Storm Events

HydraStorm supports multiple storm events per project with independent return periods for each design module — storm drains, inlets, and culverts. Each module can have its own design storm and check storm, and each storm event can carry its own frequency adjustment factor (Cf).

Why Multiple Storms?

Many jurisdictions require analysis at two or more return periods, and different design elements may use different storms. Common requirements include:

• Storm drains — Size pipes for the 10-year storm; verify HGL clearance for the 100-year storm.
• Inlets — Check spread and depth at the 10-year storm; inlet capacity may use a different return period than the pipes.
• Culverts — Analyze headwater at the 50-year or 100-year storm, independent of the storm drain design event.

Rather than maintaining separate parallel calculations, HydraStorm runs all storms on the same network in a single analysis pass, guaranteeing consistency.

Storm Configuration

Configure storm events in Project Settings > Hydrology. The storm configuration has three levels:

1. Project Storm Events

Define the storms available across the project. Each storm event has:

• Return Period (years) — e.g., 2, 5, 10, 25, 50, 100
• Adjustment Factor (Cf) — A multiplier on the runoff coefficient C for the rational method. Per TxDOT and FHWA guidance, Cf accounts for the reduced accuracy of the rational method at higher return periods. Typical values: 1.0 for storms up to 10 years, 1.1 for 25-year, 1.2 for 50-year, 1.25 for 100-year.

2. Module Assignments

Assign storms to each module. Each module independently specifies which storms it uses:

Module	Design Storm	Check Storm	Notes
Storm Drain	Required	Optional	Drives pipe sizing via the Sizing Storm selector. Design and check HGL computed independently.
Inlets	Optional	Optional	If not specified, uses the storm drain design storm. Inlet captured flows can feed into the pipe network for either storm.
Culverts	Optional	Optional	If not specified, uses the storm drain design storm. Produces independent headwater results per storm.

3. Sizing Storm

The Sizing Storm selector controls which storm event the Auto Design engine uses when selecting pipe sizes:

Option	When to Use
Design Storm	Most common. Size pipes to convey the design storm at or below full capacity. The check storm is used only for HGL verification.
Check Storm	Some jurisdictions require sizing to the check storm. This produces larger pipes that can convey the higher flows.

How It Works

When multiple storms are defined, the calculation engine runs the full hydraulic pipeline for each storm event with completely independent results:

• Each storm uses its own IDF curve (different intensity at the same Tc for different return periods).
• Each storm computes its own flows via the rational method (or uses separately provided peak flows).
• Each storm produces its own HGL/EGL profile through the pipe network.
• Inlet analysis can run at a separate storm and feed captured flows back into the pipe network at either the design or check storm.
• Culvert analysis computes headwater independently for each assigned storm event.
• All storms share the same pipe geometry (sizes, slopes, inverts) — only flow magnitudes and resulting hydraulic grade lines differ.

Storm Frequency Adjustment (Cf)

The Cf factor is a multiplier on the runoff coefficient C per TxDOT and FHWA guidance. It accounts for the fact that the rational method can underestimate peak flows at higher return periods. Each storm event carries its own Cf:

Storm	Return Period	Cf	Effective C
Design	10-year	1.00	C
Check	100-year	1.25	1.25 × C

The resulting flow is: Q = Cf × C × I × A

Inlet Flow Coupling

When the inlet module is enabled, two settings control how captured inlet flows feed into the pipe network:

• Use Inlet Flows for Design Storm — The storm drain design HGL uses captured flows from the inlet design analysis instead of raw basin flows.
• Use Inlet Flows for Check Storm — The storm drain check HGL uses captured flows from the inlet check analysis.

This coupling allows the pipe network to carry only what the inlets actually capture, resulting in a more accurate (and often more economical) design.

Profiling

When creating HGL profiles in Civil 3D via the Profile button, you can choose to profile the design storm, the check storm, or both. Each storm produces its own named profile in the drawing:

• Alignment1-HGL-10yr — design storm HGL profile
• Alignment1-HGL-100yr — check storm HGL profile

Both profiles reference the same pipe alignment, making it easy to compare hydraulic grade lines in the Civil 3D profile view.

Results

The Results Table displays columns for the selected storm. The storm drain results grid shows design storm values by default, with check storm columns visible when a check storm is configured. Key columns include:

• Q — peak flow (cfs)
• V — pipe velocity (ft/s)
• HGL DS / HGL US — hydraulic grade line elevations
• HGL Cover — clearance from HGL to rim or critical elevation
• Compliance — pass/fail summary for design criteria

Culvert results and inlet results appear in their own tabs with per-storm columns.