The MIKE SHE Couplings dialogue allows the coupling of all or some of the river branches included in the MIKE HYDRO River set-up to the MIKE SHE groundwater model. MIKE SHE will only exchange water with those branches that are listed in the MIKE SHE Couplings dialogue.
To enable coupling to MIKE SHE, tick the ‘Include MIKE SHE coupling’ check-box at the top of the MIKE SHE Couplings dialogue.
The river branches included in the coupling are those listed in the overview table at the bottom of the page. Branches are added or removed from the MIKE SHE coupling by using the Append ‘+’ and Delete’-’ buttons. The upper part of the dialogue displays the properties of the active branch in the overview table.
The properties of the MIKE SHE linkage of a specific location can be displayed in the Property view if the checkbox of that item is checked in the table. When several table items are checked, it is possible to unselect all of them at the same time by clicking on the Clear selection button above he overview table.
Include all branches
If this button is pressed all branches included in the MIKE HYDRO River set-up are copied to the MIKE SHE coupling page. Branches that should not be in the coupling can subsequently be deleted manually and remaining specifications completed. Thus you may have a large and complex hydraulic model, but couple only the main branches (or part of them) to MIKE SHE.
Note that the Include all branches feature will overwrite existing specifications.
Branch name
Name of the branch where the MIKE SHE coupling should be applied. One branch can be sub-divided into several reaches. A reason for doing so could be to allow different riverbed leakage coefficients for different reaches of the river.
Upstream chainage
Chainage of the upstream extent of a MIKE SHE coupling.
Downstream chainage
Chainage of the downstream extent of a MIKE SHE coupling. If the downstream chainage is set equal to the upstream chainage, the coupling will be performed in a single point on the river branch.
Conductance
The conductance can be defined through three different types of exchange between surface water and aquifer (described in the technical documentation of the MIKE SHE User Manual). The available options are:
· Aquifer only
· River bed only
· Aquifer + river bed
Leakage coeff.
Leakage coefficient for the riverbed lining (see MIKE SHE documentation). The leakage coefficient is relevant only if the conductance type is either 'Aquifer + river bed' or 'River bed only'.
Linear reservoir exchange
When applying the Linear Reservoir mode for Saturated Flow in MIKE SHE, exchange between the river and the baseflow can be defined individually in coupling reaches as either a Gaining river or a Losing river. That is, only one directional exchange is possible for this option.
· Gaining river. In Gaining river, the river receives water from the baseflow reservoirs in MIKE SHE
· Losing river. In Losing river, the river loses water to the baseflow reservoirs in MIKE SHE as a function of the leakage coefficient, water depth in river, bank width and length of the coupling reach.
Overland-river exchange
When the Weir formula option has been selected in MIKE SHE for Overland-River exchange calculation in MIKE SHE, the overland spilling will be calculated using the 'Villemonte Formula' weir equation. The parameters below, required for this equation, must be defined for each coupling reach.
Weir coefficient
This parameter refers to the weir coefficient in the Villemonte Formula.
Weir exponential coeff
This parameter refers to the weir exponent in the Villemonte Formula.
Min. upstream height above bank for full weir width
In the Villemonte Formula, when upstream water depth above the weir approaches zero, the flow over the weir becomes undefined. To prevent numerical problems, the flow is reduced linearly to zero when the water depth is below the minimum upstream height threshold.
Allow overbank spilling from river to overland domain
Activate this checkbox to allow overbank spilling from the river to the overland grid cells. If left unchecked, the exchange of overland flow is only one-directional. That is, from the overland model to the river.
Min. flow area for overbank spilling
The minimum flow area threshold prevents overbank spilling when the river is nearly dry. The flow area is calculated by dividing the volume of water in a coupling reach by the length of the reach. If the calculated flow area is less than the specified threshold value, overbank spilling from the river to the overland flow is excluded.
Flood area option
The Flood area option allows a number of cells from the MIKE SHE model to be flooded (being part of a river, lake, reservoir etc.) based on the water level from the river model. When this option is active, a river or a lake (with wide cross sections) may cause flooding of a number of cells in MIKE SHE. A correspondence / mapping is established between MIKE HYDRO h-points and individual cells in MIKE SHE. Subsequently a simple flood-mapping procedure is adopted to calculate water depths on the ground in MIKE SHE. The flood mapping procedure simply compares simulated water levels (in the corresponding h-point) with the ground level in the MIKE SHE cells. If the water level is higher than the ground level, flooding occurs.
Three options are available:
· No flooding
· Automatic
· Manual
The selected option may differ for each coupled reach. These options are described below.
No flooding
If the No flooding option is adopted, rivers are considered lines located between adjacent cells from the MIKE SHE model. No flooding can occur and overbank spilling is not possible, even if the water level in the river rises above the elevation of the adjacent cells in MIKE SHE.
