Design Storm

Open Design Storm Calculator

Generate synthetic rainfall hyetographs for flood modelling using industry-standard temporal storm patterns. Configure storm duration, time step, and total depth, then export to your preferred hydraulic modelling software. This guide covers the three categories of pattern (cumulative mass curves, IDF-dependent synthetic storms, and empirical regional patterns), the SCS Type I–III (US) and SA Types 1–4 distributions, Huff quartiles, the Chicago and alternating-block methods, and best practice for coupling a design storm to a rainfall-runoff model.

Overview

A design storm is a synthetic rainfall event used as input to hydrological and hydraulic models. Rather than relying on a single observed rainfall record, design storms represent a statistically derived rainfall pattern for a given return period and duration, allowing engineers to size infrastructure for a consistent level of risk.

Design storms define two critical characteristics: how much rain falls (total depth, linked to the return period) and when it falls within the storm (the temporal distribution or pattern). The temporal pattern determines the shape of the resulting hyetograph and has a significant influence on the peak discharge computed by rainfall-runoff models.

Why design storms matter

The choice of storm pattern can change peak flow estimates by 30% or more for the same total rainfall depth. Selecting an appropriate pattern for your region and catchment type is just as important as selecting the correct rainfall depth and return period.

The total rainfall depth used by the Design Storm Calculator is obtained from the Design Rainfall tool (or an equivalent IDF/DDF table). The resulting hyetograph is consumed by the DRH Method, Unit Hydrograph, and any external rainfall-runoff model such as EPA SWMM or HEC-HMS.

Getting started

The Design Storm Calculator walks you through a straightforward workflow:

1. Select a storm pattern from the pattern library (e.g. SCS Type II, Chicago Storm, Alternating Block). Patterns are grouped by region and method type.
2. Configure the storm by setting the total duration, computational time step, total rainfall depth, and return period.
3. Compute the hyetograph by clicking Calculate. The tool generates both incremental and cumulative rainfall distributions, along with peak-intensity statistics.
4. Export the results in your preferred format: CSV for general use, or native formats for EPA SWMM, HEC-HMS, InfoWorks ICM, XP-SWMM, and more.

Quick start

For a South African project, start with the SA SCS patterns (Types 1 through 4) as recommended by the SANRAL Drainage Manual. For US-based work, the SCS Type II is the most commonly applied pattern for inland areas.

Storm pattern types

The calculator supports three fundamental categories of storm pattern, each constructed differently.

Category	How it works	Examples
Cumulative distribution	A predefined cumulative mass curve is scaled to the total depth and duration. The distribution is fixed regardless of return period.	SCS Types I–III, SA SCS Types 1–4, Huff Quartiles
Intensity function (IDF-based)	Rainfall intensities are derived from an IDF curve (using Sherman or similar coefficients). The storm shape changes with return period.	Chicago Storm, Alternating Block, Nested
Empirical / custom	Based on statistical analysis of observed storms in a specific region. May combine elements of both approaches.	South African Huff, user-defined patterns

When to use each

Cumulative distribution patterns are simplest and most widely accepted for standard drainage design. IDF-based patterns are preferred when you have reliable IDF data and need the storm to reflect the local rainfall frequency relationship. Empirical patterns are used when regional studies have produced locally calibrated distributions.

Key patterns

The following are the most commonly used patterns available in the Design Storm Calculator.

SCS Types I, IA, II, and III (US)

Developed by the US Soil Conservation Service (now NRCS) for the TR-55 procedure. These 24-hour cumulative distributions represent different climatic regions of the United States:

• Type I: Pacific maritime climate with mild, wet winters (US West Coast).
• Type IA: Pacific Northwest with extended, lower-intensity wet seasons.
• Type II: Interior continental regions with intense, short-duration storms. The most commonly used type.
• Type III: Gulf Coast and Atlantic seaboard, reflecting tropical storm influence.

24-hour standard

SCS distributions are defined for a 24-hour duration. When a shorter duration is selected, the calculator extracts the most intense portion of the 24-hour curve centred on the peak.

SA SCS Types 1–4 (South Africa)

The South African adaptations of the SCS method, as specified in the SANRAL Drainage Manual. These patterns reflect South African rainfall characteristics and are the standard for local practice:

• Type 1: Interior plateau regions with convective thunderstorms (Highveld).
• Type 2: Coastal regions with moderate storm intensities.
• Type 3: Semi-arid inland regions.
• Type 4: Winter rainfall regions (Western Cape).

