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)
• b = numerator coefficient
• d = time shift (minutes)
• e = exponent (dimensionless)

The b, d, e coefficients are specific to the project location and return period. This is the same three-parameter form used by TxDOT and most published drainage criteria manuals, and the same one HydraLink uses.

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 the b, d, e 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 the b, d, e 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 — b, d, e values already computed by HydraLink.
• Raw Atlas 14 tables — intensity-duration data that HydraStorm fits using the same algorithm.

4. Manual Entry

Many drainage criteria manuals publish a table of intensity against duration rather than equation coefficients, and require you to use it. Type it in directly:

1. On the Hydrology tab, declare each return period the manual covers in the Storm Events table. Every storm you add gets its own intensity column.
2. On the IDF Curves sub-tab, fill in the Duration / Intensity Table. The standard durations are already there; Add duration covers any others the manual publishes (20, 25, 40, 45, 90 min …), up to 180 min. Leave a cell blank to omit that duration for that storm.
3. Pick the Interpolation Method the manual implies: Log-Log or Linear to read between the published rows, or b/d/e Coefficients plus Generate from Table to fit a smooth curve through them.

You can also enter the b, d, and e coefficients directly, without a table, when the manual publishes its own IDF parameters, one row per storm in the coefficient grid.

Interpolation Method

All IDF data lives in the Project Settings > Hydrology > IDF Curves sub-tab: an editable duration/intensity table with one column per storm, the interpolation method radio buttons, the per-storm equation 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.
• b/d/e Coefficients — use the fitted equation i = b / (Tc + d)e within its fitted duration window, instead of interpolating the table. Below the shortest fitted duration the intensity is held at the fit-window value (with a warning) rather than extrapolated; above the longest fitted duration the lookup hands off to the intensity table when one covers the storm.

In the two table-based modes (Log-Log, Linear), HydraStorm always reads from the intensity table, even if fitted coefficients also exist alongside it. In Coefficients mode, the fitted equation is the primary source inside its fitted duration window; the table serves the durations the equation shouldn't: storms with no coefficients, and durations beyond the fit window that the table covers.

Table clamps at both ends — and says so

A duration table has no data past its last row, and intensity must keep falling as Tc grows. Below the table's first duration, HydraStorm holds intensity at that first row's value (a standard short-duration clamp). Above the table's last duration it also holds flat — a longer Tc than the table covers gets the same intensity as the longest tabulated duration, never a lower one. A flat hold at the long end is not conservative: it over-states intensity for every Tc past the table, silently over-sizing everything downstream of a headwater basin whose Tc runs long. When this happens, HydraStorm raises one aggregate calculation warning naming the affected nodes. If a project routinely runs Tc past the table's longest duration, extend the table or switch to Coefficients mode, where the equation keeps decaying past the fit window instead of clamping.

About the Curve Fit

Two things about the fit are worth knowing:

• It matches HydraLink. HydraStorm fits b, d, and e the same way HydraLink does, so the same rainfall data produces the same coefficients in both programs, and your HydraStorm intensities will agree with the HydraLink runoff they came from.
• Only durations up to 2 hours are used. The rational method is a short-duration method, and times of concentration on a storm drain system rarely exceed 2 hours. Including longer durations would pull the fit away from the range you actually design in.

The R-squared value shown after fitting tells you how well the equation reproduces your data: values above about 0.99 are typical for Atlas 14 data. A noticeably lower value usually means the source table spans an unusually wide duration range or contains a bad row.

One Curve Per Storm

Each storm on the project's Storm Events list carries its own intensity column and its own b, d, e row, for design, check, and any additional analysis storms alike. Generate from Table fits every storm that has at least three durations, and names any it could not. For example:

Storm	Return Period	b	d	e	Fit (R²)
Design	10-year	96.3	13.8	0.874	0.996
Check	100-year	142.7	14.2	0.851	0.994

The grid's Fit (R²) column reads “—” for a row whose b/d/e were typed by hand rather than fitted — there's no residual to report against a table that was never used. 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.