New Tool Release: Design Storm Hyetograph Generator
I am excited to announce the next major addition to the CivilSheets hydrology suite: the Design Storm Hyetograph Generator! ⛈️
If you recently used our IDF Curve tool, you know how to find the average rainfall intensity for a specific duration. But what happens when you need to model exactly when that rain falls over time to route a hydrograph through a detention pond or storm sewer network?
This new web-based worksheet takes your baseline rainfall data and distributes it into a high-resolution, time-series storm profile. With built-in support for standard SCS/NRCS 24-Hour distributions and the universally adaptable Alternating Block Method, this tool instantly generates the precise data needed for dynamic simulation software like EPA SWMM or HEC-HMS.
Why Do You Need a Hyetograph?
In civil engineering, determining the "Peak Flow" (Q = CiA) using the Rational Method is often enough to size a simple storm pipe. But if you are designing a detention pond, a dam spillway, or performing dynamic flood routing, knowing just the peak flow is not enough.
You need to know the entire volume of water and exactly how fast it arrives. A hyetograph provides the minute-by-minute (or hour-by-hour) timeline of a design storm. It tells the software (like HEC-HMS) exactly how to fill up your watershed over time so you can accurately observe the routing and draw-down effects of your basins.
Two Powerful Methods Included
The tool allows you to generate time-series data using the two most widely accepted hydrological methods in the industry:
1. The Alternating Block Method (Synthesized from IDF)
This is arguably the most precise method because it generates a custom storm directly from your local Intensity-Duration-Frequency curve.
- It calculates the depth of rain for various durations (15m, 30m, 60m).
- It isolates the "incremental" depth added during each time step.
- It sorts these blocks, placing the absolute highest intensity burst precisely in the middle of your storm, with the subsequent blocks alternating to the right and left to create a perfectly balanced bell-curve of rainfall.
2. Standard SCS / NRCS 24-Hour Distributions
If you are working in the United States, you are likely required to use standard synthetic distributions developed by the NRCS (formerly SCS). The tool includes the standard dimensionless mass curves for:
- Type I & IA: Gentle, long-duration storms (Pacific maritime climates).
- Type II: Represents the vast majority of the US interior. It features a highly intense, sudden peak exactly at the 12-hour mark.
- Type III: High-intensity coastal storms driven by Gulf or Atlantic tropical weather.
Simply select your curve type, input your total 24-hour rainfall depth (from NOAA Atlas 14), and the tool will slice it into the required time increments automatically.
Pro Tips for Modelers
When using the Alternating Block method, the total volume is dictated by your IDF equation. However, sometimes regulatory bodies require you to use a specific 24-hour total depth from a map, but want you to use local IDF shapes. I added an optional Target Total Depth input. If you fill this in, the tool will mathematically scale the entire block sequence so the shape remains perfect, but the final cumulative depth exactly matches your target!
Do not arbitrarily choose a 60-minute time step for a small parking lot design! The time step ($\Delta t$) should generally be less than or equal to one-third (1/3) of your watershed's Time of Concentration ($t_c$). If your $t_c$ is 15 minutes, select the 5-Minute time step so your hydrograph routing software can properly capture the peak runoff.
Export and Run!
The absolute best feature of this tool is the Export CSV Time-Series button. With one click, you get a cleanly formatted text file containing Time vs. Intensity vs. Cumulative Depth.
You can copy those two columns directly into the "Time Series" or "Gage Data" windows of HEC-HMS or EPA SWMM and start routing your watersheds immediately.
Head over to the tool page, test out the different distributions, and let me know if it speeds up your modeling workflow. If you have questions about deriving your IDF $a, b, c$ coefficients, there is a built-in guidance panel right on the page to help you out.
Happy Modeling!
- CivilSheets
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