South African projects

For projects in South Africa, use the SA SCS pattern corresponding to your region as prescribed in the SANRAL Drainage Manual. These patterns have been calibrated against local rainfall data and are widely accepted by reviewing authorities.

Huff quartile distributions

Developed by Floyd Huff (1967) from analysis of observed storms in the US Midwest. Storms are classified by which quartile of the total duration contains the heaviest rainfall:

• 1st quartile: Peak rainfall in the first 25% of the storm. Typical of short, intense convective events.
• 2nd quartile: Peak in the 25–50% window. Common for moderate-duration storms.
• 3rd quartile: Peak in the 50–75% window.
• 4th quartile: Peak in the final 25%. Represents long-duration frontal events.

Chicago Storm (Keifer and Chu)

The Chicago method (Keifer and Chu, 1957) constructs a synthetic storm directly from the IDF curve. The resulting hyetograph has intensities that, for any duration centred on the peak, exactly match the IDF curve intensity for that duration. A parameter $r$ (the advance coefficient, typically 0.3 to 0.5) controls the position of the peak within the storm.

IDF required

The Chicago Storm requires IDF coefficients (Sherman equation: $i = a / (t + b)^c$) as input. These can be entered manually or linked from the Design Rainfall tool.

Alternating block method

The alternating block method derives incremental rainfall depths from the IDF curve for successive time intervals, then rearranges them in a symmetric pattern with the highest block at the centre and alternating blocks of decreasing intensity on either side. This produces a storm where any consecutive sequence of blocks yields a total depth consistent with the IDF curve.

South African Huff

A regionally calibrated version of the Huff distribution based on analysis of South African rainfall records. This pattern accounts for the specific storm characteristics observed across different rainfall regions of the country and is an alternative to the SA SCS patterns for practitioners who prefer the Huff approach.

IDF-dependent storms

Three patterns in the calculator derive their temporal distribution directly from an Intensity-Duration-Frequency (IDF) curve: the Chicago Storm, the Alternating Block method, and the Nested storm. Unlike cumulative distribution patterns, these storms change shape when you change the return period, because the underlying IDF relationship varies with frequency.

These methods require Sherman equation coefficients $(a, b, c)$ that define the IDF relationship:

$$i \;=\; \frac{a}{(t_d + b)^{c}}$$

Sherman equation — IDF curve

where $i$ is rainfall intensity, $t_d$ is duration, and $a$, $b$, $c$ are regionally derived coefficients for the selected return period.

Linking to Design Rainfall

If you have already computed IDF coefficients using the Design Rainfall tool, you can link them directly into the Design Storm Calculator. This avoids manual re-entry and ensures consistency between your rainfall analysis and storm generation.

Coefficient validity

Sherman coefficients are only valid within the duration range for which they were fitted. Generating a storm with a duration significantly outside this range may produce unrealistic intensity values. Always verify that your storm duration falls within the calibrated range of your IDF data.

Configuration parameters

After selecting a pattern, four key parameters must be set to define the design storm.

Parameter	Description	Guidance
Storm duration	Total length of the rainfall event.	Typically set equal to or greater than the Time of Concentration ($T_c$) of the catchment. A duration equal to $T_c$ produces the critical storm for peak flow.
Time step	Computational interval for the hyetograph.	Use 5 min for durations up to 2 hr, 10 min for 2–6 hr, 15–30 min for 6–24 hr. Finer steps capture peak intensity more accurately.
Total depth	Total rainfall depth for the event (mm or inches).	Obtained from IDF/DDF tables or the Design Rainfall tool for the chosen duration and return period.
Return period	Average recurrence interval of the design event.	Depends on the design standard and risk tolerance: 2–10 years for minor drainage, 25–100 years for major infrastructure.

Duration and Tc

Setting the storm duration equal to the Time of Concentration ensures that the entire catchment contributes to peak runoff simultaneously. For conservative designs, test durations both shorter and longer than $T_c$ to identify the critical case. See the Tc Calculator guide for $T_c$ estimation methods.

Interpreting results

The calculator produces several outputs that together describe the design storm:

• Incremental hyetograph: A bar chart showing the rainfall depth (or intensity) for each time step. This is the primary input for rainfall-runoff models. The tallest bar represents the peak-intensity interval.
• Cumulative mass curve: A line chart showing the running total of rainfall over time. This curve should be smooth and monotonically increasing. The steepest portion corresponds to the period of highest intensity.
• Peak intensity: The maximum rainfall intensity (mm/hr or in/hr) occurring in any single time step. This value is critical for inlet and pipe sizing.
• Centroid time: The time at which half the total rainfall has fallen. A centroid earlier than 50% of the duration indicates a front-loaded storm; later than 50% indicates a back-loaded storm. This affects the timing of the flood peak.

