Rainfall is the source of the ground runoff. Along with the watershed conditions, the rainfall that neither infiltrates nor gets trapped in low areas and depressions contributes to the direct surface runoff, upon which the storm drainage system design is based.
The Rainfall Library use IDF curves to provide average rainfall
intensity data for particular storm events. With a known rainfall
duration and frequency, an intensity is calculated via the IDF curve
and applied to the Rational Method to obtain the peak flow for
designing the sewer network. There are five methods to input rainfall
data: TP-40 rainfall map, Hydro-35 rainfall map, rainfall accumulation,
rainfall intensity and IDF equation coefficients. Rainfall maps
provided by
government
organizations are practical rainfall data resources for engineering
design. TP-40 maps show precipitation depths in the US for storm
durations from 1 hour to 24 hours and for recurrence intervals from 1
to 100 year. Hydro-35 maps are for the central and eastern US, and
provide rainfall data for durations as short as 5 minutes. Please refer
to HEC-12 for details on methodology for computing IDF curves from
rainmaps. In addition to TP-40 and Hydro-35 method, you can input
rainfall accumulations or intensities at various storm durations
to define IDF curves. If you already have the IDF equation coefficients
calculated, you can enter the coefficients directly to define the IDF
curves. The IDF curves are interpolated linearly between
the data points.
The IDF equation for a given return period is defined as follows.
The coefficients A, B and M are calculated by log-log regression of the
rainfall intensity and (t + B).
I = A / (t + B)M
where: I = rainfall intensity (in/hr, or mm/hr in metric)
A, B, M =
equation coefficients for a given return period
t = rainfall
duration (min.)
The Rainfall Library stores rainfall data in a library file under the ...\USER folder and is available for all projects. From the Network > Sewer Network Libraries menu in the Hydrology Module, select the Rainfall Library to open the library dialog to edit rainfall data. The dialog lists all rainfall entries by their ID and their input/edit method. New button creates a new rainfall through one of the five methods: TP-40, Hydra-35, Rainfall Accumulation, Rainfall Intensity and IDF Equation Coefficients. Edit button allows you to modify an existing rainfall, and the Delete button removes the highlighted rainfall from the library. Load and SaveAs buttons allow you to load and save the rainfall data.
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| Rainfall Library Dialog |
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| New Rainfall Dialog |
The TP-40 method is used to define IDF curves for the Western states in the US. It requires rainfall accumulations of 6-hour and 24-hour storm durations for the 2-year and 100-year storms, and the elevation of the location. In the TP-40 dialog, type the rainfall name in the Rainfall ID box. The rainfall depth can be entered either manually or from the TP-40 maps. Click on Map button to open the rainfall map, pick a state on the map to zoom in to the state map, and then pick a location to get the rainfall depth of 6-hour and 24-hour duration for 2-year, 5-year, 10-year, 25-year, 50-year and 100-year storm events. In the Elevation box, enter the surface elevation at the design location. Computation button computes the rainfall intensities and the IDF coefficients, and displays the result in the Rainfall Intensity dialog. Click on OK button to commit the rainfall entry.
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| TP-40 Rainfall Data Dialog |
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| TP-40 Rainfall Map |
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| Rainfall Intensity Results |
The Hydro-35 method is used to define IDF curves for the Central and Eastern states in the US. It requires rainfall accumulations of 5-min, 15-min and 60-min storm durations for the 2-year and 100-year storms. In the Hydro-35 dialog, type the rainfall name in the Rainfall ID box. The rainfall depth can be entered either manually or from the Hydro-35 maps. Click on Map button to open the rainfall map, pick a state on the map to zoom in to the state map, and then pick a location to get the rainfall depth. Computation button computes the rainfall intensities and the IDF coefficients at 2-year, 5-year, 10-year, 25-year, 50-year and 100-year return periods, and displays the result in the Rainfall Intensity dialog. Click on OK button to commit the rainfall entry.
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| Hydro-35 Rainfall Data |
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| Hydro-35 Rainfall Map |
This method allows you to enter the rainfall accumulations of various durations for 2-year, 5-year, 10-year, 25-year, 50-year and 100-year return periods for computing the IDF curves. Add a Duration button adds new duration entry to the spreadsheet. For the accuracy, three or more durations are required. Delete a Duration button deletes the highlighted duration entry. Computation button computes the rainfall intensities and the IDF coefficients, and displays the result in the Rainfall Intensity dialog. Click on OK button to commit the rainfall entry.
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| Customed Rainfall Accumulations |
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| New Duration Entry |
This method allows you to enter the rainfall intensities of various durations for 2-year, 5-year, 10-year, 25-year, 50-year and 100-year return periods for computing the IDF curves. Add a Duration button adds new duration entry to the spreadsheet. For the accuracy, three or more durations are required. Delete a Duration button deletes the highlighted duration entry. Computation button computes the rainfall intensities and the IDF coefficients, and displays the result in the Rainfall Intensity dialog. Click on OK button to commit the rainfall entry.
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| Customed Rainfall Intensities |
When you have the coefficients calculated, you can use this method to enter the coefficients to obtain the actual IDF curve equation. In the following spreadsheet dialog, enter the known coefficients A, B and M to create the IDF equation. Computation button computes the rainfall intensities and displays the result in the Rainfall Intensity dialog. Click on OK button to commit the rainfall entry.
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| IDF Equation Coefficients |
Rainfall Library dialog: Fill in values.
Pulldown Menu Location: Network
> Sewer Network Libraries > Rainfall Library
Keyboard Command: rainlib
Prerequisite: None
File Name: \lsp\cntr_grd.arx, \lsp\pond.arx