# HydroGeoSphere/Surface Flow

You can assign a critical depth or zero depth gradient boundary condition, as described in Section 3.7.2 to segments in the specified set. These segments should be part of the overland flow domain and are typically located on the outer boundary.

The following instructions can be used as input to the Type instruction inside the Boundary condition...End instruction group to assign critical depth boundary conditions:

## Critical depth

editSets the input type to be a critical depth boundary condition.

**• • •**

For example:

```
boundary condition
type
critical depth
segment set
outflow
end
```

This example shows that a time-value table is not mandatory. However, if you wish to turn the critical depth boundary condition on or off, then this can be accomplished by including a time value table instruction:

```
boundary condition
type
critical depth
segment set
outflow
time value table
0.0 1.0
10.0 -99999.
end
end
```

The value 1.0 at time zero is ignored, but the NODATA value -99999 at time 10.0 causes the boundary condition to be turned off and the nodes become unconstrained.

## Zero-depth gradient

editSets the input type to be a zero-depth gradient boundary boundary condition.

**• • •**

For example:

```
boundary condition
type
zero-depth gradient
segment set
outflow
end
```

If you wish to turn the zero-depth gradient boundary condition on or off, then this can be accomplished by including a time value table instruction.

**NOTE: zero-depth gradient boundaries used to include variables defining the direction of boundary gradient and the bed slope at the boundary.**