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 seperating 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 MIKE Hydro River (V1 p. 523):

·         Groundwater exchange with MIKE Hydro River (V1 p. 531) 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 MIKE Hydro River to MIKE SHE using Flood Codes (V1 p. 537) 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 MIKE Hydro River (V1 p. 539) 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.