Culvert Analysis
HydraStorm includes a culvert analysis module based on FHWA HDS-5 (Hydraulic Design of Highway Culverts) methodology. Culvert alignments are analyzed separately from the storm drain network using inlet and outlet control headwater equations. This allows you to evaluate culvert crossings within the same Civil 3D pipe network model used for your storm drain design.
Alignment Design Types
Each alignment in your pipe network is classified as one of three design types:
| Design Type | Description |
|---|---|
| Storm Drain | Standard HGL/EGL propagation with junction losses, inlet capacity, and travel time accumulation. This is the default for all alignments. |
| Culvert | FHWA HDS-5 headwater analysis. Inlet control and outlet control headwater depths are computed independently, and the controlling condition (higher headwater) governs. Culvert alignments do not contribute to rational method travel time and do not participate in inlet analysis. |
| None | Excluded from all calculations. Use this for alignments you want to ignore entirely. |
Set alignment design types in Project Settings or the Alignment Types editor.
Note: Changing an alignment from Storm Drain to Culvert removes it from the HGL/EGL network analysis. Any junction losses, inlet calculations, or travel time contributions associated with that alignment are excluded once the type is changed.
How Culvert Analysis Works
For each pipe in a culvert alignment, HydraStorm performs three steps to determine the controlling headwater:
1. Inlet Control
Headwater depth is computed using the FHWA HDS-5 inlet control equations based on the entrance type and culvert geometry. This represents the headwater depth required to push flow into the barrel. In inlet control, the barrel has more than enough capacity to convey the flow — the entrance is the bottleneck.
2. Outlet Control
Headwater depth is computed based on friction losses through the barrel, exit losses, and the tailwater elevation at the outlet. This represents the headwater depth required to push flow through and out of the barrel. In outlet control, the barrel length, roughness, and downstream conditions limit the capacity.
3. Controlling Condition
The higher of the two headwater depths governs the design:
- Inlet controlled — HWic > HWoc. The barrel capacity exceeds the entrance capacity. Increasing the barrel size will not reduce headwater; improving the entrance geometry (e.g., adding beveled edges or a wingwall) is the effective solution.
- Outlet controlled — HWoc > HWic. Barrel friction and tailwater conditions limit the capacity. Increasing the barrel size, reducing the barrel length, or lowering tailwater will reduce headwater.
The headwater elevation must remain below allowable levels, typically the roadway overtopping elevation or other project-specific criteria.
Entrance Types
The entrance type determines the inlet control headwater equation coefficients per the FHWA HDS-5 tables. Select the entrance type that best matches the field condition for each culvert pipe.
Circular Culverts
| Entrance Type | Description |
|---|---|
| Square Edge with Headwall | Conventional concrete headwall with square-edged opening |
| Groove End with Headwall | Pipe with groove (bell) end set in a headwall |
| Groove End Projecting | Pipe with groove end projecting from the fill |
| Beveled Edge | Entrance with beveled edges to improve hydraulic efficiency |
| Thin Wall Projecting | Thin-wall pipe (CMP) projecting from the fill |
| Mitered to Slope | Pipe cut to match the embankment slope |
Box (Rectangular) Culverts
| Entrance Type | Description |
|---|---|
| Square Edge | Conventional headwall with square edges on all sides |
| Groove End | Precast box section with groove end |
| Beveled Edge | Entrance with 45-degree bevels |
| Wingwall Flare 30–75 deg | Wingwalls flared 30 to 75 degrees from the barrel |
| Wingwall Flare 90 or 15 deg | Wingwalls at 90 degrees (parallel) or 15 degrees |
| Wingwall Flare 0 deg | Wingwalls parallel to the barrel (no flare) |
CMP / Steel
| Entrance Type | Description |
|---|---|
| Concrete Mitered to Slope | Concrete pipe mitered to conform to the fill slope |
| CMP Headwall | Corrugated metal pipe set in a headwall |
| Beveled Edge 15:1 | CMP with manufactured beveled entrance ring |
Choosing an Entrance Type
- The entrance type significantly affects inlet control headwater. A beveled or groove-end entrance can reduce headwater by 0.5D or more compared to a square-edged entrance.
- When in doubt, use the most conservative (least efficient) entrance type that matches the proposed construction. You can always refine later once entrance details are finalized.
- Refer to FHWA HDS-5 Chapter 3 for photographs and detailed descriptions of each entrance condition.
Multi-Barrel Culverts
Set the number of barrels per culvert pipe to model side-by-side barrel configurations. The total capacity equals the single-barrel capacity multiplied by the barrel count. Each barrel is assumed to be identical in size and slope.
Adjust the barrel count in the culvert properties panel for the selected pipe. Common configurations include twin (2) and triple (3) barrel crossings.
Note: Multi-barrel culverts divide the total design flow equally among all barrels. The headwater computation uses the per-barrel flow (Q / N) for each barrel, then reports the resulting headwater for the combined system.
