2C-1. General Information for Stormwater Hydrology


A. Introduction


Urban stormwater hydrology includes the information and procedures for estimating flow peaks, volumes, and time distributions of stormwater runoff.  The analysis of these parameters is fundamental to the design of stormwater management facilities, such as storm drainage systems for conveyance of surface runoff and structural stormwater controls for quality and quantity.  In the hydrologic analysis of a development site, there are a number of variable factors that affect the nature of stormwater runoff from the site.  Some of the factors that must be considered include:

  • Rainfall amount and storm distribution
  • Drainage area size, shape, and orientation
  • Ground cover and soil type
  • Slopes of terrain and stream channel(s)
  • Antecedent moisture condition
  • Storage potential (floodplains, ponds, wetlands, reservoirs, channels, etc.)
  • Watershed development potential
  • Characteristics of the local drainage system

 

The typical hydrologic processes of interest in urban hydrology are related to:

  • Precipitation and losses (rainfall abstractions)
  • Determination of peak flow rate
  • Determination of total runoff volume
  • Runoff hydrograph (flow vs. time)
  • Stream channel hydrograph routing and combining of flows
  • Reservoir (storage) routing

 

The practice of urban stormwater hydrology is not an exact science.  While the hydrologic processes are well-understood, the necessary equations and boundary conditions required to solve them are often quite complex.  In addition, the required data is often not available.  There are a number of empirical hydrologic methods that can be used to estimate runoff characteristics for a site or drainage subbasin; the methods presented in this section have been selected to support hydrologic site analysis for the design methods and procedures included in this chapter:

  • Rational method
  • NRCS Urban Hydrology for Small Watersheds (TR-55, 1986; WINTR-55, 2003)
  • U.S. Geological Survey (USGS) regression equations
  • Small storm hydrology methods (water quality treatment volume – WQv and water quality capture volume calculations)
  • Low-impact development (LID) hydrologic methods
  • Water balance calculations

 

These methods have been included since the applications are well-documented in urban stormwater hydrology design practice, and have been verified for accuracy in duplicating local hydrologic estimates for a range of design storms.  The applicable design equations, nomographs, and computer programs are readily available to support the methods.

 

Table 1 lists the hydrologic methods and circumstances for their use in various analysis and design applications.  Table 2 includes some limitations on the use of several of the methods.

  1. The Rational method is recommended for small, highly-impervious drainage areas, such as parking lots and roadways draining into inlets and gutters:

a.       Planning level calculations up to 160 acres.

b.      Detailed final design for peak runoff calculations of smaller homogeneous drainage areas of up to 60 acres.

  1. The NRCS Urban Hydrology for Small Watersheds (WINTR-55) has wide application for existing and developing urban watersheds up to 2000 acres.

  2. The USGS regression equations are recommended for drainage areas with characteristics within the ranges given for the equations.  The USGS equations should be used with caution when there are significant storage areas within the drainage basin, or where other drainage characteristics indicate that general regression equations might not be appropriate.

 

Table 1: Applications of hydrologic methods

 

Method

Rational method

NRCS Method

USGS Equations

Water

Quality

Volume

Water quality volume (WQv)

 

 

 

√

Channel protection volume (CPv)

 

√

 

 

Overbank flood protection (Qp5)

 

√

√

 

Extreme flood protection (Qf)

 

√

√

 

Storage facilities

 

√

√

 

Outlet structures

 

√

√

 

Gutter flow and inlets

√

 

 

 

Storm sewer piping

√

√

√

 

Culverts

√

√

√

 

Small ditches

√

√

√

 

Open channels

√

√

√

 

Energy dissipation

 

√

√

 

 

Table 2: Limitations of hydrologic methods

 

Method

Size Limitations

Comments

Rational

≤ 160 acres

Method can be used for estimating peak flows and the design of small site or subdivision storm sewer systems.  Should not be used for storage design.

NRCS

0-2000 acres

Method can be used for estimating peak flows and hydrographs for all design applications.  Can be used for low-impact development hydrologic analysis.

USGS regression

 

Method can be used for estimating peak flows for all design applications.

