IDF Data

IDF (Intensity-Duration-Frequency) data drives the rainfall intensity calculations used by the Rational Method. HydraStorm supports multiple data sources and fits all IDF data to a standard three-parameter equation for interpolation.

IDF Equation

Where:

• i = rainfall intensity (in/hr)
• Tc = time of concentration (minutes)
• E, B, D = fitted coefficients specific to the project location and return period

Data Sources

1. NOAA Atlas 14 Fetch

The fastest way to get IDF data for any location in the United States.

1. Enter latitude and longitude for your project site, or use Pick Point to click a location on the Civil 3D drawing.
2. Click Fetch NOAA Atlas 14.
3. HydraStorm downloads precipitation frequency data from the NOAA servers.
4. The curve fitting algorithm automatically fits E, B, D coefficients.
5. An R-squared value is displayed to indicate fit quality.

2. CSV Import

If you have already downloaded IDF data tables from the NOAA Atlas 14 website:

1. Click the Import CSV button.
2. Browse to the downloaded NOAA CSV file.
3. HydraStorm parses the NOAA CSV format and fits E, B, D coefficients automatically.

3. HydraLink Import

When using HydraLink mode, IDF coefficients can be imported directly from the HydraLink project file. HydraLink supports both:

• Pre-fitted coefficients — E, B, D values already computed by HydraLink.
• Raw Atlas 14 tables — intensity-duration data that HydraStorm fits using the same algorithm.

4. Manual Entry

Enter E, B, and D coefficients directly if you have them from published IDF tables or local jurisdiction standards. This is common when a municipality provides their own IDF parameters in a drainage criteria manual.

Interpolation Method

All IDF data lives in the Project Settings > Hydrology > IDF Curves sub-tab: a duration/intensity table (editable), the interpolation method radio buttons below, E/B/D coefficients with a Generate from Table button, and a log-log IDF chart. Choose how HydraStorm looks up intensity for a given Tc:

• Log-Log — interpolate the intensity table using log-log interpolation between points. The standard choice for Atlas 14 table data, and the default for any project with table data.
• Linear — interpolate the intensity table linearly between points.
• E/B/D Coefficients — use the fitted equation i = E / (Tc + B)D directly for any duration, instead of interpolating the table.

In the two table-based modes (Log-Log, Linear), HydraStorm always reads from the intensity table — even if E/B/D coefficients also exist alongside it. In Coefficients mode, the fitted equation is preferred and the table is used only as a fallback if no coefficients are present. Projects migrated from an older file version that only had coefficients are automatically upgraded to Coefficients mode.

Curve Fitting Details

HydraStorm replicates HydraLink's IDF curve fitting algorithm to ensure consistent results between the two applications:

• 3-pass grid search on the B parameter, progressively narrowing the search range with each pass for precision.
• Linearized OLS regression in log-space for the E and D parameters at each candidate B value.
• SSE evaluation in original (non-log) space to ensure the fit minimizes error in meaningful intensity units (in/hr), not in log-transformed space.
• 2-hour duration cap — only durations up to 120 minutes are used for fitting, since the rational method is applicable to short-duration storms. Longer durations would skew the fit away from the critical range.

Design vs Check Storm

When dual storm analysis is enabled, separate E, B, D coefficients are fitted (or entered) for each return period. For example:

Storm	Return Period	E	B	D
Design	10-year	96.3	13.8	0.874
Check	100-year	142.7	14.2	0.851

Each storm event uses its own IDF curve for intensity interpolation.

Verifying the Fit

After fitting or entering IDF coefficients, verify the results by checking:

• R-squared — values above 0.99 indicate an excellent fit. Values below 0.95 may warrant investigation.
• Spot-check intensities — compare the fitted curve output at common durations (5, 10, 15, 30, 60 min) against published values for your location.
• Reasonableness — intensity should decrease as duration increases. Typical 10-year, 5-minute intensities in the U.S. range from about 4 to 10 in/hr depending on location.