Conventions and units

ANUGA uses SI units throughout and a small number of conventions that are worth knowing before you build a model.

Coordinate system

The mesh lives in a 2D Cartesian plane: x points east, y points north, both in metres. Models are usually built in a projected coordinate system (e.g. a UTM zone) so that distances are metric and locally undistorted — not in latitude/longitude.

To tie the mesh to a real-world CRS, attach a Geo_reference (UTM zone or EPSG code, plus a local origin xllcorner/yllcorner). ANUGA works internally in coordinates relative to that origin to keep the numbers small and preserve floating-point precision; the origin is added back when results are written or when you request absolute coordinates. See Coordinate Reference Systems for details.

Quantities and their units

Every model tracks these quantities on the mesh:

Quantity	Units	Meaning
`elevation`	m	Bed elevation / bathymetry, measured from the vertical datum.
`stage`	m	Absolute water-surface elevation, from the same datum as elevation.
`xmomentum`	m²/s	Depth-integrated momentum in the x-direction (`depth × x-velocity`).
`ymomentum`	m²/s	Depth-integrated momentum in the y-direction (`depth × y-velocity`).
`friction`	–	Manning roughness coefficient n (dimensionless).

Depth is derived, not stored: depth = stage - elevation. A cell is dry where stage equals elevation. Because stage is measured from the datum, it is not the water depth — a common source of confusion when setting initial conditions.

Velocity is recovered from momentum as velocity = momentum / depth; in very shallow water this is regularised by velocity_protection (see below) to avoid dividing by a vanishing depth.

Wetting and drying

ANUGA models moving wet/dry fronts. A cell is treated as dry once its depth falls below minimum_allowed_height (default 1.0e-5 m); such cells carry no momentum. Only depths above minimum_storable_height (default 1.0e-3 m) are written to the SWW output, so shallow films do not clutter results.

Physical defaults and constants

Defined in anuga.config and used unless you override them:

Constant	Default	Meaning
`g`	9.8 m/s²	Gravitational acceleration.
`manning`	0.03	Default Manning friction if `friction` is not set.
`minimum_allowed_height`	1.0e-5 m	Depth below which a cell is considered dry.
`velocity_protection`	1.0e-6	Regularises velocity = momentum / depth in shallow water.

Time

Simulation time is in seconds, measured from the Unix epoch — 00:00:00 UTC on 1 January 1970 — so the default start of t = 0 corresponds to that instant. Set a real-world start with set_starttime(), which accepts either a number of seconds or a Python datetime object; use the standard library datetime and zoneinfo modules to build a timezone-aware start time (and set_timezone() to control how times are reported).

finaltime / duration in evolve() are likewise in seconds. The internal timestep is chosen automatically to satisfy the CFL stability condition and is generally much smaller than the yieldstep — see Setting up the Evolve loop.