It does look like you freeboard is small as is the dam width. One additional thing to consider is refitting you pipe with the exit end as low as possible and installing an anti-vortex plate to increase the amount of water flowing through your pipe during flooding. The pipe should be repacked well with high clay soil as should any dirt layered on top the dam. Best of luck.
Any ideas where you buy those? I looked just now and the only ones I could find were meant to go on riser style pipes and not horizontal style like mine....
All of the "dirt" around this is clay, so using clay should not be an issue.
