Spillway Link

A Spillway models the fixed, uncontrolled overflow crest on a pond or reservoir -- the concrete lip, earthen emergency spillway, or dam crest that starts passing water the moment the pond rises above a set elevation, with no gate or moving parts involved.

A Spillway always starts at a Tank -- typically one shaped as a Pond (Sloped Sides) -- since it represents the crest sitting on that reservoir's rim. It discharges using the standard weir equation rather than pipe or Manning's-equation channel flow, and like an Open Channel Conduit it is not part of the pressurized (EPANET) network.

How It's Programmed

A Spillway's flow depends only on how far the pond's water surface has risen above the spillway's crest -- it has no gate setting, schedule, or downstream dependency the way a Gate does:

Q = C · L · H^1.5

where L is the crest length, H is the head (depth of water above the crest elevation, clamped to zero when the pond is below crest), and C is the discharge coefficient, which absorbs the crest's actual shape -- sharp-crested, broad-crested, ogee, riprap-armored, etc. A pond below the crest elevation contributes no flow at all; once it rises above the crest, discharge grows with the 3/2 power of that head, the standard weir relationship.

Because a Spillway is only ever driven by the upstream pond's own head, it is inherently one-directional and free-discharging -- it cannot back up, throttle, or reverse the way a Gate or Open Channel Conduit can. If you need a controlled structure instead (one that can be opened, closed, or scheduled), use an Open Channel Gate rather than a Spillway.

UI Workflow and Configuration

To add a Spillway to your model:

1. Use the link-drawing tool starting at the Pond/Tank whose crest you're modeling, dragging to the downstream node (another Tank, an open-channel Junction, or an Outflow Node).
2. In the link dialog that appears, set Link Type to Spillway.
3. Set the Crest Elev. -- the elevation the pond must rise above before any flow occurs. Defaults to the pond's own elevation.
4. Set the Crest Length -- the length of the spillway opening/weir.
5. Set the Discharge Coeff. -- defaults to 3.33 (a typical sharp-crested weir value in US units); use a lower value (around 2.6-3.0) for a broad-crested or riprap-armored crest.
6. Confirm the connecting nodes and save.

[!NOTE] A Spillway must originate at a Tank -- the model will reject any other starting node, since a spillway crest is only meaningful sitting on a reservoir's rim.

Configuration Fields