Inlet Analysis

The inlet design module computes how much stormwater runoff is captured by each inlet and how much bypasses to downstream inlets. This lets you verify that your inlet sizing and spacing meet design standards, and it feeds realistic captured flows into the pipe network for more accurate HGL analysis.

To enable inlet analysis, go to Project Settings > Hydrology > Storm Events and check Enable inlet design in the Inlets section. Once enabled, the Inlet tab appears in the main window and the inlet storm event grid becomes available below the checkbox.

How It Works

HydraStorm processes inlets in topological order, starting at the most upstream inlets and working downstream. At each inlet the calculation follows these steps:

1. Accumulate gutter flow. The total gutter flow at the inlet is the sum of the local basin flow (from the assigned drainage area) and any bypass flow arriving from upstream inlets.
2. Compute spread and depth. Manning's equation for the gutter cross-section determines the flow spread width and curb depth for the total gutter flow.
3. Determine capture capacity. Based on the inlet type (grate, curb opening, combination, or slotted drain), the gutter geometry, and the flow conditions, HydraStorm computes how much flow the inlet intercepts using HEC-22 methods.
4. Route bypass. Flow that the inlet does not capture continues downstream to the designated bypass target inlet.
5. Feed the pipe network. Captured flow enters the storm drain system at the inlet's corresponding pipe network node.

After all inlets are processed, the captured flows are accumulated through the pipe network and used for the HGL/EGL analysis.

Inlet Types

HydraStorm supports four HEC-22 inlet categories. Each type uses different hydraulic equations to compute interception capacity.

Grate Inlets

Grate inlets intercept flow through a grated opening in the gutter. Capture depends on the grate dimensions (length and width), grate type, and the velocity of flow in the gutter. Flow is divided into frontal flow (passing directly over the grate) and side flow (alongside the grate). At higher velocities, water splashes over the grate and is not captured.

HydraStorm supports the standard HEC-22 grate types:

• P 1-7/8"
• P 1-7/8-4
• P 1-1/8
• Curved Vane
• 30° Tilt Bar
• 45° Tilt Bar (60mm transverse spacing)
• 45° Tilt Bar (85mm transverse spacing)
• Reticuline

HEC-22 does not publish an opening ratio for either 45° tilt-bar variant, and both are excluded from sag (ponded) service per the manual — use them on-grade only.

Each grate type has a characteristic splash-over velocity curve that determines what fraction of frontal flow is captured at a given gutter velocity.

Curb Opening Inlets

Curb opening inlets intercept flow through a vertical opening in the curb face. Interception depends on the opening length and height. Depressed curb openings — where the gutter is lowered at the inlet — capture significantly more flow because the depression directs water into the opening. HydraStorm accounts for the depression geometry (width and depth) when computing interception length and efficiency.

Combination Inlets

Combination inlets pair a grate with a curb opening. The grate captures first, and the curb opening provides additional interception for flow that passes alongside or splashes over the grate. A configurable credit fraction controls how much additional capture the curb opening contributes beyond the grate alone. This credit is typically set to account for the curb opening intercepting side flow that the grate misses.

Slotted Drain Inlets

Slotted drain inlets use a continuous narrow slot along the gutter line. Capture depends on the slot length and width. These are commonly used where a conventional grate or curb opening would interfere with traffic or pedestrian access.

Sag vs. On-Grade

Every inlet operates in one of two hydraulic modes, depending on its location in the roadway profile.

Mode	When It Applies	How Capacity Is Computed
On-Grade	Inlet is on a longitudinal slope where gutter flow moves past the inlet.	Interception depends on gutter flow velocity, spread width, and inlet dimensions. At higher velocities, splash-over reduces grate capture. Side flow interception decreases with velocity.
Sag (Ponding)	Inlet is at a low point where water ponds against the inlet.	Capacity is the minimum of weir flow (at shallow depths, flow spills over the inlet perimeter) and orifice flow (at greater depths, flow passes through the opening area). Sag inlets are typically more efficient because all approaching flow reaches the inlet rather than flowing past.

Toggle sag mode per inlet using the Sag Inlet checkbox in the inlet properties. When designing a sag inlet, pay close attention to the allowable ponding depth — if the depth exceeds design limits, you may need a larger inlet or multiple inlets at the sag point.

Gutter Cross-Sections

The gutter cross-section defines the shape of the roadway drainage channel. HydraStorm uses this geometry to compute flow spread, depth, and velocity using Manning's equation. Each inlet has its own gutter geometry settings.

Uniform

A constant cross-slope across the full roadway width from the crown to the curb. This is the simplest case, commonly found on residential streets. A single cross-slope value and gutter width define the section.

Composite

The gutter strip adjacent to the curb has a steeper cross-slope than the travel lane. This is the most common configuration on collector and arterial roads, where the steeper gutter slope concentrates flow near the curb and reduces spread into travel lanes. You specify both the gutter cross-slope and the roadway cross-slope, along with the gutter width.

V-Shaped

Left and right slopes meet at a V-shaped channel. Used in median ditches, some parking lot configurations, and roadside swales. You specify the left slope, right slope, and channel width.

Depressed

A depressed gutter section that creates a deeper channel near the curb to concentrate flow. The depression width and depth are specified in addition to the roadway cross-slope. This type directs more flow to the inlet and is often paired with curb opening or combination inlets.