Water quality

 

Methods used for calculating the water quality volume (WQv):

(1) Simplified method, (2) NRCS CN method, (3) water quality capture volume method.

B. Definitions


1.      Travel time (Tt) and time of concentration (Tc).  Travel time is the time it takes for water to travel from one location to another in a watershed.  Tt is a component of the time of concentration, Tc, which is the time for runoff to travel from the hydraulically most distant point of the watershed to a point of interest within the watershed.  Tc is computed by summing all the travel times for consecutive components of the drainage conveyance system.

2.      Infiltration.  Infiltration is the process through which precipitation enters the soil surface and moves through the upper soil profile. 

3.      Depression storage.  Depression storage is the natural depressions within the ground surface and landscape that collect and store rainfall runoff, either temporarily or permanently.

4.      Interception.  Interception is the storage of rainfall on foliage and other intercepting surfaces, such as vegetated pervious areas, during a rainfall event.

5.      Rainfall excess.  After interception, depression storage, and infiltration have been satisfied, rainfall excess is the remaining water available to produce runoff.

6.      Hyetograph. A hyetograph is a graph of the time distribution of rainfall over a watershed (rainfall intensity (in/hr) or volume vs. time).

7.      Hydrograph. A hydrograph is a graph of the time distribution of runoff from a watershed.

8.      Unit hydrograph.  The hydrograph resulting from 1 inch of rainfall excess generated uniformly over the watershed, at a uniform rate, for a specified period of time.  There are several types of unit hydrographs.  The use of unit hydrographs to create direct runoff hydrographs is discussed in more detail in Section 2C-7.  An example of the NRCS dimensionless unit hydrograph and the relationships to the other components presented above is shown in Figure 1.

9.      Peak discharge.  The peak discharge (peak flow) is the maximum rate of flow of water passing a given point during or after a rainfall event (or snowmelt).

10.  Runoff volume.  The runoff volume represents the volume of rainfall excess generated from the watershed area. The runoff volume is often expressed in watershed-inches or acre-feet.  The runoff volume for a rainfall event can also be represented by the area under the runoff portion of the hydrograph.

 

Figure 1:  NRCS dimensionless curvilinear unit hydrograph and equivalent triangular hydrograph


C. Concepts


The hydrologic concepts of interest with respect to the design of BMPs are closely related to the design objectives of the BMP.  Design of BMPs can be focused on peak discharge control, volume control, water quality management, pollutant removal, groundwater recharge, thermal control, or a combination of two or more of these objectives.  Each control objective has somewhat different hydrologic parameter requirements that will need to be addressed in the design of the BMP to achieve these objectives.

 

The addition of water quality considerations in the design of BMPs adds a new dimension to the hydrologic considerations for traditional BMP design.  Prior to the introduction of water quality considerations, hydrologic design methods were focused on flood event hydrology focused on storms typically ranging from the 2-year (bank-full), 5-year to 10-year (storm drainage conveyance storm), to the 100-year (floodplain storm).  Water quality considerations require a shift from flood events to annual rainfall volumes and the associated pollutant loads.  Concepts such as the rainfall frequency spectrum and small storm hydrology become important when designing for water quality.  These, along with traditional concepts, are summarized below.

1.      Large versus small storm hydrology.  Traditional practice in stormwater management has focused on flood events ranging from the 2-year to the 100-year storm.  The increased emphasis on addressing the quality of urban stormwater has resulted in the realization that small storms (i.e. <1 to 1.5 inches of rainfall) dominate watershed hydrologic parameters typically associated with water quality management issues and BMP design.  These small storms are responsible for most annual urban runoff and groundwater recharge.  Likewise, with the exception of eroded sediment, they are responsible for most pollutant wash-off from urban surfaces.  Therefore, the small storms are of most concern for the stormwater management objectives of ground water recharge, water quality resource protection, and thermal impacts control.

Medium storms, defined as storms with a return frequency of 6 months to 2 years, are the dominant storms that determine the size and shape of the receiving streams.  These storms are critical in the design of BMPs that protect stream channels from accelerated erosion and degradation.  For example, the problem with traditional detention BMPs is not the BMPs themselves, but the design guidance for BMP outlet flow control that usually does not take into account the geomorphologic character of the receiving stream.

