MIKE Hydro River Cross-sections

Whenever there is a significant change in the bed slope there should, in prin­ciple, be a cross-section defined in MIKE Hydro River. If only a few cross-sec­tions are available, it may be sufficient to estimate the cross-section shape based on neighbouring cross-sections and estimate the bank/bed elevation based on the surface topographic information in MIKE SHE or other topo­graphic maps.

Cross-sections vs. time step

However, every cross-section in MIKE Hydro River is a calculation node. The time step in MIKE Hydro River is sensitive to the Courant number, which is proportional to the distance between calculation nodes. So, if the cross-sec­tions are close together, then you may experience very short time steps in MIKE Hydro River.

Thus, if you are have very short MIKE Hydro River time steps, then you might want to check your river network to make sure you do not have cross-sec­tions that are too close together. This frequently occurs when the cross-sec­tions have been imported. If you do have cross-sections that are too close together, then you can easily eliminate one or more of them, as long as the conveyance of the different cross-sections is roughly the same. In other words, you can eliminate duplicate cross-sections if their Q/H relationships are roughly the same, even though the physical shape of the two cross-sec­tions may appear quite different. This is often the case in braided stream net­works, where the location of the main channels may move left or right, but the overall conveyance of the river bed is relatively constant.

Cross-sections versus MIKE SHE topography

In the absence of flooding, ponded water discharges to the MIKE Hydro River as overland flow. As a general rule, the topography must be higher than or equal to the bank elevation. If the bank elevation is higher than the topogra­phy, water will not be able to flow into the river in that cell, but will run laterally along the river until it reaches a place for it to flow into the river. An easy trick to see where this is happening is to run a simulation with no infiltration, ET, or detention storage and set the initial water depth at 1m. Then look at the results to find places were the water is piling up against the river links.

In the pre-processor log file, a table is create that lists all the river links where the bank elevation is different than the topography.of the adjacent cell. The critical river links with bank elevations above the topography are highlighted with the ==> symbol. This list can be surprisingly long because the river link bank elevations are interpolated from the neighbouring cross-sections. Whereas the topography is already defined. So, frequently the interpolated bank elevations do not line up precisely with the topography.

If overland flow on the flood plain is essentially absent, for example, due to infiltration or evapotranspiration, then these differences are not relevant and there is no need to modify the topography. However, if the overland to river exchange is important then you may have to carefully modify your topography file or your bank elevations so that they are consistent.

Hint In many cases, your topography is from a DEM that is different from your model grid - either because it is a .shp or xyz file, or if it is a different resolu­tion than your model grid. In this case, it may be easier to save the pre-pro­cessed topography to a dfs2 file (right click on the topography map in the pre-processed tab). Then modify and use the new dfs2 file as the topography in your model setup. The disadvantage of this, is that if you change your model domain or grid, then you will have to redo your topography modifications.

Hint You can also use one of the Flood code options to automatically modify your topography, if you have wide cross-sections or a detailed DEM of the floodplain. In this case, after you have set up your MIKE Hydro River model, you can specify a constant grid code for the whole model and let MIKE SHE calculate a modified topography based on the cross-sections or bathymetry. Then save the topography file as above and then use it as the model topogra­phy.