Open Channel Conduit Link
The Open Channel Conduit (also called a Gravity Conduit) models a length of channel or culvert where water flows by gravity rather than under pressure -- a drainage ditch, a natural or lined canal, a culvert crossing, or a stormwater channel -- using Manning's equation instead of the pressurized pipe-flow equations used elsewhere in the model.
Unlike a Pipe, an Open Channel Conduit is never assumed to run full and pressurized under normal conditions. Its cross-section can be open to atmosphere, so the model tracks the actual flow depth along its length and reports whether the channel is running subcritical, supercritical, or (if the channel is undersized for the flow) surcharged full like a pipe.
Channel Shape
Every Open Channel Conduit is assigned one of two cross-section shapes:
- Circular -- a round culvert or pipe running partially full. Defined by a single Diameter.
- Trapezoidal -- an open channel or ditch with a flat bottom and sloped sides. Defined by Bottom Width, Side Slope (H:V), and Max Depth -- the depth of the channel banks. Max Depth caps how deep the channel can hydraulically fill and is the threshold used for the depth-exceeded warning described below.
Switching the Channel Shape dropdown in the link dialog swaps in the matching set of geometry fields; the other shape's fields are hidden, not deleted, so you can flip back and forth while sizing a channel.
Flow Regime & Hydraulic Behavior
At each simulation step, the conduit solves Manning's equation for the flow, depth, and velocity that satisfy the head available between its two ends, then classifies the result:
- Subcritical -- calm, downstream-controlled flow (Froude number < 1). Typical of mild-sloped ditches and canals.
- Supercritical -- fast, upstream-controlled flow (Froude number > 1). Typical of steep chutes and spillway channels.
- Full -- the channel is undersized for the flow and runs completely full under pressure, the same way a pipe would.
- Drowned outlet -- flagged when the downstream water surface rises above the conduit's upstream invert, backing water up into the channel rather than letting it discharge freely.
For longer conduits the solver also marches a gradually-varied-flow profile along the channel in sub-reaches (spaced by the conduit's max reach length, 100 ft by default) so backwater and drawdown curve shapes show correctly in the HGL/profile chart, even though only the start, middle, and end points are plotted.
[!NOTE] An Open Channel Conduit only solves correctly when both connected nodes can supply a water-surface elevation -- currently that's a Tank, or an Inflow/Outflow Node set to elevation-based mode. If either end is a demand-based Inflow/Outflow Node, Junction, or another unsupported node type, the conduit reports zero flow rather than guessing.
UI Workflow and Configuration
To add an Open Channel Conduit to your model:
- Use the link-drawing tool the same way you would for a Pipe, dragging from the upstream node to the downstream node.
- In the link dialog that appears, set Link Type to Open Channel Conduit.
- Choose the Channel Shape (Circular or Trapezoidal) and fill in its geometry fields.
- Set Manning's n for the channel's lining/material (a rougher, unlined channel needs a higher n than a smooth-lined or paved one).
- Set the Invert Elevation Up and Invert Elevation Down -- the channel-bottom elevation at each end, which together with the length determines the channel's slope.
- Confirm the connecting nodes and save.
Double-clicking an existing Open Channel Conduit reopens the same dialog, pre-filled with its current shape, geometry, and inverts, for editing.
Configuration Fields
| Field | Description |
|---|---|
| Length | Channel length between the two connected nodes. |
| Channel Shape | Circular or Trapezoidal. |
| Diameter (Circular) | Inside diameter of a circular culvert/pipe. |
| Bottom Width (Trapezoidal) | Flat bottom width of the channel. |
| Side Slope, H:V (Trapezoidal) | Horizontal run per unit of vertical rise on each bank. |
| Max Depth (Trapezoidal) | Depth of the channel banks; also the depth-exceeded warning threshold. |
| Manning's n | Roughness coefficient for the channel lining/material. |
| Invert Elev. Up / Down | Channel-bottom elevation at the upstream and downstream ends. |
Depth & Capacity Warnings
If the simulated flow depth exceeds the channel's capacity -- the diameter for a Circular channel, or the configured Max Depth for a Trapezoidal channel -- the conduit raises a "Max channel depth exceeded" warning, the same way a Pipe raises a velocity warning, so undersized channels are flagged during a run rather than silently surcharging.
Solver Mapping & Water Quality
Open Channel Conduits are not exported to or solved by EPANET -- there is no equivalent EPANET link type for open-channel/gravity flow, so this element is solved entirely by R-THYM's own Manning's-equation and gradually-varied-flow engine, independent of the EPANET pressurized-pipe network solve. Water quality is transported along the channel as a chain of well-mixed sub-reach cells, with EPANET-style first-order bulk and wall decay kinetics applied so results stay consistent with the rest of the model even though the hydraulics are solved separately.