 

The larger, more infrequent storms have traditionally been used for the design of stormwater conveyance facilities such as storm sewers and detention basins for peak discharge control; to prevent local overbank flooding on urban streams and flooding of structures located in the floodplains of stream channels.  These storms have a return frequency of 2 years to 100 years.  For traditional urban drainage design, the 2-year to 10-year storm events are termed “minor storms,” and those with a recurrence interval >10 years are called “major storms.”  In this case, minor storms should not be confused with the concept of small storm hydrologic events as described above.  Although the larger storms may contain significant pollutant loads for a single runoff event, the contribution to the annual average pollutant load is really quite small due to the infrequency of occurrence.  In addition, longer periods of recovery are available to receiving waters between larger storm events.

 

Most rainfall events are much smaller than the design storms used for urban drainage models.  In any given area, most frequently recurrent rainfall events are small (less than 1 inch of daily rainfall).  Additional details and procedures are included in Section 2C-2.

 

A detailed discussion of small storm hydrology is presented in Section 2C-6.

2.      Rainfall frequency spectrum.  A rainfall frequency spectrum (RFS), defined as the distribution of all rainfall events (see example in Figure 4), is a useful tool placing in perspective many of the relevant hydrologic parameters.  Represented in this distribution is the rainfall volume from all storm events ranging from the smallest, most frequent events in any given year; to the largest, most extreme events, such as the 100-year frequency event, over a long duration.

 

The RFS consists of classes of frequencies, often broken down by return period ranges.  Four principal classes are typically targeted for control by stormwater management practices.  The two smallest, or most frequent, classes are often referred to as water quality storms, for which the control objectives are groundwater recharge, pollutant load reduction, and to some extent, control of channel-erosion-producing events.  The two larger, or less frequent, classes are typically referred to as quantity storms, for which the control objectives are channel erosion control, overbank control, and flood control.

 

The runoff volume is the most important hydrologic variable for water quality protection and design because water quality is a function of the capture and treatment of the mass load of pollutants.  The runoff peak rate is the most important hydrologic variable for drainage system design and flooding analysis.  Water quality facilities are designed to treat a specified quantity or volume of runoff for the full duration of a storm event, as opposed to accommodating only an instantaneous peak at the most severe portion of a storm event.  To design effective BMPs and evaluate water quality impacts in urban watersheds, it is necessary to predict the following hydrologic processes:

·         Amount and distribution of rainfall volume

·         Amount of rainfall that contributes to runoff volume, i.e., rainfall volume minus abstractions


D. Methods of runoff estimation


The Rational method (see Section 2C-4) or approved alternatives may be used in both the minor and major storm runoff computations for relatively uniform basins in land use and topography, which generally have less than 160 acres (The American Society of Civil Engineers Water Environment Federation, “Design and Construction of Urban Stormwater Management Systems,” 1992 edition, states that the Rational method is not recommended for drainage areas much larger than 100-200 acres).

 

The averaging of the significantly different land uses through the runoff coefficient of the

Rational method should be minimized where possible.  For basins that have multiple changes in land use and topography, or are larger than 160 acres, or both; the design storm runoff should be analyzed by other methods such as unit hydrographs or computer applications.  These basins should be broken down into subbasins of like uniformity and routing methods applied to determine peak runoff at specified points.  For drainage areas less than 160 acres and when routing is needed, the Modified Rational method is an acceptable method for drainage areas up to 20 acres.

 

If the Rational method is not used, TR-55, Urban Hydrology for Small Watersheds (NRCS) (see Section 2C-5), may be used for drainage areas up to 2000 acres.  For areas larger than 2000 acres, TR-20 or an approved alternative may be used.  When computer programs are used for design calculation, it is important to understand the assumptions and limits for the maximum and minimum drainage area or other limits before it is selected.


