Overland River Exchange Calculation

Generally, Overland flow will discharge into the River Link if the water elevation in the cell is higher than the bank elevation of the River Link.

Mannings equation

The rate of discharge to the river is dictated by the Mannings calculation for overland flow. However, this flow is only one way, that is ponded water will flow to the river.

There is a tick box to defined the level used to control if flow occurs or not.

· If the tick box is ON, then ponded water will flow to the river if the water level in the cell is above the water level in the River Link (default).

· If the tick box is OFF, then ponded water will flow to the river only if the water level in the cell is above the BANK elevation in the River Link. This was the only option prior to Release 2017. So, to be compatible with older simulations, the tick box must be off.

Wier formula

If you want to include overbank spilling from the river to the overland grid cells, then you must use the weir formula, which provides a mechanism for water to flow back and forth across the river bank. In this case, the bank is treated like a broad crested wier.

Note Whether or not to allow overbank spilling from the river to overland flow is made in MIKE Hydro River for each coupling reach. If you do not allow overbank spilling in MIKE Hydro River, then the overland river exchange is only one way, but uses the weir formula instead of the Mannings formula for calculating the amount of exchange flow.

If you do not use the overbank spilling option, then you can still use the flood inundation option to “flood” a flood plain. In this case, though, the flooding is not calculated as part of the overland flow, but remains part of the water balance of MIKE Hydro River. For more information on the flood inundation method see the section on Flooding from MIKE Hydro River to MIKE SHE using Flood Codes (V1 p. 537).

Threshold head difference for applying low gradient flow reduction

If the difference in water level between the river and the overland flow cell is less than this threshold, then the flow over the weir is reduced to dampen numerical instabilities. In this case, the same damping function is used as in low gradient areas. The damping function essentially increases the resistance to flow between the cell and the river link. This makes the solution more stable and allows for larger time steps. However, the resulting gradients will be artificially high in the affected cells and the solution will begin to diverge from the Mannings solution. At very low gradients this is normally insignificant, but as the gradient increases the differences can become significant.

The damping function is controlled by a minimum head difference between the river and cell below which the damping function become active. Experience suggests that you can get reasonable results with a minimum head difference between 0.05 and 0.1 metres. The default minimum head difference is 0.1. Higher values may lead to a divergence from the Mannings solution. Lower values may lead to more accurate solutions, but at the expense of numerical instabilities, smaller time steps and longer simulation times.