Reading the hyetograph

Toggle between depth (mm per time step) and intensity (mm/hr) views to suit your modelling needs. Depth is useful for direct model input, while intensity is better for comparison with IDF curves and design standards.

Export formats

The calculator exports the computed hyetograph in several formats to integrate with popular hydraulic modelling software.

Format	Extension	Use case
CSV	.csv	Universal format. Import into Excel, Python, R, or any modelling tool that accepts tabular data.
EPA SWMM	.dat	Native rainfall data file for EPA SWMM. Ready for direct import as a rainfall time series.
HEC-HMS	.gage	HEC-HMS precipitation gage format. Import as a gage record in the Time-Series Data Manager.
InfoWorks ICM	.csv	Formatted for direct import into the InfoWorks ICM rainfall event editor.
XP-SWMM	.csv	Compatible with the XP-SWMM global rainfall input format.
Full SWMM .inp	.inp	Complete SWMM input template with the rainfall time series embedded. Useful for quick standalone SWMM runs or as a starting template.

Choosing the right format

If you are unsure which format to use, start with CSV as it can be opened in any spreadsheet application and manually imported into most modelling tools. Use the native formats (SWMM .dat, HEC-HMS .gage) when you want a seamless import workflow.

Multi-return-period analysis

The calculator supports computing multiple design storms for different return periods using the same pattern and configuration. This is useful for:

• Sensitivity analysis: Understanding how peak intensity and total depth scale with return period for a given pattern.
• Dual-drainage design: Computing the minor system storm (e.g. 1:10 year) and the major system storm (e.g. 1:100 year) in a single session.
• Risk assessment: Comparing storm severity across return periods to evaluate the consequences of exceeding the design event.

For cumulative distribution patterns (SCS, Huff), only the total depth changes between return periods while the temporal shape remains constant. For IDF-dependent patterns (Chicago, Alternating Block), both the total depth and the temporal distribution change with return period, producing distinctly different hyetograph shapes.

Batch export

When multiple return periods are computed, you can export all storms at once. Each return period is included as a separate series in the export file, making it easy to set up multiple scenario runs in your hydraulic model.

Best practices

Follow these guidelines to produce reliable design-storm inputs for your flood models:

• Match duration to $T_c$: Set the storm duration equal to or slightly greater than the catchment Time of Concentration. This ensures the critical storm condition is captured. Test durations around $T_c$ to confirm which produces the highest peak discharge.
• Use appropriate time steps: Smaller time steps capture peak intensity more accurately but produce larger datasets. A time step of $T_c/5$ to $T_c/10$ is a good starting point. Ensure the time step divides evenly into the total duration.
• Verify total depth against local standards: Cross-check the rainfall depth used in the design storm with published DDF/IDF tables for your region. The storm pattern only distributes this depth in time; an incorrect total depth will invalidate all results regardless of the pattern selected.
• Use regionally appropriate patterns: Apply the storm pattern prescribed by the relevant design standard for your jurisdiction (e.g. SA SCS for South Africa, SCS Type II for US inland areas). Using a pattern from another region may misrepresent local storm characteristics.
• Document your assumptions: Record which pattern, duration, time step, and total depth you used. This information is essential for design review and future reference.

Common pitfall

Do not assume that a longer storm duration always produces a higher peak discharge. A longer duration means the same total depth is distributed over more time, potentially reducing peak intensity. The critical duration depends on the catchment response time and the shape of the storm pattern.

References

• NRCS (1986). Urban Hydrology for Small Watersheds, TR-55. United States Department of Agriculture, Natural Resources Conservation Service.
• SANRAL (2013). Drainage Manual, 6th Edition. South African National Roads Agency Limited, Pretoria.
• Chow, V.T., Maidment, D.R. & Mays, L.W. (1988). Applied Hydrology. McGraw-Hill, New York.
• Keifer, C.J. & Chu, H.H. (1957). Synthetic storm pattern for drainage design. Journal of the Hydraulics Division, ASCE, 83(4), 1–25.
• Huff, F.A. (1967). Time distribution of rainfall in heavy storms. Water Resources Research, 3(4), 1007–1019.
• Sherman, C.W. (1931). Frequency and intensity of excessive rainfalls at Boston, Massachusetts. Transactions, ASCE, 95, 951–960.
• Pilgrim, D.H. & Cordery, I. (1975). Rainfall temporal patterns for design floods. Journal of the Hydraulics Division, ASCE, 101(1), 81–95.

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Open Design Storm Calculator