2C-2. Rainfall and Runoff Analysis


A. Introduction


  1. The first step in any hydrologic analysis is an estimation of the rainfall that will fall on the site for a given time period.  The amount of rainfall can be quantified with the following characteristics:

a.       Duration (hours).  Length of time over which rainfall (storm event) occurs.

b.      Depth (inches).  Total amount of rainfall occurring during the storm duration.

c.       Intensity (inches per hour).  Depth divided by the duration.

  1. A design event is used as a basis for determining the design of a new urban storm water management project or evaluating an existing project.  It is presumed that the project will function properly if it can accommodate the design event at full capacity.  For economic reasons, some risk of failure is allowed in selection of the design event.  This risk is usually related to return period.

  2. The frequency of a rainfall event is the recurrence interval of storms having the same duration and volume (depth).  This can be expressed either in terms of exceedence probability or return period.

a.       Exceedence probability.  Probability that a storm event having the specified duration and volume will be exceeded in one given time period, typically one year.

b.      Return period.  Average length of time between events that have the same duration and volume.

 

Thus, if a storm event with a specified duration and volume has a 1% chance of occurring in any given year, then it has an exceedence probability of 0.01, and a return period of 100 years.

 

Urban stormwater projects are designed based on storm runoff, so a runoff event must be selected for design.  However, runoff data are usually not available to determine the discharge-return period or runoff volume-return period for design.  Rainfall data is available in various formats for a number of gauge stations across Iowa. 

 

Summary data can be accessed at:  http://mesonet.agron.iastate.edu/climodat/index.phtml.  Hourly (TD3240) and 15-minute (TD3260) rainfall data are available from the National Climate Data Center:  http://www.ncdc.noaa.gov/oa/ncdc.html for the National Weather Service Coop recording gauge stations in Iowa. Most all of the Coop stations in Iowa have a minimum of 60 years of hourly rainfall data, and many have 100 years on record.  A rainfall record is converted to runoff using a rainfall-runoff model. Two methods are available:  a continuous simulation approach, and the single-event design storm approach.  For the continuous simulation method, a chronological record of rainfall for the area of interest is used as input to a rainfall-runoff model of the urban watershed being considered.  The output can then be used as a chronological record of runoff to determine the maximum runoff peak and total volume for a selected design period.  The Storm Water Management Model (SWMM v.5, EPA) and HEC-HMS (Hydraulic Engineering Center, USACE) are examples of models with continuous simulation capability.  Both of these programs are available as public domain software programs.  The software programs define the format for importing the rainfall data. 

 

In the single-event design storm method, a rainfall record is analyzed to obtain a rainfall-return period relationship. Next, the storm event corresponding to a design return period is identified as the design storm.  This design storm is then used as input to a mathematical rainfall-runoff model (i.e. Rational method, NRCS WINTR-55), and the resulting output is adopted as the design runoff (peak rate and/or volume). The single-event design storm method is the most commonly-used method for smaller urban catchments and urban developments.  For assessment of larger urban stormwater systems (>1 mi2) and regional detention basins, a continuous simulation method is recommended.

 

The design storm can be described as a return period, rainfall depth, average rainfall intensity, rain duration, or a time distribution of rainfall.  Rainfall intensity refers to the time rate of rainfall (in/hr).  The intensity will vary over the duration of the event, and a plot of rainfall intensity vs. time is called a hyetograph.  The total depth of rainfall is the depth to which the rain would accumulate if it stayed in place where it fell.  The average intensity is the total rainfall depth divided by the storm duration.  Rain intensity will exhibit spatial variation, but is usually not considered for small urban watersheds (< 2000 acres).

 

The selection of the return period for design will depend on the relative importance of the facility being designed, cost (economics), desired level of protection, and damages resulting from a failure. Typical design return periods for storm sewer conveyance in Iowa (inlets and piping) vary from 2-10 years, with 5 years being most common.  For culverts, design periods of 25-50 years are typical, depending on the type and level of service for the roadway.  For detention basins, 25-100 years are common.  Additional specific design storm criteria for stormwater quality and quantity management are covered in later sections of this chapter.