River-Overland exchange is always one way with this option, namely overland to river. If this option is used, one of the three river-aquifer exchange formulations will be adopted.
Manual
The manual option allows the user to delineate the potentially flooded areas, using a T2 grid code file - the flood code file specified in MIKE SHE's user interface. If the river system considered is a very complex system with a looped or meandering network, generating a complex geometry, the best result will be obtained by creating a flood code file manually by digitizing the floodplain / lake, and using this option.
Each (potentially flooded) MIKE SHE cell is linked to the nearest MIKE HYDRO h-point on a coupling reach with the same flood code value. The flood code specified for each coupling reach must match the grid code in the flood code file. The flood code must be an integer.
Automatic
The automatic option is often useful if the geometry of rivers, lakes etc. is not too complex. This option may, for instance, be applied for wide rivers without too much meandering.
When the automatic option is chosen, MIKE SHE will automatically generate the potentially flooded areas (flood grid code map) depending on the location of the river branches and on the width and location of their cross-sections. The specified flood code is used as grid code, and the flood-mapping procedure described above is applied. Thus it is important to use unique flood codes to ensure correct mapping between MIKE HYDRO and MIKE SHE.
Flood code
As described above the flood code is used for mapping MIKE SHE cells to MIKE HYDRO h-points, and for the automatic option also for generating the flood grid codes of the coupling reach. It is important to use unique flood codes to ensure correct flood-mapping.
Bed topography
This option controls the source of the topography used in the flooded area. It should be emphasized that the flood mapping requires a good consistency between the MIKE HYDRO cross-sections and the ground levels in MIKE SHE in order to provide relevant results.
The options are:
· Use cross sections
When this option is used, the ground levels of the flooded cells are substituted with values directly interpolated from the MIKE HYDRO cross-sections. Notes on this option:
- – This interpolation uses an inverse-distance-weighted method, using points (elevations) on the MIKE HYDRO cross-sections as discrete input points. When the distance between MIKE HYDRO cross-sections is higher than half the cell size, extra cross sections are interpolated before the cells' ground levels are interpolated.
- – Please note that the ground levels are only interpolated inside the area delineated by the MIKE HYDRO cross-sections.
- – When the Manual option is used, the user defined flood area does not have to be identical to the area covered by the MIKE HYDRO cross sections. If the Automatic option is used, the area covered by the MIKE HYDRO cross-sections and the flood area will always be consistent, as the flood-area is generated (automatically) based on the MIKE HYDRO cross-sections.
- – In principle the Use cross-sections option ensures a good consistency between MIKE SHE ground levels and MIKE HYDRO cross-sections. There will, however, often be interpolation problems related to river meandering, tributary connections, etc. where wide cross-sections of separate coupling reaches overlap. Thus it is recommended to make the initial model set-up using the Use cross-section option and then subsequently retrieve and check the resulting ground levels (using the MIKE SHE Input Retrieval tool). If needed the retrieved ground levels (T2 file) can be modified (MIKE SHE Graphical Editor) and then used as input for a new set-up, now using the Use grid data option described below.
· Use Grid Data
MIKE SHE grid data is used instead of MIKE HYDRO cross-sections. This option checks whether the optional bed elevation file has been specified in MIKE SHE's user interface:
- – If the bed elevation file has been specified, the ground levels of the flooded cells are substituted with values from the specified T2 file. The option is useful when the surface elevation data of the flood areas is more detailed than the regional terrain model.
- – If the bed elevation file has NOT been specified the regional MIKE SHE surface topography is also used in flood areas. As described above the specified T2 file will often be a retrieved and modified surface topography from a previous set-up with the Use cross sections option.
Bed leakage
As described in the technical documentation of MIKE SHE, the infiltration/seepage of MIKE SHE is calculated as ordinary exchange from the overland domain to the saturated or unsaturated zone, either using full contact or reduced contact with a specified leakage coefficient.
The bed leakage option tells whether the overland-groundwater exchange option and leakage coefficient specified in MIKE SHE's user interface should also be used in the actual flood area, or substituted by the corresponding river-aquifer conductance type and Leakage coefficient specified for the coupling reach.
Options are:
· Use grid data
The overland-groundwater exchange option and leakage coefficient specified in MIKE SHE's user interface is used. Both can be single value or distributed (T2 file).
· Use river data
The MIKE SHE overland-groundwater exchange option and leakage coefficient in flood areas are substituted with the corresponding river-aquifer conductance type and leakage coefficient specified for the coupling reach. Please note that the two reduced contact options (exchange types B and C) lead to the same overland-groundwater exchange option.
The substitution is made in all cells from the flood area of the coupling reach.