Channel Flow - Technical Reference

The hydrologic components of MIKE SHE are directly coupled to DHI’s river hydraulic engine MIKE 1D. The MIKE SHE-MIKE 1D coupling enables

• the one-dimensional simulation of river flows and water levels using the fully dynamic Saint Venant equations.
• the simulation of a wide range of hydraulic control structures, such as weirs, gates and culverts.
• area-inundation modelling, using a simple flood-mapping procedure that is based on simulated river water levels and a digital terrain model.
• dynamic overland flooding flow to and from the river network.
• the full, dynamic coupling of surface and sub-surface flow processes in the river and MIKE SHE.

For technical information on MIKE 1D, please refer to either the MIKE+ River Network Modelling user guide or the MIKE 1D reference manual. Both can be found under the MIKE+ documentation index.

Note

As of release 2024, MIKE SHE can no longer be coupled to the MIKE 11 engine.

Surface Water Exchange Mechanisms

In a catchment scale model, it is usually sufficient to consider a river as a line located between model grid cells. In this case, the river-aquifer exchange can be calculated inflow to and from both sides of the river, depending on the head gradient to the adjacent groundwater cells. Further, the overland-river exchange is also along a line separating inflow over the right and left river banks.

The line assumption is generally valid if the river width is small relative to the model cells - in other words, in catchment or basin scale models. However, very often a more precise description of the interactions between rivers, flood plains, aquifers and the atmosphere (evapotranspiration) must be adopted. In this context, a reliable description of area-inundation and flood dynamics is crucial.

Thus, the MIKE SHE-River coupling considers three principally different surface water exchange mechanisms, which are described in detail in the section Coupling of MIKE SHE and a MIKE 1D River Model:

• Groundwater exchange with Rivers The river is located on the edge between two adjacent model grid cells. The river is considered a line source/sink to the groundwater and the river is a one-way sink for overland flow.
• Flooding from a MIKE 1D river model to MIKE SHE using Flood The river has a wide cross-section containing the flood plain and designated cells are “flooded” if the river water level is above the topography.
• Direct Overbank Spilling to and from Rivers The river is a line source/sink, but water above the bank elevation is allowed to flood onto the topography as overland flow.

The above options can be mixed in the river network. This allows, for example, Flood Codes in the major flood plain and overbank spilling in the upstream secondary branches, but no flooding in the upland regions with steep slopes and narrow channels. MIKE SHE also automatically converts between the line source/sink option and the flooding options. Thus, during low flow conditions, when the river is narrow (less than one grid size) and water flow is confined to the main river channel, the river-aquifer exchange method is adopted. If the river starts to flood one or more model grid cells, MIKE SHE switches to the area-inundation method or floods the grid cells directly via overbank spilling.