Additional Boundary Sources¶
Additional boundary source contributions enter the model as a source or forcing term, as either the qin in the mass balance equation, Equation (4.1a), or f in the momentum conservation equation, Equation (4.1b).
A source in Equation (4.1a), the mass balance equation, will add water into the model, while a forcing in the momentum equation will accelerate the water in the positive flow direction.
The different additional boundary sources differ in the way they connect to the model and how their contribution is distributed over a part of the network:
- Point source boundaries
- Distributed source boundaries
- Global source boundaries
Each type will be described individually in the coming sections.
Regarding negative inflows (extraction of water from the model): these should be carefully constructed since extraction of more volume than the model has available will end up with mass errors in the computations, and a model that may be unstable and fail to run to completion.
Point source boundaries¶
A point source boundary adds a source contribution to one or more points in the network. It can add contributions to nodes and reach locations.
Each point has an associated scale factor. The boundary value is multiplied with the scale factor before it is applied to the point.
The scale factor can be used by sources that have a default-normed contribution, for example per person, to scale it with the number of persons relating to a given point. It can also be used to distribute one source contribution over a number of points.
Currently supported point source boundaries:
- Inflow boundary
- Node infiltration boundary
Node infiltration is a MIKE+ boundary where the inflow to a node is proportional to the maximum surface area of the node for all water levels lower than the current water level. For manholes, node infiltration is independent of the water level (surface area is constant) while it can vary appreciably from basins.
Distributed source boundaries¶
A distributed source boundary adds a source contribution to one or more reach sections, and sometimes entire reaches in the network. It specifies either a reach section (ID, start and end chainage), or an entire reach.
A distributed boundary can distribute its contribution in three different ways. The distribution types are called
- Total value
- Value per length
- Value per area.
“Total value” distributes the value of the source so that the sum of the amount for all reach sections is equal to the source value. Each reach section receives a contribution matching the relative length of the section compared to the total length of all sections in the source.
“Value per length” assumes that the source contribution is a contribution per length, and the value of the source is multiplied by the length of the reach section.
“Value per area” assumes that the source contribution is a contribution per area unit, and the value of the source is multiplied by the surface area of the reach section. This type is only applicable for open reaches.
Currently supported distributed source boundaries:
- Inflow
- Rainfall runoff inflow. It has always the “Total Value” distribution type.
- Wind field.
Global source boundaries¶
A global source boundary is a boundary that can be applied over the entire network.
A global source boundary has a number of boundary items, each being geo-located and having an (x,y) coordinate.
If only one boundary item is specified, it is valid for the entire network, and no (x,y) point needs to be specified for the item.
If specifying more than one boundary item, each must have a unique (x,y) coordinate. When applying the global source to a specified point, the boundary item that is closest to the point is used.
On top of the global boundary items, it is possible to specify a number of distributed overrides, i.e. distributed source boundaries that override the global value.
Currently supported global source boundaries
- Rainfall
- Evaporation
- Wind field
- Ground Water Leakage
Ground Water Leakage is a MIKE 11 boundary where the discharge out of all h-grid points is a per length contribution which is proportional to 1) a constant specified in HD parameter file, 2) the water depth above ground water head, and 3) the wet perimeter of the cross section.
The ground water head can be specified as a global boundary in the MIKE 11 user interface. If there is no ground water head boundary, then the regular water depth is used.
Equation: Leakage discharge from groundwater interaction proportional to wet perimeter and depth.
where:
CL : Leakage coefficient (user defined)
P : Wet perimeter
D: Water depth above groundwater head
Dx : grid spacing
NOTE: The wet perimeter is calculated as the flow area divided by hydraulic radius. Thus, it is assumed that the flow area and the hydraulic radius and the flow area in the cross sections are the actual flow area and the proper hydraulic radius. This is not the case if Relative Resistance and/or Resistance Radius is used in the cross sections. In these cases care is needed when defining ground water leakage coefficients.