 

The design storm duration also depends on the type of project. For peak discharge design of urban storm sewers and culverts, the design storm should be the one that results in the largest peak discharge for a given return period.  For urban areas with a mix of pervious and impervious area, as the imperviousness increases, the time of concentration will decrease, and the peak runoff rate will increase.  The shorter Tc will result in a higher rainfall intensity, and will give the highest peak discharge.  As will be covered later in the Rational method for determining peak runoff rate, duration, and subsequently the rainfall intensity used for input, is dependent on the time of concentration for the catchment configuration.  For storm sewer design, a minimum duration of 5 minutes is typically specified.

 

For development of runoff hydrographs using unit hydrograph methods, a storm duration much longer than than the time of concentration is selected.  For the NRCS methods for unit hydrograph development, the duration of the storm will be almost twice the time of concentration.  For the design of detention basins, the duration of the storm should be that which yields the highest storage requirement.  The duration then becomes a function of the relative size of the detention basin, the watershed size, and the outlet configuration, and will be much longer than the duration used for peak discharge determination.  This is of particular note when the Modified Rational method is used to size detention basin volume, particularly for catchment sizes more than 15-20 acres.

 

As described later in this chapter, the design storm for management of stormwater quality is defined as the rainfall depth representing the 90% cumulative probability annual rainfall depth – this is the depth of rainfall that represents 90% of the rainfall events, based on a cumulative occurrence frequency.  These will be the rainfall events with a recurrence interval of 3-4 months and generally will be less than 1.25 inches in depth.  This water quality design storm is used to determine the water quality volume (WQv) for sizing stormwater quality BMPs.  Additional details are provided in section 2C-6.  The water quality design storm depth is determined using a cumulative frequency analysis of 24-hour precipitation event totals for the period of record for a local area.  The rainfall events with a depth of less than 0.1 inches are excluded from the analysis, since these very seldom produce measurable runoff.  The individual events are then grouped by depth intervals of 0.2 inches, and the frequency of depth occurrence tabulated to determine the cumulative rainfall depth occurrence until all of the rainfall events in the period of record are included.  The smaller rainfall events are more frequent (smaller return period) while the larger storms more infrequent (smaller number) and have a larger return period.

 

For example, 90% of the annual rainfall events recorded at the NWS Coop rainfall gauge in Ames, Iowa for the period of record from 1960-2006, are less than or equal to 1.25 inches (computation based only on those rainfall events that generate measurable runoff; rainfall events less than 0.1 inch were subtracted from the total for calculation of occurrence frequency. For all rainfall events in the total period of record (100 years for most stations in Iowa), the 90% occurrence depth is 1 inch or less.

 

A rainfall analysis for the NWS Coop gauge on the southwest edge of Ames was performed for the period of record 1960-2006.  The results are summarized in Table 1.  Rainfall data for all of the NWS Coop sites in Iowa is available from the National Climate Data Center (NCDC) http://lwf.ncdc.noaa.gov/oa/climate/climatedata.html.  The data is available in 24-hour totals recorded at 15-minute and 1-hour intervals. The frequency analysis is completed by first identifying the individual rainfall events by a separation interval (in this case, 6 hours).  This means that each rainfall event is separated from the next measurable rainfall by the selected interval.  The individual rainfall events are then grouped into discrete depth categories, as shown in the tabulated data for Ames.  The number of events in each depth category are totaled, and the depth class total is divided by the total number of rainfall events for the period of record. For the 1960-2006 period of record, there were 3,362 events with more than 0.1 inches of precipitation. Rainfall depths less than 0.1 inches usually do not produce any measurable runoff, so when these events are subtracted from the total, there are 1,999 rainfall events with greater 0.1 inches depth.  The cumulative frequency is computed by dividing the cumulative number of events at each depth category by the total number of events (1,999) to provide a percent frequency of occurrence for each depth range.

 

For the Ames data, 90.6% of the rainfall events (greater than 0.1 inch) had a depth of 1.25 inches or less.  This is termed the “90% cumulative occurrence frequency,” and is the rainfall depth recommended for determining the WQv for Iowa.  Also note, for the rainfall frequency for Ames, that the average annual rainfall for the period 1960-2006 was 31.58 inches, and the mean rainfall depth (P6) is 0.62 inches. The mean rainfall depth, P6, is used in the calculation of the water quality capture volume (WQCV) for sizing extended detention storage for water quality improvement.  The WQv is one of the unified sizing criteria discussed in Part 2B and used throughout this chapter for the sizing of stormwater quality BMPs.  The method for WQCV is discussed in more detail in Section 2C-6.

