Boundary conditions

The default outer boundary condition for the overland flow solver is a speci­fied head, based on the initial water depth in the outer nodes of the model domain. Thus, if the water depth inside the model domain is greater than the initial depth on the boundary, water will flow out of the model. If the water depth is less than the initial depth on the boundary, the boundary will act as a source of water.

There is an Extra Parameter option that allows you to define a time varying outer boundary condition for Overland Flow. In this case, the outer boundary is still a fixed head boundary, but the fixed head is defined by a time-varying dfs2 file, instead of the initial water depth.

The boundary condition to the River can be calculated based on a Mannings resistance formulation, where the length is the distance from the node to the edge of the cell, the head difference is that between the cell and the River Link, and the depth of water is the depth above the detention storage.

The boundary condition to the river can also be calculated based on a stand­ard weir formulation. The weir formulation must be used if two-way flow is calculated between the river and the cell. 

In this case, the weir formula is a standard expression based on the Ville­monte formula:

(24.18)   

where Q is the discharge over the weir, W is the weir width, C is the weir coef­ficient, k is the weir exponent, H_US is the upstream depth, H_DS is the down­stream depth and H_W is the crest level of the weir.

As the upstream water depth approaches zero, the flow over the weir becomes undefined. To prevent numerical problems, the flow is reduced line­arly to zero when the water depth is below the user-defined minimum depth for full weir flow (assumes a triangular weir cross-section).

A minimum flow area threshold is defined to reduce numerical issues with small volumes of transfer from the river to the floodplain, The flow area is cal­culated by dividing the volume of water in the Coupling Reach by the length of the Coupling Reach. If the calculated area is less than the minimum flow area, then overbank spilling from the river is excluded.

Note: the minimum flow area threshold could unduly restrict overbank spilling if you have a long Coupling Reach and a small river section where overbank spilling occurs.