Tailwater Conditions
The tailwater elevation at the culvert outlet directly affects outlet control headwater. HydraStorm provides three tailwater options:
| Tailwater Option | Description | When to Use |
|---|---|---|
| Free Outfall | No tailwater influence. The outlet discharges freely with tailwater = 0. | Culvert discharges to an open channel with no downstream backwater effect (e.g., a steep channel or drop structure). |
| Specified Elevation | Enter a fixed tailwater elevation manually. | Tailwater is known from a separate hydraulic analysis, floodplain study, or design constraint (e.g., detention pond water surface). |
| Downstream Channel | HydraStorm computes the tailwater normal depth in a downstream channel using Manning's equation. | Culvert discharges to a channel where normal depth is a reasonable approximation of tailwater. |
Downstream Channel Parameters
When the Downstream Channel option is selected, specify the following channel properties:
- Channel Shape — Rectangular, Trapezoidal, or V-Shape
- Manning's n — Channel roughness coefficient
- Slope — Channel longitudinal slope (ft/ft)
- Bottom Width — Channel bottom width (ft); not applicable for V-Shape
- Side Slopes — Horizontal to vertical ratio (H:1V); applicable to Trapezoidal and V-Shape
Normal depth is computed iteratively from Manning's equation and used as the tailwater elevation at the culvert outlet invert.
Headwall Entrance Losses on Storm Drain Pipes
Some storm drain runs begin at an open channel, ditch, or basin rather than another pipe — a headwall structure with no pipe flowing into it. A generic junction loss coefficient doesn't represent the physics at this kind of structure, so HydraStorm can apply the same FHWA HDS-5 inlet/outlet control analysis used for culvert alignments to the storm drain pipe's own upstream-most node.
Assign an entrance type in the Headwall (HDS-5) column of the Structures Table. The dropdown is only enabled for a node with no incoming pipes (the top of a run), and the list of entrance types is filtered to match that pipe's shape (circular or box). Leave the column at — None — to keep the ordinary junction-loss calculation at that node.
When a headwall entrance type is assigned, HydraStorm computes the culvert headwater for the downstream pipe — using that pipe's downstream HGL as tailwater — instead of a junction loss, and reports the resulting headwater elevation as the structure's HGL. This is independent of the alignment's design type: it applies to a dead-end structure on a Storm Drain-type alignment, not to a separate Culvert alignment.
Note: The headwall entrance type is only selectable for structures with zero incoming pipes. It has no effect once a structure has an upstream pipe connected — that structure always uses the ordinary junction-loss calculation.
Culvert Storm Events
Culvert alignments can use independent storm return periods from the storm drain network. This is common in practice — for example, you might design storm drains for a 25-year event but analyze culverts at a 50-year or 100-year event, reflecting the higher consequence of culvert overtopping.
Enable separate culvert storms in Project Settings > Hydrology. Both a culvert design storm and a culvert check storm are supported, allowing you to evaluate culvert performance at two independent return periods.
Note: When separate culvert storms are not enabled, culvert alignments use the same design and check storm events as the storm drain network.
Results
Culvert results appear in a separate tab in the results area. The results table reports the controlling headwater condition for each culvert pipe along with the key hydraulic parameters.
Design Storm Columns
| Column | Description |
|---|---|
| Size | Culvert barrel dimensions (diameter for circular, span × rise for box) |
| Length | Barrel length (ft) |
| Slope % | Barrel slope as a percentage |
| N | Number of barrels |
| Q (cfs) | Design flow (total flow to the culvert) |
| HWic (ft) | Inlet control headwater depth above the inlet invert |
| HWoc (ft) | Outlet control headwater depth above the inlet invert |
| HW (ft) | Controlling headwater depth — the maximum of HWic and HWoc |
| HW Elev (ft) | Headwater elevation (inlet invert elevation + HW depth) |
| TW (ft) | Tailwater depth at the outlet |
| Control | Controlling condition: "Inlet" or "Outlet" |
| Vout (ft/s) | Outlet velocity — important for erosion protection design at the culvert discharge point |
Check Storm Columns
When the check storm is enabled, additional columns appear to the right of the design storm results:
| Column | Description |
|---|---|
| Chk Q | Check storm flow (cfs) |
| Chk HW | Check storm controlling headwater depth (ft) |
| Chk HW Elev | Check storm headwater elevation (ft) |
| Chk Ctrl | Check storm controlling condition |
| Chk Vout | Check storm outlet velocity (ft/s) |
Interpreting Culvert Results
- Compare the HW Elev to your allowable headwater elevation (roadway overtopping elevation). If HW Elev exceeds the allowable level, increase the culvert size, add barrels, or improve the entrance type.
- High outlet velocities may require energy dissipation or erosion protection at the downstream end. Check local jurisdiction requirements for maximum allowable outlet velocity.
- A culvert that is inlet controlled under the design storm may become outlet controlled under the check storm as tailwater rises. Always verify both conditions.