There is often a fly-in measurement error in the AHN data set under bridges. This can create bumps in the DEM and slow down the flow. A new advanced option (LOWEST) can correct this, by smearing the lowest DEM value under the bridge and surroundings. This is tested in a number of areas and it solves many flow problems under (small) bridges.

Related documentation:

https://support.tygron.com/wiki/Bridge_elevation_(Water_Overlay)