Description
In MIKE+ when the water level of a node goes above ground the standard behavior of manholes with "normal" cover is to expand gradually the actual manhole surface to a fictive basin with area 1000x larger than the manhole area. The expansion starts at an elevation controlled by the engine parameter RESERVOIRHEIGHT, with a default value -0.25 m, i.e. 25 cm below ground. This works fine for manholes, because the fictive volume added at the start of the expansion is not significant. However, for basins this practice can be conflicting, because RESERVOIRHEIGHT starts expanding relative to the ground level, and above this expansion the specified basin geometry is replaced by the “standard” expanded basin, which can be problematic as the large surface of the basin may lead to unrealistic expansions with significant impact on the results.
For example, the table below describes a shallow wide basin with a surface area of 1000 square meters at a ground level (i.e. elevation 0.5 m) and a surface area of 100 square meters at 0.25m below ground (elevation 0.25 meters). A basin with this geometry should have a volume of 162.5 cubic meters - see Fig. 1.
Fig. 1 - Curve definition for a shallow basin
For this basin with a constant inflow of 10 L/s, it is expected that the basin will be full (WL = 0.5m) after 16250 seconds (4.5hours). However, with the basin following the expansion as in manholes the basin is full already after 2.5 hours - see Fig. 2.
Fig. 2 - Shallow basin water level time series.
Another example of an incorrect computation would be the implementation of a cylindrical basin with a surface area of a 100 square meters. In this case with the same elevation quotes the basin should have a maximum capacity of 50 cubic meters - see Fig. 3.
Fig. 3 - Cylindrical basin geometry curve
Similarly with a constant inflow of 10 L/s, it is expected that the basin will be full (i.e. at WL = 0.5 m) after 5000 seconds (approx. 1.4 hours). But with the expansion described above, the model needs 2.5 hours to reach full capacity in the basin. This is because the prismatic basin is replaced by a standard expansion, which in this case introduced a very large additional fictive volume to the basin which is also incorrect - see Fig. 4.
Fig. 4 - Cylindrical basin water level time series.
With MIKE+ 2025 Update1 this situation has been improved, so that basins no longer have to follow the same expansion as applied to manholes.
A new parameter has been added to the MIKE 1D Engine Configuration window. This parameter is named “Basin Cover Expansion Coefficient” and it controls the expansion factor applied to basin and soakaway nodes, with normal or spilling covers: their surface area expands to this coefficient times the area at the top of the basin geometry curve. The default value for this parameter is 1' meaning that there is no expansion by default - see Fig. 5.
Fig. 5 - MIKE 1D Engine Configuration, Basin cover expansion coefficient.
When a model created in an older version is upgraded to 2025 Update1, the value for the parameter “Basin cover expansion coefficient” will be set equal to the “Node cover expansion coefficient”, keeping the same factor as manholes, allowing backward compatibility between versions. At the same time the “Manhole cover expansion coefficient” will not be changed.
With this update a change in the engine is included for the parameter “Node cover start water level” which will no longer apply to basin and soakaway nodes. This change is reflected on the engine configuration description of this item - see Fig. 6.
Fig. 6 - MIKE 1D Engine Configuration, Node cover start water level.
In order to avoid conflicts in the storage volume computed for basin and soakaway structures, being under or over estimated as shown in the examples in this article, the start level of the expansion for basins and soakaways in case of overflow is set equal to the maximum level defined on the geometry curve or the ground level of the node, depending on which is higher.
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