Assuming this was on a spread shallow footer. Obviously failed as approx 50% of bridge failures do, by scour which undermined this abutment. Sediment is normally carried and re deposited in any rain event, especially this record event. It would have been difficult to determine how undermined such a footer had become since sediment was re deposited as normal events subside, once again covering the footer. It is a difficult problem that inspectors receive training on from the National Highway Institute called "Stream stability and scour for highway bridges". It is a complicated engineering issue that has many variables, this was probably highly erodible sandy soil. And had the abutment been better protected it could have forced flow to damage the next pier. Very fortunate that only one was hurt.

We will see what they come up with.