get a bit of pre stressed concrete ... bison slab ,,, job doneNo flooding risk, varies between a trickle and about a foot of water wide. Span in reality is 4 foot. Needs to be movable as it silts /grows in in summer.
Its crossed my mind to make a concrete mould, from a sheet of osb, two layers of mesh, then 4 inches of concrete, tamped finish
oh and hasnt got to look like scrap heap challenge!