 

 

Table 1:  Rainfall summary for Ames, IA for the period 1960-2006


B. Rainfall frequency analysis


Additional frequency analysis techniques are used to develop relationships between the average intensity, storm duration, and return period from rainfall data.  Often, the rainfall depth is used in place of the average intensity.  To establish the importance of the relationship between average intensity, duration, and frequency, the U.S. Weather Bureau compiled data for development of Intensity-Duration-Frequency (I-D-F) curves based on historic rainfall data for most localities across the country.  Herschfield (1961) developed these relationships for the entire US, and the data was published in the National Weather Service Technical Paper 40 (TP40) publication.  The Rainfall Frequency Atlas of the Midwest – Bulletin 71 (Huff and Angell, 1992), published by the Midwest Climate Center and the Illinois Water Survey, includes rainfall depth, duration, and return period frequency analysis in tabular format for the nine climate districts in Iowa (Figure 1). The Bulletin 71 summary data are provided in both rainfall depth and rainfall intensity in Tables 2 and 3 respectively.  The Bulletin 71 data includes the additional rainfall data for the additional period or record since 1960, and is recommended as the primary source for single-event design procedures.

 

Figure 1:  Climatic Sectional Codes for Iowa*

 

01 - Northwest

04 - West Central

07 - Southwest

02 - North Central

05 – Central

08 - South Central

03 - Northeast

06 - East Central

09 - Southeast

 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



Table 2: Sectional mean rainfall amounts for storm periods of 5 minutes to 10 days and recurrence intervals of 3 months to 100 years in Iowa (see Figure 2, Iowa Map)

