Tuflow and estry manual Version 3

3.17.4Recommended BC Arrangements

Hydraulic models typically have water level boundaries at the downstream end and flow boundaries at the upstream ends.

For tidal models, the only boundaries may be ocean water level boundaries with pre-defined flows into or out of the system. For flood models, there are major flows into the upstream boundaries representing the catchment runoff. Occasionally, an upstream water level boundary is used in the absence of reliable river flow estimates. Where the downstream boundary is not at a well-defined water level (eg. ocean), a stage-discharge relationship may be specified. In some situations, a hydraulic structure that is inlet controlled acts as the downstream control, in which case, the water level specified downstream of the structure has no influence on the results.

For 2D domains, water level boundaries exhibit the greatest stability (Syme 1991). Flow or velocity boundaries are difficult to specify as the flow direction and distribution across the boundary needs to be defined by the user. Wetting and drying of flow boundaries is also prone to instabilities.

Specifying boundaries oblique to the grid (ie. not parallel to the grid axes or not at 45 to the axes) is also difficult in 2D fixed grid domains. However, TUFLOW has an oblique boundary method that stabilises water level boundaries. This facility is by default on, but can be adjusted using Oblique Boundary Method. For details of the method see Syme 1991.

The recommended approach for 2D flow boundaries is to dynamically link a 1D node to a 2D HX boundary and apply the flow to the 1D node (Syme 1991). The inflow to the 1D node, generates a flow into the 2D domain across the 2D HX boundary. This combination benefits from the stability, wetting and drying performance and the oblique boundary flexibility of water level boundaries. The velocity distribution and direction across the 2D HX boundary is automatically determined by the flow regime that develops in the 2D domain.