Each inlet's gutter geometry also includes:

• Longitudinal Slope — The slope along the direction of flow in the gutter (ft/ft).
• Manning's n — Roughness coefficient for the gutter surface (typically 0.013 to 0.016 for concrete gutters).
• Cross-slope(s) — Transverse slope(s) defining the gutter and roadway section.

Roadway Templates

Roadway templates are pre-configured gutter geometries that you can apply to inlets to save time during setup. Instead of entering cross-slopes, gutter widths, and Manning's n values for each inlet individually, select a template and the geometry is filled in automatically.

HydraStorm includes built-in templates for common roadway types:

• Local Street
• Collector
• Minor Arterial
• Major Arterial
• Parking Lot
• V-Ditch

You can also create custom templates with your municipality's standard roadway sections. Custom templates are saved with the project and available in the template dropdown.

Bypass Flow Routing

Flow that an inlet does not capture — the bypass — continues along the gutter to a downstream inlet. You set the bypass target for each inlet by selecting the downstream node from the bypass target dropdown.

• If a bypass target is set, the uncaptured flow is added to the downstream inlet's total gutter flow as incoming bypass.
• If no bypass target is set, the bypass flow is reported as unmatched in the results. Unmatched bypass indicates flow that leaves the system without being captured — a potential design issue.
• Bypass chains can span multiple inlets. Flow that bypasses Inlet A routes to Inlet B, and if Inlet B also cannot capture all of it, the remainder routes to Inlet C, and so on until the flow is fully captured or reaches the end of the chain.

Coupling with the Pipe Network

When inlet analysis is enabled, the flows entering the pipe network come from the inlet capture calculations rather than (or in addition to) the raw basin flows. This coupling produces a more realistic pipe design because the storm drain system only carries what the inlets actually capture, not the full basin runoff.

Two project settings control how inlet results feed into the pipe network:

Setting	Effect
Use Inlet Flows for Design Storm	Captured flows from the design storm inlet analysis are used as the input flows for the storm drain HGL calculation. The pipe network carries only the flow that inlets intercept during the design storm.
Use Inlet Flows for Check Storm	Captured flows from the check storm inlet analysis are used as the input flows for the storm drain check HGL calculation. This is typically a larger storm to verify the system does not surcharge under extreme conditions.

This coupling means that changes to inlet sizing, spacing, or gutter geometry directly affect the pipe network hydraulics. A more efficient inlet captures more flow, which increases pipe flows and may require larger pipes — but also means less bypass and better overall system performance.

Clogging Factors

Clogging factors reduce an inlet's effective capture area to account for debris accumulation over time. A clogging factor of 0 means no clogging (full capacity), while a factor of 0.5 means 50% of the inlet area is assumed blocked.

Clogging factors can be set at three levels. The highest-priority level that has a value wins:

1. Per-inlet override — Set in the Clogging field of the inlet property panel (Bypass & Overrides section) on the Inlet tab. Overrides all other levels. Use this when a specific inlet is in a location prone to heavy debris (e.g., near trees or construction areas).
2. Per-inlet-type in the inlet library — Set on the inlet type definition in the inlet library. Applies to all inlets of that type unless overridden at the inlet level.
3. Per-category default in the inlet library's design criteria — Each inlet library carries default clogging factors per inlet category (grate, curb opening, combination, slotted drain). Applies to all inlets of that category unless overridden at the type or inlet level.

Inlet Storm Events

Inlets can be analyzed using different storm return periods than the pipe network. This is common in practice — for example, you might size pipes for a 25-year storm but check that inlet spread meets roadway criteria for a 10-year storm.

Configure the inlet design and check storm return periods in Project Settings > Hydrology. The inlet analysis uses these return periods to look up the appropriate rainfall intensity from the project's IDF data, independent of the pipe network storm events.

When dual-storm analysis is enabled, the inlet module runs twice — once for the inlet design storm and once for the inlet check storm — producing separate results for each. The pipe network receives the captured flows from whichever storm is configured via the coupling settings described above.

Results

The inlet results table displays the following columns for each inlet in the system. Use these to verify that spread, depth, and capture efficiency meet your design criteria.

Column	Description
Spread (ft)	Gutter flow spread width — the distance water extends from the curb into the roadway. Check this against your municipality's maximum allowable spread.
Depth (ft)	Water depth at the curb face. For sag inlets, this is the ponding depth that governs weir/orifice capacity.
V_g (ft/s)	Gutter flow velocity. Higher velocities reduce grate capture efficiency due to splash-over.
Q_loc (cfs)	Local basin flow — the runoff contributed by the drainage area assigned directly to this inlet.
Q_bp (cfs)	Incoming bypass flow from upstream inlets that were unable to capture all of their gutter flow.
Q_tot (cfs)	Total gutter flow at the inlet (Q_loc + Q_bp). This is the flow the inlet must handle.
Q_cap (cfs)	Flow captured by the inlet and directed into the pipe network.
Q_out (cfs)	Bypass flow that continues downstream to the next inlet (Q_tot - Q_cap).
Eff %	Capture efficiency (Q_cap / Q_tot x 100). A 100% efficient inlet captures all approaching flow. Sag inlets typically approach 100% when properly sized.