Terms as this manual and the software use them. Where the engine defines a term, the definition is the engine's.
Alternating block — A design-storm method that places the largest incremental depth from an IDF curve at the storm's centre and the rest alternately either side, so the storm reproduces the curve's average intensity over every duration (§15.7).
Autosave — A snapshot of a dirty model's text
written every few minutes to <model>.inp.autosave,
offered for recovery on the next open, removed on save (§1.6).
Backdrop — SWMM's own background image, placed in
model coordinates by [BACKDROP] DIMENSIONS, usually from a
world file (§3.9). Distinct from an
underlay.
Continuity error — The engine's mass-balance mismatch for a run, reported per section as a percentage of inflow (§16.1).
Crown — The top of a conduit's cross-section: invert plus rise.
Design basis — The IDF curve, return period, minimum Tc, junction K, minimum slope and tailwater the storm-sewer design panel needs and a SWMM model does not hold (§12.1).
Dirty — A model with changes since it was last
saved. The toolbar shows ● Unsaved.
Engine — EPA's runswmm executable, run
as a child process, identified by its reported version and the SHA-256
of the file (chapter 9).
Flooding — Water leaving a node whose water level
reached the rim (plus SurDepth). Lost from the system
unless ponding is on (§16.4).
Freeboard — Rim elevation minus HGL at a structure. Negative freeboard is surcharge to the surface.
Gesture — One map action from press to release; however many pointer events it takes, it is one undo step.
HGL — Hydraulic grade line. In the profile, a node's invert plus its reported depth at an instant; the Max HGL envelope is invert plus the run's maximum depth (chapter 11). In the design panel, the standard-step water surface (§15.4).
Invert — The bottom elevation of a node (its
Elevation) or of a conduit end (node invert plus
offset).
Lossless — The document property that parsing a file and writing it back yields identical bytes, and that an edit changes only the rows it touched (§1.6).
Offset (depth vs elevation) — A link end's height
above its node's invert (LINK_OFFSETS DEPTH) or its
absolute elevation (ELEVATION). The keyword decides which;
the numbers do not change with it (§16.6).
Ponding — With ALLOW_PONDING YES and an
Aponded area, flooded water stays on the node in a pond of
that area and drains back; the HGL can rise above the rim (§16.4).
Reporting period — One record in the
.out, every REPORT_STEP. The Time slider steps
through them.
Rim — A node's ground elevation: invert plus
MaxDepth, or the highest connecting crown when
MaxDepth is 0.
Run Status — The window that opens after a run with engine, hash, continuity, diagnostics, warnings and errors (§9.4).
Scratch copy — The copy of the model text the engine
reads when the model is dirty, unsaved, or on a non-ASCII path, under
%TEMP%\StormSewer\run\<hash>\ (§9.2).
Section — A [HEADER] block of the
.inp and its rows (chapter
17).
Subcatchment width — The width of the overland flow path; area divided by the longest overland flow length (§16.8).
Surcharge — A node's water level above the crown of its highest connecting conduit: the pipe is under pressure (§16.4). In the design panel, a design flow above a pipe's open-channel capacity.
Tc (time of concentration) — In the design panel, the larger of a pipe's inlet time and the upstream Tc plus travel time, floored at the minimum Tc (§15.3). SWMM has no Tc; its runoff timing comes from the nonlinear reservoir and the width.
Underlay — The storm-sewer workspace's PNG or DXF
drawing aid, not stored in the .inp (§7.3).
Validation findings — The live referential checks over the document, errors (the engine would refuse or misread the model) and warnings (§16.17).
World file — The six-line ESRI sidecar
(.pgw, .jgw, .wld) that places an
image in map coordinates (§17.7).