysis, take the 1-hour adjusted point depth(s) obtained from
Section 502 and multiply by the factors in Table 504. These point values
are then converted to intensities. An example showing the development of
time-intensity-frequency curves is given in Section 507.

RAINFALL DATA FOR McCARRAN AIRPORT RAINFALL
AREA

506.1 General
506.2
506.3 This section presents the point rainfall data to be used for the McCarran
Airport Rainfall Area. The data presented is applicable to those studies
507 that have their contributing drainage basin within the area presented in
Figure 513.

Rainfall Depth-Duration-Frequency

Presented in Table 505 and Figure 516 are the rainfall
depth-duration-frequency values to be used in the McCarran Airport Rainfall
Area as designated in Figure 513.

Time-Intensity-Frequency Data

Presented in Table 506 and Figure 517 are the time-intensity-frequency
values to be used in the McCarran Airport Rainfall Area as designated in
Figure 513.

EXAMPLE APPLICATIONS

507.1 Introduction

The following examples are a first in a series of example applications
pertaining to the use of this MANUAL. The series is set up to lead the
reader through the MANUAL'S design/evaluation procedures by building on
two different hypothetical design basins within the CCRFCD.

The two example basins are introduced in Figures 518 and 519 and list
general basin parameters. The first design basin, shown in Figure 518, is
located within Las Vegas Valley and was selected for non-urban, large
basin applications. The second design basin, shown in Figure 519, is also
located within Las Vegas Valley and was selected for urban, small basin
applications utilizing the McCarran Airport Rainfall Area data. Basin
modifications are presented as the examples progress in each section to
emphasize the application modeled.

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 506

Section 500 - Rainfall

Even though the basins were selected to represent actual areas within the
CCRFCD, minor changes have been made in basin parameters to enhance
the use of this MANUAL. Therefore, results obtained from the hypothetical
examples should not be construed as representative values for the actual
basin locations.

507.2 Example: 6-Hour Design Storm Distribution

Problem: Utilizing the Basin in Figure 518, determine the 100-year,
6-hour design storm distribution.

Solution:

Step 1: Determine the NOAA 100-year, 6-hour point rainfall value from
Figure 506:

For subbasin 1: P = 2.1 in For subbasin 2: P = 2.0 in
(Area = 4.73 sq mi) (Area = 6.14 sq mi)

Step 2: Determine the weighted point rainfall:

= (2.1) (4.73) + (2.0) (6.14) = 2.04 in
10.87

(One point rainfall value is used for both basins to simplify the
example calculation.)

Step 3: Determine the adjusted point rainfall value:

From Table 501: NOAA Adjustment Factor = 1.43
From Table 502: Depth-Area Reduction Factor = 0.86
Adjusted Rainfall = 2.04 x 1.43 x 0.86 = 2.51 in

Step 4: Compute the 100-year, 6-hour design storm distribution:

For a basin area of 10.87 sq mi use SDN = 4
Multiply the storm distribution percentages (Table 503 for SDN
= 4) by the adjusted rainfall depth of 2.51 in as follows:

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 507

Section 500 - Rainfall

Final 100-Year, 6-Hour Design Storm Distribution

Storm Percent of Total Storm
Time Storm Depth Depth
(min)
(SDN 4 from Table 503) (in)
0
5 0.00 0.00
10 2.00 0.05
15 5.80 0.15
20 7.50 0.19
25 9.90 0.25
30 12.60 0.32
35 13.70 0.34
40 14.50 0.36
45 14.90 0.37
50 15.10 0.38
55 15.50 0.39
60 15.60 0.39
. 15.90 0.40
. .
. . .
330 . .
335 99.20 .
340 99.30 2.49
345 99.40 2.49
350 99.70 2.49
355 99.80 2.50
360 99.90 2.50
100.00 2.51
2.51

Application: For HEC-1, the total adjusted rainfall depth of

2.51 inches is input on the

PB Card (Adjusted

Precipitation). The

cumulative storm depths as

computed above are input

on the PC Card (Storm

Distribution Number 4).

507.3 Example: Time-Intensity-Frequency Curves

Problem: Derive the 10-year and 100-year time-intensity-frequency curves
for a 10 acre subbasin within the basin presented in Figure 518.
The 10 acre subbasin is located in Section 7, T23S, R62E:

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 508

Section 500 - Rainfall

Solution:

Step 1: Determine the 6-hour and 24-hour rainfall depths:

From Figure 513 the designated basin is not within the McCarran
Airport Rainfall Area. Therefore the 2-, 5-, 10-, 25-, 50-, and
100-year, 6-hour and 24-hour point precipitation values are read
from Figures 501 through 512. For Section 7, T23S, R62E:

Return 6-Hour 24-Hour
Period Depth Depth

(yr) (in) (in)

2 1.0 1.2

5 1.2 1.6

10 1.45 2.0

25 1.75 2.4

50 1.9 2.8

100 2.1 3.0

Step 2: Determine the 1-hour rainfall depths:

Use Equations 501 and 502 to find the 2-year and
100-year, 1-hour depths:

Y2 = -0.011 + 0.942 [ (X1) (X1 / X2) ] (501)

Y100 = 0.494 + 0.755 [ (X3) (X3 / X4) ] (502)

Y2 = -0.011 + 0.942 [ (1.0) (1.0 / 1.2) ] = 0.77 in
Y100 = 0.494 + 0.755 [ (2.1) (2.1 / 3.0) ] = 1.57 in

These two points are plotted on Figure 520 and connected with a
straight line to develop 1-hour depths for the other return periods.

Step 3: Determine the adjusted 1-hour rainfall depths:

The adjusted 1-hour rainfall depths are obtained by multiplying the
rainfall depths from Figure 520 by the adjustment factors in
Table 501:

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 509

Section 500 - Rainfall

Return 1-Hour Adjustment Adjusted 1-Hour
Period Depth Factor Depth
(in)
(yr) (in) (Table 501)

2 0.77 1.00 0.77
5 1.00 1.16 1.16
10 1.15 1.24 1.43
25 1.30 1.33 1.73
50 1.40 1.39 1.95
100 1.57 1.43 2.25

Step 4:Determine the rainfall depths and intensities for durations less than 1-hour:

The values shown above are multiplied by the factors shown in Table 504 to
obtain rainfall depths for durations less than 1-hour. These depths are then
converted to intensities. The resulting rainfall depths and intensities for this
example are presented in Table 507. The resulting time-intensity-frequency
curves are shown on Figure 521 for the 10- and 100-year return period.

Application: The time-intensity-frequency curve is used to determine the
rainfall intensities (I) used in the Rational Method (Q = CIA).

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 510