Rainfall (inches) for given recurrence interval T, return period, years

*Section

Duration

3-mo

4-mo

6-mo

1-yr

2-yr

5-yr

10-yr

25-yr

50-yr

100-yr

01

10-day

2.39

2.75

3.24

4.05

4.81

5.84

6.70

8.02

9.11

10.31

01

5-day

1.90

2.15

2.49

3.11

3.77

4.68

5.43

6.61

7.60

8.75

01

72-hr

1.66

1.88

2.18

2.72

3.33

4.21

4.99

6.07

7.12

8.23

01

48-hr

1.55

1.73

2.00

2.50

3.01

3.81

4.52

5.60

6.53

7.52

01

24-hr

1.42

1.55

1.80

2.22

2.75

3.50

4.14

5.11

5.97

6.92

01

18-hr

1.34

1.46

1.69

2.09

2.59

3.29

3.89

4.80

5.61

6.50

01

12-hr

1.24

1.35

1.56

1.93

2.39

3.05

3.60

4.45

5.19

6.02

01

6-hr

1.06

1.16

1.34

1.66

2.06

2.62

3.11

3.83

4.48

5.19

01

3-hr

0.91

0.99

1.15

1.42

1.76

2.24

2.65

3.27

3.82

4.43

01

2-hr

0.83

0.90

1.04

1.29

1.59

2.03

2.40

2.96

3.46

4.01

01

1-hr

0.67

0.73

0.84

1.04

1.29

1.64

1.95

2.40

2.81

3.25

01

30-min

0.52

0.57

0.66

0.82

1.02

1.30

1.53

1.89

2.21

2.56

01

15-min

0.38

0.42

0.49

0.60

0.74

0.95

1.12

1.38

1.61

1.87

01

10-min

0.30

0.33

0.38

0.47

0.58

0.73

0.87

1.07

1.25

1.45

01

5-min

0.17

0.19

0.22

0.27

0.33

0.42

0.50

0.61

0.72

0.83

02

10-day

2.37

2.73

3.21

4.01

5.04

6.26

7.32

8.93

10.37

11.40

02

5-day

2.10

2.37

2.75

3.44

4.13

5.05

5.80

7.00

8.03

9.28

02

72-hr

1.74

1.97

2.29

2.86

3.53

4.45

5.15

6.33

7.30

8.30

02

48-hr

1.66

1.84

2.14

2.67

3.30

4.11

4.78

5.80

6.67

7.67

02

24-hr

1.51

1.65

1.91

2.36

2.98

3.72

4.38

5.33

6.14

7.07

02

18-hr

1.42

1.55

1.80

2.22

2.80

3.50

4.12

5.01

5.77

6.65

02

12-hr

1.31

1.43

1.66

2.06

2.59

3.24

3.80

4.64

5.34

6.15

02

6-hr

1.13

1.24

1.43

1.77

2.24

2.79

3.29

4.00

4.61

5.30

02

3-hr

0.97

1.06

1.22

1.51

1.91

2.38

2.80

3.41

3.93

4.52

02

2-hr

0.88

0.96

1.11

1.37

1.73

2.16

2.54

3.09

3.56

4.10

02

1-hr

0.71

0.78

0.90

1.11

1.40

1.75

2.06

2.51

2.89

3.32

02

30-min

0.56

0.61

0.70

0.87

1.10

1.38

1.62

1.97

2.27

2.62

02

15-min

0.41

0.45

0.52

0.64

0.80

1.00

1.18

1.44

1.66

1.91

02

10-min

0.32

0.35

0.41

0.50

0.63

0.78

0.92

1.12

1.29

1.48

02

5-min

0.18

0.20

0.23

0.28

0.36

0.45

0.53

0.64

0.74

0.85

03

10-day

2.49

2.87

3.38

4.22

5.04

6.17

7.07

8.29

9.20

10.19

03

5-day

2.03

2.29

2.66

3.32

3.94

4.86

5.64

6.84

7.75

8.77

03

72-hr

1.74

1.97

2.29

2.86

3.44

4.33

5.14

6.19

7.00

7.84

03

48-hr

1.61

1.79

2.07

2.59

3.20

4.02

4.69

5.62

6.34

7.09

03

24-hr

1.48

1.62

1.88

2.32

2.91

3.67

4.31

5.11

5.73

6.36

03

18-hr

1.40

1.53

1.77

2.18

2.74

3.45

4.05

4.80

5.39

5.98

03

12-hr

1.29

1.41

1.64

2.02

2.53

3.19

3.75

4.45

4.99

5.53

03

6-hr

1.11

1.22

1.41

1.74

2.18

2.75

3.23

3.83

4.30

4.77

03

3-hr

0.95

1.04

1.20

1.48

1.86

2.35

2.76

3.27

3.67

4.07

03

2-hr

0.86

0.94

1.09

1.35

1.69

2.13

2.50

2.96

3.32

3.69

03

1-hr

0.70

0.76

0.88

1.09

1.37

1.72

2.03

2.40

2.69

2.99

03

30-min

0.55

0.60

0.70

0.86

1.08

1.36

1.59

1.89

2.12

2.35

03

15-min

0.40

0.44

0.51

0.63

0.79

0.99

1.16

1.38

1.55

1.72

03

10-min

0.31

0.34

0.40

0.49

0.61

0.77

0.91

1.07

1.20

1.34

03

5-min

0.18

0.20

0.23

0.28

0.35

0.44

0.52

0.67

0.69

0.76

Source:  Bulletin 71, Rainfall Frequency Atlas of the Midwest, 1992

Table 2 (continued): Sectional mean rainfall amounts for storm periods of 5 minutes to 10 days and recurrence intervals of 3 months to 100 years in Iowa (see Figure 2, Iowa Map)

Rainfall (inches) for given recurrence interval T, return period, years

*Section

Duration

3-mo

4-mo

6-mo

1-yr

2-yr

5-yr

10-yr

25-yr

50-yr

100-yr

04

10-day

2.59

2.99

3.51

4.39

5.22

6.31

7.16

8.24

9.21

10.27

04

5-day

2.11

2.39

2.77

3.46

4.06

4.94

5.74

7.04

8.13

9.27

04

72-hr

1.79

2.02

2.34

2.93