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IM is an important parameter in the fort.15 file that defines numerical model formulation and dimension. Among other things, IM specifies whether ADCIRC is 2DDI or 3D, solution of the governing equations is implicit or explicit in space, and whether the model formulation is barotropic or baroclinic.

Shortcut IM Values

The most common combination of options used for simulation can be specified through the shortcut values

Six-digit IM Codes

For fine-grained control of various options in simulations six-digit codes for IM can be specified. Each digit represents a specific option regarding the formulation of certain terms or integration methods in the GWCE or momentum equations. The available options for each digit are specified below:

Value Digit 1: 2DDI/3D, Lateral Stress in GWCE[1] Digit 2: Advection in GWCE Digit 3: Lateral Stress in Momentum Digit 4: Advection in Momentum Digit 5: Area Integration in Momentum Digit 6: GWCE mass matrix
1 2DDI Kolar-Gray flux-based 2-part flux-based 2-part velocity-based 2-part flux-based symmetrical 2-part velocity-based symmetrical
2 2DDI 2-part flux-based row 2, cell 2 row 2, cell 3 row 2, cell 4 row 2, cell 5
3 2DDI 2-part velocity-based row 2, cell 2 row 2, cell 3 row 2, cell 4 row 2, cell 5
4 2DDI 2-part flux-based symmetrical row 2, cell 2 row 2, cell 3 row 2, cell 4 row 2, cell 5
5 2DDI 2-part velocity-based symmetrical row 2, cell 2 row 2, cell 3 row 2, cell 4 row 2, cell 5
6 3D Kolar-Gray flux-based row 2, cell 2 row 2, cell 3 row 2, cell 4 row 2, cell 5

A common code combination is IM = 111112, which uses default options (same as IM = 0), but simulates in explicit mass-lumping mode. This is a useful alternative to the (default) semi-implicit consistent GWCE mass matrix mode, which requires a matrix solve increasing computational time and memory compared to the explicit mass-lumping mode, which as about twice as fast and scales to fewer grid nodes per computational core.[2]


  1. K.M. Dresback, R.L. Kolar, R.A. Luettich, Jr., On the Form of the Momentum Equation and Lateral Stress Closure Law in Shallow Water Modeling, in: Estuar. Coast. Model., American Society of Civil Engineers, Reston, VA, 2005: pp. 399–418. doi:10.1061/40876(209)23.
  2. S. Tanaka, S. Bunya, J.J. Westerink, C. Dawson, R.A. Luettich, Scalability of an Unstructured Grid Continuous Galerkin Based Hurricane Storm Surge Model, J. Sci. Comput. 46 (2011) 329–358. doi:10.1007/s10915-010-9402-1