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How to connect parallel river branches or a river with its floodplain in MIKE+ Rivers?

Connections between parallel river branches in MIKE+ can be a bit tricky.  The aim of this Article is to use a ‘Structure Link’ River Type to move water from one river branch to another without having to define cross-sections or (in the case of simulating both Rivers and 2D Overland modules) without linking them to mesh elements. This functionality has been possible in MIKE 11 and is also available in MIKE HYDRO River using the so-called "Link Channels". Here follows an example mimicking a situation where a floodplain or a reservoir/pond or a channel/ditch runs parallel to the main river and overflows into it.

These two river bodies are close to each other as shown in Fig. 1. Here both rivers cross sections are shown at the same graph. The conceptual model is to spill water from the right channel (or floodplain) into the left one (the main river) when water level at the right channel reaches the left bank (marker 1) or vice versa.

Fig. 1 - Small channel or floodplain (top right) overflows into the main river (bottom left)

Although a structure link doesn't require cross sections to be defined, the q-point in the middle of the structure link needs to be defined using a structure (e.g. a culvert). When changing the river type to a Structure link, MIKE+ offers the option to automatically insert an 'open culvert' type of structure, which is applied to represent an overflow weir with a length to include the friction loss component.

What geometry should be used for the culvert inserted on the structure link?

Culverts are structures that allow flows from one point to another through obstructions, such as roadways. To define the culvert, it is needed to specify the Upstream and Downstream Invert levels equal to the adjacent cross-sections bank elevations. Since a culvert is defined as a structure causing a contraction loss, a friction loss (bend loss) and subsequently an expansion loss, the geometry of the culvert must be such that the culvert’s cross-sectional area at the inflow is less than the cross-sectional area of the river upstream of the culvert, and this needs to be true for all water levels. Similarly, the culvert’s cross-sectional area at the outflow end must be less than the cross-sectional area of the river immediately downstream of the culvert.  The friction loss in the structure link between the two river bodies is calculated based on the structure link length defined by the upstream and downstream chainages of the structure link.

Conclusion
Structure links are an effective way to model overflows in parallel river bodies e.g. main river and floodplain. The modeler should be aware of dimensioning the structure (e.g. culvert) in a realistic way without causing instabilities at the rivers banks.