... all of the many small bridges in the region are specifically designed to be washed away easily. It's simply cheaper to build them new ones after every serious flood.
The Icelandic director of transport states in this mornings paper that "bridges are expensive". Just rebuilding two particular bridges that are candidates for being washed away would cost roughly 25 million USD. In addition there's the disruption and inconvenience until some rudimentary bridges have been put in place.
The plan is therefore to try to save the bridges. If necessary, heavy machinery will be used to dig the roads apart the allow the flood to go that way. Rebuilding roads is a comparatively quick and painless process.