WMS:Rational Method Rainfall Intensity: Difference between revisions

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Precipitation intensity-duration-frequency (IDF) information is necessary for the specific locality in which the Rational Method will be used.  In general this is done using either HYDRO-35 or NOAA Atlas 2 data.  Using the '''IDF Curves…''' button (for either basins or outlets), WMS can be used to develop curves from either of these two data sources, statistically derived data, or directly entering an i value if the user typically computes it in another way or already knows the design value to use.
Precipitation intensity-duration-frequency (IDF) information is necessary for the specific locality in which the Rational Method will be used.  In general this is done using either HYDRO-35 or NOAA Atlas 2 data.  Using the '''IDF Curves…''' button (for either basins or outlets), WMS can be used to develop curves from either of these two data sources, statistically derived data, or directly entering an i value if typically computed in another way or the design value to use is already known.


The ''IDF Computation'' dialog (shown below) can be used to create a series of T-year IDF curves from HYDRO-35, NOAA Atlas 2, or user defined data.
The ''IDF Computation'' dialog (shown below) can be used to create a series of T-year IDF curves from HYDRO-35, NOAA Atlas 2, or user defined data.


[[Image:Rational MethodIDF.jpg|thumb|none|left|400 px|''Rational Method—IDF Computation'' dialog]]
:[[Image:Rational MethodIDF.png|thumb|none|left|400 px|''Rational Method—IDF Computation'' dialog]]


The type of data that will be used to create the IDF curves is specified with the radio group options in the upper left portion of the dialog.  The '''Define Data''' button can then be used to bring a dialog which allows entering the following values, depending on the data type specified:
The type of data that will be used to create the IDF curves is specified with the radio group options in the upper left portion of the dialog.  The '''Define Data''' button can then be used to bring a dialog which allows entering the following values, depending on the data type specified:
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The methods WMS uses to compute the IDF curve equations are discussed in Appendix A of [http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=52&id=28 FHWA HEC-12] and in [http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=2&id=6 FHWA HDS-2].
The methods WMS uses to compute the IDF curve equations are discussed in Appendix A of [http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=52&id=28 FHWA HEC-12] and in [http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=2&id=6 FHWA HDS-2].


Once the specified data has been entered the corresponding IDF-curves will be generated and plotted in the graphics window of the ''IDF Computation'' dialog. A user can then specify the recurrence interval to use for analysis by selecting it in the text list window in the upper right portion of the dialog.  After selecting the recurrence interval, the appropriate curve in the plot window will be displayed in red.  Finally, an intensity value, i, is determined by specifying a time of concentration as outlined below and then clicking on the Intensity button.  When clicking on the '''Done''' button for the ''IDF Computation'' dialog, the computed intensity value will automatically be updated in the edit field for i of the ''Rational Method'' dialog.
Once the specified data has been entered the corresponding IDF-curves will be generated and plotted in the graphics window of the ''IDF Computation'' dialog. Then specify the recurrence interval to use for analysis by selecting it in the text list window in the upper right portion of the dialog.  After selecting the recurrence interval, the appropriate curve in the plot window will be displayed in red.  Finally, an intensity value, i, is determined by specifying a time of concentration as outlined below and then clicking on the Intensity button.  When clicking on the '''Done''' button for the ''IDF Computation'' dialog, the computed intensity value will automatically be updated in the edit field for i of the ''Rational Method'' dialog.


===Computing the Time of Concentration===
===Computing the Time of Concentration===
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:<math>t_c = \frac {KL^{0.6}n^{0.6}}{i^{0.4}S^{0.3}}  </math>
:<math>t_c = \frac {KL^{0.6}n^{0.6}}{i^{0.4}S^{0.3}}  </math>


where:
:where:


* <math>tc</math> = the time of concentration.
:* ''t<sub>c</sub>'' = The time of concentration.
* <math>L</math> = overland flow length.
:* ''L'' = Overland flow length.
* <math>n</math> = Manning’s roughness coefficient.
:* ''n'' = Manning’s roughness coefficient.
* <math>i</math> =  rainfall intensity.
:* ''i'' =  Rainfall intensity.
* <math>S</math> = average slope of the overland area.
:* ''S'' = Average slope of the overland area.
* <math>K</math> = 0.93
:* ''K'' = 0.93


This method has been adopted by the FHWA for general use, but other equations can be used as defined elsewhere
This method has been adopted by the FHWA for general use, but other equations can be used as defined elsewhere


