Weir Data for overland-river exchange
The choice of using the weir formula for overland-river exchange is a global choice made in the MIKE SHE OL Computational Control Parameters (V1 p. 191) dialogue. If the weir option is chosen in MIKE SHE, then all MIKE Hydro River coupling reaches will use the weir formula for moving water across the river bank.The weir option is typically used when you want to simulate overbank spilling and detailed 2D surface flow in the flood plains. The following parameters and options are available when you specify the weir option in MIKE SHE. If you chose the Manning equation option in MIKE SHE, then these parameters are ignored.
Weir coefficient and Head exponent
The Weir coefficient and head exponent refer to the C and k terms respectively in Equation (27.8). The default values are generally reasonable. Both the weir coefficient and the head exponent are dimensionless.
Minimum upstream height above bank for full weir width
In Equation (27.8), when the 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. The EUM data type is Water Depth.
Allow overbank spilling
This checkbox lets you define which branches are allowed to flood over their banks. Thus, you can allow flooding from MIKE Hydro River only in branches with defined flood plains, or only in areas of particular interest.
If overbank spilling is not allowed for a particular branch, then the overland-river exchange is still calculated using the weir formula, but the exchange is only one way - that is from overland flow to the river.
Minimum 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 the coupling reach by the length of the reach. So, you can think of this threshold as a minimum (river volume)/(length of river) before overbank spilling can occur. The EUM data type is Flow Area, which by default is m2.
The default value is 1 m3/m length of river. This is quite a small amount of water for most reasonable rivers and should be adjusted based on the river width. For example for the default value of 1 m3/m,
· if the width of your river is 10m wide, then spilling will occur when the water level is 10cm above the bank elevation.
· if the width of your river is 200m wide, then spilling would start when the water level is only 5mm above the bank elevation.
The cell size also plays a role in determining a reasonable threshold value. When a cell is flooded, the entire cell is covered by water. If the cell size is 1000m x 1000m, then a discharge onto the flood plain of 1 m3/m of river will be only 1mm deep across the cell.
Thus, a value of 1-5 m3/m is probably reasonable for small rivers (10-20m wide) and small grid cells(50-100m). For larger rivers (+50m wide) and larger grid cells (200-500m), a value of 10-50 m3/m is probably more reasonable.