==HYDRO-35 Maps==
==HYDRO-35 Maps==
{|
<gallery widths="350px" heights="310px">
|-
Image:HYDROmap2-5.jpg|2 years 5 minutes
|[[Image:HYDROmap2-5.jpg|thumb|350 px|2 years 5 minutes]]||[[Image:HYDROmap2-15.jpg|thumb|350 px|2 years 15 minutes]]||[[Image:HYDROmap2-60.jpg|thumb|350 px|2 years 60 minutes]]
Image:HYDROmap2-15.jpg|2 years 15 minutes
|-
Image:HYDROmap2-60.jpg|2 years 60 minutes
|[[Image:HYDROmap100-5.jpg|thumb|350 px|100 years 5 minutes]]||[[Image:WMSImage392.jpg|thumb|350 px|100 years 15 minutes]]||[[Image:WMSImage394.jpg|thumb|350 px|100 years 60 minutes]]
Image:HYDROmap100-5.jpg|100 years 5 minutes
|}
Image:HYDROmap100-15.jpg|100 years 15 minutes
Image:HYDROmap100-60.jpg|100 years 60 minutes
</gallery>


==Related Topics==
==Related Topics==
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{{WMSMain}}
{{WMSMain}}
[[Category:Rational Method|R]]
[[Category:Rational Method|R]]
[[Category:Equations|R]]
[[Category:Equations|Rational method]]
[[Category:WMS Dialogs|R]]
[[Category:Rational Method Dialogs|R]]
[[Category:External Links]]
[[Category:External Links]]
[[Category:Gallery]]

Latest revision as of 21:28, 8 June 2023

Precipitation intensity-duration-frequency (IDF) information is necessary for the specific locality in which the Rational Method will be used. In general this is done using either HYDRO-35 or NOAA Atlas 2 data. Using the IDF Curves… button (for either basins or outlets), WMS can be used to develop curves from either of these two data sources, statistically derived data, or directly entering an i value if typically computed in another way or the design value to use is already known.

The IDF Computation dialog (shown below) can be used to create a series of T-year IDF curves from HYDRO-35, NOAA Atlas 2, or user defined data.

Rational Method—IDF Computation dialog

The type of data that will be used to create the IDF curves is specified with the radio group options in the upper left portion of the dialog. The Define Data button can then be used to bring a dialog which allows entering the following values, depending on the data type specified:

  • HYDRO-35 – The 2-yr 5, 15, and 60 minute rainfall values, and the 100-yr 5, 15, and 60 minute rainfall values. The HYDRO-35 maps for determining the six required rainfall values have been included below.
  • NOAA Atlas 2 – The 2-yr 6 and 24 hour rainfall values, the 100-yr 6 and 24 hr rainfall values and the mean basin elevation. When entering rainfall values they are entered as depth values and not intensity values.
  • User Defined – The recurrence interval, and the 5, 10, 15, 30 and 60 minute rainfall values. With this option only a single curve for the recurrence interval will be generated.

The methods WMS uses to compute the IDF curve equations are discussed in Appendix A of FHWA HEC-12 and in FHWA HDS-2.

Once the specified data has been entered the corresponding IDF-curves will be generated and plotted in the graphics window of the IDF Computation dialog. Then specify the recurrence interval to use for analysis by selecting it in the text list window in the upper right portion of the dialog. After selecting the recurrence interval, the appropriate curve in the plot window will be displayed in red. Finally, an intensity value, i, is determined by specifying a time of concentration as outlined below and then clicking on the Intensity button. When clicking on the Done button for the IDF Computation dialog, the computed intensity value will automatically be updated in the edit field for i of the Rational Method dialog.

Computing the Time of Concentration

A time of concentration value needs to be entered in order to determine the intensity value to be used in the Rational Method equation. This value can either be entered manually or computed from the overland flow length, Manning’s n, and slope. One equation used to compute the time of concentration from basin geometric parameters is the kinematic wave equation:

where:
  • tc = The time of concentration.
  • L = Overland flow length.
  • n = Manning’s roughness coefficient.
  • i = Rainfall intensity.
  • S = Average slope of the overland area.
  • K = 0.93

This method has been adopted by the FHWA for general use, but other equations can be used as defined elsewhere

HYDRO-35 Maps

  • 2 years 5 minutes

    2 years 5 minutes

  • 2 years 15 minutes

    2 years 15 minutes

  • 2 years 60 minutes

    2 years 60 minutes

  • 100 years 5 minutes

    100 years 5 minutes

  • 100 years 15 minutes

    100 years 15 minutes

  • 100 years 60 minutes

    100 years 60 minutes

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